Technical Support Document

for the

EPA’s Action on the 2012 Five Percent Plan

Air Division

U.S. EPA Region 9

January 14, 2014
ii

For information on this technical support document, please contact:

Scott Bohning

Office of Air Quality Analysis

U.S. Environmental Protection Agency – Region 9

75 Hawthorne Street

San Francisco, California 94105

(415) 947-4127

bohning.scott@epa.gov

Michael Flagg

Office of Air Quality Analysis

U.S. Environmental Protection Agency – Region 9

75 Hawthorne Street

San Francisco, California 94105

(415) 972-3372

flagg.michaela@epa.gov

Greg Nudd

Office of Air Planning

U.S. Environmental Protection Agency – Region 9

75 Hawthorne Street

San Francisco, California 94105

(415) 947-4107

nudd.gregory@epa.gov
iii

Table of Contents

I. Purpose of this Document............................................................................................................ 1

II. Emissions Inventories ................................................................................................................ 1

II.A. Evaluation of the 2008 PM-10 Inventory........................................................................... 1

II.A.1. Overview ..................................................................................................................... 1

II.A.1. Area Sources (Other than Windblown Dust) .............................................................. 2

II.A.2. Windblown Dust.......................................................................................................... 3

II.A.2. Nonroad Sources ......................................................................................................... 3

II.A.3. Onroad Mobile Sources............................................................................................... 4

II.B. Annual Inventories Derived from the 2008 Periodic Inventory......................................... 5

II.B.1. Base Case Inventories.................................................................................................. 5

II.C.2. Controlled Inventories................................................................................................. 5

III. Air Quality Data........................................................................................................................ 6

III.A. Requirements for Demonstrating Attainment by December 31, 2012.............................. 6

III.B. Exceptional Events............................................................................................................ 8

III.C. Tribal Data......................................................................................................................... 9

III.D. EPA Conclusion.............................................................................................................. 10

IV. Attainment Demonstration...................................................................................................... 10

IV.A. Overview......................................................................................................................... 10

IV.B. Technical Approach ........................................................................................................ 11

IV.C. Selection of Design Days................................................................................................ 14

IV.D. Modeling Domain ........................................................................................................... 15

IV.E. Estimating the Impact of Control Measures.................................................................... 15

IV.F. Results of Attainment Demonstration and EPA Assessment.......................................... 16
1

Technical Support Document

for the EPA’s Action on the 

Maricopa County (Phoenix) PM-10 Nonattainment Area 

Five Percent Plan

I. Purpose of this Document 

This Technical Support Document (TSD) is intended to support EPA’s action on the submitted 

SIP revision “Maricopa Association of Governments Five Percent Plan for PM-10 for the 

Maricopa County Nonattainment Area” submitted on May 25, 2011. (2012 Five Percent Plan). 

This TSD provides more technical detail on the topics discussed in the Federal Register notices 

for this action (see docket number EPA-R09-OAR-2013-0762). Readers should consult those 

Federal Register notices for more background and context on the topics covered in this TSD. 

II. Emissions Inventories

The 2012 Five Percent Plan included annual inventories for 2007-2012 in order to demonstrate 

compliance with the Clean Air Act 189(d) requirement for a five percent emissions reduction 

annually until the standard is attained. The annual inventories were also needed for the 

reasonable further progress and contingency measure demonstrations. These demonstrations are 

all discussed in our Federal Register notice proposing action on the plan. 

The inventories are based on the “2008 PM10 Periodic Emissions Inventory for the Maricopa 

County, Arizona Nonattainment Area, Revised June 2011” (2008 PM-10 Inventory) that was 

developed by the Maricopa County Air Quality Department (MCAQD) with assistance from the 

Maricopa Association of Governments (MAG). See Appendix A, Exhibit 1 of the 2012 Five 

Percent Plan. This document only assesses the annual inventory for PM10 for the Maricopa 

County PM10 Nonattainment Area. While the 2008 PM-10 Inventory includes typical daily data 

and data for other pollutants, these pollutants are not relevant to our action on the plan.1

II.A. Evaluation of the 2008 PM-10 Inventory

II.A.1. Overview

Table 1, below summarizes the annual PM10 emissions in the Maricopa County PM10

Nonattainment area for 2008, as provided in the 2008 PM-10 Inventory, Section 1.6. The table 

also highlights any source subcategories contributing more than 1,000 tons per year (tpy) of 

PM10 emissions.

 

1

See footnote 6 in the proposed action regarding PM10 precursors. 
2

Table 1: Overview of the 2008 PM-10 Inventory

Source Category Significant Sub-Categories

> 1,000 tpy

Annual PM10

Emissions [tpy]

Point Sources 150

Area Sources 24,521

Windblown Dust 4,815 tpy

Commercial Construction 4,057 tpy

Travel on unpaved parking lots 2,365 tpy

Offroad recreational vehicles 2,014 tpy

Road Construction 2,051 tpy

Residential Construction 1,692 tpy

Nonroad sources 1,928

Construction and mining equipment 1,249 tpy

Onroad sources 21,549

Unpaved road and alley fugitive dust 11,710 tpy

Paved road fugitive dust 6,694 tpy

Exhaust, tire and brake wear 3,144 tpy

Total Emissions 48,148

Point sources are larger industrial sources such as power plants and large manufacturing 

facilities. These sources self-report their emissions to the MCAQD. The self-reported emissions 

are quality assured and validated by MCAQD. However, as Table 1 shows, these sources are not

significant contributors to PM10 emissions in the nonattainment area. This document will focus 

on the other, more significant source categories.

II.A.1. Area Sources (Other than Windblown Dust)

Area sources are stationary sources which are too small and numerous to be treated as point 

sources. EPA has reviewed the list of area sources estimated by MCAQD and agrees that their 

list includes all of the significant PM10 area sources. The methodology for calculating PM10

emissions from area sources depends on the source, but generally one uses a combination of 

emission factors and activity data. For example, the Western Regional Air Partnership (WRAP)

Fugitive Dust Handbook2recommends using 0.42 ton PM10/acre-month for road construction. 

