Vol. 77 Wednesday, 

No. 124 June 27, 2012 

Part II 

Environmental Protection Agency 

40 CFR Part 52 

Approval and Promulgation of Implementation Plans; Arizona; Nogales 

PM10 Nonattainment Area Plan; Proposed Rule 


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38400 Federal Register / Vol. 77, No. 124 / Wednesday, June 27, 2012 / Proposed Rules 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Part 52 

[EPA–R09–OAR–2012–0458; FRL–9693–6] 

Approval and Promulgation of 

Implementation Plans; Arizona; 

Nogales PM10 Nonattainment Area Plan 

AGENCY: Environmental Protection 

Agency. 

ACTION: Proposed rule. 

SUMMARY: EPA is proposing to approve 

a state implementation plan revision 

submitted by the Arizona Department of 

Environmental Quality to address the 

moderate area PM10, particulate matter 

with an aerodynamic diameter of less 

than or equal to a nominal ten 

micrometers, planning requirements for 

the Nogales nonattainment area. 

Consistent with this proposal, EPA is 

also proposing to approve the following 

plan elements as meeting the 

requirements of the Clean Air Act: the 

Nogales nonattainment area 2008 and 

2011 emission inventories; the 

demonstration that the Nogales 

nonattainment area is attaining the 

National Ambient Air Quality Standard 

for PM10, but for international emissions 

sources in Nogales, Mexico; the 

demonstration that reasonably available 

control measures sufficient to meet the 

standard have been implemented in the 

nonattainment area; the reasonable 

further progress demonstration; the 

demonstration that implementation of 

measures beyond those needed for 

attainment meet the contingency 

measure requirement; and, the motor 

vehicle emissions budget for the 

purposes of determining the conformity 

of transportation plans, programs, and 

projects with this PM10 plan. 

DATES: Written comments must be 

received on or before July 27, 2012. 

ADDRESSES: Submit comments, 

identified by docket number EPA–R09– 

OAR–2012–0458, using one of the 

following methods: Via the Federal 

eRulemaking Portal, at 

www.regulations.gov, please follow the 

on-line instructions; via Email to 

wamsley.jerry@epa.gov; via mail or 

delivery to Jerry Wamsley, Air Planning 

Office, AIR–2, Environmental Protection 

Agency (EPA) Region IX, 75 Hawthorne 

Street, San Francisco, CA 94105–3901. 

Instructions: All comments will be 

included in the public docket without 

change and may be made available 

online at www.regulations.gov, 

including any personal information 

provided, unless the comment includes 

Confidential Business Information (CBI) 

or other information whose disclosure is 

restricted by statute. Information you 

consider to be CBI or otherwise 

protected should be clearly identified as 

such and should not be submitted 

through www.regulations.gov or email. 

www.regulations.gov is an ‘‘anonymous 

access’’ system, and EPA will not know 

your identity or contact information 

unless you provide it in the body of 

your comment. If you send an email 

directly to EPA, your email address will 

be automatically captured and included 

as part of the public comment. 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. 

Docket: The index to the docket for 

this action is available electronically at 

www.regulations.gov and in hard copy 

at EPA Region IX, 75 Hawthorne Street, 

San Francisco, California. While all 

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 at 

either location (e.g., CBI). 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: Jerry 

Wamsley, Air Planning Office, AIR–2, 

EPA Region IX, 75 Hawthorne Street, 

San Francisco, CA 94105–3901, 

telephone number: (415) 947–4111, or 

email address, wamsley.jerry@epa.gov. 

SUPPLEMENTARY INFORMATION: 

Throughout this document, wherever 

‘‘we’’, ‘‘us’’ or ‘‘our’’ are used, we mean 

EPA. We are providing the following 

outline to help locate information in 

this proposal. 

Table of Contents 

I. The PM10 National Ambient Air Quality 

Standard and the Nogales PM10

Nonattainment Area 

A. PM10 National Ambient Air Quality 

Standard 

B. Designation and Classification of PM10

Nonattainment Areas, Including the 

Nogales Nonattainment Area 

C. Clean Air Act Plan Requirements for 

Moderate PM10 Nonattainment Areas 

II. Arizona’s State Implementation Plan 

Submittal To Address PM10 Attainment 

in the Nogales Nonattainment Area 

A. Arizona’s Submittal and Clean Air Act 

Procedural Requirements 

B. Description of the Nogales 

Nonattainment Area 

III. CAA and Regulatory Requirements for 

Moderate Area PM10 Attainment Plans 

and Nonattainment Areas Influenced by 

International Transport 

A. Moderate PM10 Area Planning 

Requirements 

B. Clean Air Act Provisions and EPA 

Guidance Concerning International 

Border Areas 

1. Section 179B of the Clean Air Act 

2. The 1994 General Preamble Addendum 

3. Statutory Requirements and Guidance 

for Determining Attainment of the PM10

NAAQS 

IV. Review of the Nogales 2012 Plan 

A. Emissions Inventories 

1. Requirements for Emissions Inventories 

2. Review of the Nogales Nonattainment 

Area Emissions Inventory 

3. Proposed Action on the Nogales 

Nonattainment Area 2008 and 2011 

Emissions Inventories 

B. Section 179B Analysis and 

Demonstration of Attainment but for 

International Sources of PM10 Emissions 

1. Review of Statute and Guidance Applied 

to the Nogales Section 179B Analysis 

and Demonstration of Attainment but for 

International Sources of PM10 Emissions 

2. Review of Arizona’s Section 179B 

Analysis and Demonstration of 

Attainment but for International Sources 

of PM10 Emissions 

a. Population Growth in the Ambos 

Nogales Region 

b. Review and Comparison of U.S./Mexico 

Emission Inventories 

c. Review and Analysis of Regional 

Meteorology, Topography and Ambient 

PM10 Monitoring Data 

(i) Ambos Nogales Regional Meteorology 

and Topography 

(ii) Ambient PM10 Monitoring Network, 

Data, Analyses, and Findings 

d. Findings From Reviews of Emission 

Inventories, and Studies of Ambient 

PM10 Data, and Meteorological Data 

e. Arizona’s Demonstration of Attainment 

for the Nogales Nonattainment Area but 

for International Sources of PM10

Emissions 

(i) Daily Analysis To Demonstrate 

Attainment but for International Sources 

of PM10 Emissions 

(ii) Hourly Analysis To Demonstrate 

Attainment but for International Sources 

of PM10 Emissions 

3. Proposed Action on the Nogales 

Nonattainment Area Section 179B 

Analysis and Demonstration of 

Attainment but for International Sources 

of PM10 Emissions 

C. Reasonably Available Control Measures 

(RACM)/Reasonably Available Control 

Technology (RACT) and Adopted 

Control Strategy 

1. Requirement for RACM/RACT 

2. RACM/RACT in the Nogales 

Nonattainment Area 

D. Reasonable Further Progress 

Demonstration and Contingency 

Measures in the Nogales Nonattainment 

Area 


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1An exceedance is defined as a daily value that 

is above the level of the 24-hour standard, 150 mg/ 

m3, after rounding to the nearest 10 mg/m3 (i.e., 

values ending in five or greater are to be rounded 

up). Thus, a recorded value of 154 mg/m3 would not 

be an exceedance since it would be rounded to 150 

mg/m3; whereas, a recorded value of 155 mg/m3

would be an exceedance since it would be rounded 

to 160 mg/m3. See 40 CFR part 50, appendix K, 

section 1.0. 

2Under EPA’s ‘‘parallel processing’’ procedure, 

EPA proposes rulemaking action on a proposed SIP 

revision concurrently with the State’s public review 

process. If the State’s proposed SIP revision is 

changed, EPA will evaluate that subsequent change 

and may publish another notice of proposed 

rulemaking. If no significant change is made, EPA 

will propose a final rulemaking on the SIP revision 

after responding to any submitted comments. Final 

rulemaking action by EPA will occur only after the 

final SIP revision has been fully adopted by ADEQ 

and submitted formally to EPA for approval as part 

of the Arizona SIP. See 40 CFR part 51, appendix 

V. 

3Letter from Eric Massey, Director, Air Quality 

Division, Arizona Department of Environmental 

Quality, to Jared Blumenfeld, Regional 

Administrator, EPA, dated May 29, 2012. 

1. Reasonable Further Progress 

2. Contingency Measures 

E. Motor Vehicle Emissions Budgets for 

Transportation Conformity 

1. Requirements for Transportation 

Conformity 

2. Motor Vehicle Emissions Budget for the 

Nogales Nonattainment Area 

3. Proposed Action on the Motor Vehicle 

Emissions Budget for the Nogales 

Nonattainment Area 

VI. EPA’s Proposed Action and Request for 

Comment 

VII. Statutory and Executive Order Reviews 

I. The PM10 National Ambient Air 

Quality Standard and the Nogales PM10

Nonattainment Area 

A. PM10 National Ambient Air Quality 

Standard 

The EPA sets the National Ambient 

Air Quality Standard (NAAQS) for 

certain ambient air pollutants at levels 

required to protect human health and 

the environment. Particulate matter 

with an aerodynamic diameter less than 

or equal to a nominal ten micrometers, 

or PM10, is one of these ambient air 

pollutants for which EPA has 

established health-based standards. On 

July 1, 1987, EPA promulgated two 

primary standards for PM10: A 24-hour 

standard of 150 micrograms per cubic 

meter (mg/m3); and, an annual PM10

standard of 50 mg/m3. EPA also 

promulgated secondary PM10 standards 

that were identical to the primary 

standards. 52 FR 24634; (July 1, 1987). 

Because they are identical, we refer to 

the primary and secondary standards 

using the singular term, ‘‘standard.’’ 

Effective December 18, 2006, EPA 

revoked the annual PM10 standard but 

retained the 24-hour PM10 standard. 71 

FR 61144; (October 17, 2006). 

An area attains the 24-hour PM10

standard when the expected number of 

days per calendar year with a 24-hour 

concentration in excess of the standard 

(referred to herein as an ‘‘exceedance’’), 

is equal to or less than one,1 as 

determined in accordance with 40 CFR 

part 50, appendix K. See 40 CFR 50.6 

and 40 CFR part 50, appendix K. 

Conversely, a violation of the PM10

NAAQS occurs when the number of 

expected annual exceedances of the 

24-hour standard is greater than one. 

B. Designation and Classification of 

PM10 Nonattainment Areas, Including 

the Nogales Nonattainment Area 

Areas meeting the requirements of 

section 107(d)(4)(B) of the Clean Air Act 

(CAA or ‘‘Act’’) were designated 

nonattainment for PM10 by operation of 

law and classified ‘‘moderate’’ upon 

enactment of the 1990 Clean Air Act 

Amendments. These areas included all 

former Group I PM10 planning areas 

identified in 52 FR 29383, (August 7, 

1987), as further clarified in 55 FR 

45799, (October 31, 1990), and any other 

areas violating the NAAQS for PM10

prior to January 1, 1989. A Federal 

Register notice announcing the areas 

designated nonattainment for PM10

upon enactment of the 1990 

Amendments, known as ‘‘initial’’ PM10

nonattainment areas, was published on 

March 15, 1991, (56 FR 11101); and, a 

subsequent Federal Register document 

correcting the description of some of 

these areas was published on August 8, 

1991, (56 FR 37654). 

