2009 IMPERIAL COUNTY 

STATE IMPLEMENTATION PLAN FOR 

PARTICULATE MATTER LESS THAN 

10 MICRONS IN AERODYNAMIC DIAMETER 

FINAL 

Prepared for 

Imperial County Air Pollution Control District 

150 South 9th Street 

El Centro, CA. 92243-2801 

Prepared by 

ENVIRON International Corporation 

707 Wilshire Blvd., Suite 4950 

Los Angeles, CA 90017 

August 11, 2009
Imperial County 2009 PM10 SIP 

FINAL AUGUST 2009 ICAPCD 

Preface to the Final Imperial County 2009 PM10 SIP 

(Dated August 11, 2009) 

On May 20, 2009, the Imperial County Air Pollution Control District (ICAPCD) released the Draft 

Imperial County 2009 PM10 SIP (PM10 is small airborne particulate matter less than 10 microns 

in diameter). On June 3-4, 2009, ICAPCD held four public workshops to discuss the Draft 

Imperial County 2009 PM10 SIP (dated May 2009) and to take comments from the public. 

Based on comments made at the workshops, both verbal and written comments submitted to 

the ICAPCD, and on-going cooperative efforts with air and other agencies, ICAPCD prepared a 

Draft Final Imperial County 2009 PM10 SIP (dated July 10, 2009); it contained, among other 

revisions, the revised transportation conformity budgets that were considered for approval by 

the ICAPCD Governing Board (Chapter 6) and additional information on windblown emissions 

from vacant lands (Appendix III.B). The public hearing for the Imperial County 2009 PM10 SIP 

was scheduled for August 11, 2009. 

Since the release of the July 10, 2009 Draft Final Imperial County 2009 PM10 SIP, U.S. 

Environmental Protection Agency (EPA) staff requested additional information on, and analysis 

of, windblown emissions from vacant lands disturbed by off-highway vehicle (OHV) use. This 

resulted in minor revisions to the emission inventory and related SIP text, particularly in 

Chapters 3 and 4, and Appendix III.A and III.B. Other minor revisions, including the correction 

of some typographical errors, were made. Based on all of these revisions, ICAPCD prepared a 

Draft Final Imperial County 2009 PM10 SIP (dated July 30, 2009). An ‘Erratum Sheet’ 

documenting those revisions was prepared and released on July 31, 2009. 

On August 11, 2009, the ICAPCD Board held a public hearing and unanimously adopted the 

Imperial County 2009 PM10 SIP. The Board’s action included: 

• Approval and adoption of the Draft Final Imperial County 2009 PM10 SIP (dated July 10, 

2009), with changes as specified in the July 31, 2009 Errata Sheet; 

• Adoption of the findings in the associated Staff Report; 

• Certification of the Negative Declaration for the 2009 PM10 SIP; 

• Adoption of the transportation conformity budgets in the Imperial County 2009 PM10 SIP; 

and 

• Direction to staff to submit the Imperial County PM10 SIP and related documents to the 

California Air Resources Board for their review and action. 

This Final Imperial County 2009 PM10 SIP (dated August 11, 2009), as adopted, consists of the 

Draft Final Imperial County 2009 PM10 SIP (dated July 10, 2009), with revisions described in the 

July 31st Erratum Sheet, as well as minor corrections to footnote numbering and typographical 

errors. 
Imperial County 2009 PM10 SIP 

FINAL AUGUST 2009 i ICAPCD 

Contents 

1 Introduction ...................................................................................................................................1-1

1.1 Particulate Matter Air Pollution .....................................................................................................1-1

1.2 Imperial County.............................................................................................................................1-2

1.2.1 Geography, Population, and Land Use.........................................................................................1-2

1.2.2 Climate and Meteorology..............................................................................................................1-3

1.2.3 Atmospheric Stability and Dispersion ...........................................................................................1-3

1.3 Imperial County 2009 PM10 SIP....................................................................................................1-4

1.3.1 Background...................................................................................................................................1-4

1.3.2 Summary of Planning Requirements............................................................................................1-4

1.4 Regulatory Responsibility .............................................................................................................1-5

1.4.1 USEPA..........................................................................................................................................1-5

1.4.2 California Air Resources Board (CARB).......................................................................................1-5

1.4.3 Imperial County Air Pollution Control District (ICAPCD)...............................................................1-6

1.5 Document Organization ................................................................................................................1-6

2 Ambient Data ................................................................................................................................2-1

2.1 Background...................................................................................................................................2-1

2.1.1 Emissions from Outside the United States ...................................................................................2-1

2.1.2 Exceptional Events .......................................................................................................................2-1

2.2 Ambient Air Quality Data (2001 to 2008)......................................................................................2-2

2.2.1 Imperial County Air Monitoring Stations .......................................................................................2-2

2.2.2 Air Quality Data During 2001-2005...............................................................................................2-3

2.2.3 Ambient Air Quality in the Time Period from 2006 to 2008 ..........................................................2-4

2.2.4 Exceedences of the 24-Hour PM10 NAAQS from 2006 to 2008 ...................................................2-4

3 Inventory and Analysis of Significant Sources..............................................................................3-1

3.1 Emission Inventory........................................................................................................................3-1

3.1.1 Important Recent Adjustments to the Imperial County PM10 Emission Inventory ........................3-1

3.1.2 2005 Base Year and Future Inventories.......................................................................................3-3

3.2 Determination of Significant Sources of PM10 ..............................................................................3-6

3.2.1 Background...................................................................................................................................3-6

3.2.2 Implementation .............................................................................................................................3-6

3.2.3 Classification of PM10 Source Categories by Significance ...........................................................3-9

3.2.4 Major PM10 Stationary Sources ....................................................................................................3-9

4 CONTROL STRATEGIES AND IMPACT ANALYSIS ..................................................................4-1

4.1 Control Strategies .........................................................................................................................4-1

4.1.1 Rule 801: Construction and Earthmoving Activities......................................................................4-1

4.1.2 Rule 802: Bulk Materials...............................................................................................................4-2

4.1.3 Rule 803: Carry-Out and Track-Out..............................................................................................4-2

4.1.4 Rule 804: Open Areas ..................................................................................................................4-3

4.1.5 Rule 805: Paved and Unpaved Roads .........................................................................................4-3

4.1.6 Rule 806: Conservation Management Practices ..........................................................................4-4

4.1.7 Record of Control Implementation................................................................................................4-4

4.2 Impact Analysis and Cost Information ..........................................................................................4-5

4.2.1 Rule 801: Construction ................................................................................................................4-6
Imperial County 2009 PM10 SIP 

FINAL AUGUST 2009 ii ICAPCD 

4.2.2 Rule 802: Bulk Materials..............................................................................................................4-6

4.2.3 Rule 803: Carry-Out and Track-Out.............................................................................................4-6

4.2.4 Rule 804: Open Areas .................................................................................................................4-7

4.2.5 Rule 805: Unpaved Roads and New or Modified Paved Roads..................................................4-7

4.2.6 Rule 806: Conservation Management Practices .........................................................................4-8

4.3 BACM Determination ....................................................................................................................4-8

5 Attainment Demonstration ............................................................................................................5-1

5.1 Introduction ...................................................................................................................................5-1

5.2 Analyses of Transport Episodes ...................................................................................................5-1

5.2.1 Methods of Analysis......................................................................................................................5-3

5.2.2 Analysis for December 21, 2006...................................................................................................5-6

5.2.3 Analysis for December 25, 2006...................................................................................................5-8

5.3 Implications of Results of Transport Analyses..............................................................................5-9

6 Other Clean Air Act Requirements ...............................................................................................6-1

6.1 Contingency Measures .................................................................................................................6-1

6.2 Transportation Conformity ............................................................................................................6-3

7 Salton Sea Considerations ...........................................................................................................7-1

8 Conclusions and SIP Checklist.....................................................................................................8-1

Tables 

Table 1.1 National and State Ambient Air Quality Standards for Particulate Matter .............................1-1

Table 1.2 Contents of Imperial County 2009 PM10 SIP Document........................................................1-6

Table 2.1 PM10 Measurements in Excess of the 24-Hour NAAQS in Imperial County for the Time 

Period 2006-2008...................................................................................................................2-4

Table 3.1 Windblown Dust Model Estimatesa of PM10 Emissions (in tons/day) in Imperial County 

(Reported According to Land Use/Land Coverage)...............................................................3-2

Table 3.2 2005 Estimates of Entrained and Windblown PM10 Emission from Unpaved Roads 

(Annual Averages) .................................................................................................................3-3

Table 3.3 PM10 Emission Inventory for Imperial County in Baseline Year 2005a (tpd)..........................3-4

Table 3.4 Imperial County PM10 Emission Inventorya in 2006-2010 (tpd) .............................................3-5

Table 3.5 PM10 Measurements in Violation of the 24-Hour NAAQS in 2006-2008................................3-6

Table 3.6 Estimated Imperial County PM10 Emission Inventory for December 21 and December 

25, 2006, and Predicted Contributions of Emissions to the Measured Calexico-Grant 

Exceedences by Source Category.........................................................................................3-8

Table 3.7 Significant Source Categories in 2006-2008 .........................................................................3-9

Table 4.1 Imperial County Regulation VIII Emission Reduction Summary (Based on the 2005 

Annual Average Inventory)a ...................................................................................................4-5

Table 4.2 Comparative Stringency of Controls for Significant Source Categories ..............................4-10

Table 6.1 Imperial County PM10 Measurements > 140 μg/m3 in 2006-2008 Excluding Exceptional 

Eventsa

...................................................................................................................................6-2

Table 6.2 Imperial County PM10 Motor Vehicle Emission Budgets (tpd) ...............................................6-5

Table 8.1 SIP Checklist..........................................................................................................................8-1
Imperial County 2009 PM10 SIP 

FINAL AUGUST 2009 iii ICAPCD 

Figures 

Figure 1.1 Road map of Imperial County.................................................................................................1-3

Figure 2.1 Locations of Imperial County PM10 monitoring stations..........................................................2-3

Figure 2.2 Plots of 24-hour PM10 ambient air concentration in Imperial County during 2006-2008I. ......2-5

Figure 5.1 Satellite image showing the proximity and relative sizes of Calexico and Mexicali. ..............5-2

Figure 6.1 Imperial County PM10 non-attainment area as represented in SCAG model. ........................6-4

Appendices 

Appendix I: USEPA Finding of Failure to Attain 

Appendix II: Imperial County Air Monitoring Information: 2006-2008 

Appendix III.A: Imperial County Uncontrolled PM10 Emission Inventory 

Appendix III.B: Windblown Dust Emissions from Vacant Lands 

Appendix IV.A: Imperial County Fugitive Dust Control Program: Regulation VIII Rules 

Appendix IV.B: Cost Effectiveness Analysis of ICAPCD Regulation VIII Rules 

Appendix IV.C: Imperial County PM10 Emissions Accounting for Regulation VIII Fugitive Dust Controls 

Appendix V: Exceedences of the 24-Hour PM10 NAAQS at the US-Mexico Border: Analysis of the 

Impact of International Emissions 

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[0521191A] 
Imperial County 2009 PM10 SIP 

FINAL AUGUST 2009 iv ICAPCD 

Abbreviations and Acronyms 

AVTD Average Vehicle Trips per Day 

BACM Best Available Control Measure 

BACT Best Available Control Technology 

CAA Federal Clean Air Act 

CARB California Air Resources Board 

CMP Conservation Management Practice (agriculture) 

DM de minimis 

ICAPCD Imperial County Air Pollution Control District 

ICPWD Imperial County Public Works Department 

IID Imperial Irrigation District 

NAAQS National Ambient Air Quality Standards 

NEAP Natural Events Action Plan 

PM particulate matter 

PM10 particulate mater less than 10 microns in aerodynamic diameter 

RACM Reasonably Available Control Measure 

RFP Reasonable Future Progress 

SCAG Southern California Association of Governments (MPO) 

SIP State Implementation Plan 

SSI Size Selective Inlet 

TPA Regional Transportation Planning Agency 

tpd Tons per Day 

USEPA United States Environmental Protection Agency 

VDT Vehicle daily trips 

VDE Visible Dust Emissions 

WESTAR Western States Air Resources Council 

WRAP Western Regional Air Partnership 

μg/m3

 microgram per cubic meter 

μm micron or micrometer 
Imperial County 2009 PM10 SIP Chapter 1: Introduction 

FINAL AUGUST 2009 1-1 ICAPCD 

1 Introduction 

This document brings together the necessary data and discussion in presenting the State 

Implementation Plan (SIP) for particulate matter less than 10 microns in aerodynamic diameter, 

PM10, on behalf of the Imperial County Air Pollution Control District (ICAPCD or the District). 

This chapter provides an overview of particulate matter as an air pollutant, a brief description of 

the Imperial County area, and a discussion of the purpose, regulatory background, and 

regulatory agencies concerned with this SIP. 

1.1 Particulate Matter Air Pollution 

Particulate matter (PM) is a general term used to describe a complex group of airborne solid, 

liquid, or semi-volatile materials of various size and composition. Primary PM is emitted directly 

into the atmosphere from both human activities (including agricultural operations, industrial 

processes, construction and demolition activities, and entrainment of road dust into the air) and 

non-anthropogenic activities (such as windblown dust and ash resulting from forest fires). 

Secondary PM is formed in the atmosphere from predominantly gaseous combustion by-product 

precursors, such as sulfur and nitrogen oxides, and volatile organic compounds. The 

overwhelming majority of airborne PM in Imperial County is primary PM. The major source of 

primary PM is fugitive windblown dust, with other contributions from entrained road dust, 

farming, and construction activities. 

Particle size is a critical characteristic of PM that primarily determines the location of PM 

deposition along the respiratory system (and associated health effects) as well as the 

degradation of visibility through light scattering. In the United States, federal and state agencies 

have established two types of PM air quality standards, reported in the Table 1.1 below. PM10

corresponds to the fraction of PM no greater than 10 microns or micrometers (μm) in 

aerodynamic diameter, while PM2.5 refers to the subset of PM10 of aerodynamic diameter 

smaller than 2.5 μm, which is commonly called fine particulate matter. The state standards are 

presented for comparative purposes and are otherwise outside of the scope of this SIP 

document. 

Table 1.1 National and State Ambient Air Quality Standards for Particulate Matter

Pollutant Averaging Time California Standards National Standards 

Annual 20 μg/m3 Respirable Particulate -- 

Matter (PM10) 24-hour 50 μg/m3

 150 μg/m3

Annual 12 μg/m3 15 μg/m3 Fine Particulate 

Matter (PM2.5) 24-hour --- 35 μg/m3

 

PM air pollution has undesirable and detrimental environmental effects. PM affects vegetation, 

both directly (e.g. deposition of nitrates and sulfates may cause direct foliar damage) and 

indirectly (e.g. coating of plants upon gravitational settling reduces light absorption). PM also 

accumulates to form regional haze, which reduces visibility due to scattering of light. Agencies 

concerned with haze include the National Park Service, the U.S. Forest Service, the Western 

Regional Air Partnership (WRAP), and the Western States Air Resources Council (WESTAR). 
Imperial County 2009 PM10 SIP Chapter 1: Introduction 

FINAL AUGUST 2009 1-2 ICAPCD 

PM10 is respirable, with fine and ultrafine particles reaching the alveoli deep in the lungs, and 

larger particles depositing principally in the nose and throat area. PM10 deposition in the lungs 

results in irritation that triggers a range of inflammation responses, such as mucus secretion and 

bronchoconstriction, and exacerbates pulmonary dysfunctions, such as asthma, emphysema, 

and chronic bronchitis. Sufficiently small particles may penetrate into the bloodstream and 

impact functions such as blood coagulation, cardiac autonomic control, and mobilization of 

inflammatory cells from the bone marrow. Individuals susceptible to higher health risks from 

exposure to PM10 airborne pollution include children, the elderly, smokers, and people of all 

ages with low pulmonary/cardiovascular function. For these individuals in particular, adverse 

health effects of PM10 pollution include coughing, wheezing, shortness of breath, phlegm, 

bronchitis, and aggravation of lung or heart disease, leading for example to increased risks of 

hospitalization and mortality from asthma attacks and heart attacks.1

1.2 Imperial County 

1.2.1 Geography, Population, and Land Use 

Imperial County extends over 4,597 square miles2 in the southeastern portion of California, 

bordering Mexico to the south, Riverside County to the north, San Diego County to the west, 

and the State of Arizona to the east. The Imperial Valley runs approximately north-to-south 

through the center of the county and extends into Mexico. The terrain elevation varies from as 

low as 230 feet below sea level at the Salton Sea to the north to more than 2,800 feet above 

sea level at the mountain summits to the east. 

Imperial County’s population is about 173,000 people,3

 and its principal industries are farming 

and retail trade. Most of the population, farming, and retail trade exist in a band of land that, on 

average, comprises less than one-fourth the width of the county, stretching from the south shore 

of the Salton Sea to the Mexican border. The road network is densest within this strip, as shown 

in Figure 1.1. The rest of Imperial County is the Salton Sea and mostly dry, barren desert area 

with little or no human population. Imperial County’s population distribution and population 

growth in recent years are reported in Appendix V. 

Imperial County’s agricultural industry4

 grew to $1.37 billion in 2007, led by cattle farming at 

$334 million. More than 40 types of crops and commodities are grown in the county, ranking 

Imperial County 11th among California counties.5

 The total acreage of famed land has remained 

fairly constant at ~500,000 acres over the last decade, and nearly 25% of the county’s labor 

force works in the Agricultural Sector during the high season. 

 

1

 Additional details regarding the adverse health effects of PM can be found in the San Joaquin Valley 2006 PM10

Plan (Chapter 1, Section 1.5), available at http://www.valleyair.org/Air_Quality_Plans/06PM10.htm. 2

 Official website of Imperial County, http://www.co.imperial.ca.us/. 3

 Southern California Association of Governments, http://www.scag.ca.gov/publications/pdf/2007/SOTR07/SOTR07 

_Population.pdf 4

 Imperial County Agricultural Commissioners Office, Imperial County 2007 Agricultural Crop and Livestock Report; 

available at http://imperialcounty.net/ag/Crop%20&%20Livestock%20Reports/Crop%20&%20Livestock%20Report 

%202007%20Color.pdf 5

 California Farm Bureau Federation, http://www.cfbf.com/counties/index.cfm?id=13 
Imperial County 2009 PM10 SIP Chapter 1: Introduction 

FINAL AUGUST 2009 1-3 ICAPCD 

Figure 1.1 Road map of Imperial County

1.2.2 Climate and Meteorology 

Imperial County experiences mild and dry winters with daily maximum temperatures in the 65 to 

75oF (18-24oC) range, extremely hot summers with daily maximum temperatures of 104 to 

115oF (40-46oC), and very little rain (most of the ~3 inches annual precipitation occurs in late 

summer or midwinter). Summer weather patterns are dominated by intense heat inducing lowpressure areas over the interior desert. The flat terrain of the valley and the strong temperature 

differentials created by intense solar heating produce moderate winds and deep thermal 

convection. The sun shines, on the average, more in the Imperial County that anywhere else in 

the United States. 

Humidity is low throughout the year, ranging from 28% in summer to 52% in winter. The large 

daily oscillation of temperature produces a corresponding large variation in the relative humidity. 

Nocturnal humidity rises to 50-60%, but drops to about 10% during the day. 

Although Imperial County occasionally experience high winds of speed >30 mph (most 

frequently in April and May), wind speeds in the area are generally <10 mph. Wind statistics 

reveal that prevailing winds blow from the northwest-northeast; a secondary trend of wind 

direction from the southeast is also evident. 

1.2.3 Atmospheric Stability and Dispersion 

Air pollutant concentrations are primarily determined by the amount of pollutant emissions in an 

area and the degree to which these pollutants are dispersed in the atmosphere. Restricted 

horizontal and vertical air exchange, characteristic of low wind speeds and temperature 
Imperial County 2009 PM10 SIP Chapter 1: Introduction 

FINAL AUGUST 2009 1-4 ICAPCD 

inversions,6 constitute stagnant conditions during which pollutants can accumulate to elevated 

concentrations. 

Imperial County experiences surface inversions almost every day of the year, caused by cooling 

of the air layer in contact with the cold surface of the earth (due to radiational cooling) at night. 

Because of strong surface heating during the day, these inversions are usually broken, allowing 

pollutants to disperse more easily. Nevertheless, the atmosphere is stable enough to allow PM10

pollution to accumulate and frequently reach elevated concentrations across the southern 

border of Imperial County in the densely populated city of Mexicali, Mexico. These elevated 

pollutions levels are then observed to impact ambient air quality in the nearby city of Calexico, 

Imperial County, as documented in Chapter 2. 

1.3 Imperial County 2009 PM10 SIP 

1.3.1 Background 

In response to the opinion of the US Court of Appeals for the Ninth Circuit in Sierra Club v. 

United States Environmental Protection Agency, et al., in August 2004 the United States 

Environmental Protection Agency (USEPA) found that the Imperial Valley PM10 nonattainment 

area had failed to attain by the moderate area attainment date of December 31, 1994, and as a 

result reclassified under the Clean Air Act (CAA) the Imperial Valley from a moderate to a 

serious PM10 nonattainment area (see 69 FR 48792, August 11, 2004). Also in August 2004, 

the USEPA proposed a rule to find that the Imperial area had failed to attain the annual and 24-

hour PM10 standards by the serious area deadline of December 31, 2001. The USEPA finalized 

the rule on December 11, 2007 (see Appendix I), citing as the basis for the rule that six Imperial 

County monitoring stations were in violation of the 24-hour standard during 1999-2001. The 

USEPA’s final rule action requires the State to submit to the USEPA by December 11, 2008 

(within one year of the rule’s publication in the Federal Register) an air quality plan that 

demonstrates that the County will attain the PM10 standard as expeditiously as practicable. 

Ever since the area was designated as nonattainment for PM10 Imperial County government 

agencies and industry groups (i.e. Farm Bureau, COLAB, BIA), private and public stakeholders, 

along with the District have proactively worked to reduce PM10 emissions in order to bring the 

area into compliance with the NAAQS. These efforts culminated in the 2005 amendments of the 

District’s Regulation VIII Best Available Control Methods (BACM), adopted in advance of the 

present PM10 SIP for the purposes of accelerating BACM implementation and of meeting the 

requirements and schedule of the County’s Natural Event Action Plan (NEAP). 

1.3.2 Summary of Planning Requirements 

The 2009 Imperial County PM10 SIP is required to address and meet the following elements, 

required under the CAA of areas classified to be in serious nonattainment of the NAAQS: 

• Best available emission inventories; 

• A plan that enables attainment of the PM10 federal air quality standards; 

 

6

 A temperature inversion is simply a layer of cool air trapped below a warmer layer of air, whereby the normal 

gradient of air temperature with increasing altitude is reversed. This condition is associated with poor vertical 

mixing of air and therefore poor vertical dispersion of pollutants. 
Imperial County 2009 PM10 SIP Chapter 1: Introduction 

FINAL AUGUST 2009 1-5 ICAPCD 

• Annual reductions in PM10 or PM10 precursor emissions that are of not less than 5 percent 

from the date of SIP submission until attainment; 

• Best available control measures and best available control technologies for significant 

sources and major stationary sources7

 of PM10, to be implemented no later than 4 years 

after reclassification of the area as serious; 

• Transportation conformity and motor vehicle emission budgets in accord with the 

attainment plan; 

• Reasonable further progress and quantitative milestones; and 

• Contingency measures to be implemented (without the need for additional rulemaking 

actions) in the event that the control measure regulations incorporated in the plan cannot 

be successfully implemented or fail to give the expected emission reductions. 

1.4 Regulatory Responsibility 

Federal, state, and local agencies participate in the planning process for attaining air quality in 

compliance with NAAQS. The roles of the several agencies involved are outlined in the 

following. 

1.4.1 USEPA 

The USEPA administers the provisions of the federal CAA and other air quality related 

legislation. A principal function of the USEPA is to set national ambient air quality standards 

(NAAQS) and promulgate new regulations based on the scientific evidence of the health and 

environmental effects of pollutants. In addition, the USEPA establishes national emission limits 

for major sources of air pollution; regulates emissions from locomotives, aircraft, and other 

mobile sources most effectively controlled at the national level; inspects and monitors emission 

sources; and provides financial and technical support for air quality research and development 

programs. 

The USEPA enforces federal air quality laws. Under the CAA, the USEPA is authorized to 

require states to prepare plans to attain the NAAQS by deadlines specified in the CAA. SIPs, 

which are intended to outline specific pollution control strategies for each federal nonattainment 

area within a state, are prepared by regional and county air pollution control districts in 

collaboration with state agencies and with the USEPA, who is ultimately responsible for the SIP 

final review and approval. 

Under the CAA, the USEPA also has authority to impose sanctions for failure to submit a plan or 

failure to carry out commitments in a plan. Sanctions include (i) increased emissions offsets 

requirements for major stationary sources, and (ii) withholding of federal highway, sewage 

treatment, and air planning funds. 

1.4.2 California Air Resources Board (CARB) 

The CARB is the state agency responsible for the coordination and administration of both state 

and federal air pollution control programs in California. The CARB undertakes research, sets 

 

7

 A major stationary source is defined in a serious nonattainment area for PM10 as any source that has the potential 

to emit ≥ 70 tons per year of PM10 or PM10 precursors. 
Imperial County 2009 PM10 SIP Chapter 1: Introduction 

FINAL AUGUST 2009 1-6 ICAPCD 

state ambient air quality standards as well as emission standards for motor vehicles, provides 

technical assistance to local districts, compiles emission inventories, develops suggested 

control measures, and provides oversight of district control programs. An important function of 

the CARB is to coordinate and guide regional and local air quality planning efforts required by 

the California Clean Air Act, and to prepare and submit air quality management plans to the 

USEPA. 

1.4.3 Imperial County Air Pollution Control District (ICAPCD) 

The District shares responsibility with the CARB for ensuring that all state and federal ambient 

air quality standards are achieved and maintained within the county. The District is responsible 

for monitoring ambient air quality and has authority to regulate stationary sources and some 

area sources of emissions. The District is responsible for developing the overall attainment 

strategy for Imperial County, and therefore, is responsible for planning activities involving the 

development of emission inventories, modeling of air pollution, and quantification and 

comparison of emission reduction strategies. 

Air districts in state nonattainment areas are also responsible for developing and implementing 

transportation control measures necessary to locally achieve ambient air quality standards. In 

so doing, air districts cooperate with Regional Transportation Planning Agencies (TPAs) in the 

development of the transportation control measures adopted in the SIP. Under the conformity 

requirements of the CAA (1977, 1990), Imperial County’s TPAs cannot approve any Regional 

Transportation Plan8

 or Transportation Improvement Program9 that does not conform to the 

SIP’s purpose of expeditiously bringing the area into attainment of the NAAQS. 

1.5 Document Organization 

This document is organized as outlined in Table 1.2. 

Table 1.2 Contents of Imperial County 2009 PM10 SIP Document 

 Chapter Features 

Chapter 1 This chapter provides an overview of particulate matter as an air pollutant and a brief 

description of the Imperial County area. The purpose, regulatory background, and 

requirements for the preparation of this SIP are also discussed, as well as the roles and 

responsibilities of the regulatory agencies involved. 

Chapter 2 This chapter documents ambient PM10 air quality data in the Imperial Valley in recent 

years. 

Chapter 3 The Imperial County PM10 Emissions Inventory is described in this chapter. Based on 

this inventory, source categories that contributed significantly to violations of the 

NAAQS in 2006-2008 are identified. 

 

8

 A Regional Transportation Plan is a county’s master plan outlining policies, actions, and financial projections to 

guide investment decisions over a 20-year horizon. 9

 A Transportation Improvement Program specifies all highway and transit projects spanning a multi-year period, 

that are either regionally significant or that require federal funding or approval. 
Imperial County 2009 PM10 SIP Chapter 1: Introduction 

FINAL AUGUST 2009 1-7 ICAPCD 

Table 1.2 Contents of Imperial County 2009 PM10 SIP Document 

Chapter 4 This chapter provides an overview of Imperial County’s PM10 control program. It 

includes a description of fugitive dust rules, analyses of the control effectiveness and 

implementation costs of these rules, and analyses of the comparative stringency and 

applicability of the rules relative to similar rules adopted in other PM10 serious nonattainment areas. 

Chapter 5 Analyses presented in this chapter demonstrate attainment of the PM10 NAAQS in 

Imperial County in 2006-2008 “but-for” the impact of International Emissions from 

Mexicali. Thus, this chapter satisfies the attainment demonstration requirement of this 

SIP according to section 179B(a) of the CAA. 

Chapter 6 Additional CAA requirements concerning contingency measures and transportation 

conformity are addressed in this chapter. 

Chapter 7 The impact of PM10 emissions from the Salton Sea on Imperial County air quality in 

future years is briefly considered in this chapter. 

Chapter 8 This chapter concludes the SIP document and presents a checklist verifying that all SIP 

requirements have been addressed. 

 
THIS PAGE INTENTIONALLY LEFT BLANK 
Imperial County 2009 PM10 SIP Chapter 2: Ambient Data 

FINAL AUGUST 2009 2-1 ICAPCD 

2 Ambient Data 

2.1 Background 

As stated in the introduction, a dominant fraction of Imperial County is dry, barren desert. 

According to the latest emission inventory estimate,10 the vast majority of PM10 emissions 

impacting Imperial County originate from natural, non-anthropogenic sources (for instance, 

fugitive windblown dust from barren lands alone accounts for >55% of average daily emissions). 

During high winds, Imperial County’s desert areas can produce PM10 emissions over 50 times 

greater than the emissions from any anthropogenic source, including agricultural crop land.11

Imperial County is also bordered to the south by the densely populated city of Mexicali, Mexico. 

Mexicali comprises approximately 760,000 people within ~200 square miles, and has PM10

emissions estimated at 257 tons/day, compared with emissions of ~13 tons/day for the 

considerably smaller US town of Calexico (population ~32,000) situated just across the Mexican 

border from Mexicali. Under stagnant and light wind conditions, elevated dust concentrations in 

Mexicali can cause PM10 from Mexico to drift across the border into Calexico. 

As a result of Imperial County’s desert climate and of its shared border with the densely 

populated city of Mexicali, the primary reasons for elevated PM10 levels in Imperial County are 

thus (i) disturbance of soils by wind and human activity, (ii) transport of PM10 from Mexico, and 

occasionally, (iii) wildfires.12 

2.1.1 Emissions from Outside the United States 

Ambient PM10 concentrations in Imperial County are at times significantly impacted by 

anthropogenic and non-anthropogenic emission sources in Mexico, with whom Imperial County 

shares its southern border. These sources are beyond the reasonable control of Imperial 

County. Section 179B(d) of the CAA provides grounds for discounting contributions to PM10

exceedences due to foreign sources beyond the local nonattainment area’s control; the 

application of this provision in the present SIP is discussed in Section 5. 

2.1.2 Exceptional Events 

The USEPA promulgated a Natural Events Policy (NEP) in 1996, allowing exclusion of “PM10 air 

quality data...attributable to uncontrollable natural events from the decisions regarding an area’s 

nonattainment status.”13 This policy enabled local air districts to exclude from the decisions 

regarding an area’s attainment status documented high ambient PM10 air quality data that were 

caused by uncontrollable natural events such as (i) volcanic, seismic activity, (ii) wild land fires, 

 

10 See Chapter 3 and Appendices A-B of Draft Final Technical Memorandum, Regulation VIII BACM Analysis, 

prepared by ENVIRON International Corporation for the ICAPCD, October 2005 11 ENVIRON, Development of a Wind Blown Fugitive Dust Model and Inventory for Imperial County, California, Table 

7-1, May 2004. 12 Documented examples of wildfire natural events in Imperial County may be found in a previous report (ENVIRON, 

Technical Support Document: Exclusion of PM10 Measurements in Excess of the 24-Hour PM10 NAAQS for 

Imperial County from 2001 through 2003 Due to Natural Events and Emissions from Mexico, Volume I of II, 

November 2004) 13 Mary Nichols, USEPA. Areas Affected by PM-10 Natural Events,” June 6, 1996. This memorandum sets forth 

USEPA’s Natural Event Policy. 
Imperial County 2009 PM10 SIP Chapter 2: Ambient Data 

FINAL AUGUST 2009 2-2 ICAPCD 

and/or (iii) high wind episodes.14 The NEP has been incorporated into and superseded by the 

2007 Exceptional Event Rule (below). 

As of May 21, 2007, States petitioning the USEPA to exclude any air quality monitoring data 

from regulatory determinations related to compliance with the NAAQS must comply with the 

USEPA’s updated Exceptional Event policy.15 The rule defines an Exceptional Event as one that 

“affects air quality, is not reasonably controllable or preventable, is caused by a natural event or 

by human activity that is unlikely to recur at a particular location, and is determined by the 

USEPA to be an exceptional event.” A clear causal relationship must be established between a 

measured exceedence of a NAAQS and an exceptional event in order to exclude the 

exceedence from regulatory determination of an area’s attainment status. 

2.2 Ambient Air Quality Data (2001 to 2008) 

Under the CAA, the assessment of an area’s air quality for the preparation of a SIP is based on 

the most recent three years of complete data (2006-2008 for the present SIP). Imperial County’s 

air monitoring stations are introduced in the next section, followed in Section 2.2.2 by a brief 

overview of PM10 air quality during 2001-2005 in order to provide historical perspective. Air 

quality data of importance in the preparation of Imperial County’s 2009 PM10 SIP, corresponding 

to years 2006-2008, is subsequently presented and discussed in detail. 

2.2.1 Imperial County Air Monitoring Network 

Figure 2.1 illustrates the locations of PM10 monitoring stations16 throughout Imperial County on a 

relief map. The stations are located in the populated areas of the County, which are surrounded 

by barren and desert lands, including the lower Borrego Desert Valley to the west, and, to the 

south in Mexico, the salt flats along the dried lake bed of Laguna Salada, and the great eastern 

desert area in Mexico. We note that the Calexico-Grant station was permanently 

decommissioned after July 2007. The PM10 stations are equipped with filter-based, sizeselective inlet (SSI) monitors that meet federal performance criteria and are considered to be 

the official data source for long-term air quality planning and attainment demonstrations.17

 

14 As a requirement for data flagging and data exclusion from NAAQS compliance determination, the NEP required 

States to develop area-specific Natural Events Action Plans (NEAPs) designed to protect public health through 

public education, public notification, and efforts to minimize emissions from contributing anthropogenic sources 

during natural events. The ICAPCD satisfied this requirement by collaborating with local governments and 

stakeholders to develop the Imperial County NEAP document in 2005. (The Imperial County NEAP, which dealt 

specifically with natural events caused by high winds and wildland fires, was adopted by the ICAPCD Board of 

Directors on August 9, 2005. We note that the Imperial County NEAP development process involved the 

development of BACM measures to satisfy the requirements of controlling and abating wind-generated dust from 

anthropogenic sources.) 15 USEPA, Treatment of Data Influenced by Exceptional Events; Final Rule, Federal Register, Vol. 72, No. 55, 

March 22, 2007, p. 13560. 16 The minimum number of monitors required by EPA regulations (CFR, Part 58, Appendix D) for the purpose of PM10

air quality monitoring in an area is based on the population of the area and on the nature of the PM10 air quality in 

the area. For the Imperial County area, the 5 PM10 monitors currently operated by the CARB and the ICAPCD are 

well in excess of the 1-2 monitors needed to satisfy the Federal minimum requirements (refer to the Annual 

Network Plan for Ambient Air Monitoring, Imperial County, May 2009; available from the ICAPCD website at 

http://www.imperialcounty.net/AirPollution/). 17 It should be noted here that, in addition to the SSI units, collocated Beta Attenuation Mass (BAM) monitors were 

also in operation at the Brawley, Westmorland, and Niland stations in 2006-2008. These monitors, however, were 

used for the purpose of daily forecasting, and were not operated in accordance with the quality assurance 
Imperial County 2009 PM10 SIP Chapter 2: Ambient Data 

FINAL AUGUST 2009 2-3 ICAPCD 

Additional information about PM and meteorological monitoring stations in the Imperial Valley is 

given in Appendix II. 

Niland

Westmorland

Brawley

El Centro

C. Ethel

C. Grant

Figure 2.1 Locations of Imperial County PM10 monitoring stations.

2.2.2 Air Quality Data During 2001-2005 

The time period of 2001-2003 corresponded to dry years with below-average rainfall in Imperial 

County. The exceptional aridity of the County led to abnormally dry soils with a greater potential 

for entrainment of dust into the atmosphere, and also facilitated the wildfire events of October 

2003 (releasing tremendous amounts of ash and dust into the atmosphere). As a result, PM10

concentrations in exceedence of the 24-hour NAAQS arising from high-winds or wildfires 

occurred on 11 days during 2001-2003.18 In addition, seven additional days saw recordings of 

PM10 concentrations in excess of the 24-hour NAAQS in Calexico, as a result of transport, by 

light and variable winds, of pollution generated in Mexico. In 2004 and 2005, Imperial County 

 

procedures required to generate data suitable for regulatory compliance evaluations. A 2008 ICAPCD/ARB 

evaluation of the Imperial County 2006-2008 PM10 BAM data revealed that monitor performance had been 

inaccurate as a result of deviations from operational specifications. Consequently, the Imperial County 2006-2008 

BAM data is invalid for consideration in the area’s attainment demonstration, and unfit for submission to the 

USEPA Air Quality System (AQS). Additional information about continuous PM10 monitoring in Imperial County in 

given in Appendix II. 18 ENVIRON, Technical Support Document: Exclusion of PM10 Measurements in Excess of the 24-Hour PM10

NAAQS for Imperial County from 2001 through 2003 Due to Natural Events and Emissions from Mexico, Volume I 

of II, November 2004. 
Imperial County 2009 PM10 SIP Chapter 2: Ambient Data 

FINAL AUGUST 2009 2-4 ICAPCD 

monitors recorded exceedences of the 24-hour PM10 NAAQS on seven days, of which three 

involved high winds, and four involved high levels of PM transport from Mexico. 

2.2.3 Ambient Air Quality in the Time Period from 2006 to 2008 

SSI monitor measurements acquired during 2006-2008 are plotted by station in Figure 2.2 and 

tabulated in Appendix II. The plots reveal that PM10 air quality patterns throughout Imperial 

County fall within two groups. The first includes the Brawley, El Centro, Niland, and 

Westmorland stations, where the overwhelming majority of 24-hour ambient PM10 concentration 

measurements are well below the 150 μg/m3

 NAAQS: only 2-3% of measurements at those 

stations were above 100 μg/m3

 during 2006-2008, and only 17-31% were above 50 μg/m3. The 

second group corresponds to the Calexico stations, for which 8-16% of recordings during 2006-

2008 were above 100 μg/m3

, and 51-64% were above 50 μg/m3 (leading to higher averages and 

standard deviations relative to those at non-Calexico stations). As discussed in Chapter 5, the 

difference between the two groups is primarily caused by transport of pollution from Mexicali, 

which may be substantial even on days that do not exceed the NAAQS. 

2.2.4 Exceedences of the 24-Hour PM10 NAAQS from 2006 to 2008 

In the 2006 to 2008 time period, Imperial County experienced five days with PM10

concentrations in excess of the 24-hour NAAQS, as listed in Table 2.1. Exceedences on 

September 2, 2006, April 12, 2007, and June 5, 2007 were Exceptional Events caused by high 

winds. These events were documented, publicly noticed, and are being submitted to EPA in 

separate submittals.19 Exceedences on December 21, 2006 and December 25, 2006 arose from 

transport across the Mexican border of pollution originated in Mexicali, as discussed in Chapter 

V and Appendix V. 

Table 2.1 PM10 Measurements in Excess of the 24-Hour NAAQS in Imperial County for the 

Time Period 2006-2008 

Date Monitor Measurement (μg/m3) Comments 

12/21/2006 Calexico Grant 171 Transport from Mexico 

12/25/2006 Calexico Grant 248 Transport from Mexico 

9/2/2006 Calexico Ethel 164 High winds 

 Calexico Grant 233 High winds 

 Westmorland 167 High winds 

4/12/2007 Brawley 291 High winds 

 Westmorland 155 High winds 

6/5/2007 Brawley 281 High winds 

 Calexico Ethel 282 High winds 

 El Centro 200 High winds 

 Niland 162 High winds 

 Westmorland 226 High winds 

 

 

19 For these days, documentation satisfying the requirements of 40 CFR Parts 50 and 51 is available from the 

ICAPCD at http://www.imperialcounty.net/AirPollution/Web%20Pages/2009%20March%20Natural%20Events.htm. 

We note that these events are still under process review. 
Imperial County 2009 PM10 SIP Chapter 2: Ambient Data 

FINAL AUGUST 2009 2-5 ICAPCD 

Figure 2.2 Time-series plots of 24-hour PM10 ambient air concentration in Imperial County during 

2006-2008: SSI measurements at the El Centro, Brawley, Westmorland, Niland, Calexico-Grant and 

Calexico-Ethel stations. Note that the Calexico-Grant station was permanently decommissioned in 

August 2007. Gold-colored exceedences were affected by high wind exceptional events, and greencolored exceedences were affected by transport from Mexico. Tabulated results are given in Appendix 

II. 

0

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Jan-06 May-06 Sep-06 Feb-07 Jun-07 Nov-07 Mar-08 Aug-08 Dec-08

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Westmorland Niland

Calexico-Ethel
Imperial County 2009 PM10 SIP Chapter 3: Inventory and Analysis of Significant Sources 

FINAL AUGUST 2009 3-1 ICAPCD 

3 Inventory and Analysis of Significant Sources 

3.1 Emission Inventory 

An emission inventory (EI) for a specified criteria pollutant is an accounting of the amount of the 

pollutant that is emitted into the atmosphere by various sources over a specific period of time. 

Because pollution-generating activities are continuously changing and methods to estimate their 

impact are continuously improving, the updating of an EI is an ongoing process. In the next 

subsection, we discuss recent changes to the Imperial County PM10 inventory that reflect the 

latest, best available data.20 The resulting best estimate of the EI for the 2005 baseline year, 

along with projected growth in emissions until 2010, is presented thereafter. 

3.1.1 Important Recent Adjustments to the Imperial County PM10 Emission 

Inventory 

The EI used in the present PM10 SIP is derived from CARB’s SIP inventory Version 1.06, base 

year 2002, which has been updated to incorporate revised cattle emissions, revised windblown 

dust model results, revised SCAG activity data, and updated entrained and windblown unpaved 

road dust estimates. Here, we briefly present the revisions to (i) windblown dust estimates from 

open areas and non-pasture agricultural lands and (ii) emissions from unpaved roads. 

Additional documentation of the adjustments to the PM10 V1.06 inventory is given in Appendix 

III.A. 

Windblown Dust from Open Areas and Non-Pasture Agricultural Lands. Consistent with model 

development done for the Western Regional Air Partnership, ENVIRON developed and 

implemented a Windblown Dust Model to estimate windblown fugitive dust for specific sources 

in Imperial County.21,22 This work was conducted in light of the known importance of the 

windblown dust category in the total Imperial County PM10 Emission Inventory23 and in an effort 

to address the following inadequacies in the existing CARB methodology previously used for 

Imperial County: (i) a failure to include windblown dust emissions from all lands within Imperial 

County (e.g. from disturbed, vacant lands such as the shrublands, barren lands, and desert 

sand dunes), and (ii) an inability to use information on local soil types, vegetation coverage, 

crop types, and agricultural practices.24

For each land parcel within the modeling domain, the Windblown Dust Model (i) assesses 

emission characteristics based on soil texture and soil stability, including reservoir25 and 

reservoir recharge characteristics; (ii) assesses hourly emission factors for the land parcel 

 

20 We note that these changes have been found to be appropriate by the CARB. 21 Development of a Windblown Fugitive Dust Model and Inventory for Imperial County, California. Final Report, May 

12, 2004. Prepared by ENVIRON International Corporation and Eastern Research Group for the ICAPCD. 22 Appendix A (Technical Memorandum: Latest Revisions to the Windblown Dust Model) of the Technical 

Memorandum: Regulation VIII BACM Analysis. ENVIRON, October 2005. 23 For instance, windblown dust accounted for more than 170 of the approximately 230 tons/day of total Imperial 

County PM10 emissions in the V1.06 CARB Inventory. 24 The ARB methodology to calculate windblown emissions relies on a modified form of the wind erosion equation 

(WEA) developed by the United States Department of Agriculture (USDA) based on a single field study carried out 

in Manhattan, Kansas. 25 Reservoir characteristics refer to the limit placed on the amount of dust that can be emitted in any single wind 

episode. 
Imperial County 2009 PM10 SIP Chapter 3: Inventory and Analysis of Significant Sources 

FINAL AUGUST 2009 3-2 ICAPCD 

based on the emission characteristic profile and hourly meteorological data; and (iii) applies 

correction terms to the obtained hourly emission rates based on vegetative cover, as well as 

non-climatic corrections for agricultural lands. Windblown Dust Model estimates of windblown 

PM10 emissions from all Imperial County lands calculated using model assumptions selected 

based on CARB and USEPA input26 are reported in Table 3.1. 

We note that the model assumptions used to generate the emission estimates of Table 3.1 did 

not evaluate the impact of off-road vehicular activities on the soil stability and emission 

characteristic profiles. Consequently, the level of additional emissions resulting from 

anthropogenic disturbance of vacant lands in Imperial County was assessed (in a conservative 

manner as documented in Appendix III.B) and incorporated in the present SIP Emission 

Inventory. The corresponding annual average level of additional PM10 emissions from open 

areas is 12.2 tpd (Table III.B.4 of Appendix III.B).27The Windblown Dust Model provides a 

rigorous and comprehensive treatment of windblown dust erosion based on a thorough analysis 

and use of available information/correlations from the existing literature, and thereby enables 

considerable improvements in the accuracy of windblown PM10 emissions in Imperial County 

(relative to the previously-used CARB methodology). Nevertheless, because of limitations in 

the availability of model input information, it was necessary to make assumptions,28 especially 

for vacant lands. We see in subsequent sections of this SIP document that this uncertainty 

(related to the model input assumptions) does not affect the results of the technical analyses for 

any of the key elements of the present SIP (i.e., analysis of significant sources, rulemaking, 

attainment demonstration, and conformity). Further discussion of windblown dust emissions 

from vacant lands is given in Appendix III.B. 

Table 3.1 Windblown Dust Model Estimatesa of PM10 Emissions (in tons/day) in Imperial 

County (Reported According to Land Use/Land Coverage) 

 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual Summer Winter 

Agriculture 8.59 11.3 19.9 28.4 12.9 16.2 7.37 4.33 9.76 0.38 6.60 4.42 10.81 8.46 13.21 

Shrub/Grassland 96.1 102. 120. 243. 97.5 189. 40.6 140. 63.8 7.32 64.0 25.4 98.75 89.46 108.2 

Desert - Dunes 21.7 31.8 36.2 62.3 24.7 4.59 16.7 11.9 8.22 0.00 9.32 11.6 19.85 11.10 28.74 

Desert - Other 20.5 38.0 52.0 97.3 57.0 71.8 15.7 43.1 29.2 6.36 22.0 13.3 38.74 37.11 40.40 

Urban Open Areas 0.07 0.00 0.00 0.03 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.02 

Total 147 183 228 431 192 282 80.5 199 111 14.0 102 54.8 168 146 190 a

Calculated using 2002 meteorological data as documented in Appendix A of the Imperial County 2005 BACM 

analysis report.29

 

26 Refer to Appendix A of the Regulation VIII BACM Analysis document (ENVIRON, October 2005), and to Appendix 

III.B, Section III.B.2 for details. 27 As noted in Appendix III.B, this estimate is conservative and the actual number is likely to be smaller. Winter and 

summer contributions were calculated using the winter/summer temporal profile derived from the results of the 

windblown dust model for contributing sources (i.e., Shrubland/Grassland, Dunes, and other barren lands, see 

Table III.B.4 of Appendix III.B). Thus, the winter (181 days from November to April) and summer (184 days from 

May to October) emissions were 177.34/157.34 = 112.7% and 137.66/157.34 = 87.5% of the annual average, 

respectively. 28 E.g., assumptions about threshold velocity for the onset of emissions, reservoir recharge, surface stability, 

vegetative canopy cover, etc (refer to the May 2004 windblown dust model report for details). 29 ENVIRON, Draft Final Technical Memorandum, Regulation VIII BACM Analysis, prepared for the ICAPCD, October 

2005 
Imperial County 2009 PM10 SIP Chapter 3: Inventory and Analysis of Significant Sources 

FINAL AUGUST 2009 3-3 ICAPCD 

Dust from Unpaved Roads (Entrained and Windblown). Estimates of the windblown and 

entrained emissions from all unpaved roads in Imperial County were revised in the 2005 BACM 

analysis29 using updated emission factors for canal road windblown emissions30 and for 

unpaved road travel dust31 (refer to Appendix B of the 2005 BACM report). The same 

methodology has been used to estimate emissions from unpaved roads for the present SIP 

(Table 3.2) using the latest mileage and traffic information32 available from the county, the cities 

of Imperial County, and the Imperial Irrigation District (IID). (We note, however, that additional 

adjustments were made to the entrained emission levels from unpaved roads by incorporating 

rainfall corrections.) 

Table 3.2 2005 Estimates of Entrained and Windblown PM10 Emission from Unpaved 

Roads (Annual Averages)

Unpaved roads category Mileage Windblown (tpd) Entrained (tpd)a 

County < 50 ADTb 1155 6.64 11.11

County > 50 ADTc 199 1.14 13.40 

City Roadsb 7.5 0.03 0.07 

Maintenance (Canal) Roadsd 6148 16.32 29.57 

Federal Roadsb 139 0.37 1.34 

Farm Roads 2263 6.01 1.35 

ADT = Average Daily Trips. aNote that entrained emissions levels were adjusted to incorporate rainfall corrections. b

Average traffic taken to be 10 ADT. cAverage traffic taken to be 70 ADT. dAverage traffic taken to be 5 ADT. 

3.1.2 2005 Base Year and Future Inventories 

The updated EI for the baseline year 2005 is summarized in Table 3.3. Estimates of the EI for 

years during which emission-generating activities are mitigated are obtained by subtracting the 

projected reductions in emissions from the projected emissions valued by accounting for growth 

in activities (see Table 3.4 for “grown” EIs in 2006-2010). Control strategies and associated 

reductions are discussed in Chapter 4. 

 

30 ARB, Windblown Dust—Unpaved Roads. Areawide Source Methodologies, Section 7.13, , August 1997, 

http://www.arb.ca.gov/ei/areasrc/fullpdf/full7-13.pdf 31 CARB, Summary of Fugitive Dust and Ammonia Emission Inventory Changes for the SJVAPCD Particulate Matter 

SIP, Revision 2.1, May 2003, http://www.valleyair.org/Air_Quality_Plans/docs/2003%20PM10%20Plan 

/PDF%202003%20PM10%20Plan%20adpt%20app/App%20C-EI%20Changes.pdf 32 Communications to the ICAPCD from the ICPWD and the IID (see Attachment A to Appendix III.A for 

documentation from the ICPWD). 
Imperial County 2009 PM10 SIP Chapter 3: Inventory and Analysis of Significant Sources 

FINAL AUGUST 2009 3-4 ICAPCD 

Table 3.3 PM10 Emission Inventory for Imperial County in Baseline Year 2005a (tpd) 

Source Category Annual Average Winter Average Summer Average 

Fuel Combustion 0.41 0.35 0.48

Waste Disposal 0.00 0.00 0.00 

Cleaning/Surface Coatings 0.00 0.00 0.00 

Petroleum Production/Marketing 0.00 0.00 0.00 

Industrial Processes: 2.79 2.79 2.78 

Mineral Processes 2.63 2.62 2.64 

Food/Agriculture 0.16 0.17 0.14 

Solvent Evaporation 0.00 0.00 

Res Fuel Combustion 0.09 0.16 0.02 

Farming 9.88 11.55 8.20 

Tilling 7.10 8.77 5.42 

Harvest 0.01 0.01 0.01 

Cattle 2.77 2.77 2.77 

Construction 2.20 2.01 2.38 

Paved Road Dust 3.38 3.30 3.46 

Entrained Unpaved Road Dust: 56.85 33.71 79.98 

City/County 24.58 14.58 34.59 

Canal 29.57 17.54 41.61 

BLM/USFS 1.34 0.79 1.88 

Farm 1.35 0.80 1.90 

Windblown Dust: 212.67 223.79 201.95 

Open Areas—Urban 0.01 0.02 0.00 

Open Areas—Othersb 169.54 191.09 148.34

Unpaved Roads: 30.52 18.10 42.94 

City/County 7.82 4.64 11.00 

Canal 16.32 9.68 22.96 

BLM/USFS 0.37 0.22 0.52 

Farm 6.01 3.56 8.46 

Non-Pasture Ag Lands 10.81 13.21 8.46 

Pasture 1.79 1.37 2.20 

Fires 0.00 0.00 0.00 

Waste Burning 2.77 2.77 2.77 

Cooking 0.06 0.06 0.06 

On-Road Mobile 1.05 1.06 1.05 

Other Mobile 0.99 0.95 1.04 

Total 293 282 304 a

Entries corresponding to the summed contributions of subcategories are in italics. bGrasslands, dunes, and other 

barren lands (see Table 3.1). As documented in Appendix III.B, emissions were estimated using available 

information on the conditions of the vacant lands (e.g., desert areas of barren, grass/shrubland, and dunes). 

Reported emissions also include the conservatively-estimated contributions due to soil disturbances caused by offroad vehicle usage. 
Imperial County 2009 PM10 SIP Chapter 3: Inventory and Analysis of Significant Sources 

FINAL AUGUST 2009 3-5 ICAPCD 

Table 3.4 Imperial County PM10 Emission Inventorya in 2006-2010 (tpd) 

Source Category 2006 2007 2008 2009 2010 

Fuel Combustion 0.42 0.42 0.42 0.42 0.43

Waste Disposal 0.00 0.00 0.00 0.00 0.00 

Cleaning/Surface Coatings 0.00 0.00 0.00 0.00 0.00 

Petroleum Production/Marketing 0.00 0.00 0.00 0.00 0.00 

Industrial Processes: 2.83 2.87 2.91 2.98 3.01 

Mineral Processes 2.67 2.71 2.74 2.81 2.85 

Food/Agriculture 0.16 0.16 0.16 0.16 0.16 

Solvent Evaporation 0.00 0.00 0.00 0.00 0.00 

Res Fuel Combustion 0.09 0.09 0.09 0.09 0.10 

Farming: 10.37 10.37 10.36 10.36 10.36 

Tilling 7.10 7.09 7.09 7.09 7.08 

Harvest 0.01 0.01 0.01 0.01 0.01 

Cattle 3.26 3.26 3.26 3.26 3.26 

Construction 2.22 2.24 2.26 2.27 2.28 

Paved Road Dust 3.62 3.86 4.09 4.32 4.13 

Entrained Unpaved Road Dust: 56.84 56.84 56.84 56.84 56.84 

City/County 24.58 24.58 24.58 24.58 24.58 

Canal 29.57 29.57 29.57 29.57 29.57 

BLM/USFS 1.34 1.34 1.34 1.34 1.34 

Farm 1.35 1.35 1.35 1.35 1.35 

Windblown Dust: 212.66 212.66 212.64 212.64 212.63 

Open Areas—Urban 0.01 0.01 0.01 0.01 0.01 

Open Areas—Othersb 169.54 169.54 169.54 169.54 169.54

Unpaved Roads 30.52 30.52 30.52 30.52 30.52 

City/County 7.82 7.82 7.82 7.82 7.82 

Canal 16.32 16.32 16.32 16.32 16.32 

BLM/USFS 0.37 0.37 0.37 0.37 0.37 

Farm 6.01 6.01 6.01 6.01 6.01 

Non-Pasture Ag Lands 10.80 10.80 10.79 10.79 10.78 

Pasture 1.79 1.78 1.78 1.78 1.78 

Fires 0.00 0.00 0.00 0.00 0.00 

Waste Burning 2.75 2.73 2.71 2.69 2.67 

Cooking 0.06 0.06 0.06 0.06 0.07 

On-Road Mobile 1.01 0.93 0.88 0.82 0.77 

Other Mobile 0.98 0.96 0.95 0.95 0.94 

Total 294 294 294 294 294 a

Annual averages accounting for projected growth in emission-generating activities, but not for reductions due to 

control or mitigation of PM10 sources. Entries corresponding to the summed contributions of subcategories are in 

italics. b

Grasslands, dunes, and other barren lands (see Table 3.1). As documented in Appendix III.B, emissions 

were estimated using available information on the conditions of the vacant lands (e.g., desert areas of barren, 

grass/shrubland, and dunes). Reported emissions also include the conservatively-estimated contributions due to 

soil disturbances caused by off-road vehicle usage. 

 
Imperial County 2009 PM10 SIP Chapter 3: Inventory and Analysis of Significant Sources 

FINAL AUGUST 2009 3-6 ICAPCD 

3.2 Determination of Significant Sources of PM10 

3.2.1 Background 

USEPA policy for State Implementation Plans for serious PM10 nonattainment areas (Federal 

Register, August 16, 1994, p. 41998) instructs that Best Available Control Measures are 

required for all source categories except those that “the State [can] demonstrate [do] not 

contribute significantly to nonattainment of the NAAQS.” As a criterion for classification of PM10

sources into significant or De Minimis categories, the preamble states that “a source category... 

will be presumed to contribute significantly to a violation of the 24-hour NAAQS if its PM10

impact at the location of the expected violation would exceed 5 μg/m3

” (p. 42011). This 

language unambiguously implies that this test should be applied, for any violation, to every 

source category using information specific to the day of the violation. Violations of the 24-hour 

PM10 NAAQS recorded at Imperial County stations in 2006-2008 are reported in Table 3.5 

below.33 We note that, as discussed in Chapter 5 and Appendix V, both of the violations listed in 

Table 3.5 correspond to air quality impacts strongly affected by the transport of PM10 originating 

from emission sources in Mexicali. 

Table 3.5 PM10 Measurements in Violation of the 24-Hour NAAQS in 2006-2008 

Date Monitor Measurement (μg/m3) 

12/21/2006 Calexico Grant 171 

12/25/2006 Calexico Grant 248 

3.2.2 Implementation 

The implementation of the criterion for any specific violation requires a day-specific 

decomposition of the air quality impacts at the location of the measurement into fractional 

contributions from all relevant source categories. Thus, a day-specific emission inventory is 

needed. Significance for any source category can be conveniently assessed by comparison of 

the calculated day-specific fractional contribution to the critical fraction of (5 μg/m3

)/(violation 

measurement). We illustrate the approach below with the assessment of significance for an 

arbitrary source category X in the 248 μg/m3

 24-hour PM10 exceedence recorded at the 

Calexico Grant station on December 25, 2006. 

The critical fractional contribution for December 25, 2006 is (5 μg/m3

)/(248 μg/m3) = 2.0%, so 

any source category with fractional contribution to the Calexico Grant PM air quality 

measurement greater than 2.0% is significant. The simplest way to conduct the analysis is to 

derive a day-specific Imperial County emission inventory (for example, by making adjustments 

to the seasonal average emission levels of Table 3.3 to account for day-specific information and 

conditions), and assume that each source category makes a contribution to the measured 

ambient PM10 concentration that is proportional to its emission level.34 Note that for both days 

 

33 Table 3.6 does not include exceedences of the standard for which ICAPCD has submitted reports to seek 

exclusion of the measurements from regulatory consideration on the basis that they were influenced by 

exceptional (high winds) events. 34 This is simply an assumption of linearity. All other assumptions in this analysis are carefully outlined where they 

arise. 
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FINAL AUGUST 2009 3-7 ICAPCD 

listed in Table 3.5 above, wind speeds were very low35 and therefore emissions from windblown 

dust sources are expected to have been but a small fraction of the average numbers listed in 

Table 3.3. The fractional impact FIX of US emissions from a specific source category X at the 

Calexico Grant station is: 

 (3.1) 

where: a is the impact of US emissions from the source category X at the Grant station; 

b is the sum of the impacts of all US emissions at the Grant station; 

c is the sum of the impacts of all Mexicali emissions at the Grant station; and 

FM = c/(c+b) is the fractional impact of Mexicali emissions at the Grant station. 

Of course, the US emissions that significantly impact the Grant station may only be a small 

subset of the total US emissions derived for the entire Imperial County. However, if that small 

subset is representative of the entire Imperial County Emission Inventory (in terms of relative

contributions from all US source categories), then under the assumption of linearity mentioned 

above the ratio a/b can be simply derived from the day-specific emission inventory derived for 

the entire Imperial County. 

Such an inventory for December 21, and December 25, 2006 is given in Table 3.6. Emission 

rates were obtained assuming that (i) the winter average emissions inventory of Table 3.3 is 

representative of non-windblown emissions for Imperial County on December 21 and 25, 2006, 

and (ii) day-specific windblown emissions for these days can be estimated as a fraction (in 

Table 3.6, the fraction is simply chosen to be 0%) of the seasonal average. Note that this latter 

assumption allows convenient analysis of the sensitivity of results to the estimated levels of 

windblown emissions. 

The fractional impact of Mexicali emissions at the Grant station (i.e., the ratio FM in Equation 

3.1) can be estimated for the days in Table 3.5 using an analysis of international PM transport 

across the US-Mexico border. Such an analysis, involving a number of complementary 

approaches, is presented in detail in Appendix V. Resulting best estimates of the Mexicali 

contribution to PM10 air concentrations at the Grant station are FM = 71% for December 25, 

2006, and 61% for December 21, 2006. We note that these contributions are consistent with 

the relative ratios of non-windblown PM10 emissions for the Imperial County and the Mexicali 

municipalities according to best-available inventories.36 

 

35 For example, hourly wind speeds were < 4 mph and 24-hour average wind speeds were < 2 mph at meteorological 

stations in the Mexicali-Calexico area (see description of the December 21 and December 25 episodes in 

Appendix V) 36 According to a 2008 inventory (2005 Mexicali Emission Inventory, Draft Final Report, October 2008; prepared by 

Eastern Research Group for the USEPA, the Secretaria de Proteccion al Ambiente de Baja California, and the 

Western Governors Association), the total level of non-windblown PM10 emissions in Mexicali is 147 tpd. 

Comparison with Imperial County non-windblown PM10 winter emission of 59 tpd yields a 71:29 ratio for 

Mexicali/Imperial County PM10 emissions. 

1 1

1 1

1 1 ( 1) X

aa a FI

b c b c b b FM − −

⎛⎞ ⎛⎞ = =⋅ =⋅ ⎜⎟ ⎜⎟ + + +− ⎝⎠ ⎝⎠
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Table 3.6 Estimated Imperial County PM10 Emission Inventory for December 21 and 

December 25, 2006, and Predicted Contributions of Emissions to the Measured 

Calexico-Grant Exceedences by Source Category

Source Category Emissions (tpd) assuming 

Windblown = 0 % average 

% Contributiona

 

on December 21 

% Contributiona

on December 25 

Fuel Combustion 0.35 0.2% 0.2% 

Waste Disposal 0.00 0.0% 0.0% 

Cleaning/Surface Coatings 0.00 0.0% 0.0% 

Petroleum Production/Marketing 0.00 0.0% 0.0% 

Industrial Processes: 

Mineral Processes 2.62 1.7% 1.3% 

Food/Agriculture 0.17 0.1% 0.1% 

Solvent Evaporation 0.00 0.0% 0.0% 

Res Fuel Combustion 0.16 0.1% 0.1% 

Farming 

Tilling 8.77 5.8% 4.3% 

Harvest 0.01 0.0% 0.0% 

Cattle 2.77 1.8% 1.3% 

Construction 2.01 1.3% 1.0% 

Paved Road Dust 3.30 2.2% 1.6% 

Entrained Unpaved Road Dust 

City/County 14.58 9.6% 7.1% 

Canal 17.54 11.5% 8.6% 

BLM/USFS 0.79 0.5% 0.4% 

Farm 0.80 0.5% 0.4% 

Windblown Dust: 

Open Areas 0.00 0.0% 0.0% 

Unpaved Roads 0.00 0.0% 0.0% 

Non-Pasture Ag Lands 0.00 0.0% 0.0% 

Pasture 0.00 0.0% 0.0% 

Fires 0.00 0.0% 0.0% 

Waste Burning 2.77 1.8% 1.3% 

Cooking 0.06 0.0% 0.0% 

On-Road Mobile 1.06 0.7% 0.5% 

Other Mobile 0.95 0.6% 0.5% 

Total IC Emissions (tpd) 59 

Critical Fractional Contribution 2.9% 2.0% 

a

Estimated fractional contributions of US emissions from the specified source categories to the December 21, 2006 or 

the December 25, 2006 PM10 ambient concentrations at the Calexico Grant station, assuming that the fractional 

impact (FM in Equation 3.1) of Mexicali emissions were 61% on December 21 and 71% on December 25. Entries in 

pink indicate source categories with emissions above the significance level. 
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3.2.3 Classification of PM10 Source Categories by Significance 

The above approach was used to calculate, for each violation reported in Table 3.5, the 

fractional contribution to the violation of US emissions from each source category. The analysis 

included investigation of the sensitivity of results to the estimated level of windblown emissions 

on the days of interest. As examples, results are reported in Table 3.6 for analyses that assume 

windblown emissions on December 21 and December 25, 2006 were negligible. For both 

December 21, 2006 and December 25, 2006, we find that: 

• At zero levels of windblown emissions, the only significant sources are “Tilling”, and 

“Entrained Dust from Unpaved Roads;” 

• Any increase in the assumed level of windblown emissions results in fractional contribution 

from all source categories (other than windblown sources) that are even lower (i.e., the 

sources are even further below the significance level); and 

• For windblown sources, the only source category that becomes significant at non-zero 

levels of windblown emissions is “Open Areas” (as noted in Table 3.3, about 99% of 

emissions from these open areas are from non-populated areas such as dunes, 

grasslands, and other barren lands). This is true regardless of the assumed level of 

windblown emissions within the range of 0 to 100% of the seasonal average emission 

levels. 

We note that results of the significance analysis are the same if the calculations are run using 

the annual average inventory rather than the winter average inventory (except that all sources 

other than unpaved roads are then even further below the significance threshold). Given that 

windblown emissions can be expected to be negligible at wind speeds as low as those observed 

on both December 21 and December 25, 2006, we conclude that the only PM10 source 

categories that may have contributed significantly to non-attainment of the NAAQS in the 2006-

2008 period are those reported in Table 3.7. Although BACM is not specifically required for 

non-significant sources, emissions from most source categories contributing >1 tpd to County’s 

EI are addressed in the Imperial County PM10 control program outlined by ICAPCD Rule 420 

and amended ICAPCD Regulation VIII rules (as discussed in Chapter 4). 

Table 3.7 Significant Source Categories in 2006-2008 

Source Category 

Unpaved Roads – Entrained Dust 

Farming – Tilling Dust 

3.2.4 Major PM10 Stationary Sources 

The previous analysis determined significance for categories representing the aggregated value 

of emissions from similar sources integrated over all of Imperial County. That analysis may not 

capture localized effects of major PM10 stationary sources. (A major stationary source is defined 

in a serious nonattainment area for PM10 as any source that has the potential to emit ≥70 tons 

per year of PM10 or PM10 precursors). Stationary sources are required to implement BACT to 

control PM10 emissions (Rule 207, New and Modified Stationary Source Review), and they are 

also required to comply with 20% opacity (Rule 403, Opacity of Emissions). In addition, 
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FINAL AUGUST 2009 3-10 ICAPCD 

stationary sources will be required to mitigate fugitive dust emissions from access roads, 

construction activities, handling and transferring of Bulk Materials37 and Track-Out and CarryOut38 according to the requirements of Regulation VIII. According to the 2005 stationary source 

emission inventory, there is only one PM10 major stationary source that operates in Imperial 

County. This source, which accounted for PM10 emissions of 155.9 tons/year in 2006, 

manufactures gypsum wallboard and related products, and is located approximately 20 miles 

west from the nearest PM10 monitoring site. In addition, it underwent an expansion within the 

last ten years, during which BACT was implemented on its major sources. Its impact on Imperial 

County PM10 monitors is not significant. Regardless, this source is required to comply with all 

the above mentioned requirements and regulations; thus it meets the requirement for BACM 

and BACT, regardless of significance. 

 

37 Bulk Material is any organic and/or inorganic material consisting of or containing particulate matter with ≥ 5% silt 

content, including materials such as earth, rock, silt, sediment, sand, gravel, soil, fill, aggregate, dirt, mud, or 

debris. 38 Track-out/carry out refers to any Bulk Material that adhere to and agglomerate on the exterior surfaces of motor 

vehicles and/or equipment (including tires) that may then fall onto the pavement. 
THIS PAGE INTENTIONALLY LEFT BLANK 
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4 CONTROL STRATEGIES AND IMPACT ANALYSIS 

Reclassification of the Imperial County area to “serious” non-attainment in 2004 prompted the 

ICAPCD to begin the development of revised dust control rules at the BACM level. This 

process was begun ahead of a SIP development39 for the purpose of accelerating BACM 

implementation and to meet the requirements and schedule of the District’s NEAP40 (approved 

in August 2005). In March 2004, the ICAPCD began a review and assessment of BACMs in 

other areas. Rule development, initiated at a stakeholder meeting in October 2004, was 

conducted in a public process that involved a local Technical Advisory Committee41 as well as 

state and federal air agencies.42 The process resulted in the adoption in November 2005 of 

revised Regulation VIII fugitive dust control measures,43 which form the core of the Imperial 

County PM10 control strategy. (Note that additional controls of PM10 emissions in Imperial 

County are outlined in ICAPCD Rule 420 (beef feedlots) and Rule 710 (agricultural burning). 

These rules, which were most recently updated in October 2006 and August 2002, respectively, 

are SIP approved.) 

This chapter provides a summary of the Regulation VIII fugitive dust rules. Provisions in these 

rules were effective January 2006.44 Analyses of the control effectiveness/implementation costs 

of the rules (Section 4.2) and of the comparative stringency/applicability of the rules (relative to 

similar fugitive dust rules adopted in other PM10 serious non-attainment areas, Section 4.3) are 

also presented. 

4.1 Control Strategies 

Only a brief description of the control measures of the Regulation VIII rules are presented in this 

section; a comprehensive description of the rules is available in Appendix IV.A. 

4.1.1 Rule 801: Construction and Earthmoving Activities 

Purpose and Requirements. The purpose of Rule 801 is to reduce the amount of PM10 that is 

emitted into the air as a result of construction and other earthmoving activities, such as land 

clearing, excavating, land leveling, grading, demolishing, etc. All persons who own or operate a 

construction site or who perform any earthmoving activities are required to limit Visible Dust 

Emissions (VDE) to 20% opacity by complying with the following measures: 

• Phase work to minimize the amount of disturbed surface area at one time; 

• Apply water or chemical stabilization; 

• Construct and maintain wind barriers around the activity site; 

• Restrict vehicular access to the area by fencing or signage; 

 

39 The USEPA did not take final action regarding non-attainment of the Imperial County area until December 2007. 40 Needed to support the exclusion of natural events from attainment determination, as allowed by USEPA’s Natural 

Events Policy. 41 Including representatives from the Coalition of Labor and Business, the Farm Bureau, the Bureau of Land 

Management, Border Patrol, the IID, the ICPWD, as well as farmers and private industry stakeholders. 42 Meetings with the CARB and the USEPA were held on March 23, 2005 and on August 10, 2005. Informal 

comments were also submitted by the California Department of Transportation. 43 These rules apply to local sources. 44 The only exception is that control of County unpaved roads according to Rule 805 is phased over a 10 year period. 
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• Mitigate track out/carry out of Bulk Materials (defined in footnote on page 3-5) at the site in 

compliance with Rule 803; and 

• Transport Bulk Material to, from, and around the site in compliance with Rule 802. 

Dust Control Plan. Owners or operators of construction/earthmoving sites of ≥10 acres for 

residential developments and ≥5 acres for non-residential development are required to provide 

written notification to the ICAPCD 10 days prior to the commencement of activities, and to 

develop a dust control plan. The plan is expected to document the type and location of the 

project, the expected start and completion dates of the dust generating activities, the total area 

of land surface to be disturbed, the actual and potential sources of fugitive dust emissions on 

the site (including the location of Bulk Material handling and storage areas, paved and unpaved 

roads, entrances and exits where track out/carry out may occur, etc.), and all the fugitive dust 

control measures to be implemented before, during, and after any dust-generating activity. 

4.1.2 Rule 802: Bulk Materials 

The purpose of Rule 802 is to reduce the amount of PM10 that is emitted into the air as a result 

of outdoor handling, storage, and transport of Bulk Material.45 The rule requires implementation 

of the following controls in order to limit VDE to 20% opacity: 

• For Bulk Material handling (e.g. stacking, loading, unloading, conveying, etc.), control 

measures include spraying with water, applying and maintaining chemical stabilization, and 

protecting from wind erosion by sheltering or enclosing; 

• For Bulk Material storage, control measures include confinement of the material using a 

physical barrier (e.g. covering with tarps, plastic, etc.) and confinement by applying water 

or other chemical/organic stabilizers/suppressants; 

• For Bulk Material transport/hauling, control measures include complete covering or 

enclosing of all haul trucks loads, proper selection and maintenance of the cargo 

compartments of haul trucks to ensure no spillage or loss of Bulk Materials from holes or 

openings in the compartment’s floor, side, or tailgate, and adequate cleaning of the cargo 

compartment of all haul trucks at the delivery site after removal of Bulk Material. 

4.1.3 Rule 803: Carry-Out and Track-Out 

The purpose of Rule 803 is to reduce the amount of PM10 that is entrained in the ambient air as 

a result of Track-Out and Carry-Out46 occurring on paved public roads. The rule requires 

mitigation of the deposition of Bulk Material by tracking out/carrying out onto a paved road 

surface by implementation of the following controls: 

• Any Bulk Material tracked out or carried out onto a paved road is to be cleaned up at the 

end of the workday (or immediately if within an urban area and Track-Out or Carry-Out 

extends a cumulative distance of ≥50 feet); 

 

45 Bulk Material is any organic and/or inorganic material consisting of or containing particulate matter with ≥ 5% silt 

content, including materials such as earth, rock, silt, sediment, sand, gravel, soil, fill, aggregate, dirt, mud, or 

debris. 46 Track-out/carry out refers to any Bulk Material that adhere to and agglomerate on the exterior surfaces of motor 

vehicles and/or equipment (including tires) that may then fall onto the pavement. 
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• All sites with access to a paved road and with ≥150 Average Vehicle Trips per Day47

(AVTD) are to (i) install one or more Track-Out prevention devices and (ii) apply and 

maintain paving, chemical stabilization, or gravel for a distance of ≥50 consecutive feet, at 

access points where unpaved roads adjoin paved roads. 

4.1.4 Rule 804: Open Areas 

The purpose of Rule 804 is to reduce the amount of PM10 that is emitted from non-agricultural48

open areas, such as vacant portions of residential or commercial lots. The rule applies to any 

open area of ≥0.5 acres within urban areas, or ≥3 acres within rural areas, that contain ≥1000 

square feet of disturbed surface area. Rule 804 requires all persons who own or otherwise 

have jurisdiction over an open area to prevent vehicle use in the open area by posting “No 

Trespassing” signs or installing physical barriers to prevent trespassing. In addition, surface 

stabilization is required in open areas to limit VDE to 20% opacity by (i) applying water or dust 

suppressant(s) to all unvegetated areas, (ii) establishing vegetation on all previously disturbed 

areas, and/or (iii) paving, applying and maintaining gravel, or applying and maintaining chemical 

stabilizers/ suppressants. 

4.1.5 Rule 805: Paved and Unpaved Roads 

Purpose and Requirements. The purpose of Rule 805 is to reduce the amount of PM10 that is 

windblown or entrained from new or modified paved roads, from unpaved traffic areas and all 

non-farm49 unpaved roads, or from road construction or road modification projects in Imperial 

County. The rule requirements are the following: 

• For unpaved haul/access roads, unpaved traffic areas larger than 1 acre and with ≥75 

AVTD, unpaved roads with ≥50 AVTD, and canal roads with ≥20 AVTD, VDE must be 

limited to 20% opacity by applying at least one of the stabilization methods described 

below; 

• Parties responsible for the use of canal roads with ≥20 AVTD are further required to 

implement one of a number of additional measures that include maintenance of canal bank 

surfaces, conversion of open canals to pipeline, installation of remote-control delivery 

gates to eliminate manual gate operation by maintenance personnel in vehicles along 

canal banks, or lining of canals to eliminate maintenance associated with the control of silt 

or weed; 

• Construction of new unpaved roads is prohibited within any area with a population ≥500, 

except for temporary activity and if the road is stabilized to limit VDE to 20% opacity; 

• New or modified paved roads must be constructed with curbing adjacent to the travel 

lanes, or with shoulders of width 2-6 feet (depending on the frequency of road usage) that 

are either paved or that meet the conditions of a stabilized surface. 

Stabilization Methods. BACMs for Fugitive PM10 dust emitted from unpaved roads include 

stabilization of the unpaved surfaces by (i) paving, (ii) applying chemical stabilization as directed 

by the product manufacturer, (iii) applying and maintaining gravel, recrushed/recycled asphalt or 

 

47 Or ≥20 AVTD by vehicles with three or more axles. 48 Emissions from agricultural open areas are controlled by regulations outlined in Rule 806. 49 Emissions from agricultural unpaved roads are controlled by regulations outlined in Rule 806. 
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other material of low silt content (<5%) to a depth of three or more inches, or (iv) wetting by 

applying water one or more times daily. 