So, for every acre of land impacted by road construction for a month, one could expect 0.42 tons 

of PM10 emissions. These emission factors are combined with activity data to estimate emissions. 

For example, Maricopa County issues permits for large construction projects, and that data can 

be used to estimate how many acres were impacted by construction and for how long. Both the 

emission factors and the activity data can vary significantly in uncertainty. EPA has reviewed the 

area source emissions calculations and found that appropriate emission factors were used and 

that the most accurate possible activity data was also used.

The key area source categories in Table 1 above were carefully analyzed in the 2008 PM-10 

Inventory. MCAQD took particular care in getting the best data and emission factors for 

construction sources (see Section 3.3.9 of the 2008 PM-10 Inventory) using their databases of 

construction permits and inspections. Offroad vehicle emissions are difficult to calculate because 

 

2 WRAP Fugitive Dust Handbook, prepared by Countess Environmental for the Western Governors’ Association, 

Sept. 7, 2006.
3

the emission factors are not well established and the activity data is not easy to develop. See

Section 3.5.8 of the 2008 PM-10 Inventory. Nevertheless, MCAQD’s estimate of offroad vehicle 

emissions appears to be sufficiently accurate. Travel on unpaved parking lots is similarly 

difficult to calculate because “unpaved parking lots” are not necessarily registered or indicated as 

such on maps. Still, the MCAQD used the best and most current emission calculation methods 

and developed sufficiently accurate activity data for the purposes of the 2012 Five Percent Plan 

(see Section 3.5.9 of the 2008 PM-10 Inventory).

II.A.2. Windblown Dust 

Windblown dust is the largest component of the area source inventory and is a key source 

category relating to PM10 violations in the Maricopa County PM10 Nonattainment Area. Since 

2007, nearly all of the exceedances of the PM10 standard have been measured on days with 

elevated or high winds.3 Windblown dust is only about 10 percent of the annual inventory, but it

is a greater concern on windy days. 

MCAQD in cooperation with MAG developed a detailed method for calculating windblown dust 

emissions (see Appendix 4 of the Inventory). Levels of windblown dust emissions are 

particularly difficult to quantify.4 MCAQD and MAG developed initial inventory measurements 

using methods developed in the scientific literature, taking into account the best available data on 

land use and on the degree to which soil is disturbed. Since this method only accounts for the 

generation of the dust and not how it behaves once it is generated, it tends to overestimate the 

emissions of windblown dust that impact the area because it does not account for the 

precipitation of dust from the atmosphere and deposition on the ground. To address this issue, 

MCAQD and MAG analyzed ambient monitoring data to determine the percentage of PM10 mass 

that was measured at the monitors at various wind speeds. See Table A4-9 of Appendix 4 of the 

2008 PM-10 Inventory. This analysis showed that on an annual basis, the percentage of PM10

measured at wind speeds greater than 12 miles per hour ranged from 3.37% to 14.07% with an 

average of 7.58%. Using this data, MCAQD and MAG scaled down the windblown dust 

emissions to account for no more than 10% of the annual inventory. EPA has considered this 

method and finds it to be technically justifiable and sufficiently accurate for the purposes of this 

plan. This method would not be appropriate for an attainment demonstration, however, and it 

was not used in the attainment demonstration. 

II.A.2. Nonroad Sources

Nonroad sources are mobile sources that are not licensed or certified as highway vehicles. The 

development of this inventory is covered in Section 4 of the 2008 PM-10 Inventory. MCAQD 

used EPA’s NONROAD model (version 2008.1.0) (the current version of the model) to develop 

emissions estimates for most nonroad sources in the 2008 PM-10 Inventory. 

MCAQD used a Federal Aviation Administration model for aircraft and airport ground support 

equipment and they used surveys of railroad companies operating in the area to develop 

estimates of emissions from locomotives and support equipment. The methods and input data are 

 

3

See pages 3.8-3.12 of the 2012 Five Percent Plan

4 Appendix 4 of the 2008 PM-10 Inventory references studies regarding how the dust is generated by the wind on 

various soil types with various degrees of disturbance. EPA is not aware of any studies that attempt to quantify 

transported dust or the extent to which it is re-deposited to the ground.
4

reasonable and current for these two sources and EPA considers the resulting emissions 

estimates to be sufficiently accurate for the purposes of the 2008 Five Percent Plan. 

The most significant nonroad source is construction and mining equipment. MCAQD used 

construction employment as a measure of construction activity in order to calculate the 

construction emissions, rather than the recommended factor of the total dollar value of 

construction. Data for the preferred parameter was not available and the use of construction 

employment is an acceptable substitute, Therefore, EPA finds that this is an acceptable method 

to estimate these emissions with the NONROAD model. 

II.A.3. Onroad Mobile Sources

The process for calculating the emissions from onroad sources is documented in the 2008 PM-10 

Inventory Section 5 and in the MAG TSD, Section II. Emissions from exhaust, brake and tire 

wear were estimated using EPA’s Motor Vehicle Emissions Simulator (MOVES) model. Other 

emissions were estimated using calculation methods specified in EPA’s AP-42. 

EPA’s MOVES model was used to develop emissions estimates for vehicle exhaust, tire wear 

and brake wear. MAG used MOVES2010a, which was the recommended model at the time the 

inventory was developed. These emissions were originally estimated for the 2008 Periodic 

Emissions Inventory using the relevant data available at that time for input parameters such as 

fuel usage, vehicle population, vehicle miles traveled and others. Later, when MAG developed 

the Technical Support Document, newer data was available. Significantly, the latest vehicle 

registration data from the Arizona Department of Transportation indicated a greater proportion of 

older vehicles than expected. MAG used the same version of MOVES, which was appropriate 

but with updated data on fleet mix and the distribution of traffic. The resulting emissions 

estimate was 22.4 percent higher than was estimated in the 2008 Periodic Emissions Inventory.5

EPA finds that it was appropriate for MAG to make this change and to incorporate these higher 

estimates into the 2008 emissions estimate.