As a former ‘‘Group I’’ area, the 

Nogales nonattainment area (NA) was 

included in the March 1991 list of 

initial moderate PM10 nonattainment 

areas. Later, we codified the PM10

nonattainment designations and 

moderate area classifications in 40 CFR 

part 81 (56 FR 56694; November 6, 

1991). For ‘‘moderate’’ nonattainment 

areas, such as the Nogales NA, CAA 

section 188(c) of the 1990 Amended Act 

established an attainment date of 

December 31, 1994. On January 11, 

2011, pursuant to section 188(b)(2) of 

the CAA, we determined that the 

Nogales NA met the PM10 NAAQS as of 

the applicable attainment date, 

December 31, 1994. See 76 FR 1532; 

(January 11, 2011). The designation, 

classification, and boundaries of the 

Nogales NA are codified at 40 CFR 

81.303. 

C. Clean Air Act Plan Requirements for 

Moderate PM10 Nonattainment Areas 

Along with the new designations, 

classifications, and attainment dates, the 

CAA as amended in 1990 also 

established new planning requirements. 

States were required to develop and 

submit state implementation plan (SIP) 

revisions providing for, among other 

elements, implementation of reasonably 

available control measures (RACM) for 

control of PM10, a demonstration that 

the plan would provide for attainment 

by the applicable attainment date 

(‘‘attainment demonstration’’), and 

contingency measures, for all moderate 

PM10 nonattainment areas. See CAA 

sections 172(c) and 189(a). As discussed 

later, CAA section 179B(a) allows a 

State to submit a demonstration that the 

plan would be adequate to attain and 

maintain the standard but for emissions 

emanating from outside the United 

States in lieu of an attainment 

demonstration. CAA section 179B(a) 

does not, however, relieve qualifying 

moderate PM10 nonattainment areas of 

the other SIP requirements, including 

but not limited to RACM and 

contingency measures. 

In response, on June 14, 1993, the 

Arizona Department of Environmental 

Quality (referred to herein as ‘‘ADEQ,’’ 

‘‘Arizona,’’ or ‘‘the State’’) submitted the 

‘‘Final State Implementation Plan for 

the Nogales PM10 Nonattainment Area,’’ 

June 1993 (‘‘1993 Nogales PM10 Plan’’). 

The 1993 Nogales PM10 Plan identifies 

emissions sources located in Mexico as 

the principal sources affecting ambient 

PM10 concentrations in the area. EPA 

has not taken action on the 1993 

Nogales PM10 Plan. Today’s action 

relates to an updated plan for the 

Nogales PM10 nonattainment area that is 

intended by ADEQ, once submitted in 

final form, to supersede the 1993 

Nogales PM10 Plan. 

II. Arizona’s State Implementation Plan 

Submittal To Address PM10 Attainment 

in the Nogales Nonattainment Area 

A. Arizona’s Submittal and Clean Air 

Act Procedural Requirements 

Today’s proposed action concerns the 

Proposed State Implementation Plan for 

the Nogales PM10 Nonattainment Area 

(‘‘Nogales 2012 Plan’’), submitted by 

ADEQ on May 29, 2012. ADEQ 

concurrently requested that EPA 

‘‘parallel process’’ our review and 

proposed action on the Nogales 2012 

Plan addressing the CAA’s PM10

moderate area requirements for the 

Nogales NA.2 3 We have agreed to 

parallel process the Nogales 2012 Plan 

concurrently with the ADEQ’s public 

hearing and submittal process using our 

authority under 40 CFR part 51, 

appendix V. ADEQ’s parallel processing 

request and the Nogales 2012 Plan 

consist of the following documents: 


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4 In 2010, Nogales, Arizona had 20,017 

inhabitants and Nogales, Mexico had 212,533 

inhabitants. U.S. Census Bureau 2010 and Instituto 

Nacional de Estadistica Geografia e Informatica, 

(INEGI) 2010. 

5 ‘‘Statistical Municipal Workbook for Nogales, 

Sonora,’’ 2005 edition, INEGI. 

6The Nogales PM10 nonattainment area is subject 

to the ‘‘moderate’’ area, not the ‘‘serious’’ area, SIP 

planning requirements under the CAA. This is 

because the mandatory ‘‘bump-up’’ from 

‘‘moderate’’ to ‘‘serious’’ under CAA section 

188(b)(2) is only triggered if any area fails to attain 

the standard by the applicable attainment date (in 

this case, 1994), and the Nogales area, which was 

originally designated nonattainment for PM10 based 

on exceedances measured in the late 1980’s, 

attained the standard by 1994. Several years after 

1994, the Nogales area once again began to 

experience exceedances but such post-attainment 

date exceedances do not trigger the mandatory 

‘‘bump-up’’ provision in CAA section 188(b)(2). The 

issue of the applicability of the ‘‘bump-up’’ 

provision in CAA section 188(b)(2) to the Nogales 

area was addressed fully in EPA’s final 

determination that the Nogales area attained the 

PM10 standard by the applicable attainment date. 

See 76 FR 1532; (January 11, 2011). 

‘‘Proposed State Implementation Plan 

for the Nogales PM10 Nonattainment 

Area’’ with Appendices A–J, May 17, 

2012. The Nogales 2012 Plan, 

supporting documents, and public 

hearing information can also be found at 

ADEQ’s Web site, http:// 

www.azdeq.gov/environ/air/plan/ 

notmeet.html#nog. 

We have reviewed the ADEQ’s May 

29, 2012 parallel processing submittal 

against the completeness criteria at 40 

CFR part 51, appendix V, section 2.3.1. 

and find that the submittal is complete. 

These completeness criteria are used 

specifically for parallel processing 

submittals. Once we have received 

ADEQ’s supplemental submittal after 

the State concludes their public hearing 

process, we will use the general 

completeness criteria at 40 CFR part 51, 

appendix V, 2.0 to determine 

completeness of that submittal. Our 

completeness finding on this 

supplemental submittal will be made as 

part of our final action on this proposal. 

B. Description of the Nogales 

Nonattainment Area 

Covering 76.1 square miles, the 

Nogales NA is located within Santa 

Cruz County, Arizona, with the 

southernmost boundary of the Nogales 

NA and Santa Cruz County being the 

United States (U.S.)/Mexico border. 

Adjacent to the U.S./Mexico border, the 

city of Nogales, Arizona is 60 miles 

south of Tucson, Arizona. The city of 

Nogales, Arizona is the largest city and 

population center in the Nogales NA. 

The Nogales NA is located within the 

Sonoran Desert. This desert covers 

120,000 square miles with a minimum 

elevation of 2,500 feet above sea level 

and is in the Basin and Range 

topographic province. This topography 

is characterized by north-south 

elongated valleys surrounded by 

mountain ranges. Nogales is located in 

such a north-south valley created by the 

Nogales Wash running north to the 

Santa Cruz River. The mean elevation in 

Nogales, Arizona is 3,865 feet above sea 

level. Major highways in the Nogales, 

Arizona area are U.S. Interstate 19 

which connects Tucson, Arizona to 

Nogales, Arizona and continues south 

into Mexico, where it becomes Federal 

Highway 15, and Arizona State Route 

82, which connects Nogales, Arizona 

with Patagonia, Arizona (19 miles) and 

Sonoita (31 miles) to the northeast. 

Nogales, Mexico lies directly south of 

Nogales, Arizona across the U.S./Mexico 

border. Taken together and referred to as 

Ambos Nogales, the communities of 

Nogales, Arizona and Nogales, Mexico 

comprise the largest international 

border community in Arizona, with a 

combined population of 232,550 

inhabitants in 2010, approximately 91 

percent of whom live in Nogales, 

Mexico.4 The mean elevation in 

Nogales, Mexico is 4,265 feet above sea 

level.5

III. CAA and Regulatory Requirements 

for Moderate Area PM10 Attainment 

Plans and Nonattainment Areas 

Influenced by International Transport 

A. Moderate PM10 Area Planning 

Requirements 

The air quality planning requirements 

for moderate PM10 nonattainment areas 

are set out in subparts 1 and 4 of the 

CAA, including sections 110, 172, and 

189 of the statute. These sections will be 

discussed further during the review for 

each plan element, later in this 

proposal. Also, we have issued guidance 

in a General Preamble describing how 

we will review state submittals under 

Title I of the CAA, including moderate 

PM10 nonattainment areas. See 57 FR 

13498; (April 16, 1992) and 57 FR 

18070; (April 28, 1992). In general, 

moderate area PM10 plans must include 

the following elements: a current, 

comprehensive emissions inventory of 

emissions sources in the nonattainment 

area; provisions to ensure that 

reasonably available control measures 

and/or reasonably available control 

technologies (RACM/RACT) have been 

implemented in the nonattainment area; 

provisions demonstrating attainment of 

the PM10 NAAQS with quantitative 

milestones which show reasonable 

further progress (RFP) towards 

attainment of the NAAQS as 

expeditiously as practicable; 

contingency measures for RFP and 

attainment; and, a motor vehicle 

emissions budget for the purpose of 

determining the conformity of 

transportation programs and plans 

developed by State transportation 

agencies.6 Because the Nogales NA lies 

along the international border with 

Mexico, the CAA allows Arizona to 

submit a demonstration that the area 

would have attained the PM10 NAAQS 

but for international transport from 

Mexico in lieu of a demonstration that 

the area has attained the PM10 NAAQS. 

The statutory requirements and 

guidance for such a demonstration 

under section 179B of the CAA are 

discussed next. Under CAA section 

179B, however, other SIP requirements, 

such as RACM and contingency 

measures, among other requirements, 

continue to apply to PM10

nonattainment areas even if they quality 

for relief from the attainment 

demonstration requirement. 

B. Clean Air Act Provisions and EPA 

Guidance Concerning International 

Border Areas 

Because the southern boundary of the 

Nogales NA lies along the international 

border with Mexico and transport of 

PM10 emissions from Mexico affects air 

quality in Nogales, Arizona, there are 

specific statutory requirements in the 

CAA that apply to the Nogales NA. With 

a demonstration from Arizona showing 

that the Nogales NA would have 

attained the PM10 NAAQS, but for 

international sources of PM10, EPA may 

approve an attainment plan provided by 

the State, even if the attainment plan 

does not demonstrate attainment of the 

NAAQS. The PM10 attainment plan, 

however, must meet other requirements 

of the CAA, contingent upon meeting 

the NAAQS but for international 

transport. Such a ‘‘but for’’ attainment 

demonstration, however, must be 

consistent with statutory and regulatory 

requirements. First, we will review the 

statutory basis for a ‘‘but for’’ attainment 

demonstration. Secondly, we will 

review EPA’s published guidance on 

how such an analysis may be structured. 

Lastly, we will review how EPA 

determines whether an area’s air quality 

is meeting the PM10 NAAQS using air 

quality data gathered at monitoring sites 

in the nonattainment area and our 

application of 40 CFR part 50, 

appendix K. 