Rule Implementation. Rule 805 requires each city or county agency with primary responsibility 

for any existing unpaved road to provide to the ICAPCD (by March 31, 2006) a compliance plan 

and a compliance schedule demonstrating implementation of Rule 805 to all unpaved roads 

within its jurisdiction at an incremental rate of no less than 10% per fiscal year during the time 

period of 2006-2015. The plan identifies the control measures selected for each unpaved road 

segment, and report of yearly progress is to be made to the APCD by July 31 of each year until 

2015. 

4.1.6 Rule 806: Conservation Management Practices 

Purpose and Requirements. The purpose of Rule 806 (effective since January 1, 2006) is to 

reduce the amount of PM10 emitted from agricultural operations in Imperial County. The rule 

requires all owners or operators of Agricultural Operation Sites of ≥40 acres to implement in 

each Agricultural Parcel at least one Conservation Management Practices (CMP, described 

below) for each of the following categories: (i) land preparation and cultivation, (ii) harvest 

activities, (iii) unpaved roads, and (iv) unpaved traffic areas. Owners and operators are required 

to prepare, for each Agricultural Operation Site, a CMP Plan that must be made available to the 

ICAPCD upon request within 72 hours of the notice. 

Conservation Management Practices for Fugitive Dust (PM10). One or more of a number of 

listed CMPs must be implemented to satisfy the requirements of Rule 806. Owners or operators 

of Agricultural Operation Sites may develop and implement alternative CMPs, provided that the 

achieved PM10 emission reductions are at least equivalent to those obtained from CMPs listed 

for the applicable operation. (An alternative CMP must receive approval by the ICAPCD after 

review of its technical merit before it may be included in a CMP Plan.) A subset of the allowed 

CMPs is reported below for each category covered by Rule 806; a comprehensive listing of the 

practices is available at the ICAPCD webpage. 

• For the control of PM10 emissions from land preparation and cultivation, owners or 

operators may implement alternate tilling, non-tillage or chemical tillage, 

chemigation/fertigation, covering of crops, land fallowing, mulching, or night farming; 

• For the control of PM10 emissions from harvesting, owners or operators may implement 

green chopping, hand harvesting, night harvesting, pre-harvesting soil preparation, 

no-burning, or equipment changes/technological improvements; 

• For the control of PM10 emissions from unpaved roads and unpaved traffic areas, owners 

or operators may implement graveling, paving, restricted access, speed limits, track-out 

control, or wind barriers. 

4.1.7 Record of Control Implementation 

Any person subject to the requirements of any one of the Regulation VIII rules is required to 

compile and retain records that provide evidence of control measure application (i.e., receipts 

and/or purchase records). The records are expected to document the type of treatment or 

control measure, extent of coverage, frequency of application, and date applied. Records must 

be kept for at least two years and be made available to the APCD upon request. 
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4.2 Impact Analysis and Cost Information 

This section presents information about the emission reductions and implementation costs of 

the above Regulation VIII rules. (An analysis of cost-effectiveness is presented in Appendix IV.B 

of this SIP.) Cost estimates are based on the BACM analyses of the 1993 Imperial Valley PM10

SIP50 and the 2003 San Joaquin Valley PM10 SIP,51 on cost-information for PM10 control 

measures compiled by the CARB,52 and on information from Imperial County’s Public Works 

Department (ICPWD). Note that all costs listed in this section are in 2003 dollars; current costs 

will likely be higher. For each adopted Regulation VIII rule, estimated emissions from sources 

covered by the rule, the fraction of those emissions that are subject to control requirements, the 

control factor for those controls, and the estimated emission reductions at full implementation 

are summarized in Table 4.1. The input information and assumptions of Table 4.1 are presented 

in the ensuing sections. The expected actual emission inventories for years 2006-2010, 

accounting for mitigation of pollution by implementation of Regulation VIII rules, are reported in 

Appendix IV.C. 

Table 4.1 Imperial County Regulation VIII Emission Reduction Summary (Based on the 

2005 Annual Average Inventory)a

Regulation VIII Rule Emissions (tpd) Applicability Control Factor Reductions (tpd)

801 (Construction) 2.20 1.00 0.12 0.26 

802 (Bulk Materials) 2.63 0.10 0.5 0.13 

803 (Track-out) 3.38 0.184 0.6 0.37 

804 (Open Areas) 169.55 - - 1.20 

Urban Open Areas 0.01 1.00 0.7 0.01 

Other Open Areasb 169.54 0.01 0.7 1.19 

805 (Non-Farm Unpaved Roads)c 32.40 - - 8.73 

City/County Roads (Entrained) 24.58 0.55 0.6 8.04 

City/County Roads (Windblown) 7.82 0.15 0.6 0.69 

805 (Paved Roads) 3.38 0.01 0.8 0.03 

806 (CMPs) 25.29 - - 5.40 

Farming─Tilling 7.10 0.97 0.29 2.00 

Farming─Harvest 0.01 0.97 0.41 0.01 

Farm Unpaved Roads (Entrained) 1.35 0.97 0.23 0.30 

Farm Unpaved Roads (Windblown) 6.01 0.97 0.19 1.11 

Non-Pasture Ag Lands (Windblown) 10.81 0.97 0.19 1.99 

TOTAL 235 - - 16.12 a

Actual emissions reductions in any year vary based on projected emissions for the year and on the implementation 

schedule of the adopted rules (refer to Appendix IV.C for the implementation schedule of Rule 805). Entries in this 

table assume full implementation. b

Referring to grasslands, dunes, and other barren lands. cThe IID has estimated 

that traffic on all canal roads is presently below the 20 ADVT threshold above which Rule 805 requirements apply. 

Nevertheless, IID is controlling PM10 emissions from canal roads, although resulting reductions are not reported 

here. 

 

50 ICAPCD, State Implementation Plan for PM10 in the Imperial Valley, Final, September 28, 1993. 51 SJVAPCD, BACM/T and RACM/T Demonstration for Sources of PM10 and PM10 Precursors in the San Joaquin 

Valley Air Basin. SJVAPCD 2003 PM10 SIP, Appendix G. April 2003. 52 CARB, Proposed List of Measures to Reduce Particulate Matter – PM10 and PM2.5, October 18, 2004 
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4.2.1 Rule 801: Construction 

Rule 801 applies to any construction or other earth moving activity. Only construction at existing 

single family homes is exempt, so the rule applies to all new construction emissions. Rule 801 

upgrades the RACM controls in the previous Rule 80053 to BACM controls consistent with 

requirements in other serious nonattainment areas. Emission estimates for construction already 

include the effect of basic RACM controls, such as watering. BACM upgrades will require 

additional watering and/or stabilizing during and after construction activities. As noted in the 

South Coast 1997 AQMP, such BACM upgrades provide an additional control efficiency of 

approximately 12%, mostly from additional water and/or stabilizing during and after construction 

or other earthmoving activities. Overall reductions from Rule 801 are estimated at 0.26 tons/day. 

Estimated costs54 for certain construction-related controls include: additional use of water trucks 

($3,152 per 40 acre project);55 water sprinkler ($30 per acre); and dust control plans and related 

costs ($112/acre).56 Actual costs for compliance are subject to the control options used by the 

site, the level of dust control currently practiced, and the local material and labor costs. 

4.2.2 Rule 802: Bulk Materials 

Rule 802 applies to the handling, storage, and transport of bulk materials. There is not an 

explicit inventory category for bulk materials, although it can be assumed that the majority of 

handling, storage, and transport of these materials occurs at mineral processing facilities and, to 

some extent, at construction sites. It is assumed that 10% of the emissions from mineral 

processing facilities are related to bulk materials. Wetting of bulk material piles at transfer points 

has an estimated control efficiency of 50%. (San Joaquin estimated a 56% to 81% control 

efficiency.) Emission reductions from transfer controls have not been estimated, but most 

mineral processing plants are operating under ICAPCD permits that require control at major 

transfer points. Overall reductions from Rule 802 are estimated at 0.13 tons/day. 

Estimated costs54 for Rule 802-related controls include: Truck covers ($900 per truck);57 and 

three-sided enclosures ($830 per enclosure).58 Actual costs for compliance are subject to the 

control options used by the site, the level of dust control currently practiced, and the local 

material and labor costs. 

4.2.3 Rule 803: Carry-Out and Track-Out 

Rule 803 applies to material carried or tracked out onto paved roadways. There is not an explicit 

inventory category for track-out, although it can be assumed that a given percentage of the silt 

loading on paved road surfaces is from track-out. The USEPA guidance59 indicates that 46% of 

paved road deposition is attributable to mud and dirt carry-out. In addition, many permanent 

facilities (e.g., mineral processing facilities) currently implement track-out controls. It is assumed 

 

53 ENVIRON, Draft Regulation VIII Rules and Rule Amendments. Draft Final Technical Memorandum, Regulation VIII 

BACM Analysis, Appendix E, October 2005 54 Listed estimates are in 2003 nominal dollars. 55 SJAPCD, Final BACM Technological and Economic Feasibility Analysis, SJVAPCD 2003 PM10 SIP, April 2003. 56 SCAQMD, Rule 403 Final Staff Report, September 1992. 57 ICAPCD, State Implementation Plan for PM10 in the Imperial Valley, Final Draft, adopted September 28, 1993. 58 SJAPCD, Final BACM Technological and Economic Feasibility Analysis, SJVAPCD 2003 PM10 SIP, April 2003. 59 USEPA, Fugitive Dust Background Document and Technical Information Document for Best Available Control 

Measures, Document Number USEPA-450/2-92-004, Office of Air Quality Planning and Standards, 1992. 
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that 40% of the track-out emissions originate from construction and other temporary sites that 

have not previously been using track-out controls. Thus, 18% of paved road dust will be affected 

by new Rule 803 controls. Overall reductions from Rule 803 are estimated at 0.37 tons/day. 

Estimated costs60 for Rule 803-related controls include: paving access points ($6,000 to $8,500 

per access point);61 chemical stabilization ($984);62 gravelling ($680 to $1,360 per year per 

access point);63 and track-out control device ($3,500 to $4,800 plus maintenance costs).64 

Actual costs for compliance are subject to the control options used by the site, the level of dust 

control currently practiced, and the local material and labor costs. 

4.2.4 Rule 804: Open Areas 

Rule 804 applies to non-agricultural open areas more than 3 acres (rural) or 0.5 acres (urban). 

Review of Imperial County parcel data indicates that over 99.5% of parcels are greater than 3 

acres in size. However, 77.5% of Imperial County is desert and/or scrubland, much of which is 

under the control of BLM or other federal agencies. BLM areas are exempt from Rule 804 to the 

extent that BLM can demonstrate that Rule 804 measures are prohibited by federal or state 

laws, regulations, or approved plans concerning wilderness preservation and species 

management and recovery. (However, BLM areas are subject to other general dust control plan 

requirements per Rule 800.F.5. PM10 emission reductions arising from BLM control measures 

not mandated under Rule 804 are not reported here.) Agricultural areas, which cover 21% of 

Imperial County, are subject to Rule 806. For purposes of estimating emission reductions, it is 

assumed the applicability of Rule 804 is limited to open areas in close vicinity to urban areas, 

which represent 1.5% of Imperial County. Assuming that up to 2/3 of that area could be 

disturbed open lands, the applicability of Rule 804 is estimated to be 1% or less. The composite 

control factor is estimated to be 70% (based on control efficiencies cited in San Joaquin Valley’s 

2003 PM10 SIP). Overall reductions from Rule 804 are estimated at 1.20 tons/day. 

Estimated costs60 for Rule 804-related controls include: dust suppressants ($3,340 per acre); 

and signage ($200 per sign).65 Actual costs for compliance are subject to the control options 

used by the site, the level of dust control currently practiced, and the local material and labor 

costs. 

4.2.5 Rule 805: Unpaved Roads and New or Modified Paved Roads 

Rule 805 applies to new or modified paved roads (0.03 tpd reductions as shown in Table 4.1), 

as well as to unpaved city/county roads with >50 vehicles per day (199 miles according to the 

ICPWD; see Attachment A of Appendix III.A).66 Windblown emissions from these roads account 

 

60 Listed estimates are in 2003 nominal dollars. 61 ICAPCD, State Implementation Plan for PM10 in the Imperial Valley, Final, adopted September 28, 1993, and 

SCAQMD, Rule 403 Final Staff Report, Appendix G, February 1997. 62 SCAQMD, Rule 403 Final Staff Report, Appendix G, February 1997. 63 SJVAPCD, Final BACM Technological and Economic Feasibility Analysis, SJVAPCD 2003 PM10 SIP, April 2003. 64 SJVAPCD, Final BACM Technological and Economic Feasibility Analysis, SJVAPCD 2003 PM10 SIP, April 2003, 

and SCAQMD, Rule 403 Final Staff Report, Appendix G, February 1997 65 SJVAPCD, Final BACM Technological and Economic Feasibility Analysis, SJVAPCD 2003 PM10 SIP, April 2003. 66 Rule 805 also applies to the following: (i) unpaved canal roads with >20 ADVT (current estimates from the IID are 

that traffic is presently below that threshold on all canal roads), (ii) paved and unpaved roads under the jurisdiction 

of the U.S. Border Patrol (except to the extent that Rule 805 measures are demonstrated to be inconsistent with 

Border Patrol authority and/or mission), and (iii) paved and unpaved roads under the jurisdiction of the BLM 
Imperial County 2009 PM10 SIP Chapter 4: Control Strategies and Impact Analysis 

FINAL AUGUST 2009 4-8 ICAPCD

for 15% of the total windblown emissions from city/county unpaved roads, and entrained 

emissions from these roads account for 55% of the total entrained emissions from city/county 

unpaved roads. Assuming a composite control factor of 60%,67 reductions from unpaved roads 

obtained by implementation of Rule 805 are estimated at 8.73 tons/day. 

The ICPWD has provided the following cost information68 for the paving or gravelling of high 

ADT roads.69,70 They estimate that it would cost $2,980 to apply dust suppressant to 1 mile of 

unpaved road, $8,950 to gravel, grade, compact, and water 1 mile of unpaved road, and 

$131,200 to pave 1 mile of unpaved road. The current budget for paved road maintenance is 

$2 million per year.

4.2.6 Rule 806: Conservation Management Practices 

Rule 806 requires implementation of CMPs at all farms over 40 acres. According to information 

obtained from USDA/NRCS, parcels of size ≥ 40 acres account for approximately 96.95% of the 

area of all agricultural parcels in Imperial County. Thus, if emissions per acre of agricultural 

parcel are equal for all parcels (regardless of size), then Rule 806 applies to ~ 97% of total 

emissions from all agricultural parcels.71 If we assume that the control factors achieved in the 

San Joaquin Valley (through implementation of Rule 4550) can be applied to Imperial County, 

overall reductions from Rule 806 are ~5.40 tons/day. 

Costs for CMPs related to unpaved roads and traffic areas would be similar to control costs 

presented for Rule 803 (track-out controls) and Rule 805 (unpaved road/traffic area controls). 

Costs for CMPs for land preparation/cultivation and harvesting are highly dependent on crop 

type and the specific CMP option chosen.

4.3 BACM Determination 

As explained in Section 3.2, significant source categories (Table 3.7) are required to be 

mitigated by BACM. Whether a control strategy meets this requirement is determined through 

comparative analysis. Determination of stringency for a control strategy involves consideration 

of both rule applicability and the effectiveness of imposed control methods. In general, a control 

strategy meets the “best available” criterion if the combination of its applicability and control 

effectiveness is as stringent as or more stringent than that of similar measures implemented 

 

(except to the extent that Rule 804 measures are prohibited by federal or state laws, regulations, or approved 

plans concerning wilderness preservation and species management and recovery). PM10 emission reductions 

achieved from general dust control programs (not mandated by Rule 805) undertaken by the IID, the U.S. Border 

Patrol, and the BLM are not reported here. 67 Assuming that graveling is chosen as the primary control option and based on the control efficiency cited in San 

Joaquin Valley’s Final Draft Staff Report for Regulation VIII (May 2004). 68 Listed estimates are in 2003 nominal dollars. Actual costs for compliance will depend on how the mitigations are 

implemented and on local material and labor costs. 69 ICPWD cost estimate of compliance, August 1, 2005. 70 ICPWD letter to ICAPCD, dated August 22, 2005. 71 Agricultural parcels, which are defined as portions of real property used for carrying out specific agricultural 

operations, include non-cultivated land such as roads, vehicle/equipment traffic areas, and facilities adjacent to the 

cropland. We note that the applicability of Rule 806 reported in the 2005 BACM document incorrectly relied on a 

“net” percentage of farmland that excluded the fraction of non-cropland within parcels of size ≥ 40 acres. In other 

words, the 90% applicability reported in that document corresponds to (the total area available for land 

preparation, cultivation, and harvest that belongs to agricultural parcels ≥ 40 acres) divided by (the total Imperial 

County area designated as farmland). 
Imperial County 2009 PM10 SIP Chapter 4: Control Strategies and Impact Analysis 

FINAL AUGUST 2009 4-9 ICAPCD

elsewhere, provided there exists such a feasible strategy given local conditions, needs, and 

resources. 

The 2005 BACM analysis included a detailed evaluation72 of the stringency of Imperial County’s 

Regulation VIII rules by comparison to analogous control strategies adopted in the following 

PM10 serious nonattainment areas: San Joaquin Valley, Maricopa County, Clark County, South 

Coast, and Coachella Valley areas. Although Regulation VIII rules mitigate emissions from a 

broad range of source categories, only sources determined to be significant according to the 5 

μg/m3

 criterion (Section 3.2) are strictly subject to BACM requirements. We thus limit the scope 

of this section to a comparative analysis (Table 4.2) of rules directly relevant to the control of (i) 

entrained dust from unpaved roads (City/County and Canal), and (ii) dust generated from 

agricultural tilling (Table 3.7). 

 

72 ENVIRON, Draft Final Technical Memorandum, Regulation VIII BACM Analysis, Appendix C and Appendix D, 

October 2005. 
Imperial County 2009 PM10 SIP Chapter 4: Control Strateg

FINAL AUGUST 2009 4-10 Table 4.2 Comparative Stringency of Controls for Significant Source Categories 

Control 

Category 

Imperial San Joaquin 

Valley 

South Coast Maricopa County Clark County Disc

Unpaved Roads: (Rule) Applicability 

• (Rule 805 amended 

November 2005) 

• All new unpaved roads in 

urban areas (Ref: R805, Section E.3) 

• Unpaved haul/ access 

roads: All roads (Ref: R805, Section E.1) 

• Unpaved roads: 50 or 

more average daily vehicle trips (Ref: R805, Section E.2) 

• Canal roads: 20 or more 

ADT (Ref: R805, Section E.4) 

• (Rule 8061 

amended August 2004) 

• All new unpaved 

roads in urban areas (Ref: R8061, Section 5.2.2) 

• All unpaved road 

segments with ≥ 26 annual average vehicle daily trips must comply with requirements 5.2.1 of R8061 (pertaining to VDE emissions) 

• All unpaved public 

roads in urban areas must comply with the paving requirements of R8061, Section 5.2.3.1.3 

• (Rule 403 amended 

June 2005; Rule 1186 amended July 2008) 

• Required to meet 

the requirements of R403(d)(1) and R403(d)(3): all unpaved roads except (Ref: R403(g)(2)(B)): a) Unpaved public 

alleys as defined in R1186; or 

b) Service roads 

(i) < 50’ wide at all points, (ii) within 25’ of property line; and (iii) with < 20 VDT traffic. 

• Subject to the 

requirements of R1186(5) (mandating surface treatment of unpaved roads): All public unpaved roads that have greater than the average VDT of all unpaved roads in the corresponding jurisdiction. 

• (Rule 310.01 

amended March 2008) 

• 150 vehicles or 

more per day (Ref: R310.01, Section 302.7) 

• (Rule 91 amended 

July 2004) 

• All new unpaved 

roads/alleys in public thoroughfares; (Ref AQR Section 91.2.1.2) 

• For existing 

unpaved roads (prior to June 22, 2000), the control measures apply to roads with 150 or more vehicles per day. 

• Applica

new unas the m

• The AD

county ADT) is 150 ADCountieADT limNeverthunpavethe rule Joaquin miles),7

Imperiamiles). Regulawith resfor exisroads. 

• Only Im

separatroads, aother uProposmost st

Unpaved Roads: Control 

• For roads with 50 ADVT 

or more, limit VDE to 20% opacity and comply with 

• For unpaved roads 

with ≥ 26 ADVT: establish a 

• Except for exempt 

unpaved roads: a) Prevent dust 

• For roads with 150 

vehicles or more per day, implement 

• Implement one of 

the control measures listed 

• Require

new unas the m

73 EPA’s Technical Support Document for the San Joaquin Valley, California 2003 PM10 Plan and 2003 PM10 Plan Amendments., p. 31, 
Imperial County 2009 PM10 SIP Chapter 4: Control Strateg

FINAL AUGUST 2009 4-11 Table 4.2 Comparative Stringency of Controls for Significant Source Categories 

Control 

Category 

Imperial San Joaquin 

Valley 

South Coast Maricopa County Clark County Disc

Requirements the requirements of a 

stabilized unpaved road surface by application and/or maintenance of at least one of the following control requirements (Ref: R805, Sections E.2, F.1): a) Pave b) Apply chemical 

stabilization 

c) Apply and maintain 

gravel, asphalt, or other material of low silt content of a depth of 3 or more inches 

d) Apply water one or 

more times daily 

e) Implement permanent 

road closure 

f) Restrict unauthorized 

vehicle access 

g) Implement any other 

method that limits VDE to 20% opacity and meets the conditions of a stabilized unpaved road. 

• Stabilization standards 

(Ref: R800, Section C.35 and Section C.10 of Appendix B) a) Silt loading ≤ 0.33 

oz/ft2, or 

b) Silt content ≤ 6%. 

• Within an urban area, 

construction of a new unpaved road is 

maximum speed limit of 25 mph (and post speed limit signs); limit VDE to 20% opacity; and comply with the requirements of a stabilized unpaved road by implementing at least one of the following control measures: a) Apply water b) Apply uniform 

layer of washed gravel 

c) Apply chemical/

organic dust suppressant 

d) Apply roadmix 

(cover material with crude-oilcontaining soil) 

e) Pave f) Use any other 

approved method to limit VDE to 20% opacity and meets the condition of a stabilized unpaved road (Ref: R8061, Section 5.2.1) 

• Stabilization 

standards (Ref: 

emission that exceed 20% opacity (Ref: R403(d)(1)(B)) 

b) Prevent the 

occurrence of > 50 μg/m3 PM10

levels (in terms of the difference in upwind and downwind ambient air concentrations) (Ref: R403(d)(3)) 

• Annually treat 

unpaved public roads beginning in 1998 and continuing for each of 8 years thereafter by implementing one of the following (Ref: R1186(d)(4)): a) Pave at least 

one mile with typical roadway material 

b) Apply chemical 

stabilizers to at least two miles to maintain stabilized surface 

c) Speed control 

(15 mph) on at least three 

at least one of the following BACM (Ref: R310.01, Section 302.7.b): 

a) Pave b) Apply dust 

suppressants 

c) Uniformly 

apply and maintain surface gravel 

• BACM must meet 

the following stabilization standards (Ref: ibid, Section 302.7.a) 

a) VDE ≤20% 

opacity, and 

b) Silt loading 

≤0.33 oz/ft2 or 

c) Silt content 

≤6% 

• Compliance 

schedule requires implementation on 5 miles (of unpaved roads with ≥150 vehicles/day) per year beginning in 2008 (Ref: ibid, Section 302.7.c.(3)(b)) 

below to comply with the stabilization standard described below according to the following schedule (Ref: AQR Section 91.2.1.1): a) 1/3 of unpaved 

roads with ≥150 VDT per year over a 3 year period 

b) At the end of 

which period roads that are newly found to have ≥ 150 VDT are required to be brought into compliance within 365 days

• Applicable control 

measures are as follows: a) Pave b) Apply dust 

palliatives to meet stabilization standards 

• Stabilization 

standards (Ref: ibid, 91.2.1.4): a) VDE ≤20% 

opacity, and 

b) Silt loading 

≤0.33 oz/ft2 or 

• Imperia

areas r20% opas strinfor this 

• The sta

areas eidenticastandarsubstituμg/m3 P

impact)stringenthis req

• Althoug

controls compliasurface measurare less allowed Countycontrols standarTherefowith lowas watenot imp

• ICAPC

requireunpaveareas, sJoaquindoes nocomponas the mprovide

74 Federal Register Vol. 69, No. 102, p. 30019, May 26, 2004. 75 Federal Register Vol. 59, p. 41998, 42013, August 16, 1994. See also EPA’s Technical Support Document for the San Joaquin Valley

Plan and 2003 PM10 Plan Amendments., p. 31-32, January 27, 2004. 
Imperial County 2009 PM10 SIP Chapter 4: Control Strateg

FINAL AUGUST 2009 4-12 Table 4.2 Comparative Stringency of Controls for Significant Source Categories 

Control 

Category 

Imperial San Joaquin 

Valley 

South Coast Maricopa County Clark County Disc

prohibited, unless it meets the definition of a Temporary Unpaved Road (Ref: R805, Section E.3) 

• Cities and the County 

shall comply with Section E.2 by treating 10% of applicable roads per year over a 10 year schedule (Ref: R805, Section E.7) 

• (Note: the requirements 

for canal roads are discussed in the next row: Unpaved Roads: Canal Roads) 

R8011, Section 3.59 and Section 3.10 of Appendix B): a) VDE ≤20% 

opacity, and 

b) Silt loading 

≤0.33 oz/ft2 or 

c) Silt content 

≤6%. 

• As alternative to the 

above, obtain Fugitive PM10

Management Plan (Ref: ibid, Section 5.2.1, R8011, Section 7) with specific requirements. 

• Within an urban 

area, construction of a new unpaved road is prohibited, unless it meets the definition of a temporary unpaved Road (Ref: R8061, Section 5.2.2.) 

• Cities and the 

County shall pave an average of 20% per year of all existing unpaved public roads in urban areas form 2006 through 2010, to a cap of 5 miles per year per jurisdiction. A statement of financial hardship can be submitted if a jurisdiction cannot afford to meet the 

miles of road surface 

c) Silt content 

≤6% 

• No new unpaved 

roads are to be constructed, except for temporary use (Ref: AQR Section 91.2.1.2) 

• For unpaved roads 

with less than 150 VDT, maintain stabilized surface standards within 365 days of determination of non-stabilized surface (Note: not a SIP measure) 

rule comstringen

provide in estabsuited tsituatiourban rand accfraction city/cou3.2). Tdoes nounpave

• The sch

controls roads (demanda 10-yethat in onote tharule lacEPA’s B“where measurdependconsideactivitiefunding whethemade to availab

Given tto the ImDepartmrule’s imvery aggood faapplicamanner

• Thus, th

combinin that ttogethethe mos
Imperial County 2009 PM10 SIP Chapter 4: Control Strateg

FINAL AUGUST 2009 4-13 Table 4.2 Comparative Stringency of Controls for Significant Source Categories 

Control 

Category 

Imperial San Joaquin 

Valley 

South Coast Maricopa County Clark County Disc

requirements of this section (Ref R8061, Section 5.2.3) 

and in tschedubest eff

Unpaved Roads: Canal Roads 

For Canal Roads with ≥20 ADT, limit VDE to 20% opacity, comply with the requirements of a stabilized unpaved road, and implement at least one of the following control measures (Ref: PR805, Sections E.4 and F.2): 

a) Stock Triploid Grass 

carp in canals to reduce maintenance trips 

b) Install remote control 

delivery gates 

c) Implement Silt 

removal program to delay grading of spoil piles 

d) Implement permanent 

road closure 

e) Convert open canals 

to pipeline 

f) Line canals to 

eliminate maintenance for silt/weed control 

g) Initiate canal bank 

surface maintenance 

(No requirements specified.) 

(No requirements specified.) 

(No requirements specified.) 

(No requirements specified.) 

Canal roaby the Immaintain San Joaqcanal roanot anticiroads havthreshold general uneither doadditionasuch as tThus, Immost strinnote that addition trequiremcanal roaabove.) 

Agricultural Sources—Tilling: Conservation Management Practices (CMPs) 

• (Rule 806 amended 

November 2005) 

• CMPs requirements of 

R806 apply to commercial farms on sites ≥ 40 acres (Ref: R806, Section D.1) 

• For land preparation and 

• (Rule 4550 readopted August 

2004) 

• CMP requirements 

of R4550 apply to commercial farms on sites ≥100 acres 

• (Rule 403 amended 

June 2005; South Coast Air Basin Rule 403 Agricultural Handbook revised December 1998; Coachella Valley 

• (Rules 18-2-610 

and 18-2-611 amendments effective November 2007) 

• BMP requirements 

apply to commercial 

(No requirements for this source) 

• The ad

requirerequireMaricopand are Joaquin were ap

meeting 

76 Guide to Agricultural PM10 Best Management Practices, prepared by the Governor’s Agricultural Best Management Practices Committ
Imperial County 2009 PM10 SIP Chapter 4: Control Strateg

FINAL AUGUST 2009 4-14 Table 4.2 Comparative Stringency of Controls for Significant Source Categories 

Control 

Category 

Imperial San Joaquin 

Valley 

South Coast Maricopa County Clark County Disc

cultivation, owners/operators are required to implement at least one of the following control measures (Ref: PR806, Section E.1): a) Alternative till b) Bed/row size spacing c) Chemigation/ 

fertigation 

d) Combined operations e) Conservation 

irrigation 

f) Conservation tillage g) Cover crops h) Equipment changes i) Fallowing land j) Integrated pest 

control 

k) Mulching l) Night farming; m) Non tillage/chemical 

tillage 

n) Organic practices o) Precision farming p) Transgenic crops 

(Ref: R4550, Section 4.1.1) 

• For land 

preparation and cultivation, owners/operators are required to implement at least one of the following control measures (Ref: R4550, List of CMPs): a) Alternate till b) Bed/row size 

spacing 

c) Chemigation/ 

fertigation 

d) Combined 

operations 

e) Conservation 

irrigation 

f) Conservation 

tillage 

g) Cover crops h) Equipment 

changes 

i) Fallowing land j) Floor 

management 

k) Integrated pest 

control 

l) Mulching m) Night farming n) Non tillage/ 

chemical tillage

o) Organic 

practices 

p) Precision 

farming 

q) Transgenic 

Rule 403 Agricultural Handbook revised April 2004) 

• For agricultural 

operations within the South Coast Air Basin, with combined disturbed surface area ≥10 acres, the standards of Rule 403 apply unless Best Management Practices (BMPs) as delineated in the Rule 403 Agricultural Handbook are implemented (Ref: R403(g)(1)) 

• BMPs for Active 

activities (i.e., agricultural activities involved in disturbing the soil) require that operators/producers cease activities during wind conditions greater than 25 mph and implement at least one of the following conservation practices (Ref:R403 South Coast Air Basin Agricultural Handbook, R403 Coachella Valley 

farms on sites ≥10 acres (Ref: p. 6 of Guide to Agricultural PM10

Best Management Practices.)76

• For tillage and 

harvest, commercial farmers are required to implement at least two of the following BMPs: a) Chemical 

irrigation 

b) Combined 

operations 

c) Equipment 

modifications 

d) Green chop e) Integrated pest 

management 

f) Limited activity 

during a highwind event 

g) Multi-year 

cropping 

h) Cessation of 

night tilling 

i) Planting based 

on soil moisture 

j) Precision 

farming 

k) Reduced 

harvest activity 

l) Reduced tillage m) Tillage based 

on soil moisture 

n) Timing of tillage 

operation 

• Althoug

rules apin 2004 

Joaquin applica(which land in comparapplyincovers Imperiasize limBACM rule app

• San Jo

Coast rapplicaproceduto provineeded BACM farm sitmaintaiimplemavailabF.6 of Rthe authPlan foadequaimplemoperatoallows toversigcomplia

• The nu

CMPs/Bemissioa) 0-1 

reqpre

77 Federal Register Vol. 69, No. 102, p.30035, May 26, 2004. 
Imperial County 2009 PM10 SIP Chapter 4: Control Strateg

FINAL AUGUST 2009 4-15 Table 4.2 Comparative Stringency of Controls for Significant Source Categories 

Control 

Category 

Imperial San Joaquin 

Valley 

South Coast Maricopa County Clark County Disc

crops 

r) Transplanting 

Agricultural Handbook): a) Soil moisture 

monitoring 

b) Irrigation c) Minimum tillaged) Mulching 

o) Transgenic 

crops 

satCMem

b) 0-1 

sam

c) 0-2 d) 2 fo

pradistcon

• As note

EPA docomponas the min order adequarules paareas’ sthe SoustringenBMPs rin 2004 

that Sastringenof ICAPR4550, requireand Sochangeit can bmeets Brelates emissio
THIS PAGE INTENTIONALLY LEFT BLANK 
Imperial County 2009 PM10 SIP Chapter 5: Attainment Demonstration 

FINAL AUGUST 2009 5-1 ICAPCD 

5 Attainment Demonstration 

5.1 Introduction 

The central requirement of a SIP is to outline a plan that provides for attainment of the standard 

as expeditiously as possible (in the present case, a minimum of 5% yearly reductions of 

emissions is required). Traditionally, attainment for a specific area is demonstrated through 

modeling and is based on the highest exceedence of the standard in the relevant time period of 

analysis (the design value).78 Based on the March 2007 USEPA final rule79 giving states the 

authority to exclude from regulatory determinations air quality monitoring data related to 

qualifying exceptional events (such as, e.g., high wind natural events), the September 2, 2006, 

April 12, 2007, and June 5, 2007 exceedences were excluded from consideration in the present 

attainment demonstration.80 As a result, the design value for the present SIP is equal to the 

248 μg/m3

 value measured at the Calexico Grant station on December 25, 2006. 

In international areas, the SIP attainment requirements are designed so that only attainment 

“but-for” international emissions is required. USEPA guidelines81 state that “for PM10

nonattainment areas, section 179B(a) [of the CAA] provides that EPA must approve [a PM10] 

SIP if (i) the SIP meets all the applicable requirements under the Act other than a requirement 

that such plan or revision demonstrate attainment and maintenance of the PM10 NAAQS by the 

applicable attainment date, and (ii) the State demonstrates to EPA’s satisfaction that the SIP 

would be adequate to attain and maintain the PM10 NAAQS by the attainment date but for 

emissions emanating from outside the US.” 

An attainment demonstration for the present Imperial County 2009 PM10 SIP therefore requires 

analyses to determine the contribution of Mexican emissions to the Imperial County 

exceedences of Table 2.1. Such analyses, which are documented in detail in Appendix V and 

summarized here in Section 5.2, demonstrate that ambient air quality on December 21, 2006 

and December 25, 2006 would have attained the 24-hour PM10 NAAQS in the absence of 

impact contributions from Mexicali emissions. The consequences of these findings for the 

present attainment demonstration are discussed in Section 5.3. 

5.2 Analyses of Transport Episodes 

Excluding exceedences due to high winds exceptional events, all exceedences of the 24-hour 

PM10 federal standard recorded at Imperial County monitors between the years 2006 and 2008 

occurred at the Calexico-Grant station (Table 2.1). This station is located less than 1 mile north 

of the Mexican border, against which lies the ~3⁄4 million city of Mexicali (Figure 5.1). The 

potential for activity-related PM emissions is significantly higher in Mexicali than in Calexico, 

 

78 The design value for a nonattainment pollutant is a baseline ambient concentration used in an attainment analysis 

to determine whether projected emissions reductions are sufficient to reduce the pollutant’s concentrations to 

levels that meet federal or state standards. 79 USEPA, 40 CFR Parts 50 and 51, Treatment of Data Influenced by Exceptional Events; Final Rule, Federal 

Register, Vol. 72, No. 55, March 22, 2007, p. 13560 80 For these days, documentation satisfying the requirements of 40 CFR Parts 50 and 51 is available from the 

ICAPCD at http://www.imperialcounty.net/AirPollution/Web%20Pages/2009%20March%20Natural%20Events.htm. 81 State Implementation Plans for Serious PM10 Nonattainment Areas, and Attainment Date Waivers for PM10

Nonattainment Areas Generally; Addendum to the General Preamble for the Implementation of Title I of the Clear 

Air Act Amendments of 1990; Federal Register, August 16, 1994, p. 42000 
Imperial County 2009 PM10 SIP Chapter 5: Attainment Demonstration 

FINAL AUGUST 2009 5-2 ICAPCD 

judging by the enormously larger population and mileage of unpaved roads in Mexicali. 

Research and analyses indicate that Mexicali emissions can have high short-range impacts on 

US air quality north of the border. Historical trends have demonstrated that these impacts are 

highest when (i) stagnant atmospheric conditions result in low dispersion of pollution, and/or (ii) 

emission levels in Mexicali are particularly high. Under these conditions, PM10 air concentration 

in Mexicali can reach very elevated levels, and Mexicali PM10 “overflows” into Calexico (this 

cross-border transport is of course greatly facilitated if light winds have a southerly direction). In 

this section, we first introduce the tools used to analyze, both qualitatively and quantitatively, the 

impact of Mexican emissions on near-border Calexico stations, and then summarize the results 

of transport analyses for each of the Calexico exceedences recorded on December 21, 2006, 

and December 25, 2006. 

Figure 5.1 Satellite image showing the proximity and relative sizes of Calexico and Mexicali 

(an interactive map of the area is available online at http://wikimapia.org/#lat=32.6347491&lon=-

115.4796982&z=12&l=0&m=a&v=2). The green arrows represent the location of PM10 monitors.

El Centro 

Calexico Grant

Mexicali

Calexico Ethel
Imperial County 2009 PM10 SIP Chapter 5: Attainment Demonstration 

FINAL AUGUST 2009 5-3 ICAPCD 

5.2.1 Methods of Analysis 

For PM10 nonattainment areas, USEPA guidance82 describes the following 5 approaches to 

demonstrate “but-for” attainment in State Implementation Plans involving international border 

areas: 

• “Evaluate and quantify...changes in monitored PM10 concentrations [in the U.S., near the 

border] with predominant wind direction;” 

• “Demonstrate that local U.S. emissions ... [do] not cause the NAAQS to be exceeded;” 

• “Analyze ambient sample filters for specific types of particles emanating from across the 

border;” 

• “Inventory...sources on both sides of the border and compare the magnitude of PM10

emissions originating within the US to those emanating from outside the US;” and 

• “Perform air dispersion and/or receptor modeling to quantify the relative impacts [of US 

and international sources].” 

The guidance further instructs that states may use any number of these approaches, or other 

techniques, “depending on their feasibility and applicability, to evaluate the impact of emissions 

emanating from outside the US on the nonattainment area.” The following paragraphs outline 

our efforts to implement the 5 suggested approaches, to the extent possible, to the 

Calexico/Mexicali international area; a full description is provided in Section 2 of Appendix V. 

Approach I. Our first approach involves statistical analyses that rely on meteorology and PM10

measurements for days with characteristics similar to those on exceedence days of interest in 

order to predict the impact of Mexican emissions at Calexico monitors on the days of interest. 

The approach assumed that atmospheric dispersion, wind direction, and Mexicali emissions 

(using 24-hour average wind speed, fractional cross-border air transport from the south, and 

Mexicali PM10 concentrations as indicators) were the primary determinants of the impact of 

Mexican emissions on Calexico air quality. The following procedure was used to get a 

conservative estimate of the expected impact of pollution coming from Mexico at the CalexicoGrant station on any specified day: 

• A subset of historical days with characteristics similar to the specific day of interest was 

selected; 

• For each of the Brawley, El Centro, Niland, and Westmorland stations, we approximated 

the expected value of the difference (PM10 at Grant) – (PM10 at non-Calexico station) for 

the specific day of interest as the historical average of these differences for the days with 

similar characteristics; 

• For each of the Brawley, El Centro, Niland, and Westmorland stations, we determined the 

expected value of the difference (PM10 at Grant) – (PM10 at non-Calexico station) for days 

similar to the specific day of interest, except with no southerly flow; and 

• The results of steps 2 and 3 were subtracted. 

 

82 State implementation plans for serious PM10 nonattainment areas, and attainment date waivers for PM10

nonattainment areas generally; Addendum to the general preamble for the implementation of Title I of the Clean 

Air Act Amendments of 1990, Federal Register, Vol. 59, No. 157, Tuesday, August 16, 1994, p. 41998 
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FINAL AUGUST 2009 5-4 ICAPCD 

The results of the second and third steps are estimates of the expected “excess” PM10

concentration, for the day of interest, at the Calexico Grant station (relative to the PM10

concentration at a reference non-Calexico station) due to the impact of both US and Mexican 

emissions (step 2) and due to the impact of US emissions alone (step 3). Hence, the differences 

are estimates of the expected impact of Mexican pollution on ambient air quality at the Calexico 

Grant station on the specific day of interest. 

Approach II. As a complementary analysis to Approach I, our second quantitative approach 

relies on statistical measures to predict the impact of US emissions alone at Calexico monitors, 

based on air quality at nearby Imperial County stations and in the light of historical trends in 

same-day ambient PM10 concentrations throughout the County. Motivated by arguments that air 

qualities at Calexico and at proximate Imperial County stations are impacted by similar levels of 

US emissions and by comparable meteorology, we test the hypothesis that same-day ambient 

PM10 concentrations at Imperial County stations are correlated (and indeed comparable in 

value). The analysis reveals that PM10 concentrations at any one of the El Centro, Brawley, 

Westmorland, and Niland stations can be predicted with <25 μg/m3

 error in >95% of cases 

(<40 μg/m3

 error in >99% of cases) using same-day measurements at nearby Imperial County 

stations. Although correlations between Calexico and non-Calexico PM10 concentrations is 

diminished by the high variability in the impact of Mexicali emissions, the analysis argues that 

PM10 concentrations at Calexico stations “but-for” Mexican emissions could likewise be 

predicted with <40 μg/m3

 error in >99% of cases using same-day measurements at the El 

Centro and Westmorland stations. 

Approach III. USEPA guidelines suggest analysis of ambient sample filters for specific types of 

particles emanating from across the border. The most common method of source apportionment 

relies on elemental analysis83 of filter samples. This is referred to as receptor modeling and it 

applies chemical mass balances to apportion observed levels of pollutants in a sample to 

several independent sources of known emission characteristics (referring to the composition of 

emissions from these sources). 

A 1992-1993 Cross Border Transport Study performed receptor modeling for analysis of the 

particles collected in areas within Imperial County where exceedences had been recorded.84 

The implications of the findings of that study for international apportionment of PM10 in the 

present investigation are the following: 

• The depth of intrusion of lead85 from Mexicali into Imperial County indicates that the 

impacts of Mexicali activity-related PM are measurable up to 20 km north of Mexicali. 

Given that the distance between Calexico stations and the US-Mexicali border is <1 mile, 

the potential impacts of Mexicali emissions at Calexico monitors under conditions favorable 

to stagnation and cross-border transport are very significant. 

 

83 That is, analysis of the concentrations of chemical constituents, such as nitrate, chloride, or sulfur. 84 Chow J.C., Watson J.G., Green C.M., Lowenthal D.H., Bates B., Oslund W., Torres G., Cross-border transport and 

spatial variability of suspended particles in Mexicali and California’s Imperial Valley, Atmospheric Environment, 34, 

2000, p. 1833-1843. 85 While there was no source of lead in Imperial County, leaded gasoline was still in use in Mexicali at the time of the 

study. 
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• Although it is possible to apportion ambient PM10 in both Calexico and Mexicali to source 

categories such as geological dust or vegetative burning, the study was not able to 

apportion ambient PM10 to source sub-categories classified by international origin. In other 

words, the profiles of the various source categories contributing significantly to ambient 

PM10 concentration do not differ enough to accurately predict what portion of the PM10

captured on Imperial County filters originates in Mexico. (The same finding was confirmed 

in another study led by CARB in 2001.)86 Given the very limited use that filter analysis can 

serve in apportioning ambient PM to Mexicali versus Imperial County sources, chemical 

analyses of filter loadings is not part of the weight of evidence in the present attainment 

demonstration (and it was therefore not conducted for the December 21, 2006 and 

December 25, 2006 samples). 

Approach IV. USEPA guidelines also suggest the comparison of PM10 emission levels on both 

sides of the international border to determine the relative impact of international and domestic 

sources on air quality at the border. Given that wind speeds were very low in the 

Calexico/Mexicali area on December 21 and 25, 2006, windblown emission sources are not 

expected to have contributed significantly to ambient levels of PM10 on these days. Because 

emissions from the remaining source categories are related to human activity, in the absence of 

an accurate gridded emission inventory for the Mexicali area87 we rely on population as a metric 

to estimate the relative magnitude of cumulative emissions originating from US and international 

sources. 

Comparison of population and total PM emissions (excluding windblown) for Imperial County 

and the Mexicali Municipality87 lead to a Mexicali:Imperial per capita emission ratio of 1:2.2 

(Table V.15 of Appendix V). Using that number and assuming that the relative impacts of 

US:Mexican emissions at the Calexico-Grant station reflect the relative proportion of 

US:Mexican populations within a disc of radius R = 2-4 miles centered at the station, the 

contributions of international emissions to the December 21 and 25, 2006 Grant exceedences 

are in the range of 65% to 80% (Table V.16 of Appendix V). 

Approach V. The fifth example of analysis suggested by EPA guidelines involves air dispersion 

modeling to quantify the relative impacts of international and domestic sources on the 

nonattainment area. Unfortunately, this approach has found limited success (refer to a 2001 

modeling study88 conducted by ENVIRON for the ICAPCD) for the Calexico/Mexicali 

international area due to the lack of an adequate gridded emission inventory for Mexicali. For 

this reason, no dispersion modeling was conducted to supplement the transport analyses 

presented in the present SIP. 

We found, however, that valuable insight could be derived in an analysis that relies on the 

results of the aforementioned 2001 dispersion modeling study.88 This study considered US 

 

86 As part of the year 2001 Imperial County SIP development, CARB performed chemical mass balance receptor 

modeling using 1995-1996 PM data and new source profiles for U.S. versus Mexico gasoline combustion sources 

(which differed mainly in sulfur contents). Even using the new source profiles, the difference in the relative 

contributions between gasoline combustion sources in the United States and Mexico could not be identified. 87 A new gridded inventory of the emissions of several air pollutants in Mexicali is in progress (2005 Mexicali 

Emission Inventory, Draft Final Report, Eastern Research Group, Inc., February 27, 2009). 88 Imperial County PM10 Attainment Demonstration, ENVIRON, July 2001, included as Attachment D to Appendix V. 
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FINAL AUGUST 2009 5-6 ICAPCD 

emissions alone to assess the maximum impact of domestic emissions on PM10 ambient 

concentrations in Imperial County in 1992-1994 and 1999. Given that (i) the total levels of 

emissions on December 21, 2006 and December 25, 2006 are estimated to be significantly 

lower than the minimum level of daily emissions used in the 2001 modeling (> 147 tpd of PM10

for winter days), and (ii) that the 2001 modeling did not predict any exceedences using 4-years 

of meteorological data, we argue that new modeling (conducted in a manner consistent with the 

2001 analysis) for the 2 days above would also predict that US emissions alone would not have 

been sufficient to cause exceedences of the 24-hour PM10 NAAQS on these days. 

5.2.2 Analysis for December 21, 2006 

Description of the Episode. On December 21, 2006, the Calexico Grant SSI monitor recorded a 

24-hour PM10 measurement of 171 μg/m3

. Available SSI and BAM monitor measurements 

available at non-Calexico Imperial County stations were 3-7 times lower (in the range of 25-55 

μg/m3

), indicating that there was not a county-wide air quality problem on that day. Although the 

only information about Mexicali air quality was a 10-hour BAM reading of 262 μg/m3 at the 

UABC monitor, available measurements reveal that PM10 air quality levels in Mexicali were 

elevated just two days earlier (in the range of 115 and 286 μg/m3

 at the Conalep, Cobach, and 

Progresso stations). Overall, PM10 measurements throughout the region showed a strong 

concentration gradient from south to north and are evidence of PM10 transport from Mexico into 

Imperial County. 

Surface hourly wind data collected in both Calexico and Mexicali indicate that stagnant 

atmospheric conditions and light winds of variable (but predominantly southerly) direction were 

prevalent on that day, providing ideal conditions for low dispersion and accumulation of PM10

pollution in Mexicali, and cross-border transport of elevated levels of PM10 from Mexical into 

Calexico: 

• The 24-hour average wind speeds recorded in Calexico were ≤1.1 knots, and similar lowspeed winds were recorded in Mexicali; 

• A wind rose of hourly surface wind speeds and directions measured at Calexico Grant on 

December 21, 2006 shows that the prevailing wind directions on that day were SW, W, and 

SE. Winds of direction with a northerly component only accounted for a total of 6 hours, 

while winds having the potential to carry emissions from Mexico into Imperial County89 

prevailed for 15 hours (>60% of the day). 

Quantitative Analyses. Meteorological conditions in Calexico on December 21, 2006 were 

conducive to very low dispersion (the average and maximum wind speeds at the Calexico-Grant 

station were 0.4 knots and 2 knots, respectively), and to much transport from Mexico (85% of 

the cross-border air flow at Grant was from the south). Because the number of days with 

meteorological conditions similar to those of December 21, 2006 is extremely small, we chose 

to analyze this exceedence in the context of the larger subset of days with 24-hr wind speeds at 

Grant <1.0 knots. Relative to other days within that subset (for which the average Mexicali 

contribution to PM10 ambient concentration at Grant is in the range of 50-55 μg/m3

), we found 

that (i) wind speeds were comparatively low, (ii) southerly flow at the Grant station was 

 

89 Of direction ranging from 85 to 265 (E to WSW) based on the location of the border. 
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FINAL AUGUST 2009 5-7 ICAPCD 

comparatively more pronounced, and (iii) PM10 concentration at the Mexicali UABC station (the 

only available data point) was comparatively high. All these factors would have caused the 

impact of PM10 pollution from Mexicali on December 21, 2006 to have been significantly higher 

than the 50-55 μg/m3

 group average. Therefore, the expected air quality at Calexico Grant for 

December 21, 2006 “but for” international sources of pollution is (by subtraction of 50-55 μg/m3

from the 171 μg/m3

 observed value) significantly lower than 116-121 μg/m3, and hence in 

attainment of the federal standard. 

The PM10 air quality in Calexico that would have occurred in the absence of Mexicali pollution 

(i.e., as a result of US emissions alone) was also estimated based on observed same-day PM10

concentrations at nearby US stations, by relying on the existence of correlations in air quality 

measurements at geographically proximal locations. Using the only available same-day 

measurement of 54 μg/m3

 at El Centro, this method predicted that the PM10 concentration at the 

Calexico-Grant station on December 21, 2006 as a result of US emissions alone would have 

been 66 ± 50 μg/m3

 (so up to 116 μg/m3). This result is also consistent with attainment of the 

24-hour PM10 NAAQS at Calexico Grant on December 21, 2006 in the absence international 

pollution from Mexico. 

Additional Evidence. Unusually high levels of PM10 are emitted every year in Mexico on the 

9 consecutive nights of “Las Posadas” (December 16th through December 24th). Thousands 

throughout Mexicali in rural and urban communities participate in religious festivals celebrated 

by dinner parties involving piñatas, heavy use of fireworks, and bonfires generated by the 

combustion of wood, coal, and tires. These activities, combined with the seasonally heavy 

vehicular traffic created by the Christmas season on unpaved roads in Mexicali, are major 

contributors to elevated PM10 emission levels in the Mexicali/Calexico area. When stagnant 

and/or light southerly wind conditions prevail, such as those experienced on 

December 21, 2006, these PM10 emissions accumulate to reach levels capable of causing high 

impact at neighboring Calexico monitors. Diminished visibility and breathing difficulty attributed 

to PM10 pollution in Mexicali were pronounced enough to receive attention in Mexicali’s La 

Crónica newspaper on December 23, 2006. 

Conclusion. Quantitative statistical analyses independently confirm that the impact of US 

emissions alone at the Calexico Grant station would not have been sufficient to cause an 

exceedence of the NAAQS on December 21, 2006. This quantitative result is supported by the 

following evidence and qualitative analyses: 

• Unusually high levels of PM10 are emitted every year in Mexico between December 16 and 

24 as a result of firework shows, a high volume of vehicular traffic, and of the burning of 

wood, coal and trash. On December 21 in 2006, stagnant and low-wind conditions 

facilitated the accumulation of PM10 in ambient air in Mexicali; 

• Comparative analysis of Calexico vs. Mexicali emission inventories supports high impacts 

of Mexicali emissions under suitable meteorological conditions (Approach IV); and 

• Information derived from previous modeling analyses adds further evidence to support that 

US emissions alone would not cause exceedences at Calexico stations (Approach V). 
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5.2.3 Analysis for December 25, 2006 

Description of the Episode. On December 25, 2006, monitors in Calexico recorded 24-hour 

PM10 measurements of 248 μg/m3

 and 110 μg/m3 at the Grant station and Ethel stations, 

respectively. Available SSI and BAM monitor measurements available at non-Calexico Imperial 

County stations on that day were in the range of 12-27 μg/m3

, indicating that there was not a 

county-wide air quality problem. Instead, remarkably high PM10 ambient air quality 

measurements recorded in Mexicali on that day point to cross-border transport of PM10 from 

Mexicali into Calexico: the average of PM10 concentrations at Mexicali stations was 344 μg/m3

, 

with a maximum at the Progresso station >85% higher than the highest Imperial County 

measurement at Calexico-Grant, and a minimum at the UABC station 60% higher than the 

24-hour PM10 federal standard. 

Surface hourly wind data collected in both Calexico and Mexicali indicate that very stagnant 

atmospheric conditions and light winds of variable direction were prevalent on that day, 

providing ideal conditions for low dispersion and accumulation of PM10 pollution in Mexicali, and 

for cross-border transport of elevated levels of PM10 from Mexical into Calexico: 

• The 24-hour average wind speeds recorded in Calexico were ≤1.4 knots, and similar lowspeed winds were recorded in Mexicali; 

• A wind rose of hourly surface wind speeds and directions measured at Calexico Grant on 

December 21, 2006 shows that the prevailing wind directions on that day were ESE and 

W, although wind direction varied greatly. The low wind speeds and varying wind direction 

allowed PM10 emissions to drift from Mexicali to other areas, including Calexico. 

Quantitative Analyses. Meteorological conditions on December 25, 2006 were conducive to 

extremely low dispersion (the average and maximum wind speeds at the Grant station were 

0.5 knots and 1 knots, respectively), and to considerable transport from Mexico (41% of the 

cross-border air flow at Grant was from the south). Because the number of days with 

meteorological conditions similar to those of December 25, 2006 is extremely small, we chose 

to analyze this exceedence in the context of the larger subset of days with 24-hr wind speeds at 

Grant <1.0 knots (as was done for the December 21, 2006 analysis). Relative to other days 

within that subset, we found that (i) wind speeds were comparatively low, (ii) southerly flow at 

the Grant station was comparatively high (in the 80% percentile), and (iii) PM10 concentrations 

at all Mexicali stations were extreme. All these factors would have caused the impact of PM10

pollution from Mexicali on December 25, 2006 to have been significantly higher than the subset 

average: an estimate of 150-200 μg/m3

 was obtained using correlations based on Mexicali air 

quality for days with comparable meteorology. Our best estimate of the air quality at Calexico 

Grant on December 25, 2006 “but for” international sources of pollution is therefore (by 

subtraction of 150-200 μg/m3

 from the 248 μg/m3 observed value) <100 μg/m3, and hence in 

attainment of the federal standard. 

The PM10 air quality in Calexico that would have occurred in the absence of Mexicali pollution 

(i.e., as a result of US emissions alone) was also estimated based on observed same-day PM10

concentrations at nearby US stations, by relying on the existence of correlations in air quality 

measurements at geographically proximal locations. Using same-day measurements of 24 and 

27 μg/m3

 at El Centro and Brawley, this method predicted that the PM10 concentration at the 
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FINAL AUGUST 2009 5-9 ICAPCD 

Calexico-Grant station on December 21, 2006 as a result of US emissions alone would have 

been 31 ± 40 μg/m3

 (so up to 71 μg/m3). This result is also consistent with attainment of the 24-

hour PM10 NAAQS at Calexico Grant on December 25, 2006 in the absence of international 

pollution from Mexico. 

Additional Evidence. As described in Section 5.2.2, unusually high levels of PM10 are emitted 

every year in Mexico on the 9 consecutive nights of “Las Posadas” (December 16th through 

December 24th). Finale accentuations in the celebrations on Christmas Eve and Christmas Day 

lead to extreme levels of PM emissions from the use of legal/illegal fireworks, the burning of 

wood, coal, and tires, and from extreme levels of activity on unpaved roads.90 On those days in 

2006, stagnant atmospheric conditions enabled the accumulation of PM10 pollution in the 

Mexicali airshed, leading to the extreme PM10 concentrations recorded by all Mexicali monitors. 

Conclusion. Quantitative statistical analyses independently confirm that the impact of US 

emissions alone at the Calexico Grant station would not have been sufficient to cause an 

exceedence of the NAAQS on December 25, 2006. This quantitative result is supported by the 

following evidence and qualitative analyses: 

• Unusually high levels of PM10 are emitted every year in Mexico on Christmas Eve and 

Christmas Day as a result of firework activity, a high volume of activity on paved/unpaved 

roads, and of the burning of wood, coal and trash. On these days in 2006, stagnant and 

low-wind conditions facilitated the accumulation of PM10 in ambient air in Mexicali; 

• Comparative analysis of Calexico vs. Mexicali emission inventories supports high impacts 

of Mexicali emissions under suitable meteorological conditions (Approach IV); and 

• Information derived from previous modeling analyses adds further evidence to support that 

US emissions alone would not cause exceedences at Calexico stations (Approach V). 

The preponderance of the evidence is that the combination of extreme PM10 emissions in 

Mexicali, low atmospheric dispersion, and light winds from the south resulted on 

December 25, 2006, in a high impact of Mexicali PM10 at the Calexico Grant station, in the 

absence of which there would have been no exceedence of the federal 24-hour PM10 standard 

in Imperial County. 

5.3 Implications of Results of Transport Analyses

Quantitative and qualitative analyses summarized in the previous section have demonstrated 

that, after exclusion of PM10 measurements affected by high-wind exceptional events, ambient 

air quality on every monitored day in 2006-2008 would have been in attainment of the 24-hour 

PM10 NAAQS “but-for” international emissions from Mexicali. Consequently: 

• The transport analyses of Section 5.2 are sufficient to meet the attainment demonstration 

requirements pertaining to international areas; so that 

• Reasonable Future Progress (RFP) and milestone requirements are unnecessary, and 

specifically 

 

90 Mexican authorities are recognizing and addressing the issue by requesting communities to limit these activities for 

the sake of reducing air quality impacts. 
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• The 5% yearly emission reductions requirement does not apply to future years. 

Nevertheless, because EPA cannot exclude monitoring data influenced by international 

transport from regulatory determination related to attainment or nonattainment:91

• Imperial County cannot request attainment redesignation;92 and 

• Requirements pertaining to non-attainment areas remain, including BACM or RACM 

requirements on significant sources, non-attainment conformity, non-attainment source 

review, etc. 

Imperial County’s fugitive dust control plan designed to improve PM air quality and to meet 

regulatory BACM requirements was discussed in Chapter 4. Contingency measures and 

transportation conformity are addressed in Chapter 6. 

 

91 USEPA, 40 CFR Part 81, Finding of Failure to Attain, California—Imperial Valley Nonattainment Area; PM10; Final 

Rule; Federal Register, Vol. 72, No. 237, December 11, 2007, p. 70226. (See Appendix I of this SIP.) 92 That is, Imperial County cannot request attainment redesignation as long as one or more “but-for” transport 

exceedences continue to occur in any three-year period. 
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6 Other Clean Air Act Requirements 

6.1 Contingency Measures 

The CAA requires air quality SIPs to include a contingency plan, i.e., to outline additional 

actions to be taken by the state in the event that air quality improvements or air quality 

maintenance is not satisfactory. Specifically: 

• For a nonattainment area, the contingency plan must provide for “the implementation of 

specific measures to be undertaken if the [USEPA] Administrator finds that the 

nonattainment area has failed to make RFP toward attainment or to attain the primary 

NAAQS by the applicable statutory deadline...States must show that their contingency 

measures can be implemented with minimal further action on their part and with no 

additional rulemaking actions such as public hearings or legislative review.” 93

• For an attainment area, the contingency plan is intended to assure prompt correction of 

any future violation(s). In this case however, the plan does not need to provide fully 

adopted measures that would go into effect without further action by the state, but rather to 

outline a schedule and procedure for adoption and implementation of new measures in the 

event that air quality deteriorates to specific, predetermined levels.94

An underlying, fundamental requirement of a contingency plan is to provide for additional control 

measures intended to reduce emissions beyond the expected minimum (as established by the 

attainment demonstration) necessary for attainment or maintenance of the NAAQS. Although 

these measures do not need to be implemented until progress towards attainment has been 

shown to be unsatisfactory (in a non-attainment area) or until air quality deteriorates beyond a 

specific level (in an attainment area), there should be no penalty imposed on districts for taking 

aggressive preemptive/early action to improve air quality beyond the NAAQS standards. 

Measures that are beyond the minimum needed for attainment may be relied upon to satisfy the 

contingency requirement even if these measures are already implemented. 

We also note that USEPA has determined that the attainment demonstration, RFP, and 

contingency measure requirements of the CAA do not apply to designated nonattainment areas 

for which monitored data demonstrates that the NAAQS has already been achieved, “since 

these requirements have the purpose of helping achieve attainment of the NAAQS.”95 In light of 

Imperial County’s attainment of the 24-hour PM10 NAAQS “but-for” international emissions in 

2006-2008, contingency measures are not a required element in the Imperial County 2009 PM10

air quality plan. Nevertheless, Imperial County is addressing contingency measures in the 

present SIP to provide additional assurance that PM10 levels will remain below the standard. 

The ICAPCD has adopted regulations that have, in part, contributed to the fact that no noninternational-impact exceedences have occurred since they were adopted. Regulation VIII 

fugitive dust control regulations (cf. Chapter 4) were amended three years in advance of the 

 

93 USEPA guidelines regarding Section 172(c)(9) of the CAA, FR Vol. 59, No. 157, August 16, 1994, p. 42014. 94 Calcagni, John, Memorandum: Procedures for Processing Requests to Redesignate Areas to Attainment; USEPA, 

Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina, September 4, 1992. 95 USEPA Clean Data Policy, see FR Vol. 71, No. 91, May 11, 2006, p. 27443, and FR. Vol. 71, No. 138,July 19, 

2006, p. 40954. 
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present SIP96 for the purpose of accelerating improvements of PM air quality throughout the 

County and to meet the requirements and schedule of the County’s Natural Event Action Plan. 

Because the Regulation VIII fugitive dust control plan provides emission reductions that exceed 

the minimum necessary to bring the area into attainment, as supported by the evidence below, 

ICAPCD rules that are beyond the minimum control requirements for serious non-attainment 

areas97 qualify as contingency measures. (These include Rules 801-804, as well as the portion 

of Rule 805 targeting paved roads and the portion of Rule 806 targeting source categories other 

than Tilling.) 

• Given that the Imperial County attained the 24-hour PM10 NAAQS in 2006-2008, any 

emission reductions derived from Regulation VIII that decrease the total Imperial County EI 

below the 2006 level of 274 tpd is in excess of the minimum necessary for attainment. 

Phased implementation of Rule 805, which is expected to result in PM10 emission 

reductions (from City/County unpaved roads) that increase at a rate of 0.87 tpd per year 

from 2006 to 2015, provides such excess reductions (as shown by the steady decrease 

with time of the total Imperial County PM10 emissions inventory, cf. Table IV.C-2 of 

Appendix IV.C.) 

• In fact, PM10 air measurements “but-for” Mexicali emissions in 2006-2008 were at least 5% 

below the standard in all cases,98 as shown in Table 6.1. Therefore, some of the 

reductions achieved by implementation of Regulation VIII rules in 2006-2008 were already 

in excess of the minimum required for attainment.99 

Table 6.1 Imperial County PM10 Measurements > 140 μg/m3

 in 2006-2008 Excluding 

Exceptional Eventsa

Date Location Measurement 

(μg/m3

) 

Estimate of US 

impact (μg/m3) 

Comments 

12/25/2006 Calexico Grant 248 < 100 Documented in Chapter 5 

12/21/2006 Calexico Grant 171 < 121 (very 

conservative) 

Documented in Chapter 5 

03/13/2007 Calexico Grant 149 < 132b FMTS = 0.98, Average wind 

speed at Grant = 3.1 knots 

10/21/2007 Calexico Ethel 142 142 No southerly flow, relatively 

high winds a

Includes all SSI measurements of 2006-2008 except measurements acquired on September 2, 2006, April 12, 

2007, and June 5, 2007. b

Most conservative result obtained from Approach I of Appendix V by subtracting the last 

row of Table V.5 from the first row of Table V.8. By comparison, the most conservative result from Approach II of 

Appendix V is 69 μg/m3

, based on same-day measurements of 64 and 48 μg/m3 at El Centro and Brawley, 

respectively. 

 

96 The rules were adopted in November 2005 and implemented in January 2006. 97 I.e., BACM-level control of significant sources, which in this case are entrained dust from unpaved roads, and 

farming emissions from tilling activities. 98 Excluding measurements acquired on September 2, 2006, April 12, 2007 and June 5, 2007, which were flagged 

and excluded from regulatory consideration on the basis that they were strongly influenced by high-wind events. 99 In this light, Imperial County Regulation VIII rules can be seen to be aggressively preemptive in addressing future 

conditions conducive to possible exceedences. 
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6.2 Transportation Conformity

The transportation conformity requirements of air quality SIPs (in section 176 of the CAA) are 

intended to ensure that transportation plans, programs, and projects are consistent with the 

purpose and intent of the air quality plan. This objective is accomplished by requiring MPOs to 

conform their plans and programs to transportation emissions budgets specified in the air quality 

plan.100 

The motor vehicle emission budgets are defined101 as the “portion of the total allowable 

emissions defined in [a SIP] for a certain date for the purpose of meeting reasonable further 

progress milestones or demonstrating attainment or maintenance of the NAAQS...[that is] 

allocated to highway and transit vehicle use and emissions.” For conformity purposes, the 

motor vehicle emissions budget for PM10 includes, in addition to vehicular exhaust, tire, and 

break wear emissions, entrained dust from travel on paved and unpaved roads, as well as 

emissions from road construction. The following minimum criteria must be satisfied before EPA 

may approve a motor vehicle emission budget as adequate for transportation conformity 

purposes (40 CFR Part 93, Section 118(e)(4)): 

• The motor vehicle emissions budget must be clearly identified and precisely quantified; 

• The motor vehicle emissions budget must be consistent with and clearly related to the 

emissions inventory and the control measures in the submitted control strategy 

implementation plan...or maintenance plan; 

• The motor vehicle emissions budget, when considered together with all other emissions 

sources, must be consistent with applicable requirements for reasonable further progress, 

attainment, or maintenance; and 

• Before the control strategy implementation plan or maintenance plan is submitted to EPA, 

consultation among federal, State, and local agencies must occur; full implementation plan 

documentation must be provided to EPA; and EPA's stated concerns, if any, must be 

addressed. 

Thus, the budget for regional transportation emissions is intended to be established in 

consultation with all concerned regional transportation organizations, and must be consistent 

with the emission levels used in the demonstration of attainment, maintenance, or RFP of the air 

quality plan. 

The Imperial County transportation conformity budget is derived based on projected PM10

emissions within the SCAG Imperial County nonattainment area. (Although this area differs 

from the Imperial County area as shown in Figure 6.1, it captures the overwhelming majority of 

transportation emissions generated within the Imperial County.) Projected PM10 transportation 

emissions are obtained (using 2009 SCAG activity data as well as activity data from the 

 

100 Federal transportation conformity regulations are found in 40 CFR Part 51, subpart T, and in 40 CFR Part 93, 

subpart A, Conformity to State or Federal Implementation Plans of Transportation Plans, Programs, and Projects 

Developed, Funded or Approved Under Title 23 U.S.C. of the Federal Transit Laws. 101 40 CFR Part 93, Subpart A, §93.101—Definitions, available at http://law.justia.com/us/cfr/title40/40-

20.0.1.1.7.1.1.2.html. 
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FINAL AUGUST 2009 6-4 ICAPCD 

ICPWD) by subtracting the Regulation VIII emissions reductions from the uncontrolled PM10

inventory for the area. 

Figure 6.1 Imperial County PM10 non-attainment area as represented in SCAG model.

The projected PM10 transportation emission inventory for the 25-year horizon reveals that 

vehicular exhaust, tire, and break wear emissions are projected to decrease steadily at a slow 

rate, while controlled emissions from road construction are projected to increase marginally. 

Much larger changes in emissions over time are expected in paved road dust emissions (which 

are projected to increase by >5 tpd between 2006 and 2035), and in unpaved road dust 

emissions (which are projected to decrease by >7 tpd between 2006 and 2015 and remain 

constant in subsequent years). 

The Imperial County motor vehicle emissions budgets (i.e., the transportation conformity 

budgets), reported in Table 6.2, were chosen here to be equal to the projected levels of 

cumulative controlled emissions from the contributing source categories. We note that the 

budget for each of the 2010, 2020, 2030, and 2035 years does not exceed the level of 

transportation emissions (taken as the 3-year average rounded up to the nearest ton) in 

attainment years 2006-2008.102 

 

102 Regulation VIII PM10 emission reductions increase by > 3 tpd between 2006 and 2010. 
Imperial County 2009 PM10 SIP Chapter 6: Other Clean Air Act Requirements 

FINAL AUGUST 2009 6-5 ICAPCD 

Table 6.2 Imperial County PM10 Motor Vehicle Emission Budgets (tpd) 

 2010 2020 2030 2035 

Vehicular Exhaust, Tire, and Break Weara 0.8 0.7 0.8 0.9 

Paved Road Dust (entrained) 3.9 6.5 7.9 8.5 

Unpaved Road Dust (entrained) 24.5 24.5 24.5 24.5 

Road Construction 0.5 0.4 1.4 1.9 

Reductions from District Rules 4.5 8.8 9.1 9.2 

Total 25.1 23.3 25.5 26.5 

Motor Vehicle Emission Budgetb (annual average) 26 24 26 27 

a

EMFAC 2007 with Imperial County activity provided by SCAG April 2009. bPM10 transportation budgets are rounded 

up to the nearest tpd. 

The transportation conformity budgets of Table 6.2, which were established in consultation with 

SCAG, the Federal Highway Administration, the ICAPCD, the USEPA, and the CARB, satisfy 

the requirements established in 40 CFR Part 93, Section 118(e)(4) (see above).103 The budgets 

apply as a ceiling on transportation emissions in Imperial County in the year for which they are 

defined and for all subsequent years until another year for which a different budget is defined (or 

until a SIP revision modifies the budget). Consistency of transportation activities and 

transportation plans with the outlined budgets is required to be demonstrated in all future years 

by means of regional analyses involving all regionally significant projects and activities, as 

outlined in 40 CFR Part 93, Section 122. 

 

103 Specifically, we find that the requirement that “the motor vehicle emissions budget, when considered together with 

all other emissions sources, [be] consistent with applicable requirements for attainment” is satisfied given that (i) 

PM10 air quality in Imperial County was in attainment (but-for Mexicali emissions) of the NAAQS standard by a 

margin of at least 5% on all days in 2006-2008, (ii) the budgets for 2010, 2020, 2030, and 2035 are smaller than 

the level of emissions from the corresponding source categories in attainment year 2006-2008, and (iii) emissions 

from non-transportation sources are expected to remain approximately constant in future years. 
THIS PAGE INTENTIONALLY LEFT BLANK 
Imperial County 2009 PM10 SIP Chapter 7: Salton Sea Considerations 

FINAL AUGUST 2009 7-1 ICAPCD 

7 Salton Sea Considerations 

The Salton Sea, which is located in the northwest corner of Imperial County, lies in the bed of 

an ancient lake that has repeatedly been desiccated and reformed by flooding within the Lower 

Colorado Basin. The current Sea was formed by a break in the bank of canal carrying water 

from the Colorado to the Imperial Valley in 1905. The Sea has been sustained since then by 

agricultural drainage waters flowing from lands under cultivation in the Coachella and Imperial 

Valleys. In 2002, a water transfer agreement was executed by the IID, the Coachella Valley 

Water District, and urban water agencies in Southern California that will transfer agricultural 

water to urban areas for domestic use. This transfer will reduce drainage flows to the Salton 

Sea after 2017, the date until which IID must guarantee existing salinity levels in the Sea. 

An increase in salinity levels in the Salton Sea threatens both fish and waterfowl habitat values. 

Under legislation enacted in 2003, the CARB was required to undertake a restoration study to 

determine a preferred alternative for the restoration of the Salton Sea ecosystem and the 

permanent protection of wildlife dependent on that ecosystem. In June 2007, a final 

Programmatic EIR that analyzed each of eight alternative restoration options was certified and a 

preferred alternative recommended to the Legislature. Under all of the alternatives studied, a 

portion of the Sea bed would be exposed. These exposed areas could become sources of 

windblown dust, depending on the granularity of the exposed soils and the behavior of salt 

crystals on the soil surface. 

The control of windblown dust from exposed Sea bed has benefitted dramatically from control 

efforts tested in a similar environment at Owens Lake, California. Owens Lake was completely 

desiccated in the 1920s by the diversion of all incoming flows to an aqueduct constructed by the 

Los Angeles Department of Water and Power. Due to the highest PM10 concentrations recorded 

in the United States from windblown dust, the Owens Lake region has been subject to federal 

CAA nonattainment planning requirements since 1991. Under the most recently approved PM10

attainment plan, almost 30 square miles of lakebed surface are being treated with gravel cover, 

shallow flooding, and managed vegetation controls. The plan also calls for controls on an 

additional 13 square miles by April 2010, and testing of a trench and berm design for catching 

saltating sand particles and reducing wind shear on downwind surfaces has begun on a pilot 

basis. 

Differences in soil and wind conditions between Owens Lake and the Salton Sea suggest that 

windblown dust will not be as much of a problem at the Salton Sea as is experienced at Owens 

Lake. Salts at Owens Lake are dominated by sodium carbonate, which tends to fracture easily 

into very fine particles, while sodium chloride, which is harder and less vulnerable to abrasion, 

constitutes the majority salt at the Salton Sea. Additionally, peak wind speeds and the number 

of hours per year with wind speeds above the windblown dust generation threshold are 

substantially higher at Owens Lake than at the Salton Sea. On the basis of these two 

conditions, worse case PM10 windblown emission rates—and resultant ambient PM10

concentrations - are expected to be lower at the Salton Sea than are recorded at Owens Lake. 

Several state statutes and water use permits provide significant authority to the ICAPCD and 

the CARB to control windblown PM10 emissions from the Salton Sea. Section 2081.7 of the 

California Fish and Game Code makes the state Department of Water Resources responsible 
Imperial County 2009 PM10 SIP Chapter 7: Salton Sea Considerations 

FINAL AUGUST 2009 7-2 ICAPCD 

for any environmental impacts related to the use or transfer of water from the Imperial Valley to 

out-of-basin users that would cause declines in Salton Sea levels or increases in salinity. The 

California State Water Resources Control Board permit that authorizes transfer of agricultural 

water to urban water districts104 requires the IID to comply with all PM10 rules of the Imperial 

County Air Pollution Control District, including Rule 804. This rule requires the owner of any 

undeveloped property to maintain stabilized soil surfaces and to prevent the emission of visible 

dust in concentrations greater than those producing 20% or more opacity reduction. 

Mitigation of windblown dust from playa exposed by drops in sea level is currently being 

designed by a multi-agency working group hosted by the Department of Water Resources. 

Members of this working group include the ICAPCD and the CARB, together with other local, 

state, and federal wildlife and environmental protection agencies. The goals of the mitigation 

process are to prevent exceedences of federal ambient air quality standards as the sea level 

declines. Mitigation plans are being developed in concert with the Salton Sea Ecosystem 

Restoration Program Environmental Impact Report.105 

The requirements to control PM10 emissions from exposed seabed surfaces incorporated into 

state law and water transfer permits will mitigate potential impacts on air quality from 

implementation of the water transfer agreement. 