Emissions of fugitive dust from paved and unpaved roads were estimated using the January 2011 

updated versions of Sections 13.2.1 and 13.2.2 of the EPA Compilation of Air Pollutant 

Emission Factors, AP-42 (EPA, 2006). These are the current and best available methods for 

estimating emissions from these sources. 

The paved road dust emissions depend on the level of silt on the road (silt loading), the weight of 

the vehicles traveling on the road, the number of wet days in a year and the miles driven on the 

road. Fugitive dust emissions can be reduced by sweeping silt off the streets with PM10-certified 

street sweepers. The 2008 PM-10 Inventory details these calculations in Section 5.3.1. MAG 

updated the calculations to reflect the most current available data on vehicle weight and updated 

assumptions about the extent of street sweeping beginning on page II-11 of the MAG TSD. EPA 

finds that the methods were appropriate and the data used was the best available at the time the 

emissions were estimated. 

 

5 MAG TSD, page II-10.
5

Fugitive dust emissions from unpaved roads depend on the surface material silt content, the 

average vehicle speed, the surface material moisture content and the annual number of wet days 

per year. Emissions can be reduced by paving roads and limiting speeds on unpaved roads. 

Section 5.3.2 of the 2008 PM-10 Inventory provides information on the assumptions used. MAG 

updated these assumptions with more recent and reliable data in the MAG TSD, starting on page 

II-16. EPA finds that the methods were appropriate and the data used was the best available at 

the time the emissions were estimated.

II.B. Annual Inventories Derived from the 2008 Periodic Inventory

II.B.1. Base Case Inventories

The 2008 PM-10 Inventory (with some improvements by MAG for onroad sources) served as the 

basis for the base case and controlled inventories for 2007, 2009, 2010, 2011 and 2012. The 

methodology used to estimate these base case inventories is described in Section II of the MAG 

TSD. The first step in this process is adjusting the inventory for population and economic 

changes to convert a 2008 inventory into an inventory for a different year. The second step is to 

account for the benefits of the control measures in the plan to produce emissions inventories that 

reflect what the emissions would have been if the measures were not in place.

MAG used population and economic data from Marshall Vest of the Economic and Business 

Research Center at the University of Arizona. The economic data included population, 

manufacturing employment, and construction employment. This is a reasonable source for these

data. MAG also used data from the Arizona Agricultural Statistics Bulletins to quantify changes 

in agricultural activity in the nonattainment area. This is also a reasonable source for this 

information. MAG used the data to modify the 2008 inventory to estimate the emissions in other 

years. These calculations are shown in detail in Section II of the MAG TSD. For the nonroad 

sources whose emissions were estimated with the EPA NONROAD model, MAG used the 

NONROAD model to develop estimates for the other years. This is the appropriate approach for 

those source types. For exhaust, tire wear and brake wear, MAG updated the vehicle and vehicle 

activity and used the projection methods internal to the MOVES model to develop emissions 

estimates for the other years. This is EPA’s recommended method for this part of the emissions 

inventory. For the paved road and unpaved road fugitive dust, MAG adjusted the vehicle miles 

traveled according to their data as the local transportation agency. EPA finds that the appropriate 

methods and data were used to project the 2008 inventory and develop baseline inventories for 

2007, 2009, 2010, 2011 and 2012. 

II.C.2. Controlled Inventories

The majority of the emission reductions claimed in the 2012 Five Percent Plan are from 

improvements in rule effectiveness. Some additional reductions are claimed from the 2012 

implementation of the Dust Action General Permit. But the majority of reductions during this 

period arise from improved compliance with Maricopa County Rules 310, 310.01 and 316. The 

controlled inventories reflect the benefits of these reductions.
6

Rule effectiveness is the estimate of the extent to which a state rule in the SIP is achieving the 

intended reductions. A rule is only 100 percent effective if every impacted source is in 

compliance at all times. Often, rules are not 100 percent effective, and this aspect must be 

considered when calculating the emissions reductions from the rule. EPA provides guidance on 

rule effectiveness calculations in the document “Emissions Inventory Guidance for

Implementation of Ozone and Particulate Matter National Ambient Air Quality Standards 

(NAAQS) and Regional Haze Regulations”, EPA, August 2005 (2005 EI Guidance).

Appendix 3 of the 2008 PM-10 Inventory describes the methods MCAQD used to calculate rule 

effectiveness for rules 310, 310.01 and 316. Rule 310 most often applies to construction sites. 

Rule 310.01 typically applies to vacant lots. Rule 316 applies to sources such as sand, gravel and 

the production of concrete. MCAQD conducts thousands of inspections of Rule 310 and 301.01 

sources and hundreds of inspections of 316 sources annually. They analyze all of that data to 

determine the compliance rate. This is a robust and valid method to determine compliance rates 

and therefore rule effectiveness. This new method was developed in cooperation with EPA and is 

an improvement over previous methods of rule effectiveness calculation in Maricopa County 

which depended upon a few targeted inspections.

The resulting rule effectiveness rates are shown in Table 5-1 of the 2012 Five Percent Plan. The 

rule effectiveness rate for construction was improved from 76% in 2007 to 94% in 2012. The 

rule effectiveness rate for mining and quarrying improved from 40% in 2007 to 73% in 2012.

As explained in Section III of the MAG TSD, MAG uses the rule effectiveness results from 

MCAQD’s analysis of their inspection data to calculate the benefits of improved compliance 

with these rules. These controlled inventories were then used to demonstrate that the 5% annual 

reduction requirement had been met and that Reasonable Further Progress was being 

demonstrated. The improved compliance rates for these sources were also included in the 

inventories used in the attainment demonstration. 

III. Air Quality Data

III.A. Requirements for Demonstrating Attainment by December 31, 2012

The 2012 Five Percent Plan predicts attainment of the PM10 NAAQS by December 31, 2012. 

Generally, EPA determines whether an area’s air quality is meeting the PM10 NAAQS based on 

complete, quality assured, and certified data collected at state and local air monitoring stations 

(SLAMS) in the nonattainment area. Accordingly, EPA relies primarily on data in AQS when 

determining whether an area has attained a particular NAAQS. EPA will also consider air quality 

data from other air monitoring stations meeting the monitoring requirements in 40 CFR part 58, 

including the criteria in 40 CFR part 58, appendices A, D, and E, if appropriate.