1. Section 179B of the Clean Air Act 

For international border areas like the 

Nogales NA, CAA section 179B(a) 

provides that notwithstanding any other 

provision of law, an implementation 

plan or plan revision shall be approved 

by the Administrator if such plan or 


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7As discussed earlier, we determined that the 

Nogales NA met the PM10 NAAQS as of the 

applicable attainment date for moderate 

nonattainment areas, December 31, 1994; 

consequently, we did not reclassify the area to 

‘‘serious.’’ See 76 FR 1532; (January 11, 2011). 

8 ‘‘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,’’; 59 FR 41998, August 16, 

1994. 

revision meets all the requirements 

applicable to it other than a requirement 

that such plan or revision demonstrate 

attainment and maintenance of the 

relevant national ambient air quality 

standards by the attainment date 

specified under the applicable 

provision, or in a regulation 

promulgated under such provision, and 

the submitting State establishes to the 

satisfaction of the Administrator that the 

implementation plan of such State 

would be adequate to attain and 

maintain the relevant national ambient 

air quality standards by the attainment 

date specified under the applicable 

provision, or in a regulation 

promulgated under such provision, but 

for emissions emanating from outside of 

the United States. 

As stated above, notwithstanding any 

other provision of law, should Arizona 

establish to the satisfaction of the EPA 

Administrator that the Nogales NA 

would have attained the PM10 NAAQS 

by the applicable attainment date but for 

emissions emanating from outside the 

U.S., then the Nogales NA is not subject 

to the provisions of CAA section 

189(a)(1)(b), requiring a demonstration 

of attainment of the PM10 standards by 

the applicable attainment date.7 The 

underlying purpose of section 179B is to 

balance the requirements of the CAA in 

nonattainment areas adjacent to 

international borders affected by 

transport of pollution from foreign 

sources with the consideration that the 

State does not have the jurisdiction to 

control these foreign sources of 

pollution affecting attainment of the 

NAAQS in that State. 

2. The 1994 General Preamble 

Addendum 

As part of guidance relating to serious 

PM10 nonattainment areas (General 

Preamble Addendum), EPA included a 

discussion of the requirements 

applicable to international border 

areas.8 The General Preamble 

Addendum reviews the information and 

methods that may be used to determine 

if an international border area qualifies 

for treatment under CAA section 179B 

and to demonstrate that the area would 

attain the relevant NAAQS but for 

emissions emanating from outside the 

U.S. 

The General Preamble Addendum 

provides that ‘‘several types of 

information may be used to evaluate the 

impact of emissions emanating from 

outside the U.S.’’ The EPA will consider 

the information ‘‘for individual 

nonattainment areas on a case-by-case 

basis in determining whether an area 

may qualify for treatment under section 

179B.’’ See 59 FR 42001; (August 16, 

1994). The General Preamble 

Addendum suggests five methods that 

may be used to determine the impact of 

emissions emanating from outside the 

U.S. Below, we describe the five 

methods in general terms and later, 

when reviewing Arizona’s section 179B 

analysis and demonstration, we will 

discuss the particular applicability of 

these five methods to the analysis done 

for the Nogales NA. 

Method 1. Place several ambient PM10

monitors and a meteorological station 

measuring wind speed and direction in 

the U.S. nonattainment area near the 

international border. Evaluate and 

quantify any changes in monitored PM10

concentrations with a change in the 

predominant wind direction. 

Method 2. Comprehensively inventory 

PM10 emissions within the U.S. in the 

vicinity of the nonattainment area and 

demonstrate that those sources, after 

application of reasonably available 

controls, do not cause the NAAQS to be 

exceeded. This analysis must include an 

influx of background PM10 in the area. 

Background PM10 levels could be based 

on concentrations measured in a similar 

area not influenced by emissions from 

outside the U.S. 

Method 3. Analyze ambient sample 

filters for specific types of particles 

emanating from across the border. 

Although not required, characteristics of 

emissions from sources may be helpful 

so as to better demonstrate the causal 

relationship with and contribution to 

exceedances in the U.S. nonattainment 

area due to domestic and international 

emissions. 

Method 4. Inventory the sources on 

both sides of the border and compare 

the magnitude of PM10 emissions 

originating within the U.S. to those 

emanating from outside the U.S. 

Method 5. Perform air dispersion 

and/or receptor modeling to quantify 

the relative impacts on the 

nonattainment area of sources located 

within the U.S., and of foreign sources 

of PM10 emissions. 

As stated in the General Preamble 

Addendum, the EPA will consider the 

information ‘‘for individual 

nonattainment areas on a case-by-case 

basis in determining whether an area 

may qualify for treatment under section 

179B.’’ Because the individual 

circumstances surrounding a 

nonattainment area may differ widely 

whether by data, resources, or emissions 

sources, EPA anticipates that ‘‘the State 

may use one or more of these types of 

information or other techniques, 

depending on their feasibility and 

applicability, to evaluate the impact of 

emissions emanating from outside the 

U.S. on the nonattainment area.’’ See 59 

FR 42001; (August 16, 1994). Therefore, 

the analysis Arizona has provided for 

the Nogales NA is specific to this 

nonattainment area only and the 

timeframe, data, and circumstances 

therein, and EPA is evaluating the 

analysis as such. 

As explained earlier, the underlying 

purpose of section 179B is to balance 

the requirements of the CAA in 

nonattainment areas adjacent to 

international borders affected by 

transport of pollution from foreign 

sources with the consideration that the 

State does not have the jurisdiction to 

control these foreign sources of 

pollution affecting attainment of the 

NAAQS in that State. In this light, the 

General Preamble Addendum discusses 

several attainment plan requirements as 

applied to nonattainment areas affected 

by international transport. 

The 1994 General Preamble 

Addendum discusses the requirements 

for RACM as applied to nonattainment 

areas affected by international transport. 

In international border areas, ‘‘RACM/ 

RACT must be implemented to the 

extent necessary to demonstrate 

attainment by the applicable attainment 

date if emissions emanating from 

outside the U.S. were not included in 

the analysis.’’ See 59 FR 42001; (August 

16, 1994). As set forth in section 

179B(a)(2), a State’s moderate area PM10

plan must be ‘‘adequate’’ to attain and 

maintain the PM10 NAAQS, but for 

emissions from outside the U.S. 

Therefore, nothing in section 179B 

relieves a State from the requirement to 

address and implement RACM. 

Nonetheless, States are not required to 

implement control measures that go 

beyond what the plan demonstrates 

would otherwise be adequate for timely 

attainment and maintenance of the PM10

NAAQS but for emissions from outside 

the U.S. Furthermore, to the degree that 

the State can satisfactorily demonstrate 

that implementation of a control 

measure clearly would not advance the 

area’s attainment date, EPA may 

conclude that these control measures 

are unreasonable and do not constitute 

RACM for the nonattainment area. See 

59 FR 42001; (August 16, 1994). 


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9On March 2, 2010, EPA approved the 

availability of the Motor Vehicle Emissions 

Simulator model (MOVES2010a) in official SIP 

submissions to EPA regarding air quality and for 

certain transportation conformity analyses outside 

the state of California; see 75 FR 9411. Also see 

EPA’s Web site for more information, http:// 

www.epa.gov/otaq/models/moves/index.htm. 

The 1994 General Preamble 

Addendum also discusses the 

requirements for reasonable further 

progress (RFP) and contingency 

measures as applied to nonattainment 

areas affected by international transport. 

Section 179B(a)(1) does not relieve a 

nonattainment area of the CAA 

requirements for RFP and contingency 

measures. In international border areas, 

however, ‘‘EPA will not require the 

contingency measures for PM10 to be 

implemented after the area fails to attain 

if EPA determines that the area would 

have attained the NAAQS, but for 

emissions emanating from outside the 

U.S.’’ Conversely, to the degree that 

contingency measures are needed to 

control U.S. sources of PM10 to meet 

RFP or attainment contingency measure 

requirements but for PM10 emissions 

emanating from outside of the U.S., then 

the statutory requirements for RFP and 

contingency measures still apply. See 59 

FR 42001, 42002; (August 16, 1994). 

3. Statutory Requirements and Guidance 

for Determining Attainment of the PM10

NAAQS 

EPA determines whether an area’s air 

quality is meeting the PM10 NAAQS 

based upon air quality data gathered at 

monitoring sites in the nonattainment 

area. Then, EPA reviews the data to 

determine the area’s air quality status 

according to 40 CFR part 50, appendix 

K. Three consecutive years of clean air 

quality data (i.e., no more than one 

expected exceedance per year) is 

generally needed to show attainment of 

the 24-hour PM10 standard. As defined 

by 40 CFR part 50, appendix K, a 

complete year of air quality data is 

composed of all four calendar quarters 

with each quarter containing data from 

at least 75 percent of the scheduled 

sampling days. 

Under 40 CFR part 50, appendix K, a 

nonattainment area meets the 24-hour 

PM10 NAAQS when the expected 

number of days per calendar year with 

a 24-hour average concentration above 

150 micrograms per cubic meter (mg/m3) 

is equal to or less than one. In general, 

the number of expected exceedances at 

a site which samples every day is 

determined by recording the number of 

exceedances in each calendar year and 

then averaging them over the most 

recent three calendar years. For sites 

which do not sample every day, EPA 

requires adjusting the observed 

exceedances to account for days not 

sampled. The procedures for making 

this data adjustment are specified in 40 

CFR part 50, appendix K. 

For this review of the Nogales NA and 

the contribution of international 

emissions, the standard we will use to 

demonstrate attainment of the PM10

NAAQS, ‘‘but for’’ international 

emissions, is similar to the one 

described above: The expected number 

of days per calendar year with a 24-hour 

average concentration above 150 mg/m3

must be equal to or less than one. To 

demonstrate that the Nogales NA has 

met the PM10 standard ‘‘but for’’ 

emissions from Mexico, the State’s 

analysis must show that no more than 

three exceedances, based on data 

completeness and every day sampling, 

over the specific three-year analysis 

period, would have occurred on the U.S. 

side of the border, setting aside any 

contributions from Mexican sources of 

PM10. 

IV. Review of the Nogales 2012 Plan 

In this section, according to the 

statutory requirements and guidance 

discussed above in section III, we will 

review Arizona’s submitted Nogales 

2012 Plan and section 179B analysis 

and demonstration that the Nogales NA 

is attaining the PM10 NAAQS but for 

international emissions sources from 

Nogales, Mexico. 

A. Emissions Inventories 

1. Requirements for Emissions 

Inventories 

Section 172(c)(3) of the CAA requires 

plan submittals to include a 

comprehensive, accurate, and current 

inventory of actual emissions from all 

sources in the nonattainment area. 