 

104 Order WRO 2002-0013, In the Matter of Imperial Irrigation District’s (IID) and San Diego County Water Authority’s 

(SDCWA) Amended Joint Petition for Approval of a Long-Term Transfer of Conserved Water From IID to SDCWA 

and To Change The Point of Diversion, Place of Use, and Purpose of Use Under Permit 7643 Issued on 

Application 7482 of IID, State Water Resources Control Board, December 20, 2002 105 Salton Sea Ecosystem Restoration Program Draft and Final Programmatic Environmental Impact Reports, 

prepared for the CARB by the Department of Water Resources and the Department of Fish and Game, October 

2006 and May 2007, http://www.saltonsea.water.ca.gov/ 
Imperial County 2009 PM10 SIP Chapter 8: Conclusions and SIP Checklist 

FINAL AUGUST 2009 8-1 ICAPCD 

8 Conclusions and SIP Checklist 

A checklist of SIP requirements pertinent to the present plan (as outlined both in USEPA 

general SIP guidelines for “serious” PM10 non-attainment areas106 and in the December 11, 

2007 USEPA Final Rule107 concerning PM10 non-attainment in Imperial County) is presented in 

Table 8.1. Because the Imperial County is shown in this document to have attained the 24-hour 

PM10 NAAQS but-for international transport of Mexicali emissions in 2006-2008, RFP and 

milestone requirements are unnecessary, and specifically the 5% yearly emission reductions 

requirement does not apply to future years. As documented in Table 8.1, all remaining SIP 

requirements applicable to the 2009 Imperial County PM10 Plan have been successfully 

addressed. 

Table 8.1 SIP Checklist 

Required Elements Document Location Comments 

Emissions Inventory Chapter 3; 

Appendix III.A; 

Appendix IV.C 

CARB’s SIP inventory Version 1.06, base year 2002, 

was revised as described in Section 3.1.1 and in 

Appendix III.A. 

A plan that enables attainment of 

the PM10 federal air quality 

standards 

Chapter 4 (control 

plan); Chapter 5 

(attainment 

demonstration) 

The SIP demonstrates that Imperial County attained the 

Federal PM10 NAAQS, but-for international emissions 

from Mexico (see Chapter 5), based on 2006-2008 

monitoring data. Attainment was due, in part, to 

ICAPCD’s November 2005 adoption and subsequent 

implementation of Regulation VIII fugitive dust rules; 

those rules were based on the related 2005 BACM 

analysis. 

Annual reductions in PM10 or PM10

precursor emissions that are of no 

less than 5% until attainment 

Chapter 5, Section 

5.3; Appendix V 

Imperial County is shown in this document to have 

already attained the 24-hour PM10 NAAQS “but-for” 

transport of Mexicali emissions in 2006-2008. 

Therefore, this provision is not applicable to future 

years. 

BACM and BACT for significant 

sources and major stationary 

sources of PM10, to be 

implemented no later than 4 years 

after reclassification of the area as 

serious 

Chapter 4 Reclassification of Imperial County to serious 

nonattainment for PM10 occurred on August 2004. 

Control of fugitive PM10 emissions from the significant 

source categories identified in Section 3.2 began in 

January 2006 and meets BACM stringency, as 

established in Section 4.3. Major stationary sources 

meet the BACT requirement (Section 3.2.4). 

Transportation conformity and 

motor vehicle emission budgets in 

accord with the attainment plan 

Chapter 6, Section 

6.2 

Included. 

RFP and quantitative milestones Chapter 5, Section 

5.3 

These requirements are not applicable in the present 

SIP since Imperial County is already in attainment (butfor international emissions) based on air quality data 

from 2006 to 2008. 

Contingency measures Chapter 6, Section 

6.1 

Included. 

 

106 FR Vol. 59, No. 157, August 16, 1994, p. 42002 107 FR Vol. 72, No. 237, December 11, 2007, p. 70222 (reported in Appendix I) 
THIS PAGE INTENTIONALLY LEFT BLANK 
Imperial County 2009 PM10 SIP Appendix I: Finding of Failure to Attain 

A PPENDIX I 

FINDING OF FAILURE TO ATTAIN 

FINAL AUGUST 2009 ICAPCD 
Imperial County 2009 PM10 SIP Appendix I: Finding of Failure to Attain 

Appendix I 

Finding of Failure to Attain 

In response to the opinion of the US Court of Appeals for the Ninth Circuit in Sierra Club v. 

United States Environmental Protection Agency, et al., in August 2004 the United States 

Environmental Protection Agency (USEPA) found that the Imperial Valley PM10 nonattainment 

area had failed to attain by the moderate area attainment date of December 31, 1994, and as a 

result reclassified under the Clean Air Act the Imperial Valley from a moderate to a serious PM10

nonattainment area (see 69 FR 48792, August 11, 2004). Also in August 2004, the USEPA 

proposed a rule to find that the Imperial area had failed to attain the annual and 24-hour PM10

standards by the serious area deadline of December 31, 2001. The USEPA finalized the rule on 

December 4, 2007; this Appendix is the Federal Register publication of the USEPA’s finding of 

failure to attain and requirement for SIP submittal. 

FINAL AUGUST 2009 I-i ICAPCD 
70222 Federal Register / Vol. 72, No. 237 / Tuesday, December 11, 2007 / Rules and Regulations 

1Effective December 18, 2006, EPA revoked the 

annual PM–10 standard. 71 FR 61144 (October 17, 

2006). References to the annual standard in this 

proposed rule are for historical purposes only. EPA 

is not taking any regulatory action with regard to 

this former standard. 

contained in any of the voluntary 

national model codes acceptable upon 

review by RHS.’’ 

■ B. Revising the third sentence in the 

definition for ‘‘Replacement housing’’ to 

read ‘‘The overall condition of the unit 

or dwelling must meet Thermal 

Standards adopted by the locality/ 

jurisdiction for new or existing 

structures and applicable development 

standards for new or existing housing 

recognized by RHS in subpart A of part 

1924 or standards contained in any of 

the voluntary national model codes 

acceptable upon review by RHS.’’ 

PART 3550—DIRECT SINGLE FAMLY 

HOUSING LOANS AND GRANTS 

■ 5. The authority citation for part 3550 

continues to read as follows: 

Authority: 5 U.S.C. 301; 42 U.S.C. 1480. 

Subpart B—Section 502 Origination 

§ 3550.57 [Amended] 

■ 6. Section 3550.57(c) is amended by 

adding the word ‘‘and’’ after the word 

‘‘systems;’’ and by removing ‘‘and meet 

the thermal performance requirements 

for existing dwellings of 7 CFR part 

1924, subpart A’’. 

Subpart C—Section 504 Origination 

and Section 306C Water and Waste 

Disposal Grants 

§ 3550.106 [Amended] 

■ 7. Section 3550.106(b) is amended by 

removing the words ‘‘or thermal 

performance standards’’. 

Dated: November 28, 2007. 

Russell T. Davis, 

Administrator, Rural Housing Service. 

[FR Doc. 07–6009 Filed 12–10–07; 8:45 am] 

BILLING CODE 3410–XV–P 

DEPARTMENT OF DEFENSE 

Office of the Secretary 

32 CFR Part 68 

Provision of Free Public Education for 

Eligible Children Pursuant to Section 

6, Public Law 81–874 

AGENCY: Department of Defense. 

ACTION: Final rule. 

SUMMARY: The Department of Defense is 

removing 32 CFR Part 68, ‘‘Provision of 

Free Public Education for Eligible 

Children Pursuant to Section 6, Public 

Law 81–874.’’ The part has served the 

purpose for which it was intended and 

is no longer valid. 

DATES: Effective Date: December 11, 

2007. 

FOR FURTHER INFORMATION CONTACT: L.M. 

Bynum, 703–696–4970. 

SUPPLEMENTARY INFORMATION: DoD 

Directive 1342.16 was originally 

codified as 32 CFR Part 68. This 

Directive was canceled by DoD Directive 

1342.20. Copies of DoD Directive 

1342.20 may be obtained at http:// 

www.dtic.mil/whs/directives/. 

List of Subject in 32 CFR Part 68 

Elementary and secondary education, 

Government employees, Military 

personnel. 

■ Accordingly, by the authority of 10 

U.S.C. 301, title 32 of the Code of 

Federal Regulations is amended by 

removing part 68: 

PART 68—[REMOVED] 

Dated: December 5, 2007. 

L.M. Bynum, 

Alternate OSD Federal Register Liaison 

Officer, DoD. 

[FR Doc. 07–6006 Filed 12–10–07; 8:45 am] 

BILLING CODE 5001–06–M 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Part 81 

[EPA–R09–OAR–2005–CA–0017; FRL– 

8504–2] 

Finding of Failure To Attain; 

California—Imperial Valley 

Nonattainment Area; PM–10 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Final rule. 

SUMMARY: EPA is finding that the 

Imperial Valley serious PM–10 

nonattainment area did not attain the 

24-hour particulate matter (PM–10) 

National Ambient Air Quality Standard 

(NAAQS) by the deadline mandated in 

the Clean Air Act (CAA), December 31, 

2001. In response to this finding, the 

State of California must submit a 

revision to the California State 

Implementation Plan (SIP) that provides 

for attainment of the PM–10 standard in 

the Imperial Valley area and at least five 

percent annual reductions in PM–10 or 

PM–10 precursor emissions until 

attainment as required by CAA section 

189(d). The State must submit the SIP 

revision by December 11, 2008. 

DATES: Effective Date: This finding is 

effective on January 10, 2008. 

ADDRESSES: EPA has established docket 

number EPA–R09–OAR–2006–0583 for 

this action. The index to the docket is 

available electronically at http:// 

www.regulations.gov and in hard copy 

at U.S. Environmental Protection 

Agency Region IX, 75 Hawthorne Street, 

San Francisco, CA 94105–3901. While 

documents in the docket are listed in 

the index, some information may be 

publicly available only at the hard copy 

location (e.g., copyrighted material), and 

some may not be publicly available in 

either location (e.g., Confidential 

Business Information). To inspect the 

hard copy materials, please schedule an 

appointment during normal business 

hours with the contact listed in the FOR

FURTHER INFORMATION CONTACT section. 

FOR FURTHER INFORMATION CONTACT: 

Adrienne Priselac, EPA Region IX, (415) 

972–3285, priselac.adrienne@epa.gov. 

SUPPLEMENTARY INFORMATION: 

Throughout this document ‘‘we,’’ ‘‘us,’’ 

and ‘‘our’’ refer to EPA. 

I. Background 

On August 11, 2004, EPA reclassified 

under the Clean Air Act (CAA or the 

Act) the Imperial Valley PM–10 

nonattainment area (Imperial area) from 

moderate to serious in response to the 

opinion of the U.S. Court of Appeals for 

the Ninth Circuit in Sierra Club v. 

United States Environmental Protection 

Agency, et al., 346 F.3d 955 (9th Cir. 

2003), amended 352 F.3d 1186, cert. 

denied, 542 U.S. 919 (2004). See 69 FR 

48792 (August 11, 2004). 

Also on August 11, 2004 (69 FR 

48835), EPA proposed to find under the 

CAA that the Imperial area failed to 

attain the annual 1 and 24-hour PM–10 

standards by the serious area deadline 

of December 31, 2001. Our proposed 

finding of failure to attain was based on 

monitored air quality data for the PM– 

10 NAAQS from January 1999 through 

December 2001. A summary of these 

data was provided in the proposed rule 

and is not reproduced here. 

EPA has the responsibility, pursuant 

to sections 179(c) and 188(b)(2) of the 

Act, of determining within 6 months of 

the applicable attainment date (i.e., June 

30, 2002), whether the Imperial area 

attained the PM–10 NAAQS. Because 

the June 30, 2002 date has passed, EPA 

is required to make that determination 

as soon as practicable. Delaney v. EPA, 

898 F.2d 687 (9th Cir. 1990). 

Section 179(c)(1) of the Act provides 

that attainment determinations are to be 

based upon an area’s ‘‘air quality as of 


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Federal Register / Vol. 72, No. 237 / Tuesday, December 11, 2007 / Rules and Regulations 70223 

2Pursuant to appendix K, attainment of the 24- 

hour PM–10 NAAQS is achieved when the 

expected number of exceedances of the 24-hour 

NAAQS (150 mg/m3) per year at each monitoring 

site is less than or equal to one. A total of three 

consecutive years of clean air quality data is 

generally necessary to show attainment of the 24- 

hour standard for PM–10. A complete year of air 

quality data, as referred to in 40 CFR part 50, 

appendix K, is comprised of all four calendar 

quarters with each quarter containing data from at 

least 75 percent of the scheduled sampling days. 

3Section 188(e) provides for a one-time extension 

of the attainment deadline for serious PM–10 

nonattainment areas if certain conditions are met. 

However such an extension cannot extend beyond 

December 31, 2006. Because that date has now 

passed, a section 188(e) extension for the Imperial 

area is unavailable under any circumstances. 

Nevertheless we address in this final rule the 

comments we received relating to section 188(e) 

insofar as doing so enables us to fully respond to 

those comments. For example, here a discussion of 

section 188(e) is relevant to the District’s claim, 

among others, that EPA’s action subjects the area to 

more stringent requirements than otherwise would 

have been imposed. 

4See Washington, DC, 68 FR 3410, 3413 (January 

24, 2003). See also Santa Barbara, California, 62 FR 

65025 (December 10, 1997); Phoenix, Arizona, 62 

FR 60001 (November 6, 1997); and Dallas-Fort 

Worth, Texas, 63 FR 8128 (February 18, 1998). 

5The District also cites Georgetown University 

Hospital v. Bowen in which a federal agency 

reissued a procedurally defective rule and gave it 

retroactive effect. Both the D.C. Circuit and the U.S. 

Supreme Court invalidated the action, finding, 

among other things, that under the APA legislative 

rules must be given future effect only. 821 F.2d 759 

(D.C. Cir. 1987); 488 U.S. 204 (1988). 

the attainment date,’’ and section 

188(b)(2), which is specific to PM–10, is 

consistent with that requirement. EPA 

determines whether an area’s air quality 

is meeting the PM–10 NAAQS based 

upon air quality data gathered at 

monitoring sites in the nonattainment 

area and entered into EPA’s Air Quality 

System (AQS) database. These data are 

reviewed to determine the area’s air 

quality status in accordance with EPA 

regulations at 40 CFR part 50, appendix 

K.2 For details about EPA’s proposed 

failure to attain finding, please see the 

proposed rule. 

II. EPA’s Responses to Comments on the 

Proposed Rule 

EPA received eight comment letters 

on the proposed finding. Summaries of 

the comments and EPA’s responses are 

set forth below. 

1. Retroactive Finding of Failure To 

Attain Is Unlawful 

The Imperial County Air Pollution 

Control District (District or ICAPCD) 

claimed that EPA’s proposed finding 

that the Imperial area failed to attain the 

serious area deadline of December 31, 

2001, issued the same day as the 

reclassification of the area from 

moderate to serious, constitutes an 

unlawful and unjust retroactive 

rulemaking in that the area would be at 

once reclassified and punished for 

failing to meet the requirements of the 

new classification. The District strongly 

urged EPA to refrain from finalizing any 

rule that makes a nonattainment finding 

under these circumstances. 

In support of its position that this 

type of rulemaking is illegal under the 

Administrative Procedure Act (APA), 

the District cited a number of federal 

court decisions and EPA rulemakings. 

The District believes that these 

decisions and rulemakings support its 

position that the nonattainment finding 

could create liabilities and penalties for 

missing long past deadlines associated 

with serious nonattainment areas and/or 

impose more rigorous requirements than 

would otherwise be justified, e.g., the 

requirement under CAA section 189(d) 

to submit a revised plan in 12 months 

rather than the 18 months allowed 

under section 189(b)(2) when a 

moderate area fails to meet its 

attainment deadline. 

Response: At bottom, the argument 

that the District makes is that if the 

Imperial area had been reclassified as 

the CAA envisioned, the area would not 

now be subject to the requirements of 

section 189(d). In other words, EPA 

would have found that the area failed to 

attain the moderate area deadline of 

December 31, 1994 well before the 

serious area deadline of December 31, 

2001. Consequently, the serious area 

plan for the Imperial area would have 

been due 18 months from the 

reclassification pursuant to section 

189(b)(2) instead of being subject to the 

12-month deadline in section 189(d). 

Furthermore, the argument goes, if the 

State had been able to demonstrate that 

attainment by 2001 was impracticable 

the area would have been able to avail 

itself of the attainment date extension 

provisions of section 188(e),3 thereby 

potentially avoiding both the 

substantive and procedural 

requirements of section 189(d) entirely. 

Instead, the District argues, EPA’s action 

has illegally circumvented the statutory 

scheme by precluding the area from 

taking advantage of allegedly more 

lenient submittal and substantive 

requirements. 

The cases and EPA actions cited by 

the District, however, do not support its 

position. With respect to the Imperial 

PM–10 nonattainment area, EPA 

reclassified it from moderate to serious 

and immediately proposed to find that 

the area had failed to attain the serious 

area deadline. The result of these 

actions is that the State will be required 

to submit in the future a plan for the 

area under CAA section 189(d). In 

contrast, in Sierra Club v. EPA, 356 F.3d 

296 (D.C. Cir. 2004), EPA set a 

prospective submittal date pursuant to 

CAA section 182(i) upon reclassification 

of the Washington, D.C. ozone 

nonattainment area from serious to 

severe because the severe area plan 

submittal deadline in the CAA had 

already passed. Similarly, in several 

other ozone reclassification actions, 

EPA also determined that where a 

submittal date had passed and was 

therefore impossible to meet, the 

Agency could administratively establish 

a later date. EPA’s reasoning in these 

cases was that to do otherwise would 

have subjected these areas to an 

immediate finding of failure to submit 

and the immediate initiation of 

sanctions clocks.4

In the case of Washington, DC, EPA 

stated in its final rule that ‘‘the 

Administrative Procedure Act * * * 

requires that before a rule takes effect, 

persons affected will have advance 

notification of its requirements. A 

failure to meet an obligation, especially 

one accompanied by sanctions, cannot 

occur in advance of the imposition of 

that obligation.’’ 68 FR at 3414. The 

Court of Appeals agreed, quoting EPA, 

‘‘that adopting petitioner’s suggestion 

[that EPA retain the original submittal 

deadlines] ‘would give the 

reclassification retroactive effect by 

holding the States in default of their 

submission obligations before the events 

necessary to trigger that obligation 

(reclassification) * * * occurred.’ ’’ 356 

F.3d at 309. 

In Sierra Club v. Whitman, 130 

F.Supp. 2d. 78 (D.D.C. 2001), cited by 

the D.C. Circuit in Sierra Club v. EPA 

above and the District in its comment 

letter, and affirmed in Sierra Club v. 

Whitman, 285 F.3d 63, 68 (D.C. Cir 

2002), the plaintiffs sought to compel 

EPA to backdate a nonattainment 

determination to the date on which the 

Agency was statutorily required to make 

such a determination. In affirming the 

District Court’s denial of the relief 

sought, the D.C. Circuit opined that: 

Although EPA failed to make the 

nonattainment determination within the 

statutory time frame, Sierra Club’s proposed 

solution only makes the matter worse. 

Retroactive relief would likely impose large 

costs on the States, which would face fines 

and suits for not implementing air pollution 

prevention plans in 1997, even though they 

were not on notice at the time. 

Id. at 68.5

In the instant case, however, by giving 

the State the benefit of a future plan 

submittal deadline for the Imperial area, 

EPA’s action is consistent with the 

holdings of the cases and with the EPA 

regulatory actions cited by the District. 


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70224 Federal Register / Vol. 72, No. 237 / Tuesday, December 11, 2007 / Rules and Regulations 

6Our rationale for this plan submittal deadline is 

discussed in the proposed rule. See at 69 FR at 

48837. 

7 59 FR 41998 (August 16, 1994) (‘‘State 

Implementation Plans for Serious PM–10 

Nonattainment Areas, and Attainment Date Waivers 

for PM–10 Nonattainment Areas Generally; 

Addendum to the General Preamble for the 

Implementation of Title I of the Clean Air Act 

Amendments of 1990’’ (Addendum)). 

8 Development of a Wind Blown Fugitive Dust 

Model and Inventory for Imperial County, 

California, ENVIRON International Corporation and 

Eastern Research Group, 2004 (Wind Blown Dust 

Study). 

9Technical Memorandum: Latest Revisions of the 

Windblown Dust Study, ENVIRON International 

Corporation, September 20, 2005 (Revised Study), 

attached as Appendix A to Draft Final Technical 

Memorandum, Regulation VIII BACM Analysis, 

ENVIRON, October 2005 (Regulation VIII BACM 

Analysis). 

10With respect to the section 188(f) waiver of 

serious area requirements, EPA cautions that while 

the District in its comment appears to characterize 

the predominant issue in the Imperial area to be 

nonanthropogenic sources, the District has 

identified anthropogenic PM–10 source categories 

that contribute significantly to peak 24-hour average 

PM–10 values in the area. See Regulation VIII 

BACM Analysis. 

Under section 189(d), the State must 

submit a plan revision for the Imperial 

area ‘‘within 12 months after the 

applicable attainment date. * * *’’ That 

date was December 31, 2002. However, 

because, at the time of EPA’s proposed 

finding of failure to attain, that date had 

already passed, EPA proposed that the 

section 189(d) plan revision be due 

‘‘within one year of publication of a 

final finding of nonattainment pursuant 

to CAA section 179(d).’’ 69 FR at 48837. 

Thus, rather than invoking the long past 

submittal deadline in section 189(d), 

EPA looked to another provision of the 

Act to supply a prospective deadline. In 

doing so, EPA alleviated the problem of 

imposing a retroactive deadline without 

imposing immediate sanctions. 

While it is true, as the District points 

out, that a serious PM–10 area 

proceeding initially under section 

189(b) instead of section 189(d) would 

in theory have had more time to submit 

a plan (18 rather than 12 months), in 

both instances the submittal deadlines 

are prospective and not retroactive. 

Furthermore, as we point out in our 

response to comment #3 below, the 

section 189(d) plan that the State is now 

required to submit is actually due later 

than the serious area plan would have 

been due under the scenario preferred 

by the District. Therefore, the retroactive 

penalty the District complains of with 

respect to the plan submittal deadline 

simply does not exist. 

Moreover, while it is also true that, as 

a result of EPA’s nonattainment finding, 

the Imperial area must comply with the 

substantive requirements of CAA 

section 189(d) instead of those of 

section 188(e), this consequence cannot 

be construed as ‘‘punishment.’’ Under 

both sections 189(d) and 188(e), 

implementation of best available control 

measures (BACM) under section 

189(b)(1) and attainment of the PM–10 

standards as expeditiously as 

practicable are required. In addition, 

while the respective substantive 

requirements of sections 188(e) and 

189(d) are different, neither are 

necessarily more onerous than the other. 

See Corrected Brief of Respondent EPA, 

pages 40–42, in Association of Irritated 

Residents, et al. v. EPA, 423 F.3d 989 

(9th Cir. 2005). Only if the State fails to 

submit the new plan in the future could 

sanctions come into play. Thus the 

substantive consequences here of EPA’s 

nonattainment finding are not in fact 

retroactive, nor do they impose a 

penalty. 

For the reasons discussed in its 

proposed finding, EPA is legally 

compelled to finalize the nonattainment 

finding with the result that section 

189(d) applies to the Imperial area. The 

section 189(d) plan is due within one 

year of publication of this final finding 

of nonattainment.6

2. Waive the Attainment Date and 

Related Requirements 

Several commenters suggested that 

instead of finding that the Imperial area 

failed to attain the serious area 

attainment date, EPA should waive that 

date and the related submittal 

requirements and penalties to reduce 

the burden of the Agency’s action on 

Imperial County. While two 

commenters who suggested this 

approach did not describe EPA’s legal 

authority to grant a waiver, one 

commenter, the District, cited CAA 

section 188(f) as providing EPA with the 

authority to waive a specific attainment 

date where the Agency determines that 

nonanthropogenic sources contribute 

significantly to violations in the area 

and to waive any requirement 

applicable to any serious PM–10 area 

where anthropogenic sources do not 

contribute significantly to violations. 

The District stated that in the Imperial 

area, dry soil from vast barren lands are 

entrained by high winds producing an 

impact on the monitors. The District 

asserted that EPA has determined that 

this type of dust raised by high wind 

events constitutes a nonanthropogenic 

source of PM–10 pursuant to section 

188(f) and, citing a May 30, 1996 EPA 

memorandum, that monitoring data 

impacted by such events may be 

excluded from consideration in 

attainment decisions. 

Response: Congress recognized in the 

Clean Air Act that there may be areas 

where the NAAQS may never be 

attained because of PM–10 emissions 

from nonanthropogenic sources, and 

that the imposition in such areas of 

certain state planning requirements may 

not be justified. Therefore, under 

section 188(f), Congress provided a 

means for EPA to waive a specific date 

for attainment and certain control and 

planning requirements when specified 

conditions are met in a nonattainment 

area. Section 188(f) provides two types 

of waivers. First, EPA may, on a caseby-case basis, waive any PM–10 

nonattainment planning requirement 

applicable to any serious nonattainment 

area where EPA determines that 

anthropogenic sources of PM–10 do not 

contribute significantly to violation of 

the standards in the area. Second, EPA 

may waive a specific date for attainment 

of the standards where EPA determines 

that nonanthropogenic sources of PM– 

10 contribute significantly to the 

violation of the standards in the area.7

In the Addendum, EPA set forth 

threshold levels for determining 

whether areas qualify for waivers under 

section 188(f). Addendum at 42004– 

42005. 

In its comment letter, the District 

included and discussed a report 8 that it 

characterized as showing that 

windblown dust from barren lands 

represents over 92% or 792 tons per day 

(tpd) of the total PM–10 inventory in 

Imperial County. The District 

maintained that ‘‘high winds frequently 

entrain large amounts of this dry soil 

into the ambient air, producing a 

documented impact on County 

monitors.’’ As a result of comments 

provided to the District by EPA and the 

California Air Resources Board (CARB), 

the Windblown Dust Study was revised 

in 2005.9 The Revised Study concluded, 

among other things, that there are 157 

tpd of fugitive dust emissions from 

barren lands. Revised Study at A–15. 

The Windblown Dust Study and the 

Revised Study are primarily inventories 

of windblown dust emissions in 

Imperial County. These documents do 

not address the requirements of section 

188(f) and EPA’s guidance on that 

provision. Therefore they do not 

provide sufficient analysis and 

documentation to support a waiver of 

either the December 31, 2001 attainment 

deadline or any of the serious area 

requirements. However, the section 

188(f) waivers, if the conditions for 

them can be met, are available to the 

State in the context of the section 189(d) 

serious area plan.10

The May 30, 1996 memorandum cited 

by the District is entitled ‘‘Areas 

Affected by PM–10 Natural Events’’ and 


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Federal Register / Vol. 72, No. 237 / Tuesday, December 11, 2007 / Rules and Regulations 70225 

11On March 22, 2007, EPA issued a final rule, 

intended to replace the NEP, governing the review 

and handling of air quality data influenced by 

exceptional events. 72 FR 13560. The rule became 

effective on May 21, 2007 and is codified at 40 CFR 

50.1, 50.14 and 51.920. 72 FR 13560, 13580–13581. 

However, as discussed below, the 1999–2001 data 

relevant to this final action are not eligible for 

exclusion under the transition policy for the rule 

because the State did not meet the provisions of the 

NEP that were applicable at the time of the 

exceedances. See 72 FR 49046, 49048 (August 27, 

2007). 

12The AIRS database is the predecessor to the 

AQS database. 

13Note that even if adequate documentation had 

been submitted for the flagged events, the Imperial 

area would not have attained the PM–10 standard 

because of the number of unflagged exceedances. 

See ‘‘Imperial valley PM10 Exceedances 1999– 

2001,’’ Excel Spreadsheet, Bob Pallarino, EPA. 

is from Mary Nichols, Assistant 

Administrator for Air and Radiation to 

EPA Regional Division Directors 

(Natural Events Policy or NEP). This 

policy provides, among other things, 

that EPA believes it is appropriate to 

exclude air quality data attributable to 

uncontrollable natural events from the 

Agency’s decisions regarding an area’s 

attainment status. NEP at p. 2.11 In the 

case of high winds, under the NEP EPA 

considers ambient PM–10 

concentrations due to dust raised by 

unusually high winds as due to 

uncontrollable natural events (and thus 

excludable from attainment 

determinations) if either (1) the dust 

originated from nonanthropogenic 

sources or (2) the dust originated from 

anthropogenic sources controlled with 

BACM. NEP at pp. 4–5. 

The NEP sets forth a process for 

declaring an exceedance as due to 

natural events and for documenting a 

natural events claim. NEP at pp. 7–10. 

Where a state believes that natural 

events caused the NAAQS exceedances 

it must establish through supporting 

documentation a clear causal 

relationship between the exceedance 

and the natural event. The amount and 

type of documentation must be 

sufficient to demonstrate that the 

natural event occurred and that it 

impacted a particular monitoring site in 

such a way as to cause the PM–10 

concentrations measured. The 

documentation also should provide 

evidence that, absent the natural event 

emissions, concentrations at the 

monitoring site would not cause an 

exceedance. 

Under the NEP, when air quality data 

affected by a natural event are submitted 

to EPA for inclusion into the AIRS 

database,12 the state is to request that a 

flag be placed on the data to indicate 

that a natural event was involved. NEP 

at 8–9. A number of exceedances in 

1999–2001 in the Imperial area were 

flagged as high wind and other natural 

events. Under the NEP, the 

documentation supporting a natural 

events flag was required to be submitted 

no later than 180 days from the time the 

exceedance occurred. However no 

documentation with respect to the 

1999–2001 exceedances was submitted 

to EPA.13 Because the State did not 

comply with the provisions of the NEP, 

the flagged 1999–2001 data cannot be 

excluded as affected by natural events 

from EPA’s determination of whether 

the Imperial area attained the PM–10 

standard by December 31, 2001. 

3. EPA Should Grant a 5-Year Extension 

To Allow More Time To Develop Plan 

Several commenters opposing our 

proposed action stated that our 

proposed time frame for the 

development and submittal of a serious 

area PM–10 plan, including a CAA 

section 189(d) plan, was too short, and 

that EPA should grant a 5-year 

extension of the attainment date for the 

Imperial area to provide time for 

preparation, submittal and 

consideration of an attainment 

demonstration. Of the commenters 

making this request, only the District 

cited any legal authority for a 5-year 

extension: ‘‘* * * The District requests 

that EPA withdraw its proposed 12- 

month deadline for the County’s serious 

area SIP submittal * * * and instead 

grant a five-year extension under 

Section 188(e) to allow sufficient time 

for preparation, submittal and 

consideration of the County’s final PM– 

10 attainment demonstration.’’ The 

District characterized the 12-month plan 

submittal schedule as ‘‘abbreviated’’ and 

as a ‘‘penalty.’’ One of the commenters 

suggesting the 5-year extension 

approach urged EPA to utilize our 

discretion under the CAA to extend the 

time allowed to prepare a plan so that 

unwarranted imposition of additional 

measures could be avoided. 

Another commenter stated that 

although a preferable outcome would 

have been an extension of the 

attainment date, it was clear that no 

attainment date extension was in place, 

and thus, the finding of failure to attain 

by EPA was mandatory under the Clean 

Air Act with the one-year deadline for 

an attainment demonstration. 

Response: CAA section 188(e) 

provides that, upon application by a 

state, EPA may extend the attainment 

deadline for a serious PM–10 

nonattainment area no more than 5 

years beyond, in this case, December 31, 

2001, if: (a) Attainment by that date 

would be impracticable; (b) the state has 

complied with all requirements and 

commitments in the implementation 

plan for the area; and (c) the state 

demonstrates that the plan contains the 

most stringent measures (MSM) in the 

plan of any state or are achieved in 

practice in any state, and can feasibly be 

implemented in the area. The state must 

submit at the time of its extension 

application a demonstration of 

attainment by the most expeditious 

alternative date practicable. 

As stated above, the Imperial area is 

no longer eligible for an attainment date 

extension under section 188(e) because 

that extension cannot extend beyond 

2006. Regardless, the attainment date 

extension provided for in section 188(e) 

does not relate in any way to the 

submittal date for a serious area plan. 

Rather, under the Act, submittal dates 

for serious area PM–10 plans are 

initially governed by subpart 4 of part 

D of the CAA, i.e, either by section 

189(b)(2) or 189(d). As explained in the 

proposed rule, EPA believes that section 

189(d) applies to the Imperial area’s 

situation. 69 FR at 48837. In the first 

instance, EPA looked to this provision, 

which applies exclusively to PM–10 

nonattainment areas, for the applicable 

submittal date for the Imperial area’s 

section 189(d) plan. Because the 

deadline for plan submittal under that 

section, December 31, 2002 has passed, 

EPA looked to subpart 1 of part D of the 

CAA in order to determine 

Congressional intent. Section 179(d) 

requires submittal of a plan revision 

within one year after EPA publishes a 

notice of a finding of failure to attain. 

In case of the Imperial area, the 

application of the deadline provided for 

in section 179(d) has already resulted in 

a significantly longer time for submittal 

of the serious area plan than the 

deadline that would otherwise have 

applied. If the Imperial area had been 

reclassified to serious prior to the end 

of 2001, it would have been subject to 

section 189(b)(2). As such, the deadline 

for submittal of a serious area plan 

would be 18 months from the date of the 

reclassification. The effective date of the 

reclassification here was September 10, 

2004; therefore, the alternative to the 

due date provided in section 179(d) 

would result in the plan having been 

due by March 10, 2006. Instead, the 

area’s serious area plan is not due until 

one year from publication of the Federal 

Register notice of this action. EPA 

knows of no legal theory that would 

allow the Agency to provide the 5 years 

apparently sought by the commenters 


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70226 Federal Register / Vol. 72, No. 237 / Tuesday, December 11, 2007 / Rules and Regulations 

14We note that subpart 4 of part D of title I which 

contains the Act’s provisions specific to PM–10 

does not have a provision that is analogous to 

section 182(i) which grants EPA considerable 

latitude to adjust submittal and other schedules 

upon an ozone area’s reclassification. See also 

section 187(f). 

for the development and submittal of a 

serious area PM–10 plan.14

4. Economic Hardship 

A number of commenters claimed that 

an EPA finding of failure to attain 

would result in adverse economic 

consequences for Imperial County. One 

commenter stated that the County has 

one of the poorest economies in the 

State, that EPA’s finding will place an 

undue hardship on an economy that is 

already on the brink of breaking, and 

that the Agency should take economic 

justice into account. Another 

commenter suggested that another set of 

government-imposed regulations would 

place an unnecessary financial hardship 

on area companies and could possibly 

disrupt farming operations. Another 

commenter cited the County’s high 

unemployment rate that would increase 

under severe emission control 

requirements that undermine an 

agriculture-dependent economy. The 

commenters attributed these perceived 

hardships to various factors they believe 

to be related to a nonattainment finding: 

the five percent and BACM 

requirements applicable to serious PM– 

10 attainment areas; the inability of the 

County to control Mexican emissions; 

and the prevalence of high wind natural 

events. We address each of these factors 

below. 

A. Five Percent and BACM 

Requirements 

A number of commenters opposed to 

our proposed rule requested that EPA 

reduce or remove entirely the proposed 

requirement that Imperial County 

submit a plan that achieves at least 5 

percent annual reductions in PM–10 or 

PM–10 precursor emissions as required 

by CAA section 189(d). Some 

commenters stated that this requirement 

was not feasible or was too burdensome 

for Imperial County. Another 

commenter attributed severe economic 

consequences to the serious area plan 

requirements for expeditious 

implementation of BACM. 

Response: As stated above and in the 

proposed rule, EPA is legally compelled 

to finalize the nonattainment finding 

with the result that the 5 percent 

requirement of section 189(d) applies. 

Under section 189(b)(1)(B), the serious 

area PM–10 plan for the Imperial area is 

required to provide for the expeditious 

implementation of BACM. This 

requirement applies as a result of the 

Imperial area’s reclassification to 

serious which was mandated by the U.S. 

Court of Appeals for the Ninth Circuit 

in Sierra Club v. U.S. Environmental 

Protection Agency, et al., 346 F.3d 955 

(9th Cir. 2003), amended 352 F.3d 1186, 

cert. denied, 542 U.S. 919 (2004). 

Therefore BACM would have to be 

implemented in the Imperial area even 

in the absence of EPA’s finding that the 

area failed to attain the PM–10 

standards by the end of 2001. 

EPA has defined BACM as: ‘‘* * * 

The maximum degree of emissions 

reduction of PM–10 and PM–10 

precursors from a source * * * which is 

determined on a case-by-case basis, 

taking into account energy, 

environmental, and economic impacts 

and other costs, to be achievable for 

such source through application of 

production processes and available 

methods, systems, and techniques for 

control of each such pollutant.’’ 

Addendum at 42010. Therefore, while 

EPA cannot take into account the 

general economy of a nonattainment 

area in determining what statutory 

requirements apply in a serious 

nonattainment area, it can consider the 

cost of reducing emissions from a 

particular source category and costs 

incurred by similar sources that have 

implemented emission reductions. In 

addition, where the economic feasibility 

of a measure depends on public 

funding, an appropriate consideration is 

past funding of similar activities as well 

as availability of funding sources. Id. at 

42013. Nevertheless, the CAA still 

requires that the State submit a plan for 

the Imperial area to, among other things, 

attain the PM–10 NAAQS as 

expeditiously as practicable. Moreover, 

there are economic benefits to attaining 

the NAAQS. 

B. Mexican Emissions 

Several commenters felt that the 

economic hardship was a result of the 

failure of EPA, in its proposed action, to 

consider the fact that significant 

amounts of particulate matter air 

pollution in Imperial County emanate 

from the large and growing city of 

Mexicali, Mexico. Many commenters 

opposing our proposed rule stated that 

EPA ignored the fact that emissions 

from Mexico are one of the reasons that 

poor air quality exists in Imperial 

County. Some commenters pointed out 

that in the past, EPA has agreed that 

Imperial County would have attained 

the PM–10 NAAQS but for emissions 

from Mexico (e.g., EPA’s approval of 

CAA section 179B demonstration; 66 FR 

53106, October 2001). Additionally, the 

commenters claimed that the PM–10 

plan needs to include consideration of 

how emissions from Mexico impact the 

attainment of the PM–10 NAAQS in 

Imperial County. 

Response: As explained in our 

proposed rule, EPA has the 

responsibility, pursuant to CAA sections 

179(c) and 188(b)(2), to determine 

within 6 months of the applicable 

attainment date whether a PM–10 

nonattainment area attained the 24-hour 

NAAQS. Section 179(c)(1) of the Act 

provides that determinations of failure 

to attain are to be based upon an area’s 

‘‘air quality as of the attainment date,’’ 

and section 188(b)(2) is consistent with 

this requirement. EPA determines 

whether an area’s air quality is meeting 

the PM–10 NAAQS based upon air 

quality data gathered at monitoring sites 

in the nonattainment area and entered 

into EPA’s AQS database. These data are 

reviewed to determine the area’s air 

quality status in accordance with EPA 

regulations at 40 CFR part 50, appendix 

K. 69 FR at 48836. Thus, neither the 

CAA nor EPA regulations authorize the 

Agency to consider the economic 

circumstances of an area in making a 

finding of attainment or nonattainment; 

the determination is to be made solely 

on the basis of the ambient air quality 

in the area. Similarly, neither the CAA 

nor EPA regulations allow EPA to ignore 

the actual attainment status of an area 

based on the influx of a pollutant from 

another country. The attainment status 

is intended to reflect the actual ambient 

pollutant levels. 

Section 179B(d) of the Act does allow 

a moderate PM–10 nonattainment area 

to avoid a reclassification to serious if 

a state establishes to the satisfaction of 

EPA that such an area would have 

attained but for emissions emanating 

from outside the United States. EPA did 

approve such a demonstration for the 

Imperial area but that approval was 

overturned by the Ninth Circuit in 

Sierra Club. See the discussion of this 

case and its aftermath, 69 FR at 48835. 

The State can, however, take the effect 

of Mexican emissions into account in 

addressing the CAA section 189(d) 

attainment demonstration requirement. 

See CAA section 179B(a) and the 

Addendum at 42000–42002. In this 

regard, note that section 179B does not 

provide authority to exclude monitoring 

data influenced by international 

transport from regulatory 

determinations related to attainment 

and nonattainment. Thus, even if EPA 

approves a section 179B ‘‘but for’’ 

demonstration for an area, the area 

would continue to be designated as 

nonattainment and subject to the 

applicable requirements, including 

nonattainment new source review, 


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15See footnote 11. 

16Under EPA’s NEP, if natural events caused 

ambient concentrations of PM–10 that exceeded the 

NAAQS in an area, the State was responsible for 

developing a NEAP meeting certain specified 

requirements to address future events. NEP at 5–8. 

Under EPA’s exceptional events rule NEAPs are not 

required, although similar requirements apply 

under 40 CFR 51.920. 72 FR at 13581. 

nonattainment conformity, and other 

measures prescribed for nonattainment 

areas by the CAA. 

C. High Wind Events 

Several commenters felt that the 

economic hardship was a result of the 

failure of EPA’s proposal to consider the 

fact that significant amounts of 

particulate matter air pollution in 

Imperial County are the result of high 

wind natural events. To support their 

claims, commenters cited the Wind 

Blown Dust Study. 

Response: As discussed in our 

response to comment #2, EPA will 

under certain circumstances exclude 

from attainment determinations ambient 

PM–10 concentrations due to dust 

raised by unusually high winds. 

However, the State did not provide 

documentation to support the flagged 

high wind events from 1999–2001 and 

the data are therefore not eligible for 

exclusion here.15 Moreover, as noted 

previously, even if the State had met the 

provisions of EPA’s NEP that were 

applicable at the time of the relevant 

exceedances, the Imperial area would 

not have attained the PM–10 standard 

by December 31, 2001. The State can, 

however, if it meets the requirements of 

EPA’s exceptional events rule, take 

future unusually high winds into 

account in developing its CAA section 

189(d) attainment demonstration. See 72 

FR at 13565–13566 and 13576–13577. 

5. Governmental Entities Should Work 

Together 

One commenter urged EPA to 

immediately initiate a coordinated effort 

involving the federal government, 

Mexican government counterparts and 

County officials to develop a federally 

funded international plan to reduce 

emissions. Another commenter 

requested that, given the short time 

provided in the CAA to develop and 

submit a plan in this case, and the need 

for the plan to consider international 

transport, and perhaps, 

nonanthropogenic sources, EPA be 

involved early in the plan development 

to ensure a timely plan submittal. One 

commenter also stated that EPA needs 

to work with other governmental 

agencies to implement reasonable 

policies for controlling PM–10 pollution 

in the Imperial area. 

Response: EPA agrees with the 

commenters who encourage 

governmental entities to work together 

to address air pollution from Mexicali to 

Imperial County. Reducing air pollution 

anywhere along the U.S./Mexico border 

requires binational cooperation and 

coordination. Since 1983, EPA has been 

working with the Mexican Government 

and other stakeholders to reduce air 

pollution along the border region. 

Pursuant to the 1983 La Paz Agreement, 

the U.S. and Mexico developed the 

Border XXI Program and more recently 

its successor, the Border 2012 U.S.- 

Mexico Environmental Program. 

Through these programs, EPA and 

Mexico have worked together with 

border tribal, state, and local 

governments, as well as academia and 

the general public, to improve our 

understanding of the relative impacts of 

contributing international sources of air 

pollution and have developed and 

implemented cost-effective control 

strategies to reduce those emissions. 

EPA continues to implement the 

Border 2012 regionally-based border 

program in the Mexicali-Imperial area. 

We are active participants in the 

Imperial/Mexicali Air Quality Task 

Force which provides a forum for the 

federal, state, and local governments to 

discuss and analyze with community 

stakeholders how to improve air quality 

in the binational region. EPA continues 

to fund numerous projects that study 

and manage air pollution in various 

crossborder airsheds like the Imperial/ 

Mexicali area. In addition to supporting 

the District’s work to develop its PM–10 

plan, EPA also provides direct funding 

for the Mexicali-Imperial Air Quality 

Task Force for binational public forums 

to discuss the air quality of the 

Mexicali-Imperial region, and to carry 

out projects, including projects to 

monitor air quality (especially in 

Mexico), to demonstrate retrofit 

equipment technologies for diesel 

trucks, and to provide real time air 

quality information to residents of 

Imperial County. 

Regarding the comment that EPA be 

involved early in the development of 

the air quality plan, we intend to 

provide guidance and assistance to the 

District and the State to support a 

technically sound and timely submittal. 

Lastly, regarding the need to develop 

reasonable policies, EPA has worked 

closely with the State and District to 

improve the PM–10 emissions inventory 

for the Imperial area, to develop a 

natural events action plan (NEAP),16

and to develop rules to control certain 

sources of fugitive dust in the 

nonattainment area. 

6. Finding of Failure To Attain Is 

Mandatory Under the CAA and Fully 

Supported by Ambient Monitoring Data 

One commenter stated that the 

proposal correctly reflects that the 

Imperial Valley is a serious PM–10 

nonattainment area that has missed its 

attainment date and does not have an 

extension of the attainment date in 

place. The same commenter stated 

further that EPA correctly assessed that 

areas in situations like this have oneyear to submit a plan including a 5 

percent plan. Another commenter who 

agreed with EPA’s proposed rule stated 

that EPA’s proposal had omitted some 

statutory requirements (e.g., BACM 

implemented expeditiously, major 

source cutoffs), and reserved the right to 

comment further on EPA’s proposed 

action on the PM–10 SIP. 

Response: EPA agrees with comments 

supporting the proposal. We did not 

include a comprehensive list of the CAA 

requirements applicable to the Imperial 

area, but expect the plan to address all 

of them. See Section III below. 

7. PM–10 Is Not a Regulated Pollutant 

One commenter, California 

Cattlemen’s Association (CCA), notes 

that the U.S. Court of Appeals for the 

District of Columbia Circuit in 

American Trucking Ass’n v. Browner 

vacated EPA’s 1997 PM–10 standard 

because it included both coarse and fine 

PM and therefore was ‘‘inherently 

confounded.’’ CCA claims that the 1987 

standard suffers from the same defect. 

Therefore, CCA argues, there is no 1987 

standard and, as a result, the Imperial 

area cannot be out of compliance with 

it. CCA states that if EPA’s response is 

that the 1987 standard was re-instituted 

in a final rule (65 FR 80776; December 

22, 2000), there was not sufficient notice 

as that rule was noticed within a ruling 

for Ada County, Idaho (65 FR 39321; 

June 26, 2000). Also, CCA believes that 

because the same problem exists with 

the 1987 standard as the 1997 standard, 

simply reinstating the old standard was 

not the court’s intention. Finally, CCA 

discusses EPA’s then current process of 

revising the PM NAAQS and finds, 

among other things, similar 

confounding problems in measurements 

contained in studies that EPA is using 

to consider setting its new NAAQS. 

Response: In a portion of American 

Trucking Ass’ns v. EPA, 175 F. 3d 1027, 

not later reversed by the Supreme Court, 

the D.C. Circuit held that, although 

there was ‘‘ample support’’ for EPA’s 

decision to regulate coarse-fraction 

particles, EPA had not provided a 


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70228 Federal Register / Vol. 72, No. 237 / Tuesday, December 11, 2007 / Rules and Regulations 

17For a brief discussion of these requirements, 

see our proposed approval of the San Joaquin 

Valley PM–10 plan at 69 FR 5413, 5414 (February 

4, 2004). See also the final rule at 69 FR 30006 (May 

26, 2004). 

reasonable justification for its choice of 

PM–10 as an indicator for coarse 

particles, especially given that PM–10 

includes not only coarse particles but 

PM fine as well. 175 F. 3d at 1054–55. 

Pursuant to the D.C. Circuit’s 

decision, EPA deleted 40 CFR 50.6(d), 

the regulatory provision controlling the 

transition from the pre-existing 1987 

PM–10 standards to the 1997 PM–10 

standards. 65 FR 80776. EPA proposed 

this deletion in the context of a 

proposed rule to rescind a finding, made 

prior to the D.C. Circuit’s vacatur of the 

1997 standards, that the 1987 PM–10 

standards no longer applied in Ada 

County, Idaho. As EPA explained in the 

proposed rule, the Ada County finding 

was based on the existence of the 1997 

standards as well as the transition 

policy. Because the court vacated those 

standards, leaving in place the finding 

would have resulted in no federal 

protection from high levels of coarse 

particulate matter pollution. Finding 

that result untenable, EPA concluded 

that it was appropriate to restore the 

pre-existing PM–10 standards with 

respect to Ada County. 65 FR at 39323. 

As is clear from the final rule, however, 

the 1987 standards were never revoked 

with respect to the rest of the country. 

Therefore, although EPA deleted 40 CFR 

50.6(d)(as required by the mandate of 

ATA I), the pre-existing NAAQS 

continue to apply. 65 FR at 80777. If 

CCA believes that insufficient notice 

was provided in connection with this 

final action, it was required under CAA 

section 307(b)(1) to file a petition for 

review of that action in the U.S. Court 

of Appeals within 60 days of December 

22, 2000. CCA did not do so and is 

therefore foreclosed from raising this 

issue now. 

Moreover, to the extent that CCA 

raises issues with respect to the preexisting 1987 PM–10 standards, we note 

that those standards were upheld in 

Natural Resources Defense Council, 

Inc., et al. v. EPA, et al., 902 F.2d 962 

(D.C. Cir. 1990). In any case, the 1987 

standards do not use PM–10 as an 

indicator exclusively for coarse 

particles, but rather are intended to 

address both PM–2.5 and PM–10–2.5, 

i.e. both fine and coarse particles. 52 FR 

24634, 24639 (July 1, 1987). Thus, any 

concerns that PM–10 may be an 

inappropriate indicator for coarse 

particles exclusively are inapplicable to 

the 1987 standard. 

When CCA submitted its comment 

letter in 2004, EPA was in the process 

of developing proposed regulations to 

again address thoracic coarse particles. 

The Agency subsequently finalized such 

regulations in 2006. 71 FR 61144 

(October 17, 2006). CCA’s concerns 

regarding new standards for PM–10, 

including putative confounding 

problems, were properly raised in the 

context of that rulemaking. In fact, 

challenges to the use of PM–10 as an 

indicator for coarse particles, as well as 

challenges to the scientific bases for the 

2006 final rule have been raised by 

various petitioners in the pending D.C. 

Circuit cases (American Farm Bureau 

Fed. et al. v. EPA and consolidated 

cases) challenging the rule. CCA can, 

and is, pursuing its concerns in that 

forum. 

III. Final Action 

EPA is finding that the Imperial area 

failed to attain the 24-hour PM–10 

NAAQS by the December 31, 2001 

attainment deadline and is requiring the 

State to submit under section 189(d) of 

the Act ‘‘plan revisions which provide 

for attainment of the PM–10 air quality 

standards and, from the date of such 

submission until attainment, for an 

annual reduction in PM–10 or PM–10 

precursor emissions within the area of 

not less than 5 percent of the amount of 

such emissions as reported in the most 

recent inventory prepared for such 

area.’’ The plan must be submitted to 

EPA no later than one year from the 

publication of this final rule. 

The pollutant-specific requirements 

for moderate and serious PM–10 

nonattainment areas are found in 

section 189 of the CAA, and the general 

planning and control requirements for 

nonattainment plans are found in CAA 

sections 110 and 172. In addition to the 

attainment demonstration and 5 percent 

annual reductions requirements 

referenced above, the PM–10 plan for 

the Imperial area must include the 

following elements: 17

• Transportation conformity and 

motor vehicle emissions budgets; 

• Emissions inventories; 

• Best available control measures for 

significant sources of PM–10; 

• Reasonably available control 

measures for significant sources of PM– 

10; 

• Control requirements applicable to 

major stationary sources of PM–10 

precursors pursuant to section 189(e); 

and 

• Reasonable further progress and 

quantitative milestones. 

The District must also revise its new 

source review (NSR) rule to reflect the 

serious area definitions for major new 

sources in CAA section 189(b)(3) and 

must make any changes in its Title V 

operating permits program necessary to 

reflect the change in the major source 

threshold from 100 tpy for moderate 

areas to 70 tpy for serious areas. 

Revisions to the NSR and Title V rules 

must also be submitted no later than one 

year from the publication of this final 

rule. 

IV. Statutory and Executive Order 

Reviews 

Under Executive Order 12866 (58 FR 

51735, October 4, 1993), this final action 

is not a ‘‘significant regulatory action’’ 

and therefore is not subject to review by 

the Office of Management and Budget. 

For this reason, this action is also not 

subject to Executive Order 13211, 

‘‘Actions Concerning Regulations That 

Significantly Affect Energy Supply, 

Distribution, or Use’’ (66 FR 28355, May 

22, 2001). This action merely makes a 

determination based on air quality data 

and does not impose any additional 

requirements. Accordingly, the 

Administrator certifies that this final 

rule will not have a significant 

economic impact on a substantial 

number of small entities under the 

Regulatory Flexibility Act (5 U.S.C. 601 

et seq.). Because this rule does not 

impose any additional enforceable duty, 

it does not contain any unfunded 

mandate or significantly or uniquely 

affect small governments, as described 

in the Unfunded Mandates Reform Act 

of 1995 (Pub. L. 104–4). This rule also 

does not have tribal implications 

because it will not have a substantial 

direct effect on one or more Indian 

tribes, on the relationship between the 

Federal Government and Indian tribes, 

or on the distribution of power and 

responsibilities between the Federal 

Government and Indian tribes, as 

specified by Executive Order 13175 (65 

FR 67249, November 9, 2000). This 

action also does not have Federalism 

implications because it does not have 

substantial direct effects on the States, 

on the relationship between the national 

government and the States, or on the 

distribution of power and 

responsibilities among the various 

levels of government, as specified in 

Executive Order 13132 (64 FR 43255, 

August 10, 1999). 

This action merely makes a 

determination based on air quality data 

and does not alter the relationship or 

the distribution of power and 

responsibilities established in the CAA. 

Executive Order 12898 establishes a 

Federal policy for incorporating 

environmental justice into Federal 

agency actions by directing agencies to 

identify and address, as appropriate, 

disproportionately high and adverse 


rfrederick on PROD1PC67 with RULES
Federal Register / Vol. 72, No. 237 / Tuesday, December 11, 2007 / Rules and Regulations 70229 

human health or environmental effects 

of their programs, policies, and 

activities on minority and low-income 

populations. Today’s action involves 

determinations based on air quality 

considerations. It will not have 

disproportionately high and adverse 

effects on any communities in the area, 

including minority and low-income 

communities. This rule also is not 

subject to Executive Order 13045 

‘‘Protection of Children from 

Environmental Health Risks and Safety 

Risks’’(62 FR 19885, April 23, 1997), 

because it is not economically 

significant. The requirements of section 

12(d) of the National Technology 

Transfer and Advancement Act of 1995 

(15 U.S.C. 272 note) do not apply. This 

rule does not impose an information 

collection burden under the provisions 

of the Paperwork Reduction Act of 1995 

(44 U.S.C. 3501 et seq.). 

Under section 307(b)(1) of the Clean 

Air Act, petitions for judicial review of 

this action must be filed in the United 

States Court of Appeals for the 

appropriate circuit by February 11, 

2008. Filing a petition for 

reconsideration by the Administrator of 

this final rule does not affect the finality 

of this rule for the purposes of judicial 

review nor does it extend the time 

within which a petition for judicial 

review may be filed, and shall not 

postpone the effectiveness of such rule 

or action. This action may not be 

challenged later in proceedings to 

enforce its requirements. (See section 

307(b)(2).) 

List of Subjects in 40 CFR Part 81 

Environmental protection, Air 

pollution control, National parks, 

Wilderness areas. 

Authority: 42 U.S.C. 7401 et seq. 

Dated: November 30, 2007. 

Laura Yoshii, 

Acting Regional Administrator, Region IX. 

[FR Doc. E7–23943 Filed 12–10–07; 8:45 am] 

BILLING CODE 6560–50–P 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Part 271 

[EPA–R01–RCRA–2007–0999; FRL–8504–4] 

Rhode Island: Final Authorization of 

State Hazardous Waste Management 

Program Revisions 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Immediate final rule. 

SUMMARY: The State of Rhode Island has 

applied to EPA for final authorization of 

certain changes to its hazardous waste 

program under the Resource 

Conservation and Recovery Act (RCRA). 

EPA has determined that these changes 

satisfy all requirements needed to 

qualify for final authorization, and is 

authorizing the State’s changes through 

this immediate final action. 

DATES: This final authorization will 

become effective on February 11, 2008 

unless EPA receives adverse written 

comment by January 10, 2008. If EPA 

receives such comment, it will publish 

a timely withdrawal of this immediate 

final rule in the Federal Register and 

inform the public that this authorization 

will not take immediate effect. 

ADDRESSES: Submit your comments, 

identified by Docket ID No. EPA–R01– 

RCRA–2007–0999, by one of the 

following methods: 

• http://www.regulations.gov: Follow 

the on-line instructions for submitting 

comments. 

• E-mail: biscaia.robin@epa.gov. 

• Fax: (617) 918–0642, to the 

attention of Robin Biscaia. 

• Mail: Robin Biscaia, Hazardous 

Waste Unit, EPA New England—Region 

1, One Congress Street, Suite 1100 

(CHW), Boston, MA 02114–2023. 

• Hand Delivery or Courier: Deliver 

your comments to Robin Biscaia, 

Hazardous Waste Unit, Office of 

Ecosystem Protection, EPA New 

England—Region 1, One Congress 

Street, 11th Floor, (CHW), Boston, MA 

02114–2023. Such deliveries are only 

accepted during the Office’s normal 

hours of operation, and special 

arrangements should be made for 

deliveries of boxed information. 

Instructions: Identify your comments 

as relating to Docket ID No. EPA–R01– 

RCRA–2007–0999. EPA’s policy is that 

all comments received will be included 

in the public docket without change and 

may be made available online at 

http://www.regulations.gov, including 

any personal information provided, 

unless the comment includes 

information claimed to be Confidential 

Business Information (CBI) or claimed 

to be other information whose 

disclosure is restricted by statute. Do 

not submit information that you 

consider to be CBI or otherwise 

protected through http:// 

www.regulations.gov or e-mail. The 

http://www.regulations.gov Web site is 

an ‘‘anonymous access’’ system, which 

means EPA will not know your identity 

or contact information unless you 

provide it in the body of your comment. 

If you send an e-mail comment directly 

to EPA without going through http:// 

www.regulations.gov, your e-mail 

address will be automatically captured 

and included as part of the comment 

that is placed in the public docket and 

made available on the Internet. If you 

submit an electronic comment, EPA 

recommends that you include your 

name and other contact information in 

the body of your comment and with any 

disk or CD–ROM you submit. If EPA 

cannot read your comment due to 

technical difficulties and cannot contact 

you for clarification, EPA may not be 

able to consider your comment. 

Electronic files should avoid the use of 

special characters, any form of 

encryption, and be free of any defects or 

viruses. For additional information 

about EPA’s public docket visit the EPA 

Docket Center homepage at http:// 

www.epa.gov/epahome/dockets.htm. 

Docket: EPA has established a docket 

for this action under Docket ID No. 

EPA–R01–RCRA–2007–0999. All 

documents in the docket are listed on 

the http://www.regulations.gov Web 

site. Although it may be listed in the 

index, some information might not be 

publicly available, e.g., CBI or other 

information whose disclosure is 

restricted by statute. Certain other 

material, such as copyrighted material, 

is not placed on the Internet and will be 

publicly available only in hard copy 

form. Publicly available docket 

materials are available either 

electronically through http:// 

www.regulations.gov or in hard copy at 

the following two locations: (i) EPA 

Region 1 Library, One Congress Street— 

11th Floor, Boston, MA 02114–2023; by 

appointment only; tel: (617) 918–1990; 

and (ii) Rhode Island Department of 

Environmental Management, 235 

Promenade St., Providence, RI 02908– 

5767, by appointment only through the 

Office of Technical and Customer 

Assistance, tel: (401) 222–6822. 

FOR FURTHER INFORMATION CONTACT: 

Robin Biscaia, Hazardous Waste Unit, 

EPA New England—Region 1, One 

Congress Street, Suite 1100 (CHW), 

Boston, MA 02114–2023; telephone 

number: (617) 918–1642; fax number: 

(617) 918–0642, e-mail address: 

biscaia.robin@epa.gov. 

SUPPLEMENTARY INFORMATION: 

A. Why Are Revisions to State 

Programs Necessary? 

States which have received final 

authorization from EPA under RCRA 

section 3006(b), 42 U.S.C. 6926(b), must 

maintain a hazardous waste program 

that is equivalent to, consistent with, 

and no less stringent than the Federal 

program. As the Federal program 

changes, States must change their 


rfrederick on PROD1PC67 with RULES
Imperial County 2009 PM10 SIP Appendix II: IC Air Monitoring Information: 

2006-2008 

A PPENDIX II 

IMPERIAL COUNTY AIR MONITORING INFORMATION: 

2006-2008

FINAL AUGUST 2009 ICAPCD 
Imperial County 2009 PM10 SIP Appendix II: IC Air Monitoring Information: 

2006-2008 

Appendix II 

Imperial County Air Monitoring Information: 

2006-2008 

This appendix provides detailed information about the PM10 and meteorological monitoring 

networks (Section II.1 and II.2, respectively) in Imperial County. Ambient PM10 measurements 

acquired at all filter-based (SSI) monitors in Imperial County are also reported in tabulated 

format (Section II.3). 

FINAL AUGUST 2009 II-i ICAPCD 
Imperial County 2009 PM10 SIP Appendix II: IC Air Monitoring Information: 

2006-2008 

II.1 Imperial County PM10 Air Monitoring 

II.1.1 Filter-Based Monitors 

During the 2006-2008 time period, the Imperial County Air Pollution Control District (ICAPCD) 

operated filter-based, size-selective inlet (SSI) PM10 monitors at six stations located in the 

populated areas of the County (Figure II.1): Calexico-Grant, Calexico-Ethel, El Centro, Brawley, 

Westmorland, and Niland. These SSI monitors meet federal performance criteria and are 

considered to be the official data source for long-term air quality planning and attainment 

demonstration. Detailed information about these individual monitors is provided in this section. 

Niland

Westmorland

Brawley

El Centro

C. Ethel

C. Grant

Figure II.1 Locations of Imperial County PM10 SSI stations.

Calexico Grant. The Calexico Grant station, a hi-volume sampler, is located on top of a trailer 

at 900 Grant Street in a residential area with homes lying immediately to the east, west, and 

north of the site. Grant Street is a fairly busy street with visible dust accumulations on the street. 

The station is found on the southwest side of the city of Calexico, approximately 0.5 miles west 

of the central business district. To the south lies the Calexico International Airport and the 

US-Mexican border (0.75 miles south), and to the southeast lies Anza Road, with a large 5-acre 

unpaved lot, and the New River. The Calexico-Grant monitor was permanently 

decommissioned in August 2007. 

FINAL AUGUST 2009 II-1 ICAPCD 
Imperial County 2009 PM10 SIP Appendix II: IC Air Monitoring Information: 

2006-2008 

Calexico Ethel. The Calexico Ethel station, a hi-volume sampler, is located on top of a trailer at 

1029 Ethel Street southeast of the Calexico High School parking lot. A residential community 

surrounds the Ethel station on the southeast, west, and northwest sides. The football athletic 

field and the school buildings including tennis courts and a baseball field lie to the north and 

northeast, respectively. The Mexican border is approximately 0.75 miles to the south. The 

monitoring inlet is approximately 10.5 feet above ground level (AGL). 

El Centro. The El Centro station, a hi-volume sampler, is located on the rooftop of the old 

County Jail Building at 150 S. 9th Street. The El Centro station is located in close proximity to a 

commercial area within the center of the city and is north of Calexico. The site has been 

classified as an urbanized location with large agricultural areas to the east and west of the city. 

Westmorland. The Westmorland station, a hi-volume sampler, is located on top of a trailer at 

202 West 1st Street in Westmorland. The city of Westmorland is a rural community with a 

population under 2,000 people. The site is located on the southwest section of the community 

within the wastewater treatment plant. Westmorland is located 14.9 miles northwest of the city 

of Brawley and 16.5 miles southwest of the city of Calipatria. Residential and agricultural areas 

lie within 33 feet and 1,312 feet of the site, respectively. The monitoring inlet is approximately 16 

feet AGL. 

Niland. The Niland station, a hi-volume sampler, is located on the rooftop of a trailer at 7711 

English Road within a rural area, approximately 2 miles southwest from the city of Niland. The 

Niland site is the most northerly positioned of the Imperial County monitoring stations and is 

located near the Salton Sea and Imperial County’s border with Riverside County. The 

monitoring inlet is approximately 10.5 feet AGL. 

Brawley. The Brawley monitor, a hi-volume sampler, is located on the roof of a two-story 

building at 220 Main Street in downtown Brawley next to a commercial area. The closest street 

is approximately 100 feet away and the site experiences approximately 5,000 vehicle passages 

a day. Agricultural land adjoins Brawley on the east, north, and west sides. It is the second site 

north of Calexico (the first is El Centro). 

II.1.2 Continuous Monitors 

In addition to the PM10 SSI units, the ICAPCD also operated Beta Attenuation Mass (BAM) 

monitors at the Brawley, Westmorland, and Niland station in 2006-2008. These monitors were 

used for the purpose of daily forecasting, and were not operated in accordance with the quality 

assurance procedures required to generate data suitable for regulatory compliance evaluations. 

A 2008 ICAPCD/ARB evaluation of the Imperial County 2006-2008 PM10 BAM data revealed 

that monitor performance had been inaccurate as a result of deviations from operational 

specifications. Consequently, the Imperial County 2006-2008 BAM data was unfit for 

submission to the USEPA Air Quality System (AQS). 

FINAL AUGUST 2009 II-2 ICAPCD 
Imperial County 2009 PM10 SIP Appendix II: IC Air Monitoring Information: 

2006-2008 

To overcome inaccurate BAM monitor performance, in June 2008 the ICAPCD suspended the 

operation of all three of the Brawley, Westmorland, and Niland BAM units to allow for their 

recalibration. Operation of continuous PM10 monitors (either BAM monitors or Tapered Element 

Oscillating Microbalance (TEOM) monitors) at the Brawley, Westmorland, and Niland stations is 

expected to have resumed by Summer 2009. The CARB and the ICAPCD are also 

implementing a new monitor network surrounding the Salton Sea. Comprising 5 additional 

TEOM monitoring units positioned around the Sea as shown in Figure II.2, operation of this 

network by the ICAPCD is expected to begin by Summer 2009 as well. 

Figure II.2 Proposed Continuous PM10 Monitor network around the Salton Sea.

FINAL AUGUST 2009 II-3 ICAPCD 
Imperial County 2009 PM10 SIP Appendix II: IC Air Monitoring Information: 

2006-2008 

II.2 Meteorological Stations 

Figure II.3 shows the location of meteorological monitoring stations from which data may be 

obtained in the analysis of the transport and dispersion of atmospheric pollutants. The 

meteorological stations are divided into four categories based on the agency or organization 

that operates them, as discussed in detail below. The geographic coordinates, anemometer 

height, and anemometer type of the meteorological stations shown in Figure II.3 are reported in 

Table II.1. 

Figure II.3 Meteorological Stations in and around Imperial County.

FINAL AUGUST 2009 II-4 ICAPCD 
Imperial County 2009 PM10 SIP Appendix II: IC Air Monitoring Information: 

2006-2008 

Table II.1 Meteorological Station Location Information

Meteorological 

Station Owner/Operator Latitude Longitude Anemometer 

Height (m) Anemometer Type 

Calexico East 32o 40' 27" 115o 23' 28" 

El Centro 32o 47' 32" 115o 33' 47" 

Calexico Ethel 32o 40' 34" 115o 28' 59" 

Calexico Grant 32o 40' 26" 115o 30' 59" 

Niland 33o 12' 49" 115o 32' 43" 

Westmorland 33o 1' 57" 115o 37' 25" 

Indio 33o 42' 30" 116o 12' 57" 

Palm Springs 

Fire Station 33o

 49' 10" 116o 29' 24" 

10 

Cobach 32o 38' 25" 115o 30' 21" 6 

Institute of 

Technology 32o

 37' 17" 115o 23' 52" 10 

UABC 

CARB1

32o

 37' 45" 115o 26' 45" 10 

Information 

Unavailable 

Calipatria 33o 02' 37" 115o 24' 56" 

Meloland 32o 48' 24" 115o 26' 46" 

Salton Sea 

East 33o 13' 12" 115o 34' 48" 

Salton Sea 

West 33o 19' 38" 115o 57' 00" 

Seeley 32o 45' 34" 115o 43' 54" 

UC-Mex 32o 24' 40" 115o 11' 50" 

UC-San Luis 32o 29' 34" 114o 49' 34" 

2 

Westmorland 

California DWR 

CIMIS 

33o 04' 39" 115o 36' 50" Inactive 

Met-One, Model 

014A, Three-cup, 

0-100 mph, ± 0.25 

mph 

Imperial County 

Airport FAA/NOAA 32o 55' 3" 115o 34' 43" 10 

Standard 

Automated 

Surface 

Observation 

System 

El Centro Naval 

Air Facility US Navy Information Unavailable 

1

 These stations may be operated by the local air districts 

FINAL AUGUST 2009 II-5 ICAPCD 
Imperial County 2009 PM10 SIP Appendix II: IC Air Monitoring Information: 

2006-2008 

II.2.1 ICAPCD Monitoring Stations 

The ICAPCD Stations are operated by the Imperial County Air Pollution Control District. All 

ICAPCD stations record wind speed and direction from anemometers and wind vanes mounted 

on 33 foot (10 meter) towers. A general description of each ICAPCD site can be found in 

Section 2 of this report. Additionally, information about all air monitoring stations in CARB’s 

network is available at www.arb.ca.gov/qaweb/site.php. This site includes maps of each site, 

GPS coordinates, site photos, pollutants monitored, and site surveys. The site surveys list indepth monitoring information such as traffic descriptions, calibration dates, distances to trees 

and obstacles, and residence times. In some cases, real-time and recent observations are also 

reported. 

In addition to wind speed and wind direction, PM10 is monitored at the ICAPCD Stations of El 

Centro, Niland, Westmorland, Calexico Grant, Calexico Ethel and Calexico East. Note that the 

Brawley PM10 monitoring station does not gather meteorological data and is therefore not shown 

in Figure II.3. 

The meteorological sensors operated by ICAPCD are routinely subjected to performance audits 

by CARB’s Quality Assurance Section. The parameters audited include wind speed, wind 

direction, ambient temperature, relative humidity, barometric pressure, and total solar radiation. 

The criteria used for auditing meteorological sensors can be found in Ambient Monitoring 

Guidelines for Photochemical Assessment Monitoring Stations (PAMS) (USEPA, 1994) and 

Quality Assurance Handbook for Air Pollution Measurement Systems, Volume IV: 

Meteorological Measurements (USEPA, March 1995). These documents specify that the 

sensors must meet the following criteria: 

Parameters Criteria

Wind Speed Starting Threshold: less than 0.5 m/s 

Accuracy: +/- 0.2 m/s + 5% 

Wind Direction Starting Threshold: less than 0.5 m/s 

Accuracy: +/- 5 degrees 

II.2.2 Mexicali Monitoring Stations 

The Mexicali Stations are operated by the Mexican Secretaría de Medio Ambiente y Recursos 

Naturales (SEMARNAT). The Institute of Technology and UABC stations monitor wind speed 

and direction at 33 feet (10 meters) while the Cobach station monitors at 19.8 feet (6 meters). 

These Mexicali Stations, as well as the Progresso and Conalep stations, measure PM10, 

however the latter two stations do not gather meteorological data. The meteorological sensors 

on the Mexicali stations are also routinely audited by CARB, as described above. Additional 

FINAL AUGUST 2009 II-6 ICAPCD 
Imperial County 2009 PM10 SIP Appendix II: IC Air Monitoring Information: 

2006-2008 

information about these monitoring sites can be found on the CARB website 

(www.arb.ca.gov/qaweb/site.php). 

II.2.3 CIMIS Monitoring Stations 

The California Department of Water Resources (DWR) operates a network of over 120 

automated weather stations in the state of California as part of its California Irrigation 

Management Information System (CIMIS). CIMIS was developed in 1982 by DWR and the 

University of California, Davis to assist irrigators in managing their water resources efficiently. 

CIMIS weather stations collect weather data on a minute-by-minute basis, calculate hourly and 

daily values and transmit that information to a central database at the DWR headquarters in 

Sacramento each day. The data are analyzed for quality, apportioning appropriate flags as 

necessary, and recorded in the central database, which can be accessed at 

http://wwwcimis.water.ca.gov/cimis/welcome.jsp. 

The CIMIS stations measure wind speeds using a Met-One (Model 014A) three-cup 

anemometer, with a magnet activated reed switch whose frequency is proportional to wind 

speed, mounted 6.6 feet (2.0 meters) above the ground. They measure wind speeds from 0-100 

mph, with an accuracy of 0.25 mph. Wind direction is measured by a Met-One (Model 024A) 

wind vane mounted 6.6 feet (2.0 meters) above the ground. The threshold speed for the wind 

vane is 1 mph and has ± 5% accuracy. Most of the CIMIS stations are sited on standardized 

grass surfaces. 

II.2.4 Other Monitoring Stations 

For large scale regional natural events, meteorological data from the Palm Springs and Indio 

stations in the Coachella Valley can be used. These stations, which are also PM10 monitoring 

stations, are operated by the South Coast Air Quality Management District (SCAQMD) and are 

routinely audited by CARB. Both of these stations monitor wind speed and direction using 

sensors mounted on a 33 foot (10 meter) tower. More information about these monitoring sites 

can also be found on the CARB website (www.arb.ca.gov/qaweb/site.php). 

To supplement the data from these stations, wind speed and direction information was also 

collected from the Imperial County Airport and the El Centro Naval Air Facility. The Imperial 

County Airport (ICA) is located north of El Centro. Data from the meteorological station at ICA 

were obtained from the Western Regional Climate Center (WRCC). Meteorological conditions 

are monitoring using a standard automated surface observation system and wind speed and 

direction data are collected using sensors mounted on a 33 foot (10 meter) tower. 

The El Centro Naval Air Facility (NAF) is located northeast of Seeley and 5 miles northwest of El 

Centro. Information about the type and location of the meteorological station at this facility were 

unavailable. 

FINAL AUGUST 2009 II-7 ICAPCD 
Imperial County 2009 PM10 SIP Appendix II: IC Air Monitoring Information: 

2006-2008 

FINAL AUGUST 2009 II-8 ICAPCD 

II.2.5 Impact of Anemometer Height on Wind Speed Readings 

The surface of the Earth exerts a frictional drag on the air blowing just above it, therefore the 

wind speed an anemometer measures depends on the height of that anemometer. Wind speeds 

can be adjusted to a consistent reference height using the following wind power law: 

p

measured

reference

reference measured

z

z

uu ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⋅=

where ureference is the estimated wind speed at the reference height (zreference), umeasured is the wind 

speed measured at the anemometer height (zmeasured), and p is the wind profile exponent.1

 The 

wind profile exponent can range from 0.07 to 0.55 depending on the Pasquill stability category 

and land use surrounding the monitor. 

CIMIS stations measure wind speed and direction at an average height of approximately 6.6 

feet above ground level, while many other meteorological stations, including most of the CARB 

audited stations, measure wind speeds at a height of 33 feet (10 meters) above ground level. 

The wind speeds from CIMIS stations reported herein are the actual wind speeds measured at 

the monitor. To compare these wind speeds to those measured at higher (e.g. 10 meter) 

monitors, the CIMIS wind speeds need to be scaled up by a factor of 1.12 to 2.42, depending on 

Pasquill stability category. 