Attainment of the 24-hour PM10 standard is determined by calculating the average number of 

expected exceedances of the standard over a three-year period. Specifically, the 24-hour PM10 

standard is attained when the expected number of exceedances averaged over a three-year period 
7

is less than or equal to one at each monitoring site within the nonattainment area. Generally, 

three consecutive years of complete air quality data are required to show attainment of the 

24-hour PM10 standard. Furthermore, the monitor must provide sufficient data to perform the 

required calculations in 40 CFR part 50, appendix K. To be considered complete, valid 

measurements must be made for 75 percent of all the scheduled sampling dates in each quarter of 

the year, and generally, three years of representative monitoring data that meet the 75 percent 

criterion should be utilized, where available. Data not meeting these criteria may also suffice to 

show attainment; however, such exceptions must be approved by the appropriate Regional 

Administrator in accordance with EPA guidance.

For this action, EPA is primarily relying on 2010-2012 design values for monitoring stations 

located within the nonattainment area. During the 2010-2012 time period the Maricopa County 

Air Quality Division (MCAQD) operated fifteen PM10 SLAMS, while the Arizona Department 

of Environmental Quality (ADEQ) and Pinal County Air Quality Control District (PCAQCD) 

operated an additional three PM10 monitoring stations in the area. In addition to the State and 

local monitoring agency PM10 networks, there are a number of Tribal PM10 monitors operated by 

the Salt River Pima-Maricopa Indian Community (SRPMIC) and the Fort McDowell Yavapai 

Nation (Fort McDowell) within the nonattainment area. Table 2 summarizes the SLAMS PM10

data collected over the past three years.

Table 2: Summary of 2010-2012 SLAMS PM10 Data for the Phoenix Nonattainment Area

Monitoring Site

AQS Site 

Identification 

Number

Highest 24-hour 

PM10 Concentration 

(μg/m3)

3-Year 

Average of 

Expected 

Exceedancesa

2010 2011 2012 2010-2012

West Phoenix 04-013-0019-1 86 139 189 0.3

Mesab04-013-1003-1 86 127 64 0.0*

North Phoenixb04-013-1004-1 44 132 140 0.0

Glendale 04-013-2001-1 92 141 136 0.0

Central Phoenix 04-013-3002-4 106 144 137 0.0

South Scottsdaleb04-013-3003-1 37 119 102 0.0*

Greenwood 04-013-3010-1 158 148 145 0.3

South Phoenix 04-013-4003-1 120 168 144 0.3

West Chandler 04-013-4004-1 76 126 402 0.7

Tempec04-013-4005-1 NA NA 145 0.0*

Higley 04-013-4006-1 83 152 162 0.3

West 43rd Avenue 04-013-4009-1 112 150 209 0.7

Dysart 04-013-4010-1 81 136 127 0.0

Zuni Hills 04-013-4016-1 70 411 147 0.3

Bethuned 04-013-8006-1 71 30 NA 0.0*

Durango Complex 04-013-9812-1 111 151 144 0.0

JLG Supersitee 04-013-9997-3 74 150 120 0.0

04-013-9997-4 NA 151 120 0.0*

Apache Junctione 04-021-3002-1 66 125 89 0.0

04-021-3002-3 NA 173 130 0.8*

* Completeness criterion not met for all quarters during the 2010-2012 time period.

a Excludes Exceptional Events concurred upon by EPA.
8

b Hourly data was also submitted under POC 2 for limited periods of time during monitor changes. See AQS Raw 

Data Report, August 2, 2013.

c Monitor began operating on March 1, 2012.

d SPM shutdown June 30, 2011. See ANP approval letter dated December 1, 2011, from Matthew Lakin, Manager, 

Air Quality Analysis Office, EPA Region IX, to Eric Massey, Air Division, ADEQ.

e Both the JLG Supersite POC 4 and Apache Junction POC 3 monitors began operating in January and August of 

2011, respectively

During the 2010-2012 time period, only six monitors within the Maricopa County Nonattainment 

Area do not meet the completeness criterion for all quarters. Both the Mesa and South Scottsdale 

monitors have incomplete data for the 2010-2012 time period. Specifically, the Mesa monitor 

has complete data for 8 of the required 12 quarters, and the South Scottsdale monitor has 

complete data for 11 of the required 12 quarters. While complete data should be used whenever 

possible, data not meeting these criteria may also suffice to show attainment. In the case of the 

Mesa monitor, the incomplete data was primarily due to a temporary shutdown and relocation 

due to construction at the monitoring site, while the incomplete data at the South Scottsdale 

monitor was due to a small number of samples that were not collected during August and 

September of 2011.6These monitoring sites are not the high concentration sites in the 

nonattainment area, and the number of currently operating monitors in the area far exceeds 

EPA’s regulatory requirements, and therefore, the incomplete data does not inhibit EPA’s ability 

to determine area has attained the 24-hour PM10 NAAQS. Based on data from these monitoring 

stations, EPA has determined that the area has attained the 24-hour PM10 NAAQS for the 2010-

2012 time-period, as the 3-year average of expected exceedances for all the monitoring stations 

are less than one.

III.B. Exceptional Events

On March 14, 2012, February 13, 2013, and January 28, 2013 ADEQ submitted eighteen high 

wind PM10 exceptional events demonstrations for a total of one hundred thirty-three exceedances 

occurring over twenty seven-days in the years 2011 and 2012 at monitors within the Phoenix 

nonattainment area. EPA has reviewed the documentation provided by ADEQ to demonstrate 

that the exceedances on these days meet the criteria for an exceptional event under the 

Exceptional Events Rule (EER). EPA has concurred, based on the weight of evidence, that one 

hundred thirty-one of the exceedances were caused by high wind exceptional events and found 

that ADEQ has successfully made seventeen of the eighteen of the demonstrations referred to in 

40 CFR 58.14 to EPA’s satisfaction.

7

ADEQ has also recently submitted, in accordance with EPA guidance, a letter of intent for a 

number of PM10 exceedances that occurred in 2013.8The letter outlines that ADEQ will be 

preparing exceptional events demonstrations for three event days (April 8, 2013, June 30, 2013, 

and July2, 2013) that will cover twenty-four exceedances of the 24-hour PM10 NAAQS. ADEQ 

subsequently submitted a notice of intent for an additional three days (August 17, 2013, August 

 

6

See AQS Raw Data Report, August 2, 2013 and letter from Ben Davis, Air Monitoring Manager, MCAQCD to 

Michael Flagg, Air Quality Analysis Office, EPA Region IX, dated January 13, 2012.