2. Review of the Nogales Nonattainment 

Area Emissions Inventories 

Arizona submitted emissions 

inventories for the Nogales NA for the 

years 2008 and 2011. These emissions 

inventories were calculated using 

information from version 1.5 of EPA’s 

2008 National Emission Inventory (NEI) 

and the NEI emissions estimates for 

Santa Cruz County, Arizona. A Nogales 

NA 2008 emissions inventory was 

scaled from the larger Santa Cruz 

County emissions inventory using a 

combination of population and land 

allocation ratios. A specific point 

source’s location was the basis for 

assigning point sources to the Nogales 

NA emissions inventory. On-road motor 

vehicle PM10 emissions for 2008 and 

2011 were calculated using County-level 

data for 2008 and 2011 and the 

MOVES2010a model.9 The larger and 

remaining portions of the 2011 

emissions inventory, particularly area 

sources, were calculated from the 2008 

emissions inventory according to 

estimates of population and economic 

growth. An overview of the Nogales NA 

2008 and 2011 emissions inventories is 

provided here; for detailed results and 

a complete discussion of the 

methodology used to produce the 

emission inventories, see ‘‘PM10

Emission Inventories for 2008 and 2011, 

Nogales Non-Attainment Area, Santa 

Cruz County, Arizona’’, in Appendix B 

of the Nogales 2012 Plan. 

EPA’s NEI database contains 

information about sources that emit 

criteria air pollutants and their 

precursors, and hazardous air 

pollutants. The database includes 

estimates of annual air pollutant 

emissions, including PM10, from point, 

nonpoint, and mobile sources in the 50 

states, including Arizona, and 

specifically Santa Cruz County. 

Collaborating with the states, EPA 

develops the emissions inventory and 

releases an updated version of the NEI 

database every three years. A complete 

description of the development of the 

2008 NEI may be found at the following 

URL: http://www.epa.gov/ttn/chief/net/ 

2008inventory.html. 

In calculating PM10 emissions from 

on-road mobile sources in Santa Cruz 

County, Arizona used the MOVES2010a 

version dated September 23, 2010 

(hereafter referred to as ‘‘MOVES’’). This 

is the current version of the MOVES 

model. MOVES allows the use of 

county-specific data concerning factors 

such as the average speed distribution of 

on-road vehicles, daily vehicle miles 

traveled, and road types among others 

in place of national default values. The 

MOVES model requires the use of 

county-specific data for SIP purposes. In 

this instance, the MOVES calculation 

was performed using input data from 

the 2008 NEI for Santa Cruz County. 

Similar MOVES model runs were 

completed to estimate 2011 on-road 

mobile source PM10 emissions. 

Although EPA has no specific 

guidance on assigning emissions 

sources from a county level of analysis 

to a smaller area within that county, for 

the Nogales NA emissions inventory, 

Arizona used a combination of 

population ratios, land area ratios, and 

point source locations within the 

Nogales NA to determine the 

appropriate allocation of county-wide 

emissions to the Nogales NA. See Table 


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10U.S. Census, Quickfacts, Santa Cruz County, 

Arizona. 11 2010 U.S. Census population estimates. 

12 Ibid. 13Arizona Department of Commerce Profile: 

Santa Cruz County Arizona, May 10, 2011, http:// 

www.azcommerce.com/doclib/commune/ 

SantaCruzpercent20county.pdf. 

1 for the specific population and land 

allocation ratios used to scale PM10

emissions from the County to the 

Nogales NA level. 

TABLE 1—SUMMARY OF LAND AREA AND 2008 POPULATION ALLOCATION RATIOS

Santa Cruz 

County Nogales NA Allocation ratio (percent) 

Land Area (square miles) ...................................................................................................... 10 1,237 .6 76 .1 6 .15 

2008 Population ..................................................................................................................... 11 43,091 12 23,735 55 .1 

The State used data from the U.S. 

Census Bureau to estimate the 2008 

population of the Nogales NA 

population and Santa Cruz County. A 

land area-weighted emission ratio was 

developed using U.S. Census geographic 

data and confirmed with Arizona 

Commerce Authority data.13 Some 

source categories, such as agricultural 

emissions, are likely to be proportional 

to land area; consequently, they are 

logically allocated by the land area ratio. 

To confirm whether specific point 

sources in the Santa Cruz County 

emissions inventory should be included 

in the Nogales NA emissions inventory, 

ADEQ and EPA used visual inspections 

with location information, such as 

satellite photography using Google 

Earth. 

As shown in Table 2, in 2008, the 

majority of PM10 emissions in the 

Nogales NA came from fugitive dust 

from four source categories: Unpaved 

road dust, road construction, 

commercial/industrial/institutional 

construction, and paved road dust. The 

estimated emissions inventory for 2011 

only differed slightly as total emissions 

decreased from 1,524 tons per year (tpy) 

in 2008 to 1,521 tpy in 2011, due 

primarily to implementation of new and 

cleaner engine standards for diesel 

engines. Little or no growth in 

population or economic activity 

occurred from 2008 to 2011. From 2008 

to 2011, the emissions estimated for five 

of the top six source categories remain 

unchanged, except for residential wood 

burning which increased by two tons 

per year. Again, in 2011 as in 2008, 

these six source categories account for 

approximately 95 percent of all PM10

emissions in the Nogales NA. 

TABLE 2—2008 AND 2011 NOGALES NA PM10 EMISSIONS INVENTORIES

[Tons per year] 

Source category 2008 2011 

Dust—Unpaved Road Dust ............................................................................................................................. 865 865 

Dust—Road Construction ................................................................................................................................ 267 267 

Dust—Commercial/Industrial/Institutional Construction ................................................................................... 143 143 

Dust—Paved Road Dust ................................................................................................................................. 121 121 

Fuel Combustion—Residential—Wood ........................................................................................................... 24 26 

Dust—Residential Construction ....................................................................................................................... 24 24 

Waste Disposal—Residential Garbage Burning .............................................................................................. 23 25 

All other sources .............................................................................................................................................. 57 50 

Total .......................................................................................................................................................... 1,524 1,521 

Note: All other sources include emissions from source categories such as all on-road mobile and off-road mobile, all commercial and industrial 

fuel combustion, agriculture, land clearing and burning activities. 

Source: Table 5 in ‘‘PM10 Emission Inventories for 2008 and 2011, Nogales Non-Attainment Area, Santa Cruz County, Arizona,’’ Appendix B of 

the Nogales 2012 Plan. Table 5 also provides a detailed listing of all source categories. Due to rounding, totals may not reflect exactly the sum 

of each source category. 

3. Proposed Action on the Nogales 

Nonattainment Area 2008 and 2011 

Emissions Inventories 

We propose to find that the Nogales 

NA emissions inventories for 2008 and 

2011 are comprehensive, accurate, and 

current inventories of actual emissions 

from all sources in the nonattainment 

area and that they meet the 

requirements of section 172(c)(3) of the 

CAA. The State has provided a 2008 

base year and 2011 future year 

emissions inventory comprehensively 

addressing all source categories in the 

Nogales NA. The State also used the 

most recent iteration of mobile source 

emissions modeling tool, MOVES2010a, 

in developing its emissions inventories. 

Consequently, we are proposing to find 

that the emissions inventories provided 

by Arizona meet the requirements of 

section 172(c)(3) and provide an 

adequate basis for the attainment 

demonstration under section 179B, and 

the State’s RACM/RACT and RFP 

demonstrations. 

B. Section 179B Analysis and 

Demonstration of Attainment but for 

International Sources of PM10 Emissions 

1. Review of Statute and Guidance 

Applied to the Nogales Section 179B 

Analysis and Demonstration of 

Attainment but for International Sources 

of PM10 Emissions 

As discussed earlier, the General 

Preamble Addendum provides that 

‘‘several types of information may be 

used to evaluate the impact of emissions 

emanating from outside the U.S.’’ The 

EPA will consider the information ‘‘for 

individual nonattainment areas on a 

case-by-case basis in determining 


emcdonald on DSK67QTVN1PROD with PROPOSALS2
38406 Federal Register / Vol. 77, No. 124 / Wednesday, June 27, 2012 / Proposed Rules 

14PM2.5, also called fine particulate, refers to 

particulate matter with an aerodynamic diameter 

less than or equal to 2.5 micrometers. PM10 includes 

both PM2.5 and the particulates with aerodynamic 

diameter between 2.5 and 10 micrometers, which is 

referred to as PM10-2.5. This larger fraction is called 

‘‘coarse’’ particulate. While fine particles originate 

mostly from combustion sources and secondary 

aerosol generation processes, coarse particles 

usually originate from mechanical activities and 

fugitive source categories. 

15HYSPLIT is the ‘‘Hybrid Single Particle 

Lagrangian Integrated Trajectory’’ Model, developed 

and maintained by the National Oceanic and 

Atmospheric Administration; see 

www.arl.noaa.gov/HYSPLIT_info.php for more 

information. 

whether an area may qualify for 

treatment under section 179B.’’ See 59 

FR 42001; (August 16, 1994). The 

General Preamble Addendum suggests 

five methods that may be used to 

determine the impact of emissions 

emanating from outside the U.S. and 

explains that ‘‘the State may use one or 

more of these types of information or 

other techniques, depending on their 

feasibility and applicability, to evaluate 

the impact of emissions emanating from 

outside the U.S. on the nonattainment 

area.’’ See 59 FR 42001; (August 16, 

1994). Below, we discuss these five 

methods for evaluating the effects from 

transport of international pollution and 

the applicability of these methods to the 

Nogales NA, as presented in the Nogales 

2012 Plan. 

Method 1. Place several ambient PM10

monitors and a meteorological station 

measuring wind speed and direction in 

the U.S. nonattainment area near the 

international border. Evaluate and 

quantify any changes in monitored PM10

concentrations with a change in the 

predominant wind direction. 

The State reviewed the ambient PM10

data, meteorology, and topography in 

the Ambos Nogales area. Arizona 

maintains a monitor in Nogales, Mexico, 

as well as three monitors in Nogales, 

Arizona. The Nogales, Arizona monitors 

are divided as follows: Two monitors 

measure ambient PM10 levels; and one 

monitor measures ambient PM2.5

levels.14 Arizona also has two reference 

monitors at increasing distances from 

the Nogales NA. Arizona’s complete 

analysis of the ambient data, 

meteorology, and topography is 

provided in Appendix D of the Nogales 

2012 Plan and is discussed below in 

section IV.B.2.c of this proposal. This 

method provided useful information to 

understand emissions sources and PM10

concentrations in the Nogales NA. 

Method 2. Comprehensively inventory 

PM10 emissions within the U.S. in the 

vicinity of the nonattainment area and 

demonstrate that those sources, after 

application of reasonably available 

controls, do not cause the NAAQS to be 

exceeded. This analysis must include an 

influx of background PM10 in the area. 

Background PM10 levels could be based 

on concentrations measured in a similar 

area not influenced by emissions from 

outside the U.S. 

This method implies the use of an air 

quality model to demonstrate that 

emissions within the U.S. do not create 

a violation of the NAAQS. Although a 

comprehensive, area-wide inventory of 

PM10 emissions is available for Nogales, 

Arizona, information about the spatial 

and temporal distribution of those 

emissions required to support air 

quality modeling is not readily available 

and would require significant effort to 

develop. Furthermore, given the 

complex topography of the Ambos 

Nogales area, it is not feasible to 

develop an adequate demonstration 

using available modeling tools. 