 

1 User’s Guide for the Industrial Source Complex (ISC3) Dispersion Models: Volume II – Description of 

Model Algorithms. U.S. Environmental Protection Agency, Document Number EPA-454/B-95-003b, 

September 1995. 
Imperial County 2009 PM10 SIP Appendix II: IC Air Monitoring Information: 

2006-2008 

II.3 2006-2008 PM10 Air Quality Data 

2006 Data 

Sampling 

Day 

Sampling 

Date Brawley Calexico Ethel 

Calexico 

Grant El Centro Niland Westmorland 

Tue 5-Jan 21 34 23 25 17 23 

Wed 11-Jan 47 78 101 57 26 46 

Tue 17-Jan 37 113 64 22 

Thu 19-Jan 16 

Mon 23-Jan 26 39 28 43 15 

Wed 25-Jan 59 

Sun 29-Jan 51 85 102 59 48 

Sat 4-Feb 45 72 140 36 23 34 

Fri 10-Feb 55 81 77 61 33 46 

Thu 16-Feb 35 37 43 34 36 43 

Wed 22-Feb 29 45 62 20 13 35 

Tue 28-Feb 15 39 36 22 16 

Mon 6-Mar 40 100 60 19 38 

Sun 12-Mar 5 5 6 5 

Wed 15-Mar 53 13 

Sat 18-Mar 16 10 11 8 11 11 

Tue 21-Mar 14 

Fri 24-Mar 28 61 26 13 23 

Sat 25-Mar 41 

Thu 30-Mar 28 31 26 20 

Wed 5-Apr 132 92 85 57 118 

Tue 11-Apr 34 38 24 16 21 

Mon 17-Apr 36 32 26 17 27 32 

Sun 23-Apr 53 12 15 11 11 16 

Tue 25-Apr 33 

Sat 29-Apr 31 42 50 76 33 25 

Fri 5-May 37 46 51 41 39 

Thu 11-May 45 78 90 39 57 

Wed 17-May 57 80 89 47 60 

Thu 18-May 51 

Tue 23-May 27 30 43 18 18 24 

Thu 25-May 24 

Mon 29-May 34 36 43 25 33 40 

Wed 31-May 34 

Sun 4-Jun 43 58 91 53 48 46 

Sat 10-Jun 34 44 68 43 42 41 

Fri 16-Jun 35 50 62 31 29 43 

Thu 22-Jun 56 61 84 57 42 58 

Wed 28-Jun 113 79 76 122 

Fri 30-Jun 55 

Tue 4-Jul 42 44 73 50 38 55 

Mon 10-Jul 57 50 82 50 49 70 

Sun 16-Jul 98 74 104 136 

Tue 18-Jul 37 

Sat 22-Jul 51 41 62 59 47 92 

Fri 28-Jul 31 30 41 36 37 39 

Thu 3-Aug 37 33 63 43 31 44 

FINAL AUGUST 2009 II-9 ICAPCD 
Imperial County 2009 PM10 SIP Appendix II: IC Air Monitoring Information: 

2006-2008 

FINAL AUGUST 2009 II-10 ICAPCD 

Sampling 

Day 

Sampling 

Date Brawley Calexico 

Ethel 

Calexico 

Grant El Centro Niland Westmorland 

Wed 9-Aug 54 36 52 48 61 

Tus 15-Aug 47 52 

Mon 21-Aug 70 105 65 48 84 

Wed 23-Aug 64 28 34 42 

Sun 27-Aug 70 40 58 31 36 35 

Wed 30-Aug 60 60 100 

Sat 2-Sep 127 164 233 146 99 167 

Fri 8-Sep 75 33 68 67 40 Invalidated 

Thu 14-Sep 106 80 95 96 116 59 

Wed 20-Sep 37 33 45 29 28 68 

Tue 26-Sep 40 49 64 40 34 79 

Mon 2-Oct 33 34 37 24 25 

Sun 8-Oct 26 38 52 24 24 

Tue 10-Oct 70 

Sat 14-Oct 25 18 17 40 

Fri 20-Oct 42 91 35 28 32 

Tue 24-Oct 20 

Thu 26-Oct 27 33 33 19 34 

Sat 28-Oct 50 

Wed 1-Nov 60 73 130 56 40 54 

Fri 3-Nov 67 

Tue 7-Nov 39 134 110 40 36 50 

Mon 13-Nov 42 63 120 58 30 45 

Sun 19-Nov 36 92 41 30 23 31 

Sat 25-Nov 46 39 39 37 50 

Fri 1-Dec 40 40 14 38 

Thu 7-Dec 47 31 32 

Sun 10-Dec 57 

Wed 13-Dec 41 62 58 43 22 31 

Tue 19-Dec 22 44 103 36 12 28 

Thu 21-Dec 171 54 

Mon 25-Dec 27 110 248 24 

Sun 31-Dec 47 71 101 67 39 42 

2007 Data 

Sampling 

Day 

Sampling 

Date Brawley 

Calexico 

Ethel 

Calexico 

Grant El Centro Niland Westmorland 

Sat 6-Jan 29 46 78 31 22 28 

Fri 12-Jan 33 39 29 22 20 25 

Mon 16-Jan 34 64 42 

Thu 18-Jan 52 22 31 

Sat 20-Jan 22 

Wed 24-Jan 36 60 31 12 26 

Tue 30-Jan 21 54 57 36 15 24 

Thu 1-Feb 13 

Mon 5-Feb 38 70 90 34 27 43 

Sun 11-Feb 30 32 28 99 64 

Sat 17-Feb 36 31 34 31 23 22 

Wed 21-Feb 64 
Imperial County 2009 PM10 SIP Appendix II: IC Air Monitoring Information: 

2006-2008 

FINAL AUGUST 2009 II-11 ICAPCD 

Sampling 

Day 

Sampling 

Date Brawley 

Calexico 

Ethel 

Calexico 

Grant El Centro Niland Westmorland 

Fri 23-Feb 60 41 50 22 35 68 

Thu 1-Mar 30 36 46 29 16 35 

Wed 7-Mar 82 89 59 51 68 

Sat 10-Mar 84 

Tue 13-Mar 48 127 149 64 42 

Thu 15-Mar 58 

Mon 19-Mar 44 52 58 40 50 56 

Sun 25-Mar 23 47 61 32 18 24 

Fri 30-Mar 49 

Sat 31-Mar 54 106 127 55 29 

Fri 6-Apr 49 35 45 44 49 51 

Thu 12-Apr 291 124 147 121 108 155 

Wed 18-Apr 38 44 27 29 

Tue 24-Apr 23 51 79 21 27 22 

Mon 30-Apr 46 63 77 53 37 49 

Sun 6-May 30 25 13 

Wed 9-May 31 

Sat 12-May 51 36 43 25 38 37 

Fri 18-May 51 87 52 35 56 

Tue 22-May 61 

Thu 24-May 43 67 77 41 41 49 

Wed 30-May 41 49 46 61 

Tue 5-Jun 281 282 200 162 226 

Thu 7-Jun 35 30 

Mon 11-Jun 89 116 34 65 

Sun 17-Jun 48 41 50 36 73 

Wed 20-Jun 50 

Sat 23-Jun 52 60 66 55 57 

Tue 26-Jun 73 48 

Fri 29-Jun 46 63 62 52 44 

Thu 5-Jul 71 75 65 53 111 

Wed 11-Jul 78 62 91 47 92 

Tue 17-Jul 58 81 58 50 

Mon 23-Jul 64 54 111 60 62 58 

Wed 25-Jul 30 

Sun 29-Jul 47 38 47 41 46 44 

Sat 4-Aug 55 57 47 44 

Fri 10-Aug 53 62 36 57 

Mon 13-Aug 62 

Thu 16-Aug 52 41 63 44 51 

Wed 22-Aug 55 94 59 73 

Fri 24-Aug 96 

Tue 28-Aug 43 46 32 32 30 

Wed 29-Aug 58 

Mon 3-Sep 61 60 45 40 

Wed 5-Sep 51 

Sun 9-Sep 48 45 43 57 39 

Tue 11-Sep 45 

Sat 15-Sep 53 65 59 46 44 

Wed 19-Sep 

Decommissi

oned Unit 

117 53 
Imperial County 2009 PM10 SIP Appendix II: IC Air Monitoring Information: 

2006-2008 

FINAL AUGUST 2009 II-12 ICAPCD 

Sampling 

Day 

Sampling 

Date Brawley 

Calexico 

Ethel 

Calexico 

Grant El Centro Niland Westmorland 

Fri 21-Sep 17 15 13 15 11 

Thu 27-Sep 69 55 41 24 34 

Wed 3-Oct 37 45 40 25 30 

Tue 9-Oct 56 100 77 36 23 

Mon 15-Oct 42 67 36 28 33 

Sun 21-Oct 117 142 94 57 71 

Sat 27-Oct 54 103 30 51 51 

Fri 2-Nov 46 35 37 22 38 

Thu 8-Nov 58 101 70 59 

Wed 14-Nov 29 70 27 21 33 

Tues 20-Nov 70 110 98 46 

Mon 26-Nov 67 117 75 31 45 

Wed 28-Nov 30 

Sun 2-Dec 13 8 9 

Tue 4-Dec 106 

Sat 8-Dec 21 14 12 12 11 

Wed 12-Dec 49 

Fri 14-Dec 35 52 36 8 

Thu 20-Dec 30 40 37 19 27 

Wed 26-Dec 20 70 44 26 

2008 Data 

Sampling 

Day 

Sampling 

Date Brawley Calexico Ethel El Centro Niland Westmorland 

Tue 1-Jan 23 25 26 18 18 

Mon 7-Jan 27 14 9 

Wed 9-Jan 9 

Sun 13-Jan 11 11 5 12 

Tue 15-Jan 74 

Wed 16-Jan 38 

Sat 19-Jan 29 46 23 14 22 

Fri 25-Jan 22 36 30 15 18 

Thu 31-Jan 24 60 29 6 20 

Wed 6-Feb 28 74 45 62 28 

Tue 12-Feb 25 59 20 9 25 

Mon 18-Feb 42 76 34 29 34 

Sun 24-Feb 31 43 36 28 41 

Sat 1-Mar 45 64 53 57 43 

Fri 7-Mar 28 85 28 15 28 

Thu 13-Mar 38 88 50 34 51 

Wed 19-Mar 33 67 38 26 32 

Tue 25-Mar 34 51 33 44 37 

Mon 31-Mar 30 58 28 31 29 

Sun 6-Apr 42 29 18 34 38 

Sat 12-Apr 23 35 18 21 27 

Fri 18-Apr 33 77 39 36 41 

Thu 24-Apr 30 37 25 22 35 

Wed 30-Apr 137 104 88 99 137 

Tue 6-May 29 49 32 26 26 
Imperial County 2009 PM10 SIP Appendix II: IC Air Monitoring Information: 

2006-2008 

FINAL AUGUST 2009 II-13 ICAPCD 

Sampling 

Day 

Sampling 

Date Brawley Calexico Ethel El Centro Niland Westmorland 

Mon 12-May 111 111 42 83 97 

Sun 18-May 27 58 29 26 

Sat 24-May 11 21 47 8 

Wed 28-May 18 

Fri 30-May 22 30 19 31 20 

Thu 5-Jun 52 75 44 41 45 

Wed 11-Jun 49 81 50 53 51 

Tus 17-Jun 48 71 43 44 47 

Mon 23-Jun 45 59 34 30 32 

Sun 29-Jun 36 45 32 45 37 

Sat 5-Jul 51 59 48 50 

Fri 11-Jul 43 53 42 35 42 

Thu 17-Jul 43 57 44 42 

Wed 23-Jul 36 58 32 48 44 

Fri 25-Jul 46 

Tue 29-Jul 36 47 45 45 

Mon 4-Aug 37 52 41 41 43 

Sun 10-Aug 40 24 23 31 

Sat 16-Aug 43 47 25 71 48 

Fri 22-Aug 40 48 33 49 

Tue 26-Aug 52 

Thu 28-Aug 28 35 32 33 41 

Wed 3-Sep 28 42 26 33 30 

Tue 9-Sep 45 45 41 34 53 

Mon 15-Sep 51 67 28 53 

Sun 21-Sep 22 34 21 27 22 

Sat 27-Sep 32 64 20 27 28 

Fri 3-Oct 60 47 26 55 

Thu 9-Oct 52 62 32 53 

Wed 15-Oct 42 43 24 65 

Tue 21-Oct 52 73 33 40 49 

Mon 27-Oct 53 61 38 35 47 

Wed 29-Oct 72 50 

Sun 2-Nov 79 32 49 83 

Tue 4-Nov 44 34 

Sat 8-Nov 37 63 56 33 

Fri 14-Nov 46 65 26 24 31 

Thu 20-Nov 52 82 54 42 63 

Wed 26-Nov 27 33 22 18 26 

Tue 2-Dec 44 42 23 33 

Thu 4-Dec 99 

Mon 8-Dec 55 33 41 

Sun 14-Dec 15 18 12 10 14 

Tue 16-Dec 17 6 11 

Sat 20-Dec 31 27 22 6 17 

Fri 26-Dec 24 33 122 20 

Sat 27-Dec 22 
THIS PAGE INTENTIONALLY LEFT BLANK 
Imperial County 2009 PM10 SIP Appendix III.A: IC PM10 Uncontrolled Emissions Inventory 

A PPENDIX III.A 

IMPERIAL COUNTY UNCONTROLLED PM10 EMISSIONS 

INVENTORY

FINAL AUGUST 2009 ICAPCD 
Imperial County 2009 PM10 SIP Appendix III.A: IC PM10 Uncontrolled Emissions Inventory 

Appendix III.A 

Imperial County PM10 Uncontrolled Emissions Inventory 

The emission inventory used in the Imperial County 2009 PM10 SIP is derived from CARB’s SIP 

inventory Version 1.06 (SIP V1.06), base year 2002. Since SIP V1.06 was developed, changes 

have occurred which impact the emissions inventory. To ensure that the Imperial County SIP is 

based on the best available data, appropriate adjustments were implemented to update the SIP 

V1.06 inventory. These adjustments fall in two categories: (i) adjustments to incorporate new 

methodology and updated information (e.g., throughputs, activity data, etc.), and (ii) adjustments 

to incorporate emission reductions arising from the implementation of new control measures. 

This appendix discusses updates to the Imperial County PM10 emissions inventory that fall 

within the first category. Adjustments to the inventory to reflect reductions resulting from 

ICAPCD’s fugitive dust control measures (Regulation VIII rules) are discussed in Chapter IV. 

FINAL AUGUST 2009 III.A-i ICAPCD 
Imperial County 2009 PM10 SIP Appendix III.A: IC PM10 Uncontrolled Emissions Inventory 

FINAL AUGUST 2009 III.A-1 ICAPCD 

III.1 Summary of Adjustments to the Imperial County PM10 Emission Inventory 

Adjustments to the Imperial County PM10 Emissions Inventory to reflect updated information 

and/or methodology are summarized in Table 1. Overall, these adjustments resulted in a >20% 

increase in total PM10 emissions in Imperial County in 2005 from 232 tpd1

 to 281 tpd. Additional 

details concerning the adjustments are presented in subsequent subsections. 

Table 1: Summary of Adjustments to the Imperial County PM10 Inventory (Excluding Control 

Reductions) 

EIC Category Description External Adjustments 

620-618-0262-0101 

Farming Operations - 

Livestock Husbandry – 

Dairy Cattle 

1. Reduce emissions by incorporating reductions 

from Rule 420. 

2. Revise emissions, based on updated cattle 

population data and 2004 methodology. 

3. Revise Total PM emissions by changing PM 

profile from #900 (Unspecified) to #423 

(Livestock Operations Dust) with lower PM10

fractions. 

620-618-0262-0103 

Farming Operations - 

Livestock Husbandry – 

Feedlot Cattle 

1. Reduce emissions by incorporating reductions 

from Rule 420. 

2. Revise emissions, based on updated cattle 

population data and 2004 methodology. 

3. Revise Total PM emissions by changing PM 

profile from #900 (Unspecified) to #423 

(Livestock Operations Dust) with lower PM10

fractions. 

645-638-5400-0000 Unpaved Road Travel Dust 

- City and County Roads 

Increase emissions, based on higher average daily 

trip (ADT) estimates from the Imperial County Public 

Works Dept. 

645-640-5400-0000 

Unpaved Road Travel Dust 

- US Forest and Park 

Roads 

Decrease emissions, based on keeping forest road 

miles at 1993 levels, rather than growing them. 

645-644-5400-0000 Unpaved Road Travel Dust 

- B.L.M. Roads 

Decrease emissions, based on keeping BLM road 

miles at 1993 levels, rather than growing them. 

645-648-5400-0000 

Unpaved Road Travel Dust 

– Unspecified (Canal 

Roads) 

Increase emissions, based on increased canal road 

miles from the Imperial County Public Works Dept. 

Replace default growth factors with zero growth. 

650-650-5400-0000 

Fugitive Windblown Dust - 

Dust from Agricultural 

Lands (Non-Pasture) 

Decrease emissions, based on lower estimates 

from the 2004 Windblown Dust Study. 

650-652-5400-0000 

Fugitive Windblown Dust - 

Dust from Unpaved Roads 

and Associated Areas 

Increase emissions, based on the 2004 Windblown 

Dust Study and ARB’s latest (1997) emission factor. 

III.2 Farming Operations – Livestock Husbandry – Dairy Cattle & Feedlot Cattle 

(EICs 620-618-0262-0101 and 620-618-0262-0103) 

In the original SIP V1.06 inventory, EIC 620-618-0262-0101 had zero emissions and did not 

appear in forecast scenarios. This was corrected so that the PM10 Imperial County SIP inventory 

includes emissions for EIC 620-618-0262-0101. 

 

1

 This number corresponds to the 2004 projection from SIP Inventory V1.06 
Imperial County 2009 PM10 SIP Appendix III.A: IC PM10 Uncontrolled Emissions Inventory 

FINAL AUGUST 2009 III.A-2 ICAPCD 

In 2002, the Imperial County APCD revised Rule 420, Beef Feedlots, to reduce PM emissions. 

Rule 420 became effective in 2004, resulting in a 50% reduction of PM emissions. Rule 420 was 

also revised in 2006, although with no change in the percent reduction. The SIP inventory V1.06 

has been adjusted to reduce PM emissions 50% for EICs 620-618-0262-0101 and 

620-618-0262-0103, beginning in 2004. 

In 2004, ARB published an updated methodology for Livestock Husbandry which included new 

emission factors for dairy cattle and feedlot cattle.2

 In 2005, the Imperial County APCD obtained 

updated cattle population data for 2004, as documented in the Best Available Control Measure 

(BACM) Analysis.3

 These changes resulted in external adjustments for EIC 620-618-0262-0101 

that increased uncontrolled PM10 emissions slightly from zero tpd to 0.01 tpd in 2004. 

When calculating PM10 emissions for dairy cattle, the BACM Analysis specifically excluded the 

population of 21,452 heifers. However, in a subsequent Imperial County APCD letter,4

 heifers 

were grouped with feedlot cattle under EIC 620-618-0262-0103. To ensure consistency, ARB 

staff used the approach in the Imperial County APCD letter and grouped heifers with feedlot 

cattle to determine the 2004 emissions adjustment. For EIC 620-618-0262-0103, the updated 

methodology and population data resulted in an 18% increase of uncontrolled PM10 emissions 

from 4.66 to 5.52 tpd. When 50% reductions from Rule 420 were applied, PM10 emissions 

decreased in 2004. The SIP inventory V1.06 has been adjusted to reduce 2004 and 2005 PM10

emissions for EIC 620-618-0262-0101 to 0.006 tpd and 620-618-0262-0103 to 2.76 tpd. 

For the years 2006 and 2007, Imperial County APCD provided updated cattle population data.4

 

The SIP inventory V1.06 has been adjusted to include the following from 2006 to 2010: 

(1) For EIC 620-618-0262-0101, the growth factor increased from 1 (i.e., no growth) to 1.452 

and the emissions increased from 0.006 to 0.009 tpd, beginning in 2006; and 

(2) For EIC 620-618-0262-0103, the growth factor increased from 1 (i.e., no growth) to 1.177 

and the emissions increased from 2.76 to 3.25 tpd, beginning in 2006. 

III.3 Unpaved Road Travel Dust-City and County Roads (EIC 645-638-5400-0000) 

For the SIP Inventory V1.06, emissions from unpaved road travel dust were based on an 

emission factor of 2.27 lbs PM10/vehicle mile traveled (VMT) using ARB’s 1997 methodology.5

 

In 2003, ARB updated this emission factor to 2.0 lbs PM10/VMT for the San Joaquin Valley PM10

 

2

 Air Resources Board. Areawide Source Methodologies, Section 7.6, Livestock Husbandry, May 2004, 

http://www.arb.ca.gov/ei/areasrc/fullpdf/FULL7-6.PDF. 3

 Environ International Corporation, “Draft Final Technical Memorandum, Regulation VIII BACM Analysis”, prepared 

for the Imperial County APCD, October 2005. 4

 Letter from Imperial County APCD (Reyes Romero) to ARB (Andrew Deleo), regarding an emission inventory 

update for Livestock Husbandry, dated April 9, 2008. 5

 Air Resources Board. Areawide Source Methodologies, Section 7.6, Unpaved Road Dust (Non-Farm Roads), 

August 1997, http://www.arb.ca.gov/ei/areasrc/fullpdf/full7-10prev.pdf. 
Imperial County 2009 PM10 SIP Appendix III.A: IC PM10 Uncontrolled Emissions Inventory 

FINAL AUGUST 2009 III.A-3 ICAPCD 

SIP.6 In 2005,7 and later in 2008,8 the Imperial County APCD obtained updated information 

from the Imperial County Public Works Department regarding the miles of unpaved city and 

county roads. The updated emission factor and road data resulted in a ~50% increase9

 in travel 

road dust emissions from unpaved city and county roads to 25.6 tpd (24.6 tpd after correcting 

for rainfall). 

III.4 Unpaved Road Travel Dust – US Forest and Park Roads & BLM Roads 

(EICs 645-640-5400-0000 and 645-644-5400-0000) 

For the SIP Inventory V1.06, emissions from unpaved road travel dust were based on an 

emission factor of 2.27 lbs PM10/vehicle mile traveled (VMT) using ARB’s 1997 methodology.5

 

In 2003, ARB updated this emission factor to 2.0 lbs PM10/VMT for the San Joaquin Valley PM10

SIP.6

 Also, Imperial County APCD requested revision of emissions estimates for these source 

categories to reflect no growth in the mileage of forest roads (USFS roads) and Bureau of Land 

Management roads (BLM roads) after 1993.7

 The updated emission factor and road data result 

in a ~40% decrease in 2004 PM10 emissions from 2.3 to 1.4 tpd (1.3 tpd after correcting for 

rainfall). The SIP inventory V1.06 has been externally adjusted to account for this decrease. 

III.5 Fugitive Windblown Dust – Dust from (Non-Pasture) Agricultural Lands 

(EIC 650-650-5400-0000) 

In 2004, ENVIRON completed an updated windblown dust study for Imperial County.7 The 

results of this study were used to develop revised emission estimates that amount to an 88% 

decrease in PM10 emissions from 91.5 to 10.8 tpd. In addition, the windblown dust study 

resulted in changes to the temporal factors for this category. The Summer Temporal Factor 

changed from 1.539 to 0.782 and the Winter Temporal Factor changed from 0.462 to 1.221. The 

SIP inventory V1.06 has been externally adjusted to account for the decreased emissions and 

the temporal factor changes. 

III.6 Fugitive Windblown Dust – Dust from Unpaved Roads and Open Areas (EIC 

650-652-5400-0000) 

The SIP inventory V1.06 apparently used an older emission factor than is contained in ARB’s 

1997 methodology.10 Emissions estimates for this category were revised based on the 1997 

emission factor. The results of the revised windblown dust study (see section III.5) were also 

 

6

 Air Resources Board. “Summary of Fugitive Dust and Ammonia Emission Inventory Changes for the SJVU APCD 

Particulate Matter SIP”, Revision 2.1, May 2003, http://www.valleyair.org/Air_Quality_Plans/docs/ 

2003%20PM10%20Plan/PDF%202003%20PM10%20Plan%20adpt%20app/App%20C-EI%20Changes.pdf. 7

 Environ International Corporation, “Draft Final Technical Memorandum, Regulation VIII BACM Analysis”, prepared 

for the Imperial County APCD, October 2005. 8

 Communications between the ICAPCD and the Imperial County Public Works Department; refer to Attachment A of 

this Appendix. 9 Primarily as a consequence of higher average daily trip estimates from the Imperial County Public Works 

Department. 10 Air Resources Board. Areawide Source Methodologies, Section 7.13, Windblown Dust – Unpaved Roads, August 

1997, http://www.arb.ca.gov/ei/areasrc/fullpdf/full7-13.pdf. 
Imperial County 2009 PM10 SIP Appendix III.A: IC PM10 Uncontrolled Emissions Inventory 

FINAL AUGUST 2009 III.A-4 ICAPCD 

used to develop revised annual and seasonal emission estimates for open areas (i.e., 

grasslands, dunes, barren lands).11 Finally, additional PM10 emissions due to disturbance of 

soils as a result of off-road vehicle usage on Imperial County vacant lands were calculated as 

documented in Appendix III.B. The revised emission factor and updated studies resulted in a 

>150% increase in PM10 emissions from these categories to 200.1 tpd. 

III.7 Unpaved Road Travel Dust – Unspecified (IID Maintenance Roads) (EIC 645-

648-5400-0000) 

For the SIP Inventory V1.06, emissions from unpaved road travel dust were based on an 

emission factor of 2.27 lbs PM10/vehicle mile traveled (VMT) using ARB’s 1997 methodology.12

In 2003, ARB updated this emission factor to 2.0 lbs PM10/VMT for the San Joaquin Valley PM10

SIP.13 In 2005,14 and later in 2008,15 Imperial County APCD obtained updated information from 

the Imperial County Public Works Department regarding the miles of unpaved roads along 

canals. The updated emission factor and road data increased 2005 PM10 emissions ~140% to 

30.7 tpd (29.6 tpd after correcting for rainfall), primarily as a consequence of higher inventory of 

canal road miles. 

The default growth factors for unspecified unpaved roads are based on vehicle miles traveled by 

Light Duty Trucks and Medium Duty Trucks (i.e., LDT&MDT_VMT), and correspond to a 24% 

mileage growth from 2002 to 2010. The ICAPCD suggested that these default growth factors 

are not appropriate for IID maintenance roads, given that (i) these roads are only used for canal 

maintenance and irrigation duties, and (ii) IID is installing concrete lining on many canals, which 

reduces the use of canal roads for maintenance purposes. For these reasons, ICAPCD 

requested that zero growth be assumed for this source category after 2002. (We note that there 

is a precedent for applying a zero growth assumption to unpaved road dust: for the San Joaquin 

Valley Unified Air Pollution Control District 2003 PM10 SIP, ARB determined16 that the growth 

should be set to zero for dust from unpaved roads.) 

 

11 Note here that the 0.593 and 1.407 winter and summer temporal factors were retained for windblown dust from 

unpaved roads, since the windblown dust study did not estimate emissions from roads. 12 Air Resources Board. Areawide Source Methodologies, Section 7.6, Unpaved Road Dust (Non-Farm Roads), 

August 1997, http://www.arb.ca.gov/ei/areasrc/fullpdf/full7-10prev.pdf. 13 Air Resources Board. “Summary of Fugitive Dust and Ammonia Emission Inventory Changes for the SJVU APCD 

Particulate Matter SIP”, Revision 2.1, May 2003, http://www.valleyair.org/Air_Quality_Plans/docs/ 

2003%20PM10%20Plan/PDF%202003%20PM10%20Plan%20adpt%20app/App%20C-EI%20Changes.pdf. 14 Environ International Corporation, “Draft Final Technical Memorandum, Regulation VIII BACM Analysis”, prepared 

for the Imperial County APCD, October 2005. 15 Communications between the ICAPCD and the Imperial Irrigation District. 16 Air Resources Board. “Summary of Fugitive Dust and Ammonia Emission Inventory Changes for the SJVU APCD 

Particulate Matter SIP, Revision 2.1”. May 2003. 
Imperial County 2009 PM10 SIP Appendix III.A: IC PM10 Uncontrolled Emissions Inventory 

III.8 Baseline and Projected Imperial County PM10 Emissions 

The 2005 baseline and the projected 2006-2010 Imperial County PM10 emission inventories 

derived using the above adjustments are given in Tables III.1-III.4 below. 

Table III.1 PM10 Emission Inventory for Imperial County in Baseline Year 2005a

 (tpd)

Source Category Annual Average Winter Average Summer Average 

Fuel Combustion 0.41 0.35 0.48

Waste Disposal 0.00 0.00 0.00

Cleaning/Surface Coatings 0.00 0.00 0.00

Petroleum Production/Marketing 0.00 0.00 0.00

Industrial Processes: 2.79 2.79 2.78

Mineral Processes 2.63 2.62 2.64 

Food/Agriculture 0.16 0.17 0.14 

Solvent Evaporation 0.00 0.00

Res Fuel Combustion 0.09 0.16 0.02

Farming 9.88 11.55 8.20

Tilling 7.10 8.77 5.42 

Harvest 0.01 0.01 0.01 

Cattle 2.77 2.77 2.77 

Construction 2.20 2.01 2.38

Paved Road Dust 3.38 3.30 3.46

Entrained Unpaved Road Dust: 56.85 33.71 79.98

City/County 24.58 14.58 34.59 

Canal 29.57 17.54 41.61 

BLM/USFS 1.34 0.79 1.88 

Farm 1.35 0.80 1.90 

Windblown Dust: 212.67 223.79 201.95

Open Areas—Urban 0.01 0.02 0.00 

Open Areas—Othersb 169.54 191.09 148.34

Unpaved Roads: 30.52 18.10 42.94 

City/County 7.82 4.64 11.00 

Canal 16.32 9.68 22.96 

BLM/USFS 0.37 0.22 0.52 

Farm 6.01 3.56 8.46 

Non-Pasture Ag Lands 10.81 13.21 8.46 

Pasture 1.79 1.37 2.20 

Fires 0.00 0.00 0.00

Waste Burning 2.77 2.77 2.77

Cooking 0.06 0.06 0.06

On-Road Mobile 1.05 1.06 1.05

Other Mobile 0.99 0.95 1.04

Total 293 282 304 a

Entries corresponding to the summed contributions of subcategories are in italics. bGrasslands, dunes, and other 

barren lands (see Table 3.1 of the main document). As documented in Appendix III.B, emissions were estimated 

using available information on the conditions of the vacant lands (e.g., desert areas of barren, grass/shrubland, and 

dunes). Reported emissions also include the conservatively-estimated contributions due to soil disturbances 

caused by off-road vehicle usage. 

FINAL AUGUST 2009 III.A-5 ICAPCD 
Imperial County 2009 PM10 SIP Appendix III.A: IC PM10 Uncontrolled Emissions Inventory 

Table III.2 Imperial County PM10 Annual-Average Emission Inventorya in 2006-2010 (tpd)

Source Category 2006 2007 2008 2009 2010

Fuel Combustion 0.42 0.42 0.42 0.42 0.43

Waste Disposal 0.00 0.00 0.00 0.00 0.00

Cleaning/Surface Coatings 0.00 0.00 0.00 0.00 0.00

Petroleum Production/Marketing 0.00 0.00 0.00 0.00 0.00

Industrial Processes: 2.83 2.87 2.91 2.98 3.01

Mineral Processes 2.67 2.71 2.74 2.81 2.85 

Food/Agriculture 0.16 0.16 0.16 0.16 0.16 

Solvent Evaporation 0.00 0.00 0.00 0.00 0.00

Res Fuel Combustion 0.09 0.09 0.09 0.09 0.10

Farming: 10.37 10.37 10.36 10.36 10.36

Tilling 7.10 7.09 7.09 7.09 7.08 

Harvest 0.01 0.01 0.01 0.01 0.01 

Cattle 3.26 3.26 3.26 3.26 3.26 

Construction 2.22 2.24 2.26 2.27 2.28

Paved Road Dust 3.62 3.86 4.09 4.32 4.13

Entrained Unpaved Road Dust: 56.84 56.84 56.84 56.84 56.84

City/County 24.58 24.58 24.58 24.58 24.58 

Canal 29.57 29.57 29.57 29.57 29.57 

BLM/USFS 1.34 1.34 1.34 1.34 1.34 

Farm 1.35 1.35 1.35 1.35 1.35 

Windblown: 212.66 212.66 212.64 212.64 212.63

Open Areas-Urban 0.01 0.01 0.01 0.01 0.01 

Open Areas-Othersb 169.54 169.54 169.54 169.54 169.54

Unpaved Roads 30.52 30.52 30.52 30.52 30.52 

City/County 7.82 7.82 7.82 7.82 7.82 

Canal 16.32 16.32 16.32 16.32 16.32 

BLM/USFS 0.37 0.37 0.37 0.37 0.37 

Farm 6.01 6.01 6.01 6.01 6.01 

Non-Pasture Ag Lands 10.80 10.80 10.79 10.79 10.78 

Pasture 1.79 1.78 1.78 1.78 1.78 

Fires 0.00 0.00 0.00 0.00 0.00

Waste Burning 2.75 2.73 2.71 2.69 2.67

Cooking 0.06 0.06 0.06 0.06 0.07

On-Road Mobile 1.01 0.93 0.88 0.82 0.77

Other Mobile 0.98 0.96 0.95 0.95 0.94

Total 294 294 294 294 294 a

Annual averages accounting for projected growth in emission-generating activities, but not for reductions due to 

control or mitigation of PM10 sources. Entries corresponding to the summed contributions of subcategories are in 

italics. b

Grasslands, dunes, and other barren lands (see Table 3.1 of the main document). As documented in 

Appendix III.B, emissions were estimated using available information on the conditions of the vacant lands (e.g., 

desert areas of barren, grass/shrubland, and dunes). Reported emissions also include the conservatively-estimated 

contributions due to soil disturbances caused by off-road vehicle usage. 

FINAL AUGUST 2009 III.A-6 ICAPCD 
Imperial County 2009 PM10 SIP Appendix III.A: IC PM10 Uncontrolled Emissions Inventory 

Table III.3 Imperial County PM10 Winter-Average Emission Inventorya in 2006-2010 (tpd)

Source Category 2006 2007 2008 2009 2010

Fuel Combustion 0.35 0.35 0.35 0.36 0.36

Waste Disposal 0.00 0.00 0.00 0.00 0.00

Cleaning/Surface Coatings 0.00 0.00 0.00 0.00 0.00

Petroleum Production/Marketing 0.00 0.00 0.00 0.00 0.00

Industrial Processes: 2.84 2.88 2.91 2.99 3.02

Mineral Processes 2.67 2.70 2.74 2.81 2.84 

Food/Agriculture 0.17 0.18 0.18 0.18 0.18 

Solvent Evaporation 0.00 0.00 0.00 0.00 0.00

Res Fuel Combustion 0.16 0.16 0.17 0.17 0.17

Farming: 12.04 12.04 12.04 12.03 12.03

Tilling 8.77 8.77 8.76 8.76 8.76 

Harvest 0.01 0.01 0.01 0.01 0.01 

Cattle 3.26 3.26 3.26 3.26 3.26 

Construction 2.04 2.05 2.07 2.08 2.09

Paved Road Dust 3.52 3.76 3.99 4.21 4.03

Entrained Unpaved Road Dust: 33.71 33.71 33.71 33.71 33.71

City/County 14.58 14.58 14.58 14.58 14.58 

Canal 17.54 17.54 17.54 17.54 17.54 

BLM/USFS 0.79 0.79 0.79 0.79 0.79 

Farm 0.80 0.80 0.80 0.80 0.80 

Windblown: 223.77 223.77 223.76 223.76 223.74

Open Areas-Urban 0.02 0.02 0.02 0.02 0.02 

Open Areas-Othersb 191.09 191.09 191.09 191.09 191.09

Unpaved Roads 18.10 18.10 18.10 18.10 18.10 

City/County 4.64 4.64 4.64 4.64 4.64 

Canal 9.68 9.68 9.68 9.68 9.68 

BLM/USFS 0.22 0.22 0.22 0.22 0.22 

Farm 3.56 3.56 3.56 3.56 3.56 

Non-Pasture Ag Lands 13.19 13.19 13.18 13.18 13.17 

Pasture 1.37 1.37 1.37 1.37 1.37 

Fires 0.00 0.00 0.00 0.00 0.00

Waste Burning 2.75 2.73 2.71 2.69 2.67

Cooking 0.06 0.06 0.06 0.06 0.07

On-Road Mobile 1.02 0.93 0.88 0.83 0.78

Other Mobile 0.94 0.91 0.91 0.90 0.90

Total 283 283 284 284 284 a

Winter (November-April) averages accounting for projected growth in emission-generating activities, but not for 

reductions due to control or mitigation of PM10 sources. Entries corresponding to the summed contributions of 

subcategories are in italics. b

Grasslands, dunes, and other barren lands (see Table 3.1 of the main document). As 

documented in Appendix III.B, emissions were estimated using available information on the conditions of the vacant 

lands (e.g., desert areas of barren, grass/shrubland, and dunes). Reported emissions also include the 

conservatively-estimated contributions due to soil disturbances caused by off-road vehicle usage. 

FINAL AUGUST 2009 III.A-7 ICAPCD 
Imperial County 2009 PM10 SIP Appendix III.A: IC PM10 Uncontrolled Emissions Inventory 

FINAL AUGUST 2009 III.A-8 ICAPCD 

Table III.4 Imperial County PM10 Summer-Average Emission Inventorya in 2006-2010 (tpd)

Source Category 2006 2007 2008 2009 2010

Fuel Combustion 0.48 0.49 0.49 0.49 0.49

Waste Disposal 0.00 0.00 0.00 0.00 0.00

Cleaning/Surface Coatings 0.00 0.00 0.00 0.00 0.00

Petroleum Production/Marketing 0.00 0.00 0.00 0.00 0.00

Industrial Processes: 2.82 2.87 2.90 2.97 3.01

Mineral Processes 2.68 2.71 2.75 2.82 2.86 

Food/Agriculture 0.14 0.15 0.15 0.15 0.15 

Solvent Evaporation 0.00 0.00 0.00 0.00 0.00

Res Fuel Combustion 0.02 0.02 0.02 0.02 0.02

Farming: 8.69 8.69 8.69 8.69 8.68

Tilling 5.42 5.42 5.42 5.41 5.41 

Harvest 0.01 0.01 0.01 0.01 0.01 

Cattle 3.26 3.26 3.26 3.26 3.26 

Construction 2.41 2.43 2.44 2.46 2.48

Paved Road Dust 3.71 3.96 4.20 4.43 4.24

Entrained Unpaved Road Dust: 79.98 79.98 79.98 79.98 79.98

City/County 34.59 34.59 34.59 34.59 34.59 

Canal 41.61 41.61 41.61 41.61 41.61 

BLM/USFS 1.88 1.88 1.88 1.88 1.88 

Farm 1.90 1.90 1.90 1.90 1.90 

Windblown: 201.93 201.93 201.92 201.92 201.91

Open Areas-Urban 0.00 0.00 0.00 0.00 0.00 

Open Areas-Othersb 148.34 148.34 148.34 148.34 148.34

Unpaved Roads 42.94 42.94 42.94 42.94 42.94 

City/County 11.00 11.00 11.00 11.00 11.00 

Canal 22.96 22.96 22.96 22.96 22.96 

BLM/USFS 0.52 0.52 0.52 0.52 0.52 

Farm 8.46 8.46 8.46 8.46 8.46 

Non-Pasture Ag Lands 8.44 8.44 8.43 8.43 8.43 

Pasture 2.20 2.20 2.20 2.20 2.20 

Fires 0.00 0.00 0.00 0.00 0.00

Waste Burning 2.75 2.73 2.71 2.69 2.67

Cooking 0.06 0.06 0.06 0.06 0.07

On-Road Mobile 1.00 0.92 0.87 0.82 0.77

Other Mobile 1.03 1.00 1.00 0.99 0.99

Total 305 305 305 306 305 a

Summer (May-October) averages accounting for projected growth in emission-generating activities, but not for 

reductions due to control or mitigation of PM10 sources. Entries corresponding to the summed contributions of 

subcategories are in italics. b

Grasslands, dunes, and other barren lands (see Table 3.1 of the main document). As 

documented in Appendix III.B, emissions were estimated using available information on the conditions of the vacant 

lands (e.g., desert areas of barren, grass/shrubland, and dunes). Reported emissions also include the 

conservatively-estimated contributions due to soil disturbances caused by off-road vehicle usage. 
Imperial County 2009 PM10 SIP Appendix III.A: IC PM10 Uncontrolled Emissions Inventory 

Attachment A 

Mileage and Traffic Information for Imperial County 

Unpaved County Roads 

This attachment is an excerpt of a yearly report from the Imperial County Public Works 

Department providing the best available estimates of the mileage of unpaved county roads with 

average daily traffic above and below 50 VDT. 

FINAL AUGUST 2009 ICAPCD 
FINAL AUGUST 2009 ICAPCD
FINAL AUGUST 2009 ICAPCD
THIS PAGE INTENTIONALLY LEFT BLANK 
Imperial County 2009 PM10 SIP Appendix III.B: Windblown 

Dust Emissions From Vacant Lands 

A PPENDIX III.B 

WINDBLOWN DUST EMISSIONS FROM

VACANT LANDS

FINAL AUGUST 2009 ICAPCD 
Imperial County 2009 PM10 SIP Appendix III.B: Windblown 

Dust Emissions From Vacant Lands 

Appendix III.B 

Windblown Dust Emissions from Vacant Lands 

This appendix provides additional information about windblown dust estimates from vacant 

lands, as calculated by the windblown dust model (ENVIRON, 2004 and later revisions). In 

addition, it considers the impact of off-road activity on the emission inventory for these areas by 

identifying those state and federal lands where off-road activities are allowed and estimating the 

additional levels of emissions (relative to the estimates of Table 3.1 of the SIP document) that 

arise from these lands due to anthropogenic soil disturbance unaccounted for in prior modeling 

assumptions (see Section III.B.2 of this appendix for details). 

FINAL AUGUST 2009 III.B-i ICAPCD 
Imperial County 2009 PM10 SIP Appendix III.B: Windblown 

Dust Emissions From Vacant Lands 

III.B.1 Vacant Lands Classification 

Nearly two-thirds of the land area in Imperial County (1.89 out of 2.87 million acres) falls under 

the jurisdiction of state and federal agencies, including the Bureau of Land Management (BLM), 

the U.S. Fish and Wildlife Service (USFWS), the Bureau of Reclamation, the State of California, 

and the U.S. Military (Figure III.B.1). The overwhelming majority of these areas corresponds to 

vacant lands. The remaining third of Imperial County (labeled as “unclassified” in Figure III.B.1) 

corresponds primarily to parts of the Salton Sea and to agricultural lands (although a fraction of 

these private areas is expected to also be vacant lands), and will not be discussed in this 

appendix. 

In Imperial County, BLM land is divided into 3 classes: 

• Multiple Use Class I areas are areas where Off-Highway Vehicles (OHV) usage is 

permitted. This class includes the Imperial County Sand Dunes to the east, as well as 

the Plaster City, Superstition Mountains, and Arroyo Salado open areas to the west 

(Figure III.B.1). We note that part of the Sand Dunes is currently in administrative 

closure. 

• Multiple Use Class L areas (also called limited-usage areas) are closed to cross-country 

OHV usage, but vehicle travel is allowed on approved/signed routes of travel. This class 

includes: 

o Approximately 455,000 acres of Critical Environmental Concern areas (for which 

road traffic is limited to street legal vehicles). 

o Approximately 465,000 acres where roads are open to all vehicles. 

• Multiple Use Class C areas (also called wilderness areas) are closed to all motorized 

vehicle activity, while open to horseback and foot travel (196,000 acres, plus part of the 

dune areas under administrative closure). 

Approximately 100 thousand acres of vacant lands are under the jurisdiction of the State of 

California. Parts of the following two state parks are located within Imperial County: 

• The Anza-Borrego Desert State Park (west of Superstition Mountains and Plaster City in 

Figure III.B.1) is closed to OHV usage; while 

• The Ocotillo Wells State Vehicular recreation Area (parts of which lie in the north-west 

corner of Imperial County, Figure III.B.1) is open for off-highway exploration and 

recreation. 

Nearly 420 thousand acres of vacant lands are under the jurisdiction of the military. These 

areas are not open to cross-country vehicle use. Lands under the jurisdiction of the Bureau of 

Reclamation and of the U.S. Fish and Wildlife Service (USFWS) account for 110 thousand 

acres, but (owing to the nature/location of these lands, see Figure III.B.1) for only 1.6 tpd of 

windblown PM10 according the results of the Windblown Dust Model. 

FINAL AUGUST 2009 III.B-1 ICAPCD 
Imperial County 2009 PM10 SIP Appendix III.B: Windblown 

Dust Emissions From Vacant Lands 

FINAL AUGUST 2009 III.B-2 ICAPCD 

Table III.B.1 Vacant/Open Lands in Imperial County Under the Jurisdiction of State and Federal 

Agencies

Agency Open to Vehicle Use Area (103 acres) PM10 Emissions (tpd)a

BLM 1,260 83 

 Open Areas OHV 136 8.1 

 Sand Dunes OHV 85 7.5 

 Plaster City OHV 27 0.02 

 Superstition Mountains OHV 13 0.2 

 Arroyo Salado/Ocotillo Wells OHV 11 0.4 

 Limited Areas On trails only 915 69 

 Wilderness Areas No 209 5.4 

State 100 4.3 

 Ocotillo Wells SVRA OHV 28 0.7 

 Other No 72 3.6 

Military No 418 32 

Bureau of Reclamation No 65 0.3 

USFWS No 45 1.3 

Totalb for State/Federal Lands - 1,888 121 a

Windblown dust model emission estimates for non-road, non-pasture land sources using assumptions described in 

Section III.B.2. b

Acreage totals may not be exact due to rounding. Emissions from the remaining 985,000 acres in 

Imperial County predicted by the Windblown Dust Model are 47 tpd (mostly from private sources including agricultural 

lands at 10.8 tpd). Other windblown PM10 emissions not accounted for by the Windblown Dust Model are 32 tpd from 

unpaved roads and pasture lands, for an overall annual average total of ~200 tpd of windblown PM10 emissions in 

Imperial County. 

III.B.2 Modeled Windblown Dust Emissions Estimates

The Windblown Dust Model1

 determines emissions for each land parcel as a function of 

wind speed based on soil emission characteristics such as soil type (derived from the State 

Soil Geographic Database, see Figure III.B.2), Land Use/Land Cover (LULC, derived from 

survey data from the Department of Water Resources and from the U.S. Geological Survey 

National Land Cover Data, see Figure III.B.3), surface stability, and reservoir 

characteristics. Of these characteristics, soil type and surface stability play dominant roles. 

For example, comparative inspection of these figures with results of the windblown dust 

model (Figure III.B.4) reveals that: 

• Sandy, stable soil (either with shrubland or bare rock) emits at very low rates; while 

• Sandy, unstable soil (e.g., dunes) emits at much higher rates; and 

• Sandy loam soils generally emit at tremendously higher rates than sandy soils, and can 

have high emissions even when stable. (We note that most of the sandy loam soil in IC 

corresponds to Shrubland LULC, which explains why this category accounts for such a 

 

1

 Development of a Windblown Fugitive Model and Inventory for Imperial County, California. Final 

Report, May 12, 2004. Prepared by ENVIRON International Corporation and Eastern Research Group 

for the ICAPCD. 
Imperial County 2009 PM10 SIP Appendix III.B: Windblown 

Dust Emissions From Vacant Lands 

FINAL AUGUST 2009 III.B-3 ICAPCD 

high fraction of the total windblown dust emissions within the county as seen in Table 3.1 

of the SIP document). 

Sensitivity analyses2

 have shown that model results are also particularly sensitive to the value 

of the threshold velocity above which soils begin to emit PM10 into the atmosphere. For 

example, emission rates were approximately reduced or increased by a factor of 2 when 

threshold velocities were increased from 15 to 20 mph or decreased from 15 to 10 mph, 

respectively. 

Model Assumptions in Calculation of Imperial County Windblown PM10 Emissions Best 

Estimates. The Windblown Dust Model provides a rigorous and comprehensive treatment 

of windblown dust erosion based on a thorough analysis and use of available information/ 

correlations from the existing literature. Use of the model enables considerable 

improvements in the accuracy of windblown PM10 emissions in Imperial County relative to 

the previously-used CARB methodology (refer to Section 3.1 of the SIP document). 

Nevertheless, because of limitations in the availability of model input information, it was 

necessary to make assumptions about stability and reservoir characteristics, vegetative 

cover, and threshold velocities. Best estimates of windblown dust emissions in Imperial 

County calculated using the windblown dust model3

 (Table 3.1 of the SIP document) were 

obtained assuming: 

• All soils are stable except for the sand dunes (this assumption was made in response to 

comments by CARB and USEPA on the results of the initial study); 

• Stable soils emit for 1 hr; unstable soils for 10 hrs; 

• Threshold velocities of 15 mph; 

• Vegetative canopy cover as listed in Table III.B.2. 

Table III.B.2 Default Vegetation Cover Percentages for Each LULC Type Used in 

Revised Calculation of Windblown Dust Emissions 

Land Use/Land Coverage (LULC) Category Vegetation Cover (%) 

Urban 55 (stable)/0 (unstable) 

Shrubland 11 

Grassland 23 

Mixed Shrub/Grassland 17 

Forest 55 

Barren Lands (Excluding Sand Dunes) 9 

Sand Dunes 0 

As discussed in Section 3.1 of the SIP document, the uncertainty (related to the validity of the 

above assumptions) in accurately representing windblown emissions in Imperial County does 

 

2

 Ibid, Section 8. 

3

 See Appendix A (Technical Memorandum: Latest revisions to the Windblown Dust Model) of the 

Technical Memorandum: Regulation VIII BACM Analysis. ENVIRON, October 2005. 
Imperial County 2009 PM10 SIP Appendix III.B: Windblown 

Dust Emissions From Vacant Lands 

not affect the result of any of the analyses for the key elements of the SIP (DM analysis, 

rulemaking, attainment demonstration, or conformity). 

III.B.3 Estimated Impact of Anthropogenic Disturbance of Vacant Lands Beyond 

Model Assumptions 

We acknowledge that anthropogenic disturbance of soil can lead to higher emissions in wind 

events. 

• The anthropogenic contribution should be estimated as the incremental level of 

emissions relative to the native baseline, i.e., the difference in windblown emissions from 

disturbed soil relative to emissions from native soil; 

• In some cases, this difference is zero. For instance, the sand dunes are fully unstable 

and have 0% vegetative canopy cover in the native state. Therefore, vehicle off-roading 

on these lands does not alter any of the parameters that impact emissions, such as soil 

type, LULC, stability, or reservoir characteristics; 

• In most cases, a higher emission factor would be appropriate for vacant lands disturbed 

by humans. 

Unfortunately, it was not possible to acquire sufficient information to very accurately describe 

both (i) the native state and (ii) the extent of changes in the soil characteristics (such as stability 

and reservoir capacity, canopy cover, etc) for Imperial County lands that are significantly 

disturbed by anthropogenic activity (such as the BLM Open Areas). Nevertheless, in response 

to particular interest in the level of emissions from vacant lands that is due to off-road 

anthropogenic activity, and to establish a complete emission inventory, we conducted additional 

analyses to estimate conservative estimates of the incremental levels of windblown dust 

emissions that might be obtained using assumptions of much more severe disturbance of the 

soils subject to OHV usage. The results of these analyses are summarized in Table III.B.3. 

FINAL AUGUST 2009 III.B-4 ICAPCD 
Imperial County 2009 PM10 SIP Appendix III.B: Windblown 

Dust Emissions From Vacant Lands 

FINAL AUGUST 2009 III.B-5 ICAPCD 

Table III.B.3 Conservative Estimates of Additionala PM10 Emissions Due to Off-Road 

Anthropogenic Disturbanceb

 of Federal and State Vacant Lands in Imperial County.

Area (103 Agency acres) 

Total area Open to Vehicle use

Conservative Estimate of Additionala

PM10 Emissions (tpd) 

BLM 1,260 136 7.5 

 Open Areas 136 136 5.8

 Sand Dunes 85 85 0.9 

 Plaster City 27 27 0.6

 Superstition Mountains 13 13 0.3

 Arroyo Salado/Ocotillo Wells 11 11 4.0

 Limited Areas 915 Negligible Negligible 

 Wilderness Areas 209 Negligible Negligible 

State 100 28 6.3 

 Ocotillo Wells SVRA 28 28 6.3

 Other 72 Negligible Negligible 

Military 418 Negligible Negligible 

Bureau of Reclamation 65 Negligible Negligible 

USFWS 45 Negligible Negligible 

Total ~1,888 165 ~ 12.1 tpd a

Relative to windblown dust model results established in Section 3.1.1 of the SIP document and Section III.B.2 of this 

appendix. b

This analysis only relates to anthropogenic disturbance of soils due to authorized OHV usage excluding 

military OHV usage. This is expected to be the dominant mechanism of soil disturbance of Imperial County vacant 

lands. 

The reasoning is as follows: 

• Superstition Mountains and Plaster City: 

o The soil texture (Figure III.B.5) is mostly sand (which is the soil texture of the 

Sand Dunes), bedrock (which is assumed by the model to generate no PM10

emissions), and a small fraction of sandy loam (which is expected to emit more 

than sand for a specified level of disturbance). 

o The model results (based on the assumptions listed in the previous section, 

Figure III.B.6) show that these areas emit much less than the Sand Dunes per 

acre of land. This is a consequence of the difference in stability and canopy 

cover between these open areas and the sand dunes. 

o Anthropogenic disturbance of these lands can be expected to generate some 

level of instability. Based on information provided by the BLM,4

 we assume here 

that up to 16% of these lands are disturbed by OHV usage. Using this 

assumption, conservative estimates of emissions may be obtained using the rate 

of emissions per acre predicted by the model for the Sand Dune LULC 

classification (corresponding to a fully unstable surface with no vegetation); 

results5

 obtained in this manner are reported in Table III.B.3. This analysis is 

 

4

 Communication to the ICAPCD from the El Centro Field Office of the Bureau of Land Management, 

United States Department of the Interior, July 22, 2009. 5

 The average rate of PM10 emissions from the Sand Dunes based on the information in Table 3.1 of the 

SIP document is 1.3×10-4 tons/day/acre. (Note that this emission rate is higher that the average rate 
Imperial County 2009 PM10 SIP Appendix III.B: Windblown 

Dust Emissions From Vacant Lands 

FINAL AUGUST 2009 III.B-6 ICAPCD 

conservative because (i) the assumption of 16% disturbance is conservative, (ii) 

the extent of disturbance for the disturbed fraction will probably be much less 

than that of the dunes, and (iii) the remaining vegetative cover is certain to be 

greater than 0. 

• Sand Dunes: 

o Of the 85,000 acres of Sand Dune area open to OHV usage, approximately 

58,000 acres correspond to the Sand Dune LULC classification (Figure III.B.7), 

with emissions of 6.6 tpd. These areas are unstable with no vegetation in the 

native state; therefore, human activities on these areas do not reduce the level of 

stability or of vegetative canopy cover, and as a result cause no change in 

windblown dust emissions from these areas. 

o The remaining 27,000 acres correspond primarily to Shrubland LULC (Figure 

III.B.7) on the same soil texture, i.e., sand (Figure III.B.7), accounting for a 

balance of 0.9 tpd of PM10 emissions according to model predictions (Table 

III.B.1). Information provided by the BLM (refer to footnote 4 above) indicates 

that approximately 50% of these lands are disturbed by OHV usage. If we apply 

the rate of PM10 emissions for fully disturbed Sand Dunes LULC to this area in an 

analysis similar to that above for Plaster City and Superstition Mountains, a 

conservative estimate of the excess PM10 emissions (relative to model estimates 

of Table III.B.1) due to OHV usage in the Sand Dune Open Area is therefore 

50% × 1.3×10-4 tpd/acre × 27×103

 acres – 0.9 tpd = 0.9 tpd. 

• Arroyo Salado and Ocotillo Wells: 

o The windblown dust emissions from the Arroyo Salado area (the portion of the 

Ocotillo Wells SVRA under the jurisdiction of the BLM according to Figure III.B.1) 

are dominated by the small fraction featuring sandy loam soil (Figures III.B.5 and 

III.B.6). If we conservatively assume that up to 50% of the area is disturbed and 

that the disturbed soils emit at rates up to 20 times6

 as high as the same soils in 

 calculated using the information in Table III.B.1 for the Sand Dune Open Area; the difference is primarily 

due to the fact that a portion of the Sand Dune Open Area corresponds to Shrubland LULC, which emits 

at much lower rates.) Applying this factor to the Plaster City and Superstition Mountain areas and 

assuming that the baseline emissions from these lands in the native (undisturbed) state are negligible 

(this is the most conservative analysis), the excess emissions due to anthropogenic activity from these 

areas are 16% × 1.3×10-4 tpd/acre × 27×103

 acres = 0.6 tpd for the Plaster City Open Area and 16% × 

1.3×10-4 tpd/acre × 13×103

 acres = 0.3 tpd for the Superstition Mountains Open Area. 6

 We rely here on information contained in the reference of footnote 1 (ENVIRON, May 12, 2004). Based 

on the data in Figures 2.6 and 2.7 of that report, hourly emission rates for sandy loam soils (soil group 

code = 2, see Table 3.2) are on average twice as high for unstable than for stable surfaces (1-3 times 

as high depending on wind speed). Because the Windblown Dust Model assumed that unstable soils 

could emit for up to 10 times a long as stable soils (10 hours instead of 1 hour), in wind events ≥ 10 

hours with continuous wind speeds above the 15 mph entrainment threshold, unstable sandy loam soils 

may be expected (according to the model) to emit PM10 levels ~ 20 times a high as stable sandy loam 

soils. Note that in wind events of shorter duration (in terms of the consecutive number of hours with 
Imperial County 2009 PM10 SIP Appendix III.B: Windblown 

Dust Emissions From Vacant Lands 

FINAL AUGUST 2009 III.B-7 ICAPCD 

the natural state, the excess PM10 emissions due to anthropogenic disturbance of 

the Arroyo Salado area are 50%×0.4 tpd×20 = 4 tpd. (Note that in the calculation 

of this conservative upper bound we have again assumed that emissions in the 

native, undisturbed state are negligible.) 

o The windblown dust emissions from the remainder of the Ocotillo Wells SVRA 

are also dominated by the small fraction featuring sandy loam soil. Following the 

same analysis as above, a conservative upper bound of the excess PM10

emissions (relative to the model prediction) due to anthropogenic disturbance are 

50%×0.7 tpd×20 -0.7 tpd = 6.3 tpd. 

To complete the above analysis, we next assess the levels of incremental PM10 emissions due 

to anthropogenic disturbance of soils as a result of OHV usage on non-OHV-designated vacant 

lands. In doing so, we again apply the emission rate per acre for the Sand Dunes LULC to the 

estimated acreage of areas subject to non-designated vehicle off-roading. The results are 

incorporated in Table III.B.4, which summarizes the results of our windblown dust analyses 

according to LULC (enabling cross-referencing and comparison with results of Table 3.1 of the 

SIP document). 

III.B.4 Summary 

According to the foregoing analyses, a conservative estimate of additional7 windblown PM10

emissions due to anthropogenic activities8

 on Imperial County vacant lands is 12.2 tpd. (The 

actual number is likely to be smaller, based on the nature of the conservative assumptions used 

in the above analysis.) This can be compared to the estimated ~200 tpd annual average 

emissions for windblown PM10 emissions, which includes ~32 tpd of windblown dust emissions 

from unpaved roads and pasture land (as estimated using CARB’s methodology) and ~168 tpd 

from other windblown dust sources (as estimated by the windblown dust model). A summary of 

results according to LULC is given in Table III.B.4. 

 wind speeds above the threshold), the ratio of emissions for disturbed vs. undisturbed soils is 

correspondingly smaller. 7

 Relative to windblown dust model results established in Section 3.1.1 of the SIP document and Section 

III.B.2 of this appendix, and obtained using assumptions of much more severe levels of disturbance of 

the soils subject to OHV usage. 8

 Focusing on OHV usage on Imperial County vacant lands, and excluding disturbance of vacant lands 

due to military activities on lands under military jurisdiction. 
Imperial County 2009 PM10 SIP Appendix III.B: Windblown 

Dust Emissions From Vacant Lands 

FINAL AUGUST 2009 III.B-8 ICAPCD 

Table III.B.4 Summary Table of Modeled PM10 Emissions (Reported According to LULC) and 

Conservative Estimates of Additional PM10 Emissions Resulting from Assumptions of 

More Severe Impacts of OHV Usage on Soils Disturbed by OHV Usage.

Category Areaa 

(103

 acres) 

Modeled annual 

PM10 Emissionsb

(tpd) 

Conservative Estimate of additionalc

PM10 emissions due to OHV usage 

(tpd) 

Agriculture 559 10.81 

 Designated for OHV usage 0.3 Negligible 

 Closed to OHV usage 558 10.81 

- 

Desert-Dunes 151 19.85 - 

 Designated for OHV usage 58 7.77 - 

 Closed to OHV usage 93 12.08 - 

 Undesignated OHV usage 0.3 - 

Desert-Other 588 38.74 11.0 

 Designated for OHV usage 66 1.43 11.0d 

 Closed to OHV usage 522 37.31 0.04 

 Undesignated OHV usage 0.3 0.04e 

Shrubland/Grassland 1337 98.75 1.1 

 Designated for OHV usage 39 0.80 1.1f

 Closed to OHV usage 1298 97.95 0.03

 Undesignated OHV usage 0.2 0.03e 

Urban Open Areas 40 0.01 

 Designated for OHV usage 1 Negligible 

 Closed to OHV usage 39 0.01 

- 

Total 2675 168.16 12.2 a

Areas where undesignated OHV usage takes place were conservatively estimated to be equal to 0.5% of the 

designated OHV usage area for the specified category. b

Emissions estimates of the windblown dust model as 

described in Section 3.1.1 of the SIP document and Section III.B.2 of this appendix. c

Relative to windblown dust 

model results established in Section 3.1.1 of the SIP document and Section III.B.2 of this appendix. d

Corresponding 

to additional emissions (Table III.B.3) from the Ocotillo Wells SVRA, Arroyo Salado Open Area, Superstition 

Mountains Open Area, and ~2/3 of the Plaster City Open Area as seen in Figure III.B.7. eAdditional emissions due to 

OHV usage in undesignated areas closed to OHV usage were calculated by applying the emission rate for the Sand 

Dunes LULC to the areas where undesignated OHV usage may take place (e.g., 1.3×10-4 tpd/acre × 0.2×103 acres = 

0.03 tpd for Shrubland/Grasslands). f

Corresponding to additional emissions (Table III.B.3) from the Sand Dunes 

Open Area and ~1/3 of the Plaster City Open Area as seen in Figure III.B.7. 
Sand Dunes

Plaster City

Superstition Mountains

Arroyo Salado

Octotillo Wells SVRA

1:650,00

0 1 5 0 

Scale in Kilom

1. Off Highway Vehicle Are does not include areas c through administrative c

NOTES:

DRAWN BY:

MMM

DATE:

APPROVED: REVISED:

CONTRACT NUMBER

07/08/09

0521191A

Federal and State Ma

Lands in Imperial C

Projection set to:

NAD 1983 CTM
1:625,00

0 1 5 0 

Scale in Kilom

DRAWN BY:

MMM

DATE:

APPROVED: REVISED:

CONTRACT NUMBER

07/08/09

0521191A

Soil Texture Classific

Projection set to: in Imperial Count

NAD 1983 CTM
1:725,0

0 10 

5

Scale in Kilom

DRAWN BY:

MMM

DATE:

APPROVED: REVISED:

CONTRACT NUMBER

07/08/09

0521191A

Land Cover/Land U

Projection set to: Classification in Imperia

NAD 1983 CTM
1:700

0 10 

5

Scale in Kilo

DRAWN BY:

MMM

DATE:

APPROVED: REVISED:

CONTRACT NUMBER

07/08/09

0521191A

Modeled Windblown Du

Projection set to: Emissions in Imperial 

NAD 1983 CTM
1:675,000

0 10 

5

Scale in Kilomet

DRAWN BY:

MMM

DATE:

APPROVED: REVISED:

CONTRACT NUMBER

07/08/09

0521191A

Soil Texture Classifica

in Imperial County 

Showing Areas Open 

Off-Highway Vehicle U

Projection set to:

NAD 1983 CTM
1:67

0 10 

5

Scale in K

DRAWN BY:

MMM

DATE:

APPROVED: REVISED:

CONTRACT NUMBER

07/08/09

0521191A

Modeled Windblown Dust PM10 

in Imperial County 

Showing Areas Open 

Off-Highway Vehicle Us

Projection set to: NAD 1983 CTM
1:725,0

0 10 

5

Scale in Kilom

DRAWN BY:

MMM

DATE:

APPROVED: REVISED:

CONTRACT NUMBER

07/08/09

0521191A

Land Cover/Land Use Clas

in Imperial County

Showing Areas Open 

Off-Highway Usag

Projection set to: NAD 1983 CTM
THIS PAGE INTENTIONALLY LEFT BLANK 
Imperial County 2009 PM10 SIP Appendix IV.A: ICAPCD Regulation VIII Rules 

A PPENDIX IV.A 

IMPERIAL COUNTY FUGITIVE DUST CONTROL PROGRAM:

REGULATION VIII RULES

FINAL AUGUST 2009 ICAPCD 
Imperial County 2009 PM10 SIP Appendix IV.A: ICAPCD Regulation VIII Rules 

Appendix IV.A 

Imperial County Fugitive Dust Control Program: 

Regulation VIII Rules 

This Appendix reports the Imperial County APCD Regulation VIII rules addressing mitigation of 

fugitive dust as found in the ICAPCD Rulebook (available online at 

http://www.co.imperial.ca.us/AirPollution/Forms%20&%20Documents/RULEBOOK/APCD%20R

ules%20Feb%203%202009.pdf). 

 

FINAL AUGUST 2009 IV.A-i ICAPCD 
RULE 800 GENERAL REQUIREMENTS FOR CONTROL OF FINE PARTICULATE 

MATTER (PM-10) 

(Adopted 10/10/94; revised 11/25/96; revised 11/08/2005) 

A. General Description 

The purpose of this regulation is to reduce the amount of fine Particulate Matter (PM-10) 

entrained in the ambient air as a result of emissions generated from anthropogenic 

(man-made) Fugitive Dust (PM-10) sources generated from within Imperial County by 

requiring actions to prevent, reduce, or mitigate PM-10 emissions. The Rules contained 

within this Regulation have been developed pursuant to United States Environmental 

Protection Agency guidance for Serious PM10 Non Attainment Areas. 

B. Applicability 

The requirements of this rule shall apply to any Active Operation, and/or man-made or 

man-caused condition or practice capable of generating Fugitive Dust (PM-10) as 

specified in this Regulation except those determined exempt as defined in Part E of this 

Rule. The definitions, exemptions, requirements, administrative requirements 

recordkeeping requirements, and test methods set forth in this rule are applicable to all 

the rules under Regulation VIII (Fugitive Dust Requirements) of the Rules and 

Regulations of the Imperial County Air Pollution Control District. 

C. Definitions 

 For the purpose of this Regulation, the following terms are defined: 

 C.1 ACTIVE OPERATION: Activities capable of generating Fugitive Dust (PM-10), 

including but not limited to, Earthmoving Activities, Construction activities,

Unpaved Roads, Track-Out/Carry-Out, Bulk Material storage and transport, 

Unpaved Haul/Access Roads. 

 C.2 AGGREGATE MATERIALS: Consists of sand, Gravel, quarried stone and/or 

rock fragments that are typically used in Construction. Aggregates may be 

natural, artificial or recycled. 

 C.3 ANEMOMETRS: Are devices used to measure wind speed and direction in 

accordance with manufacturer’s performance standards, maintenance and 

calibration criteria. 

 C.4 ANNUAL AVERAGE DAILY VEHICLE TRIPS: annual average 24-hour total 

of all vehicles counted on a road. 

 C.5 APCD: The Imperial County Air Pollution Control District. 

C.6 APCO: The Imperial County Air Pollution Control Officer. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
C.7 AVERAGE VEHICLE TRIPS PER DAY: Means the average number of vehicles 

that cross a given point surface during a specific 24-hour period as determined by 

the most recent Institute of Transportation Engineers trip generation manual, tube 

counts, or observations. 

C.8 BLM: The Bureau of Land Management. 

C.9 BP: The United States Border Patrol. 

 C.10 BULK MATERIAL: Earth, rock, Silt, sediment, sand, Gravel, soil, fill, 

Aggregate, dirt, mud, debris, and other organic and/or inorganic material 

consisting of or containing Particulate Matter with five percent or greater Silt 

content. For the purpose of this Regulation, the Silt content level is assumed to be 

5 percent or greater, unless the Person responsible for the Active Operation 

conducts the applicable laboratory tests and demonstrate that the Silt content is 

less than 5 percent. Active Operations seeking to determine if the Silt content is 

less than five percent are required to conduct the laboratory analysis in 

accordance with ASTM method C-136-a (Standard Test Method for Sieve 

analysis of Fine and Coarse Aggregates), or other equivalent test methods 

approved by EPA, ARB, and the APCD. 

 C.11 CANAL BANK: A rise of land on either side of an irrigation canal. 

 

 C.12 CHEMICAL STABILIZATION/SUPPRESSION: A means of Fugitive Dust 

(PM-10) control implemented to mitigate PM-10 emissions by applying 

petroleum resins, asphaltic emulsions, acrylics, adhesives, or any other materials 

approved for use by the California Air Resources Board (CARB), U.S. 

Environmental Protection Agency (U.S. EPA) and/or the APCO. 

 C.13 CONSTRUCTION: Any on-site mechanical activities preparatory to or related to 

the building, alteration, rehabilitation, or demolition of an improvement on real 

property, including, but not limited to, land clearing, excavation related to 

construction, land leveling, grading, cut and fill grading, and the erection or 

demolition of any structure. As used in Regulation VIII, a construction site may 

encompass several contiguous parcels, or may encompass only a portion of one 

parcel, depending on the relationship of the property boundaries to the actual 

construction activities.

 C.14 DESIGNATED REPRESENTATIVE: The agent for a Person. The Designated 

Representative shall be responsible for and have the full authority to implement 

BACM on behalf of the Person. 

 C.15 EARTHMOVING ACTIVITIES: The use of any equipment for an activity that 

may generate Fugitive Dust emissions, including, but not limited to, cutting and 

filling, grading, leveling, excavation, trenching, loading or unloading of Bulk 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
Materials, demolishing, drilling, adding to or removing bulk of materials from 

open storage piles, weed abatement through disking, and back filling. 

 C.16 FUGITIVE DUST: The Particulate Matter entrained in the ambient air which is 

caused from man-made and natural activities such as, but not limited to, 

movement of soil, vehicles, equipment, blasting, and wind. This excludes 

Particulate Matter emitted directly in the exhaust of motor vehicles or other fuel 

combustion devices, from portable brazing, soldering, or welding equipment, pile 

drivers, and stack emissions from stationary sources. 

 C.17 GRAVEL: Gravel travelways shall have a three (3) inch minimum depth 

Stabilized Surface. The travelway shall have a relative compaction of not less 

than 95% as determined by Test Method No. California 216 of State of California, 

Business and Transportation Agency Department of Transportation, and 

conforming to the following grading: 

 3⁄4” Maximum 

 Sieve Designation Percent Passing 

 1” 100 

 3⁄4” 90-100 

 #4 35-60 

 #30 10-30 

 #200 

 2-9 

 Reference: California Department of Transportation Standard Specification 

 Section 26/class II Aggregate Base 

 

 C.18 HAUL/ACCESS ROAD: Any on-site road used for commercial, industrial, 

institutional, and/or governmental traffic. 

C.19 HAUL TRUCK: Any fully or partially open-bodied licensed motor vehicle used 

for transporting Bulk Material for industrial or commercial purposes. 

 C.20 IMPLEMENT OF HUSBANDRY: An unlicensed vehicle which is used 

exclusively in the conduct of Agricultural Operations. An Implement of 

Husbandry does not include a vehicle if its existing design is primarily for the 

transportation of persons or property on a highway, unless specifically designated 

as such by some other provision of the Vehicle Code of California. 

 C.21 NON-RESIDENTIAL AREA: Any unpaved vehicle and equipment traffic area 

operated at any commercial, manufacturing or government sites. 

 C.22 MODIFIED PAVED ROAD: Any Paved Road that is widened or improved so as 

to increase traffic capacity. This term does not include road maintenance, repair, 

chip seal, pavement or roadbed rehabilitation that does not affect roadway 

geometrics, or surface overlay work. 

 C.23 OFF-FIELD AGRICULTURAL SOURCE: Any Agricultural Source or activity at 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
an Agricultural Source that falls into one or more of the following categories: 

 C.23.a Outdoor handling, storage and transport of Bulk Material; 

 C.23.b Paved Road; 

 C.23.c Unpaved Road; or 

 C.23.d Unpaved Traffic Area. 

 C.24 OFF-ROAD VEHICLE: Any nonstationary device, powered by an internal 

combustion engine or motor, used primarily off the highways to propel, move, or 

draw persons or property including any device propelled, moved, or drawn 

exclusively by human power, and used in, but not limited to, any of the following 

applications: marine vessels, construction/farm equipment, utility and lawn and 

garden equipment, off-road motorcycles, and off-highway vehicles. 

 C.25 ON-FIELD AGRICULTURAL SOURCE: Any Agricultural Source or activity at 

an Agricultural Source that is not an Off-Field Agricultural Source, including (but 

not limited to) the following: 

 C.25.a Activities conducted solely for the purpose of preparing land for the 

growing of crops or the raising of fowl or animals, such as brush or timber 

clearing, grubbing, scraping, ground excavation, land leveling, grading, 

turning under stalks, disking, or tilling; 

 C.25.b Drying or pre-cleaning of agricultural crop material on the field where it 

was harvested; 

 C.25.c Handling or storage of agricultural crop material that is baled, cubed, 

pelletized, or long-stemmed, on the field where it was harvested, and the 

handling of fowl or animal feed materials at sites where animals or fowl 

are raised; 

 C.25.d Disturbances of cultivated land as a result of fallowing, planting, 

fertilizing or harvesting. 

C.26 OPEN AREA: Any of the following described in Subsection C.26.a through 

C.26.c of this rule. For the purpose of this rule, vacant portions of residential or 

commercial lots and contiguous parcels that are immediately adjacent to and 

owned and/or operated by the same individual or entity are considered one open 

area. An open area does not include any Unpaved Traffic Area as defined in this 

rule. 

 

C.26.a An un-subdivided or undeveloped land adjoining a developed (or partially 

developed) residential, industrial, institutional, governmental, or 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
commercial area. 

C.26.b A subdivided residential, industrial, institutional, governmental, or 

commercial lot, which contains no approved or permitted building or 

structures of a temporary or permanent nature. 

C.26.c A partially developed residential, industrial, institutional, governmental, or 

commercial lot and contiguous lots under common ownership. 

C.27 PARTICULATE MATTER: Any material, except uncombined water, which 

exists in a finely divided form as a liquid or solid at 60 degrees F and one 

atmosphere pressure. 

C.28 PAVED ROADS: An improved street, highway, alley, public way, that is covered 

by concrete, asphaltic concrete, or asphalt. 

C.29 PERSON: Any individual, public or private corporation, partnership, association, 

firm, trust, estate, municipality, or any other legal entity whatsoever which is 

recognized by law as the subject of rights and duties, who is responsible for an 

Active Operation. 

 C.30 PM-10: Particulate Matter with an aerodynamic diameter smaller than or equal to 

a nominal 10 microns as measured by the applicable State and Federal reference 

test methods. 

 C.31 RECREATIONAL USE: The use of motorized vehicles on public lands. 

 C.32 RURAL: Areas not classified as urban constitute “rural.” 

 

 C.33 SILT: Any Aggregate Material with a particle size less than 75 micrometers in 

diameter as measured by a No. 200 sieve as defined in ASTM D-2487 and as 

tested by ASTM-C-136 or other equivalent test methods approved by EPA, ARB, 

and the APCD. 

 C.34 STABILIZED SURFACE: Any disturbed surface area or open bulk storage pile 

that is resistant to wind blown Fugitive Dust emissions. A surface is considered 

to be stabilized if it meets at least one of the following conditions specified in this 

Section and as determined by the test methods specified in Appendix B, Section 

A, B and D-G tests of this rule: 

 C.34.a A visible crust; or 

 C.34.b A threshold friction velocity (TFV) for disturbed surface areas corrected 

for non-erodible elements of 100 centimeters per second or greater; or 

 C.34.c A flat vegetative cover of at least 50 percent that is attached or rooted 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
vegetation; or unattached vegetative debris lying on the surface with a 

predominant horizontal orientation that is not subject to movement by 

wind; or 

 C.34.d A standing vegetative cover of at least 30 percent that is attached or rooted 

vegetation with a predominant vertical orientation; or 

 C.34.e A standing vegetative cover that is attached or rooted vegetative with a 

predominant vertical orientation that is at least 10 percent and where the 

TFV is at least 43 centimeters per second when corrected for non-erodible 

elements; or 

 C.34.f A surface that is greater than or equal to 10 percent of non-erodible 

elements such as rocks, stones, or hard-packed clumps of soil. 

C.35 STABILIZED UNPAVED ROAD: Any Unpaved Road or unpaved 

vehicle/equipment traffic area surface which meets the definition of Stabilized 

Surface as determined by the test method in Appendix B, Section C of this rule, 

and where VDE is limited to 20% opacity. 

C.36 TACTICAL TRAINING: Training conducted by the U.S. Department of Defense, 

the U.S. military services, or its allies for combat, combat support, combat service 

support, tactical or relief operations. Examples include but are not limited to 

munitions training. 

C.37 TEMPORARY UNPAVED ROAD: Any Unpaved Road surface which is created 

to support a temporary or periodic activity and the use of such road surface is 

limited to vehicle access for a period of not more than six months during any 

consecutive three-year period. 

C.38 THRESHOLD FRICTION VELOCITY (TFV): The corrected velocity necessary 

to initiate soil erosion as determined by the test method specified in Appendix B, 

Section D, of this rule. The lower TFV, the greater the propensity for fine 

particles to be lifted at relatively low wind speeds. 

C.39 TRACK-OUT/CARRY-OUT: Any and all Bulk Materials that adhere to and 

agglomerate on the exterior surfaces of motor vehicles and/or equipment 

(including tires) that may then fall onto the pavement. 

 C.40 TRACK-OUT PREVENTION DEVICE: A Gravel pad, grizzly, wheel wash 

system, or a paved area, located at the point of intersection of an unpaved area 

and a Paved Road that prevents or controls Track-Out. 

 

 C.41 UNPAVED ROADS: Streets, alley ways, or roadways that are not covered by one 

of the following: concrete, asphaltic concrete, asphalt, or other similar materials 

specified by the U.S.EPA, CARB and/or the APCO. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
 C.42 UNPAVED TRAFFIC AREA: Any nonresidential area that is: 

 C.42.a Not covered by asphalt, recycled asphalt, asphaltic concrete, concrete, or 

concrete pavement, and 

 C.42.b Used for fueling and servicing; shipping, receiving and transfer; or 

parking or storing equipment, haul trucks, vehicles, and any conveyances. 