7

See concurrence letters dated September 6, 2012, May 6, 2013,and July 1, 2013 from Jared Blumenfeld, Regional 

Administrator, EPA Region IX, to Eric Massey, Air Division, ADEQ

8

See letter dated September 11, 2013from Eric Massey, Air Division, ADEQ, to, Deborah Jordan, Air Division 

Director, EPA Region IX.
9

26, 2013, and October 9, 2013) that will cover six more exceedances of the 24-hour PM10 

NAAQS.9 While EPA has determined that the area has attained the NAAQS by December 31, 

2012 with data from 2010-2012, ADEQ submission and EPA approval of the 2013 exceptional 

events identified in the letter of intent will be critical to ensuring the area continues to attain the 

24-hour PM10 NAAQS with data from 2011-2013. 

III.C. Tribal Data

As mentioned above, there are a number of Tribal monitoring stations located within the 

Maricopa County Nonattainment Area. Specifically, the SRPMIC operates three PM10

monitoring stations and Fort McDowell operates one station in the northeastern portion of the 

nonattainment area. Table 3 summarizes the Tribal PM10 design values and Table 4 summarizes

the individual exceedance data collected over the past three years.

Table 3: Summary of 2010-2012 Tribal PM10 Data for the Maricopa County Nonattainment Area

Monitoring Site

AQS Site 

Identification 

Number

Highest 24-hour 

PM10 Concentration 

(μg/m3)

3-Year 

Average of 

Expected 

Exceedances

2010 2011 2012 2010-2012

Fort McDowell – Yuma Frank 04-013-5100-1 62 34 NA 0.0*

04-013-5100-3 NA 559 125 2.3*

SRPMIC – Senior Center 04-013-7020-1 59 163 126 1.9

SRPMIC – Lehi 04-013-7022-1 55 167 150 2.0

SRPMIC – High School 04-013-7024-1 75 216 90 2.2

* Non-regulatory monitor, for informational purposes only.

Table 4: Summary of 2010-2012 Tribal PM10 Exceedances

Exceedance Date

AQS Site 

Identification 

Number

Monitoring Site

24-hour PM10

Concentration 

(μg/m3)s

7/2/11 04-013-7024-1 SRPMIC – High School 216

7/5/11 04-013-5100-3 Fort McDowell – Yuma Frank 559*

7/6/11 04-013-5100-3 Fort McDowell – Yuma Frank 401*

7/8/11 04-013-7020-1 SRPMIC – Senior Center 163

7/8/11 04-013-7022-1 SRPMIC – Lehi 167

7/8/11 04-013-5100-3 Fort McDowell – Yuma Frank 329*

8/28/11 04-013-5100-3 Fort McDowell – Yuma Frank 305*

9/2/11 04-013-5100-3 Fort McDowell – Yuma Frank 195*

11/4/11 04-013-5100-3 Fort McDowell – Yuma Frank 237*

* Non-regulatory monitor, for informational purposes only.

 

9

See email dated December 2, 2013 from Bradley Busby, Evaluation Unit Manager, ADEQ, to Michael Flagg, Air 

Quality Analysis Office, Air Division, EPA Region IX.
10

As shown in Table 3, the PM10 data from Tribal monitors within the nonattainment area continue 

to violate the 24-hour PM10 NAAQS. The PM10 data from SRPMIC has met the monitoring 

requirements in 40 CFR part 58, including the criteria in 40 CFR part 58, appendices A, D, and 

E, and may be used in determining whether the area attains the PM10 NAAQS. Two of the three 

exceedance dates in 2011 measured on SRPMIC monitors were coincident with an EPA 

approved high wind exceptional event on July 8, 2011 that affected monitors located in the 

eastern portion of the nonattainment area. The remaining exceedance date of July 2, 2011 was 

not associated with an EPA approved exceptional event. Nearby monitors on tribal land operated 

by SRPMIC (Senior Center) and SLAMS operated by MCAQD did not measure exceedances of 

the PM10 NAAQS. Preliminary analysis of nearby monitoring data and information obtained 

from SRPMIC monitoring staff suggest that the contribution from the storm was small and that 

local sources are likely responsible for the exceedance at the High School station on July 2, 

2011. Pursuant to 40 CFR 49.10, however, EPA cannot disapprove a state SIP submittal because 

of the “failure to address air resources within the exterior boundaries of an Indian Reservation or 

other areas within the jurisdiction of an Indian tribe.” Therefore, we did not further consider 

these exceedances as part of this proposed action to approve the 2012 Five Percent Plan.

The PM10 data from Fort McDowell is considered “informational” and does not fully meet the 

monitoring requirements of 40 CFR part 58. Thus it is not appropriate for use in determining if 

an area has attained the PM10 NAAQS. However, it is important to note that five of the six 

exceedance dates in 2011 correspond to dates where EPA has concurred with ADEQ’s 

exceptional events demonstrations (July 5, 2011, July 8, 2011, August 28, 2011, September 2, 

2011, and November 4, 2011) and the remaining exceedance date of July 6, 2011 at Fort 

McDowell is likely associated with the large dust storm event on July 5, 2011. Thus, the 

monitoring data from Fort McDowell is consistent with the other SLAMS information 

considered for EPA’s proposed action. 