Method 3. Analyze ambient sample 

filters for specific types of particles 

emanating from across the border. 

Although not required, characteristics of 

emissions from foreign sources may be 

helpful so as to better demonstrate the 

causal relationship with and 

contribution to exceedances in the U.S. 

nonattainment area due to international 

emissions. 

This method is unlikely to produce 

useful information for the Nogales NA 

because the large proportion of crustal 

PM sources on either side of the 

international border far outweigh any 

specific stationary or combustion-based 

PM source that could be identified by a 

filter-based analysis, and differentiating 

between Arizona and Mexican sources 

of crustal material is not feasible. Also, 

specific local and international point 

source emissions information, such as 

source-specific signature emissions 

compounds, was not available with 

which to correlate the filter analyses 

results. 

Method 4. Inventory the sources on 

both sides of the border and compare 

the magnitude of PM10 emissions 

originating within the U.S. to those 

emanating from outside the U.S. 

Arizona provided two emissions 

inventories: The first emissions 

inventory, discussed above, describes 

the PM10 sources and estimates PM10

emissions in and around the Nogales 

NA, Arizona; and, the second inventory 

describes the PM10 sources and 

estimates PM10 emissions in and around 

Nogales, Mexico. The Nogales NA PM10

emissions inventory is provided in 

Appendix B and the Nogales 

Municipality, Mexico emissions 

inventory is provided in Appendix C of 

the Nogales 2012 Plan. The results of 

both inventories are discussed below in 

section IV.B.2.b. of this proposal. Also, 

as a basis for these analyses, Arizona 

reviewed population estimates and 

relative population differences for these 

areas, which is further discussed in 

section IV.B.2.a. of this proposal. 

Method 5. Perform air dispersion and/ 

or receptor modeling to quantify the 

relative impacts on the nonattainment 

area of U.S. and foreign sources of PM10

emissions. 

As discussed above, the information 

necessary to support air dispersion or 

receptor modeling is not readily 

available for the Nogales, Arizona area, 

nor is it available for the Nogales, 

Mexico area. For example, neither 

ADEQ, nor EPA, had available a gridded 

emissions inventory or a data set from 

an extensive monitoring array of 

ambient PM10 values and meteorological 

data derived from observations on 

multiple exceedance days. 

Backward wind trajectory analysis 

using the HYSPLIT model was 

considered, based on Eta Data 

Assimilation System (EDAS) gridded 

meteorological data, but again, neither 

Arizona nor EPA pursued this 

analysis.15 Previously, EPA performed 

such an analysis for the Nogales, 

Arizona area and found the resulting 

wind trajectories to be inconclusive. 

The EDAS has a 40-kilometer grid 

resolution; in contrast, the valley 

containing Nogales is 20 kilometers 

wide at its widest point. As a result, the 

EDAS data were not of a fine enough 

resolution to portray the south-to-north 

valley air drainage flows that are a key 

feature of local Nogales meteorology; 

consequently, further use of HYSPLIT 

model results for purposes of this 

section 179B analysis was rejected by 

the State and EPA. 

To summarize, the State analyzed 

ambient PM10 levels in and around the 

Nogales NA, the local meteorology 

associated with exceedances of the PM10

standards, and sources of PM10

emissions on either side of the 

international border. These analyses are 

consistent with Methods 1 and 4 

described by the General Preamble 

Addendum. The State examined method 

3, but did not pursue this avenue of 

investigation because it was unlikely 

that definitive results could be 

produced given the large crustal source 

emissions on either side of the 

international border. 

Initially, the State did not pursue 

Methods 2 and 5 because it did not have 

the data and the models required for 

this type of analysis. Instead, the State 

used the available information 

consistent with methods 1 and 4, to 


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16See 76 FR 1532; (January 11, 2011) for our 

determination that the Nogales NA attained the 

PM10 NAAQS by December 31, 1994. 

17The 1995 Nogales, Arizona population estimate 

was interpolated from 1990 and 2000 U.S. Census 

figures; the 1990 population estimate was 19,489. 

demonstrate if the Nogales NA would 

have attained the standard, but for 

international emissions. 

As stated in the General Preamble 

Addendum, EPA will consider the 

information ‘‘for individual 

nonattainment areas on a case-by-case 

basis in determining whether an area 

may qualify for treatment under section 

179B.’’ See 59 FR 42001; (August 16, 

1994). Because the individual 

circumstances surrounding a 

nonattainment area may differ widely 

whether by data, resources, or emissions 

sources, EPA anticipates that ‘‘the State 

may use one or more of these types of 

information or other techniques, 

depending on their feasibility and 

applicability, to evaluate the impact of 

emissions emanating from outside the 

U.S. on the nonattainment area.’’ See 59 

FR 42001; (August 16, 1994). The 

analysis the State has provided for the 

Nogales NA is specific to this 

nonattainment area only and the 

timeframe, data, and circumstances 

therein, and EPA evaluated the analysis 

as such. 

2. Review of Arizona’s Section 179B 

Analysis and Demonstration of 

Attainment but for International Sources 

of PM10 Emissions 

a. Population Growth in the Ambos 

Nogales Region 

In producing emissions inventories, 

Arizona reviewed recent 2010 

population information from the U.S. 

Census Bureau and Mexican Census 

data from the Instituto Nacional de 

Estadistica Geografia e Informatica 

(INEGI). While population estimates, by 

themselves, are not direct indicators of 

emissions activity, they provide an 

indication of relative human activity 

and resulting PM10 emissions on either 

side of the international border. Table 3 

provides a comparison of the 

populations residing in the Nogales NA 

and the Nogales Municipality, Mexico. 

The Nogales NA population estimate 

includes persons residing in the city of 

Nogales, Arizona, and the surrounding 

community of Rio Rico within the Santa 

Cruz County portion of the 

nonattainment area. 

TABLE 3—2010 POPULATION: NOGALES NA, ARIZONA AND NOGALES MUNICIPALITY, MEXICO

Area Population Percent 

Nogales NA, Arizona ............................................................................................................................................... 24,059 9.8 

Nogales Municipality, Mexico .................................................................................................................................. 220,292 90.2 

Total .................................................................................................................................................................. 244,351 100 

Source: INEGI & U.S. Census. 

Although the Nogales Municipality is 

a larger land area than the Nogales NA, 

a large proportion of the Municipality’s 

population is concentrated within the 

city of Nogales, Mexico and the 

surrounding area. In sum, 90.2 percent 

of the 2010 population in the Ambos 

Nogales area can be attributed to the 

Mexican side of the international 

border. 

It is also instructive to examine 

population change since 1995, when the 

Nogales NA met the PM10 NAAQS along 

with the subsequent observed 

exceedances of the PM10 NAAQS.16

Table 4 shows population estimates for 

1995, 2000, 2005, and 2010, while Table 

5 shows the annual number of expected 

exceedances of the PM10 NAAQS since 

1998, the first year the Nogales NA 

recorded exceedances after meeting the 

PM10 standard in 1994. The Nogales NA 

did not record exceedances of the PM10

standard from 1995 to 1997. 

TABLE 4—NOGALES, ARIZONA AND NOGALES MUNICIPALITY, MEXICO POPULATIONS: 1995, 2000, 2005 AND 2010 17

1995 2000 2005 2010 

Nogales, Arizona ............................................................................................. 20,184 20,878 20,421 20,837 

Nogales Municipality, Mexico .......................................................................... 133,491 159,787 193,517 220,292 

Source: INEGI & U.S. Census. 

Between 1995 and 2010, Nogales, 

Arizona population increased 

approximately three percent, and has 

fallen slightly since 2000. The 2010 

Nogales NA population at 24,059 

persons is marginally larger than the 

city of Nogales because the 

nonattainment area estimate includes 

portions of the Rio Rico communities in 

the northernmost portion of the 

nonattainment area. In contrast, the 

Nogales Municipality, Mexico 

population has increased 65 percent in 

the 1995 to 2010 timeframe. With the 

exceptions of 2000 and 2004, 

exceedances of the PM10 standard have 

been recorded since 1998 in the Nogales 

NA. The largest number of expected 

exceedances, 47.9, was recorded in 

2006. See Table 5. 

TABLE 5—NOGALES, ARIZONA EXPECTED EXCEEDANCES OF 24-HOUR PM10 NAAQS FROM 1998–2010 

Monitor frequency 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 

1 in 6 day ................................... 13.5 15.5 0.0 6.9 6.1 12.3 0.0 17.9 20.0 6.1 6.6 0.0 0.0 

Continuous ................................. .......... .......... .......... .......... .......... .......... .......... 29.6 47.9 14.0 13.2 2.0 * 8.5 

* There were no quarters in 2010 where there was a complete data set per 40 CFR part 50, appendix K; see section IV.B.2.c. for a discussion 

of 2010 data. 

Source for expected exceedance data: EPA Air Quality System Database. 


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18Two methods were used to scale point source 

emissions from 1999 to 2008 and 2011 generating 

the high and low estimates for point source PM10: 

For the low estimate, National point source 

emissions growth; and, for the high estimate, 

population based allocation ratio. The starting 1999 

baseline for point source emission was 0.9 tpy and 

the high estimate, therefore, assumes an increase of 

three orders of magnitude compared to the low 

estimate. No point sources in the Nogales 

Municipality, Mexico have been identified as 

operating at a level of emissions consistent with the 

high estimate, but lacking source specific data to 

adjudicate the difference in estimates, the high 

estimate was reported as an upper bound. See 

Appendix C of the Nogales 2012 Plan for the 

Nogales Municipality Emissions Inventory for a 

complete discussion. 

To summarize, population estimates 

since 1995 show the Nogales NA 

population remaining relatively 

constant while the Nogales 

Municipality, Mexico population has 

steadily increased to the present where 

9 of 10 people in the Ambos Nogales 

area reside in Mexico. Over the same 

timeframe, after attaining the PM10

NAAQS in 1994 through 1997, expected 

exceedances of the PM10 NAAQS in the 

Nogales NA increased to a high of 47.9 

in 2006 and the area does not meet the 

NAAQS today. The dramatic differential 

population increase in Nogales, Mexico 

compared to Nogales, Arizona and the 

surrounding nonattainment area 

supports the inference that a large and 

growing proportion of PM10 emissions 

in the Ambos Nogales area emanates 

from outside of the Nogales NA and the 

U.S. 

b. Review and Comparison of U.S./ 

Mexico Emissions Inventories 

Both the Nogales NA and the Nogales 

Municipality, Mexico have similar 

contributing sources of PM10, primarily 

fugitive dust from unpaved and paved 

roads, as well as combustion sources 

and construction. The Nogales NA 

emissions inventories were presented 

above in section IV.A.2 of this proposal. 

While less detailed than the Nogales NA 

emissions inventories, the Nogales 

Municipality, Mexico emissions 

inventories shows that the largest 

contributing sources of PM10 emissions 

are from unpaved and paved road dust 

followed by residential wood 

combustion and other area sources. 