 C.43 URBAN AREA: An area within an incorporated city boundary or within 

unincorporated areas completely surrounded by an incorporated city. 

 C.44 VDE: Visible dust emissions. Dust emissions that are visible to an observer. 

 C.45 VMT: Vehicle miles traveled. 

C.46 WIND GUST: Is the maximum instantaneous wind speed as measured by an 

anemometer. 

D. Compliance Schedule 

D.1 Existing sources subject to this Regulation shall comply with its requirements no 

later than 90 days after its adoption date. 

D.2 New sources subject to this Regulation shall comply with its requirements prior to 

initiation of activity. 

D.3 The BLM and BP shall each comply with the following compliance schedule: 

 D.3.a Submit a draft dust control plan addressing all applicable portions of this 

Regulation including section F.5 within three (3) months of the adoption 

date of this rule, to which the APCO shall respond within 60 days; 

 D.3.b Submit a final dust control plan addressing all APCO comments within 

two (2) months after receiving APCO’s comments, which the APCO shall 

transmit to CARB and U.S. EPA for 45-day review and comment; 

 D.3.c Implement all final dust control plan elements within six (6) months of 

submittal; and 

D.3.d Submit an updated dust control plan every two calendar years by the 

procedures described in D.3.a to D.3.c. The updated plans shall be 

transmitted to the District no later than 90 days after the end of the 

calendar year and, in addition to information required of the initial plan, 

shall include a summary of actions taken to prevent or mitigate PM10 

emissions during the previous two years. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
E. Exemptions 

The following activities are exempt from provisions of this Regulation: 

E.1 Actions required by the Federal or State Endangered Species Act or any order 

issued by a court or governmental agency. 

E.2 Off-Field Agricultural Sources necessary to minimize or respond to adverse 

effects on agricultural crops caused during freezing temperatures as declared by 

the National Weather Service. 

 E.3 Emergency maintenance of flood control channels and water spreading basins. 

E.4 Any emergency operation activities performed to ensure public health and safety. 

Emergency activities lasting more than 30 days shall be subject to this Regulation, 

except where compliance would limit the effectiveness of the emergency activity 

performed to ensure public health and safety. 

E.5 Blasting operations permitted by the California Division of Industrial Safety. 

Other activities performed in conjunction with blasting are not exempt from 

complying with the provisions of this rule. 

 E.6 The Recreational Use of public lands covered by the most recent BLM dust 

control plan that complies with Rule 800, including but not limited to Off-Road 

Vehicles, all-terrain vehicles, trucks, cars, motorcycles, motorbikes or 

motorbuggies. 

 E.7 The following military training activities conducted by the Department of 

Defense: (1) military Tactical Training, (2) maintenance, repair, and removal of 

targets and munitions associated with military Tactical Training, (3) open areas on 

active military ranges, including but not limited to designated impact areas, 

landing zones, and bivouac areas. Other activities performed in conjunction with 

military Tactical Training are not exempt from complying with the provisions of 

this rule.

F. General Requirements 

F.1 Materials used for Chemical Stabilization of soils, including petroleum resins, 

asphaltic emulsions, acrylics, and adhesives shall not violate State Water Quality 

Control Board standards for use as a soil stabilizer. Materials accepted by the 

California Air Resources Board (ARB) and the United States Environmental 

Protection Agency (EPA), and which meet State water quality standards, shall be 

considered acceptable to the ICAPCD. 

F.2 Any material prohibited for use as dust Suppressant by EPA, the ARB, or other 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
applicable law, rule, or regulation is also prohibited under Regulation VIII. 

F.3 Use of hygroscopic materials may be prohibited by the APCD in areas lacking 

sufficient atmospheric moisture of soil for such materials to effectively reduce 

Fugitive Dust emissions. The atmospheric moisture of soil is considered to be 

sufficient if it meets the application specifications of the hygroscopic product 

manufacturer. Use of such materials may be approved in conjunction with 

sufficient wetting of the controlled area. 

F.4 Any use of dust Suppressants or gravel pads, and paving materials such as 

asphalt or concrete for paving, shall comply with other applicable District Rules. 

F.5 Bureau of Land Management (BLM) Requirements 

The BLM shall prepare a dust control plan to minimize PM10 emissions for 

sources under the control of BLM. The dust control plan shall include at a 

minimum the following: 

F.5.a A stipulation that all new authorizations for point and area stationary 

emission sources obtain all necessary permits and satisfy all applicable 

SIP provisions, including Regulation VIII specific control measures; 

F.5.b A summary of: the total miles of BLM roads that are paved, paved with 

unpaved shoulders, and unpaved roads with 50 or more average vehicle 

trips per day, including length and level of usage of each such road; the 

priority for control of road segments based on annual and episodic (e.g. 

event) usage; the plans for control of PM-10 emissions from these roads; 

the location and extent (e.g. acreage) of open areas disturbed by legal and 

illegal Recreational Use; the priority for control of these open areas based 

on annual and episodic (e.g. event) usage; the plans for control of PM-10 

emissions from these areas; 

F.5.c BLM must demonstrate in its dust control plan that Unpaved Roads, 

parking, and Open Areas are controlled pursuant to the applicability and 

requirements of Rules 804 and 805 except where measures are 

demonstrated by BLM to be prohibited by federal or state laws, 

regulations, or approved plans concerning wilderness preservation and 

species management and recovery. 

 

F.5.d Where compliance with any control measure in Rules 804 and 805 is 

prohibited pursuant to F.5.c, the dust control plan must discuss and 

commit to implement other possible control measures, such as vehicle 

speed limits. 

F.5.e The dust control plan must describe all PM-10 control measures that will 

be implemented, such as restricted use areas, stabilization of Unpaved 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
Traffic Areas and current Recreation Area Management Plan (RAMP) 

measures, to reduce PM10 emissions during off-road events and/or 

competitions on public land and include all those measures that are 

feasible and not prohibited by the laws, regulations and plans described in 

F.5.c; 

F.5.f Use BLM-standard road design and drainage specifications when 

maintaining existing roads or authorizing road maintenance and new road 

construction; and

F.5.g Include public educational information on reducing PM-10 emissions with 

BLM open area literature (e.g. identification of restricted areas and/or 

applicable speed limits) and on related information signs in heavily used 

areas. 

F.6. Border Patrol (BP) Requirements 

The BP shall prepare a dust control plan designed to minimize PM10 emissions 

from sources under the control of the BP. The dust control plan shall include 

those dust control measures found in Rules 804 and 805. The dust control plan 

shall include the following fugitive dust control measures: 

F.6.a A stipulation that all new authorizations for point and area stationary 

emission sources obtain all necessary permits and satisfy all applicable 

SIP provisions, including Regulation VIII specific control measures; 

F.6.b Implement alternatives to tire-dragging that result in fewer PM10 

emissions, unless BP demonstrates such alternatives to be inconsistent 

with the monitoring of immigration across the U.S.-Mexico border; and 

F.6.c Control dust emissions from certain Unpaved Roads and routes owned or 

operated by the BP as identified through general BP planning consistent 

with Rule 805 unless those dust control measures are demonstrated to be 

inconsistent with BP authority and/or mission. 

G. Administrative Requirements 

 G.1 Test Methods 

G.1.a Determination of VDE Opacity 

Opacity observations to determine compliance with VDE standards shall 

be conducted in accordance with the test procedures for “Visual 

Determination of Opacity” as described in Appendix A of this rule. 

Opacity observations for sources other than unpaved traffic areas (e.g., 

roads, parking areas) shall be conducted per Section B of Appendix A and 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
shall require 12 readings at 15-second intervals. 

G.1.b Determination of Stabilized Surface 

Observations to determine compliance with the conditions specified for a 

stabilized surface, in any inactive disturbed surface area, whether at a 

work site that is under construction, at a work site that is temporarily or 

permanently inactive, or on an open area and vacant lot, shall be 

conducted in accordance with the test methods described in Appendix B of 

this rule. If a disturbed surface area passes any of the applicable Appendix 

B-Section A, B and D-G tests, then the surface shall be considered 

stabilized. 

G.1.c Determination of Soil Moisture Content 

Soil moisture content shall be determined by using ASTM Method D2216-

98 (Standard Test Method for Laboratory Determination of Water 

[Moisture] Content of Soil and Rock by Mass), or other equivalent test 

methods approved by the EPA, ARB, and the APCO. 

G.1.d Determination of Silt Content for Bulk Materials 

Silt content of a Bulk Material shall be determined by ASTM Method 

C136a (Standard Test Method for Sieve Analysis of Fine and Coarse 

Aggregates), or other equivalent test methods approved by EPA, ARB, 

and the APCD. 

G.1.e Determination of Silt Content for Unpaved Roads and Unpaved 

Vehicle/Equipment Traffic Areas 

 Silt Content for Unpaved Roads and Unpaved Traffic Areas shall be 

determined by using Section C of Appendix B of this Rule or other 

equivalent test methods approved by EPA, ARB, and the APCO. 

G.1.f Determination of Threshold Friction Velocity (TFV) 

 TFV shall be determined by using Section D of Appendix B of this Rule 

or other equivalent test methods approved by EPA, ARB, and the APCO. 

H. Record of Control Implementation 

Any Person subject to the requirements of this rule shall compile and retain records that 

provide evidence of control measure application and compliance with this rule (i.e., 

receipts and/or purchase records). Such Person shall describe, in the records, the type of 

treatment or control measure, extent of coverage, and date applied. For control measures 

which require multiple daily applications, recording the frequency of application will 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
fulfill the recordkeeping requirements of this rule (i.e., water being applied three times a 

day and the date) Records shall be maintained and be readily accessible for two years 

after the date of each entry and shall be provided to the APCD upon request. 

I. Violations 

Failure to comply with any provisions of this rule shall constitute a violation of 

Regulation VIII. Failure to comply with the provisions of an APCO approved dust 

control plan shall also constitute a violation of this Regulation. Regardless of whether an 

APCO approved dust control plan is being implemented or not, or whether a Person 

responsible for an Active Operation(s) is complying with an approved dust control plan, 

the Person is still subject to the requirements of Regulation VIII at all times. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
APPENDIX A 

Visual Determination of Opacity 

SECTION A Test Method For Unpaved Roads and Unpaved Traffic Areas 

SECTION B Test Method For Time-Averaged Regulations 

SECTION A TEST METHOD FOR UNPAVED ROADS AND UNPAVED TRAFFIC 

AREAS 

A. Opacity Test Method. The purpose of this test method is to estimate the percent opacity of 

Fugitive Dust plumes caused by vehicle movement on Unpaved Roads and Unpaved 

Traffic Areas. This method can only be conducted by an individual who has current 

certification as a qualified observer. 

A.1 Step 1: Stand at least 16.5 feet from the fugitive dust source in order to provide a 

clear view of the emissions with the sun oriented in the 140° sector to the back. 

Following the above requirements, make opacity observations so that the line of 

vision is approximately perpendicular to the dust plume and wind direction. If 

multiple plumes are involved, do not include more than one plume in the line of 

sight at one time. 

A.2 Step 2: Record the Fugitive Dust source location, source type, method of control 

used, if any, observer’s name, certification data and affiliation, and a sketch of the 

observer’s position relative to the Fugitive Dust source. Also, record the time, 

estimated distance to the Fugitive Dust source location, approximate wind direction, 

estimated wind speed, description of the sky condition (presence and color of 

clouds), observer’s position to the Fugitive Dust source, and color of the plume and 

type of background on the visible emission observation form both when opacity 

readings are initiated and completed. 

A.3 Step 3: Make opacity observations, to the extent possible, using a contrasting 

background that is perpendicular to the line of vision. Make opacity observations 

approximately 1 meter above the surface from which the plume is generated. Note 

that the observation is to be made at only one visual point upon generation of a 

plume, as opposed to visually tracking the entire length of a dust plume as it is 

created along a surface. Make two observations per vehicle, beginning with the 

first reading at zero seconds and the second reading at five seconds. The zerosecond observation should begin immediately after a plume has been created above 

the surface involved. Do not look continuously at the plume but, instead, observe 

the plume briefly at zero seconds and then again at five seconds. 

A.4 Step 4: Record the opacity observations to the nearest 5% on an observational 

record sheet. Each momentary observation recorded represents the average opacity 

of emissions for a 5-second period. While it is not required by the test method, 

EPA recommends that the observer estimate the size of the vehicles which generate 

dust plumes for which readings are taken (e.g. mid-size passenger car or heavy-duty 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
truck.) and take the approximate speeds the vehicles are traveling when the readings 

are being taken. 

A.5 Step 5: Repeat Step 3 (Section A.3. of this appendix) and Step 4 (Section A.4. of 

this appendix) until you have recorded a total of 12 consecutive opacity readings. 

This will occur once six vehicles have driven on the source in your line of 

observation for which you are able to take proper readings. The 12 consecutive 

readings must be taken within the same period of observation but must not exceed 1 

hour. Observations immediately preceding and following interrupted observations 

can be considered consecutive. 

A.6 Step 6: Average the 12 opacity readings together. If the average opacity reading 

equals 20% or lower, the source is in compliance with the opacity standard 

described in the applicable rule. 

SECTION B TEST METHOD FOR VISUAL DETERMINATION OF OPACITY OF 

EMISSIONS FROM SOURCES FOR TIME-AVERAGED REGULATIONS 

B. Applicability. This method is applicable for the determination of the opacity of emissions 

from sources of visible emissions for time-averaged regulations. A time-averaged 

regulation is any regulation that requires averaging visible emission data to determine the 

opacity of visible emissions over a specific time period. 

B.1 Principle. The opacity of emissions from sources of visible emissions is determined 

visually by a qualified observer who has received certification. 

B.2 Procedures. A qualified observer who has been certified shall use the following 

procedures for visually determining the opacity of emissions. 

B.2.a Position. Stand at a position at least 5 meters from the Fugitive Dust 

source n order to provide a clear view of the emissions with the sun 

oriented in the 140° sector to the back. Consistent as much as possible 

with maintaining the above requirements, make opacity observations from 

a position such that the line of sight is approximately perpendicular to the 

plume and wind direction. The observer may follow the Fugitive Dust 

plume generated by mobile earthmoving equipment, as long as the sun 

remains oriented in the 140° sector to the back. As much as possible, if 

multiple plumes are involved, do not include more than one plume in the 

line of sight at one time. 

B.2.b Field Records. Record the name of the site, Fugitive Dust source type 

(i.e., pile, material handling (i.e., transfer, loading, sorting)), method of 

control used, if any, observer’s name, certification data and affiliation, 

and a sketch of the observer’s position relative to the Fugitive Dust source. 

Also, record the time, estimated distance to the Fugitive Dust source 

location, approximate wind direction, estimated wind speed, description of 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
the sky condition (presence and color of clouds,) observer’s position 

relative to the fugitive dust source, and color of the plume and type of the 

background on the visible emission observation form when opacity 

readings are initiated and completed. 

B.2.c Observations. Make opacity observations, to the extent possible, using a 

contrasting background that is perpendicular to the line of sight. For 

storage piles, make opacity observations approximately 1 meter above the 

surface from which the plume is generated. For extraction operations and 

the loading of haul trucks in open-pit mines, make opacity observations 

approximately one meter above the rim of the pit. The initial observation 

should begin immediately after a plume has been created above the 

surface involved. Do not look continuously at the plume, but instead 

observe the plume momentarily at 15-second intervals. For Fugitive Dust 

from Earthmoving equipment, make opacity observations approximately 1 

meter above the mechanical equipment generating the plume. 

B.2.d Recording Observations. Record the opacity observations to the nearest 

5% every 15 seconds on an observational record sheet. Each momentary 

observation recorded represents the average opacity of emissions for a 15-

second period. If a multiple plume exists at the time of an observation, do 

not record an opacity reading. Mark an “x” for that reading. If the 

equipment generating the plume travels outside of the field of observation, 

resulting in the inability to maintain the orientation of the sun within the 

140° sector or if the equipment ceases operating, mark an “x” for the 15 – 

second interval reading. Readings identified as “x” shall be considered 

interrupted readings. 

B.2.e Data Reduction For Time-Averaged Regulations. For each set of 12 or 24 

consecutive readings, calculate the appropriate average opacity. Sets must 

consist of consecutive observations, however, readings immediately 

preceding and following interrupted readings shall be deemed consecutive 

and in no case shall two sets overlap, resulting in multiple violations. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
APPENDIX B 

Determination of Stabilization 

SECTION A Test Methods for Determining Stabilization 

SECTION B Visible Crust Determination 

SECTION C Determination of Silt Content for Unpaved Roads and Unpaved 

Vehicle/Equipment Traffic Areas 

SECTION D Determination of Threshold Friction Velocity 

SECTION E Determination of Flat Vegetative Cover 

SECTION F Determination of Standing Vegetative Cover 

SECTION G Rock Test Method 

SECTION A TEST METHODS FOR DETERMINING STABLIZATION 

 The test methods described in Section B through Section G of this appendix shall be used 

to determine whether an area has a Stabilized Surface. Should a disturbed area contain 

more than one type of disturbance, soil, vegetation, or other characteristics, which are 

visibly distinguishable, test each representative surface separately for stability, in an area 

that represents a random portion of the overall disturbed conditions of the site, according 

to the appropriate test methods in Section B through Section G of this appendix, and 

include or eliminate it from the total size assessment of disturbed surface area(s) 

depending upon test method results. 

SECTION B VISIBLE CRUST DETERMINATION 

B.1 Where a visible crust exists, drop a steel ball with a diameter of 15.9 millimeters (0.625 

inches) and a mass ranging from 16-17 grams from a distance of 30 centimeters (one 

foot) directly above (at a 90° angle perpendicular to ) the soil surface. If blowsand is 

present, clear the blowsand from the surfaces on which the visible crust test method is 

conducted. Blowsand is defined as thin deposits of loose uncombined grains covering 

less than 50% of a site which have not originated from the representative site surface 

being tested. If material covers a visible crust, which is not blowsand, apply the test 

method in Section D of this appendix to the loose material to determine whether the 

surface is stabilized. 

B.2 A sufficient crust is defined under the following conditions: once a ball has been dropped 

according to section B.1 of this appendix, the ball does not sink into the surface, so that it 

is partially or fully surrounded by loose grains and, upon removing the ball, the surface 

upon which it fell has not been pulverized, so that loose grains are visible. 

B.3 Drop the ball three times within a survey area that measures 1 foot by 1 foot and that 

represents a random portion of the overall disturbed conditions of the site. The survey 

area shall be considered to have passed the Visible Crust Determination Test if the results 

of at least two out of the three times that the ball was dropped, met the criteria in section 

B.2 of this appendix. Select at least two other survey areas that represent a random 

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portion of the overall disturbed conditions of the site, and repeat this procedure. If the 

results meet the criteria of section B.2 of this appendix for all of the survey areas tested, 

then the site shall be considered to have passed the Visible Crust Determination Test and 

shall be considered sufficiently crusted. 

B.4 At any given site, the existence of a sufficient crust covering one portion of the site may 

not represent the existence or protectiveness of a crust on another portion of the site. 

Repeat the visible crust test as often as necessary on each random portion of the overall 

conditions of the site for an accurate assessment. 

SECTION C DETERMINATION OF SILT CONTENT FOR UNPAVED ROADS AND 

UNPAVED VEHICLE/EQUIPMENT TRAFFIC AREAS 

 The purpose of this test method is to estimate the silt content of the trafficked parts of 

Unpaved Roads and Unpaved vehicle/equipment Traffic Areas. The higher the Silt 

content, the more fine dust particles that are released when vehicles travel on Unpaved 

Roads and Unpaved vehicle/equipment Traffic Areas. 

C.1 Equipment: 

C.1.a. A set of sieves with the following openings: 4 millimeters (mm), 2mm, 1mm, 

0.5mm and 0.25 mm, a lid, and collector pan. 

C.1.b A small whisk broom or paintbrush with stiff bristles and dustpan 1 ft. in width 

(the broom/brush should preferably have one, thin row of bristles no longer than 

1.5 inches in length.) 

C.1.c A spatula without holes. 

C.1.d A small scale with half-ounce increments (e.g., postal/package scale.) 

C.1.e A shallow, lightweight container (e.g., plastic storage container.) 

C.1.f A sturdy cardboard box or other rigid object with a level surface. 

C.1.g A basic calculator. 

C.1.h Cloth gloves (optional for handling metal sieves on hot, sunny days.) 

C.1.i Sealable plastic bags (if sending samples to a laboratory.) 

C.1.j A pencil/pen and paper. 

C.2 Step 1: Look for a routinely traveled surface, as evidenced by tire tracks. Only collect 

samples from surfaces that are not damp due to precipitation or dew. This statement is 

not meant to be a standard in itself for dampness where watering is being used as a 

control measure. It is only intended to ensure that surface testing is done in a 

representative manner. Use caution when taking samples to ensure personal safety with 

respect to passing vehicles. Gently press the edge of a dustpan (1 foot in width) into the 

surface four times to mark an area that is 1 square foot. Collect a sample of loose surface 

material into the dustpan, minimizing escape of dust particles. Use a spatula to lift 

heavier elements such as gravel. Only collect dirt/Gravel to an approximate depth of 3/8 

inch or 1 cm in the 1 square foot area. If you reach a hard, underlying subsurface that is 

<3/8 inch in depth, do not continue collecting the sample by digging into the hard 

surface. In other words, you are only collecting a surface sample of loose material down 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
to 1 cm. In order to confirm that samples are collected to a 1cm depth, a wooden dowel 

or other similar narrow object at least one-foot in length can be laid horizontally across 

the survey area while a metric ruler is held perpendicular to the dowel. (Optional: At this 

point, you can choose to place the sample collected into a plastic bag or container and 

take it to an independent laboratory for silt content analysis. A reference to the procedure 

the laboratory is required to follow is at the end of this section.) 

C.3 Step 2: Place a scale on a level surface. Place a lightweight container on the scale. Zero 

the scale with the weight of the empty container on it. Transfer the entire sample 

collected in the dustpan to the container, minimizing escape of dust particles. Weigh the 

sample and record its weight. 

C.4 Step 3: Stack a set of sieves in order according to the size openings specified above, 

beginning with the largest size opening (4mm) at the top. Place a collector pan 

underneath the bottom (0.25mm) sieve. 

C.5 Step 4: Carefully pour the sample into the sieve stack, minimizing escape of dust 

particles by slowly brushing material into the stack with a whiskbroom or brush. On 

windy days, use the trunk or door of a vehicle as a wind barrier. Cover the stack with a 

lid. Lift up the sieve stack and shake it vigorously up and down and sideways for at least 

1 minute. 

C.6 Step 5: Remove the lid from the stack and disassemble each sieve separately, beginning 

with the top sieve. As you remove each sieve, examine it to make sure that all of the 

material has been sifted to the finest sieve through which it can pass (e.g., material in 

each sieve (besides the top sieve that captures a range of larger elements) should look the 

same size.) If this is not the case, re-stack the sieves and collector pan, cover the stack 

with the lid, and shake it again for at least 1 minute. You only need to reassemble the 

sieve(s) that contain material, which require further sifting. 

C.7 Step 6: After disassembling the sieves and collector pan, slowly sweep the material from 

the collector pan into the empty container originally used to collect and weigh the entire 

sample. Take care not to minimize escape of dust particles. You do not need to do 

anything with material captured in the sieves – only the collector pan. Weigh the 

container with the materials from the collector pan and record its weight. 

C.8 Step 7: If the source is an unpaved road, multiply the resulting weight by 0.38. If the 

source is an Unpaved vehicle/equipment Traffic Area, multiply the resulting weight by 

0.55. The resulting number is the estimated silt loading. Then, divide the total weight of 

the sample you recorded earlier in Step 2 (Section C.4) and multiply by 100 to estimate 

the percent Silt content. 

C.9 Step 8: Select another two routinely traveled portions of the Unpaved Road or Unpaved 

vehicle/equipment Traffic Area and repeat this test method. Once you have calculated 

the silt loading and percent silt content of the 3 samples collected, average your results 

together. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
C.10 Step 9: Examine Results. If the average silt loading is less than 0.33 oz/ft2

, the surface is 

STABLE. If the average silt loading is greater than or equal to 0.33 oz/ft2, then proceed 

to examine the average percent Silt content. If the source is an Unpaved Road and the 

average percent Silt content is 6% or less, the surface is STABLE. If the source is an 

unpaved parking lot and the average percent Silt content is 8% or less, the surface is 

STABLE. If your field test results are within 2% of the standard (for example, 4%-8% 

Silt content on an Unpaved Road) it is recommended that you collect 3 additional 

samples from the source according to Step 1 (section C.2) and take them to an 

independent laboratory for Silt content analysis. 

C.11 Independent Laboratory Analysis: You may choose to collect samples from the source, 

according to Step 1 (section C.2) and send them to an independent laboratory for Silt 

content analysis rather than conduct the sieve field procedure. If so, the test method the 

laboratory is required to use is: “Procedures For Laboratory Analysis for Surface/Bulk 

Dust Loading Samples,” (Fifth Edition, Volume 1, Appendix C.2.3 “Silt Analysis,” 

1995,) AP-42, Office of Air Quality Planning & Standards, U.S. Environmental 

Protection Agency, Research Triangle Park, North Carolina. 

SECTION D DETERMINATION OF THRESHOLD FRICTION VELOCITY (TFV) 

 For disturbed surface areas that are not crusted or vegetated, determine threshold friction 

velocity (TFV) according to the following sieving field procedure (based on a 1952 

laboratory procedure published by W.S. Chepil). 

D.1 Obtain and stack a set of sieves with the following openings: 4 millimeters (mm), 2 mm, 

1 mm, 0.5 mm, and 0.25 mm or obtain and stack a set of standard/commonly available 

sieves. Place the sieves in order according to size openings, beginning with the largest 

size opening at the top. Place a collector pan underneath the bottom (0.25 mm) sieve. 

Collect a sample of loose surface material from an area at least 30 cm by 30 cm in size to 

a depth of approximately 1 cm using a brush and dustpan or other similar device. Only 

collect soil samples from dry surfaces (i.e. when the surface is not damp to the touch). 

Remove any rocks larger than 1 cm in diameter from the sample. Pour the sample into 

the top sieve (4 mm opening) and cover the sieve/collector pan unit with a lid. Minimize 

escape of particles into the air when transferring surface soil into the sieve/collector pan 

unit. Move the covered sieve/collector pan unit by hand using a broad, circular arm 

motion in the horizontal plane. Complete twenty circular arm movements, ten clockwise 

and ten counterclockwise, at a speed just necessary to achieve some relative horizontal 

motion between the sieves and the particles. Remove the lid from the sieve/collector pan 

unit and disassemble each sieve separately beginning with the largest sieve. As each 

sieve is removed, examine it for loose particles. If loose particles have not been sifted to 

the finest sieve through which they can pass, reassemble and cover the sieve/collector pan 

unit and gently rotate it an additional ten times. After disassembling the sieve/collector 

pan unit, slightly tilt and gently tap each sieve and the collector pan so that material 

aligns along one side. In doing so, minimize escape of particles into the air. Line up the 

sieves and collector pan in a row and visibly inspect the relative quantities of catch in 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
order to determine which sieve (or whether the collector pan) contains the greatest 

volume of material. If a visual determination of relative volumes of catch among sieves 

is difficult, use a graduated cylinder to measure the volume. Estimate TFV for the sieve 

catch with the greatest volume using Table 1 of this appendix, which provides a 

correlation between sieve opening size and TFV. 

Table 1. Determination of Threshold Friction Velocity (TFV) 

Tyler Sieve No. ASTM 11 

Sieve No. 

Opening 

(mm) 

TFV 

(cm/s) 

5 5 4 135 

9 10 2 100 

16 18 1 76 

32 35 0.5 58 

60 60 0.25 43 

Collector Pan --- --- 30 

D.2 Collect at least three soil samples which represent random portions of the overall 

conditions of the site, repeat the above TFV test method for each sample and average the 

resulting TFVs together to determine the TFV uncorrected for non erodible elements. 

Non-erodible elements are distinct elements, in the random portion of the overall 

conditions of the site, that are larger than 1 cm in diameter, remain firmly in place during 

a wind episode, and inhibit soil loss by consuming Section of the shear stress of the wind. 

Non-erodible elements include stones and bulk surface material but do not include flat or 

standing vegetation. For surfaces with non-erodible elements, determine corrections to 

the TFV by identifying the fraction of the survey area, as viewed from directly overhead, 

that is occupied by non-erodible elements using the following procedure. Select a survey 

area of 1 meter by 1 meter that represents a random portion of the overall conditions of 

the site. Where many non-erodible elements lie within the survey area, separate the nonerodible elements into groups according to size. For each group, calculate the overhead 

area for the non-erodible elements according to the following equations: 

Average Dimensions = 

(Average Length) x ( Average Width) Eq. 1 

Overhead Area = 

(Average Dimensions) x (Number of Elements) Eq. 2 

Total Overhead Area = 

Overhead Area Of Group 1 + Overhead Area of Group 2 (etc) 

Eq. 3 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
Total Frontal Area = 

Total Overhead Area/2 Eq. 4 

Percent Cover of Non-Erodible Elements = 

(Total Frontal Area/Survey Area) x 100 Eq. 5 

 

 Note: Ensure consistent units of measurements (e.g., square meters or square inches when 

calculating percent cover). 

Repeat this procedure on an additional two distinct survey areas that represent a random 

portion of the overall conditions of the site and average the results. Use Table 2 of this 

appendix to identify the correction factor for the percent cover of non-erodible elements. 

Multiply the TFV by the corresponding correction factor to calculate the TFV corrected 

for non-erodible elements. 

Table 2. Correction Factors for Threshold Friction Velocity 

Percent Cover of Non-Erodible Elements Correction Factor 

Greater than or equal to 10% 5 

Greater than or equal to 5% and less than 10% 3 

Less than 5% and greater than or equal to 1% 2 

Less than 1% None 

SECTION E DETERMINATION OF FLAT VEGETATIVE COVER 

Flat vegetation includes attached (rooted) vegetation or unattached vegetative debris 

lying on the surface with a predominant horizontal orientation that is not subject to 

movement by wind. Flat vegetation, which is dead but firmly attached, shall be 

considered equally protective as live vegetation. Stones or other aggregate larger than 1 

centimeter in diameter shall be considered protective cover in the course of conduction 

the line transect test method. Where flat vegetation exists conduct the following line 

transect test method. 

E.1 Line Transect Test Method. Stretch a 100 foot measuring tape across a survey area that 

represents a random portion of the overall conditions of the site. Firmly anchor both ends 

of the measuring tape into the surface using a tool such as a screwdriver, with the tape 

stretched taut and close to the soil surface. If vegetation exists in regular rows, place the 

tape diagonally (at approximately a 45° angle) away from a parallel or perpendicular 

position to the vegetated rows. Pinpoint an area the size of a 3/32 inch diameter brazing 

rod or wooden dowel centered above each 1 foot interval mark along one edge of the 

tape. Count the number of times that flat vegetation lies directly underneath the 

pinpointed area at 1 foot intervals. Consistently observe the underlying surface from a 

90° angle directly above each pinpoint on one side of the tape. Do not count the 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
underlying surface as vegetated if any portion of the pinpoint extends beyond the edge of 

the vegetation underneath in any direction. If clumps of vegetation or vegetative debris 

lie underneath the pinpointed area, count the surface as vegetated, unless bare soil is 

visible directly below the pinpointed area. When 100 observations have been made, add 

together the number of times a surface was counted as vegetated. This total represents 

the percent of flat vegetations cover (e.g., if 35 positive counts were made, then 

vegetation cover is 35%.) If the survey area that represents a random portion of the 

overall conditions of the site is too small for 100 observations, make as many 

observations as possible. Then multiply the count of vegetated surface areas by the 

appropriate conversion factor to obtain percent cover. For example, if vegetation was 

counted 20 times within a total of 50 observations, divide 20 by 50 and multiply by 100 

to obtain a flat vegetation cover of 40%. 

E.2 Conduct the line transect test method, as described in section E.1 of this appendix, an 

additional two times on areas that represent a random portion of the overall conditions of 

the site and average results. 

SECTION F DETERMINATION OF STANDING VEGETATIVE COVER. 

Standing vegetation includes vegetation that is attached (rooted) with a predominant 

vertical orientation. Standing vegetation, which is dead but firmly rooted, shall be 

considered equally protective as live vegetation. Conduct the following standing 

vegetation test method to determine if 30% cover or more exists. If the resulting percent 

cover is less than 30% but equal to or greater than 10%, then conduct the test in Section 

D; “Determination Of Threshold Friction Velocity (TFV,) of this appendix in order to 

determine if the site is stabilized, such that the standing vegetation cover is equal to or 

greater than 10%, where threshold friction velocity, corrected for non-erodible elements, 

is equal to or greater than 43cm/second. 

F.1 For standing vegetation that consists of large, separate vegetative structures (e.g., shrubs 

and sagebrush,) select a survey area that represents a random portion of the overall 

conditions of the site that is the shape of a square with sides equal to at least 10 times the 

average height of the vegetative structures. For smaller standing vegetation, select a 

survey area of three feet by three feet. 

F.2 Count the number of standing vegetative structures within the survey area. Count 

vegetation, which grows in clumps as a single unit. Where different types of vegetation 

exist and/or vegetation of different height and width exists, separate the vegetative 

structures with similar dimensions into groups. Count the number of vegetative 

structures in each group within the survey area. Select an individual structure within 

each group that represents the average height and width of the vegetation in the group. If 

the structure is dense (e.g., when looking at it vertically from base to top there is little or 

zero open air space within its perimeter,) calculate and record its frontal silhouette area, 

according to Equation 6 of this appendix. Also, use Equation 6 of this appendix to 

estimate the average height and width of the vegetation if the survey area is larger than 

nine square feet. Otherwise, use the procedure in section F.3 of this appendix to calculate 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
the frontal silhouette area. Then calculate the percent cover of standing vegetation 

according to Equations 7, 8, and 9 of this appendix. 

Frontal Silhouette Area = 

(Average Height) x (Average Width) Eq. 6 

Frontal Silhouette Area Of Group= 

(Frontal Silhouette Area Of Individual Vegetative Structure) x 

(Number Of Vegetation Structures Per Group) 

Eq. 7 

Total Frontal Silhouette Area = 

Frontal Silhouette Area Of Group 1 + Frontal Silhouette Area Of 

Group 2 (etc.) 

Eq. 8 

Percent Cover Of Standing Vegetation = 

(Total Frontal Silhouette Area/Survey Area) x 100 Eq. 9 

Percent Open Space = 

[(Number Of Circled Gridlines Within The Outlined Area 

Counted That Are Not Covered By Vegetation/Total Number Of 

Gridline Intersections Within The Outlined Area) x 100] 

Eq.10 

Percent Vegetative Density = 

100 – Percent Open Space Eq. 11 

Vegetative Density = 

Percent Vegetative Density/100 Eq. 12 

Frontal Silhouette Area = 

[Max. Height x Max. Width] x [Vegetative Density/.04]o.5 Eq. 13 

Note: Ensure consistent units of measurement (e.g., square meters or square inches when 

calculating percent cover.) 

F.3. Vegetative Density Factor. Cut a single, representative piece of vegetation (or 

consolidated vegetative structure) to within 1cm of surface soil. Using a white paper grid 

or transparent grid over white paper, lay the vegetation flat on top of the grid (but do not 

apply pressure to flatten the structure.) Grid boxes of 1 inch or 1⁄2 inch squares are 

sufficient for most vegetation when conducting this procedure. Using a marker or pencil, 

outline the shape of the vegetation along its outer perimeter, according to Figure B, C, or 

D of this appendix, as appropriate. (Note: Figure C differs from Figure D primarily in 

that the width of vegetation in Figure C is narrow at its base and gradually broadens to its 

tallest height. In Figure D, the width of the vegetation generally becomes narrower from 

its midpoint to its tallest height.) Remove the vegetation, count and record the total 

number of gridline intersections within the outlined area, but do not count gridline 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
intersections that connect with the outlined shape. There must be at least 10 gridline 

intersections within the outlined area and preferably more than 20, otherwise, use smaller 

grid boxes. Draw small circles (no greater than a 3/32 inch diameter) at each gridline 

intersection counted within the outlined area. Replace the vegetation on the grid within 

its outlined shape. From a distance of approximately 2 feet directly above the grid, 

observe each circled gridline intersection. Count and record the number of circled 

gridline intersections that are not covered by any piece of the vegetation. To calculate 

percent vegetative density, use Equations 10 and 11 of this appendix. If percent 

vegetative density is equal to or greater than 30, use an equation (one of the equationsEquations 16, 17, or 18 of this appendix) that matches the outline used to trace the 

vegetation (Figure B, C, or D) to calculate its frontal silhouette area. If percent 

vegetative density is less than 30, use Equations 12 and 13 of this appendix to calculate 

the frontal silhouette area. 

Figure B. Cylinder 

Frontal Silhouette Area = Maximum Height x Maximum Width Eq.16 

Figure C. Inverted Cone 

Frontal Silhouette Area = Maximum Height x 1⁄2 Maximum Width Eq. 17 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
Figure D. Upper Sphere 

Frontal Silhouette Area = (3.14 x Maximum Height x 1⁄2 Maximum Width)/2 Eq.18 

SECTION G ROCK TEST METHOD 

The Rock Test Method, which is similar to Section D, Test Methods For StabilizationDetermination Of Threshold Friction Velocity (TFV) of this appendix, examines the 

wind-resistance effects of rocks and other non-erodible elements on disturbed surfaces. 

Non-erodible elements are objects larger than 1 centimeter (cm) in diameter that remain 

firmly in place even on windy days. Typically, non-erodible elements include rocks, 

stones, glass fragments, and hardpacked clumps of soil lying on or embedded in the 

surface. Vegetation does not count as a non-erodible element in this method. The 

purpose of this test method is to estimate the percent cover of non-erodible elements on a 

given surface to see whether such elements take up enough space to offer protection 

against windblown dust. For simplification, the following test method refers to all nonerodible elements as ‘rocks.” 

G.1 Select a 1 meter by 1 meter survey area that represents the general rock distribution on 

the surface. A 1 meter by 1 meter area is slightly greater than a 3 foot by 3 foot area. 

Mark-off the survey area by tracing a straight, visible line in the dirt along the edge of a 

measuring tape or by placing short ropes, yard sticks, or other straight objects in a square 

around the survey area. 

 

G.2 Without moving any of the rocks or other elements, examine the survey area. Since 

rocks >3/8 inch (1cm) in diameter are of interest, measure the diameter of some of the 

smaller rocks to get a sense of which rocks need to be considered. 

G.3 Mentally group the rocks >3/8 inch (1cm) diameter lying in the survey area into small, 

medium, and large size categories. Or, if the rocks are all approximately the same size, 

simply select a rock of average size and typical shape. Without removing any of the 

rocks from the ground, count the number of rocks in the survey area in each group and 

write down the resulting number. 

G.4 Without removing rocks, select one or two average-size rocks in each group and measure 

the length and width. Use either metric units or standard units. Using a calculator, 

multiply the length times the width of the rocks to get the average dimensions of the 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
rocks in each group. Write down the results for each rock group. 

G.5 For each rock group, multiply the average dimensions (length times width) by the number 

of rocks counted in the group. Add the results from each rock group to get the total rock 

area within the survey area. 

G.6 Divide the total rock area, calculated in section G.5 of this appendix, by two (to get 

frontal area.) Divide the resulting number by the size of the survey area (make sure the 

units of measurement match,) and multiply by 100 for percent rock cover. For example, 

the total rock area is 1,400 square centimeters divide 1,400 by 2 to get 700. Divide 700 

by 10,000 (the survey area is 1 meter by 1 meter, which is 100 centimeters by 100 

centimeters or 10,000 centimeters) and multiply by 100. The result is 7% rock cover. If 

rock measurements are made in inches, convert the survey area from meters to inches (1 

inch = 2.54 centimeters.) 

G.7 Select and mark-off two additional survey areas and repeat the procedures described in 

section G.1 through section G.6 of this appendix. Make sure the additional survey areas 

also represent the general rock distribution on the site. Average the percent cover results 

from all three survey areas to estimate the average percent of rock cover. 

G.8 If the average rock cover is greater than or equal to 10%, the surface is stable. If the 

average rock cover is less than 10%, follow the procedures in section G.9 of this 

appendix. 

G.9 If the average rock cover is less than 10%, the surface may or may not be stable. Follow 

the procedures in Section D.3 Determination Of Threshold Friction Velocity (TFV) of 

this rule and use the results from the rock test method as a correction (i.e., multiplication) 

factor. If the rock cover is at least 1%, such rock cover helps to limit windblown dust. 

However, depending on the soil’s ability to release fine dust particles into the air, the 

percent rock cover may or may not be sufficient enough to stabilize the surface. It is also 

possible that the soil itself has a high enough TFV to be stable without even accounting 

for rock cover. 

G.10 After completing the procedures described in Section G.9 of this appendix, use Table 2 of 

this appendix to identify the appropriate correction factor to the TFV, depending on the 

percent rock cover. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
RULE 801. CONSTRUCTION AND EARTHMOVING ACTIVITIES

(Adopted 11/08/2005) 

A. Purpose 

The purpose of this rule is to reduce the amount of fine Particulate Matter (PM-10) 

entrained in the ambient air as a result of emissions generated from Construction and 

other Earthmoving Activities by requiring actions to prevent, reduce, or mitigate PM-10 

emissions. 

B. Applicability 

This rule applies to any Construction and other Earthmoving Activities, including, but not 

limited to, land clearing, excavation related to construction, land leveling, grading, cut 

and fill grading, erection or demolition of any structure, cutting and filling, trenching, 

loading or unloading of bulk materials, demolishing, drilling, adding to or removing bulk 

of materials from open storage piles, weed abatement through disking, back filling, travel 

on-site and travel on access roads to and from the site. 

C. Definitions 

The definitions of terms found in Rule 800 (General Requirements for Control of Fine 

Particulate Matter (PM-10) shall apply to this rule. 

D. Exemptions 

 In addition to the exemptions listed in Rule 800, Section E, the following exemptions are 

established for this rule: 

 D.1 Construction or demolition at existing single family residential dwellings. 

 D.2 The 20% opacity limit of Sections E.1.a and E.2.b shall not apply when Wind 

Gusts exceed 25 miles per hour, provided that at least one of the following control 

measures is implemented for each applicable Fugitive Dust source type: 

 D.2.a Cease dust generating activities for a period of one hour after Wind Gusts 

last exceed the threshold. If operations cease for the remainder of the day, 

stabilization measures must be implemented. 

 D.2.b Apply water or dust Suppressants once per hour. 

 D.2.c Apply water to maintain 12% soil moisture content. 

D.2.d Construct fences 3-5 feet high with 50% or less porosity, and must be 

done in conjunction with another measure, as above. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
E. Requirements 

E.1 Construction sites and Earthmoving Activities: 

E.1.a All Persons who own or operate a Construction site shall comply with the 

requirements of Section F.1 so as to limit VDE to 20% opacity and 

comply with the conditions for a Stabilized Surface when applicable. 

E.1.b. All Persons who perform any Earthmoving Activities shall comply with 

the requirements of Section F.1 so as to limit VDE to 20% opacity. 

E.1.c All Persons who own or operate a Construction site of 10 acres or more in 

size for residential developments or 5 acres or more for non-residential 

developments shall develop a dust control plan. The dust control plan shall 

be made available to the APCD upon request. The dust control plan shall 

comply with the requirements of Section F. 

E.1.d The owner or operator required to develop a dust control plan shall 

provide written notification to the APCD within 10 days prior to the 

commencement of any Construction activities via fax or mail. The 

requirement to develop a dust control plan shall apply to all such activities 

conducted for residential and non-residential (e.g., commercial, industrial, 

or institutional) purposes or conducted by any governmental entity. 

Regardless of whether a dust control plan is in place or not the owner or 

operator is still subject to comply with all requirements of the applicable 

rules under Regulation VIII at all times. 

F. Best Available Control Measures for Fugitive Dust (PM-10) 

 F.1 Construction and Earthmoving Activities shall comply with the following 

requirements: 

 F.1.a Pre-Activity: 

 F.1.a.1 Pre-water site sufficient to limit VDE to 20% opacity, and 

 F.1.a.2 Phase work to minimize the amount of disturbed surface area at 

any one time. 

 F.1.b During Active Operations: 

 F.1.b.1 Apply water or Chemical Stabilization as directed by product 

manufacturer to limit VDE to 20% opacity, or 

 F.1.b.2 Construct and maintain wind barriers sufficient to limit VDE to 

20% opacity. If utilizing wind barriers, control measure F.1.b.1 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
above shall be implemented. 

 F.1.b.3 Apply water or Chemical Stabilization as directed by product 

manufacturer to unpaved haul/access roads and Unpaved Traffic 

Areas sufficient to limit VDE to 20% opacity and meet the 

conditions of a Stabilized Unpaved Road. 

 F.1.c Temporary Stabilization During Periods of Inactivity: 

 F.1.c.1 Restrict vehicular access to the area by fencing or signage; and 

 F.1.c.2 Apply water or Chemical Stabilization, as directed by product 

manufacturer, sufficient to comply with the conditions of a 

Stabilized Surface. If an area having 0.5 acres or more of 

disturbed surface area remains unused for seven or more days, the 

area must comply with the conditions for a Stabilized Surface area. 

 

 F.1.d Track Out/Carry Out of Bulk Materials at the site shall be mitigated in 

compliance with Rule 803. 

 F.1.e Unpaved Roads and Unpaved Traffic Areas at the site shall comply with 

Rule 805. 

 F.1.f Bulk Material handling operations at the site shall comply with Rule 802. 

 F1.g Material transport of Bulk Material to, from, or around the site shall 

comply with Rule 802. 

 F.1.h Haul trucks transporting Bulk Material to, from, or around the site shall 

comply with Rule 802. 

F.2 Dust Control Plan: 

 F.2.a Retain a copy of the dust control plan at the project site. 

 F.2.b Comply with the requirements of the approved dust control plan. 

 F.2.c A dust control plan shall contain all of the following information: 

 1. Name, address, and phone number of the Person responsible for the 

preparation, submittal, and implementation of the dust control plan and 

responsible for the project site. 

 2. A plot plan which shows the type and location of each project. 

 3. The total area of land surface to be disturbed, estimated daily 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
throughput volume of earthmoving in cubic yards, and total area in 

acres of the entire project site. 

 4. The expected start and completion dates of dust generating and soil 

disturbance activities to be performed on the site. 

 5. The actual and potential sources of Fugitive Dust emissions on the site 

and the location of Bulk Material handling and storage areas, Paved 

and Unpaved Roads, entrances and exits where Track Out/Carry Out 

may occur, and Unpaved Traffic Areas. 

6. Dust Suppressants to be applied, including: product specifications; 

manufacturer's usage instructions (method, frequency, and intensity of 

application); type, number, and capacity of application equipment; and 

information on environmental impacts and approvals or certifications 

related to appropriate and safe use for ground application. 

7. Specific surface treatment(s) and/or control measures utilized to 

control Track Out/Carry Out, and sedimentation where unpaved and/or 

access points join paved public access roads. 

8. The dust control plan should describe all Fugitive Dust control 

measures to be implemented before, during, and after any dust 

generating activity. 

G. Record of Control Implementation 

Any Person subject to the requirements of this rule shall compile and retain records that 

provide evidence of control measure application (i.e., receipts and/or purchase records). 

Such Person shall describe, in the records, the type of treatment or control measure, 

extent of coverage, and date applied. For control measures which require multiple daily 

applications, recording the frequency of application will fulfill the recordkeeping 

requirements of this rule (i.e., water being applied three times a day and the date) Records 

shall be maintained and be readily accessible for two years after the date of each entry 

and shall be provided to the APCD upon request. 

 

 H. Violations 

Failure to comply with any provisions of this rule shall constitute a violation of 

Regulation VIII. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
RULE 802. BULK MATERIALS 

(Adopted 11/08/2005) 

A. Purpose 

The purpose of this regulation is to reduce the amount of fine Particulate Matter (PM-10) 

entrained in the ambient air as a result of emissions generated from outdoor handling, 

storage, and transport of Bulk Material by requiring actions to prevent, reduce, or 

mitigate PM-10 emissions. 

B. Applicability 

This rule applies to the outdoor handling, storage, and transport of Bulk Material, 

including, but not limited to, earth, rock, silt, sediment, sand, gravel, soil, fill, Aggregate 

Materials, dirt, mud, debris, and other organic and/or inorganic material consisting of or 

containing Particulate Matter with five percent or greater silt content. 

C. Definitions 

The definitions of terms found in Rule 800 (General Requirements for Control of Fine 

Particulate Matter (PM-10) shall apply to this rule. 

D. Exemptions 

 In addition to the exemptions listed in Rule 800, Section E, the following exemptions are 

established for this rule: 

D.1 Outdoor storage, transport, or handling of Bulk Materials (including, but not 

limited to, organic or inorganic fertilizer, grains, seed, soil amendments, and feed)

which would be damaged by wetting with water or by the application of Chemical 

Stabilization/Suppression, provided owners/operators demonstrate to the 

satisfaction of the APCO that none of the control measures required by this rule 

can be implemented to limit VDE to 20% opacity or provide a Stabilized Surface, 

as defined in Rule 800. 

D.2 Outdoor storage or handling of any Bulk Material at a single site where no 

material is actively being added or removed at the end of the workday or 

overnight and where the total material stored is less than 100 cubic yards. 

D.3 Transport of a Bulk Material in an outdoor area for a distance of twelve feet or 

less with the use of a chute or conveyor device. 

D.4 Transport/hauling of Bulk Materials when conducted within the boundaries of a 

premises, are exempt from the requirements specified in Sections F.3.a and F.3.d.

 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
E. Requirements 

 E.1 Bulk Material handling: no Person shall cause, suffer, allow or engage in any 

Bulk Material handling operation including, but not limited to stacking, loading, 

unloading, conveying and reclaiming of Bulk Material, for industrial or 

commercial purposes without complying with one or more of the requirements of 

Section F.1 so as to limit VDE to 20% opacity. 

 E.2 Bulk Material storage: no Person shall cause, suffer, allow or engage in any Bulk 

Material storage, for industrial or commercial purposes without complying with 

one or more of the requirements of Section F.2 so as to limit VDE to 20% opacity. 

 E.3 Material transport: no Person shall cause, suffer, allow or otherwise engage in the 

transportation of Bulk Materials for industrial or commercial purposes, without 

complying with all of the requirements of Section F.3 so as to limit VDE to 20% 

opacity. 

E.4 Haul Trucks: no Person shall cause, suffer, allow or otherwise engage in the use 

or operation of any Haul Truck, for industrial or commercial purposes, of 

transporting or storing Bulk Material without complying with all of the 

requirements of Section F.3 so as to limit VDE to 20% opacity. 

F. Best Available Control Measures for Fugitive Dust (PM-10) 

 F.1 BULK MATERIAL HANDLING/TRANSFER: 

 F.1.a Spray with water prior to handling and/or at points of transfer; or. 

 F.1.b Apply and maintain Chemical Stabilization, or 

 F.1.c Protect from wind erosion by sheltering or enclosing the operation and 

transfer line. 

 F.2 BULK MATERIAL STORAGE 

 F.2.a When storing Bulk Materials, comply with the conditions for a Stabilized 

Surface; or 

 F.2.b Cover Bulk Materials stored outdoors with tarps, plastic, or other suitable 

material and anchor in such a manner that prevents the cover from being 

removed by wind action, or 

 F.2.c Construct and maintain barriers with less than 50% porosity. If utilizing 

fences or wind barriers, apply water or chemical/organic 

stabilizers/suppressants, or 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
 F.2.d Utilize a 3-side structure with a height at least equal to the height of the 

storage pile and with less than 50% porosity. 

 F.3. MATERIAL TRANSPORT/HAULING: 

 F.3.a Completely cover or enclose all Haul Truck loads of Bulk Material. 

 

F.3.b Haul Trucks transporting loads of Aggregate Materials shall not be 

required to cover their loads if the load, where it contacts the side, front, 

and back of the cargo container area remains six inches from the upper 

area of the container area, and if the load does not extend, at its peak, 

above any part of the upper edge of the cargo container area (As defined in 

Section 23114 of the California Vehicle Code for both public and private 

roads). 

 F.3.c The cargo compartment(s) of all Haul Trucks are to be constructed and 

maintained so that no spillage and loss of Bulk Material can occur from 

holes or other openings in the cargo compartment's floor, side, and/or 

tailgate. Seals on any openings used to empty the load including, but not 

limited to, bottom-dump release gates and tailgates to be properly 

maintained to prevent the loss of Bulk Material from those areas. 

 

 F.3.d The cargo compartment of all Haul Trucks is to be cleaned and/or washed 

at delivery site after removal of Bulk Material. 

G. Record of Control Implementation 

Any Person subject to the requirements of this rule shall compile and retain records that 

provide evidence of control measure application (i.e., receipts and/or purchase records). 

Such Person shall describe, in the records, the type of treatment or control measure, 

extent of coverage, and date applied. For control measures which require multiple daily 

applications, recording the frequency of application will fulfill the recordkeeping 

requirements of this rule (i.e., water being applied three times a day and the date) Records 

shall be maintained and be readily accessible for two years after the date of each entry 

and shall be provided to the APCD upon request.

 

H. Violations 

Failure to comply with any provisions of this rule shall constitute a violation of 

Regulation VIII. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
RULE 803. CARRY-OUT AND TRACK-OUT 

(Adopted 11/08/2005) 

A. Purpose 

The purpose of this regulation is to reduce the amount of fine Particulate Matter (PM-10) 

entrained in the ambient air as a result of emissions generated from Track-Out and CarryOut by requiring actions to prevent, reduce, or mitigate PM-10 emissions. 

 

B. Applicability 

This rule applies to all sites that are subject to Regulation VIII where Track-Out or CarryOut has occurred or may occur on paved public roads or the paved shoulders of a paved 

public road. 

C. Definitions 

The definitions of terms found in Rule 800 (General Requirements for Control of Fine 

Particulate Matter (PM-10) shall apply to this rule. 

D. Exemptions: 

 In addition to the exemptions listed in Rule 800, Section E, the following exemptions are 

established for this rule: 

 

D.1 Agricultural Operation Sites defined in and subject to Rule 806, Conservation 

Management Practices, are exempt from the requirements specified in Sections 

F.1.b and F.1.c. 

D.2 Any operation site that operates no more than 10 days within a 90 days period at 

each location is exempt from the requirements specified in Sections F.1.b and 

F.1.c. 

E. Requirements 

E.1 Track Out/Carry Out: any Person who causes the deposition of Bulk Material by 

tracking out or carrying out onto a Paved Road surface shall comply with the 

requirements of Section F.1, as specified, to prevent or mitigate such deposition. 

F. Best Available Control Measures for Fugitive Dust (PM-10) 

 F.1 TRACK OUT/CARRY OUT: 

 F.1.a. Clean up any Bulk Material tracked out or carried out onto a Paved Road 

on the following time-schedule: 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
(1) Within urban areas, immediately, when Track-Out or Carry-Out 

extends a cumulative distance of 50 linear feet or more; and 

(2) At the end of the workday, for all other Track-Out or Carry-Out.

 F.1.b In addition to F.1.a, all sites with access to a Paved Road and with 150 or 

more Average Vehicle Trips per Day, or 20 or more Average Vehicle 

Trips per Day by vehicles with three or more axles shall install one or 

more Track-Out Prevention Devices or other APCO approved Track-Out 

control device or wash down system at access points where unpaved 

traffic surfaces adjoin Paved Roads; or 

 F.1.c In addition to F.1.a, all sites with access to a Paved Road and with 150 or 

more Average Vehicle Trips per Day, or 20 or more Average Vehicle 

Trips per Day by vehicles with three or more axles shall apply and 

maintain paving, Chemical Stabilizeation, or at least 3 inch depth of 

Gravel (using Gravel or other low Silt (<5%) content material), for a 

distance of 50 or more consecutive feet at access points where Unpaved 

Roads adjoin Paved Roads. 

G. Record of Control Implementation 

Any Person subject to the requirements of this rule shall compile and retain records that 

provide evidence of control measure application (i.e., receipts and/or purchase records). 

Such Person shall describe, in the records, the type of treatment or control measure, 

extent of coverage, and date applied. Records shall be maintained and be readily 

accessible for two years after the date of each entry and shall be provided to the APCD 

upon request. 

H. Violations 

Failure to comply with any provisions of this rule shall constitute a violation of 

Regulation VIII. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
RULE 804 OPEN AREAS 

(Adopted 11/08/2005) 

A. Purpose 

The purpose of this regulation is to reduce the amount of fine Particulate Matter (PM-10) 

entrained in the ambient air as a result of emissions generated from Open Areas by 

requiring actions to prevent, reduce, or mitigate PM-10 emissions. 

 

B. Applicability 

 This rule shall apply to any open area having 0.5 acres or more within urban areas, or 3.0 

acres or more within rural areas; and contains at least 1000 square feet of disturbed 

surface area. 

C. Definitions 

The definitions of terms found in Rule 800 (General Requirements for Control of Fine 

Particulate Matter (PM-10) shall apply to this rule. 

D. Exemptions 

 In addition to the exemptions listed in Rule 800, Section E, the following exemptions are 

established for this rule: 

 D.1 Agricultural Operation Sites defined in and subject to Rule 806, Conservation 

Management Practices. 

E. Requirements 

E.1 Open Areas: all Persons who own or otherwise have jurisdiction over an Open 

Area shall comply with one or more of the requirements of Section F.1 to comply 

with the conditions of a Stabilized Surface at all times and limit VDE to 20% 

opacity. 

E.2 Vehicle use in Open Areas: within 30 days following initial discovery of evidence 

of trespass, a Person who owns or otherwise has jurisdiction over an Open Area 

shall prevent unauthorized vehicle access by posting "No Trespassing" signs or 

installing physical barriers such as fences, gates, posts, and/or appropriate barriers 

to effectively prevent access to the area. 

F. Best Available Control Measures for Fugitive Dust (PM-10) 

 F.1 OPEN AREAS 

F.1.a Apply and maintain water or dust suppressant(s) to all unvegetated areas. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
 

 F.1.b Establish vegetation on all previously disturbed areas. 

F.1.c Pave, apply and maintain Gravel, or apply and maintain Chemical 

Stabilizers/Suppressants. 

G. Record of Control Implementation 

Any Person subject to the requirements of this rule shall compile and retain records that 

provide evidence of control measure application (i.e., receipts and/or purchase records). 

Such Person shall describe, in the records, the type of treatment or control measure, 

extent of coverage, and date applied. For control measures which require multiple daily 

applications, recording the frequency of application will fulfill the recordkeeping 

requirements of this rule (i.e., water being applied three times a day and the date) Records 

shall be maintained and be readily accessible for two years after the date of each entry 

and shall be provided to the APCD upon request.

 

H. Violations 

Failure to comply with any provisions of this rule shall constitute a violation of 

Regulation VIII. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
RULE 805 PAVED AND UNPAVED ROADS 

(Adopted 11/08/2005) 

A. Purpose 

The purpose of this regulation is to reduce the amount of fine Particulate Matter (PM-10) 

entrained in the ambient air as a result of emissions generated from new or existing 

public or private Paved or Unpaved Road, road construction project, or road modification 

project by requiring actions to prevent, reduce, or mitigate PM-10 emissions. 

 

B. Applicability 

This rule applies to any new or existing public or private Paved or Unpaved Road, road 

construction project, or road modification project. 

C. Definitions 

The definitions of terms found in Rule 800 (General Requirements for Control of Fine 

Particulate Matter (PM-10) shall apply to this rule. 

D. Exemptions 

 In addition to the exemptions listed in Rule 800, Section E, the following exemptions are 

established for this Rule: 

 

D.1 Paved and unpaved driveways serving one single family residential dwelling. 

D.2 Agricultural Operation Sites defined in and subject to Rule 806, Conservation 

Management Practices. 

E. Requirements 

E.1 Unpaved Haul/Access Roads: No Person shall cause, suffer or allow the 

operation, use, or maintenance of any unpaved Haul/Access Road without 

complying with one or more of the requirements of Section F.1 so as to limit VDE 

to 20% opacity. 

E.2 Unpaved Roads: On any Unpaved Road segment with 50 or more Average 

Vehicle Trips per Day, the owner/operator shall limit VDE to 20% opacity, as 

determined by the test methods for “Visual Determination of Opacity” in Rule 

800, Appendix A, and comply with the requirements of a Stabilized Unpaved 

Road by application and/or maintenance of at least one of the requirements of 

Section F.1. 

E.3 The construction of any new Unpaved Road is prohibited within any area with a 

population of 500 or more unless the road meets the definition of a Temporary 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
Unpaved Road. The Temporary Unpaved Road shall meet the definition of a 

Stabilized Unpaved Road as determined by the test methods in Rule 800, 

Appendix B, Section C, and where VDE is limited to 20% opacity. 

E.4 Canal Roads: all Persons who cause, suffer or allow the operation, use or 

maintenance of any Canal Road with 20 or more Average Vehicle Trips per Day 

shall comply with one or more of the requirements of Section F.1 to comply with 

the requirements of a Stabilized Unpaved Road and limit VDE to 20% opacity, as 

determined by the test methods in Rule 800, Appendix A, and shall also comply 

with one or more of the requirements of Section F.2. 

E.5 Unpaved Traffic Areas: All Persons who cause, suffer or allow the operation, use 

or maintenance of any Unpaved Traffic Area larger than one (1) acre and with 75 

or more Average Vehicle Trips per Day shall comply with one or more of the 

requirements of Section F.3 and limit VDE to 20% opacity. 

E.6 Paved Roads: any new or Modified Paved Roads shall comply with the 

requirements of section F.4. 

E.7 Requirements for Existing Unpaved Public Roads in City and Rural Areas: 

Each city or county agency with primary responsibility for any existing Unpaved 

Road shall take the following actions: 

E.7.a By January 1, 2006 provide the APCD with a list of all Unpaved Roads 

under its jurisdiction in any city or Rural area(s), including data on length 

of, and Average Vehicle Trips per Day on, each Unpaved Road segment. 

E.7.b By March 31, 2006 the County Public Works Department shall provide 

the APCD with a compliance plan. The compliance plan shall include a 

compliance schedule indicating that during the period 2006 through 2015 

a 10% per each fiscal year, beginning July 1 and ending June 30, of all 

Unpaved Roads subject to the requirements of this rule will comply with a 

20% VDE and comply with the requirements of a Stabilized Unpaved 

Road (Treatment in excess of the annual requirement can be credited 

toward future year requirements). The plan shall identify the control 

measures implemented or that will be implemented at each Unpaved Road 

segment with 50 or more Average Vehicle Trips per Day. 

E.7.c By July 31 of each year, 2007 through 2016, the County Public Works 

Department shall submit to the APCD the total number of Unpaved Road 

miles which were mitigated during the previous fiscal year, and the 

percentage of cumulative miles relative to the schedule provided pursuant 

to Section E.7.b. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
F. Best Available Control Measures for Fugitive Dust (PM-10) 

 F.1 UNPAVED ROADS, INCLUDING UNPAVED HAUL AND ACCESS ROADS: 

 F.1.a Pave. 

 F.1.b Apply Chemical Stabilization as directed by product manufacturer to 

control dust on Unpaved Roads. 

 

 F.1.c Apply and maintain Gravel, recrushed/recycled asphalt or other material 

of low Silt (<5%) content to a depth of three or more inches. 

 

 F.1.d Wetting. Apply water one or more times daily

 F.1.e Permanent road closure 

 F.1.f Restrict unauthorized vehicle access. 

F.1.g Any other method that effectively limits VDE to 20% opacity and meets 

the conditions of a Stabilized Unpaved Road. 

 F.2 CANAL ROADS: 

 F.2.a Stocking of Triploid Grass Carp in canals to reduce maintenance vehicle 

trips along Canal Banks to mechanically remove aquatic weeds. 

 F.2.b Installation of remote control delivery gates to eliminate manual gate 

operation by maintenance personnel in vehicles along Canal Banks. 

F.2. c Implement Silt removal program to delay grading of spoil piles deposited 

on Canal Bank after cleaning operations until the next cleaning operation 

to eliminate vehicle access to Canal Bank. 

 F.2.d Permanent road closure. 

 F.2.e Conversion of open canals to pipeline. 

 F.2.f Lining canals to eliminate maintenance for Silt/weed control. 

 F.2.g Canal Bank surface maintenance. 

 F.3 UNPAVED TRAFFIC AREAS: 

 F.3.a Pave. 

F.3.b Apply Chemical Stabilization as directed by product manufacturer to 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
control dust on Unpaved Roads. 

F.3.c Apply and maintain Gravel, recrushed/recycled asphalt or other material 

of low silt (<5%) content to a depth of three or more inches. 

 

 F.3.d Wetting. Apply water one or more times daily. 

F.4. NEW OR MODIFIED PAVED ROADS 

Any Person having jurisdiction over, or ownership of, public or private Paved 

Roads shall construct, or require to be constructed, all new or Modified Paved 

Roads in conformance with the Imperial County Public Works Department 

guidelines for width of shoulders and median shoulders as specified below: 

F.4.a New arterial roads or streets or modifications to existing arterial roads or 

streets shall be constructed with paved shoulders that meet following 

widths: 

 

Annual Average Daily 

Vehicle Trips 

Minimum Paved or Stabilized 

Shoulder Width in Feet 

 1-2000 2 

 Greater than 2000 6 

F.4.b New or modified collector roads or streets or local roads or streets shall be 

constructed with paved shoulders that meet following widths: 

 

Annual Average Daily 

Vehicle Trips 

Minimum Paved or Stabilized 

Shoulder Width in Feet 

 1-2000 2 

 Greater than 2000 4 

F.4.c A curbing adjacent to and contiguous with the travel lane or paved 

shoulder or a road may be constructed, in lieu of meeting the paved 

shoulder width standard listed in Sections F.4.a and F.4.b. Any road 

paving projects constructing curbing in County road right of ways shall be 

approved by the Director of Public Works Department prior to 

construction. 

F.4.d Intersections, auxiliary entry lanes, and auxiliary exit lanes may be 

constructed adjacent to and contiguous with the roadway, in lieu of 

meeting the paved shoulder width standard in Sections F.4.a and F.4.b. 

F.4.e New Paved Road construction or modifications to an existing Paved Road 

that are required to comply with California Environmental Quality Act 

(CEQA) and National Environmental Policy Act (NEPA) determinations 

regarding environmental, cultural, archeological, historical, or other 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
considerations addressed in such documents, are exempt from the paved 

shoulder width requirements specified in Section F.4.a. 

F.4.f Whenever any Paved Road which has projected Annual Average Daily 

Vehicle Trips of 500 or more is constructed, or modified with medians, the 

medians shall be constructed with paved shoulders having a minimum 

width of four feet adjacent to the traffic lanes unless: 

F.4.f1 The medians of roads having speed limits set at or below 45 miles 

per hour are constructed with curbing; or 

F.4.f2 The medians are landscaped and maintained with grass or other 

vegetative ground cover to comply with the definition of Stabilized 

Surface. 

 F.4.g In lieu of complying with the paving or vegetation requirements a Person 

may apply oils or other Chemical Stabilizers/Suppressants to the required 

width of shoulder and median areas as specified in Sections F.4.a and 

F.4.b. The material shall be reapplied and maintained to limit VDE to 20% 

opacity and fulfill conditions for a Stabilized Surface. 

G. Record of Control Implementation 

Any Person subject to the requirements of this rule shall compile and retain records that 

provide evidence of control measure application (i.e., receipts and/or purchase records). 

Such Person shall describe, in the records, the type of treatment or control measure, 

extent of coverage, and date applied. For control measures which require multiple daily 

applications, recording the frequency of application will fulfill the recordkeeping 

requirements of this rule (i.e., water being applied three times a day and the date) Records 

shall be maintained and be readily accessible for two years after the date of each entry 

and shall be provided to the APCD upon request. 

H. Violations 

Failure to comply with any provisions of this rule shall constitute a violation of 

Regulation VIII. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
RULE 806 CONSERVATION MANAGEMENT PRACTICES 

(Adopted 11/08/2005) 

A. Purpose 

The purpose of this regulation is to reduce the amount of fine Particulate Matter (PM-10) 

entrained in the ambient air as a result of emissions generated from Agricultural 

Operation Sites by requiring Conservation Management Practices to prevent, reduce, or 

mitigate PM-10 emissions. 

B. Applicability 

This rule applies to Agricultural Operation Sites located within the Imperial County. 

Effective on and after January 1, 2006, an owner/operator shall implement the applicable 

CMPs selected for each Agricultural Operation Site. 

C. Definitions 

 

In addition to the definitions of terms in Rule 800 (General Requirements for Control of 

Fine Particulate Matter (PM-10), the following definitions shall govern the 

implementation of this rule: 

 

 C.1 AGRICULTURAL OPERATIONS: The growing and harvesting of crops for the 

primary purpose of earning a living. 

C.2 AGRICULTURAL OPERATION SITE: One or more agricultural parcels that 

meet the following: 

C.2.a Are under the same or common ownership or operation, or which are 

owned or operated by entities which are under common control; and 

C.2.b Are located on one or more contiguous or adjacent properties wholly 

within Imperial County. 

C.3 AGRICULTURAL PARCEL: A portion of real property used by an owner or 

operator for carrying out a specific agricultural operation. Roads, 

vehicle/equipment traffic areas, and facilities, on or adjacent to the cropland are 

part of the agricultural parcel. 

C.4 ALTERNATIVE TILLING: Rotate tillage leaving residue on soil. Tilling 

alternative rows for weed management and wind blown dust allows for 

approximately 50% reduction in field activity in addition to stabilizing soil 

surface and reducing soil compaction. 

C.5 BALING/LARGE BALES: Using balers to harvest crop. It reduces PM 

emissions from crops traditionally harvested by chopping, truck, passes and 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
residue burning. 

C.6 BED/ROW SIZE OR SPACING: Increase or decrease the size of the planting bed 

area (can be done for field and permanent crops). Spacing adjustments reduce the 

number of passes and soil disturbance by increasing plant density/canopy through 

reduction of row width to contain PM within the canopy. 

C.7 CHEMIGATION/FERTIGATION: Application of chemicals through an 

irrigation system. Each application reduces the need to travel in the field for 

application purposes, thus reducing the number of passes and soil disturbance 

while increasing the efficiency of the application. 

C.8 CHIPS/MULCHES, ORGANIC MATERIALS, POLYMERS, ROAD OIL & 

SAND: Application of any nontoxic chemical or organic dust suppressant that 

meets all specification required by any federal, state, or local water agency and is 

not prohibited for use by any applicable regulations. 

C.9 COMBINED OPERATION: To combine equipment, to perform several 

operations during one pass. The reduction in the number of passes necessary to 

cultivate the land will result in fewer disturbances to the soil. Other benefits are 

reduction of soil compaction and time to prepare fields, both of which can be 

precursors to additional tillage requirements. 

C.10 CONSERVATION IRRIGATION: To conserve the quantity of water use, e.g.: 

drip, sprinkler, buried/underground line. Conserving water reduces weed 

population, which in turn reduces the need for tillage as well as reduces soil 

compaction. 

C.11 CONSERVATION MANAGEMENT PRACTICE (CMP): An activity or 

procedure that prevents, reduces, or mitigates PM-10 normally emitted by, or 

associated with, an agricultural activity. 

C.12 CONSERVATION MANAGEMENT PRACTICES PLAN (CMP PLAN): A 

document prepared by the owner or operator of an Agricultural Operation site that 

lists the selected CMPs for implementation. The CMP Plan also contains, but is 

not limited to, contact information for the owner or operator, a description of the 

Agricultural Operation Site and locations of Agricultural Parcels, and other 

information describing the extent and duration of CMP implementation. 

C.13 CONSERVATION TILLAGE (e.g.: no tillage, minimum tillage): Types of tillage 

that reduce loss of soil and water in comparison to Conventional Tillage. It 

reduces the number of passes and amount of soil disturbance. It improves soil 

because it retains plant residue and increases organic matter. 

C.14 COVER CROPS: Use seeding or natural vegetation/regrowth of plants to cover 

soil surface. It reduces soil disturbance due to wind erosion and entrainment. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
C.15 EQUIPMENT CHANGES/TECHNOLOGICAL IMPROVEMENTS: To modify 

the equipment such as tilling; increase equipment size; modify land planing and 

land leveling; matching the equipment to row spacing; granting to new varieties 

or other technological improvements. It reduces the number of passes during an 

operation, thereby reducing soil disturbance. 

C.16 FALLOWING LAND: Temporary or permanent removal from production. 

Eliminates entire operation/passes or reduces activities. 

C.17 GRAVEL: Placing a layer of Gravel with enough depth to minimize dust 

generated from vehicle movement and to dislodge any excess debris which can 

become entrained. 

C.18 GREEN CHOP: The harvesting of a forage crop without allowing it to dry in the 

field. It reduces multiple equipment passes in-field as well as reduces soil 

disturbance and soil compaction. 

C.19 HAND HARVESTING: Harvesting crop by hand. It reduces soil disturbance due 

to machinery passes. 

C.20 INTEGRATED PEST MANAGEMENT: A decision process that uses a 

combination of techniques including organic, conventional and biological farming 

concepts to suppress pest problems. It creates beneficial insect habitat that 

reduces the use of herbicides/pesticides thereby reducing number of passes for 

spraying. It also reduces soil compaction and the need for additional tillage. 

C.21 MECHANICAL PRUNING: Using a machine instead of hand labor to prune 

(Applies as an Unpaved Road CMP only). It reduces vehicle trips, thereby 

reducing PM emissions. 

C.22 MULCHING: Applying or leaving plant residue or other material to soil surface. 

It reduces entrainment of PM due to winds as well as reduces weed competition 

thereby reducing tillage passes and compaction. 

C.23 NIGHT FARMING: Operate at night, if practical, when moisture levels are 

higher and winds are lighter. It decreases the concentration of PM emissions 

during daytime and the increased ambient humidity reduces PM emissions during 

the night. 

C.24 NIGHT HARVESTING: Implementing cultural practices at night, or at times or 

high humidity. It reduces PM by operating when ambient air is moist, thereby 

reducing PM emissions. 

C.25 NO BURNING: Switching to a crop/system that would not require waste burning. 

It reduces emissions associated with burning. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
 

C.26 NON TILLAGE/CHEMICAL TILLAGE: Use flail mower, low volume sprayers 

or heat delivery systems (as harvest pre-conditioner). It reduces soil compaction 

and stabilizes soil through elimination or reduction of soil tillage passes. 

C.27 ORGANIC PESTICIDES: Use biological control methods or non-chemical 

control methods. It reduces chemical use, thereby reducing passes. 

C.28 PAVING: To pave currently Unpaved Roads. 

C.29 PRECISION FARMING (GPS): Using satellite navigation to calculate position in 

the field, therefore manage/treat selective area. It reduces overlap and allows 

operations to occur during inclement weather conditions and at night thereby 

generating less PM. 

C.30 PRE-HARVEST SOIL PREPARATION: Applying a light amount of water or 

stabilizing material to soil prior to harvest (when possible). It reduces PM 

emissions at harvest. 

C.31 RESTRICTED ACCESS: To restrict public access to private roads. It reduces 

vehicle traffic and thus reduces associated fugitive dust. 

C.32 SHED PACKING: Packing commodities in a covered or closed area. It reduces 

field traffic, thereby reducing PM emissions. 

C.33 SHUTTLE SYSTEM/LARGE CARRIER: Multiple bin/trailer. Haul multiple or

larger trailers/bins per trip thereby reducing emissions through reduced passes. 

C.34 SPEED LIMITS: Enforcement of speeds that reduce visible dust emissions. The 

dust emissions from unpaved roads are a function of speed meaning reducing 

speed reduces dust. 

C.35 TRACK-OUT CONTROL: Minimize any and all material that adheres to and 

agglomerates on all vehicle and equipment from unpaved roads and falls onto a 

paved public road or the paved shoulder of a paved public road. 

C.36. TRANSGENIC CROPS: Use of GMO or Transgenic crops such as “herbicideready.” It reduces need for tillage or cultivation operations, as well as reduces 

soil disturbance. It can also reduce the number of chemical applications. 

C.37 WATER APPLICATION: Application of water to unpaved roads and traffic 

areas. 

C.38 WIND BARRIER: Artificial or vegetative wall/fence that disrupts the erosive 

flow of wind over unprotected land. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
D. Requirements for Agricultural Operation Sites: 

D.1 All Persons who own or operate an Agricultural Operation Site of forty (40) acres 

or more in size shall implement in each Agricultural Parcel at least one of the 

Conservation Management Practices listed in Section E.1 for each of the 

following categories: 

D.1a Land preparation and cultivation; 

D.1.b Harvest activities; 

D.1.c Unpaved Roads; 

D.1.d Unpaved Traffic Areas 

D.2 The owner or operator of an Agricultural Operation Site may implement more 

than one Conservation Management Practices for one or more of the categories. 

D.3 The owner or operator of an Agricultural Operation Site shall ensure that the 

implementation of each selected Conservation Management Practices does not 

violate any other local, state, or federal law. 