III.D. EPA Conclusion

Considering SLAMS and tribal data collected from 2010-2012, EPA believes that the Phoenix 

PM10 nonattainment area has attained the 24-hour PM10 NAAQS by the December 31, 2012 

deadline in the 2012 Five Percent Plan. EPA also acknowledges that future concurrence on 2013 

exceptional events demonstrations is critical to ensuring the area continues to attain the 24-hour 

PM10 NAAQS with data from 2011-2013.

IV. Attainment Demonstration

IV.A. Overview

Although monitoring data shows that the Phoenix area already attains the 24-hour PM10

NAAQS, an attainment demonstration based on modeling provides assurance that the NAAQS 

will continue to be attained based on the control measures that are in place. MAG and ADEQ 

developed an attainment demonstration using a proportional rollback approach that takes into 

account the timing and geographic location of emissions contributing to NAAQS exceedances. 
11

The State’s analysis adequately demonstrates that controls implemented during 2007-2012 

brought the area into attainment, and will continue to assure NAAQS compliance.

IV.B. Technical Approach

EPA guidance allows considerable latitude in the choice of attainment demonstration approach 

for fugitive dust-dominated PM10 nonattainment areas. The approach used by the State is a form 

of proportional rollback and a microinventory method. It was concurred upon in advance by 

EPA, and satisfies the available guidance.

EPA has issued several guidance documents and memoranda that describe PM10 modeling 

procedures. However, some of this available guidance is rather dated, it is not comprehensive,

and is sometimes aimed only at Moderate Area, rather than Serious Area PM10 plans. 

Sometimes, too, it assumes that the emissions to be modeled are predominantly from wellquantified point sources, such as industrial stacks, rather than from the more poorly characterized 

fugitive dust emissions. Interpretation and judgment is therefore needed in applying the 

guidance; also, the guidance explicitly recognizes that case-by-case evaluations of SIP modeling 

may be needed at times.

The term “rollback” refers to the assumption that the PM10 concentration is directly proportional 

to emissions. To predict the ambient effect of an emissions change, the concentration can be 

scaled, or “rolled back”, by the same percentage that emissions reduce. In “proportional 

rollback”, each source or each source category is rolled back separately, since emissions from 

each will have a different level of control, and in general a different degree of dispersion. As in

simple rollback, the ambient contribution of each individual source category scales with its 

emissions. However, the proportion that each category contributes to the total concentration may 

be different than its proportion of the total emissions. Thus, a source with only small emissions 

may nevertheless have a large contribution to the concentration, for example if it is nearby. A 

change in a source’s emissions causes a change in total concentration in proportion to that 

source’s particular contribution.

The PM10 SIP Development Guideline (EPA-450/2-86-001, June 1987) (“PSDG”), chapter 4 

states that, in order of preference, the modeling used in the demonstration can be 1) a 

combination of receptor and dispersion models, 2) dispersion models alone, or 3) two receptor 

models alone if dispersion modeling is inappropriate. Section 6.4.2 put forward alternatives such 

as the Chemical Mass Balance (CMB) receptor model and the microinventory method.10

However, generally receptor models such as CMB cannot distinguish between the many source 

categories that create fugitive dust, like roadways, vacant lots, construction activities, etc. Thus 

for fugitive dust-dominated areas, a CMB analysis does little but confirm conclusions from the 

emission inventory, and the recommendation of a combination of receptor and dispersion model 

 

10The microinventory method is the intensive inventorying and modeling of emissions in a small area that 

contributes to exceedances, and is a reasonable one for assessing pollutants like PM10, which has relatively sharp 

spatial gradients as dust settles out with distance from the source. It is also cited in the Receptor Model Technical 

Series, Volume I, Overview of Receptor Model Application to Particulate Source Apportionment, EPA-450-4-81-

016a, July 1981, p. 27.
12

does not seem appropriate. And in any case, previous work11 has already showed the 

preponderance of fugitive dust in PM10 concentrations in the Phoenix area. Thus, the threeoption typology of approaches cited in the PSDG is not directly applicable.

The Guideline On Air Quality Models (Revised), 2005, 40 CFR §51 Appendix W (“GAQM”) 

deals with model use in general, including recommendations for input preparation. It also 

addresses PM10 specifically in §5.2.2.2(d):

“Under certain conditions, recommended dispersion models may not be reliable. 

In such circumstances, the modeling approach should be approved by the 

Regional Office on a case-by-case basis.” 

Earlier versions of GAQM (in then-numbered section §7.2.2(d)) followed this with: 

“For example, where there is no recommended air quality model and area sources 

are a predominant component of PM-10, an attainment demonstration may be 

based on rollback of the apportionment derived from two reconciled receptor 

models, if the strategy provides a conservative demonstration of attainment.” 

In addition, GAQM §5.2.2.2(e) states: 

“In some unique cases a model developed specifically for the situation may be 

needed. Due to the difficult nature of characterizing and modeling fugitive dust 

and fugitive emissions, it is recommended that the proposed procedure be cleared 

by the Regional Office for each specific situation before the modeling exercise is 

begun.”

Finally, the EPA memorandum, “PM10 SIP Demonstrations for Small Isolated Areas with 

Spatially Uniform Emissions” (Robert Bauman & Joseph Tikvart, July 5, 1990), is relevant. 

While Phoenix is not a “small isolated area”, the reasoning in this memorandum applies because 

of the predominance of area sources in the Salt River area and in Phoenix generally, for which 

the memorandum allowed the use of proportional rollback as a form of attainment demonstration 

modeling.

All of these help justify the acceptability of the State approach.

The latitude for judgment allowed in EPA guidance is especially important for fugitive dust 

PM10 areas. It is widely recognized that fugitive dust emissions are markedly uncertain, and can 

have large temporal and spatial variability. The uncertainty of these crucial inputs can make 

dispersion modeling problematic, since the model output is only as good as the model input. 