Because Nogales Municipality, Mexico 

specific data could not be found to 

calculate unpaved and paved road 

emissions, the State reviewed other 

U.S./Mexico border emissions 

inventories to identify data for use in 

these calculations. Given the range of 

data generated and used by these U.S./ 

Mexico border emissions inventories, 

low and high estimates were calculated 

for the unpaved and paved road source 

categories. Much of the difference 

between the low and high estimates of 

Nogales Municipality emissions is 

attributed to the low and high estimates 

of unpaved and paved road emissions. 

A high estimate for point sources was 

included because the State did not have 

readily available source-specific 

information providing a precise estimate 

for stationary point sources of PM10 in 

the Nogales Municipality, Mexico.18

The methods for calculating these 

estimates are discussed in ‘‘2008 and 

2011 PM10 Emission Inventories, 

Nogales Municipality, Sonora, Mexico’’ 

in Appendix C of the Nogales 2012 Plan. 

The Nogales Municipality, Mexico 

emissions inventories for 2008 and 2011 

are presented in Table 6. 

TABLE 6—PM10 EMISSIONS INVENTORIES FOR NOGALES MUNICIPALITY, MEXICO FOR 2008 AND 2011 

[Tons per year] 

Source category Range 2008 2011 

Point Sources .................................. .......................................................... Low Estimate ................................... 1.1 1.1 

High Estimate .................................. 305 390 

Area Sources ................................... Unpaved Road ................................ Low Estimate ................................... 2,144 2,308 

High Estimate .................................. 5,521 5,944 

Paved Road ..................................... Low Estimate ................................... 53 57 

High Estimate .................................. 646 696 

Agricultural Tilling ............................ .......................................................... 0.8 0.8 

Agricultural Burning ......................... .......................................................... 1.6 1.6 

Residential Wood Combustion ........ .......................................................... 176 47 

Open Burning of Waste ................... .......................................................... 55 56 

Construction Activities ..................... .......................................................... 23 24 

Remaining Area Sources ................ .......................................................... 159 150 

Mobile Sources ................................ .......................................................... .......................................................... 80 85 

Nonroad Sources ............................. .......................................................... .......................................................... 20 27 

Total .......................................... .......................................................... Low Estimate ................................... 2,713 2,757 

Total .......................................... .......................................................... High Estimate .................................. 6,987 7,420 

Emissions are rounded to the nearest ton/year, or to the nearest tenth of a ton/year for emissions less than 10 tons/year. 

Source: Table 18 from ‘‘2008 and 2011 p.m.10 Emission Inventories, Nogales Municipality, Sonora, Mexico’’ in Appendix C of the Nogales 

2012 Plan. 

A review of the emissions inventory 

data by relative percentage and relative 

ratio provides two ways of considering 

the data. A comparison of 2008 and 

2011 Nogales Municipality, Mexico low 

emission inventory estimates with the 

Nogales NA 2008 and 2011 emission 

inventory estimates shows a 36/64 

percent split in total combined U.S./ 

Mexico emissions inventories between 

emissions from the Nogales NA, Arizona 

and Nogales Municipality, Mexico 

areas, respectively. To characterize the 

relative difference by ratio using the low 

emissions estimate for the Nogales 

Municipality, Mexico, for every one ton 

of PM10 emissions produced annually in 

Nogales NA, there is an estimated 1.8 

tons produced in Nogales Municipality. 

Similarly, a comparison of 2008 and 

2011 Nogales Municipality high 

emission inventory estimates suggests 

that there is an 18/82 percent split in 

total combined U.S./Mexico emissions 

inventories between emissions from the 

Nogales NA, Arizona and Nogales 

Municipality, Mexico areas, 


emcdonald on DSK67QTVN1PROD with PROPOSALS2
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19See Tables 6–9 from ‘‘Clean Air Act, Section 

179B Attainment Determination for the Nogales, 

Arizona PM10 Nonattainment Area’’ in Appendix A 

of the Nogales 2012 Plan for the presentation of the 

data underlying this relative percentage and relative 

ratio presentation. 

20For a listing of the 29 exceedance days by year 

and observed 24-hour concentrations, see Tables 

1–3 in ‘‘Analysis of Ambient PM10 Levels, 

Topography, and Meteorological Data in Nogales, 

Arizona: 2007–2009’’ in Appendix D of the Nogales 

2012 Plan. 

21See, in particular, Section 3 of ‘‘Analysis of 

Ambient PM10 Levels, Topography, and 

Meteorological Data in Nogales, Arizona: 2007– 

2009’’, in Appendix D of the Nogales 2012 Plan. 

22See Figure 18, Long Aerial and Elevation 

Transect of Nogales Arizona and Nogales, Sonora, 

in ‘‘Analysis of Ambient PM10 Levels, Topography, 

and Meteorological Data in Nogales, Arizona: 2007– 

2009’’, in Appendix D of the Nogales 2012 Plan. 

23See Figure 19, Short Aerial and Elevation 

Transect of Nogales, Arizona and Nogales, Sonora, 

from ‘‘Analysis of Ambient PM10 Levels, 

Topography, and Meteorological Data in Nogales, 

Arizona: 2007–2009’’, in Appendix D of the Nogales 

2012 Plan. 

24See Figure 17, Elevated Topographical View of 

Ambos Nogales Area from Northwest Perspective 

with Nogales, Sonora Highlighted and International 

Border in Red Line, from ‘‘Analysis of Ambient 

PM10 Levels, Topography, and Meteorological Data 

in Nogales, Arizona: 2007–2009’’, in Appendix D of 

the Nogales 2012 Plan. 

25Observations of PM10 concentrations, wind 

direction, wind speed and temperature were taken 

at the Nogales, Arizona Post Office site; hourly 

temperature observations were taken at the Nogales 

International Airport, 7.6 miles from the Nogales 

Post Office monitoring site and within the Nogales 

NA. 

26These monitors are described in detail in 

Section 2 of ‘‘Analysis of Ambient PM10 Levels, 

Topography, and Meteorological Data in Nogales, 

Arizona: 2007–2009’’, in Appendix D of the Nogales 

2012 Plan. Also, see Figure 2 of the same document 

for a map of their locations. 

respectively. Again, to characterize the 

relative difference by ratio using the 

high emissions estimate for the Nogales 

Municipality, Mexico, for every one ton 

of PM10 emissions produced annually in 

Nogales NA, there is an estimated 4.6 

tons produced in Nogales Municipality, 

Mexico.19

In summary, a comparison of the 

State’s 2008 and 2011 emissions 

inventory data shows for every one ton 

of PM10 produced in the Nogales NA, 

there was between 1.8 and 4.6 tons of 

PM10 emissions produced annually in 

the Nogales Municipality, Mexico, 

depending on the choice of either the 

low or the high estimate of Nogales 

Municipality, Mexico emissions. The 

emission sources appear to be similar, 

with the majority of emissions from 

fugitive dust sources, such as 

reentrained unpaved and paved road 

dust. 

c. Review and Analysis of Regional 

Meteorology, Topography and Ambient 

PM10 Monitoring Data 

In its review of the ambient PM10

data, meteorological data, and through 

its analyses, Arizona found that the 

Ambos Nogales area’s meteorology and 

topography influence the observed 

exceedances of PM10 NAAQS and there 

is a definite south-to-north directional 

component to the ambient air quality 

data underlying the exceedances of the 

PM10 NAAQS. Over the 2007–2009 

timeframe, there were 29 exceedances at 

the Nogales, Arizona Post Office (Model: 

Met One BAM 1020) monitor.20

(i) Ambos Nogales Regional Meteorology 

and Topography 

The State’s analysis of ambient 

concentration and meteorological data 

identified 26 of the 29 exceedances as 

having nearly identical diurnal patterns; 

the three exceptions were January 1, 

2007, May 22, 2008, and January 1, 

2009.21 For each of the 26 days, there is 

a strong pattern of decreasing PM10

concentrations in the early morning. 

Generally, the wind speeds are low and 

variable overnight and wind direction 

starts southerly but becomes 

increasingly variable into the daylight 

morning hours. The majority of days 

have a pronounced PM10 increase and 

drop-off between 6:00 a.m. and 9:00 

a.m., suggesting a reproducible direct 

PM10 source, noting the times 

correspond to a morning commute 

pattern. The PM10 concentrations reach 

their lowest points between 10:00 a.m. 

and 4:00 p.m., with corresponding 

increases in ambient temperature and 

wind speed observed during those 

times. Usually, northerly winds 

accompany these increases in 

temperature and wind speed. As 

temperatures and wind speeds drop in 

the evening hours, a pronounced spike 

in PM10 concentration is then observed 

beginning between 4:00 p.m. and 6:00 

p.m., with concentrations remaining 

high for several hours and gradually 

dropping off towards midnight. The 

afternoon spike in PM10 concentrations 

correlates with a significant drop in 

temperature and wind speed, and 

generally a shift to low and variable 

southerly (from the south) winds. 

Looking at the topography from south 

to north, the highest elevation of a 

primary roadway transect is at 4,331 feet 

above sea level at the southern edge of 

Nogales, Mexico, falling to the 

international border at 3,933 feet, 

continuing to the northern edge of the 

Nogales NA at 3,425 feet, and elevation 

continues to fall along the Santa Cruz 

River watershed to the north to 

approximately 3,100 feet.22 Across this 

largest 48.5-mile local transect, the 

elevation falls approximately 1,200 feet 

from south to north, i.e., from Nogales, 

Mexico, through the Nogales NA, and to 

the north towards Tucson, Arizona. 

In examining a smaller 14.8-mile 

transect along a similar primary 

roadway route, the State found that 

elevation declines on a south-to-north 

axis across two sub-transects centering 

on the international border. The 

Nogales, Mexico sub-transect shows an 

elevation drop of 201 feet over 4.8 miles 

to the international border where there 

is a slight leveling; starting at 4,134 feet 

above sea level at the Nogales, Mexico 

urban boundary and dropping to 3,933 

feet at the international border. The 

Nogales, Arizona sub-transect shows an 

elevation drop of 508 feet over 10 miles, 

from the international border to the 

northern boundary of the Nogales NA; 

starting at 3,933 feet and dropping to 

3,425 feet.23 In sum, looking at a southto-north transect along the Nogales 

Wash, elevations fall from south to 

north with the highest elevations 

occurring in the Nogales, Mexico area. 

Looking at the general topography of the 

Ambos Nogales area from a northwest 

perspective in Arizona to the southeast 

into Mexico, there is a funnel created as 

the Nogales Wash falls from higher 

southern elevations to the international 

border along the route of the Alvaro 

Obrego ́n Boulevard and into Nogales, 

Arizona.24 Small side canyons extend 

off of the Nogales Wash bottom and into 

the surrounding hills between the 

international border and south of the 

Nogales, Mexico city center, and to a 

lesser extent into Nogales, Arizona as 

elevations drop moving to the north. 

(ii) Ambient PM10 Monitoring Network, 

Data, Analyses, and Findings 

As suggested by method 1 from the 

General Preamble Addendum, the State 

analyzed hourly observations of PM10

concentrations, wind direction, wind 

speed and temperature.25 First, we will 

provide an overview and review of the 

Nogales, Arizona monitoring network. 