D.4 The owner or operator of an Agricultural Operation Site may develop alternative 

CMPs. The owner or operator shall submit to the APCD a technical evaluation of 

the alternative CMPs, demonstrating that the alternative CMP achieves PM-10 

emission reductions that are at least equivalent to other CMPs available for the 

applicable operation. The APCD will review the technical evaluation, and the 

alternative CMP must receive approval by the APCD before being included in the 

CMP Plan. 

D.5 The owner or operator shall prepare a CMP Plan for each Agricultural Operation 

Site. The CMP Plan shall be made available to the APCD upon request. The 

CMP Plan shall be provided to the APCD within 72 hours of notice to the owner 

or operator. 

E. Conservation Management Practices for Fugitive Dust (PM-10) 

E.1 The owner or operator of an Agricultural Operation Site shall implement at least

one of the following CMPs in each Agricultural Parcel to reduce PM10 emissions 

from land preparation and cultivation: 

 E.1.a Alternate Till, 

 E.1.b Bed/Row Size Spacing, 

 E.1.c Chemical/Fertigation, 

 E.1.d Combined Operations, 

 E.1.e Conservation Irrigation, 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
 E.1.f Conservation Tillage, 

 E.1g Cover Crops, 

 E.1.h Equipment Changes/Technological Improvements, 

 E.1.i Fallowing Land, 

 E.1.j Integrated Pest Control, 

 E.1.k Mulching, 

 E.1.l Night Farming, 

 E.1.m Non Tillage /Chemical Tillage, 

 E.1.n Organic Pesticides, 

 E.1.o Precision Farming (GPS), or 

 E.1.p Transgenic Crops 

 E.2 The owner or operator of an Agricultural Operation Site shall implement at least

one of the following CMPs in each Agricultural Parcel to reduce PM10 emissions 

from harvesting: 

 E.2.a Baling /Large Bales 

 E.2.b Combined Operations 

 E.2.c Equipment Changes/Technological Improvements 

 E.2.d Green Chop 

 E.2.e Hand Harvesting 

 E.2.f Fallowing Land 

 E.2.g Night Harvesting 

 E.2.h No Burning 

 E.2.i Pre-Harvesting Soil Preparation 

 E.2.j Shed Packing 

 E.2.k Shuttle System/Large Carrier 

E.3 The owner or operator of an Agricultural Operation Site shall implement at least

one of the following CMPs for each Unpaved Road to reduce PM10 emissions: 

 E.3.a Chips/Mulches, Organic Materials, polymers, road oil and sand, 

 E.3.b Gravel 

 E.3.c Paving, 

 E.3.d Restricted access 

 E.3.e Speed limit 

 E.3.f Track-out control 

 E.3.g Water 

 E.3.h Wind barrier 

E.4 The owner or operator of an agricultural operation site shall implement at least 

one of the following CMPs for each unpaved traffic area to reduce PM10 

emissions: 

 E.4.a Chips/Mulches, Organic Materials, Polymers, Road Oil and Sand, 

 E.4.b Gravel 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
 E.4.c Paving, 

 E.4.d Restricted Access 

 E.4.e Speed Limit 

 E.4.f Track-Out Control 

 E.4.g Water 

 E.4.h Wind Barrier 

 

F. CMP Plan Preparation 

An owner or operator shall prepare a CMP Plan for each Agricultural Operation Site. 

Each CMP Plan shall include, but is not limited to, the following information: 

F.1 The name, business address, and telephone number of the owner or operator 

responsible for the preparation and implementation of the CMP Plan. 

F.2 The signature of the owner or operator and the date that the CPM Plan was 

signed. 

 

F.3 The location of the Agricultural Operation Site: cross roads; canal and gate 

number. 

F.4 The crop grown at each location covered by the CMP Plan, total acreage for each 

crop, the length (miles) of unpaved roads, and the total area (acres or square feet) 

of the unpaved equipment and traffic areas to be covered by the CMP Plan, and. 

F.5 The CMPs implemented or planned for implementation. 

F.6 Other relevant information as determined by the APCD. 

 

G. Violations 

Failure to comply with any provisions of this rule shall constitute a violation of 

Regulation VIII. Failure to comply with the provisions of a CMP Plan shall also 

constitute a violation of Regulation VIII. 

H. Record of Control Implementation 

Any Person subject to the requirements of this rule shall maintain a copy of the CMP 

Plan and any supporting documentation necessary to confirm implementation of the 

CMPs. An owner or operator implementing alterative CMPs shall maintain a copy of 

technical evaluation for alternative CMPs and documentation of APCD approval of 

alternative CMPs. Records shall be maintained for two years after the date of each entry 

and shall be provided to the APCD upon request. 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
CONSERVATION MANAGEMENT PRACTICES PLAN 

Farm Name:____________________________________ Owner/Operator: ______________________TelephTotal Farm Acreage:___________________ Address:_____________________________________________Canal & Gate*:___________________________________Crossroads*:_______________________________*List all canals & gates, as well as crossroads, associated to this agricultural operation site at the reverse of this page. In alocation covered by the CMP plan, total acreage for each crop, the length (miles) of unpaved roads, and the total area (acreequipment and traffic areas to be covered by the CMP Plan. Select one or more CMPs from each category: 

Land Preparation and Harvesting Unpaved Roads Cultivation 

 Alternative Till 

 Bailing/Large Bales 

 Dust Suppressants 

 

 Bed/Row Size Spacing 

 Combined Operations 

 Gravel 

 

 Chemical Fertigation 

 Equipment Changes 

 Paving 

 

 Combined Operations 

 Green Chop 

 Restricted Access 

 

 Conservation Irrigation 

 Hand Harvesting 

 Speed Limit 

 

 Cover Crops 

 Fallowing Land 

 Track-out Control 

 

 Equipment Changes 

 Night Harvesting 

 Water 

 

 Fallowing Land 

 Pre-Harvesting Land Prep 

 Wind Barriers 

 

 Integrated Pest Control 

 Shuttle System/Large Carrier 

 Other 

 

 Mulching 

 Shed Packing 

 Night Farming 

 Other 

 Non-Tillage/Chemical Tillage  Organic Pesticides  Precision Farming (GPS)  Transgenic Crops  Other 

I hereby certify that: I am the owner or operator of the agricultural operation site on which this CMP Plan will be implemented; I have a copy of 

Signature:____________________________________Date:__________________________

Imperial County 2009 PM10 SIP Appendix IV.A ICAFINAL AUGUST 2009 
CONSERVATION MANAGEMENT PRACTICES PLAN 

Agricultural Parcel ID ____________________ 

Canal & Gate: _______________________________________________________________ 

Crossroads: __________________________________________________________________ 

Crop Grown: _________________________________________________________________ 

Total Acreage: _______________________________________________________________ 

Approx. Length (miles) of unpaved roads: ________________________________________ 

Approx. Unpaved Equipment Traffic Areas (acres or square feet): ___________________ 

CMPs Selected:_______________________________________________________________ 

Agricultural Parcel ID ____________________ 

Canal & Gate: ________________________________________________________________ 

Crossroads: __________________________________________________________________ 

Crop Grown: _________________________________________________________________ 

Total Acreage: _______________________________________________________________ 

Approx. Length (miles) of unpaved roads: ________________________________________ 

Approx. Unpaved Equipment Traffic Areas (acres or square feet): ___________________ 

CMPs Selected:_______________________________________________________________ 

Agricultural Parcel ID ____________________ 

Canal & Gate: _______________________________________________________________ 

Crossroads: __________________________________________________________________ 

Crop Grown: _________________________________________________________________ 

Total Acreage: _______________________________________________________________ 

Approx. Length (miles) of unpaved roads: ________________________________________ 

Approx. Unpaved Equipment Traffic Areas (acres or square feet): ___________________ 

CMPs Selected:_______________________________________________________________ 

Agricultural Parcel ID ____________________ 

Canal & Gate: ________________________________________________________________ 

Crossroads: __________________________________________________________________ 

Crop Grown: _________________________________________________________________ 

Total Acreage: _______________________________________________________________ 

Approx. Length (miles) of unpaved roads: ________________________________________ 

Approx. Unpaved Equipment Traffic Areas (acres or square feet): ___________________ 

CMPs Selected:_______________________________________________________________ 

Imperial County 2009 PM10 SIP Appendix IV.A ICAPCD Regulation VII Rules FINAL AUGUST 2009 ICAPCD
THIS PAGE INTENTIONALLY LEFT BLANK 
Imperial County 2009 PM10 SIP Appendix IV.B: Cost Effectiveness Analysis of 

ICAPCD Regulation VIII Rules 

APPENDIX IV.B 

COST EFFECTIVENESS ANALYSIS OF

ICAPCD REGULATION VIII RULES 

Note: This cost-effectiveness analysis was originally presented in the 2005 BACM Report prepared in 

support of the November 2005 adoption of the Regulation VIII amendments. 

FINAL AUGUST 2009 ICAPCD 
Imperial County 2009 PM10 SIP Appendix IV.B: Cost Effectiveness Analysis of 

ICAPCD Regulation VIII Rules 

FINAL AUGUST 2009 IV.B-i ICAPCD 

Appendix IV. B 

Cost Effectiveness of ICAPCD Regulation VIII Rules 

Tables IV.B-1 though IV. B-6 of this appendix document the cost-effectiveness of the measures 

in each Regulation VIII rule. These estimates of cost-effectiveness for the individual control 

measures in Regulation VIII are based on the 2004 CARB staff report1

 for SB 656 and the 2003 

San Joaquin Valley PM10 SIP2

. Actual cost-effectiveness estimates for controls in Imperial 

County are expected to be the same or less cost-effective than those listed in the tables. Based 

on the rural nature of Imperial County, it would be expected that the emission reductions 

associated with these measures will be the same or smaller than those assumed in San Joaquin 

Valley and other areas. For example, activity levels associated with emissions (e.g., VMT for 

paved roads, ADVT for unpaved roads) will be lower in Imperial County than other areas, 

resulting in less emissions (and emission reductions) for these sources. Control costs in 

Imperial County may also be higher in some cases. More current information is available for 

Rule 805 implementation on county roads. Based on information from Imperial County’s Public 

Works Department,3

 assuming that 10% of the 119 miles of applicable high ADT roads are 

treated per year for ten years, and a 4% interest rate, the cost-effectiveness of Rule 805 for 

county roads is approximately $795/ton. (The cost-effectiveness of paving would be 

approximately $7,100/ton, but the absolute cost ($2,600,000) is greater than the entire paved 

and unpaved road maintenance budget for Imperial County). These estimates are comparable 

to estimates in the SB 656 staff report ($344/ton to $12,300/ton for stabilizing, gravelling and 

paving)4

 and in other non-attainment area BACM analyses ($2,100/ton to $5,900/ton for paving 

in the San Joaquin Valley)5

. 

Certain dust control measures, as identified in Senate Bill 656, are not included in proposed 

Regulation VIII amendments based on technical reasons such as the source is not present 

(e.g., winter non-skid sand) or the control cannot be applied effectively in Imperial County (e.g. 

PM10-certified sweepers). These measures are identified in Table IV.B-7. No potential BACM 

measures have been excluded based on cost-effectiveness, although a cost-effectiveness 

analysis of Rule 805 provisions for unpaved parking lots and other traffic areas is being 

conducted. 

 

1

 CARB Staff Report, Proposed List of Measures to Reduce Particulate Matter – PM10and PM2.5

(Implementation of Senate Bill 656, Sher 2003), October 18, 2004. 2

 SJVAPCD 2003 PM10 SIP, Appendix G, BACM/T and RACM/T Demonstration for Sources of PM10 and 

PM10 Precursors in the San Joaquin Valley Air Basin, SJVAPCD, April 2003. 3

 Imperial County Public Works cost estimate of compliance, August 1, 2005. 4

 CARB Staff Report, Proposed List of Measures to Reduce Particulate Matter – PM10and PM2.5

(Implementation of Senate Bill 656, Sher 2003), October 18, 2004. 5

 Final BACM Technological and Economic Feasibility Analysis, SJVAPCD 2003 PM10 SIP, April 2003. 
Imperial County 2009 PM10 SIP Appendix I

Re

Table IV.B-1 Cost-effectiveness of measures in Proposed Rule 801 – Construction and Demolitiona 

Rule 801 SB 656 Measure No. SB 656 ($/ton) SJVUA

E.1.a-b Limit VDE to 20% opacity for sites of > 1 acre 24-26.a Watering $301/ton See below F1.a. 

Pre-Activity: Pre-water site and phase work to reduce amount of distributed surface area 

F1.b 

During Active Operation: apply water or chemical stabilizer; or construct and maintain a wind barrier 

F.1.b.3 

Apply water or chemical stabilizer to unpaved haul/access roads and unpaved vehicle/equipment traffic areas 

F.1.c 

Periods of Inactivity: restrict vehicular access; and apply water or chemical stabilizer. If area > 0.5 acres of disturbed surface area remains unused for 3 7 days, area must comply with conditions for stabilized surface area 

24-26.b, 

39, and 40 

RACM to BACM upgrade: $197/ton [NOTE: based on SCAQMD minor upgrades. Not appropriate comparison]

Apply water and/or end of day: $7,222Prohibit Demolition wind>25 mph: $847Dust Monitoring: $231,000-$339,00012% soil moisture f$21,600-$56,000/to15 mph speed limit posting of speed limDust Control PlansRequire notification operations $2,480-

 a Construction and demolition source categories are below the DM level. FINAL AUGUST 2009 IV.B-1 
Imperial County 2009 PM10 SIP Appendix I

Re

Table IV.B-2 Cost-effectiveness of measures in Proposed Rule 802 – Bulk Materialsa

Rule 802 SB 656 Measure No. SB 656 ($/ton) SJVUA

E 

Limit VDE to 20% opacity for bulk material handling, material transport, and haul trucks 

28.a $1,151/ton (handling) to $28,293/ton (storage) None Reported 

F.1.a Spray with water prior F.1.b Apply and maintain chemical stabilizer 28.b and 41a. 

RACM to BACM upgrade: $197/ton 

F.1.c 

Protect from wind erosion by sheltering or enclosing the operation and transfer line 

28.a See 28.a. above 

F.1.d 

Cover bulk materials stored outdoors with tarps, plastic, or other material 

28.a See 28.a. above 

Require Construcenclosures with 5$659,000/ton 

F.2.a 

Completely cover or enclose all Haul Truck loads of Bulk Material 

28.a See 28.a. above 

F.2.b, c, d Material transport: cover, freeboard, housekeeping 28.b See 28.b. above 

a Bulk materials source categories are below the DM level. FINAL AUGUST 2009 IV.B-2 
Imperial County 2009 PM10 SIP Appendix I

Re

Table IV.B-3 Cost-effectiveness of measures in Proposed Rule 803 – Track-out and Carry-outa

Rule 803 SB 656 Measure No. SB 656 ($/ton) SJVU

F.1.a 

Rapidly clean up any bulk material tracked out or carried out onto a paved road surface by the end of the day 

29-30 

Manual Sweeping: $3,54/tonControl devices installed at access points to public roads: $13,700 to $322,000/ton Length of paved interior roads: $7,930 to $186,000/ton 

All sites with access to a paved road and with > 150 ADT, or > 20 ADT by vehicles with > 3 axles shall: 

F.1.b Install one or more Track-Out Prevention Devices or wash down system at access points; or F.1.c 

Apply and maintain paving, chemical stabilization, or gravel for a distance of 50 or more consecutive feet at access points 

29.b <$100/ton 

Impose Rule $44,100-$387Require trackbe 25 ft long $13,700-$322Require pave100 ft long an$7,930-$186,Gravel pads: 

a Track-out and carry-out source categories are below the DM level. FINAL AUGUST 2009 IV.B-3 
Imperial County 2009 PM10 SIP Appendix I

Re

Table IV.B-4 Cost-effectiveness of measures in Proposed Rule 804 – Open Areasa

Rule 804 SB 656 Measure No. SB 656 ($/ton) SJVU

E.1 Limit open areas to VDE of 20% opacity 31.a Watering: $7020/ton E.2 

Prevent unauthorized vehicle access by posting "No Trespassing" signs or installing physical barriers such as fences, gates, posts, and/or appropriate barriers to prevent access 

31.a Watering: $7,020/ton None Report

F.1.a Apply and maintain water or dust suppressant to all undefeated areas F.1.b Establish vegetation on all previously disturbed 

areas 

F.1.c Pave, apply and maintain gravel or apply and maintain chemical stabilizers/suppressants 

27, 31, and 42 

27b. RACM to BACM upgrade: $197/ton 31a. Watering: $7,020/ton 31b. RACM to BACM upgrade: $197/ton 42. $697/ton 

Impose Rule urban parcels that have a leof disturbed sImpose Rule immediately adisturbance: 

a Open areas source categories are below the DM level. FINAL AUGUST 2009 IV.B-4 
Imperial County 2009 PM10 SIP Appendix I

Re

Table IV.B-5 Cost-effectiveness of measures in Proposed Rule 805 – Paved and Unpaved Roadsa

Rule 805 – Unpaved Roads SB 656 Measure No. SB 656 ($/ton) SJVU

F.1.a Pave or F.1.b Apply chemical stabilizers or F.1.c 

Apply and maintain gravel, recrushed/recycled asphalt or other material of low silt content to a depth of > 3 inches or 

F.1.d Apply water one or more time daily or F.1.F Permanent road closure or F.1.f 

Any other method to meet VDE of 20% opacity and meets conditions of a stabilized unpaved road 

35-36 

35. Apply water, gravel, chemical or dust suppressant, or pave: $344 to $12,293/ton 36a. -Apply water, dust suppressant, gravel, pave: $56 to $1,481/ton -Paving: $2,160 to $5,920/ton 36b. $958/ton 

Limit speed tRequire roadpaved: $2,16Impose Rule 

F.2.a through g Canal Roads measures None Not Estimated None ReportUnpaved Traffic Areas F.3.a Pave or F.3.b Apply chemical stabilizers or F.3.c 

Apply and maintain gravel, recrushed/recycled asphalt or other material of low silt content to a depth of 3 3 inches or 

F.3.d Apply water one or more time daily 

35-36 See 35-36 above See 35-36 ab

a The unpaved roads source category is above the DM level. FINAL AUGUST 2009 IV.B-5 
Imperial County 2009 PM10 SIP Appendix I

Re

Table IV.B-5 Cost-effectiveness of measures in Proposed Rule 805 – Paved and Unpaved Roadsa (Cont

Rule 805 – New and Modified Paved Roads SB 656 Measure No. SB 656 ($/ton) SJVU

F.4.1 

New or modified paved roads with projected ADT > 500 vehicles shall construct paved shoulders of 4 ft (500-3000 vehicle trips) or 8 ft (> 3000 vehicle trips) 

32 

F.4.2 

In lieu of F.4.1, a curbing adjacent to and contiguous with the travel lane or paved shoulder or road may be constructed 

32 

F.4.3 

In lieu of F.4.1, intersections, auxiliary entry lanes and auxiliary exit lanes may be constructed adjacent to and contiguous with the roadway 

32 

F.4.4 

Medians shall be constructed with paved shoulders having a minimum width of 4 ft. adjacent to traffic lanes for projects ADT > 500, unless speed limit < 45 mph with curbing or landscaped medians 

32 

F.6 In lieu of paving or vegetation requirement, may apply oils or other chemical stabilizers 32 

32a. 4 ft. Paved shoulder on 50% of highest ADT existing paved roads: $7,290- $11,300/ton 32b. Curb and Gutter road shoulder: $5577/ton 

4ft paved shonew/modified $13,800-$5084ft paved shoADT existing $11,300/ton Require winddeposition to 24 hrs: $2,85

a The unpaved roads source category is above the DM level. FINAL AUGUST 2009 IV.B-6 
Imperial County 2009 PM10 SIP Appendix I

Re

Table IV.B-6 Cost-effectiveness of measures in Proposed Rule 806 – Conservation Management Practic

Rule 806 SB 656 Measure No. SB 656 ($/ton) SJVU

D.1 

Shall implement at least one of the following practices if own/operate a commercial farm of > 40 acres: land preparation and cultivation; harvest activities; unpaved roads; or unpaved equipment operation yards. 

D.4 Prepare and submit a CMP application for each agricultural operation site 

43.b 

High-wind tilling prohibition and stabilization of fallow fields: $134/ton 

E.1 Land Preparation and Cultivation 43.c $8/ton E.2 Harvesting 43.b None provided E.3 Unpaved Farm Roads 43.a and d $958/ton E.4 Equipment Traffic Areas 43.d $958/ton 

Overall: $8 to $2,500/t

a The tilling agricultural operations source category is above the DM level. FINAL AUGUST 2009 IV.B-7 
Imperial County 2009 PM10 SIP Appendix I

Re

FINAL AUGUST 2009 IV.B-8 Table IV.B -7 Senate Bill 656 Measures Not in the Proposed Regulation VIII Amended Rules 

SB656 Cost-Effectiveness Comment 

33 Requires use of certified PM10 efficient street 

$1,119/ton (1996$) A Rule 1186-certified sweeper is $37,000 more expensive than a conventional sweeper. 

As noted in the SJVUAPCD “BACM/BACDemonstration for Sources of PM10 and the San Joaquin Valley Air Basin,” (dated of these units will result in safety problemrural roads in flat terrain.” The overwhelin Imperial County are freeways and ruraFor maximum efficiency, sweepers must mph. 

34 

Requires vacuum-street sweeping on roads to remove sand and cinders that are placed on the road during winter storms as an antiskid material. . 

$350/ton (1996$) (assumes 2,400 lb/day winter-day emission reductions) 

This Great Basin Unified APCD measure Imperial County, where anti-skid materia

37a. 

and 

37b. 

Weed Abatement Activities Pre-activity Requirements: 1) Pre-watering to limit VDE opacity to 20%; or 2) phasing work to reduce amount of disturbed surface area. Apply water during active operations to limit VDE to 20% opacity. Apply water or chemical stabilizers to meet conditions of stabilized surface. 

Not estimated Emissions from this source are not quan

de minimis. 

38 Defines windblown dusts NA No specific requirements. 

41b. Additional bulk material control requirements for Coachella Valley source $352 - $462 /ton (1992 $) Controls specific to Coachella Valley blo

does not exist in Imperial County. 
Imperial County 2009 PM10 SIP Appendix IV.C: IC PM10 Emissions Accounting for 

Regulation VIII Fugitive Dust Controls 

A PPENDIX IV.C 

IMPERIAL COUNTY PM10 EMISSIONS ACCOUNTING FOR 

REGULATION VIII FUGITIVE DUST CONTROLS

FINAL AUGUST 2009 ICAPCD 
Imperial County 2009 PM10 SIP Appendix IV.C: IC PM10 Emissions Accounting for 

Regulation VIII Fugitive Dust Controls 

Appendix IV.C 

Imperial County PM10 Emissions 

Accounting for Regulation VIII Fugitive Dust Controls 

Imperial County fugitive dust control rules (Regulation VIII rules) were adopted in November 

2005 and became fully effective in January 2006—with the notable exception that Rule 805 

allows the mitigation of unpaved roads to be phased over a 10-year schedule (Table IV.C-1). 

This Appendix reports best estimates/projections of the Imperial County “grown and controlled” 

PM10 emission inventories for years 2006-2010 (Tables IV.C-2 to IV.C-4), calculated by 

subtracting emission reductions due to control and mitigation of PM10 sources (described in 

Chapter 4 of the SIP document) from the “grown” emission inventories presented in Appendix 

III.A. For controlled PM10 emissions from unpaved roads, entries in Tables IV.C-2 to IV.C-4

were calculated based on the expected implementation schedule of Rule 805 (Table IV.C-1). 

FINAL AUGUST 2009 IV.C-i ICAPCD 
Imperial County 2009 PM10 SIP Appendix IV.C: IC PM10 Emissions Accounting for 

Regulation VIII Fugitive Dust Controls 

Table IV.C-1 Implementation Schedule Required by Rule 805 for Mitigation of Unpaved 

County/City Roads,a and Associated Reductions

2006 2007 2008 2009 2010 2012 2015 

Unpaved road miles to be treatedb 19.9 39.8 59.7 79.6 99.5 139.3 199 

Expected reductionsc (tpd) 0.87 1.75 2.62 3.49 4.36 6.11 8.73 

a

Although Rule 805 also applies to unpaved canal roads with >20 ADVT, the Imperial Irrigation District has estimated 

that traffic is presently below that threshold on all canal roads. bBased on an estimate by the Imperial County Public 

Works Department of 199 miles of unpaved city/county roads (see Attachment A of Appendix III.A). c

Based on a 60% 

composite control factor assuming graveling is chosen as the primary control option (ENVIRON, Draft Final Technical 

Memorandum, Regulation VIII BACM Analysis, prepared for the ICAPCD, October 2005, p. 42). 

FINAL AUGUST 2009 IV.C-1 ICAPCD 
Imperial County 2009 PM10 SIP Appendix IV.C: IC PM10 Emissions Accounting for 

Regulation VIII Fugitive Dust Controls 

Table IV.C-2 Imperial County PM10 “Grown and Controlled” Annual Average Emission 

Inventorya

 in 2006-2010 (tpd) 

Source Category 2006 2007 2008 2009 2010

Fuel Combustion 0.42 0.42 0.42 0.42 0.43

Waste Disposal 0.00 0.00 0.00 0.00 0.00

Cleaning/Surface Coatings 0.00 0.00 0.00 0.00 0.00

Petroleum Production/Marketing 0.00 0.00 0.00 0.00 0.00

Industrial Processes: 2.70 2.74 2.77 2.84 2.87

Mineral Processes 2.54 2.57 2.61 2.67 2.71 

Food/Agriculture 0.16 0.16 0.16 0.16 0.16 

Solvent Evaporation 0.00 0.00 0.00 0.00 0.00

Res Fuel Combustion 0.09 0.09 0.09 0.09 0.10

Farming: 8.51 8.51 8.51 8.50 8.50

Tilling 5.10 5.10 5.10 5.09 5.09 

Harvest 0.01 0.01 0.01 0.01 0.01 

Cattle 3.26 3.26 3.26 3.26 3.26 

Construction 1.96 1.97 1.98 2.00 2.01

Paved Road Dust 3.19 3.40 3.61 3.81 3.64

Entrained Unpaved Road Dust: 55.74 54.93 54.13 53.33 52.52

City/County 23.78 22.98 22.17 21.37 20.56 

Canal 29.57 29.57 29.57 29.57 29.57 

BLM/USFS 1.34 1.34 1.34 1.34 1.34 

Farm 1.05 1.05 1.05 1.05 1.05 

Windblown: 208.52 208.45 208.37 208.30 208.22

Open Areas-Urban 0.00 0.00 0.00 0.00 0.00 

Open Areas-Othersb 168.35 168.35 168.35 168.35 168.35

Unpaved Roads 29.43 29.36 29.29 29.22 29.15 

City/County 7.75 7.68 7.61 7.55 7.48 

Canal 16.32 16.32 16.32 16.32 16.32 

BLM/USFS 0.37 0.37 0.37 0.37 0.37 

Farm 4.90 4.90 4.90 4.90 4.90 

Non-Pasture Ag Lands 8.81 8.81 8.80 8.80 8.79 

Pasture 1.79 1.78 1.78 1.78 1.78 

Fires 0.00 0.00 0.00 0.00 0.00

Waste Burning 2.75 2.73 2.71 2.69 2.67

Cooking 0.06 0.06 0.06 0.06 0.07

On-Road Mobile 1.01 0.93 0.88 0.82 0.77

Other Mobile 0.98 0.96 0.95 0.95 0.94

Total 286 285 284 284 283

a

Annual averages accounting for projected growth in emission-generating activities, but not for reductions due to 

control or mitigation of PM10 sources. Entries corresponding to the summed contributions of subcategories are in 

italics. b

Grasslands, dunes, and other barren lands (see Table 3.1 of the main document). As documented in 

Appendix III.B, emissions were estimated using available information on the conditions of the vacant lands (e.g., 

desert areas of barren, grass/shrubland, and dunes). Reported emissions also include the conservatively-estimated 

contributions due to soil disturbances caused by off-road vehicle usage. 

FINAL AUGUST 2009 IV.C-2 ICAPCD 
Imperial County 2009 PM10 SIP Appendix IV.C: IC PM10 Emissions Accounting for 

Regulation VIII Fugitive Dust Controls 

Table IV.C-3 Imperial County PM10 “Grown and Controlled” Winter Average Emission 

Inventorya

 in 2006-2010 (tpd)

Source Category 2006 2007 2008 2009 2010

Fuel Combustion 0.35 0.35 0.35 0.36 0.36

Waste Disposal 0.00 0.00 0.00 0.00 0.00

Cleaning/Surface Coatings 0.00 0.00 0.00 0.00 0.00

Petroleum Production/Marketing 0.00 0.00 0.00 0.00 0.00

Industrial Processes: 2.70 2.75 2.78 2.85 2.88

Mineral Processes 2.53 2.57 2.60 2.67 2.70 

Food/Agriculture 0.17 0.18 0.18 0.18 0.18 

Solvent Evaporation 0.00 0.00 0.00 0.00 0.00

Res Fuel Combustion 0.16 0.16 0.17 0.17 0.17

Farming: 9.75 9.75 9.74 9.74 9.74

Tilling 6.30 6.30 6.30 6.30 6.29 

Harvest 0.01 0.01 0.01 0.01 0.01 

Cattle 3.26 3.26 3.26 3.26 3.26 

Construction 1.79 1.80 1.82 1.83 1.84

Paved Road Dust 3.11 3.32 3.52 3.72 3.56

Entrained Unpaved Road Dust: 33.05 32.58 32.10 31.62 31.15

City/County 14.10 13.62 13.15 12.67 12.19 

Canal 17.54 17.54 17.54 17.54 17.54 

BLM/USFS 0.79 0.79 0.79 0.79 0.79 

Farm 0.62 0.62 0.62 0.62 0.62 

Windblown: 219.51 219.47 219.42 219.38 219.33

Open Areas-Urban 0.01 0.01 0.01 0.01 0.01 

Open Areas-Othersb 189.75 189.75 189.75 189.75 189.75

Unpaved Roads 17.45 17.41 17.37 17.33 17.29 

City/County 4.60 4.56 4.52 4.47 4.43 

Canal 9.68 9.68 9.68 9.68 9.68 

BLM/USFS 0.22 0.22 0.22 0.22 0.22 

Farm 2.91 2.91 2.91 2.91 2.91 

Non-Pasture Ag Lands 10.76 10.76 10.75 10.75 10.74 

Pasture 1.37 1.37 1.37 1.37 1.37 

Fires 0.00 0.00 0.00 0.00 0.00

Waste Burning 2.75 2.73 2.71 2.69 2.67

Cooking 0.06 0.06 0.06 0.06 0.07

On-Road Mobile 1.02 0.93 0.88 0.83 0.78

Other Mobile 0.94 0.91 0.91 0.90 0.90

Total 275 275 274 274 273

a

Winter (November-April) averages accounting for projected growth in emission-generating activities, but not for 

reductions due to control or mitigation of PM10 sources. Entries corresponding to the summed contributions of 

subcategories are in italics. b

Grasslands, dunes, and other barren lands (see Table 3.1 of the main document). As 

documented in Appendix III.B, emissions were estimated using available information on the conditions of the vacant 

lands (e.g., desert areas of barren, grass/shrubland, and dunes). Reported emissions also include the 

conservatively-estimated contributions due to soil disturbances caused by off-road vehicle usage. 

FINAL AUGUST 2009 IV.C-3 ICAPCD 
Imperial County 2009 PM10 SIP Appendix IV.C: IC PM10 Emissions Accounting for 

Regulation VIII Fugitive Dust Controls 

FINAL AUGUST 2009 IV.C-4 ICAPCD 

Table IV.C-4 Imperial County PM10 “Grown and Controlled” Summer Average Emission 

Inventorya

 in 2006-2010 (tpd)

Source Category 2006 2007 2008 2009 2010

Fuel Combustion 0.48 0.49 0.49 0.49 0.49

Waste Disposal 0.00 0.00 0.00 0.00 0.00

Cleaning/Surface Coatings 0.00 0.00 0.00 0.00 0.00

Petroleum Production/Marketing 0.00 0.00 0.00 0.00 0.00

Industrial Processes: 2.69 2.73 2.76 2.83 2.86

Mineral Processes 2.55 2.58 2.61 2.68 2.71 

Food/Agriculture 0.14 0.15 0.15 0.15 0.15 

Solvent Evaporation 0.00 0.00 0.00 0.00 0.00

Res Fuel Combustion 0.02 0.02 0.02 0.02 0.02

Farming: 7.27 7.27 7.27 7.27 7.27

Tilling 3.90 3.90 3.89 3.89 3.89 

Harvest 0.01 0.01 0.01 0.01 0.01 

Cattle 3.26 3.26 3.26 3.26 3.26 

Construction 2.12 2.14 2.15 2.16 2.18

Paved Road Dust 3.27 3.48 3.70 3.90 3.73

Entrained Unpaved Road Dust: 78.42 77.29 76.16 75.03 73.90

City/County 33.46 32.33 31.20 30.06 28.93 

Canal 41.61 41.61 41.61 41.61 41.61 

BLM/USFS 1.88 1.88 1.88 1.88 1.88 

Farm 1.48 1.48 1.48 1.48 1.47 

Windblown: 197.79 197.70 197.59 197.49 197.39

Open Areas-Urban 0.00 0.00 0.00 0.00 0.00 

Open Areas-Othersb 147.30 147.30 147.30 147.30 147.30

Unpaved Roads 41.40 41.30 41.21 41.11 41.01 

City/County 10.91 10.81 10.71 10.62 10.52 

Canal 22.96 22.96 22.96 22.96 22.96 

BLM/USFS 0.52 0.52 0.52 0.52 0.52 

Farm 6.90 6.90 6.90 6.90 6.90 

Non-Pasture Ag Lands 6.89 6.89 6.88 6.88 6.87 

Pasture 2.20 2.20 2.20 2.20 2.20 

Fires 0.00 0.00 0.00 0.00 0.00

Waste Burning 2.75 2.73 2.71 2.69 2.67

Cooking 0.06 0.06 0.06 0.06 0.07

On-Road Mobile 1.00 0.92 0.87 0.82 0.77

Other Mobile 1.03 1.00 1.00 0.99 0.99

Total 297 296 295 294 292

a

Summer (May-October) averages accounting for projected growth in emission-generating activities, but not for 

reductions due to control or mitigation of PM10 sources. Entries corresponding to the summed contributions of 

subcategories are in italics. b

Grasslands, dunes, and other barren lands (see Table 3.1 of the main document). As 

documented in Appendix III.B, emissions were estimated using available information on the conditions of the vacant 

lands (e.g., desert areas of barren, grass/shrubland, and dunes). Reported emissions also include the 

conservatively-estimated contributions due to soil disturbances caused by off-road vehicle usage. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 ICAPCD 

A PPENDIX V 

EXCEEDENCES OF THE 24-HOUR PM10 NAAQS AT THE 

US-MEXICO BORDER: ANALYSIS OF THE IMPACT OF 

INTERNATIONAL EMISSIONS 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V.i ICAPCD 

Table of Contents 

Page 

1 Introduction and Background 1

2 Toolkit for Analysis of Potential International Transport Events 5

2.1 Introduction: USEPA Guidelines 5

2.2 Statistical Analyses 6

2.2.1 Introduction 6

2.2.2 Approach I: Statistical Analyses of Impact of Mexican Emissions 6

2.2.3 Approach II: Statistical Analyses of the Impact of US Emissions 17

2.3 Other Analyses 23

2.3.1 Approach III: Analysis of Sample Filters 23

2.3.2 Approach IV: Comparison of Emission Inventories on Both Sides of the Border 26

2.3.3 Approach V: Discussion of Dispersion Modeling Results 28

3 Discussion and Analysis of Potential Transport Episodes 31

3.1 December 21, 2006 31

3.1.1 Description of the Episode 31

3.1.2 Statistical Analyses 35

3.1.3 Additional Evidence 36

3.1.4 Weight of Evidence 37

3.2 December 25, 2006 38

3.2.1 Description of the Episode 38

3.2.2 Statistical Analyses 43

3.2.3 Additional Evidence 44

3.2.4 Weight of Evidence 46

4 Conclusion 47

List of Tables 

Table V.1 PM10 Measurements in Excess of the 24-Hour NAAQS in Imperial County for the Time 

Period 2006-2008 1

Table V.2 Imperial County Population by Cities in January 2005 2

Table V.3 Imperial County Population Growth Since 1999 2

Table V.4 Effect of 24-Hour Wind Speed at the Grant Station (FMTS = 0-1) 10

Table V.5 Effect of Wind Direction on Days when Average Wind Speed at the Grant Station ≥1.5 

Knots 11

Table V.6 Effect of Wind Direction on Days when Average Wind Speed at the Grant station <1.0 

Knots 12

Table V.7 Excess PM10 Concentrations at Grant, PM10 Concentrations in Mexicali, and CrossBorder Wind Direction on Days when Average Wind Speed at Grant <1.0 Knots 13

Table V.8 Estimates of Excess PM10 at Grant as a Result of US Emissions Alone (All Days with 

Average Wind Speed at the Grant Station ≥1.0 Knots) 14

Table V.9 Effect of Wind Speed at the Calexico Ethel station (FMTS = 0-1) 15

Table V.10 Effect of Wind Direction on Days when Average Wind Speed at Ethel ≥2.5 knots 16

Table V.11 Effect of Wind Direction on Days when Average Wind Speed at Ethel <1.75 knots 16
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V.ii ICAPCD 

Table V.12 Estimates of Excess PM10 at Ethel as a Result of US Emissions Alone (All Days in 2002-

2007) 16

Table V.13 PM10 Emission Inventory for Imperial County in 2005 18

Table V.14 Imperial County Winter PM10 Emission Inventory (2005)a 26

Table V.15 Imperial County vs. Mexicali Municipality Population and PM10 emissions (2005) 27

Table V.16 Population within distance R of the Calexico Grant Station (2005), and Associated 

Relative Impacts at Calexico Grant 28

Table V.17 PM10 air quality at Mexicali monitors on December 21, 2006 31

Table V.18 Hourly Wind Speeds (WS, knots) and Wind Directions (WD) at US stations near Calexico 

on December 21, 2006 33

Table V.19 Hourly Wind Speeds (WS, in knots) and Wind Directions (WD) recorded in Mexicali on 

December 21, 2006 33

Table V.20 PM10 air quality at Imperial County monitors on December 25, 2006 38

Table V.21 PM10 air quality at Mexicali SSI monitors on December 25, 2006 39

Table V.22 Hourly Wind Speeds (WS, knots) and Wind Directions (WD) at US stations near Calexico 

on December 25, 2006 41

Table V.23 Hourly Wind Speeds (WS, in knots) and Wind Directions (WD) recorded in Mexicali on 

December 25, 2005 41

List of Figures 

Figure V.1 Plot of PM10 concentrations at the El Centro station vs. average PM10 concentrations at 

the Brawley and Westmorland stations (left), and plot of PM10 concentrations at the Grant 

station vs. average PM10 concentrations at the El Centro and Brawley stations (right). 21

Figure V.2 Spatial Distribution of 24-Hour PM10 Concentrations in Imperial County and Mexicali for 

December 21, 2006 32

Figure V.3 Calexico-Grant 36-hour back trajectory ending at 12 pm PST on December 21, 2006 34

Figure V.4 Calexico-Grant Wind Rose Analysis for December 21, 2006 35

Figure V.5 Dust emissions being created in Mexicali due to lack of paving. 37

Figure V.6 Spatial Distribution of 24-Hour PM10 Concentrations in Imperial County and Mexicali for 

December 25, 2006 40

Figure V.7 Calexico-Grant 36-hour back trajectory ending at 12 pm PST on December 25, 2006 42

Figure V.8 Calexico-Grant Wind Rose Analysis for December 25, 2006 43

Figure V.9 Mexicali Newspaper article published on December 24, 2006 45

Figure V.10 Mexicali Newspaper article published on December 25, 2006 45

List of Attachments 

Attachment A: Supporting Figures for Appendix V 

Attachment B: US Area Directly South of the Calexico-Grant Station 

Attachment C: Photographs of PM Filters 

Attachment D: 2001 PM10 Attainment Demonstration Report 

p:\i\imperial\sip 2008\2009 ic pm10 sip\2009 pm10 sip document\final august 18 2009\uncontrolled\appendix v_final draft_11aug09.doc
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-1 ICAPCD 

1 Introduction and Background 

Introduction. Section 179B(d) of the federal Clean Air Act (CAA) provides that, 

notwithstanding any other provision of law, any State that establishes to the satisfaction of 

USEPA that a PM10 nonattainment area in such State would have attained the PM10 National 

Ambient Air Quality Standard (NAAQS) by the applicable attainment date but for emissions 

emanating from outside the United States shall not be subject to the provisions of CAA Section 

188(b)(2). During the years 2006 to 2008 there were two days in Imperial County with recorded 

station concentrations in excess of the 24-hour NAAQS1

 (Table V.1). Exceedences on these 

days occurred at the Calexico-Grant monitor, which is less than 1 mile from the Mexican border. 

Note that the December 25, 2006 exceedence is the design value for this SIP. 

Table V.1 PM10 Measurements in Excess of the 24-Hour NAAQS in 

Imperial County for the Time Period 2006-2008 

Date Monitor Measurement (μg/m3

) 

December 21, 2006 Calexico Grant 171 

December 25, 2006 Calexico Grant 248 

The analysis conducted for these days indicates that PM10 emissions from Mexico impacted US 

PM10 monitors in Calexico, California. Research and analysis on the build up and transport 

characteristics of PM10 emissions indicates that under stagnant and very light wind conditions, 

elevated PM10 emissions from Mexican sources can contribute to PM10 levels in Calexico due to 

“overflowing” of PM10 across the border. This Appendix describes the tools used to analyze, 

both quantitatively and qualitatively, the impact of Mexican emissions on near-border Imperial 

County stations. Also included are detailed descriptions of the events of December 21, 2006 

and December 25, 2006, as well as technical analyses of the international PM10 transport 

contributions to the exceedences. The analyses show, through both quantitative and qualitative 

approaches, that concentrations at Calexico stations on each of these days would not have 

exceeded the NAAQS but for international transport of PM10 from Mexico. 

Although in 2006-2008 exceedences were only observed at the Grant station, we extend the 

scope of this Appendix to enable analysis of the impact of Mexicali emissions at the CalexicoEthel station as well. The reasons for this are two-fold. First, an understanding of the impact of 

transported Mexicali PM at the Calexico-Ethel station is important in its own right (either for 

analysis of historical exceedences or for analysis of future elevated PM measurements at 

Ethel). Second, results at the Ethel station provide a valuable element of comparison for results 

at the Grant station. 

 

1

 These are in addition to exceedences due to high winds exceptional events recorded on September 2, 2006, April 

12, 2007, and June 5, 2007 (documented in separate reports). PM10 in Imperial County is reported on a 1 day-in-6 

schedule
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-2 ICAPCD 

Background. Imperial County is located at the southeastern corner of California and occupies 

4,597 square miles. It borders Mexico to the south, Riverside County to the north, San Diego 

County to the west and the state of Arizona to the east. The area known as the Imperial Valley 

runs approximately northwest-southwest through the center of the county and extends into 

Mexico. In 2005, the total population for Imperial County was estimated at ~162,000 people, 

and its three most populated cities were El Centro (41,000 people), Calexico (36,200 people), 

and Brawley (24,000 people). Imperial County’s population distribution and population growth in 

recent years are reported in Tables V.2 and V.3. 

Table V.2 Imperial County Population by Cities in January 2005 

City/County Population 1/1/2005 (in thousands) 

Imperial County 161.6 

Brawley 24.0 

Calexico 36.2 

Calipatria 7.9 

El Centro 41.0 

Holtville 5.7 

Imperial 9.6 

Westmorland 2.4 

Balance of County 34.8 

Source: State of California, Department of Finance2

Table V.3 Imperial County Population Growth Since 1999 

Year Population (July 1, in thousands) 

1999 140.8 

2000 143.6 

2001 146.2 

2002 150.0 

2003 154.8 

2004 159.5 

2005 164.3 

2006 169.9 

Source: State of California, Department of Finance2

Across the US-Mexico border from Calexico is the city of Mexicali, of estimated population 

>600,000 inhabitants.2

 As analyzed in Section 2.3.2, the potential for PM10 emissions south of 

the border is expected to be high given the greater activity south of the border (since activity is 

generally a function of population) and existence of multiple fugitive dust sources, including 

vacant lands and significant areas with predominantly unpaved roads. 

 

2

 Population Demographics for the Imperial and Mexicali Valleys, California Center for Border and Regional 

Economic Studies, CCBRES Bulletin, Vol. 8, No. 3 & 4, March/April 2007 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-3 ICAPCD 

Description of Conditions Generally Characteristic of Transport Events. Historical trends 

have demonstrated that the impacts of Mexicali emissions on Calexico air quality are highest 

under the following scenario: 

• Emission levels in Mexicali are high; 

• Stagnant atmospheric conditions result in low dispersion of pollution; 

• PM10 air concentrations in Mexicali reach high levels; 

• Mexicali PM10 “overflows” into Calexico and/or light winds from a generally southerly 

direction transport Mexicali pollution over short distances. 

Purpose. The purpose of this Appendix is two-fold. First, it describes the “toolkit” used to 

assess the potential for international transport under specific meteorological conditions; second, 

it applies the tools to the identified exceedence episodes in 2005-2007 to determine both 

qualitatively and quantitatively the contribution of Mexican emissions to those exceedences. An 

attainment demonstration “but for” the contribution of Mexican emissions would require that 

none of the identified exceedences would have occurred but for emissions outside of the United 

States (US).

Appendix Organization. This Appendix is organized as follows: 

• Section 2 presents the toolkit used in the analyses of the potential transport events. The 

toolkit is based on the five approaches described in the 1994 Federal Register guidance 

regarding CAA Section 179B. For the first two approaches, this Appendix presents novel 

statistical analyses that produce a quantitative assessment of the minimal contribution of 

Mexican emissions to US-side PM10 levels (Approach I) and the maximum expected 

impact on US-side PM10 levels from US emissions (Approach II). These complementary 

approaches are used to quantitatively assess specific exceedence episodes as to 

whether they would have exceeded the NAAQS but for emissions from outside of the 

US. The other three approaches are qualitative assessments used to see if their results 

are consistent with the quantitative results of the first 2 approaches. 

• Section 3 presents a discussion and analysis of each exceedence that was potentially 

impacted by emissions outside of the US. For each exceedence episode, this section 

presents the following:

o A description of the episode, including PM10 measurements in Imperial 

County/Mexicali and meteorological conditions (surface winds, back trajectory 

analysis, and wind rose analysis); 

o Results of statistical analyses (Approaches I and II) to estimate the impacts of 

emissions originating from across the border and the impact of US emissions 

alone; 

o Additional evidence, including the results of the qualitative Approaches III, IV and 

V, as well as day-specific information on activity or emissions, if any; and 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-4 ICAPCD 

o A Weight of Evidence (WOE) analysis of the exceedence to determine if it would 

have shown attainment of the NAAQS but for emissions from outside the US. 

• Section 4 presents the conclusions of this attainment demonstration. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-5 ICAPCD 

2 Toolkit for Analysis of Potential International Transport 

Events

2.1 Introduction: USEPA Guidelines 

For PM10 nonattainment areas, section 179B(a) of the CAA provides that the USEPA must 

approve a SIP if (i) it meets all the applicable requirements under the Act other than the 

requirement to demonstrate attainment and maintenance of the PM10 NAAQS by the applicable 

attainment date, and (ii) the state demonstrates to the satisfaction of the USEPA that the SIP 

would be adequate to attain and maintain the federal standard by the attainment date but for 

emissions emanating from outside the US. USEPA guidance3

 to demonstrate “but-for” 

attainment in State Implementation Plans involving international border areas describes the 

following 5 approaches: 

1. “Evaluate and quantify...changes in monitored PM10 concentrations [in the U.S., near the 

border] with predominant wind direction;” 

2. “Demonstrate that local U.S. emissions ... [do] not cause the NAAQS to be exceeded;” 

3. “Analyze ambient sample filters for specific types of particles emanating from across the 

border;” 

4. “Inventory...sources on both sides of the border and compare the magnitude of PM10

emissions originating within the US to those emanating from outside the US;” and 

5. “Perform air dispersion and/or receptor modeling to quantify the relative impacts [of US 

and international sources].” 

The guidance further instructs that states may use any number of these approaches, or other 

techniques, “depending on their feasibility and applicability, to evaluate the impact of emissions 

emanating from outside the US on the nonattainment area.” The following sections document 

our efforts to implement the 5 suggested approaches, to the extent possible, to the 

Calexico/Mexicali international area. We begin in Section 2.2 with statistical analyses designed 

to quantitatively assess the impact of both Mexican and US emissions based on historical 

patterns in meteorology and air quality (Approaches I and II). Sample filter analysis, relative 

emission inventories, and air dispersion modeling (Approaches III, IV, and V) are then 

addressed in Section 2.3. 

NOTE: For ease of presentation, detailed plots and figures for this section are in Attachment A 

of this Appendix. Those plots and figures are referenced as Figure V.A.(figure number). 

 

3

 State implementation plans for serious PM10 nonattainment areas, and attainment date waivers for PM10

nonattainment areas generally; Addendum to the general preamble for the implementation of Title I of the Clean Air 

Act Amendments of 1990, Federal Register, Vol. 59, No. 157, Tuesday, August 16, 1994, p. 41998 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-6 ICAPCD 

2.2 Statistical Analyses 

2.2.1 Introduction 

Approaches I and II are complementary analyses that rely on meteorology and PM10

measurements for the exceedence days of interest and for days with similar characteristics to 

predict the impact of both US and Mexican emissions at Calexico monitors on exceedence 

days. 

Algebraically, PM10 concentrations on any given day at either one of the Calexico stations are 

the sum of impacts of both US and Mexican emissions (Equation 1). Each of those impacts has 

a component that can be predicted based on historical patterns of meteorology and air quality 

using information about the day of interest and about days with similar characteristics, and can 

be decomposed into an expected value4

 plus a term capturing the error of prediction (Equation 

2). 

(1) 

(2) 

where Y and Z refer to the impact of US and Mexican emissions (respectively) at the Calexico

station of interest, E[Y] and E[Z] are the predictable component of those impacts based on 

historical patterns and using information about the day of interest and about days with similar 

characteristics, and the terms eY and eZ capture the errors of prediction for those impacts. The 

subscript i indicates that reference is made to parameter values for a specific day i. 

Approach II estimates E[Yi] and a range for Yi using PM10 measurements at other Imperial 

County stations on the day of interest as the explanatory variables (i.e. as predictor variables). 

Approach I estimates E[Zi] based on PM10 measurements in Imperial County and Mexicali on 

days with characteristics similar to the day of interest (in terms of meteorology, and, to a lesser 

extent, Mexicali air quality). Thus, the two approaches are complementary both (i) because their 

results depend primarily on complementary data,5

 and (ii) because they focus on 

complementary components of PM10 concentrations in Calexico. 

2.2.2 Approach I: Statistical Analyses of Impact of Mexican Emissions 

Purpose and Introduction. The first approach listed in the USEPA preamble is to “evaluate 

and quantify...changes in monitored PM10 concentrations [in the US nonattainment area near 

 

4

 In the notation used throughout Approaches I and II, E[] refers to the expected value of the parameter within 

brackets. Note that E[] is a linear operator, i.e. E[A+B] = E[A] + E[B] for any variables A and B. 5

 I.e., the day of interest in Approach II, and days similar to the day of interest in Approach I. 

10 i

 i

,

,

 at Calexico impact of US emissions at Calexico

 impact of Mexican emissions at Calexico

i.e., PM 

or PM

day i day

day

Calexico i i i

Calexico i

PM

Y Z

=

+

= +

, , [] [ ] = ++ + EY e EZ e i Yi i Zi
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-7 ICAPCD 

the international border] with changes in the predominant wind direction”. Accordingly, this 

section presents the results of statistical analyses of PM10 concentration data in Imperial County 

as a function of meteorological conditions (and to a lesser extent, PM10 concentrations in 

Mexicali). 

As stated in the previous section, PM10 concentration at Calexico monitors on any given day is 

the sum of the impact of US emissions (Y) and of the impact of Mexican emissions (Z). The 

impact of Mexican emissions depends primarily on the following three factors: Mexicali 

emissions, atmospheric dispersion, and wind direction. We will use the 24-hour average wind 

speed at Calexico stations (denoted VC) as a metric related to dispersion, and the Fraction of 

total cross-border air Mass Transport that is from the South6

 (FMTS) to quantify wind direction. 

Under stagnant conditions, PM10 concentrations in Mexicali are expected to be a reasonable 

measure of Mexicali emissions. The present analysis is motivated by the expectation that, 

contrary to the impact of Mexican emissions at Calexico stations, the impact of US emissions at 

Imperial County stations does not depend strongly on wind direction,7

 all other things equal. For 

a given set of conditions:8

10 10 , , , ,, Calexico V FMTS MexicaliPM V FMTS V FMTS MexicaliPM C C C PM = + Y Z (3) 

If there is zero air flux from Mexico into the US, then there cannot be impact of Mexican 

emissions on Calexico monitors (except through simple diffusion), so ZFMTS=0 ≈ 0. Therefore, the 

expected value of the impact of Mexican emissions at Calexico for a given set of conditions is 

approximately the difference: 

( )

10

10

10

10

, , , 0

, , , , 0 , 0

, , , , 0

, ,

[ ] [ ]

[ ][ ][ ][ ]

[ ] [ ] [ ]

[ ]

C C

C C C C

C C C

C

Calexico V FMTS MexicaliPM Calexico V FMTS

V FMTS V FMTS MexicaliPM V FMTS V FMTS

V FMTS MexicaliPM V FMTS V FMTS

V FMTS MexicaliPM

E PM E PM

EY EZ EY EZ

E Z EY EY

E Z

=

= =

=

−

=+ − −

= +−

≈

 (4) 

where “expected values” refer to values obtained by averaging over a very large number of days 

with the specified characteristics. Note that in Equations 3 and 4 (and likewise in Equations 5-6 

 6

 FMTS is the fraction of the daily total cross-border air flux that corresponds to air flow from Mexico into IC. Crossborder air flux is directly proportional to the component of wind speed that is perpendicular to the border. Thus, if n

is a unit vector perpendicular to the border and pointing toward Mexico, and v denotes the hourly average wind 

speed vector, FMTS = (∑v·n>0 v·n)/∑|v·n|, where the first sum is over all the hours of the day when the wind 

direction has some “southerly” component, and the second sum is over all hours of the day. Geometrical 

representations of the calculations are shown in Figures V.A.1 and V.A.2. 7

 We show on page V.14 that this expectation is not strictly valid, but that the estimates of Z obtained by assuming 

that the impacts of US emissions are independent of wind direction are conservative. 8

 In the notation that follows, vertical bars are used to indicate conditioning. Thus, the last term of Equation 3 refers 

to the impact of Mexican emissions under the set of conditions defined by specified values of the parameters VC, 

FMTS, and Mexicali PM10 concentrations. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-8 ICAPCD 

below), VC and FMTS are calculated based on wind velocity measurements at the Calexico 

station of interest. 

Thus, the impact of Mexicali emissions at Calexico monitors under specific conditions might be 

studied by comparing Calexico air quality for a set of days with the specified characteristics 

relative to days when FMTS = 0, all other things equal. However, a better approach is to study 

the differences in same-day PM10 measurements between the Calexico stations and other 

Imperial County stations, rather than the absolute values of Calexico ambient air quality 

measurements. For example, for a specific set of conditions, the same-day difference in air 

quality measurements at Calexico and El Centro is: 

10 10 , , , ,, , ( ) Calexico ElCentro V FMTS MexicaliPM V FMTS V FMTS MexicaliPM V FMTS C C C C PM PM − = Y Z + −W (5) 

where W is the impact of US emissions at El Centro (assumed here to be largely independent of 

both wind direction at Calexico and of Mexicali air quality). Therefore, the expected value of the 

impact of Mexican emissions at Calexico for a given set of conditions can also be approximated 

by the difference: 

( ) ( )

10

10

10

, , , 0

, , , , 0 , 0 ,

, ,

[( ) ] [( ) ]

 [] [ ] [ ][ ] [ ]

[ ]

C C

C C C C C

C

Calexico ElCentro V FMTS MexicaliPM Calexico ElCentro V FMTS

V FMTS MexicaliPM V FMTS V FMTS V FMTS V FMTS

V FMTS MexicaliPM

E PM PM E PM PM

E Z EY EY EW EW

E Z

=

= =

− − −

= +− + −

≈

 (6) 

The advantage of estimating 10 , , [ ] V FMTS MexicaliPM C E Z by using same-day PM10 differences 

between stations rather than the difference in PM10 measurements at the same Calexico station 

on different days is that better estimates are obtained for data sets of limited size.9

 

Our objective in the present approach is to estimate the contribution of Mexican emissions to 

Calexico PM10 concentrations on December 21, 2006 and December 25, 2006, based on the 

pattern of PM10 concentrations for historical days with similar characteristics. The analysis 

focuses on the difference in PM10 measurements between the Calexico stations and the other 

Imperial County stations; these differences10 are investigated as a function of (i) 24-hour 

average wind speed at Calexico (VC), (ii) wind direction (FMTS), and (iii) PM10 concentrations in 

Mexicali. The next subsection outlines data selection and availability. Following subsections 

document general results of the statistical analyses at the Calexico Grant and Calexico Ethel 

stations. 

 

9

 This is because the impact of Mexican emissions on Calexico air quality corresponds to a large fraction of the 

same-day PM10 difference between Calexico and other IC stations, but to a smaller fraction of the absolute value of 

PM10 concentrations at Calexico. 10 That is, the differences of same-day measurements between the Calexico Grant (or the Calexico Ethel) station and 

any one of the El Centro, Brawley, Westmorland, or Niland stations. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

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FINAL AUGUST 2009 V-9 ICAPCD 

Data Selection and Availability. The impact of Mexicali emissions at Calexico stations has 

evolved in time. Reasons for this include the tremendous growth of the city of Mexicali over the 

last decade, and increasing effects in recent years to control PM emissions in Mexicali (e.g., by 

increasing public awareness). It is probable that emission reductions in Mexicali have played an 

important role in attainment of the NAAQS at Calexico in 2008. Our goal is to analyze the 

December 2006 exceedences in the context of historical patterns descriptive of the actual time 

frame in which the exceedences occurred. As a result, there is a compromise to be made 

between the size of the database used in the statistical analysis (which database can be made 

larger by including increasingly older or more recent data) and the quality of the data in 

accurately representing the conditions of the December 2006 time period of interest. In this 

analysis, we choose to consider historical data for years 2001-2007. Within this group: 

• Days for which PM10 measurements were not available in Calexico were not included in 

the analysis; 

• Given that the potential transport days in question for this analysis have very low wind 

speeds, days with high winds were not included in the analysis.11 The PM10

concentrations measured on these days are largely influenced by windblown dust in the 

Imperial Valley, and are not representative of the phenomenon of cross-border transport 

on low-wind days that we seek to understand and model. 

Data availability is as follows: 

• Wind direction data at the Grant station for a large fraction of 200212 were corrupt and 

thus unavailable; 

• The Grant Station was decommissioned in August 2007; and 

• Meteorological data for the Calexico-Ethel station in 2001 were not available. 

We are therefore considering the time period from January 2001 to July 2007 at Calexico Grant, 

and from January 2002 to December 2007 at Calexico Ethel. 

Our objectives are to obtain an understanding of the effects of wind speed, wind direction, and 

Mexicali PM10 concentrations on the impact of Mexicali emissions at Calexico PM10 monitors. 

Although we seek a qualitative understanding of these effects over a broad range of conditions, 

we are particularly interested in a quantitative understanding for stagnant atmospheric 

conditions in order to analyze the December 21, 2006 and December 25, 2006 episodes. 

Consequently, the following analyses are not intended to provide a comprehensive 

 

11 These included days for which abnormally high PM10 measurements due to high winds were recorded at multiple 

stations far from the border (usually, two more 24-hr PM10 measurements above 100 μg/m3

 at any of the El Centro, 

Brawley, Westmorland, and Niland stations, on days with concurrently high winds), as well as days when maximum 

hourly wind speed AND 24-hour average wind speed at either of the Calexico stations were in excess of 18 knots 

and 12 knots, respectively. 12 The recorded wind direction data at the Grant station for the time intervals of May 2 to June 15, 2002, August 24 to 

September 12, 2002, and September 29, 2002 to January 15, 2003 are corrupt. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

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FINAL AUGUST 2009 V-10 ICAPCD 

documentation of air quality trends under all conditions, but rather to place emphasis on results 

pertinent to days with low wind speeds. 

Analysis at the Grant Station 

Effects of Wind Speed on PM10 at Calexico Grant. Wind speed is a key determinant of the level 

of atmospheric dispersion. Beginning with a simple analysis of the effect of wind speed alone 

(i.e., ignoring the effect of wind direction and of Mexicali emission levels), we find that on days 

with stagnant conditions in Calexico (i.e., 24-hour average wind speeds of 0 to 1 knots), PM10

concentrations in Calexico are higher than those observed at other Imperial County stations 

(Table V.4). Plots of PM10 concentrations differences vs. average speed show an inverse 

relationship between the excess PM10 concentrations at Grant and the 24-hour average of wind 

speed at the Grant station (Figure V.A.3), so that the highest PM10 concentration differences are 

typically measured on low wind speed days. Given that the PM10 concentrations measured on 

low wind speed days are predominantly due to anthropogenic, activity-related sources rather 

than to windblown dust, the above results are consistent with our expectations that (i) there are 

more anthropogenic sources of PM10 in Mexicali than in Imperial County, and that (ii) this 

difference in emissions density has a more pronounced impact at low levels of atmospheric 

dispersion. 

Table V.4 Effect of 24-Hour Wind Speed at the Grant Station (FMTS = 0-1) 

average speeda [PM10] difference between Grant and other IC stationsb # points in 

range (knots) Brawley El Centro Niland West. sampling 

0-1 57±51 61±47 64±39 53±38 ≥24 

1-1.5 36±33 42±36 55±40 45±38 ≥40 

1.5-2 30±23 30±25 38±27 28±23 ≥50 

2-3 23±22 22±23 23±26 16±20 ≥59 

3-5 19±25 20±21 18±29 11±26 ≥69 

5-10 14±28 17±18 21±29 12±27 ≥48 a

24-hour average wind speed at the Calexico Grant station. bNumbers on the left are sample averages of the daily 

differences in 24-hour PM10 measurements at the Grant station and the other Imperial County station indicated, in 

μg/m3

. The numbers on the right correspond to the samples’ standard deviations about the averages. 

To test this understanding, it is useful to continue with an analysis of the effect of wind speed on 

days with predominantly northerly flow in Calexico/Mexicali (for which the impact of Mexicali 

emissions is minimized). Plots of the PM10 difference between the Grant station and nonCalexico Imperial County stations as a function of average 24-hour wind speed at Grant on days 

with FMTS = 0-0.03 are given in Figure V.A.4. The plots clearly show downward trends, i.e., that 

“excess” PM10 concentrations at Grant (PM10 at Grant minus PM10 at non-Calexico Imperial 

County stations) when FMTS ≈ 0 are higher at lower wind speeds. There are two potential 

explanations for this trend. First, if indeed US emissions are slightly higher at Calexico than 

throughout the remainder of Imperial County, then the impact of this difference will be higher at 

low atmospheric dispersion. Second, measurements of wind direction are not very accurate 

under very stagnant conditions, resulting in random errors in the calculation of FMTS at low 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-11 ICAPCD 

wind speeds. In other words, the lower the 24-hour average wind speed at Grant, the more 

likely it is that there was actually some impact of Mexican emissions at Calexico Grant in spite 

of the calculated value of FMTS in the 0 to 0.03 range. 

To find out the magnitude of the effect of 24-hour wind speed on the impact of US emissions 

alone, we investigated results at non-Calexico stations (control study). The plots of Figure V.A.5 

show no clear or pronounced relationship between the average impact of US emissions at the 

non-Calexico monitors and the 24-hour average wind speed at the monitors (there is an 

downward trend at El Centro, but upward trends at Westmorland and Niland). Likewise, the 

difference in PM10 concentrations between the Westmorland and Niland stations (which is ~10 

μg/m3

 on average) does not seem to depend strongly on 24-hour average wind speed at 

Westmorland. These results at non-Calexico stations suggest that the difference in the impact of 

US emissions alone on Imperial County stations is largely insensitive to average wind speed, 

and that therefore the higher values of (PM10 at Grant – PM10 at other Imperial County stations) 

shown in Figure V.A.4 and in Table V.4 at low values of VC are predominantly the effect of the 

impact of Mexican emissions. 

Effects of Wind Direction on PM10 at Calexico Grant. Although the study of cross-border air 

transport focuses on the differences in air quality at Calexico resulting from changes in wind 

direction, the results of the previous subsection suggest that when assessing the ambient air 

quality in Calexico it is necessary to conduct a data analysis based on both wind direction AND 

wind speed. As stated previously, the fraction of total cross-border air mass transport that is 

from the south (FMTS) was used to quantify northerly/southerly wind direction. The effect of 

FMTS on the PM10 concentration difference between Calexico Grant and other Imperial County 

stations is shown for different conditions of atmospheric dispersion in Tables V.5 and V.6. At 

higher FMTS, the difference becomes larger, which is consistent with the hypothesis that PM10

concentrations at the Calexico Grant station are impacted by emissions from Mexicali. Also, the 

smaller the atmospheric dispersion, the greater the impact of Mexicali emissions can be (as 

evidenced by the higher values in Table V.6 compared to the values of Table V.5). 

Table V.5 Effect of Wind Direction on Days when Average Wind Speed at the Grant Station ≥1.5 

Knots 

south wind [PM10] difference between Grant and other IC stations # points in 

(FMTSa) Brawley El Centro Niland West. sampling 

0-0.03 1±19 12±12 11±19 2±20 ≥31 

0.03-0.9 22±25 22±23 25±29 17±25 ≥141 

0.9-1 32±20 28±19 34±27 23±21 ≥46 a

Calculated using wind direction and wind speed measurements at the Calexico Grant Station 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

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FINAL AUGUST 2009 V-12 ICAPCD 

Table V.6 Effect of Wind Direction on Days when Average Wind Speed at the Grant station <1.0 

Knots 

south wind [PM10] difference between Grant and other IC stations # points in 

(FMTSa) Brawley El Centro Niland West. sampling 

0-0.1 30±32 38±23 47±27 31±30 ≥8 

0.1-0.4 49±28 52±27 61±29 54±28 11 

0.4-1 122±67 109±66 114±51 100±43 ≥4 a

Calculated using wind direction and wind speed measurements at the Calexico Grant Station 

The differences in PM10 measurements between the Calexico Grant and other Imperial County 

stations when FMTS is close to 0 are expected to largely represent the differences in the impact 

of US emissions alone on the several stations.13 In other words, the results of the first row of 

Table V.5 indicate that on days when wind speed at Grant ≥1.5 knots, the impact of US 

emissions alone cause PM10 at Grant to be higher than at non-Calexico stations by an average 

of 1-12 μg/m3

. This result suggests that US emissions are slightly higher in the areas 

surrounding Calexico than throughout the rest of Imperial County (although as stated in footnote 

13, some of the 1-12 μg/m3

 excess may in reality be attributable to carried-over Mexican 

emissions). 

Effects of PM10 Concentrations in Mexicali on PM10 at Calexico Grant. The cross-border 

transport of PM10 into Imperial County is the result of cross-border transport of air containing 

PM10 originating from Mexicali sources. We investigated the PM10 concentrations in Mexicali and 

Calexico for correlations between air quality in Mexicali and air quality in Calexico. Our analysis 

in this subsection focuses on days when the highest impact of Mexican emissions at Calexico 

Grant is expected to occur, i.e., on days when average wind speed at Grant <1.0 knots (i.e., the 

days from which the entries of Table V.6 were derived). Relevant information for the 26 days of 

that data set is reported in Table V.7, where the days were sorted in ascending order based on 

the average difference of the PM10 concentration at Grant relative to other non-Calexico Imperial 

County stations (the “excess” PM10 concentration). 

Correlations between PM10 concentration in Mexicali and “excess” PM10 concentrations at Grant 

(relative to other Imperial County stations) are visualized by plotting the “excess” PM10

concentrations at Grant (i.e., the 2nd column of Table V.7) against the PM10 concentrations at 

Mexicali monitoring sites (i.e., the last 6 columns of Table V.7). The data show a consistent 

trend such that higher PM10 concentrations in Mexicali correspond to higher “excess” PM10

concentrations at the Calexico Grant station, with significantly better correlations if the 8 days 

with FMTS <0.1 (i.e., mostly northerly flow) are omitted. Results are plotted in Figure V.A.6; for 

 13 Note that this is conservative because there may in fact be some PM10 from Mexican sources on the US side (e.g., 

transported the day prior) even when the calculated value of FMTS for the day of interest is ~0. For convenience, 

we use the term “impact of US emissions alone” in this document to refer to the conservative estimates of the 

contribution of US sources obtained from the statistical analyses when FMTS ~0. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

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FINAL AUGUST 2009 V-13 ICAPCD 

reference the locations of Imperial County and Mexicali monitoring stations are shown in 

Figure V.A.7. 

Table V.7 Excess PM10 Concentrations at Grant, PM10 Concentrations in Mexicali, and CrossBorder Wind Direction on Days when Average Wind Speed at Grant <1.0 Knots 

 Grant Station [PM10] at Mexicali Stations 

Date excess PM10a wind speedb FMTS Cobach Carretera UABC Progresso ITM Conalep

1-Dec-06 9 0.8 0.05 160 112 72 372 - 174 

19-Nov-06 11 0.9 0.09 179 103 69 436 47 161 

27-Aug-06 15 0.8 0.07 244 63 - 192 44 152 

26-Sep-06 16 0.9 0.04 138 - 82 241 64 162 

12-Nov-05 18 0.7 0.17 147 - 67 - 46 104 

13-Dec-06 24 0.6 0.04 191 94 91 336 72 268 

25-Sep-05 25 0.8 0.13 113 56 58 274 - 68 

18-Dec-05 26 0.6 0.14 190 84 113 221 108 131 

10-Feb-06 28 0.6 0.29 218 101 159 373 - - 

24-Mar-06 39 1.0 0.07 145 39 - 194 - 89 

31-Dec-06 52 0.9 0.62 252 113 188 429 102 127 

5-Feb-07 55 0.8 0.11 - 63 - 421 - 97 

11-Jan-06 57 0.7 0.21 214 120 106 401 - 137 

6-Nov-05 62 0.6 0.37 218 - - 366 - - 

30-Nov-05 63 0.7 0.02 - 114 138 366 100 - 

6-Dec-05 65 0.9 0.17 210 74 102 399 78 136 

7-Nov-06 69 0.4 0.07 - - - - - - 

13-Nov-06 76 1.0 0.64 198 156 197 297 68 97 

24-Dec-05 77 0.6 0.23 - - - - - - 

1-Nov-06 78 0.5 0.27 - - 106 - - 213 

19-Dec-06 79 0.9 0.06 138 55 75 287 48 116 

30-Dec-05 104 0.6 0.56 - 87 117 310 72 - 

4-Feb-06 106 0.9 0.32 246 119 159 405 103 125 

21-Dec-06 117 0.4 0.85 - - - - - - 

12-Dec-05 156 0.6 0.97 - 147 - 391 139 130 

25-Dec-06 223 0.5 0.41 437 266 240 466 274 305 a

Reported numbers correspond to the difference between the 24-hour PM10 measurement at the Grant station and 

the average of the 24-hour PM10 measurements at the El Centro, Brawley, Westmorland, and Niland stations on the 

day of interest (in μg/m3

). b24-hour average wind speed at the Grant station (in knots). 

Estimation of Mexicali’s Contribution to Measured PM10 Concentrations at Calexico Grant. 

According to Equation 6, the expected impact of Mexican emissions at Grant under a set of 

specific conditions may be expressed as: 

( ) ( )

10 10 , , , , , 0

,0 , , , 0

[ ] [( ) ] [( ) ]

 [ ] [ ] [ ] [ ]

C C C

C C CC

V FMTS MexicaliPM Calexico ElCentro V FMTS MexicaliPM Calexico ElCentro V FMTS

V FMTS V FMTS V FMTS V FMTS

E Z E PM PM E PM PM

EY EY EW EW

=

= =

= − − −

+ −+ −

where W is the impact of US emissions at the El Centro station,14 and VC and FMTS are 

calculated based on wind velocity measurements at the Calexico Grant station. 

 

14 Similar equations can be written by reference to the Brawley, Westmorland, and Niland stations. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

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FINAL AUGUST 2009 V-14 ICAPCD 

In the case of homogeneous surface density of PM10 emissions in the areas surrounding a 

station, air quality at the station is expected to be independent of wind direction. Plots of PM10

measurements at non-Calexico Imperial County stations as a function of wind direction (Figures 

V.A.8 and V.A.9) reveal instead that the impacts on air quality at the El Centro, Brawley, 

Westmorland, and Niland stations are higher for southerly flow than for northerly flow.15 As a 

consequence, the term , , 0 [ ][ ] EW EW V FMTS C C − V FMTS= in the above equation is ≥0 for all values of 

FMTS. Furthermore, because there is only a very narrow band of US territory south of the 

Calexico stations, we expect the impact of US emissions at Calexico stations to be higher for 

northerly flow than for southerly flow.16 Accordingly, we can reasonably expect 

,0 , [ ][ ] EY EY V FMTS C C = − V FMTS ≥0 also, so that approximating the impacts of Mexican emissions at 

Grant using 10 10 , , , , , 0 [ ] [( ) ] [( ) ] V FMTS MexicaliPM C Calexico ElCentro V FMTS MexicaliPM C Calexico ElCentro V FMTS C E Z = − E PM PM − − E PM PM =

consistently underestimates the contribution of Mexico to PM pollution at Grant. 

Next, we found in the previous subsection that the difference in the impact of US emissions 

alone on Imperial County stations is largely insensitive to average wind speed. Consequently, 

the difference17

, 0 [( ) ] Grant Non Calexico IC station V FMTS C E PM PM − = is largely independent of VC.

18 As a result, 

we will use a single value to approximate the difference , 0 [( ) ] Grant Non Calexico IC station V FMTS C E PM PM − = at 

all values of VC. These single values are calculated in the present case by averaging the 

“excess” PM10 at Calexico for all days with FMTS = 0-0.03 and VC >1.0 knots,19 and are 

reported in Table V.8. 

Table V.8 Estimates of Excess PM10 at Grant as a Result of US Emissions Alone (All Days with 

Average Wind Speed at the Grant Station ≥1.0 Knots) 

south wind [PM10] difference between Grant and other IC stations # points in 

(FMTSa) Brawley El Centro Niland West. sampling 

0-0.03 2±18 11±12 11±18 2±19 ≥35 a

Calculated using wind direction and wind speed measurements at the Calexico Grant Station 

 

15 Note that the trends are less pronounced in Figure V.A.8 than in Figure V.A.9, consistent with the expectation that 

the influence of wind direction on W decreases with decreasing wind speed. 16 The reasoning for this argument is the following: (i) outside of Calexico, there are no US sources to the south, but 

all of Imperial County to the north; (ii) within Calexico, inspection of Figure V.A.10 also shows more activity-related 

sources north of the stations (or at least a comparable number of sources north and south), and (iii) in the direct 

vicinity of the Grant station (where both the December 21 and December 25, 2006 exceedences were recorded), 

activity-related PM10 emissions are once again expected to be comparatively lower south of the station, where the 

only source is the low-traffic Calexico airport (servicing only a handful of small planes that land and take-off daily 

on a paved runway) and other activities are limited. A more detailed analysis of the area south of the Grant station 

is presented in Attachment B of this Appendix. 17 This difference is the expected difference of the impact of US emissions alone at the Grant station relative to the 

impact of US emissions alone and a non-Calexico station, at a specified value of VC. 18 Except at very low values of wind speeds when calculated values of FMTS are less accurate and the difference 

may include impacts of Mexicali emissions as well. 19 This restriction is to minimize calculation errors resulting from less accurate calculated values of FMTS for VC <1.0 

knots. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-15 ICAPCD 

The following procedure is then used to get a conservative estimate of the expected impact of 

pollution coming from Mexico at the Grant station on any specified day: 

1. Identify the subset of historical days with characteristics similar to the specific day of 

interest (in terms of VC, FMTS, and if possible Mexicali PM10 concentrations); 

2. For each of the Brawley, El Centro, Niland, and Westmorland stations, approximate the 

expected value of the difference (PM10 at Grant) – (PM10 at non-Calexico station) for the 

specific day of interest as the historical average of these differences for the days with 

similar characteristics (this result corresponds to the expected excess PM10 at Grant due 

to the impact of both US and Mexican emissions); and 

3. Subtract the bolded values from Table V.8 (which correspond approximately—and 

conservatively—to excess PM10 at Grant due to the impact of US emissions alone). 

This procedure will be used in Section 3 to analyze whether the PM10 exceedences measured 

on December 21, 2006 and December 25, 2006 would have occurred “but-for” emissions from 

Mexico. 

Analysis at the Calexico Ethel Station 

In this section, we pattern our analysis after the analysis conducted for the Calexico Grant 

station, but limit our discussion the effect of wind speed and wind direction on excess PM10

concentrations at Ethel (relative to PM10 concentrations at other non-Calexico stations). 