Previous modeling experience in the Phoenix area has amply demonstrated this. Accurately 

estimating emissions for dispersion modeling of widespread area sources of fugitive dust is much 

more difficult than for industrial point sources of gaseous pollutants, which were the archetypes 

for much of the modeling guidance. Point sources are relatively few in number, and their 

emissions are relatively constant in time and can be directly measured. By contrast, fugitive dust 

 

11 MAG, 2008. PM-10 Source Attribution and Deposition Study. March, 2008. Available on web page

http://www.azmag.gov/Projects/Project.asp?CMSID2=1120&MID=Environmental%20Programs . Also Watson, 

Chow, Lurman, and Stefan. “Ammonium Nitrate, Nitric Acid, and Ammonia Equilibrium in Wintertime Phoenix, 

Arizona”, Journal of the Air and Waste Management Association, 44:405-412, April 1994
13

sources are numerous and vary in space and time, with indirectly measured emissions stemming 

from human activity and location-specific winds that are known with poor temporal and spatial 

resolution. In addition, fugitive dust sources are generally ground-based, whereas dispersion 

model algorithms are generally based on studies of elevated sources (i.e., emitting from a point 

above the ground, such as the exit of a smoke stack). For all of these technical reasons, case by 

case judgment is needed in assessing PM10 attainment demonstrations for fugitive-dust 

dominated areas. 

The State’s attainment demonstration approach is described in the 2012 Five Percent Plan 

Chapter 6, “Attainment Demonstration”, and the Plan’s own TSD (Appendix B, Exhibit 1: 

Technical Document in Support of the MAG 2012 Five Percent Plan for PM-10 for the Maricopa 

County Nonattainment Area, Maricopa Association of Governments, especially chapter V, 

Attainment Modeling). The general approach used by the State was to 1) determine the source 

areas contributing to PM10 NAAQS exceedances using meteorological and dispersion modeling 

analyses, 2) develop hourly PM10 emissions inventories for those source areas, and then 3) “roll 

back” or scale the monitored PM10 concentration in proportion to the declining source emissions, 

taking into account their distance from the exceeding monitor. (This is a “modeling” approach in 

the general sense of a scientific model of concentrations, rather than in the narrow sense of a 

dispersion model.) The concentration scaling was done hour-by-hour, since emissions vary 

hourly, in proportion to changes due to control measures during 2007-2012, including increased 

rule effectiveness improvements. The results were averaged to get a 24-hour concentration. A 

background concentration was held constant during the scaling, reflecting the effect of distant 

sources not affected by local control measures.

The source-to-monitor distance was taken into account via an inverse distance factor (1/d). 

Analyses of data from temporary monitors and the relationships between their concentrations 

showed that it was crucial to include some kind of distance dependence to reflect the actual 

effect of source emissions at the monitors (Plan TSD, V-7). The 1/d factor was determined from 

AERMOD dispersion model runs showing how concentrations fall off with distance from a 

source, reflecting increased dispersion and dilution as the pollutant plume travels. The power 

function that fits the concentration curve, i.e. the best exponent for distance d, varied depending 

on which set of distances was used, but appeared to center on 1/d (Source Weighting Analysis, 

Plan TSD, V-21). While there is some danger of “over-fitting” the exponent to the AERMOD 

curve, this was a rational approach, and the data probably do not justify a more complex function 

than 1/d. (Also, the 1/d was used even for the low-wind day inventories, though it was derived 

using modeling for high wind speeds.) The rollback of emissions with 1/d distance dependence 

served as a substitute for actually modeling the emissions with AERMOD. As noted in the Plan 

TSD (“Conceptual Model”, Plant TSD V-5), rollback has the advantage of mathematical 

simplicity in calculation time and in arriving at a set of source emission reductions needed for 

attainment. It also has the advantage of relying on actual measured concentrations, instead of on 

estimates from a steady state model that does not incorporate deposition, which would be 

important for PM10.

As mentioned above, rollback incorporates a background concentration that is held constant 

during the overall concentration scaling, since it is due to sources outside the area not affected by 

local controls. Hourly background concentrations were developed by State using monitors on the 
14

upwind (west) edge of the area, the Buckeye monitor and the temporary Arlington monitor. 

Hourly concentration data and wind speed and direction data help the State choose a 

concentration that was indeed arriving from outside the area, reflected high wind conditions 

(defined as above 12 mph), and excluded values for which Arlington measured higher than 

Buckeye. (Buckeye is downwind of Arlington, and so should register the effects of any local 

emissions between the two when background pollution is transported into the area (Plan TSD, V33).) Using 400 concentration pairs at the sites, the median of 21.9 μg/m3 was chosen as a 

representative background; using the median avoided overemphasizing the lower concentrations 

from relatively low-wind hours, were more frequent. The State followed a reasonable and 

thorough procedure in choosing a background concentration.

EPA believes that the modified rollback attainment demonstration approach used by the State 

meets the relevant guidance, and is technically sound based on extensive analysis of the sources 

and meteorological conditions that lead to NAAQS exceedances in the Maricopa County PM10

Nonattainment Area.

IV.C. Selection of Design Days

The goal of an attainment demonstration is to show that the NAAQS will be met at all times and 

places in the nonattainment area. Depending on the form of the NAAQS, a particular measured 

concentration may exceed the numerical value of the standard one or more times per year (for 

PM10, 150 μg/m3 may be exceeded once per year, on an average basis). But because it is often 

impractical to model every day of the year (for example for highly variable fugitive dust PM10

emissions that would have to be estimated for 8760 separate hours), attainment demonstrations 

instead focus on a smaller set of days that have experienced, or are conducive to, the highest 

concentrations. Designing a plan’s control strategy around these “design days”, to ensure that 

controlled concentrations are below the numerical value of the NAAQS, is an economical, but 

still conservative, approach to attainment demonstrations. Note that other considerations beside 

high concentration must be considered in choosing design days, such as the availability of data 

needed to analyze the day, how widespread NAAQS exceedances were, the feasibility of 

analyzing the day given limitations of models, and other regulatory considerations.