Second, we will examine the State’s 

review of the ambient PM10 data for 

2007–2009. Finally, we will review the 

findings from the State’s analyses of the 

ambient PM10 and meteorological data. 

Ambient PM10 and Meteorological 

Monitoring Network. There are five 

ambient air monitors in the vicinity of 

Ambos Nogales that the State 

considered for this analysis.26 Within 

the nonattainment area, the Nogales, 

Arizona Post Office is the primary 

violating monitor location for PM10.

Arizona operates two PM10 monitors 

there, along with a PM2.5 monitor. The 

Nogales, Arizona Post Office monitoring 

site is 0.3 miles north of the border and 

this monitoring site is 0.9 miles 

northeast of the Nogales, Mexico Fire 


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27See EPA’s ‘‘Arizona Department of 

Environmental Quality Technical System Audit’’ 

final October 2005; ‘‘Technical System Audit 

Report, Arizona Department of Environmental 

Quality, Air Quality Division, Ambient Air Quality 

Monitoring Program,’’ final September 2010; and 

‘‘Pima County Department of Environmental 

Quality, Technical System Audit’’ final February 

2009. Final reports for the April 2012 TSA of ADEQ 

and September 2011 TSA of PDEQ are not yet 

complete. 

28See ADEQ’s ‘‘State of Arizona Air Monitoring 

Network Plan For the Year 2011, Final Report’’ 

dated August 2, 2011 and EPA’s approval letter 

from Matthew Lakin, Manager of EPA Region 9’s 

Air Quality Analysis Office, to Eric Massey, 

Director of the Air Quality Division of Arizona 

Department of Environmental Quality, dated 

December 1, 2011. 

29The NAAQS for all pollutants can be found at 

www.epa.gov/air/criteria.html. 

30This monitor is formally designated as AQS ID: 

04–023–0004, POC 3. 

31For a list of the 29 exceedance days by year and 

observed 24-hour concentrations at all five Nogales 

area monitors, see Tables 1–3 in ‘‘Analysis of 

Ambient PM10 Levels, Topography, and 

Meteorological Data in Nogales, Arizona: 2007– 

2009’’ in Appendix D of the Nogales 2012 Plan. 

32For the Exceptional Events Rule see ‘‘Treatment 

of air quality monitoring data influenced by 

exceptional events’’; 40 CFR 50.14. 

33See Section 4.5 in ‘‘Analysis of Ambient PM10

Levels, Topography, and Meteorological Data in 

Nogales, Arizona: 2007–2009’’, in Appendix D of 

the Nogales 2012 Plan. 

34See Figures 4, 5, 6, and 7 in ‘‘Analysis of 

Ambient PM10 Levels, Topography, and 

Meteorological Data in Nogales, Arizona: 2007– 

2009’’, in Appendix D of the Nogales 2012 Plan. 

35See Figure 4 in ‘‘Analysis of Ambient PM10

Levels, Topography, and Meteorological Data in 

Station monitoring site. The Green 

Valley and Corona de Tucson 

monitoring sites are approximately 35 

and 45 miles away from the U.S./ 

Mexico border, respectively. The 

Nogales Post Office and the Nogales, 

Mexico Fire Station monitors are 

operated by ADEQ. The Corona de 

Tucson and the Green Valley monitors, 

located near Tucson, Arizona, are 

operated by the Pima County 

Department of Environmental Quality 

(PDEQ). 

Also, Arizona operates a 

meteorological data collection station at 

the Nogales, Arizona Post Office 

monitoring site. Wind speed 

observations discussed in its analyses 

were collected at that location. 

Temperature observations were 

collected at the Nogales International 

Airport, located approximately six miles 

northeast of the Nogales, Arizona Post 

Office monitoring site and within the 

nonattainment area. 

EPA performed independent 

Technical System Audits (TSAs) of 

ADEQ’s ambient air monitoring program 

in December 2004, September 2009, and 

April 2012 and TSAs of PDEQ’s ambient 

monitoring program in June 2008 and 

September 2011, per requirements in 40 

CFR part 58, appendix A, section 2.5.27

We assessed ADEQ and PDEQ’s 

compliance with established regulations 

governing the collection, analysis, 

validation, and reporting of ambient air 

quality data and concluded that ADEQ 

and PDEQ have a robust ambient air 

monitoring program, with an 

appropriate quality system in place for 

collecting ambient air monitoring data. 

EPA reviewed and subsequently 

approved the 2011 ADEQ annual 

monitoring network plan on December 

1, 2011.28 We found that ADEQ’s 2011 

monitoring network plan was complete 

and met the requirements for annual 

network plans described in 40 CFR 

58.10. 

Ambient PM10 Data for 2007–2009. 

The 24-hour PM10 NAAQS is based on 

the number of expected exceedances 

greater than 150 mg/m3 averaged over 

three years.29 For this analysis, the State 

considered the most recent and most 

complete three-year data range 

available: 2007–2009. There was a large 

period of missing data at the Nogales, 

Arizona Post Office PM10 federal 

equivalency method (FEM)/special 

purpose monitor between March 16 and 

October 27, 2010. Consequently, we 

concur with the State that 2007 to 2009 

is the most appropriate timeframe for 

this section 179B analysis and 

attainment demonstration. At the 

Nogales, Arizona Post Office monitors, 

PM10 data completeness for each quarter 

within the 2007–2009 timeframe is 

greater than 75 percent. 

In the 2007–2009 period, there were 

29 exceedances at the Nogales, Arizona 

Post Office, FEM/special purpose 

monitor.30 31 Of those exceedances, 14 

occurred in 2007, 13 in 2008, and two 

in 2009. Twenty-seven of the twentynine exceedances were observed in the 

October through March annual 

timeframe. Twenty-four hour PM10

concentrations on exceedance days 

varied between 155 and 238 mg/m3, with 

some hourly measurements reaching 

900 mg/m3. Arizona has not flagged any 

of these 2007, 2008, or 2009 exceedance 

days for potential exclusion from air 

quality planning considerations under 

EPA’s Exceptional Events Rule.32 The 

State focused on the data from the 

Nogales, Arizona Post Office FEM/Met 

One BAM 1020 monitor for the 

following reasons: it is comparable to 

the NAAQS; it has recorded all the 

exceedances in the area; it has recorded 

hourly ambient values; and, it has a 

sufficiently complete dataset for 

comparison to the NAAQS. 

The State did not use 2010 and 2011 

data for its detailed meteorological 

analysis and attainment demonstration 

for two reasons. First, the 2010 dataset 

did not meet the completeness criteria 

specified in 40 CFR part 50, appendix 

K; no quarter in 2010 had complete 

data. This was due to a large data gap 

from March 16 to October 27 resulting 

from poor quality assurance and control 

results. Second, at the time of this 

analysis, the 2011 dataset had yet to be 

entered completely into the EPA’s Air 

Quality System (AQS) database and 

certified by Arizona. As stated earlier, a 

complete year of air quality data, as 

defined by 40 CFR part 50, appendix K, 

comprises all four calendar quarters 

with each quarter containing data from 

at least 75 percent of the scheduled 

sampling days. While the 2010 and 2011 

ambient data do not provide the basis 

for the State’s attainment 

demonstration, the State examined this 

data and found no information to 

contradict its conclusions using the 

2007–2009 data set.33

The State reviewed the 2010 and 2011 

data to see how ambient PM10 levels 

compared to the 2007–2009 dataset. In 

2010, the Nogales, Arizona Post Office 

(FRM/Met One BAM 1020) monitor 

recorded six exceedances of the 24-hour 

PM10 NAAQS; these 24-hour average 

ambient values ranged from 159 mg/m3

to 191 mg/m3. There was one exceedance 

of the PM10 standard in 2011. Arizona 

has not flagged any of these 2010 or 

2011 exceedances for potential 

exclusion from air quality planning 

considerations under EPA’s Exceptional 

Events Rule. 

Analyses of 2007–2009 Ambient PM10

Data, Meteorological Data and Findings. 

To understand and characterize the 

ambient PM10 data and meteorological 

data from the Nogales NA on the 29 

exceedance days chosen for this 

analysis, the State conducted two initial 

studies: an examination of hourly 

ambient PM10 concentrations, hourly 

wind speed observations, and hourly 

temperatures; and, several analyses of 

hourly wind direction observations and 

hourly ambient PM10 concentrations. 

The first study of hourly observations 

of ambient PM10 concentrations, wind 

speeds, and temperatures on the 29 

exceedance days involved line plots of 

these three variables over the 24 hour 

exceedance day.34 These line plots 

showed a relatively tight grouping 

among the three subject variables across 

29 exceedance days except for three 

days that were distinct from the rest. 

The line plot of hourly PM10

concentrations versus time of day for all 

exceedance days identified January 1, 

2007, May 22, 2008, and January 1, 2009 

as having a significantly different 

diurnal pattern.35 The remaining 26 of 


emcdonald on DSK67QTVN1PROD with PROPOSALS2
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Nogales, Arizona: 2007–2009’’, in Appendix D of 

the Nogales 2012 Plan. 36See Figure 5 in ‘‘Analysis of Ambient PM10

Levels, Topography, and Meteorological Data in 

Nogales, Arizona: 2007–2009’’, in Appendix D of 

the Nogales 2012 Plan. 

37See Figure 6 in ‘‘Analysis of Ambient PM10

Levels, Topography, and Meteorological Data in 

Nogales, Arizona: 2007–2009’’, in Appendix D of 

the Nogales 2012 Plan. 

38See Figure 7 in ‘‘Analysis of Ambient PM10

Levels, Topography, and Meteorological Data in 

Nogales, Arizona: 2007–2009’’, in Appendix D of 

the Nogales 2012 Plan. 

39Throughout these analyses and this document, 

the term ‘‘southerly wind direction quadrant’’ refers 

to wind originating from between 135 and 224 

degrees on a compass rose. Similarly, the term ‘‘all 

other wind direction quadrants’’ refers to the 

remaining 270 degrees of wind direction between 

225 and 134 degrees on a compass rose. 

40See Figures 8 and 9 in ‘‘Analysis of Ambient 

PM10 Levels, Topography, and Meteorological Data 

in Nogales, Arizona: 2007–2009’’, in Appendix D of 

the Nogales 2012 Plan. 

41See Figures 11 and 12 in ‘‘Analysis of Ambient 

PM10 Levels, Topography, and Meteorological Data 

in Nogales, Arizona: 2007–2009’’, in Appendix D of 

the Nogales 2012 Plan. 