Effects of Wind Speed on PM10 at Calexico Ethel. PM10 concentrations at the Calexico-Ethel 

station were also found to be much higher than those observed at other Imperial County 

stations on days with low atmospheric dispersion (Table V.9).20 The plots of PM10

concentrations differences vs. average speed (Figure V.A.11) again show the inverse 

relationship between excess ambient PM10 in Calexico (relative to other Imperial County 

stations) and wind speed at Calexico. This relationship is consistent with that illustrated in the 

same plots for the Grant station (Figure V.A.3). 

Table V.9 Effect of Wind Speed at the Calexico Ethel station (FMTS = 0-1) 

average speeda [PM10] difference between Ethel and other IC stations # points in 

range (knots) Brawley El Centro Niland West. sampling 

0-1.75 57±28 49±27 75±34 61±33 ≥18 

1.75-2.25 31±19 29±21 44±24 34±21 ≥33 

2.25-3 22±28 23±29 33±30 23±28 ≥51 

3-5 17±19 16±19 20±22 13±19 ≥90 

5-15 6±21 11±14 10±22 4±20 ≥75 a

24-hour average wind speed at the Calexico Ethel station. 

 

20 Note that wind speeds were generally higher at Calexico Ethel, so the intervals of wind speeds in the first column of 

Table V.9 and in subsequent analyses for the Ethel station were not chosen to match those for the Grant station. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-16 ICAPCD 

Effects of Wind Direction on PM10 at Calexico Ethel. The effect of FMTS on the PM10

concentration differences between Calexico Ethel and non-Calexico Imperial County stations is 

shown for different conditions of atmospheric dispersion in Tables V.10 and V.11 below. Results 

are qualitatively similar to those at Grant. 

Table V.10 Effect of Wind Direction on Days when Average Wind Speed at Ethel ≥2.5 knots 

south wind [PM10] difference between Ethel and other IC stations # points in 

(FMTSa) Brawley El Centro Niland West. sampling 

0 -2±20 9±14 9±15 -2±20 ≥30 

0-0.1 10±17 17±20 20±22 12±20 ≥48 

0.1-1 19±23 16±23 20±28 14±23 ≥117 a

Calculated using wind direction and wind speed measurements at the Calexico Ethel Station 

Table V.11 Effect of Wind Direction on Days when Average Wind Speed at Ethel <1.75 knots 

south wind [PM10] difference between Ethel and other IC stations # points in 

(FMTSa) Brawley El Centro Niland West. sampling 

0-0.35 46±15 47±24 65±22 53±23 ≥10 

0.35-1 71±35 51±35 95±49 77±46 ≥6 a

Calculated using wind direction and wind speed measurements at the Calexico Ethel Station 

Estimation of Mexicali’s Contribution to Measured PM10 Concentrations at Calexico Ethel. 

Following the reasoning outlined for analysis at the Grant station, we assess that the average 

“excess” PM10 concentration at Ethel21 due to US emissions alone is the average difference 

between same-day PM10 measurements at Calexico Ethel and non-Calexico stations on days 

when FMTS ≈ 0 (and VC is sufficiently high for accurate estimates of FMTS). Because there 

were no days with calculated FMTS = 0 and average wind speed at Ethel <2.5 knots, and 

because ≥30 points is a reasonable sample size, the entries in the first row of Table V.10 

(reproduced in Table V.12) will be used as an approximation for all values of VC of the excess 

PM10 at Ethel due to US emissions alone (conservative).

Table V.12 Estimates of Excess PM10 at Ethel as a Result of US Emissions Alone (All Days in 

2002-2007) 

south wind [PM10] difference between Ethel and other IC stations # points in 

(FMTS) Brawley El Centro Niland West. sampling 

0 -2±20 9±14 9±15 -2±20 ≥30 

 

21 That is, the difference between PM10 measurements at Ethel and PM10 measurements at non-Calexico Imperial 

County stations. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-17 ICAPCD 

In order to conservatively estimate the expected impact of Mexicali emission sources at the 

Ethel station on any specified day of interest, the following steps may be repeated at Ethel: 

1. Identify the subset of historical days with meteorological conditions at Ethel similar to 

those on the specific day of interest (in terms of VC and FMTS at Ethel); 

2. For each of the Brawley, El Centro, Niland, and Westmorland stations, approximate the 

expected value of the difference (PM10 at Ethel) – (PM10 at non-Calexico station) for the 

specific day of interest as the historical average of these differences for the days with 

similar meteorology (this result corresponds to the expected excess PM10 at Ethel due to 

the impact of both US and Mexican emissions); and 

3. Subtract the bolded values from Table V.12 (which correspond approximately to excess 

PM10 at Ethel due to the impact of US emissions alone). 

2.2.3 Approach II: Statistical Analyses of the Impact of US Emissions 

Discussion of the Approach. The second approach listed in the USEPA preamble is to 

“demonstrate that the impact of [US sources alone] on the nonattainment area... does not 

cause the NAAQS to be exceeded,” after accounting for the influx of background PM10 into the 

area. The USEPA guidelines suggest beginning with a comprehensive inventory of PM10

emissions within the U.S. in the vicinity of the nonattainment area. With an emissions inventory, 

air dispersion modeling can be used to estimate the impact of these emissions on air quality. 

The use of dispersion modeling to evaluate whether US emissions alone would have caused 

exceedences is discussed in Approach V. In this section, the emissions inventory is analyzed in 

conjunction with the monitoring data collected throughout Imperial County to determine if US 

emissions alone would have led to the measured exceedances at the Calexico Grant and Ethel 

stations. This approach (without modeling)22 is consistent with USEPA guidance. 

Analysis of the Imperial County Emission Inventory: Spatial Distribution of US Emission 

Sources. The air quality measured by any monitor is primarily a function of two sets of inputs. 

These are the levels of emissions and the meteorology within the areas surrounding the monitor 

(the latter governs the build-up and transport of pollution). In general, the impact of emission 

sources on the monitor decrease with increasing distance from the monitor: for the Imperial 

County stations, “background” US PM10 concentrations are the impact of sources from within 

Imperial County (and in some cases from surrounding US counties as well), while the greater 

portion of PM10 concentrations measured at any station is likely the result of local sources. 

Because of the flat terrain and geographical proximity among the several Imperial County 

monitoring stations, meteorological conditions relevant to air quality at one Imperial County 

station are generally comparable to meteorological conditions relevant to air quality at another 

Imperial County station (this approximation is of course better for stations within close proximity, 

 

22 EPA has stated (Federal Register, Volume 66, Number 203, October 19, 2001, p. 53107) that implementation of 

their second approach was intended to be conducted without modeling. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-18 ICAPCD 

such as the Westmorland and Brawley stations, or the Calexico and El Centro stations). 

Furthermore, US emissions within the populated portion of the Imperial County (this populated 

portion is clearly distinguishable from the desert portion on satellite photographs such as Figure 

V.A.12) are also relatively evenly distributed throughout the county. This is so because the 

dominant sources of PM10 emissions within the populated area of the Imperial Valley 

(accounting for ~ 89% of emissions23 according to the 2005 Imperial County Emission Inventory, 

Table V.13) are unpaved roads and agricultural lands, and these sources are distributed evenly 

throughout the populated portion of Imperial County. 

Table V.13 PM10 Emission Inventory for Imperial County in 2005 

Source Category Annual Average (tpd) 

Fuel Combustion 0.41

Waste Disposal 0.00 

Cleaning/Surface Coatings 0.00 

Petroleum Production/Marketing 0.00 

Industrial Processes 2.79 

Solvent Evaporation 0.00 

Res Fuel Combustion 0.09 

Farming 9.88 

Construction 2.20 

Paved Road Dust 3.38 

Entrained Unpaved Road Dust 56.85 

Windblown Dust 212.67 

Open Areas—Urban 0.01 

Open Areas—Grasslands, Dunes, Barren Lands 169.54 

Unpaved Roads 30.52 

Non-Pasture Ag Lands 10.81 

Pasture 1.79 

Fires 0.00 

Waste Burning 2.77 

Cooking 0.06 

On-Road Mobile 1.05 

Other Mobile 0.99 

Total excluding windblown PM10 from 

Grasslands, Dunes, and Other Barren Lands 136 

Therefore, we expect that PM10 concentrations on any given day would be more or less equal at 

the six Imperial County stations (located in the suburban/rural areas of the Imperial Valley, as 

shown in Figure V.A.12) if Mexicali had emission levels similar to Imperial County (e.g. 

excluding excess Mexican emissions). Random differences between same-day PM10

concentrations at the several stations are expected to occur as a result of the spatial differences 

in meteorology and as a result of random fluctuations in local emissions about their local 

averages. Systematic differences between same-day PM10 concentrations at any two stations 

may also exist if local emissions are on average comparatively higher or lower in the general 

areas surrounding one of the stations. For example, we expect PM10 concentrations at Niland to 

 

23 According to Table V.13: Farming, Entrained Unpaved Road Dust, and Windblown Dust from Unpaved Roads and 

Agricultural lands (pasture and non-pasture) account for 110 tpd or 89% of the 124 tpd total Imperial County PM10

inventory after exclusion of windblown dust from grasslands, dunes, and other barren lands (which are outside of 

the populated part of the Imperial Valley). 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-19 ICAPCD 

be generally somewhat lower than PM10 concentrations at other Imperial County stations, as a 

result of lower levels of activity in the area (there is essentially no agricultural activity to the 

North, East, or West of Niland, and the area is very sparsely populated). 

The above arguments outline the logic for the following two key concepts. First, air quality at 

Imperial County stations on any day and except for excess Mexican emissions is expected to be 

more or less the same, although there may be some systematic differences. Second, PM10

concentrations at Imperial County stations are expected to be correlated. This implies that the 

measurement at any station can be predicted to some extent by the values of the 

measurements at other stations. The nature of these correlations and the implications are 

analyzed below. Analyses involved the same set of PM10 data used in Approach I, except that 

Calexico PM10 measurements characteristic of strong impact from Mexico were also excluded24

(this is because in this approach we seek to find correlations in the impact of US emissions 

alone at the several Imperial County stations). 

Analysis of Historical PM10 Measurements: Comparative Analysis for Non-Calexico 

Stations. We begin by conducting a comparative analysis of daily measurements at the El 

Centro, Brawley, Westmorland, and Niland stations. Because the impact of Mexicali PM10

emissions is expected to be small at these stations (in comparison to their impact on Calexico 

monitors), this analysis will be used to test/validate the expectations derived in the previous 

subsection (i.e., expectations derived from analysis of the Imperial County Emission Inventory). 

Plots of PM10 concentration at one station vs. PM10 concentration at another are given in 

Figures V.A.13-V.A.16. We find positive correlations (R2

 values ranging from 0.32-0.56) in all 

cases, with correlations generally decreasing with increasing distance between stations,25 as 

expected. Note that except for regressions between Niland and El Centro or between Niland 

and Brawley, all correlations had R2

 values ≥0.49. 

Our interest is to capture as much information as possible in predicting PM10 concentrations at a 

specified location based on ambient air quality measurements at nearby stations. Because this 

task must be accomplished in a manner that will be useful later in our analysis of Calexico 

stations, multiple linear regression is not a good choice here. For the sake of simplicity, we 

suggest instead the averaging of measured PM10 concentrations at nearby stations.26 

 

24 Calexico PM10 measurements were excluded for the following days: January 7, 2001, November 3, 2001, 

November 21, 2001, January 14, 2002, February 13, 2002, November 22, 2002, December 29, 2003, March 11, 

2005, December 12, 2005, December 21, 2006, and December 25, 2006. For the above days in 2001-2003, an 

analysis of cross-border transport was described in the November 2004 report “Technical support document: 

exclusion of PM10 measurements in excess of the 24-hour PM10 NAAQS for Imperial County from 2001 through 

2003 due to natural events and emissions from Mexico” prepared by ENVIRON for the ICAPCD. 25 For example, R2

 values for correlations with the El Centro station decrease as we progress northward from Brawley 

to Westmorland and Niland. The closest station (Brawley) exhibited the strongest correlation (R2

 = 0.53), while the 

most distant station (Niland) exhibited the worst (R2

 = 0.32). 26 The motivation for predicting PM10 concentration at a specified location (station A) using the average of PM10

measurements at nearby stations (for example, let us refer to the closest two stations as stations B and C) is 

outlined as follows. ¶Correlation in same-day PM10 measurements for any pair of Imperial County stations simply 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-20 ICAPCD 

Regression of PM10 concentrations at a specified location on the average of concentrations at 

the two closest stations (Figures V.A.17-V.A.20) gave improved correlations in all cases (except 

at Niland). Note however that regression on the average of PM10 concentrations at the three 

closest stations was not a further improvement.

Figures V.A.17-V.A.20 also show that the relationships are well described27 by direct linear 

proportionality—a relationship simple enough to enable convenient analysis of the impact of US 

emissions at Calexico stations. The slope of the linear fits (i.e., the proportionality coefficients) 

indicate that local emissions in the areas surrounding the El Centro and Brawley stations are 

approximately the same, with slightly higher emissions impacting the Westmorland station 

(~10% higher), and slightly lower emissions impacting the Niland station (~20% lower). 

The scatter of the right-side plots of Figures V.A.17-V.A.20 represents the limitations, due to 

random processes affecting meteorology differences and emission differences, in accurately 

predicting PM10 concentration at a given station based on the average of same-day PM10

concentrations at its two nearest stations. The inaccuracy in prediction is <25 μg/m3

 in >95% of 

cases28 and <40 μg/m3

 in >99% of cases. The use of the regressions of Figures V.A.17-V.A.20 

in predicting PM10 concentrations if PM10 air quality data is only available at one of the two 

nearest stations results in slightly higher residuals, as expected (Figure V.A.21): the error of 

prediction becomes <30 μg/m3

 in >95% of cases29 and <50 μg/m3 in >99% of cases. 

Comparative analysis of PM10 measurements at non-Calexico stations therefore validates the 

expectations of the previous section, i.e., that air quality at all Imperial County stations on any 

day and except for excess Mexican emissions is more or less the same, and that because of 

correlation the measurement at any station can be predicted to a large degree by the values of 

the measurements at other stations. Even though these expectations have only been validated 

for the non-Calexico stations, based on similarity at Calexico we expect that they would hold as 

well for the Calexico stations in the absence of Mexican emissions. 

 

implies that PM10 concentrations at one station tend to be lower (or higher) than average if same-day PM10

concentrations at the other station are lower (or higher) than average. Given that the yearly averages of PM10 air 

quality are comparable at all non-Calexico IC stations, the result is that same-day PM10 concentrations for any two 

non-Calexico stations are comparable. The difference in same-day PM10 concentrations between non-IC stations 

has a predictable component (for example, Niland tends to have slightly better air quality while Westmorland tends 

to have slightly poorer air quality), as well as a non-predictable or random component. Extreme random 

differences in PM10 concentrations between stations A and B primarily occur when unusual local activity impacts 

one of the stations. If this is the case at station B, then according to the laws of probability is it unlikely that an 

extreme random difference also concurrently exists between PM10 concentrations at stations A and C, so that in 

general we expect fewer, less extreme random differences between the measurement at station A and the average 

of same-day measurements at station B and C. This in indeed what we observe (i.e., better correlations for all 

stations except at Niland). 27 This is confirmed both by the high correlation coefficients and by the homogeneous distribution of the scatter about 

the linear fits through the origin (this did not hold at Niland as well as it did at the El Centro, Brawley, and 

Westmorland stations). 28 The standard deviations of the residuals in Figures V.A.17-V.A.20 are 10-13 μg/m3

. 29 The standard deviations of the residuals in Figure V.A.21 are 12-16 μg/m3

. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-21 ICAPCD 

Analysis of Historical PM10 Measurements: Comparative Analysis of Calexico Air Quality 

Relative to Non-Calexico Air Quality. Plots of PM10 concentrations at the Grant station vs. 

same-day PM10 concentrations at non-Calexico stations are shown in Figure V.A.22, and similar 

plots for the Ethel station are given in Figure V.A.23. Linear regressions analysis (in parallel 

manner to that conducted above) reveals important similarities and differences compared with 

the results for non-Calexico stations. As before, we find that correlations improve with 

decreasing distance between stations,30 and that scatter is evenly distributed on both sides of 

the linear fits through the origin for regression of PM10 at Grant or Ethel on the average of PM10

measurements at the closest two non-Calexico stations (Figure V.A.24). On the other hand, we 

find that: 

• scatter around the linear regressions is far more pronounced, and therefore the 

correlation coefficients are much lower (i.e. R2

 values are in the range of 0.09-0.43, 

rather than 0.32-0.56); and 

• the relationships deviate significantly from 1:1 (i.e., PM10 concentration at Grant and 

Ethel are generally 59% and 47% greater than the average of same-day measurements 

at El Centro and Brawley as shown on Figure V.A.24). 

y = 0.95x

R2

 = 0.58

0

50

100

150

200

0 50 100 150

Average PM10 at Brawley and West. (μg/m3)

PM10 at El Centro ( μg/m3

)

y = 1.59x

R2

 = 0.37

0

50

100

150

200

0 50 100 150

Average PM10 at El Centro and Brawley (μg/m3)

PM10 at Calexico Grant ( μg/m3

)

 

Figure V.1 Plot of PM10 concentrations at the El Centro station vs. average PM10

concentrations at the Brawley and Westmorland stations (left), and plot of PM10

concentrations at the Grant station vs. average PM10 concentrations at the El 

Centro and Brawley stations (right). 

 

30 For both Calexico Grant and Calexico Ethel, values of R2

 are significantly greater with El Centro than with Brawley 

or Westmorland, and significantly lower with Niland (see Figures V.A.22-V.A.23). 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

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FINAL AUGUST 2009 V-22 ICAPCD 

Both these differences, illustrated in Figure V.1 above (using the Grant and the El Centro 

stations as examples), are predominantly the result of the impact of Mexicali emissions. 

However, we determined in Section 2.2.2 that a portion of the incremental PM10 concentrations 

at Calexico stations (relative to non-Calexico stations) is due to US emissions alone. 

Conservative estimates of the impact of US emissions alone (assuming no carry-over of 

Mexican PM10) can be obtained by analysis of days with FMTS ≈ 0, as before. For the group of 

days referenced in Table V.8, we find that the average of PM10 concentrations at the Grant 

station is 23 % higher (39.8 μg/m3

) than the average of all PM10 measurements at the El Centro 

and Brawley stations (32.4 μg/m3

). Likewise, for the group of days referenced in Table V.12, we 

find that the average of PM10 concentrations at the Ethel station is 10 % higher (38.8 μg/m3

)

than the average of all PM10 measurements at the El Centro and Brawley stations (35.4 

μg/m3

).31 Using the notation introduced earlier, 

PM [ ] [ ] Calexico i i Y i i Z i ,, , = ++ + EY e EZ e , 

we therefore conservatively expect the impact of US emissions at Calexico and the error of 

prediction to be: 

3 3

,

3 3

,

[ ] 1.23 and -40 g/m 40 g/m at Grant

[ ] 1.10 and -40 g/m 40 g/m at Ethel

i i Y i

i i Y i

EY X e

EY X e

= μ < < μ

= μ < < μ

where Xi is the average of same-day PM10 measurements at the El Centro and Brawley stations. 

Note that the errors of prediction in the above expressions (± 40 μg/m3

) were taken to be the 

same as those observed for similar predictions of PM10 concentrations at any non-Calexico 

station.32 

The decomposition of the total PM10 measurements at Calexico stations (PMGrant,i = 1.59 Xi + ei, 

with -75 μg/m3

 <ei <75 μg/m3 in 99% of cases, and PMEthel,i = 1.47 Xi + ei, with -75 μg/m3 <ei <75 

μg/m3

 in 99% of cases, as shown in Figure V.A.24) are then:33

3 3

, , ,

, ,

1.23 + with -40 g/m 40 g/m

0.36 + 

Grant i i YGrant i YGrant i

Grant i i ZGrant i

Y Xe e

Z Xe

= μ < < μ

= (7) 

 

31 Measurements at the Niland and Westmorland stations were not included in these calculations to allow consistent 

comparison with the regressions of Figures V.A.17-V.A.20. 32 The rationale for this assumption is that there would be no large dissimilarity between the Calexico area and areas 

surrounding other Imperial County monitors in the absence of emissions from Mexico. 33 Note that no information can be deduced about the error of prediction for the impact of Mexican emissions (eZ,i) 

since Z and Y are not expected to be uncorrelated variables. Both experience and good judgment however 

indicate that this error term can be very large on days when favorable meteorological conditions combine with high 

emission levels in Mexicali to cause high PM10 cross-border transport into Calexico. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

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FINAL AUGUST 2009 V-23 ICAPCD 

3 3

, , ,

, ,

1.10 + with -40 g/m 40 g/m

0.37 + 

Ethel i i YEthel i YEthel i

Ethel i i ZEthel i

Y Xe e

Z Xe

= μ < < μ

= (8) 

To summarize, it is the impact of Mexican emissions at Calexico stations that causes the slopes 

of the linear regressions of Figures V.A.24 to deviate so significantly from 1. In addition, the 

scatter around the linear fits is greater than seen in Figures V.A.17-V.A.20 (as evidenced by 

lower values of R2

) because of the component of Mexican impacts that doesn’t correlate with 

PM10 concentrations at El Centro or Brawley (i.e., the part eZ,i of Zi

 that doesn’t correlate with 

Xi). 

Method for Analysis of Potential “But-For” Exceedences. Estimates of YGrant and YEthel on 

exceedance days will be made using information about same-day measurements at other 

stations (using Equations 7 or 8). Based on relationships in PM10 measurements between nonCalexico stations (page V.20), the prediction errors will be assumed to be ± 40 μg/m3

 if the 

average of measurements at El Centro and Brawley is used, and ± 50 μg/m3

 if measurements at 

only one of these two stations are available for prediction. 

2.3 Other Analyses 

2.3.1 Approach III: Analysis of Sample Filters 

Background and Limitations. USEPA guidelines suggest analysis of ambient sample filters 

for specific types of particles emanating from across the border. The most common method of 

source apportionment relies on elemental analysis of filter samples (i.e. analysis of the 

concentrations of chemical constituents, such as nitrate, chloride, or sulfur). This is referred to 

as receptor modeling and it applies chemical mass balances to apportion observed levels of 

pollutants in a sample to several independent sources of known emission characteristics 

(referring to the composition of emissions from these sources). 

A 1992-1993 Cross Border Transport Study performed receptor modeling for analysis of the 

particles collected in areas within Imperial County where exceedences have been recorded.34 

Important results of this study are summarized here (Note: the page numbering in this 

subsection corresponds to that of a final report by the Desert Research Institute):35

 

34 Chow J.C., Watson J.G., Green C.M., Lowenthal D.H., Bates B., Oslund W., Torres G., Cross-border transport and 

spatial variability of suspended particles in Mexicali and California’s Imperial Valley, Atmospheric Environment, 34, 

2000, p. 1833-1843. 35 Draft Final Report “Imperial Valley/Mexicali Cross-Border PM10 Transport Study”, prepared by Chow J.C. and 

Watson J.G., Desert Research Institute, for Bates B., USEPA, Region IX, April 21, 1995. Report available from the 

USEPA website at http://www.epa.gov/ttncatc1/cica/other2_e.html. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

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FINAL AUGUST 2009 V-24 ICAPCD 

1. The study showed that it was possible to conduct source apportionment of ambient PM10

to the extent that contributions could generally36 be determined from source categories 

exhibiting sufficient differences, such as (i) geological dust, (ii) motor vehicle exhaust, 

(iii) vegetative burning, (iv) charbroil cooking, (v) marine aerosol, (vi) various industrial 

sources, and (vii) secondary aerosol. On the other hand, source sub-types within these 

categories typically “could not be distinguished by commonly measured chemical 

species...For example, within the geological dust category, it [was] not possible to 

distinguish contributions from resuspended road dust, windblown dust, and agricultural 

tilling from each other” (p. 3-1). 

2. The study determined that geological dust (70-90%), motor vehicle exhaust (10-15%), 

and vegetative burning (4-8%) accounted for the highest contribution of PM10

concentrations. Contributions from primary marine aerosol, secondary ammonium 

nitrate, and secondary ammonium sulfate were distinguishable but low (2-3% each). 

Although the relative source mix for both Mexicali and Calexico ambient air was similar 

on average, the absolute PM10 mass concentrations and source contributions in Mexicali 

were approximately twice as high (on average, but up to 3-7 times higher in 

December/January, see p. 5-6, 7-11, and 9-5). 

3. One important tracer that was hoped to help apportion ambient PM10 into international 

contributions from Imperial County and from Mexico was lead. This was so because 

transportation fuel sold in Mexico still contained lead at the time of the study (leaded 

gasoline appears to have been completely phased out in Mexico in 1998),37 while on the 

other hand there was no source of lead in Imperial County. Average ambient 

concentrations of lead in Calexico were found to be 0.038 ± 0.048 μg/m3

, approximately 

half of those in Mexicali (0.097 ± 0.074 μg/m3

, p. 5-4). These high Pb concentrations in 

Calexico provide strong evidence of cross-border PM transport from Mexico into Imperial 

County. 

4. However, although there was no lead in Calexico that originated (initially) from the US 

side of the border, the presence of lead in any given air sample from Calexico could not 

be directly connected to international transport on the day the sample was taken. This is 

because PM10 emissions arising from transportation activity (i.e. motor vehicle exhaust 

and entrained/re-suspended road dust) on the US side also contained lead. Results of 

the study showed that the difference in the lead content for Mexicali (0.10 ± 0.3% μg/m3

) 

vs. Imperial County (0.06 ± 0.06% μg/m3

) motor vehicle exhaust profiles38 were too small 

to use both profiles in the same CMB analysis. 

 

36 We note that even these source profiles could not easily be differentiated in all cases. For instance, it was often 

difficult to distinguish motor vehicle emissions from vegetative burning emissions because both types of emissions 

were composed mainly of elemental and organic carbon. Ibid, p. 7-11. 37 http://economicsbulletin.vanderbilt.edu/2008/volume18/EB-08R00002A.pdf 38 Source profiles for motor vehicle exhaust were constructed from ground-based roadside sampling at various traffic 

intersections and highway on/off ramps. In this way, it was hoped that automotive source profiles which 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

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FINAL AUGUST 2009 V-25 ICAPCD 

The implications of the findings of the 1992-1993 Cross-Border Transport Study for international 

apportionment of PM10 in the Mexicali/Calexico area are the following: 

1. The gradient of ambient lead concentration from Mexicali to northern Imperial County 

can serve to provide some evidence concerning the depth of intrusion of Mexican 

emissions into Imperial County. The study found that lead was present in ~2 times 

smaller concentrations in Calexico air samples relative to Mexicali air samples, in lower 

concentrations still at El Centro and Holtville, and only in trace amounts at Brawley and 

Niland (p. 5-13). We conclude that the impacts of Mexicali activity-related PM decrease 

strongly with distance39 and are negligible over ranges >15-20 km. Given however that 

the distance between Calexico stations and the US-Mexicali border is <1 mile, the 

potential impacts of Mexicali emissions at Calexico monitors under conditions favorable 

to stagnation and cross-border transport are very significant. 

2. Although it is possible to apportion ambient PM10 in both Calexico and Mexicali to source 

categories such as geological dust or vegetative burning, it was not possible to apportion 

ambient PM10 to source sub-categories classified by international origin. In other words, 

it was not possible to distinguish, for example, between geological dust from Imperial 

County vs. geological dust from Mexico, or to accurately apportion motor vehicle exhaust 

PM into contributions from Mexican and US sources in spite of the difference in fuel lead 

content. This finding (i.e., that the chemical profiles of emission sources on both sides of 

the border do not differ enough to accurately predict what portion of the PM10 captured 

on Imperial County filters originates in Mexico) was confirmed in another study led by 

CARB in 2001.40 

Filter Analysis for Source Apportionment in the Present “But-For” Transport Analysis. 

Color photographs of SSI filters corresponding to Calexico measurements on December 21, 

2006 and December 25, 2006 are shown in Attachment C. Visual analysis reveals a higher 

loading of organic/elemental carbon on these filters than is usually observed, consistent with 

high levels of combustion-source PM. Given the very limited use that filter analysis can serve in 

apportioning ambient PM to Mexicali versus Imperial County sources, chemical analyses of the 

filter loadings were not conducted.41 Consequently, analysis of filters will not be part of the 

weight of evidence in the present attainment demonstration. 

 

represented the actual mixture of vehicles in operation were obtained. This method was chosen because of the 

inherent difficulty of measuring mobile source particulate emissions. Note that these profiles are unavoidably 

affected by vehicle-related suspended road dust. 39 This is of course explained by increasing dispersion of pollution with distance. We can then also infer that high 

impacts associated with long-range transport (which we know occurs during e.g. high wind events) can only result 

from levels of emissions much higher than those due to human activity. 40 As part of the year 2001 Imperial County SIP development, CARB performed chemical mass balance receptor 

modeling using 1995-1996 PM data and new source profiles for U.S. versus Mexico gasoline combustion sources 

(which differ mainly in sulfur contents). Even using the new source profiles, the difference in the relative 

contributions between gasoline combustion sources in the United States and Mexico could not be identified. 41 Technical limitations preclude speciation analyses on these filters. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

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2.3.2 Approach IV: Comparison of Emission Inventories on Both Sides of the 

Border 

USEPA guidelines suggest the comparison of PM10 emission levels on both sides of the 

international border to determine the relative impact of international and domestic sources on air 

quality at the border. A qualitative comparison of emission sources on both sides of the border 

for the Calexico/Mexicali area reveals that the potential for PM10 emissions is expected to be far 

greater south of the border. This is a consequence of (i) greater activity south of the border 

(since activity is generally a function of population), and (ii) the existence of a large number of 

fugitive dust sources such as vacant lands and unpaved roads in Mexicali (Figure V.A.25). 

Because the only violations of the PM10 NAAQS in Imperial County during 2006-2008 occurred 

on days with extremely low wind speeds (refer to Sections 3.1.1 and 3.2.1 for a description of 

the December 21 and December 25, 2006 episodes), windblown emissions sources are 

expected to have made insignificant contributions to these violations. Therefore, a comparative 

analysis of windblown PM sources north and south of the US/Mexico border would not be 

relevant. Note also that on days with little-to-no wind, farming sources contribute < 20% of the 

total winter emissions in Imperial County (Table V.14). Because emissions from the remaining 

source categories are related to human activity, in the absence of an accurate gridded emission 

inventory for the Mexicali area42 we use population as a metric to estimate the relative 

magnitude of cumulative emissions originating from US and international sources. 

Table V.14 Imperial County Winter PM10 Emission Inventory (2005)a 

Source Category Winter Average (tpd) 

Fuel Combustion 0.35

Industrial Processes 2.79 

Res Fuel Combustion 0.16 

Farming 11.55 

Construction 2.01 

Paved Road Dust 3.30 

Entrained Unpaved Road Dust 33.71 

Windblown Dust 223.79 

Open Areas—Urban 0.02 

Open Areas—Othersb 191.09 

Unpaved Roads 18.10 

Non-Pasture Ag Lands 13.21 

Pasture 1.37 

Waste Burning 2.77 

Cooking 0.06 

On-Road Mobile 1.06 

Other Mobile 0.95 

Total excluding Windblown 59 

a

Source categories contributing less than 0.01 tpd were excluded from this table. b

Sources of dust from grasslands, dunes, and other barren lands undisturbed by 

humans. 

 

42 A new gridded inventory of the emissions of several air pollutants in Mexicali is in progress (2005 Mexicali 

Emission Inventory, Draft Final Report, Eastern Research Group, Inc., February 27, 2009). 
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FINAL AUGUST 2009 V-27 ICAPCD 

In 2005, Imperial County had a total population of 164,000 people (Background Section, Table 

V.3) and a total winter PM10 emission inventory (excluding windblown PM10) of 59 tpd (Table 

V.14). In comparison, the Mexicali municipality43 had a population of 878,00044 and a total PM10

emission inventory (excluding windblown PM10) of 147 tpd.45 These numbers correspond to 

Mexicali municipality:Imperial County ratios of 5.4:1 for population, 2.5:1 for total PM10

emissions, and 1:2.2 for activity-related PM10 emissions per capita (Table V.15). 

Table V.15 Imperial County vs. Mexicali Municipality Population and PM10 emissions (2005) 

 Imperial County Mexicali Municipality Mexicali:Imperial

Population 164,000 878,000 5.4:1 

PM10 emissions* (tons/day) 59 147 2.5:1 

Per capita emissions* (tons/year) 0.13 0.061 1:2.2 

* Except windblown dust emissions 

If we now assume that: 

1. impacts at the Calexico-Grant station were dominated by emissions originating within a 

disc of radius R centered around the station;46

2. the greater average distance to the Grant station for Mexicali sources (owing to the fact 

that the station is ~ 1 mile north of the border) was offset by the slight southerly wind 

flows observed on the days of interest;47 and 

3. the ratio of US to Mexicali emissions within the R-mile radius can be estimated using the 

ratio of populations and the 1:2.2 activity-related PM emissions per capita proportion 

derived for the entire Mexicali Municipality and Imperial County area; 

then we can estimate the relative impact of US and Mexicali emissions at the Calexico Grant 

station on December 21, 2006 and December 25, 2006 simply as the weighted ratio of US and 

Mexican populations within the circle of radius R. For stagnant atmospheric conditions and 

slight southerly wind flow such as those observed on December 21 and 25, 2006, it is 

reasonable to expect that a disc of radius R = 2-4 miles captures the majority of emissions 

pertinent to air quality at Calexico-Grant. Given that the 2005 populations of Calexico and 

Mexicali (cities) were 37,000 and 632,000, population estimates (based on a map of population 

density such as Figure V.A.27) within circles of radii ranging from R = 2 miles to R =5 miles are 

given in Table V.16. The relative impact of US and Mexicali emissions at the Grant station 

estimated using this approach and based on the population estimates of Table V.16 ranges 

from ~2:1 to ~3.5:1 for 2≤R (miles)≤4 miles. We note that this result is consistent with best 

estimates derived in Section 3 for the December 21, and December 25, 2006 exceedences. 

 

43 The distinction between the city and the municipality of Mexicali is shown in Figure V.A.26 44 Population Demographics for the Imperial and Mexicali Valleys, California Center for Border and Regional 

Economic Studies, CCBRES Bulletin, Vol. 8, No. 3 & 4, March/April 2007 45 2005 Mexicali Emission Inventory, Final Report, Eastern Research Group, February 27, 2008. 46 As a result of the fact that emission impacts decrease with increasing distance from the source for relatively 

stagnant atmospheric conditions. 47 Again, refer to the description of the December 21 and December 25, 2006 episodes in Sections 3.1.1 and 3.2.1 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

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Table V.16 Population within distance R of the Calexico Grant Station (2005), and 

Associated Relative Impacts at Calexico Grant

R (miles) US Population Mexican Population Mexicali:Imperial Impacts

2 30,000 632,000 × 0.2 1.9:1 (~65:35) 

3 38,000 632,000 × 0.3 2.3:1 (~70:30) 

4 40,000 632,000 × 0.5 3.6:1 (~80:20) 

5 42,000 632,000 × 0.75 5.1:1 (~85:15) 

2.3.3 Approach V: Discussion of Dispersion Modeling Results 

Scope of the Approach. The fifth example of analysis that states may use in a 179B “but-for” 

attainment demonstration is to “perform air dispersion and/or receptor modeling to quantify the 

relative impacts on the nonattainment area of sources located within the US and of foreign 

sources of PM10 emissions.” This approach involves the use of an international emission 

inventory coupled with information gathered from meteorological stations, air quality monitoring 

stations, and analysis of filters. 

Because of very high technical challenges, the use of receptor modeling to determine the 

contribution from Mexican sources to Imperial County PM10 exceedences is limited. Therefore, 

receptor modeling of the transport episodes will not be part of the weight of evidence in the 

present attainment demonstration. 

Air dispersion modeling was conducted in 200148 to assess the maximum impact of US 

emissions on PM10 ambient concentrations in Imperial County in 1992-1994 and 1999. Our 

objective here will be to show that the results the 2001 modeling exercise can be used to 

provide supporting evidence that, in the absence of some unusual occurrence(s) resulting in 

unusually high levels of US-emissions (i.e., well beyond that reflected in the best inventory 

available to us), emissions from Imperial County would not have been sufficient to cause the 

observed PM10 exceedences that occurred on December 21, 2006 and December 25, 2006. 

Description of the 2001 Modeling Analysis. A brief summary of the 2001 modeling analysis 

is as follows: 

• Modeling was conducted using CALMET/CALPUFF; 

• Information about emissions in Imperial County relied on the SCOS inventory (1997 

emissions) with 2-km and 5-km resolutions (that inventory was adjusted for seasonal 

variations and scaled up according to population increases; USEPA 49 found this scaling 

to be adequate); 

 

48 Imperial County PM10 Attainment Demonstration, ENVIRON report prepared for the ICAPCD, July 2001. Included 

as Attachment D of this Appendix. 49 Clean air Act Finding of Attainment; California-Imperial Valley Planning Area; Particulate Matter of 10 Microns or 

less. Federal Register 66 (203) October 19, 2001, p. 53108. 
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• The Mexicali inventory was found to be grossly inadequate, therefore it was not included 

in the modeling; 

• PM10 background concentration of 25 μg/m3

 were assumed in all cases (USEPA50 also 

found this to be conservative); 

• Modeling was conducted for each day for which exceedences were recorded in Imperial 

County in 1992-1994, and for 4 full years (1992, 1993, 1994, and 1999). Reported 

results included yearly maximums and yearly averages of PM10 concentration at each 

station for each of the above 4 years. 

Limitations of the 2001 Modeling Analysis. USEPA found that the modeling inventory used 

was inadequate to find the maximum 24-hour PM10 concentrations resulting from US emissions 

alone. This is because all calculations were performed based on seasonal-average emissions. 

Therefore, the analysis did not demonstrate that on peak days—when emissions might be 

significantly greater than the seasonal average—the NAAQS would not have been exceeded in 

the absence of emissions from Mexico. (Note here that the greatest variations from the average 

inventory are in wind-related emissions, and that all Calexico exceedences currently under 

consideration are low-wind days.) 

Given the sensitivity of the 24-hour PM10 NAAQS to the modeling inputs, and the small margin 

in demonstrating attainment (i.e., the maximum predicted 24-hour PM10 concentration in 1994 

was 140.5 μg/m3

 at Grant),51 USEPA concluded that the 2001 modeling analysis could not be 

confidently relied upon for a conservative estimate of the maximum 24-hour PM10 concentration 

in the 4-year time period modeled. 

Implications of the Results of the 2001 Modeling Analysis in the Present But-For 

Demonstration. In spite of the above limitation, USEPA found that the 2001 modeling 

procedure was adequate to assess the annual average PM10 concentration in Imperial County 

from US emissions.52 The reason for this different position was that USEPA found that the USinventory used in the model was “the best available inventory and information at [the] time” to 

represent annual average PM10 emissions. 

We infer that USEPA seemed satisfied with the 2001 modeling results obtained when using an 

acceptable US emission inventory. That implies that the 2001 modeling approach itself provides 

acceptable predictions of reality “but-for” Mexican emissions given a sufficiently accurate set of 

inputs. 

 

50 Clean air Act Finding of Attainment; California-Imperial Valley Planning Area; Particulate Matter of 10 Microns or 

less. Federal Register 66 (203) October 19, 2001, p. 53108. 51 In other words, the maximum predicted PM10 concentration at Calexico Grant in 1994 might have exceeded the 

federal standard if emission inputs as little as 8% higher had been used. 52 Clean air Act Finding of Attainment; California-Imperial valley Planning Area; Particulate Matter of 10 Microns or 

less. Federal Register 66 (203) October 19, 2001, p. 53108. EPA stated that “the use of [the modeling inventory 

developed for the 2001 modeling] to represent average annual PM10 concentrations is an acceptable approach.” 
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FINAL AUGUST 2009 V-30 ICAPCD 

Let us now consider US-emissions in Imperial County on the exceedence days under 

consideration (December 21, 2006 and December 25). As discussed previously, wind speeds 

on these days were very low.53 Even if we assume that windblown dust emissions on these 

days were ~25% of the seasonal winter average (when in reality windblown emissions were 

likely zero), then the total levels of emissions on these days were <115 tons/day according to 

Table V.14. We base the argument below on the observation that the levels of emissions on the 

4 exceedence days of present interest were therefore less than those used for any day modeled 

in the 2001 exercise (>147 tons/day of PM10 for winter days, and >339 tons/day of PM10 for 

summer days). 

Assume that meteorological conditions of December 21, 2006 were similar to meteorological 

conditions on any one of the days modeled in the 2001 exercise (denoted as Day X); this seems 

reasonable given the nearly 1500 days covered by the 4 full years of modeling. Given that the 

level of emissions on December 21, 2006 was less than the level of emissions on Day X, we 

conclude that an acceptable prediction of the December 21, 2006 ambient air quality has lower 

PM10 concentrations at the Imperial County monitors than values predicted by the 2001 

modeling exercise for Day X. Because PM10 concentrations predicted by the 2001 modeling for 

all days modeled (including day X) were below 150 μg/m3

 at all Imperial County stations,54 we 

can conclude that acceptable dispersion modeling predictions for December 21, 2006 would 

not exceed the NAAQS in the absence of Mexican emissions. 55 The same argument also holds 

for December 25, 2006. 

 

53 For example, maximum hourly wind speeds at the Calexico Grant station on December 21, 2006,and December 

25, 2006 were 2 and 1 knots, respectively (with 24-hour averages of 0.4 and 0.5 knots); and maximum hourly wind 

speeds at the Calexico Ethel station were 2 and 3 knots, respectively (with 24-hour averages of 1.1 and 1.4 knots). 54 The highest predictions of 24-hour PM10 concentrations in the 4-year period were 86 μg/m3

, 131 μg/m3, 141 μg/m3, 

113 μg/m3

, and 100 μg/m3 at the Brawley, Calexico-Ethel, Calexico-Grant, El Centro, Niland, and Westmorland 

stations, respectively. 55 Note that this conclusion has been reached without additional modeling, relying only on the following 3 

assumptions: (i) the 2001 model itself produces acceptable predictions of air quality in IC “but-for” Mexican 

emissions given sufficiently accurate meteorological and emissions input data, (ii) there is at least one of the nearly 

1500 days modeled that has meteorological conditions similar to those of December 21, 2006, and (iii) the 

emission inventory in Imperial County on December 21, 2006 was lower than the inventory for day X. 
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FINAL AUGUST 2009 V-31 ICAPCD 

3 Discussion and Analysis of Potential Transport Episodes 

3.1 December 21, 2006 

3.1.1 Description of the Episode 

On December 21, 2006, one filter-based monitor in the ICAPCD recorded an exceedence of the 

PM10 NAAQS of 150 μg/m3. Because SSI measurements for that day were only acquired at the 

Calexico-Grant and the El Centro station,56 24-hour average BAM PM10 results at the Niland, 

Westmorland, and Brawley stations are also reported here (Table V.17). It should be noted that 

at the present time, BAM data is polled by the CARB for inclusion into the Air Quality Index for 

forecasting purposes only. 

Table V.17 PM10 air quality at Mexicali monitors on December 21, 2006 

Monitor AIRS No. SSI Monitor (μg/m3) BAM monitor (μg/m3) 

Niland 60254004 - 25 

Westmorland 60254003 - 52 

Brawley 60250007 - 55 

El Centro 60251003 54 - 

Calexico-Grant 60250004 171 - 

The entries in Table V.17 are arranged to visually demonstrate the gradient of PM10

concentrations from north to south. As can be seen, the measurement registered at Calexico 

Grant is at least seven times as high than the measurement recorded at the most northerly 

monitoring station of Niland, and three times as high as the measurement recorded a few miles 

north at the El Centro station. The measurements recorded at all non-Calexico stations are in 

the range of 25-55 μg/m3

 and do not indicate that there was a county-wide PM10 problem on 

December 21, 2006. 

Because December 21, 2006 was not a scheduled run day, no SSI measurements were 

acquired at Mexicali stations, and the only information about Mexicali air quality on that day is a 

10-hour BAM reading of 262 μg/m3 at the UABC monitor. However, SSI measurements acquired 

on December 19, 2006 (115, 137, and 286 μg/m3 at the Conalep, Cobach, and Progresso 

monitors) reveal that PM10 air quality levels in Mexicali were elevated just two days earlier. The 

spatial distribution of the 24-hour PM10 concentrations that were measured at Imperial County 

and Mexicali monitoring sites on December 21, 2006 is shown in Figure V.2. The strong PM10

concentration gradient from south to north provides strong evidence of PM10 transport from 

Mexico into Imperial County. 

 

56 On occasion, scheduled filter runs are voided due to operational malfunctions. These missing data points require 

the acquisition of “make-up” measurements on days that fall outside the normal one-in-six day schedule. 

December 21, 2006 was such a “make-up” day. 
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Niland FRM = 25 μg/m3

Westmorland FRM = 52 μg/m3

Brawley FRM = 55 μg/m3

El Centro FRM = 54 μg/m3

Calexico Ethel FRM = no data

Calexico Grant FRM = 171 μg/m3

Mexicali UABC BAM = 262 μg/m3

Mexicali ITM = No Data

Conalep = No Data

Progresso = No Data

Mexicali Cobach = No Data

Caretera San Felipe = No data

Mexicali

Figure V.2 Spatial Distribution of 24-Hour PM10 Concentrations in Imperial County and 

Mexicali for December 21, 2006 

Meteorological Data: Surface Winds. Hourly wind data collected on December 21, 2006 at 

the Imperial Airport and at Calexico monitoring stations are presented in Table V.18, while 

surface wind data recorded in Mexicali are reported in Table V.19. The information in these 

tables confirms that stagnant and light winds were prevalent on that day, with 24-hour average 

wind speeds in Calexico <1.5 knots and similarly low-wind conditions in Mexicali. Combined 

with air flow from the south (as discussed below), these were ideal meteorological conditions in 

the border region to cause PM10 pollution in Mexicali to impact Calexico monitors. 
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Table V.18 Hourly Wind Speeds (WS, knots) and Wind Directions (WD) at US stations near 

Calexico on December 21, 2006 

Imperial Airport Calexico-Grant Calexico-Ethel

Time WS WD WS WD WS WD

0 0 Calm 0.1 SE 1 S

1 0 Calm 0.1 N 1 NW

2 3 SW 0.1 ESE 1 WNW

3 0 Calm 0.1 WD 1 SSW

4 3 West 1 WD 2 WSW

5 0 Calm 0.1 ENE 0.1 SW

6 0 Calm 0.1 N 1 NW

7 0 Calm 0.1 SE 1 SE

8 0 Calm 0.1 SW 1 WSW

9 0 Calm 0.1 SW 1 WSW

10 0 Calm 0.1 SSE 1 E

11 0 Calm 0.1 SW 1 NE

12 0 Calm 1 S 1 NW

13 0 Calm 0.1 WSW 0.1 N

14 0 Calm 1 SW 1 NW

15 4 East 0.1 W 1 E

16 0 Calm 0.1 NNE 2 ENE

17 0 Calm 1 ENE 2 E

18 0 Calm 0.1 ESE 1 NE

19 0 Calm 0.1 SW 1 SW

20 2 SW 0.1 SE 1 SSE

21 0 Calm 2 ESE 1 ESE

22 0 Calm 1 E 1 SSE

23 0 Calm 0.1 NNW 1 SW

24-hr avg 0.5 0.4 1.1 

Table V.19 Hourly Wind Speeds (WS, in knots) and Wind Directions (WD) recorded in Mexicali 

on December 21, 2006 

Mexicali Airport Mexicali-UABC Mexicali-Cobach Mexical-ITM Mexicali-Carretera

Time WS WD WS WD WS WD WS WD WS WD

0 0 - 2 - 1 ENE 3 SSW 0 -

1 0 - 1 - 1 NE 1 ESE 0 -

2 3 S 1 - 2 ENE 2 WSW 0 -

3 0 - 1 - 2 SSE 2 WSW 0 -

4 0 - 2 - 2 E 2 W 1 -

5 0 - 1 - 1 NE 2 NW 1 -

6 0 - 2 - 1 NE 2 ENE 1 -

7 0 - 1 - 1 ENE 2 SE 0 -

8 0 - 1 - 2 WNW 1 W 1 -

9 0 - 2 - 3 WNW 1 NW 1 -

10 0 - 2 - 2 ESE 3 ENE 2 -

11 - - 2 - 2 ESE 2 WNW 1 -

12 0 - 3 - 3 ESE 2 WNW 1 -

13 0 - 2 - 3 SW 3 WNW 1 -

14 0 - 3 - 3 NNE 3 NW 1 -

15 - - 2 - 2 ENE 3 NE 0 -

16 - - 2 - 2 NE 3 NNE 0 -

17 - - 3 - 2 NE 3 ENE 0 -

18 - - 2 - 1 E 3 ESE 0 -

19 - - 2 - 1 ESE 2 SSW 0 -

20 - - 2 - 1 ENE 2 S 1 -

21 - - 2 - 2 E 3 ESE 0 -

22 - - 1 - 2 ENE 2 SE 0 -

23 0 - 2 - 2 NNW 1 NE 0 -

24-hr avg 0.2 1.8 1.8 2.2 0.7 
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Meteorological Conditions: Back Trajectory Analysis. 36-hour back trajectories arriving at 

the Calexico Grant station at 6 am, 12 pm, 6 pm, and 12 am PST were created for December 

21, 2006. Examination of these trajectories shows very similar paths, so only the back trajectory 

ending at 12 pm is included here (Figure V.3). The figure indicates that the air parcel that 

“arrived” at Calexico Grant at 12 pm on December 21, 2006 was carried by winds of low speeds 

and variable direction in a circle around Calexico and along the border between Calexico and 

Mexicali. The relatively short length of the trajectory line provides a strong sign of stagnation, 

consistent with the measured wind data presented above. The winds of low speed and variable 

direction allowed PM10 emissions to accumulate within the Calexico/Mexicali area, thereby 

causing high PM10 ambient concentration in both areas. 

Figure V.3 Calexico-Grant 36-hour back trajectory ending at 12 pm PST on December 

21, 2006 

Meteorological Conditions: Wind Rose Analysis. A wind rose of hourly surface wind speeds 

and directions measured at the Calexico Grant station on December 21, 2006 was also created 

(Figure V.4). The figure shows that the prevailing wind directions on that day were SW, W, and 

SE. Winds of direction with a northerly component only accounted for a total of 6 hours, while 

winds having the potential to carry emissions from Mexico into Imperial County57 prevailed for 

 

57 Of direction ranging from 85 to 265 (E to WSW) based on the location of the border. 
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15 hours (>60% of the day). Combined with low atmospheric dispersion resulting from low wind 

speeds, this result strongly supports the hypothesis that elevated PM10 concentrations in 

Mexicali on that day were transported (by overflow and advection) across the US/Mexico border 

to impact Calexico monitors. 

Figure V.4 Calexico-Grant Wind Rose Analysis for December 21, 2006 

3.1.2 Statistical Analyses 

Approach I. Meteorological conditions on December 21, 2006 were conducive to extremely low 

dispersion (the average and maximum wind speeds at the Calexico Grant station were 0.4 knots 

and 2 knots, respectively), and to significant transport from Mexico (FMTS = 0.85 at Grant). The 

24-hour PM10 measurement at Grant was 171 μg/m3

; therefore the station reading would not 

have exceeded the NAAQS but-for international emissions if the Mexicali contribution was ≥17 

μg/m3

. 

Because the number of days with meteorological conditions similar to those of December 21, 

2006 is extremely small (cf. third row of Table V.6), we choose to analyze this exceedence in 

the context of the larger subset of days with 24-hr wind speeds at Grant <1.0 knots (first row of 

Table V.4, Table V.7). For that subset, the expected value of the contribution from Mexico to air 

quality at the Grant station is (by subtracting the bolded values from Table V.8 from the first row 

of Table V.4) in the range of 50-55 μg/m3

. 
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FINAL AUGUST 2009 V-36 ICAPCD 

By inspection of the information in Table V.7, we observe that: 

• 24-hour average wind speed at the Grant station on December 21, 2006 was the lowest 

of all days within the group; 

• FMTS (or southerly flow) at the Grant station on December 21, 2006 was the second 

highest in the group; and 

• Although the only available measurement of PM10 concentration in Mexicali on that day 

was a 10-hour average BAM monitor recording of 262 μg/m3

 at the UABC station, that 

measurement is the highest within its category for all the days in the table. 

All three of these factors would cause the impact of PM10 pollution from Mexicali on December 

21, 2006 to be significantly higher than the 50-55 μg/m3

 average for this group of days, 

consistent with the results of the third row of Table V.6 (pertaining to days with 24-hour average 

wind speeds < 1.0 knots and FMTS > 0.4). Note that a prediction of “excess” PM10

concentrations at the Grant station (relative to other Imperial County stations) given the 10-hour 

BAM 262 μg/m3

 measurement at UABC and using the linear regression of Figure V.A.6 is ~160 

μg/m3

. 

Based on this information, we conclude that the position of the December 21, 2006 

measurement in Table V.7 does not seem out of order, and that the “excess” PM10 at Grant on 

that day was therefore in the range of ~110-150 μg/m3

 (corresponding to the range of “excess” 

PM10 for days most similar to December 21, 2006 according to the table). After subtracting the 

bolded values from Table V.8,58 this assessment results in a best estimate (according to 

Approach I) of the impact of Mexicali emissions to the December 21, 2006 exceedence in the 

range of 

~100-150 μg/m3

. This expected international contribution is far in excess of the 17 μg/m3 that 

the Calexico-Grant station reading was above the NAAQS. 

Approach II. The 24-hour PM10 measurement at Calexico Grant was 171 μg/m3

 on December 

21, 2006. The only other Imperial County measurement available on that day is at El Centro (54 

μg/m3

), so we will use that number to predict the impact of US emissions. The resulting 

predicted value of YGrant for December 21, 2006 using Equation 7 with ׀eY,Grant׀> 50 μg/m3

 is 66 ± 

50 μg/m3

 (so up to 116 μg/m3). According to this conservative analysis, US emissions alone 

would not have been sufficient to exceed the 24-hour PM10 NAAQS on December 21, 2006. 

3.1.3 Additional Evidence 

The Season of Festivals in Mexico. From December 16th through December 24th, the country 

of Mexico celebrates a deeply rooted religious and social reenactment of the birth of Jesus for 

nine consecutive nights called “Las Posadas.” This yearly Christian tradition reenacts the trials 

and tribulations experienced by Mary and Joseph on their travel back to Jerusalem where 

 

58 Which correspond to excess PM10 at Grant due to the impact of US emissions alone, as explained in Section 2.2.2. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-37 ICAPCD 

ultimately Jesus would be born. These festivals (Posadas) are celebrated by thousands 

throughout Mexicali in rural and urban neighborhoods where whole communities host atypical 

dinner-parties with strong religious and social intonations involving very familiar symbols such 

as piñatas, bonfires, fireworks and food for the nine consecutive nights. 

These “Posadas” are purposely celebrated at night to allow for maximum community 

participation. Bonfires, as mentioned above, are provided by the host of each Posada to help 

keep guests warm, and are created by the burning of wood, coal, and tires. In addition to 

bonfires, heavy usage of legal and illegal fireworks is very common throughout the duration of 

the Posadas. These activities, combined with the seasonally heavy vehicular traffic created by 

the Christmas Season on unpaved roads in Mexicali, are major contributors to elevated PM10

emission levels in the Mexicali/Calexico area. When stagnant and light wind conditions occur, 

such as those experienced on December 21, 2006, these PM10 emissions accumulate to reach 

levels capable of causing high impact at neighboring Calexico monitors. Historical trends show 

that this occurs in the concurrent presence of stagnant conditions and/or light, southerly winds. 

Figure V.5 is a photograph published in Mexicali’s La Crónica newspaper on December 23,

2006. The caption below the photograph expresses concern over the growing amount of dust 

created by the lack of watering of unpaved roads in Mexicali. The article directly correlates the 

lack of visibility and diminished breathing capabilities to the increased amounts of dust particles. 

Figure V.5 Dust emissions being created in Mexicali due to lack of paving. 

3.1.4 Weight of Evidence 

Both Approach I and Approach II independently confirm that the impact of US emissions alone 

at the Calexico Grant station would not have been sufficient to cause an exceedence (see 

previous sections). Note that the best estimate from Approach II (i.e., US contribution = 66 

μg/m3

, corresponding to a Mexican contribution of 171 μg/m3 - 66 μg/m3 = 105 μg/m3) is 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-38 ICAPCD 

consistent with more conservative estimates from Approach I (which predicted a Mexican 

contribution in the range of 100-150 μg/m3

). Based on this information, our best estimate of the 

fractional contribution of Mexicali emissions to the December 21, 2006 Calexico Grant 

measurement is 1 – 66/171 = 61%; this number is used in the analysis of the significance of 

emission source categories as outlined in Section 3.2 of the main SIP document. 

All qualitative analyses and evidence presented in this report are consistent with the 

conclusions of the above quantitative estimates. The occurrence of intense Mexican holiday 

celebrations is supportive evidence of abnormally high levels of PM10 emissions in Mexicali on 

December 21, 2006. Also, comparative analysis of Calexico vs. Mexicali emission inventories 

(Approach IV) and information derived from previous modeling analyses (Approach V) add 

further evidence to support claims (i) of high impacts of Mexicali emissions on Calexico air 

quality, and (ii) that Calexico-Grant station reading would not have exceeded the 24-hour PM10

NAAQS at Calexico monitors but-for Mexican sources. 

3.2 December 25, 2006 

3.2.1 Description of the Episode 

On December 25, 2006, one filter-based monitor in Imperial County recorded an exceedence of 

the 24-hour PM10 NAAQS (Table V.20). Note that because SSI measurements for that day were 

not acquired at the Westmorland or Niland stations, 24-hour BAM PM10 results at these two 

stations are reported instead. 

Table V.20 PM10 air quality at Imperial County monitors on December 25, 2006 

Monitor AIRS No. SSI Monitor (μg/m3) BAM Monitor (μg/m3) 

Niland 60254004 12 

Westmorland 60254003 27 

Brawley 60250007 27 

El Centro 60251003 24 

Calexico-Ethel 60250005 110 

Calexico-Grant 60250004 248 

The entries in Table V.20 are arranged to visually demonstrate the change in PM10

concentrations from north to south. The measurements recorded at stations from El Centro to 

Niland (in the range of 12 to 27 μg/m3

) do not indicate that there was a county-wide PM10

problem on December 25, 2006. Rather, evidence of cross-border transport of PM10 to account 

for the remarkably high measurements at Calexico stations can be inferred from inspection of 

air quality measurements recorded in Mexicali on that day (Table V.21). 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

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FINAL AUGUST 2009 V-39 ICAPCD 

Table V.21 PM10 air quality at Mexicali SSI monitors on December 25, 2006 

Monitor AIRS No. Concentration (μg/m3) 

Mexicali-UABC 00020012 240 

Mexicali-Cobach 00020014 437 

Mexicali-ITM 00020010 274 

Mexicali-Progresso 00020015 466 

Mexicali Conalep 00020011 305 

Mexicali-Carretera 00020018 266 

Spatial Plot. Figure V.6 visually demonstrate the spatial distribution of the PM10 concentrations 

measurements acquired on December 25, 2006. The average PM10 concentration at Mexicali 

stations was 344 μg/m3

, with a maximum at the Progresso station >85% higher than the highest 

Imperial County measurement of 248 μg/m3

 at Calexico-Grant, and a minimum at the UABC 

station >115% higher than the 110 μg/m3

 measurement at the Calexico-Ethel station. Note also 

that within Imperial County, PM10 measurements recorded at the Calexico-Grant and CalexicoEthel stations were 11 times and 5 times as high, respectively, as the average of measurements 

at non-Calexico stations. This strong PM10 concentration gradient from Mexicali to El Centro 

provides strong evidence of cross-border PM10 transport from Mexico into Imperial County. 

Meteorological Data: Surface Winds. Hourly wind data collected on December 25, 2006 at 

the Imperial Airport and at Calexico monitoring stations are presented in Table V.22, while 

surface wind data recorded in Mexicali are reported in Table V.23. The information in these 

tables confirms that stagnant and light winds were prevalent on that day, with 24-hour average 

wind speeds in Calexico <1.5 knots and similarly low-wind conditions in Mexicali. Combined 

with air flow from the south (as discussed below), these were ideal meteorological conditions in 

the border region to cause PM10 pollution in Mexicali to impact Calexico monitors. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-40 ICAPCD 

Niland FRM = No Data

Westmorland FRM = No Data

Brawley FRM = 27 μg/m3

El Centro FRM = 24 μg/m3

Calexico Ethel FRM = 110 μg/m3

Calexico Grant FRM = 248 μg/m3

Mexicali UABC FRM = 240 μg/m3

ITM FRM = 274 μg/m3

Conalep FRM = 305 μg/m3

Progresso FRM = 466 μg/m3

Mexicali Cobach = 437 μg/m3

Caretera San Felipe FRM = 266 μg/m3

Mexicali

Figure V.6 Spatial Distribution of 24-Hour PM10 Concentrations in Imperial County and 

Mexicali for December 25, 2006 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-41 ICAPCD 

Table V.22 Hourly Wind Speeds (WS, knots) and Wind Directions (WD) at US stations 

near Calexico on December 25, 2006 

Imperial Airport Calexico-Grant Calexico-Ethel

Time WS WD WS WD WS WD

0 0 Calm 0.1 W 2 W

1 4 West 1 SW 2 WSW

2 3 West 1 WSW 2 WSW

3 0 Calm 0.1 W 3 WNW

4 3 WSW 0.1 E 2 W

5 0 Calm 1 ESE 1 SSE

6 0 Calm 1 ESE 1 SSW

7 4 WSW 1 ENE 2 E

8 0 Calm 0.1 ESE 2 E

9 0 Calm 0.1 N 0.1 NW

10 0 Calm 0.1 W 1 W

11 0 Calm 0.1 NNW 1 WNW

12 0 Calm 0.1 NW 1 NE

13 4 East 1 ENE 1 E

14 4 SE 0.1 NE 2 NE

15 0 Calm 1 NNE 2 ENE

16 0 Calm 1 N 2 NE

17 0 Calm 0.1 NE 1 NNE

18 3 West 0.1 ESE 1 ESE

19 3 NNE 1 SSW 1 SW

20 0 Calm 0.1 N 1 NNE

21 4 WSW 0.1 E 1 NNE

22 3 WSW 1 W 0.1 SSW

23 5 West 1 WNW 2 WSW

24-hr avg 1.7 0.5 1.4 

Table V.23 Hourly Wind Speeds (WS, in knots) and Wind Directions (WD) recorded in Mexicali 

on December 25, 2005 

Mexicali Airport Mexicali-UABC Mexicali-Cobach Mexical-ITM Mexicali-Carretera

Time WS WD WS WD WS WD WS WD WS WD

0 0 - 2 - 1 NE 3 WNW 0 -

1 0 - 2 - 1 ESE 3 SW 0 -

2 0 - 2 - 1 ENE 3 W 1 -

3 0 - 2 - 1 ENE 2 WSW 0 -

4 0 - 1 - 1 E 2 ESE 0 -

5 - - 1 - 2 ESE 3 ESE 0 -

6 0 - 1 - 1 ENE 2 SSE 0 -

7 0 - 2 - 1 ENE 3 NE 0 -

8 - - 2 - 1 ENE 2 E 0 -

9 - - 2 - 2 NE 2 NNW 0 -

10 6 W 2 - 2 NNW 2 W 1 -

11 0 - 2 - 2 NNW 2 W 1 -

12 0 - 2 - 3 WNW 3 NW 1 -

13 0 - 3 - 3 N 4 NNE 1 -

14 - - 3 - 2 E 3 NE 1 -

15 5 NNW 2 - 2 NE 3 NE 0 -

16 0 - 2 - 2 NE 3 ENE 0 -

17 - - 2 - 1 ENE 2 E 0 -

18 - - 1 - 1 E 2 NNW 0 -

19 - - 2 - 1 E 3 W 0 -

20 - - 1 - 1 E 2 WSW 0 -

21 0 - 3 - 2 ENE 2 NNE 1 -

22 - - 1 - 2 WNW 2 NNE 1 -

23 0 - 2 - 2 E 2 WNW 0 -

24-hr avg 0.7 1.9 1.7 2.5 0.5 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-42 ICAPCD 

Meteorological Conditions: Back Trajectory Analysis. As before, 36-hour back trajectories 

arriving at the Calexico Grant station at 6 am, 12 pm, 6 pm, and 12 am PST were created for 

December 25, 2006. Examination of these trajectories shows very similar paths, so only the 

back trajectory ending at 12 pm is included here (Figure V.7). The figure indicates that the air 

parcel that “arrived” at Calexico Grant at 12 pm on December 25, 2006 was carried by winds of 

low speeds and variable direction. The major transport mechanism for this episode was high 

levels of Mexicali PM10 “overflowing” into Calexico. The relatively short length of the trajectory 

line provides a strong sign of stagnation, consistent with the above analysis of wind speed data. 

The winds of low speed and variable direction allowed PM10 emissions to accumulate within the 

Calexico/Mexicali area, thereby causing high PM10 ambient concentrations. 

Figure V.7 Calexico-Grant 36-hour back trajectory ending at 12 pm PST on December 

25, 2006 

Meteorological Conditions: Wind Rose Analysis. A wind rose of hourly surface wind speeds 

and directions measured at the Calexico Grant station on December 25, 2006 was also created 

(Figure V.8). The figure shows that the prevailing wind directions on that day were ESE and W, 

although wind direction varied greatly. Consequently, the low wind speeds and varying wind 

directions allowed PM10 emissions to drift from Mexicali to other areas including Calexico. The 

major transport mechanism for this episode was high levels of Mexicali PM10 “overflowing” into 

Calexico. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-43 ICAPCD 

Figure V.8 Calexico-Grant Wind Rose Analysis for December 25, 2006 

3.2.2 Statistical Analyses 

Approach I. Meteorological conditions on December 25, 2006 were conducive to extremely low 

dispersion (the average and maximum wind speeds at the Grant station were 0.5 knots and 1 

knots, respectively), and to considerable transport from Mexico (FMTS = 0.41 at Grant). The 24-

hour PM10 measurement at Grant was 248 μg/m3

; therefore if the Mexicali contribution was ≥94 

μg/m3 then the station would not have exceeded the NAAQS but for Mexican emissions. 

The analysis for this day is similar to the analysis for the December 21, 2005 Grant 

measurement. By inspection of the information of Table V.7, we observe that (i) 24-hour 

average wind speed at the Grant station on December 25, 2006 was low relative to values for 

other days in this group, and (ii) southerly flow at the Grant station on December 25, 2006 was 

higher than that for most days within this group, and (iii) PM10 concentrations at all Mexicali 

stations on December 25, 2006 were remarkably higher than those observed on all other days 

within the group. All these factors would cause the impact of PM10 pollution from Mexicali on 

December 25, 2006 to be significantly higher than the 50-55 μg/m3

 average for the group of 

days represented in Table V.7. For example, predictions of “excess” PM10 concentrations at the 

Grant station (relative to other Imperial County stations) given measured PM10 concentrations in 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

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FINAL AUGUST 2009 V-44 ICAPCD 

Mexicali on December 25, 2006 using the linear regressions of Figure V.A.6 and are in the 

range of 125 μg/m3

-225 μg/m3 (with an average of 185 μg/m3).59 

Our best prediction of the impact of Mexican emissions to the December 25, 2006 Grant 

measurement using the present analysis is thus in the range of 150 μg/m3

-200 μg/m3. This is far 

in excess of the 94 μg/m3

 that the station reading was above the NAAQS. 

Approach II. The 24-hour PM10 measurement at Calexico Grant was 248 μg/m3

 on December 

25, 2006. Measurements at the El Centro and Brawley stations were 24 μg/m3

 and 27 μg/m3, 

so we will use their average as the best available predictor of US impacts at Calexico. The 

resulting predicted value of YGrant for December 25, 2006 using Equation 7 is 31 ± 40 μg/m3

 (so 

up to 71 μg/m3

). According to this conservative analysis, US emissions alone would not have 

been sufficient to exceed the 24-hour PM10 NAAQS on December 25, 2006. 

3.2.3 Additional Evidence 

Christmas Holiday. On December 25, 2006, the United States and Mexico celebrated 

Christmas. Unlike the US, Mexico begins its seasonal celebration several days before with the 

cultural traditions of Las Posadas. As mentioned in Section 3.1.3, the traditional celebrations 

begin nine days before Christmas and continue on with finale accentuations on Christmas Eve 

and Christmas Day. The events are chronicled in newspapers on both sides of the border. 

Published articles in Mexicali not only describe the deteriorating condition of air quality, but also 

provide evidence of the volume of fireworks and burning that takes place during these festive 

times. For example, every year articles are published depicting Mexican authorities confiscating 

voluminous amounts of illegal fireworks during the holiday season. For every illegal firework that 

is confiscated there are hundreds more that are not. It is not uncommon to see heavy clouds of 

white smoke covering Mexicali and Calexico the morning after Christmas Eve and Christmas 

Day. The large amount of fireworks and traditional bonfires associated with the celebrations of 

the Las Posadas has caused a rippling effect on Mexicali’s air quality over the last couple of 

decades. Figures V.9 and V.10 are copies of newspaper articles published by La Voz de la 

Frontera on December 24th and December 25th, respectively. The article depicted in Figure V.9 

is a public announcement by the Governor of Baja California, Eugenio Walther, requesting the 

community to not burn tires or fireworks in order to help reduce air quality impacts. The article 

depicted by Figure V.10 reports that the burning of tires, wood and the setting of fireworks 

contributed directly to the poor air quality for Christmas day. The article further explains that the 

visible dense white cloud covering Mexicali during the early morning hours was a direct result of 

the activities associated with the previous night. The sharp contrast between the recorded 

concentrations of PM10 emissions in Mexicali and those of the Imperial County suggest that 

 

59 Of course, the December 25, 2006 Mexicali PM10 concentrations corresponds to highly influential points in a 

number of cases. A more conservative approach might use leave-one-out cross-validation: it gives a range of 90 

μg/m3

-240 μg/m3 and an average of 145 μg/m3 for the “excess” PM10 concentration at Grant. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-45 ICAPCD 

emissions created in Mexicali were transported into Imperial County, with greatest impact in the 

Calexico area. 

Mexicali 

Una Navidad limpia, pide el gobernador 

La Voz de la Frontera 

24 de diciembre de 2006 

Por José MERCADO 

El licenciado Eugenio Elorduy Walther, gobernador del estado, hizo un llamado a la ciudadanía para que en esta 

navidad no lleven a cabo la quema de llantas o cohetes por los problemas de contaminación que provocan, y que 

procuren no exponerse a las balas perdidas por el peligro que significan. 

"Todos queremos llegar a esta navidad y el año nuevo", indicó el mandatario estatal y añadió a la lista de 

recomendaciones evitar el viajar en auto con una persona que haya consumido alcohol "porque el mayor número 

de muertes violentas en el estado lo provocan los accidentes de tránsito". 

Entrevistado después de la toma de protesta del nuevo secretario de Educación y Bienestar Social del Estado, 

licenciado Óscar Ortega Vélez, que sustituye a José Gabriel Posada Gallego, Elorduy Walther comentó de la 

necesidad de conservar la armonía de la familia en estas fechas de fin de año. 

"Es importante hacer un llamado especial para los automovilistas para que si toman procuren mejor no manejar 

por el peligro que significa para ellos, su familia y la sociedad en general. Y las personas, que si ven a alguien que 

insiste en manejar si se encuentra tomado, mejor no se suban al auto".

Figure V.9 Mexicali Newspaper article published on December 24, 2006 

Mexicali 

Una Navidad "Casi Blanca": Elorduy 

La Voz de la Frontera 

26 de diciembre de 2006 

Por José MERCADO 

En lo que consideró una Nochebuena "casi blanca", el gobernador del Estado licenciado Eugenio Elorduy Walther 

dijo tener conocimiento de 258 accidentes de tránsito en toda la entidad, con dos personas muertas en Mexicali y 

Rosarito. 

Indicó que afortunadamente no se registraron ejecuciones, aunque reconoció que esta ciudad amaneció el día 25 

con un marcado nivel de contaminación, provocada por la quema de llantas, leña o cohetes. Al amanecer se 

apreciaba una nube densa de humo, indicó. 

Referente a los accidentes de tránsito, indicó que solamente en las ciudades de Mexicali y Rosarito hubo 

resultados funestos, pero el resto del estado se mantuvo dentro de un nivel de aceptabilidad. 

Recordó del llamado que se hizo a la población bajacaliforniana, cuando se les advirtió que lo más importante de 

la familia es lo que no se compra en la botica, como es el caso de mantener una buena relación de familia. 

En relación a los niveles de contaminación, el titular del Ejecutivo, se refirió, a quienes llevaron a cabo la quema 

de llantas o cohetes, como personas que no se dan cuenta de que es el mismo aire que respiramos todos y que a 

final de cuentas nos afecta en la salud por igual a todos. 

Indicó que afortunadamente durante la noche del 24 y para amanecer el día 25, "al menos hasta el momento en 

que recibí esta información", dijo, no se presentaron problemas por ejecuciones en toda la entidad, lo que es un 

buen síntoma.

Figure V.10 Mexicali Newspaper article published on December 25, 2006 
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FINAL AUGUST 2009 V-46 ICAPCD 

3.2.4 Weight of Evidence 

Both Approach I and Approach II independently confirm that the impact of US emissions alone 

at the Calexico Grant station would not have been sufficient to cause an exceedence on 

December 25, 2006 (see previous sections). Note that the most conservative estimate from 

Approach II (i.e., US contribution = 71 μg/m3

, corresponding to a Mexican contribution of 248 

μg/m3

 - 71 μg/m3 = 177 μg/m3) is consistent with the best estimates from Approach I (which 

predicted a Mexican contribution in the range of 150-200 μg/m3

). Based on this information, our 

best estimate of the fractional contribution of Mexicali emissions to the December 25, 2006 

Calexico Grant measurement is 1 – 71/248 = 71%; this number is used in the analysis of the 

significance of emission source categories as outlined in Section 3.2 of the main SIP document. 

All qualitative analyses and evidence presented in this report are consistent with the 

conclusions of the above quantitative estimates. The occurrence of intense Mexican holiday 

celebrations is supportive evidence of abnormally high levels of PM10 emissions in Mexicali on 

December 25, 2006. Also, comparative analysis of Calexico vs. Mexicali emission inventories 

(Approach IV) and information derived from previous modeling analyses (Approach V) add 

further evidence to support claims (i) of high impacts of Mexicali emissions on Calexico air 

quality, and (ii) that the Calexico-Grant station reading would not have exceeded the 24-hour 

PM10 NAAQS at Calexico monitors but-for Mexican sources. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 V-47 ICAPCD 

4 Conclusion 

The primary purpose of a SIP is to outline a plan designed to improve air quality in an area and 

to demonstrate how that plan provides for attainment of air quality standards as expeditiously as 

possible (this is referred to as an attainment demonstration). In international areas, the SIP 

attainment requirements are designed so that only attainment “but-for” international emissions is 

required. USEPA guidelines60 state that “for PM10 nonattainment areas, section 179B(a) [of the 

CAA] provides that EPA must approve [a PM10] SIP if (i) the SIP meets all the applicable 

requirements under the Act other than a requirement that such plan or revision demonstrate 

attainment and maintenance of the PM10 NAAQS by the applicable attainment date, and (ii) the 

State demonstrates to EPA’s satisfaction that the SIP would be adequate to attain and maintain 

the PM10 NAAQS by the attainment date but for emissions emanating from outside the US.” 

An attainment demonstration for the present Imperial County 2009 PM10 SIP therefore requires 

analyses to determine the contribution of international emissions to the 2006-2008 Imperial 

County exceedences (Table V.1). This appendix described the tools developed and used to 

analyze, both quantitatively and qualitatively, the impact of Mexicali emissions on near-border 

Imperial County stations. The results demonstrate that ambient air quality on December 21, 

2006 and December 25, 2006 would have attained the 24-hour PM10 NAAQS in the absence of 

impact contributions from Mexicali emissions. Given that there were no other exceedences of 

the PM10 NAAQS in 2006-2008,61 the transport analyses of the present appendix are therefore 

sufficient to meet the attainment demonstration requirements of the Imperial County 2009 PM10

SIP. 

 

60 State Implementation Plans for Serious PM10 Nonattainment Areas, and Attainment Date Waivers for PM10 

Nonattainment Areas Generally; Addendum to the General Preamble for the Implementation of Title I of the Clear 

Air Act Amendments of 1990; Federal Register, August 16, 1994, p. 42000 61 After exclusion of measurements recorded on September 2, 2006, April 12, 2007, and June 5, 2007. As these 

measurements were strongly affected by high-wind exceptional events, the ICAPCD has submitted to the USEPA 

documentation seeking exclusion of these PM10 data from regulatory determinations, as allowed by the March 

2007 USEPA Exceptional Event rule (Treatment of Data Influenced by Exceptional Events; Final Rule, 40 CFR 

Parts 50 and 51; Federal Register, March 22, 2007, p. 13560) . 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 ICAPCD 

Attachment A 

Supporting Figures for Appendix V 

Figure V.A.1 Graphical representation and illustration of the calculation of FMTS. 