Starting from the full set of PM10 exceedances in the base year of 2007, the year just before new 

control measures were implemented, the State eliminated days that would be likely to qualify as 

having exceedances due to Exceptional Events. Such days could be either caused by human 

activity unlikely to recur at a given location, or by natural events, such as high wind, sufficient to 

overcome reasonable control measures in place on emissions. The State has had extensive 

experience in classifying such candidate Exceptional Event days and has found they are typically 

caused by high wind during a frontal passage or during thunderstorm outflow. These conditions 

are identifiable by specific patterns of using meteorological and concentration data, with frontal 

passages having long periods of high wind over a large areas, and thunderstorm outflows being 

of shorter duration and more localized, but also gustier (Plan TSD, V-42). It is reasonable to 

exclude such days from consideration as design days, since by definition they are not within the 

power of the State to ameliorate via emission controls.
15

The design days chosen were May 4, 2007 for the Salt River area, with 197 μg/m3 monitored at 

the West 43rd Street monitor, and June 6, 2007 for the nonattainment area as a whole, with 181.0 

μg/m3at Higley and 225.7 μg/m3at West 43rd Street. These days satisfied the criteria of having 

among the highest concentrations and being high-wind days, but not candidate Exceptional 

Events. EPA agrees that these days are a good choice for design day analysis.

IV.D. Modeling Domain

A modeling domain is a geographic area chosen for analysis. In the State’s approach, this was 

the area covering emission sources contributing to PM10 exceedances for a design day. The 

contributing source area was determined by analyzing back trajectories using wind data to 

showing transport paths from upwind source locations to the West 43rd Street monitor; the 

timing of changes in PM10 concentration and in wind speed and direction; the alignment of 

monitors with each other, with wind direction, and with possible source areas; and AERMOD 

dispersion model runs for different wind speeds that showed the distances over which sources 

were likely to have a significant effect on concentration (Plan TSD, p.V-7 on insights from 

temporary monitors, and V-53, domain discussion). 

Back trajectories were generally from the west-southwest, the predominant wind direction, 

following the valley of the Salt River, and formed the central segmented axis of the domain for a 

given modeled day. So, the axis of the domain ran from the West 43d Street monitor upwind 

some 20 miles toward the west-southwest, terminating beyond developed areas, whether urban or 

agricultural. The domain width was set at 2 miles, 1 mile on either side of the trajectory axis, 

reflecting the plume width determined using the AERMOD dispersion model. An example area 

source was modeled with AERMOD, to find that distance that the plume concentration falls to 

1% of its value just downwind of the source; the plume width at that distance was about 1 mile 

(Plan TSD, p.V-57). (The “plume width” was defined somewhat unconventionally, as the angle 

subtended by the plume when it first had an off-axis non-zero concentration.)

Separate domains were developed for the Salt River area, and for the Phoenix nonattainment area 

as whole. In addition, additional “domains” were needed to provide emissions locations for lowwind hours, since the trajectory-based domains are geared to high-wind hours. These domains 

were larger areas encompassing either the Salt River area (36 mi2), the Higley monitor (16 mi2), 

or the Central City (40 mi2), encompassing the Central Phoenix, Greenwood, Super Site, and 

West Phoenix monitors and the surrounding urban area.

These various domains include all the violating monitors, the emission sources leading to PM10

exceedances, and mixes of source types that are typical of the full nonattainment area. As in 

previous plans, the State applies control measures to the full nonattainment area. By focusing 

analysis on the subareas causing the high concentrations, yet having attainment area-wide 

controls, the State addresses the PM10 problem adequately.

IV.E. Estimating the Impact of Control Measures
16

As discussed above, the State used a rollback approach, in which a change in PM10 emissions 

causes a proportional change in the monitor PM10 concentrations. The proportion is determined 

partly by the emissions, and partly by a 1/d distance factor. There was a 2007 emission rate and a 

distance associated with each 10 acre parcel in the high-wind model domain. The inverse 

distance-weighted sum over all the parcels gives the total (weighted) emissions contributing to 

the monitored concentration. Changing the emissions to 2012 levels for the various activities and 

land uses in each parcel gives a new total emissions value; the percent change in that total yields 

the percent change to apply to the original monitored concentration, giving the projected 2012 

concentration. This approach is done for each hour, and the 24-hour average taken for the final 

24-hour PM10 concentration.

Emissions were based on industrial and agricultural activity present, and also on wind speed for a 

given hour, since that drives fugitive emissions from bare soil areas. For each 5 mph increment 

of wind speed, a different emission factor was used, one for stabilized land and one for disturbed 

land. (2012 Five Percent Plan TSD, V-62) Since the proportion of stabilized and disturbed land 

is difficult to know for the many, many individual parcels in the domain, rule effectiveness for 

the relevant fugitive dust control measures was used as a surrogate for disturbance level. These 

measures were Rule 310, earthmoving activities, Rule 310.01, open and vacant areas, and Rule 

316, sand and gravel sites. The increasing rule effectiveness over the years 2007-2012 (2012 

Five Percent Plan TSD, V-65) provided much of the overall emission reductions that lead to 

decreased PM10 concentrations for 2012.

This method for estimating the impact of control measures is inherent in the modified rollback 

approach, which EPA believes is a reasonable one, as stated earlier.

IV.F. Results of Attainment Demonstration and EPA Assessment

The attainment demonstration itself consists of carrying out the emissions change and totaling 

calculation just described to arrive at a projected 2012 PM10 concentration. The individual hourly 

concentration changes for the various monitor and design days are shown in a series of tables in 

the Plan (2012 Five Percent Plan TSD, V-80 – 88). In each case, the resulting 2012 PM10

concentration meets the NAAQS. In one case, the West 43rd Street monitor for the June 6, 2007 

design day, the end concentration is 153.8 μg/m3, just barely below the 154 μg/m3that is 

considered to in compliance with the NAAQS, using regulatory rounding conventions for PM10

data (2012 Five Percent Plan TSD, Table V-44, p.V-88). For other days and monitors, the end 

concentration is well below the NAAQS. This demonstrates attainment for the Phoenix PM10

nonattainment area.

Its extensive reliance on real-world meteorological and monitoring data in deriving the approach 

lends confidence to the Plan’s conclusions. EPA believes that the attainment demonstration in 

the 2012 Five Percent Plan is technically sound, and meets the available guidance for PM10.