Figure V.A.2 Example calculation of FMTS for using wind data at the Calexico Grant station acquired on 

December 21, 2006. 

Figure V.A.3 Effect of wind speed on excess PM10 concentration at the Calexico Grant station (2001-2007 

data). 

Figure V.A.4 Effect of wind speed on excess PM10 concentration at the Calexico Grant station when FMTS 

= 0-0.03 (2001-2007 data). 

Figure V.A.5 Effect of wind speed on the impact (or relative impact) of US emissions at non-Calexico 

Imperial County monitors (2001-2007 data) 

Figure V.A.6 Plot of excess PM10 concentration at Calexico Grant (relative to other Imperial County 

stations) vs. ambient PM10 concentration at the Mexicali Cobach, UABC, ITM, and Progresso 

stations on days when the 24-hour average wind speed at Grant is <1.0 knot/hour and there 

is some southerly wind flow (FMTS >0.1). 

Figure V.A.7 Map of Imperial County showing the location of Imperial County and Mexicali PM10 monitoring 

sites. 

Figure V.A.8 Plot of PM10 concentration at non-Calexico Imperial County stations vs. wind direction at the 

Calexico Grant station (2001-2007 data). 

Figure V.A.9 Plot of PM10 concentration at non-Calexico Imperial County stations vs. wind direction at the 

Calexico Grant station, on days when 24-hour average wind speed at the Grant station was ≤

2 knots (2001-2007 data). 

Figure V.A.10 Satellite image showing the location of the Calexico Grant and Calexico Ethel air monitoring 

stations within Calexico and relative to the US/Mexico border. The dashed red line is parallel 

to the border and separates US areas south and north of Calexico stations. 

Figure V.A.11 Effect of wind speed on excess PM10 concentration at the Calexico Ethel station (2002-2007 

data). 

Figure V.A.12 Satellite image showing land use in Imperial County (an interactive map of the area is 

available online at http://www.wikimapia.org/#lat= 

32.9337761&lon=115.5075073&z=10&l=0&m=a). The green arrows represent the location of 

PM10 monitors. 

Figure V.A.13 Plots of PM10 concentrations at the El Centro station vs. same-day PM10 concentrations at 

Brawley, Westmorland, and Niland (2001-2007 data) 

Figure V.A.14 Plots of PM10 concentrations at the Brawley station vs. same-day PM10 concentrations at 

Westmorland, El Centro, and Niland (2001-2007 data) 

Figure V.A.15 Plots of PM10 concentrations at the Westmorland station vs. same-day PM10 concentrations 

at Brawley, El Centro, and Niland (2001-2007 data) 

Figure V.A.16 Plots of PM10 concentrations at the Niland station vs. same-day PM10 concentrations at 

Westmorland, Brawley, and El Centro (2001-2007 data) 

Figure V.A.17 Plots of PM10 concentration at the El Centro station vs. the average of same-day PM10

concentrations at neighboring stations (left plots). The plots on the right are the residuals of 

the plots on the left. (2001-2007 data.) 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 ICAPCD 

Figure V.A.18 Plots of PM10 concentration at Brawley station vs. the average of same-day PM10

concentrations at neighboring stations (left plots). The plots on the right are the residuals of 

the plots on the left. (2001-2007 data.) 

Figure V.A.19 Plots of PM10 concentration at the Westmorland station vs. the average of same-day PM10

concentrations at neighboring stations (left plots). The plots on the right are the residuals of 

the plots on the left. (2001-2007 data.) 

Figure V.A.20 Plots of PM10 concentration at the Niland station vs. the average of same-day PM10

concentrations at neighboring stations (left plots). The plots on the right are the residuals of 

the plots on the left. (2001-2007 data.)

Figure V.A.21 Residual plots: Measured PM10 values minus PM10 values predicted from measurements at 

only one neighboring station using the correlations of the top plots of Figures V.A.17-V.A.20 

(2001-2007 data) 

Figure V.A.22 Plots of PM10 concentrations at the Calexico Grant station vs. same-day PM10 concentrations 

at El Centro, Brawley, Westmorland, and Niland (2001-2007 data) 

Figure V.A.23 Plots of PM10 concentrations at the Calexico Ethel station vs. same-day PM10 concentrations 

at El Centro, Brawley, Westmorland, and Niland (2001-2007 data)

Figure V.A.24 Plots of PM10 concentrations at the Calexico Grant and Calexico Ethel stations vs. the 

average of same-day PM10 concentrations at El Centro and Brawley (left plots). The plots on 

the right are the residuals of the plots on the left. (2001-2007 data.) 

Figure V.A.25 Pavement Coverage of Mexicali Roads, according to the ERG 2005 Mexicali Emission 

Inventory Report. 

Figure V.A.26 Municipality of Mexicali (~ 5,300 square miles, 120 miles from North to South) 

Figure V.A.27 Satellite image showing the proximity and relative sizes of Calexico and Mexicali (an 

interactive map of the area is available online at http://wikimapia.org/#lat=32.6347491&lon=-

115.4796982&z=12&l=0&m=a&v=2). The green arrows represent the location of PM10

monitors. In this figure, the yellow circle centered at the Calexico-Grant station has a radius 

of 4 miles. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 ICAPCD 

n

G

v

G

n v⋅ = G G component of wind direction that corresponds 

to air flow from Mexico into Imperial County The border has a ~ 5 degree tilt 

compared to a strictly east-west 

direction

The following illustrates the calculation of FMTS for a hypothetical 4-hour period:

n

G

1 v

G

2 p

2 v

G

4 v

G 3 v

G

1 p

3 p

4 p For this particular case:

3 4

1234

p p FMTS 0.54

pppp

+ = +++ 

N

Figure V.A.1 Graphical representation and illustration of the calculation of FMTS. 

Error! Objects cannot be created from editing field codes. is an hourly wind 

direction vector (pointing in the direction from which the wind in 

coming), and Error! Objects cannot be created from editing field codes. is a 

normal unit vector perpendicular to the US-Mexico border and pointing 

toward Mexicali. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 ICAPCD 

northerly flow ∑ n v 0.61 ⋅ =

G G

12 am 12 pm 11 pm

southerly flow ∑ n v 3.41 ⋅ =

G G

Figure V.A.2 Example calculation of FMTS for using wind data at the Calexico Grant 

station acquired on December 21, 2006. The arrows are hourly values of 

the cross-border flow of air (i.e. Error! Objects cannot be created from 

editing field codes. in Figure V.A.1) representing the extent of both 

southerly (arrows pointing upward) and northerly (arrows pointing 

downward) air flow. On December 21, 2006, FMTS = 3.41/(3.41 + 0.61) = 

0.85. 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

-100

-50

0

50

100150200250

0.0 2.0 4.0 6.0 8.0 

24-Hour Average Wind Speed at Gra

PM10 at Grant - PM10 at Brawley (

μg/m

3

)

-100

-50

0

50

100150200250

0.0 2.0 4.0 6.0 8.0 

24-Hour Average Wind Speed at Gra

PM10 at Grant - PM10 at Niland (

μg/m

3

)

-100

-50

0

50

100150200250

0.0 2.0 4.0 6.0 8.0 10.0 12.0

24-Hour Average Wind Speed at Grant (knots)

PM10 at Grant - PM10 at Westmorland (

μg/m

3

)

-100

-50

0

50

100150200250

0.0 2.0 4.0 6.0 8.0 10.0 12.0

24-Hour Average Wind Speed at Grant (knots)

PM10 at Grant - PM10 at El Centro (

μg/m

3

)

Figure V.A.3 Effect of wind speed on excess PM10 concentration at the Calexico Grant station (2001-
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

-100

-50

0

50

100

0 2 4 6 

24-Hour Average Wind Speed at Gr

PM10 Grant - PM10 Brawley (

μg/m

3

)

-100

-50

0

50

100

0 2 4 6 

24-Hour Average Wind Speed at Gr

PM10 Grant -PM10 Niland (

μg/m

3

)

-100

-50

0

50

100

0 2 4 6 810

24-Hour Average Wind Speed at Grant (knots)

PM10 Grant - PM10 Westmorland (

μg/m

3

)

-100

-50

0

50

100

0 2 4 6 810

24-Hour Average Wind Speed at Grant (knots)

PM10 Grant - PM10 El Centro (

μg/m

3

)

Figure V.A.4 Effect of wind speed on excess PM10 concentration at the Calexico Grant station when 

2007 data). 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

0

50

100150200

0.0 3.0 6.0 

24-Hour Average Wind Speed at Westmo

PM10 at Westmorland (

μg/m

3

)

-100

-50

0

50

100

0.0 3.0 6.0 

24-Hour Average Wind Speed at Westmo

PM10 at West. - PM 10 at Niland (

μg/m

3

)

0

50

100150200

0.0 3.0 6.0 9.0 12.0

24-Hour Average Wind Speed at Niland (knots)

PM10 at Niland (

μg/m

3

)

0

50

100150200

0.0 3.0 6.0 9.0 12.0

24-Hour Average Wind Speed at El Centro (knots)

PM10 at El Centro (

μg/m

3

)

Figure V.A.5 Effect of wind speed on the impact (or relative impact) of US emissions at non-Calexico 

monitors (2001-2007 data) 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 ICAPCD 

y = 0.70x - 22.77

R2

 = 0.46

0

50

100

150

200

250

0 100 200 300 400 500

Ambient PM10 at UABC (μg/m3

)

Excess PM10 at Grant (μg/m3

)

y = 0.65x - 77.66

R2

 = 0.85

0

50

100

150

200

250

0 100 200 300 400 500

Ambient PM10 at Cobach (μg/m3

)

Excess PM10 at Grant (μg/m3

)

y = 0.41x - 71.29

R2

 = 0.26

0

50

100

150

200

250

0 100 200 300 400 500

Ambient PM10 at Progresso (μg/m3

)

Excess PM10 at Grant (μg/m3

)

y = 0.81x + 2.07

R2

 = 0.71

0

50

100

150

200

250

0 100 200 300 400 500

Ambient PM10 at ITM (μg/m3

)

Excess PM10 at Grant (μg/m3

)

y = 0.87x - 19.64

R2

 = 0.70

0

50

100

150

200

250

0 100 200 300 400 500

Ambient PM10 at Carretera (μg/m3

)

Excess PM10 at Grant (μg/m3

)

y = 0.71x - 20.57

R2

 = 0.56

0

50

100

150

200

250

0 100 200 300 400 500

Ambient PM10 at Conalep (μg/m3

)

Excess PM10 at Grant (μg/m3

)

Figure V.A.6 Plot of excess PM10 concentration at Calexico Grant (relative to other 

Imperial County stations) vs. ambient PM10 concentration at the Mexicali 

Cobach, UABC, ITM, and Progresso stations on days when the 24-hour 

average wind speed at Grant is <1.0 knot/hour and there is some southerly 

wind flow (FMTS >0.1). 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 ICAPCD 

Niland

Westmorland

Brawley

El Centro

Calexico Ethel

Calexico Grant

Mexicali UABC

Mexicali ITM

Mexicali Conalep

Mexicali Progresso

Mexicali Cobach

Mexicali Caretera

Mexicali

Figure V.A.7 Map of Imperial County showing the location of Imperial County and 

Mexicali PM10 monitoring sites. 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

0

50

100150

0.0 0.2 0.4 0.6 

FMTS at Grant 

PM10 Brawley (

μg/m

3

)

0

50

100150

0.0 0.2 0.4 0.6 

FMTS at Grant

PM10 Niland (

μg/m

3

)

0

50

100150

0.0 0.2 0.4 0.6 0.8 1.0

FMTS at Grant

PM10 Westmorland (

μg/m

3

)

0

50

100150

0.0 0.2 0.4 0.6 0.8 1.0

FMTS at Grant

PM10 El Centro (

μg/m

3

)

Figure V.A.8 Plot of PM10 concentration at non-Calexico Imperial County stations vs. wind direction a

Grant station (2001-2007 data). 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

0

50

100150

0.0 0.2 0.4 0.6 

FMTS at Grant (knots)

PM10 Brawley (

μg/m

3

)

0

50

100150

0.0 0.2 0.4 0.6 

FMTS at Grant (knots)

PM10 Niland (

μg/m

3

)

0

50

100150

0.0 0.2 0.4 0.6 0.8 1.0

FMTS at Grant (knots)

PM10 Westmorland (

μg/m

3

)

0

50

100150

0.0 0.2 0.4 0.6 0.8 1.0

FMTS at Grant (knots)

PM10 El Centro (

μg/m

3

)

Figure V.A.9 Plot of PM10 concentration at non-Calexico Imperial County stations vs. wind direction a

Grant station, on days when 24-hour average wind speed at the Grant station was ≤ 2 k

data). 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

Figure V.A.10 Satellite image showing the location of the Calexico Grant and Calexico Ethel air monit

Calexico and relative to the US/Mexico border. The dashed red line is parallel to the borUS areas south and north of Calexico stations. 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

-100

-50

0

50

100150200

0.0 3.0 6.0 

24-Hour Average Wind Speed at Eth

PM10 at Ethel - PM10 at Brawley (

μg/m

3

)

-100

-50

0

50

100150200

0.0 3.0 6.0 

24-Hour Average Wind Speed at Eth

PM10 at Ethel - PM10 at Niland (

μg/m

3

)

-100

-50

0

50

100150200

0.0 3.0 6.0 9.0 12.0

24-Hour Average Wind Speed at Ethel (knots)

PM10 at Ethel - PM10 at Westmorland (

μg/m

3

)

-100

-50

0

50

100150200

0.0 3.0 6.0 9.0 12.0

24-Hour Average Wind Speed at Ethel (knots)

PM10 at Ethel - PM10 at El Centro (

μg/m

3

)

Figure V.A.11 Effect of wind speed on excess PM10 concentration at the Calexico Ethel station (2002-2
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 ICAPCD 

Figure V.A.12 Satellite image showing land use in Imperial County (an interactive map 

of the area is available online at

http://www.wikimapia.org/#lat=32.910721 

&lon=-115.5267334&z=10&l=0&m=a). The green arrows represent the 

location of PM10 monitors. 

El Centro

Brawley

Westmorland

Niland

C. Grant C. Ethel
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

0

50

100150200250300

0 50 

PM10 at Nila

PM10 at Grant (

μg/m

3

)

R2 = 0.49

0

50

100150200250300

0 50 100 150

PM10 at Westmorland (μg/m3)

PM10 at Grant (

μg/m

3

)

R2 = 0.53

0

50

100150200250300

0 50 100 150

PM10 at Brawley (μg/m3)

PM10 at El Centro (

μg/m

3

)

Figure V.A.13 Plots of PM10 concentrations at the El Centro station vs. same-day PM10 concentrations 

Westmorland, and Niland (2001-2007 data) 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

0

50

100150200250300

0 50 

PM10 at Nila

PM10 at Grant (

μg/m

3

)

R2 = 0.53

0

50

100150200250300

0 50 100 150

PM10 at El Centro (μg/m3)

PM10 at Grant (

μg/m

3

)

R2 = 0.56

0

50

100150200250300

0 50 100 150

PM10 at Westmorland (μg/m3)

PM10 at Brawley (

μg/m

3

)

Figure V.A.14 Plots of PM10 concentrations at the Brawley station vs. same-day PM10 concentrations a

Centro, and Niland (2001-2007 data) 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

0

50

100150200250300

0 50 

PM10 at Nila

PM10 at Grant (

μg/m

3

)

R2 = 0.49

0

50

100150200250300

0 50 100 150

PM10 at El Centro (μg/m3)

PM10 at Grant (

μg/m

3

)

R2 = 0.56

0

50

100150200250300

0 50 100 150

PM10 at Brawley (μg/m3)

PM10 at Westmorland (

μg/m

3

)

Figure V.A.15 Plots of PM10 concentrations at the Westmorland station vs. same-day PM10 concentrati

Centro, and Niland (2001-2007 data) 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

0

50

100150200250300

0 50 

PM10 at El Cen

PM10 at Grant (

μg/m

3

)

R2 = 0.32

0

50

100150200250300

0 50 100 150

PM10 at Brawley (μg/m3)

PM10 at Grant (

μg/m

3

)

R2 = 0.49

0

50

100150200250300

0 50 100 150

PM10 at Westmorland (μg/m3)

PM10 at Niland (

μg/m

3

)

Figure V.A.16 Plots of PM10 concentrations at the Niland station vs. same-day PM10 concentrations at 

Brawley, and El Centro (2001-2007 data) 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

y = 1.01x

R2 = 0.57

0

50

100150

0 50 100 150

Average PM10 at B W N (μg/m3)

PM10 El Centro (

μg/m

3

)

-100

-50

0

50

100

0 25 50 75 100 

Average PM10 at B W N (μg/m

Residuals El Centro vs B,W,N (

μg/m

3

)

-100

-50

0

50

100

0 25 50 75 100 

Average PM10 at B and W (μg/

Residuals El Centro vs B,W (

μg/m

3y = 0.95x

) 

R2 = 0.58

0

50

100150

0 50 100 150

Average PM10 at B and W (μg/m3)

PM10 El Centro (

μg/m

3

)

Figure V.A.17 Plots of PM10 concentration at the El Centro station vs. the average of same-day PM10 c

neighboring stations (left plots). The plots on the right are the residuals of the plots on data.) 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

y = 1.01x

R2 = 0.48

0

50

100150

0 50 100 150

Average PM10 at EC W N (μg/m3)

PM10 Brawley (

μg/m

3

)

-100

-50

0

50

100

0 25 50 75 100 

Average PM10 at EC W N (μg/m

Residuals Brawley vs EC,W,N (

μg/m

3

)

-100

-50

0

50

100

0 25 50 75 100 

Average PM10 at EC and W (μg

Residuals Brawley vs EC,W (

μg/m

3y = 0.95x

) 

R2 = 0.53

0

50

100150

0 50 100 150

Average PM10 at EC and W (μg/m3)

PM10 Brawley (

μg/m

3

)

Figure V.A.18 Plots of PM10 concentration at the Brawley station vs. the average of same-day PM10 co

neighboring stations (left plots). The plots on the right are the residuals of the plots on data.) 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

y = 1.09x

R2 = 0.66

0

50

100150

0 50 100 150

Average PM10 at EC B N (μg/m3)

PM10 Westmorland (

μg/m

3

)

-100

-50

0

50

100

0 25 50 75 100 

Average PM10 at EC B N (μg/m

Residuals West. vs EC, B, N (

μg/m

3

)

-100

-50

0

50

100

0 25 50 75 100 

Average PM10 at B and N (μg/

Residuals West. vs B, N (

μg/m

3 y = 1.11x

) 

R2 = 0.67

0

50

100150

0 50 100 150

Average PM10 at B and N (μg/m3)

PM10 Westmorland (

μg/m

3

)

Figure V.A.19 Plots of PM10 concentration at the Westmorland station vs. the average of same-day PM

neighboring stations (left plots). The plots on the right are the residuals of the plots on data.) 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

y = 0.83x

R2 = 0.46

0

50

100150

0 50 100 150

Average PM10 at EC B W (μg/m3)

PM10 Niland (

μg/m

3

)

-100

-50

0

50

100

0 25 50 75 100 

Average PM10 at EC B W (μg/m

Residuals Niland vs EC, B, W (

μg/m

3

)

-100

-50

0

50

100

0 25 50 75 100 

Average PM10 at B and W (μg/

Residuals Niland vs B, W (

μg/m

3) y = 0.81x

R2 = 0.46

0

50

100150

0 50 100 150

Average PM10 at B and W (μg/m3)

PM10 Niland (

μg/m

3

)

Figure V.A.20 Plots of PM10 concentration at the Niland station vs. the average of same-day PM10 conc

neighboring stations (left plots). The plots on the right are the residuals of the plots on data.) 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

-100

-50

0

50

100

0 50 100 150

PM10 at Brawley/Niland (μg/m3)

Residuals Westmorland vs B and N (

μg/m

3

)

-100

-50

0

50

100

0 25 50 75 

PM10 at Brawley/Westmo

Residuals Niland vs B or W (

μg/m

3

)

-100

-50

0

50

100

0 25 50 75 

PM10 at Westmorland/El C

Residuals Brawley vs W, EC (

μg/m

3) 

-100

-50

0

50

100

0 50 100 150

PM10 at Brawley/Westmorland (μg/m3)

Residuals El Centro vs B and W (

μg/m

3

)

Figure V.A.21 Residual plots: Measured PM10 values minus PM10 values predicted from measurements 

neighboring station using the correlations of the top plots of Figures V.A.17-V.A.20 (200
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

0

50

100150200250300

0 

PM

PM10 at Grant (

μg/m

3

)

R2 = 0.24

0

50

100150200250300

0 50 100 150

PM10 at Westmorland (μg/m3)

PM10 at Grant (

μg/m

3

)

R2 = 0.24

0

50

100150200250300

0 50 100 150

PM10 at Brawley (μg/m3)

PM10 at Grant (

μg/m

3

)

R2 = 0.43

0

50

100150200250300

0 50 100 150

PM10 at El Centro (μg/m3)

PM10 at Grant (

μg/m

3

)

Figure V.A.22 Plots of PM10 concentrations at the Calexico Grant station vs. same-day PM10 concentra

Brawley, Westmorland, and Niland (2001-2007 data) 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

0

50

100150200250300

0 

PM1

PM10 at Grant (

μg/m

3

)

R2 = 0.23

0

50

100150200250300

0 50 100 150

PM10 at Westmorland (μg/m3)

PM10 at Grant (

μg/m

3

)

R2 = 0.21

0

50

100150200250300

0 50 100 150

PM10 at Brawley (μg/m3)

PM10 at Grant (

μg/m

3

)

R2 = 0.31

0

50

100150200250300

0 50 100 150

PM10 at El Centro (μg/m3)

PM10 at Ethel (

μg/m

3

)

E1

Figure V.A.23 Plots of PM10 concentrations at the Calexico Ethel station vs. same-day PM10 concentra

Brawley, Westmorland, and Niland (2001-2007 data) 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

y = 1.47x

R2 = 0.23

0

50

100150200

0 50 100 150 200

Average PM10 at EC and B (μg/m3)

PM10 Ethel (

μg/m

3

)

-100

-50

0

50

100

0 25 50 75 

Average PM10 at EC and 

Residuals Ethel vs EC, B (

μg/m

3

)

-100

-50

0

50

100

0 25 50 75 

Average PM10 at EC and 

Residuals Grant vs EC, B (

μg/m

3) y = 1.59x

R2 = 0.37

0

50

100150200

0 50 100 150 200

Average PM10 at EC and B (μg/m3)

PM10 Grant (

μg/m

3

)

Figure V.A.24 Plots of PM10 concentrations at the Calexico Grant and Calexico Ethel stations vs. the a

PM10 concentrations at El Centro and Brawley (left plots). The plots on the right are the 

plots on the left. (2001-2007 data.) 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 ICAPCD 

Figure V.A.25 Pavement Coverage of Mexicali Roads, according to the ERG 2005 

Mexicali Emission Inventory Report. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 ICAPCD 

Figure V.A.26 Municipality of Mexicali (~5,300 square miles, 120 miles from North to 

South) 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

Figure V.A.27 Satellite image showing the proximity and relative sizes of Calexico and Mexicali (an in

area is available online at http://wikimapia.org/#lat=32.6347491&lon=-115.4796982&z=12The green arrows represent the location of PM10 monitors. In this figure, the yellow circ

Calexico-Grant station has a radius of 4 miles. 

El Centro 

Calexico 

Mexicali
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 ICAPCD 

Attachment B 

US Area Directly South of the Calexico Grant Station 

Satellite images of Calexico/Mexicali reveal the presence of an open area directly south 

of the Calexico Grant station that appears as though it may have the potential to 

generate high levels of fugitive dust. A high resolution satellite image (Figure V.B.1) 

shows that this area is the location of the Calexico International Airport. The airport site 

covers ~305 acres of land and includes a single east-west runway 4670 feet long and 75 

feet wide.62 Owned and operated by the City of Calexico, the airport site is situated 

within the southwest portion of the city limits directly along the US-Mexican border, and 

is only separated from the border by a ~0.2 mile × 1.5 mile strip of land guarded by the 

US border patrol. 

In order to develop a qualitative understanding of the role that the open area directly 

south of Grant may have played in the 24-hour PM10 exceedences recorded on 

December 21, 2006 and December 25, 2006, we note the following: 

1. Public access to the airport area is limited to vehicular traffic on paved roads to 

and from the airport terminal. For security reasons, the airport area is 

surrounded by a fence (Figure V.B.2) intended to prevent public access to other 

parts of the airport area. On December 21 and 25, 2006, the airport had air 

traffic; therefore, normal security procedures were in place. The only activitybased emissions from the airport area thus corresponded to vehicular/airplane 

traffic on paved roadways/runways; 

2. There is no evidence of unusual activity by the border patrol on the border strip; 

and 

3. Because of very low wind speeds in Calexico (and indeed throughout the 

Imperial Valley) on December 21 and 25, 2006, windblown dust from the 

Calexico airport area and from the border patrol area south of the airport would 

not have been a significant source contributor. 

For December 21 and 25, 2006, activity-based emissions and windblown fugitive dust 

emissions from the open area directly south of the Calexico Grant station are therefore 

expected to have been negligible relative to activity-based emissions throughout the 

remainder of Calexico, and more particularly relative to activity-based emissions 

throughout Mexicali. 

 

 

62 http://www.calexico.ca.gov/index.php?option=com_content&task=view&id=60&Itemid=90. 
Imperial County 2009 PM10 SIP Appendix V: Exceed

NAAQ

FINAL AUGUST 2009 

Figure V.B.1 Satellite image showing an enlarged view of the area directly south of the Calexico Gran

airport is fenced in within the boundaries delineated by the orange line. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 ICAPCD 

Figure V.B.2 Photographs of the boundary limits of the Calexico International 

Airport documenting the presence of fencing to restrict public 

access. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 ICAPCD 

Attachment C 

Photographs of PM10 Filters 

Color photographs of SSI filters corresponding to Calexico-Grant measurements 

acquired December 21, 2006 and December 25, 2006 are shown in Figures V.C.1 and 

Figures V.C.2. As references in the visual analysis of these samples, photographs of 

Calexico-Grant filters corresponding days with average PM10 air quality are also given in 

Figures V.C.1 and V.C.2. The December 21 and 25, 2006 filters appear dirtier, with 

higher surface coarseness and darker hue, as a consequence of a higher loading of 

organic/elemental carbon consistent with high levels of combustion-source PM. 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 ICAPCD 

Figure V.C.1 Color photograph of 24-hour SSI filters used at the Calexico Grant 

station: 12-21-06 (171 μg/m3

, top) compared with 6-15-05 (53 μg/m3, 

bottom) 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 ICAPCD 

Figure V.C.2 Color photograph of 24-hour SSI filters used at the Calexico Grant 

station: 12-25-06 (248 μg/m3

, top) compared with 9-7-05 (68 μg/m3, 

bottom) 
Imperial County 2009 PM10 SIP Appendix V: Exceedences of the 24-Hour PM10 

NAAQS at the US-Mexico Border 

FINAL AUGUST 2009 ICAPCD 

Attachment D 

Imperial County PM10 2001 Attainment Demonstration 

This attachment is the report documenting the air dispersion modeling conducted by 

ENVIRON in 2001 to assess the maximum impact of US emissions on PM10 ambient 

concentrations in Imperial County in 1992-1994 and 1999. 
Imperial County

PM10 Attainment Demonstration

Prepared for:

Imperial County Air Pollution Control District 

150 South Ninth Street

El Centro, CA 92243 

Prepared by:

6001 Shellmound Street, Suite 700

Emeryville, CA 94608

July 2001

FINAL AUGUST 2009 ICAPCD
: i

TABLE OF CONTENTS

ABBREVIATIONS AND ACRONYMS..................................................................................... ii

I. MODELING OVERVIEW .................................................................................................. 1

II. MODELS ............................................................................................................................... 1

III. MODEL INPUT DATA........................................................................................................ 2

III.A. METEOROLOGICAL DATA.............................................................................................. 2

III.B. MODELING EMISSIONS INVENTORY .............................................................................. 3

III.C. RECEPTOR LOCATIONS .................................................................................................. 6

III.D. BACKGROUND CONCENTRATIONS ................................................................................. 6

III.E. DESIGN VALUE DETERMINATION .................................................................................. 6

IV. MODELING RESULTS....................................................................................................... 7

IV.A. FULL-YEAR MODELING RESULTS ................................................................................. 7

IV.B. EXCEEDANCE DAY MODELING RESULTS ...................................................................... 7

APPENDIX A Supporting Analyses for Attainment Demonstration

TABLES

Table 7-1 Sources of meteorological data

Table 7-2 Determination of modeling inventory adjustment factors

Table 7-3 Imperial County population data, 1990-2000

Table 7-4 Lowest percentiles of a distribution of observed PM10 data

Table 7-5 Predicted concentrations at Imperial County monitors for full-year runs (μg/m3)

Table 7-6 Predicted concentrations for exceedance days between 1992 and 1994

Table 7-7 Imperial County 24-hour PM10 exceedance days, 1992-1994

FINAL AUGUST 2009 ICAPCD
: ii

ABBREVIATIONS AND ACRONYMS

BLM U.S. Department of Interior Bureau of Land Management

CAA Federal Clean Air Act

CARB California Air Resources Board

CMB chemical mass balance

CRP Conservation Reserve Program of the Food Security Act of 1985

DPW Department of Public Works

EPA U.S. Environmental Protection Agency

IC Imperial County

ICAPCD Imperial County Air Pollution Control District

IID Imperial Irrigation District

LAER Lowest Achievable Emissions Rate

NAA nonattainment area

NAAS Naval Auxiliary Air Station

NAAQS National Ambient Air Quality Standards

NCDC National Climatic Data Center

NO3 nitrates

NOx nitrogen oxides

NSR New Source Review

NWCG National Wildlife Coordinating Group

PM particulate matter

PM10 particulate mater less than 10 μm in aerodynamic diameter

RACM Reasonably Available Control Measure

RACT Reasonably Available Control Technology

SCS U.S. Department of Agriculture Soil Conservation Service

SFS sequential filter sampler

SIP State Implementation Plan

SO2 sulfur dioxide

SO4 sulfates

SSI size-selective inlet

TSP total suspended particulates (less than 30 μm in aerodynamic diameter)

μg/m microgram per cubic meter

μm micron or micrometer

VMT vehicle miles traveled

FINAL AUGUST 2009 ICAPCD
: 1

I. Modeling Overview

The CAA (in Section 172) requires that Imperial County must have attained the PM10 standard

by 1994. The CAA also allows (in Section 179B) that an area may demonstrate that it would

have attained a NAAQS but for emissions which originate outside the United States. In

accordance with these provisions, this attainment plan demonstrates that Imperial County would

have been in attainment by 1994 (design years 1992-1994), but for international transport of

PM10 emissions emanating from Mexico. The CALMET/CALPUFF modeling system was

applied in order to make this demonstration over the time period of interest.

The modeling conducted in this demonstration only includes emissions generated within

Imperial County. This limitation resulted from deficiencies in the quality and availability of data

representing sources located in Mexicali, the closest Mexican urban area to the border with

Imperial County. EPA has previously allowed states to demonstrate attainment of the PM10

NAAQS but for emissions transported from Mexico using only emissions generated within the

United States. In 1993, EPA approved the PM10 attainment plan for El Paso, Texas in which

only emissions generated on the U.S. side of the border were modeled.

All exceedance days between 1992 and 1994 were modeled in order to show that Imperial has

attained the PM10 standard but for emissions from Mexico. That is, all days over a three-year

period in which Imperial County monitors measured PM10 concentrations exceeding the NAAQS

standard were modeled. There were 9 such days during this time period.

In addition to the exceedance days, complete year model runs were performed for 1992, 1993,

1994, and 1999. These complete year runs were performed for two reasons: (1) to have a more

complete meteorological time frame including the 5:6 days when PM10 data are not available

with which to demonstrate that the PM10 exceedances in Imperial County would not have

occurred but for emissions from Mexico; and (2) the 1999 year was used for a

CALMET/CALPUFF model performance evaluation to evaluate the adequacy of the Imperial

County and Mexicali emissions inventory.

II. Models

PM10 exceedances in the Imperial Valley are mainly attributable to primary PM10 due to

geological dust (~60%), followed by carbon (~25%) (Chow and Watson, 1997). There are also

small components due to secondary PM (sulfate, nitrate, and organics). Exceedances of the

PM10 standard sometimes occur under stagnation conditions. Thus, a model for simulating PM

in the Imperial Valley needs to have the following attributes:

• Ability to simulate the impacts of primary PM10 under stagnant as well as more

organized transport conditions;

• Ability to treat point and area sources, and primary and secondary PM; and

• Ability to perform source attribution to identify the contributions of United States

versus Mexican sources.

For treating just primary PM, the ISC and AERMOD steady-state Gaussian plume models are

the EPA-recommended tools. However, these models do not treat stagnation conditions nor do

FINAL AUGUST 2009 ICAPCD
: 2

they treat secondary PM. There are no recommended models for treating secondary

PM/stagnation conditions. We considered using an advanced three-dimensional grid model such

as UAM-AERO, UAM-AERO/LT, CMAQ, or CAMx, but these models have high data and

resource requirements and are difficult to use for source attribution. To meet the above attributes

we chose the CALMET/CALPUFF modeling system as the most appropriate model that can

meet the SIP submittal time constraints imposed by the present litigation against EPA.

We also investigated the possibility of using receptor models, such as the Chemical Mass

Balance (CMB) model. The 1992-1993 Cross Border Transport Study performed receptor

modeling and found geological dust the highest contributor (70-90%) followed by motor vehicle

exhaust (10-15%) with vegetative burning sometimes contributing also (10%). As part of the

year 2001 Imperial County SIP development, CARB performed CMB receptor modeling using

1995-1996 PM data and new source profiles for U.S. versus Mexico gasoline combustion

sources (which differ mainly in sulfur contents). According to CARB, the latest Imperial County

CMB receptor modeling indicated that a vast majority of the PM is due to geological material

(Woodhouse, 2000; personal communication). Even using the new source profiles, the

difference in the relative contributions between gasoline combustion sources in the United States

and Mexico could not be identified.

III. Model Input Data

This section describes the input data used in the modeling performed for this attainment

demonstration. For each data type, the source of the input data is discussed, along with any

modifications to this data that were required for its use in this demonstration.

III.A. Meteorological Data

Meteorological data was obtained from a variety of sources. Surface data archived by the

National Climatic Data Center (NCDC) was obtained for Imperial Airport. Upper air data was

also obtained from NCDC for the San Diego (Miramar) and Tucson areas. Additional surface

data was acquired from the CARB for stations located at Calexico-Ethel, Calexico-East, CBTIS,

COBACH, ITM, and UABC. The latter four of these sites are located in Mexicali, Mexico.

Table 7-1 shows the sources of all meteorological data used in the modeling in this attainment

demonstration. Missing data substitutions were performed using methods developed by

Atkinson and Lee (Atkinson and Lee, 1992).

FINAL AUGUST 2009 ICAPCD
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Table 7-1. Sources of meteorological data.

Station Name Start End

Imperial Airport 1992 2000

Calexico-Ethela 1995 1999

Calexico-East 1996 2000

CBTIS 1997 2000

COBACH 1997 2000

ITM 1997 2000

UABC 1997 2000

Notes:

a

 Meteorological data is available for 2000 at this site, but it was not obtained in time to

incorporate it into the modeling.

III.B. Modeling Emissions Inventory

The modeling inventory is updated less frequently than the inventory used for planning purposes,

since emissions must be separated by source type (i.e., point sources, area sources, and mobile

sources), and then entered into a geographic grid to account for the location of each source. The

most recent gridded modeling inventory available was prepared by CARB as part of the Southern

California Ozone Study (SCOS), using 1997 emissions data. CARB hired a SCOS emissions

inventory contractor who has prepared a gridded emissions inventory for a domain including the

Imperial Valley at 2-km and 5-km resolutions.

The 1997 modeling inventory used in this analysis has a 5-km resolution and included all of

western Imperial County and Mexicali. The CARB SCOS modeling inventory was for a typical

summer day in 1997 and included TSP, NOx, SOx, VOC, and CO (the VOC and CO emissions

were not used in this study). This modeling inventory was adjusted for use in the SIP analysis.

First, since this inventory contained emissions of total suspended particulate (TSP), all emissions

were adjusted down to represent only PM10. The ratio used in this adjustment was 1.93, which

was calculated by dividing the total TSP emissions to total PM10 emissions from Imperial County

in 1995 (see Table 7-2).

The 1997 inventory was also prepared for a typical summer day, which is not appropriate when

modeling days in other seasons. Summer months in Imperial County experience the highest

winds, and hottest and driest days, so higher levels of PM10 are most likely during these months.

In order to account for this, 1995 emissions were compared for the summer and winter seasons.

The ratio of these emissions was used to scale the modeling inventory for use in winter months.

For the spring and fall months, the inventory was adjusted assuming that emissions during these

months are an average of summer day and winter day emissions. Thus, the total adjustment

factor for any day is the product of the factor for converting TSP to PM10 and the factor used to

convert from a summer day to the day of interest. The adjustment factors used in this analysis,

along with the 1995 Imperial County emissions used to determine these, are listed in Table 7-2.

FINAL AUGUST 2009 ICAPCD
: 4

The NOx and SOx SCOS inventories obtained from CARB were included in this modeling

demonstration to account for the formation of secondary particulate. Please note that the NOx

and SOx emissions exhibited much less seasonal variation and were a much smaller component

to the Imperial County PM concentrations, so no scaling of the SOx and NOx SCOS modeling

inventories were performed.

Table 7-2. Determination of modeling inventory adjustment factors.

Season 1995 TSP

(tons/day)

1995 PM10

(tons/day)

Adjustment Factor:

TSP to PM10

Adjustment Factor:

Seasonal

Annuala 468.00 243.36 -- --

Summera 654.43 339.17 1.93 1

Wintera 282.28 147.93 1.93 2.29

Spring/Fallb 468.36 243.55 1.93 1.39

Notes:

a

 1995 emissions inventory obtained from CARB website (www.arb.gov/app/emsinv). Results

using the 2000 CARB emissions produce almost identical scaling factors.

b Emissions for spring/fall day calculated as the average of summer and winter day emissions

An additional adjustment was required to allow the use of the 1997 gridded emissions data for all

years between 1992 and 2000. In order to account for the change in emissions over time, the

1997 inventory has been scaled using changes in population. Thus, we assume that the change in

emissions is roughly proportional to the change in population over time. This should be a

reasonable assumption since the vast majority of PM10 emissions in Imperial County are from

area sources such as unpaved roads, paved roads, and agriculture. Thus, changes in population

are more likely to affect overall PM10 emissions than would be the case if the emissions were

mainly from industrial sources.

Population data used to scale emissions was obtained from three different sources, as shown in

Table 7-3. For years 1990 and 1999, where multiple data points were available, the average of

all data was used. For years where no data was available, the population for that year was

interpolated assuming steady population growth of 3.3%.

FINAL AUGUST 2009 ICAPCD
: 5

Table 7-3. Imperial County population data, 1990-2000.

Year CCBRESa CDOFb U.S. Censusc Average

1990 110,934 -- 109,303 110,119

1991 -- -- -- 112,899

1992 -- -- -- 116,613

1993 -- -- -- 120,450

1994 -- -- -- 124,413

1995 -- -- -- 128,506

1996 -- -- -- 132,734

1997 -- -- -- 137,101

1998 144,051 -- -- 142,831

1999 145,287 144,500 145,287d 145,025

2000 -- 145,300 -- 147,683

Notes:

a

 Data from California Center for Border and Regional Economic Studies

 (http://www.ccbres.sdsu.edu)

b

 Data from California Department of Finance (http://www.dof.ca.gov)

c

 Data from U.S. Census Bureau (http://www.census.gov)

d

 When the model runs were started, 2000 U.S. Census data was not available, so the

Census’ 1999 projected population was used. The 2000 Census population for Imperial

County is 142,361, so the population estimates used in the modeling are conservative,

since higher population gives higher emissions estimates.

The 1997 SCOS modeling inventory also contained emissions for Mexicali. During 1998, a joint

U.S.-Mexico study was performed to prepare a quality-assured inventory for Mexicali (Radian,

1999). Although the Radian Mexicali emissions inventory represented the best emissions

database ever prepared for Mexicali, the emissions still appear low compared to emissions from

U.S. cities, especially given the much larger number of unpaved roads and higher silt loadings on

the streets in Mexicali. The SCOS 1997 summer baseline emissions inventory for the Mexicali

portion of the domain was compared to the Radian Mexicali inventory. The SCOS Mexicali

PM10 and NOx emissions were half of those in the Radian inventory, and the SCOS SOx

Mexicali emissions were only a quarter of what was in the Radian inventory.

After some investigation, we discovered that the SCOS Mexicali inventory was based on an old

1990 placeholder database that was projected to 1997. Mexicali has undergone significant

amounts of growth over the last decade, particularly to the west. When the 1999

CALMET/CALPUFF model evaluation run was performed, the observed PM10 concentrations at

the Progresso site in western Mexicali were underpredicted by more than a factor of three

(Morris, Lu, and Tai, 2001). Thus, the SCOS Mexicali inventory was judged to be inadequate

for use in the SIP modeling.

FINAL AUGUST 2009 ICAPCD
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III.C. Receptor Locations

In modeling runs performed for this analysis, receptors were placed at all existing PM10 State and

local monitoring sites in the Imperial County Air Basin, including those located in Mexicali.

III.D. Background Concentrations

A background PM10 concentration must be added to the CALPUFF modeling results. Ideally,

monitoring sites within the modeling domain that are upwind of all emission sources should be

used to obtain a background concentration. Unfortunately, none of the monitors in Imperial

County can be classified over a 24-hour period as being upwind of all emission sources.

In order to determine an approximate PM10 background concentration, the frequency distribution

of observed PM10 concentrations at all Imperial and Mexicali monitors from 1992 to 2000 were

analyzed. Table 7-4 summarizes the observed PM10 concentrations in the lowest percentiles (5%

to 30%). Assuming that background levels accounts for about half of the lowest measured

concentrations, a clean background could be approximated as 10 μg/m3. Our modeling results,

however, will be based on an annual and 24-hr background concentration of 25 μg/m3 in order to

be conservative. (Bohnenkamp, 2001)

Table 7-4. Lowest percentiles of a distribution of observed PM10 data

Percentile Monitor Value (μg/m3

)

5% 19

10% 25

15% 29

20% 32

25% 36

30% 39

III.E. Design Value Determination

Based on the 1:6 day sampling schedule for 1994, Imperial County is required to show that all

exceedances can fall under the ‘but for’ exemption for that year. Thus, all days that showed an

exceedance at any monitor were modeled. All other exceedance dates between 1992 and 1994

were also modeled to show that Imperial County has maintained attainment but for emissions

from outside the U.S. The modeling results from 1992, 1993, 1994, and 1999 were also used to

show attainment of the annual NAAQS standard.

FINAL AUGUST 2009 ICAPCD
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IV. Modeling Results

IV.A. Full-Year Modeling Results

Full-year model runs were performed for the years 1992, 1993, 1994, and 1999. The predicted

maximum 24-hr and annual average concentrations were below the NAAQS at all monitoring

sites for each of these years. The CALPUFF modeling results for these years are summarized in

Table 7-5. For the years 1992, 1993, and 1994, the maximum 24-hour and annual averages were

predicted at the Calexico-Grant monitoring site. The highest 24-hr concentration modeled at this

site during this period was 140.4 μg/m3. The annual average at Calexico-Grant, calculated as the

average of the quarterly averages for a period of three years (1992-1994), was 41.6 μg/m3

. For

1999, the maximum predicted 24-hr concentration was 111.6 μg/m3

 at the El Centro site. The

annual average for 1999 was 43.8 μg/m3 at Calexico-Grant.

Table 7-5. Predicted concentrations at Imperial County monitors for full-year runs (μg/m3)

Year Averaging

Time

Brawley CalexicoEthelCalexicoGrantEl

Centro

Niland Westmorland

1992 Max. 24-hr 69.7 87.8 92.4 86.2 61.2 80.2

Annual 34.2 39.7 40.5 40.0 30.5 35.8

1993 Max. 24-hr 80.5 101.7 102.3 94.2 77.7 80.2

Annual 34.0 41.9 42.8 41.9 30.0 35.2

1994 Max. 24-hr 85.9 131.3 140.5 112.9 100.3 89.2

Annual 34.9 40.8 41.6 41.0 31.9 36.9

1999 Max. 24-hr 77.7 102.5 108.4 111.7 65.7 76.1

Annual 33.8 42.6 43.8 42.1 30.3 35.5

Note: modeling assumes a background concentration of 25 μg/m3.

IV.B. Exceedance Day Modeling Results

Each day between 1992 and 1994 that had observed exceedances of the 24-hr NAAQS was

modeled using only Imperial County emissions. The results show that the predicted

concentrations are below the 24-hr PM10 NAAQS for every exceedance day during this period.

Table 7-6 shows the predicted values for each exceedance day. The highlighted values show

monitors that measured an exceedance on a given day. For comparison, the actual values

measured on these days can be found in Table 7-7.

FINAL AUGUST 2009 ICAPCD
: 8

The maximum predicted 24-hour concentrations for Brawley, Calexico-Ethel, Calexico-Grant,

Niland, and Westmorland are 79.7 μg/m3, 101.7 μg/m3, 102.1 μg/m3, 124.0 μg/m3, 62.3 μg/m3,

and 88.7 μg/m3

, respectively. These results confirm that Imperial County would be in attainment

of the NAAQS standards but for emissions originating outside the United States.

FINAL AUGUST 2009 ICAPCD
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Table 7-6. Predicted concentrations for exceedance days between 1992 and 1994.

Predicted concentration (μg/m3

)

Date Brawley CalexicoEthel

CalexicoGrant El Centro Niland Westmorland

10/9/92 20.5 37.9 38.3 45.5 2.3 18.2

1/19/1993 30.3 48.1 49.6 43.8 26.2 29.5

1/25/1993 26.3 30.4 30.8 28.5 25.8 25.9

8/23/1993 45.8 101.7 102.1 91.8 31.6 43.7

1/20/94 8.5 18.8 20.7 22.6 1.3 6.4

7/7/1994 56.4 53.3 53.8 65.8 31.6 54.4

8/6/1994 46.3 31.3 31.5 37.8 56.0 54.9

10/17/1994 31.7 49.4 52.7 47.0 26.9 32.0

12/16/1994 26.6 34.1 35.6 30.6 25.3 26.4

Table 7-7. Imperial County 24-hour PM10 exceedance days, 1992-1994 (D – dichotomous

sampler).

Observed concentration (μg/m3

)

Date Brawley CalexicoEthel

CalexicoGrant El Centro Niland Westmorland

10/9/92 78 208 D

01/19/93 162 

01/25/93 175

08/23/93 98 253 D 166, 168 D 

1/20/94 88 156 D 55 D

07/07/94 58 165 62 62

08/06/94 126 258 182 119 73

10/17/94 44 113 159 49 

12/16/94 60 153 56 37 

FINAL AUGUST 2009 ICAPCD
: R-1

REFERENCES

Atkinson and Lee, 1992: Dennis Atkinson and Russell F. Lee, “Procedures for Substituting

Values for Missing NWS Meteorological Data for Use in Regulatory Air Quality Models,”

available on EPA SCRAM website (http://www.epa.gov/scram001/surface/missdata.txt)

July 7, 1992.

Bohnenkamp, 2001: telephone conversation with Carol Bohnenkamp, U.S. Environmental

Protection Agency, Region IX, 2001.

CARB, 1997: California Air Resources Board, Emissions Inventory Procedural Manual,

Volume III: Methods for Assessing Area Source Emissions in California, updated 1997.

(http://www.arb.ca.gov/emisinv/areasrc/index0.htm)

Census, 2000: U.S. Census Bureau “Census 2000” website (http://www.census.gov)

Chow and Watson, 1997: Judith C. Chow and John G. Watson, Desert Research Institute,

“Imperial Valley/Mexicali Cross Border PM-10 Transport Study,” January 1997.

EPA, 1985: U.S. Environmental Protection Agency, OAQPS, Compilation of Air Pollutant

Emission Factors, AP-42, Fourth Edition, Research Triangle Park, NC, September 1985.

EPA, 1986: U.S. Environmental Protection Agency, OAQPS, “Identification, Assessment, and

Control of Fugitive Particulate Emissions,” EPA-600/8-86-023, Research Triangle Park,

NC, August 1986.

EPA, 1987: U.S. Environmental Protection Agency, OAQPS, “PM-10 SIP Development

Guide,” EPA-450/2-86-001, Research Triangle Park, NC, June 1987.

EPA, 1988: U.S. Environmental Protection Agency, OAQPS, “Control of Open Fugitive Dust

Sources,” EPA 450/3-88-008, Research Triangle Park, NC, September 1988.

EPA, 1991: U.S. Environmental Protection Agency, OAQPS, “PM-10 Moderate Area SIP

Guidance: Final Staff Work Product,” memorandum from John Calcagni, Director, Air

Quality Division, Research Triangle Park, NC, April 2, 1991.

EPA, 1992a: U.S. Environmental Protection Agency, “General Preamble for the Implementation

of Title I of the Clean Air Act Amendments of 1990,” 57 Fed. Reg. 13498, April 16, 1992.

EPA, 1992b: U.S. Environmental Protection Agency, OAQPS, “Procedures for Emission

Inventory Preparation, Volume III: Area Sources,” EPA-450/4-81-026c (Revised), 1992.

EPA, 1994a: U.S. Environmental Protection Agency, “State Implementation Plans for Serious

PM-10 Nonattainment Areas; Addendum to the General Preamble for the Implementation

of Title I of the Clean Air Act Amendments of 1990,” 59 Fed. Reg. 41998, August 16,

1994.

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EPA, 1994b: U.S. Environmental Protection Agency, Region X, “Approval and Promulgation of

State Implementation Plans: Idaho,” 59 Fed. Reg. 43745, August 25, 1994.

EPA, 1998: U.S. Environmental Protection Agency, Region IX, “Technical Support Document

for U.S. EPA’s Proposed Federal Implementation Plan for the Phoenix Nonattainment

Area, RACM/RACT Analysis,” March 1998.

IC, 1985: Imperial County Planning Department, 1985 Land Use Plan.

IC, 2001: Imperial County Agricultural Commissioner, Sealer of Weights and Measures,

“Annual Crop Report for 2000,” April 2001.

ICAPCD, 1998: Imperial County Air Pollution Control District, Stationary Source Inventory

records, 1997 emissions.

ICAPCD, 2000: Imperial County Air Pollution Control District, Agricultural Burning Records

database, 1995-2000 records.

ICAPCD, 2001: Imperial County Air Pollution Control District, personal communication with

Brad Poiriez, May 2001.

ICDPW, 2001: Imperial County Department of Public Works, Memorandum to Brad Poirez -

ICAPCD, June 20, 2001.

ICPBD, 2000: Imperial County 2000-2005 Housing Element, Imperial County Planning/

Building Department, March 2001.

IID, 2001: Imperial Irrigation District, Ten-year plan for irrigation canal undergrounding,

communication with Imperial County Air Pollution Control District, 2001.

MAG, 1997: Maricopa Association of Governments, “Economic, Small Business, and

Consumer Impact Statement,” 1997.

MAG, 1999: Maricopa Association of Governments, “MAG 1999 Serious Area Particulate Plan

for PM-10 for the Maricopa County Nonattainment Area,” June 1999.

Morris R., E. Lu, and E. Tai. 2001. “PM10 Background, Emissions Evaluation, and Preliminary

Modeling and Evaluation”. Memorandum prepared for the Imperial County PM10 SIP

Development Team dated April 8, 2001. ENVIRON International Corporation, Novato,

California.

NCDC, 2001: National Climatic Data Center website (http://www.ncdc.noaa.gov)

FINAL AUGUST 2009 ICAPCD
: R-3

NWCG, 1985: National Wildlife Coordinating Group, Prescribed Fire and Fire Effects

Working Team, “Prescribed Fire Smoke Management Guide,” PMS 420-2, NFES 1279,

February 1985.

Pechan, 1993: Pechan, E.H. and Associates, Inc. “State Implementation Plan for PM-10 in the

Imperial Valley,” Adopted September 1993.

SCS, 1981: U.S. Department of Agriculture Soil Conservation Service, Soil Survey of Imperial

County California, Imperial Valley Area, in cooperation with the University of California

Agricultural Experiment Station and the Imperial Irrigation District, October 1981.

TACB, 1991: Texas Air Control Board, “1991 PM-10 State Implementation Plan for Moderate

Area - El Paso,” November 1991.

CONEPO, 2001: Consejo Estatal de Población de Baja California website listing data from the

Mexican Census 2000 (http://www.baja.gob.mx/organizacion)

Wallerstein, 1997: Letter from Barry. R. Wallerstein, Deputy Executive Office, South Coast Air

Quality Management District to Lynn Terry, Assistant Executive Officer, California Air

Resources Board, January 22, 1997.

Woodhouse and Gouze, 1997: Luis F. Woodhouse and Steve C. Gouze, California Air

Resources Board, “PM10 State Implementation Plan Attainment Demonstration for the

Imperial County Air Pollution Control District,” February 1997.

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Appendix A

Supporting analyses for the determination that attainment

would have been achieved but for emissions originating

outside of the U.S.

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I. Introduction

In the main report, the results of dispersion modeling of PM10, SOX, and emissions from Imperial

County were reported. These results suggest that emissions from Imperial County were

insufficient to cause the observed PM10 exceedances that occurred in Imperial County between

1992 and 1994. For reasons discussed in the main report and summarized below, only the

emission sources in Imperial County were included in the dispersion modeling. Performing

dispersion modeling using U.S. sources alone to demonstrate that PM10 exceedances would not

have occurred “but for” contributions from International sources has been demonstrated in the

past as an adequate and sufficient test for an EPA approvable “but for”PM10 attainment

demonstration (TACB, 1991). The modeling results presented in the main report fulfill the

standard set by the aforementioned PM10 attainment demonstration. Qualitative information and

analyses that support the conclusion that Imperial County would attain the PM10 standard “but

for” contributions from international transport are presented here in Appendix A.

One of the difficulties in making the “but for” determination is that there is limited monitoring

and emissions inventory data available in Imperial County and even less data for the areas just

south of the border in Mexico. And although the number of monitoring sites and the emissions

inventory have been improving over the last 5 years in Imperial County and Mexicali, further

improvements would yield a more precise quantification of the impacts on PM10 concentrations

in Imperial County due to emissions from Mexico. In the original Imperial County Data

Analysis and Modeling Protocol, dispersion modeling using both U.S. and Mexican emissions

was proposed in order to apportion the U.S. versus Mexico contributions to each PM10

exceedance to make the “but for” determination. Emissions data were to be based on the

California Air Resources Board’s (CARB) extensive and resource intensive emissions updates

performed as part of the Southern California Ozone Study (SCOS). However, the database

received in Spring 2001 was based on a 1990 placeholder inventory, not the more recent 1996

emissions inventory for Mexicali (Radian, 2000). This database was determined to be unsuitable

for modeling. Thus, in order to perform dispersion modeling of both U.S. and Mexican sources,

the redevelopment of a modeling inventory for Mexico would be required.

Multiple data analysis techniques, that qualitatively show that emissions from Mexico contribute

to concentrations measured on Imperial County’s exceedance days, have been developed to

support the quantitative dispersion modeling results of U.S emission sources. These data

analysis results are less quantitative than the modeled attainment demonstration in the main

report of the main report. The analyses, however, do offer corroborative qualitative evidence to

the dispersion modeling that the PM10 exceedances in Imperial County would not have occurred

“but for” contributions from Mexico. The results of the analyses are grouped by exceedance

day at the end of this appendix.

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II. Procedures

The series of data analysis techniques developed to illustrate whether or not emissions from

Mexico contribute to exceedances of the 24-hour PM10 NAAQS in Imperial County include the

following:

• Comparison of PM10 emissions and population in all of Imperial County with those

just in the city of Mexicali, Mexico;

• Emissions uncertainty analysis that analyzes whether the dispersion modeling

suggests that violations of the PM10 standard could be attributable to Imperial County

sources if the emissions were varied within their range of uncertainties;

• Spatial maps of 24-hour PM10 concentrations on Imperial County exceedance days

from which the potential transport paths can be inferred based on concentrationgradients. For Imperial County, the monitors are aligned such that only a strict south

to north transport path can be shown;

• Wind roses of 24-hour hourly wind speeds and wind directions at the closet surface

meteorological site to the Imperial County PM10 exceedance site. Transport from

Mexico can be inferred when southerly wind components are present;

• Back trajectory analysis showing the synoptic air flow over the last 36-hours. The

plots show the likely geographic source of the PM10 emissions that contributed to the

exceedance based on upper-level winds; and

• Meteorological stagnation classification. Under stagnant conditions, the wind rose

and back trajectory analyses are less reliable than other periods with stronger winds.

By themselves, each of these data analysis techniques has limitations and uncertainties and only

provides qualitative supporting evidence. However, taken in total along with the dispersion

modeling provided in the main report, the analyses provide a weight of evidence that the PM10

exceedances in Imperial County would not have occurred “but for” contributions due to

emissions that originated in Mexico.

Imperial County versus Mexicali Emission Inventory Comparisons

The dispersion modeling presented in the main report suggests that emissions from Imperial

County are insufficient by themselves to cause the observed PM10 exceedances that occurred in

Imperial County from 1992 to 1994. In this section, the possible sources that may have led to the

high PM10 concentrations measured in Imperial County are discussed. The other potential

explanations for the PM10 exceedances are:

1. Contamination or other monitoring artifacts leading to false positive exceedances;

2. Undocumented or episodic emission sources in the U.S.;

3. Transport from U.S. emission sources outside of Imperial County; and/or

4. Contributions of emissions from Mexico.

PM10 monitoring is an inexact science with many more possibilities of contamination,

interference, or other phenomena leading to false positive exceedances than other criteria

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pollutants (e.g., ozone, SO2, etc.). Thus, isolated exceedances, without supporting evidence from

the other PM10 observations that elevated PM10 concentrations were present in the area, must be

considered with caution.

The dispersion modeling showed that emissions in Imperial County are insufficient to cause

exceedances of the PM10 standard suggesting that there must be other emission sources either

non-routine episodic emissions (e.g., upsets) or emissions originating outside of Imperial County

(in the U.S. or outside the U.S.). Within Imperial County, the following episodic emissions

could contribute to PM10 concentrations:

• Vegetative burning;

• Wind blown dust due to high winds; and

• Fugitive dust due to INS tire dragging operations

A correlation between PM10 exceedances in the Imperial County and agricultural burning or

wildfires in the Imperial Valley, however, could not be determined. The ICAPCD agricultural

burning records show that of the 159 days with PM10 monitoring data between 1999 and 2000,

there are 128 days with agricultural burning in the county; and only 31 days without. Out of the

17 exceedance days in this period, 12 had some type of agricultural burning, 5 did not. Of the

142 days with monitoring data that did not show an exceedance, 116 had some type of

agricultural burning, only 26 did not. These results suggest that there is no correlation between

exceedance days and agricultural burning taking place in Imperial County.

Furthermore, if extremely high winds produced sufficient wind blown dust to cause PM10

exceedances at Imperial County monitors, it is likely that high PM10 concentrations would be

measured throughout the Imperial Valley. As discussed in more detail below, such wide spread

high PM10 occurrences tend to only occur when winds are from the south, where emissions from

Mexico are most likely impacting the measured concentrations.

Finally, the fugitive dust emissions due to the INS tire dragging operations are sufficiently far

away from the Calexico Grant and Ethel sites and are expected to have a minimal impact at these

sites, and at other sites further north in Imperial County.

If U.S. sources outside of Imperial County contributed significantly to PM exceedances in

Imperial County, then such sources would likely cause severe local exceedances, outside of

Imperial County also. There is no evidence of this.

The largest emissions source outside of the U.S. that is not accounted for in the dispersion

modeling is the City of Mexicali. Radian compiled a 1996 emissions inventory for Mexicali that

represents the best current estimate of Mexicali emissions (Radian, 2000). However, it is also

recognized that this inventory is much lower quality than that available for the U.S. sources and

that the emissions in Mexicali are underreported. Table A-1 below compares PM10 emissions

and other pertinent parameters in Imperial County with those in the City of Mexicali. In 1996,

emissions within the City of Mexicali itself exceed those in all of Imperial County. The

emissions density in Mexicali is 20 times greater than that of Imperial County.

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Based on the data from the Radian report, one of the largest PM10 emissions source in the

Imperial Valley is the City of Mexicali. This emissions source is not accounted for in the

dispersion modeling and is the most logical source that would lead to the Imperial County

exceedances.

Table A-1. Comparison of Imperial County and Mexicali Emissions

Imperial

County

City of

Mexicali

1996 PM10 Emissions (tons/day)1 246 257

Population (2000)2 142,361 622,617

Area (mi2)

3 4060 200

Growth Rate (1990-2000)2 30% 42%

1. Air Resources Board emissions inventory and Radian, 2000.

2. US CENSUS bureau and Radian, 2000.

3. CERES and Topographic Maps.

Emission Uncertainties

The dispersion modeling results presented in the main report suggest that emissions from the

U.S. alone are insufficient to cause PM10 exceedances in Imperial County. However, the

Imperial County emissions inventory has a degree of uncertainty. This is especially true for the

fugitive dust PM10 emission sources (e.g., unpaved roads, agricultural, etc.) that dominate the

Imperial County PM10 emissions inventory. For some emission sources (e.g., combustion),

however, emission uncertainties are fairly low and well known.

The Imperial County dispersion modeling results from 1992 to 1994 were examined in order to

determine the level of emissions uncertainty necessary for emissions from Imperial County alone

to cause a PM10 exceedance and a violation of the PM10 standard. Ideally, the Imperial County

emissions should be varied within their range of uncertainties and a Monte Carlo or other

statistical technique should be used to estimate the probability that emissions from Imperial

County alone could cause an exceedance or violation of the PM10 standard. However, since such

emissions uncertainty bounds are not currently available, a simple worst case analysis was

performed to estimate the level of emissions increase necessary for Imperial County emissions

alone to cause an exceedance or violation of the PM10 standard.

For the 24-hour PM10 NAAQS and a 25 μg/m3

 background, the Imperial County PM10 emissions

inventory would have to be biased 86%, 62%, and 8% higher in order to cause a PM10

exceedance during, 1992, 1993, and 1994, respectively. Note that the 25 μg/m3 background,

itself, is conservatively high.

For the annual PM10 NAAQS and a 25 μg/m3

 background, the Imperial County PM10 emissions

inventory would have to be biased 61%, 40%, and 50% higher in order to cause a violation in

1992, 1993, and 1994, respectively. Using the modeling results from 1992 to 1994, the Imperial

County emissions inventory would have to be biased 50% high in order to have a violation of the

24-hour PM10 NAAQS.

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In summary, although the uncertainty in the Imperial County emissions inventory is not

quantified, the analysis presented above indicates that a 46% and 50% positive bias of the

Imperial County PM10 emissions inventory is required in order to show a violation of the annual

and 24-hour PM10 NAAQS, respectively, due to emissions from Imperial County. Since the

procedures used to generate the Imperial County emissions inventory are the same as those used

throughout California, it is unlikely that the Imperial County emissions inventory possess such a

large bias. Thus, the emissions uncertainty analysis supports the finding that the Imperial

County emissions alone are insufficient to cause violations of the PM10 NAAQS.

Spatial Plots

Spatial plots were prepared showing the spatial distribution of 24-hour PM10 concentrations that

were measured at each Imperial County monitoring site, as well as sites in Mexicali when

available, for each Imperial County exceedance day from 1992 through 1994. On days in which

transport is from the south and is aligned along the Mexicali-Imperial County PM10 monitoring

network then a PM10 concentration gradient from south to north provides evidence of transport

from Mexico.

On many of these exceedance days, spatial plots provide evidence that the Imperial County

exceedance was likely caused by emissions emanating from Mexico. Strong evidence includes

days where concentrations are concurrently high in Mexicali (or higher than) and in Imperial

County or days where there is a decreasing concentration gradient as one moves from south to

north. The spatial plots show the following for Imperial County 24-hour PM10 exceedance days

from 1992 to 1994:

• The spatial plots for 7 out of 9 days show spatial PM10 concentration gradients going

from higher to lower as one moves from south to north;

• Zero out of nine exceedance days show concurrent high PM10 concentrations in both

Mexicali and in Imperial County (no data was available in Mexicali between 1992

and 1994);

• Only 2 out of 9 days do not show a south to north increasing concentration gradient;

Both of these days have only have one data point so no trend can not be determined.

On some exceedance days, the spatial plots do not conclusively show that transport from

Mexicali is contributing to the Imperial County PM10 exceedances. In these cases, however, the

potential for emissions from Mexico impacting Imperial County monitors still exists for three

reasons. First, the Mexicali monitoring network is very sparse and there is a large area directly

to the south of Calexico in Mexicali without any monitors. This gap allows the possibility that

the Mexicali monitors may miss significant emission sources in Mexicali. Second, emissions

may circumvent the sparse monitoring network and circle around to impact only northern

Imperial County. Third, there are emission sources outside the U.S. not in Mexicali that can also

impact the Imperial County monitors without recording high PM10 concentrations in Mexicali

(e.g., Laguna Salada).

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Wind Rose Analysis

Wind roses of hourly surface wind speed and direction from the closest meteorological station to

the monitor showing an exceedance for the 24-hour exceedance day are also included. These

wind roses provide evidence of potential transport from Mexico during all Imperial County PM10

exceedance days. An hourly wind direction was classified as having the potential of carrying

emissions from Mexico into Imperial County if it has a southerly component (i.e. wind direction

ranging from 90 degrees thru 270 degrees).

Every exceedance day had at least 14 hours where the measured wind direction indicated that

emissions from Mexico would be carried into Imperial County. That is, all Imperial County

PM10 exceedance days had a fraction of the time where winds are out of the south from Mexico.

On average, each exceedance day had 16 hours of wind that would carry emissions from outside

of the U.S. into Imperial County (i.e., on average, on Imperial County exceedance days, over half

of the day exhibited hourly wind directions that implied transport from Mexico). These results

overwhelmingly support the hypothesis that emissions from Mexico are impacting Imperial

County monitors and are significantly contributing to PM10 concentrations on PM10 exceedance

days.

Back Trajectory Analysis

Back trajectories, showing the most likely source of origination of air parcels that arrived at the

Imperial County monitor during the 24-hour PM10 exceedance day, were also created for each

exceedance date. These were created using the HYSPLIT model on the Air Resources

Laboratory (ARL) of the National Oceanic and Atmospheric Administration (NOAA) website.

Based on user input information (date, time, location, wind heights), the model calculates and

plots the back trajectory using wind fields developed by National Weather Service (NWS).

These wind fields were based on the initialization portion of the NWS weather forecast models

and represent synoptic flow regimes. Thus, local surface wind variations are likely not be

present in these analyses. The 36-hour back trajectory plots were generated for each exceedance

day in four hour increments from the location of the Imperial County monitor measuring the

highest PM10 concentration of the day. The trajectories demonstrate 1) when the air passes

through Mexico and then into Imperial County; 2) when the air might have circumvented the

monitors in Calexico and impacted the northern monitors in Imperial County; and 3) when air

may have come from Mexico, but not necessarily through Mexicali.

Stagnation Classification

Wang and Angell (1999) have developed procedures to classify days as having stagnant air.

They define a major air pollution episode day as a stagnation event if it lies in a 4-day period

when the average sea-level geostrophic wind speed is less than 8 m/s. Wang and Angell equate

this to a surface wind speed (10 m above ground level) of less than 3.2 m/s. Using Wang and

Angell’s criteria, our exceedance days were classified as stagnation events if the average wind

speed for the day was less than 3.2 m/s. The 4-day criterion was excluded because the

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exceedance days were not required to be major pollution episodes. A listing of exceedance days

for Imperial County and the corresponding surface wind speeds on those days can be found in

Table A-2.

Under stagnant air conditions, low wind speeds lead to poor dispersion of pollutants such as

PM10. Furthermore, the extrapolation of low local wind speeds to a regional level is not

necessarily as accurate as that made for higher local wind speeds. Therefore, wind roses and

back trajectories for exceedance days that can be classified as a stagnation event should be

analyzed with more care. During stagnation events, it is possible that the wind rose and/or back

trajectory may not accurately represent the impact of emissions from Mexico on Imperial

County.

For all of the exceedance days where the back trajectories do not show air coming from Mexico,

the day can be classified as a stagnant air event. All exceedance dates showed a fraction of hours

with winds blowing from the south towards the north. There are 5 out of 9 exceedance dates

where the back trajectories did not indicate air being carried from outside the U.S. into Imperial

County. Each of these 5 dates could be classified as stagnant air days (1/19/93, 1/25/93, 1/20/94,

10/17/94, 12/16/94). Under these conditions, the dispersion modeling is probably the best

indicator of which sources contributed to the exceedances since dispersion modeling can address

both stagnant and non-stagnant conditions.

Table A-2. Wind speeds for each exceedance day, 1992-1994.

Sampling Date Average Wind Speed (m/s) Meteorological Station

10/9/92 2.3 Imperial Airport

1/19/93 1.8 Imperial Airport

1/25/93 2.8 Imperial Airport

8/23/93 4.0 Imperial Airport

1/20/94 1.9 Imperial Airport

7/7/94 2.5 Imperial Airport

8/6/94 3.9 Imperial Airport

10/17/94 1.5 Imperial Airport

12/16/94 2.8 Imperial Airport

III. Results

The spatial plot, wind rose, and back trajectory analyses results provide an indication that

emissions from outside the U.S. impact monitors in Imperial County. Four out of the nine

exceedance dates had positive indicators from all three of these analyses. The results for the

remaining five Imperial County exceedance days from the three data analysis techniques (spatial

maps, wind roses, and back trajectories) were not as explicit, but the analyses still suggest that on

all exceedance days, emission sources from outside the U.S. contributed to the PM10 exceedances

measured in Imperial County. These results combined with the dispersion modeling results in

the main report, which suggest that emissions from the U.S. alone were insufficient to cause the

PM10 exceedance, provide a weight of evidence that the 24-hour PM exceedances in Imperial

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County would not have occurred “but for” the contributions of emissions originating from

Mexico. A general summary is given below followed by a detailed account for each exceedance

day.

There were 7 out of 9 spatial plots that suggest that Imperial County monitors were being

impacted by emissions coming from outside the U.S. Of the remaining two plots, both (1/19/93,

1/25/93) had only one data point (i.e., only one monitor recorded a concentration on each of

these dates). All 9 exceedances dates show at least 14 hours with wind directions that could have

carried emissions from Mexico to the impacted Imperial County monitor. Four out of nine

exceedance dates have back trajectories that show air passing through Mexico and being carried

to the impacted Imperial County monitor. Of the remaining five days, they can be classified as

stagnant air days.

Listing by exceedance day

October 9, 1992 (208 μg/m3

 at Calexico-Grant dichotomous sampler)

There is a high value along the border and a lower value at the more northerly Brawley monitor.

The wind rose shows that there are 14 out of 24 hours with southerly wind directions that have

the potential to carry emissions from Mexico into Imperial County. The back trajectories

suggest that air passes through Mexicali and then into Imperial County. The day, however, is

classified as a stagnant air day. The proximity to the border, the spatial distribution, wind rose,

and back trajectory suggest that emissions from Mexico contribute to this exceedance.

January 19, 1993 (162 μg/m3

 at Brawley)

There is only one data point for the spatial plot (Brawley). The wind rose shows that there are 15

out of 24 hours with southerly wind directions that have the potential to carry emissions from

Mexico into Imperial County. The back trajectories based on upper-air synoptic wind data

suggest much higher winds and back trajectories that do not pass through Mexicali. The day is

classified as a stagnant air day, therefore there is more separation between the surface winds and

the winds aloft and greater weight is given to the surface data. The wind rose suggests that

emissions from Mexico contributed to the concentration measured at Brawley. Because the

PM10 exceedance on this day occurs when only one site was operating, there is insufficient

information to draw any additional conclusions. However, given the closeness of the exceedance

to the standard and the presence of surface winds with a southerly wind component for over half

the day, it appears likely that this exceedance would not have occurred but for transport from

Mexico.

January 25, 1993 (175 μg/m3

 at Brawley)

There is only one data point for the spatial plot (Brawley). The wind rose shows that there are 14

out of 24 hours with wind directions that have the potential to carry emissions from Mexico into

Imperial County. The back trajectories suggest very high wind speeds that are obviously

decoupled from the surface winds, which are very low. The day is classified as a stagnant air

day. The wind rose suggests that emissions from Mexico contributed to the concentration

measured at Brawley. This day is very similar in character with January 23, 1993 and the same

conclusions apply.

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August 23, 1993 (166 μg/m3 at El Centro, 168 μg/m3 at El Centro dichotomous sampler, 253

μg/m3 at Calexico-Grant dichotomous sampler)

The spatial plot only has two data points at El Centro and Brawley but the more southerly

monitor (El Centro) shows a much higher concentration. The dichotomous sampler in CalexicoGrant shows an even higher value. The wind rose shows that there are 19 out of 24 hours with

wind directions that have the potential to carry emissions from Mexico into Imperial County and

the back trajectory also shows that winds aloft carried air from Mexico into Imperial County.

The day is not classified as a stagnation day suggesting that the wind rose and back trajectory are

accurate. All analyses suggest that emissions from Mexico are impacting monitors in Imperial

County.

January 20, 1994 (156 μg/m3 at Calexico-Grant dichotomous sampler)

There is a high value along the border and a lower value at the more northerly El Centro and

Brawley monitors. The wind rose shows that there are 15 out of 24 hours with southerly wind

directions that have the potential to carry emissions from Mexico into Imperial County. The

back trajectories suggest that air does not pass through Mexico. The day, however, is classified

as a stagnant air day. The proximity to the border, the spatial distribution, and wind rose,

suggest that emissions from Mexico contribute to this exceedance.

July 7, 1994 (165 μg/m3

 at Calexico-Grant)

The spatial plot shows an exceedance at Calexico-Grant that is more than two times higher than

any monitor in Imperial County. The wind rose shows that there are 17 out of 24 hours with

wind directions that have the potential to carry emissions from Mexico into Imperial County and

the back trajectory also shows that winds aloft carried air from Mexico into Imperial County.

The day is classified as a stagnation day suggesting that the wind rose and back trajectory should

be interpreted cautiously. The analyses, however, still strongly suggest that emissions from

Mexico are impacting monitors in Imperial County.

August 6, 1994 (258 μg/m3 and 182 μg/m3 at Calexico-Ethel and Calexico-Grant, respectively)

The spatial plot shows exceedances at Calexico-Ethel and Grant, one of which is more than

double the concentrations measured at more northerly sites in Imperial County. The wind rose

shows that there are 18 out of 24 hours with wind directions that have the potential to carry

emissions from Mexico into Imperial County and the back trajectory also shows that winds aloft

carried air from Mexico into Imperial County. The day is not classified as a stagnation day. All

analyses suggest that emissions from Mexico are impacting monitors in Imperial County.

October 17, 1994 (159 μg/m3 at Calexico-Grant)

The spatial plot shows decreasing concentrations moving north from Calexico. The wind rose

shows that there are 14 out of 24 hours with wind directions that have the potential to carry

emissions from Mexico into Imperial County. The back trajectories do not show air being

carried from outside of the U.S. to the impacted location. The day, however, is classified as a

stagnant air day, therefore there is more separation between the surface winds and the winds aloft

and greater weight is given to the surface data. The single high PM10 concentration at CalexicoGrant, close proximity to Mexico, and wind rose strongly suggest that this exceedance is likely

due to emissions from Mexico.

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December 16, 1994 (153 μg/m3 at Calexico-Ethel)

The spatial plot shows decreasing concentrations moving north from Calexico. The wind rose

shows that there are 14 out of 24 hours with wind directions that have the potential to carry

emissions from Mexico into Imperial County. The back trajectories do not show air being

carried from outside of the U.S. to the impacted location. The day, however, is classified as a

stagnant air day, therefore there is more separation between the surface winds and the winds aloft

and greater weight is given to the surface data. The single high PM10 concentration at CalexicoEthel, close proximity to Mexico, and wind rose suggest that this exceedance is potentially due

to emissions from Mexico.

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REFERENCES

California Environmental Resources Evaluation System (CERES) website (http://ceres.ca.gov/)

California Center for Border and Regional Economic Studies website

(http://www.ccbres.sdsu.edu/).

Census, 2000: U.S. Census Bureau “Census 2000” website (http://www.census.gov).

National Oceanic and Atmospheric Administration (NOAA) website. Air Resources Laboratory.

(http://www.arl.noaa.gov/ready/hysplit4.html),

Radian International, 2000: “Mexicali Air Emissions Inventory,” February 2000.

TACB, 1991: Texas Air Control Board, “1991 PM-10 State Implementation Plan for Moderate

Area - El Paso,” November 1991.

Wang and Angell, 1999. “Air Stagnation Climatology for the United States (1948-1998)”

(http://www.arl.noaa.gov/ss/climate/stag.html).

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