42See Table 12 for all estimated values on all 

exceedance days in ‘‘Clean Air Act, Section 179B 

Attainment Determination for the Nogales PM10

Nonattainment Area’’ in Appendix A of the Nogales 

2012 Plan. 

the 29 observed exceedances have 

nearly identical diurnal patterns.36 Line 

plots of hourly wind speed versus time 

of day for all exceedance days show 

wind speeds were eight miles per hour 

(mph) or below for all exceedance days, 

with the exception of May 22, 2008, 

when elevated wind speeds were 

observed.37 Line plots of hourly 

temperatures versus time of day for all 

exceedance days show a distinct diurnal 

heating and cooling pattern with no 

particular day deviating substantially 

from the others.38

In a second set of analyses of ambient 

PM10 concentrations and wind direction 

on exceedance days, the State found 

that high PM10 concentrations are 

associated with wind direction from a 

southerly quadrant, or southerly air 

flows, more often than what is typically 

observed on non-exceedance days. Also, 

the State found that the largest number 

of hourly ambient values above 150 mg/ 

m3 and the highest ambient values, 

including those markedly above 150 mg/ 

m3, originated from a southerly wind 

direction quadrant.39 These 

observations suggest a greater influence 

on ambient PM10 concentrations from 

sources in Mexico during these hours of 

southerly wind direction. 

Beginning with wind rose analyses, 

the State determined that the prevailing 

wind direction was from the south, and 

to a lesser degree, from the west 

southwest directions on non-exceedance 

days, but almost primarily from the 

south on exceedance days.40 Following 

with pollution rose studies that link 

hourly ambient PM10 concentration and 

wind direction observations, these 

studies showed a significant percentage 

of values greater than 150 mg/m3

originating from the southerly wind 

direction quadrant.41 A presentation of 

the Figure 11 pollution rose data in 

tabular form is provided in Table 7. The 

largest proportion of hourly values 

above 150 mg/m3 and the highest hourly 

concentrations were found in the 

southerly wind direction quadrant. 

When ambient PM10 values above 150 

mg/m3 were sorted by 100 mg/m3

increments to 550 mg/m3 and greater, the 

analysis showed that within each 

increment above 150 mg/m3, 71 to 92 

percent of the ambient PM10

observations were from the southerly 

wind quadrant. Again, these 

observations suggest a greater influence 

on ambient PM10 concentrations from 

sources in Mexico during these hours of 

southerly wind direction. 

TABLE 7—HOURLY AMBIENT PM10 CONCENTRATIONS SORTED BY CONCENTRATION AND WIND DIRECTION, 2007–2009 

EXCEEDANCE DAYS

Wind direction quadrant 

Range of ambient concentration values (microgram/m3) 

<150 

(percent) 

150–250 

(percent) 

250–350 

(percent) 

350–450 

(percent) 

450–550 

(percent) 

>=550 

(percent) 

Share of all 

wind direction 

observations 

Northerly NW to NNE ........................ 27 6 3 3 3 0 17 

Easterly NE to ESE ........................... 15 16 16 11 3 8 14 

Southerly SE to WSW ....................... 41 71 72 84 92 92 57 

Westerly SW to WNW ....................... 18 6 8 3 3 0 12 

Total ............................................ 100 100 100 100 100 100 100 

Source: Table 11 in ‘‘Clean Air Act, Section 179B Attainment Determination for the Nogales PM10 Nonattainment Area’’ in Appendix A of the 

Nogales 2012 Plan. 

Finally, in a third analysis, the State 

examined the wind direction and hourly 

PM10 concentrations on each 

exceedance day to determine two 

average ambient values for each 

exceedance day: one value for the 

southerly wind quadrant and a second 

value representing all other wind 

direction quadrants.42 The results 

showed that two of the 29 exceedance 

days, January 1, 2007 and January 26, 

2008, have an average ambient 

concentration greater than 150 mg/m3 for 

the ‘‘all other wind direction’’ 

quadrants. The ratio of the southerly 

quadrant concentration to the ‘‘all other 

direction’’ quadrant concentration 

ranges from 0.86 to one to 11 to one, 

with an average ratio value of 3.83 to 

one. Only one day, January 1, 2007, has 

a ratio value less than 1.0 to one; i.e., 

the ‘‘all other direction’’ quadrants’ 

share exceeds the southerly quadrant 

share. This analysis also suggests a 

greater influence on ambient PM10

concentrations from sources in Mexico 

during these hours of southerly wind 

direction. 

To summarize, the State analyzed 

hourly ambient concentrations on 

exceedance days and found that high 

PM10 concentrations are associated with 

wind direction from a southerly 

quadrant, or southerly air flows, more 

often than what is typically observed on 

non-exceedance days. The State found 

that the largest number of hourly 

ambient values above 150 mg/m3 and the 

highest ambient values, including those 

markedly above 150 mg/m3, originated 

from a southerly wind direction 

quadrant. These studies of hourly 

ambient data confirm these general 

findings; however, the January 1, 2007 

and January 26, 2008 exceedance days 

may be exceptions. Also, due to the 

differing meteorology exhibited on May 

22, 2008 and January 1, 2009, these days 

are marked for further study. All four of 

these exceedance days are reviewed and 

discussed further, below. 


emcdonald on DSK67QTVN1PROD with PROPOSALS2
38412 Federal Register / Vol. 77, No. 124 / Wednesday, June 27, 2012 / Proposed Rules 

43See Figure 3 in ‘‘Analysis of Ambient PM10

Levels, Topography, and Meteorological Data in 

Nogales, Arizona: 2007–2009’’ in Appendix D of the 

Nogales 2012 Plan. 

44See Figures 7 and 14 in ‘‘Analysis of Ambient 

PM10 Levels, Topography, and Meteorological Data 

in Nogales, Arizona: 2007–2009’’ in Appendix D of 

the Nogales 2012 Plan. 

45See Table 11 above. For a visual representation 

of this data, see the pollution roses in Figures 11 

and 12, ‘‘Analysis of Ambient PM10 Levels, 

Topography, and Meteorological Data in Nogales, 

Arizona: 2007–2009’’ in Appendix D of the Nogales 

2012 Plan. 

46For a graphical depiction of the interplay 

between ambient PM10 concentrations, wind speed, 

and temperatures described by the conceptual 

model, see Figure 3 in ‘‘Clean Air Act, Section 179B 

Attainment Determination for the Nogales PM10

Nonattainment Area’’ in Appendix A of the Nogales 

2012 Plan. As explained in the footnote to Figure 

3, although the diurnal emissions pattern of the 

January 26, 2008 exceedance day is very similar to 

the 25 exceedance days summarized by the 

conceptual model other parts of the discussion may 

not be consistent with the observed data from 

January 26, 2008. 

d. Findings From Reviews of Emission 

Inventories, and Studies of Ambient 

PM10 Data, and Meteorological Data 

From the State’s analyses, the Nogales 

NA emissions inventories, the Nogales 

Municipality, Mexico emissions 

inventories, and the 2007–2009 ambient 

data and meteorological analyses, the 

State made the findings listed below. 

• The majority of exceedances, 79 

percent, occurred in the October to 

January timeframe, mostly in 

November.43 Also, given the high desert 

environment and winter light regime, 

temperatures usually drop dramatically, 

20 degrees Fahrenheit over the 3–4 

hours after sunset.44

• From the Nogales NA and Nogales 

Municipality, Mexico emission 

inventories, the State estimated 

pollution loads may differ by a ratio of 

1.8 (low estimate)—4.6 (high estimate) 

to one on a south-to-north basis in 

relation to the international border. 

• The largest sources of PM10

emissions in the Ambos Nogales area 

are reentrained dust from unpaved and 

paved roads. 

• Overall, elevations drop 

approximately 709 feet across the entire 

south-to-north local transect, from the 

southernmost edge of the Nogales, 

Mexico urban boundary to the Nogales 

NA northern boundary line. 

• Of the 29 exceedance days in 2007– 

2009, 26 of those days showed a similar 

pattern of ambient PM10 concentrations, 

wind speeds, wind direction, and 

temperature variation over a 24-hour 

period; the three exceptions were 

January 1, 2007, May 22, 2008, and 

January 1, 2009. 

• On exceedance days, the largest 

proportions, 71–92 percent, of hourly 

values exceeding 150 mg/m3 and almost 

all of the highest observed PM10

concentrations of observations above 

450 mg/m3, 92 percent, are associated 

with a southerly wind direction 

quadrant.45

• The ambient PM10 concentration 

attributed to the southerly wind 

quadrant exceeds 150 mg/m3 on all 29 

exceedance days. In contrast, two 

exceedance days from the ‘‘all other 

wind direction’’ quadrants show a value 

greater than 150 mg/m3: January 1, 2007, 

and January 26, 2008. 

• Only one of 29 exceedance days 

shows the concentration attributed to 

the ‘‘all other wind direction’’ quadrants 

greater than that of the concentration 

attributed to the southerly wind 

quadrant: January 1, 2007. 

• On exceedance days, the average 

ratio of the southerly wind quadrant 

share of 24-hour ambient PM10 values to 

all other wind quadrants share of 

ambient values is 3.83 to one. This ratio 

is relatively consistent with the 

estimated pollution loads ratio of 1.8– 

4.6 to one, from south-to-north across 

the international border. This 

comparison of the hourly ambient PM10

value/wind direction ratio and the 

pollution load ratios suggests that the 

pollution load ratios and the low and 

high emissions inventory estimates are 

both conservatively low and high 

estimates of ambient conditions. 

Upon review of the ambient PM10 data, 

meteorology, and the State’s analyses, 

we concur with the State’s findings 

listed above. 

e. Arizona’s Demonstration of 

Attainment for the Nogales 

Nonattainment Area but for 

International Sources of PM10 Emissions 

(i) Daily Analysis to Demonstrate 

Attainment but for International Sources 

of PM10 Emissions 

As described above, 26 of the 29 

2007–2009 exceedances showed a 

similar pattern of ambient PM10

concentrations, wind speeds, wind 

direction, and temperature variation 

over a 24-hour period; the exceptions 

were January 1, 2007, May 22, 2008, and 

January 1, 2009. Two of these days, 

January 1, 2007, and January 1, 2009, 

with higher early morning PM10

concentrations, only vary from the 

diurnal profile of PM10 concentrations 

observed for the other exceedances, but 

have similar meteorological and 

concentration patterns throughout the 

rest of the day. Two of the 29 

exceedance days, January 1, 2007, and 

January 26, 2008, had high average 

ambient concentrations during hours 

when the wind was out of directions 

other than the south. Thus, there are 25 

exceedance days that are equivalent and 

can be considered as a group, setting 

aside the dissimilar exceedance days 

listed above, January 1, 2007, January 

26, 2008, May 22, 2008, and January 1, 

2009. 

A Conceptual Model of 2007–2009 

Exceedance Days. Considering these 25 

similar exceedance days, the State 

explained how the elements of pollution 

loads and sources, temperature changes, 

and wind direction may contribute to 

producing the majority of observed 

ambient PM10 values exceeding the 

NAAQS in Nogales, Arizona.46 The data 

concerning January 1, 2007, January 26, 

2008, May 22, 2008, and January 1, 2009 

are reviewed later in more detail in this 

daily analysis. 

Within the cited Figure 3, the State 

shows the average PM10 concentration, 

wind speed, and temperature across 26 

similar exceedance days and including 

25 of those days in the conceptual 

model. The 24-hour pattern of these 

variables on these 25 days is similar. 

Beginning at midnight, the data indicate 

that there is a strong pattern of 

decreasing PM10 concentrations from 

the previous day’s high values into the 

early morning hours. Then, there is a 

pronounced PM10 increase and drop-off 

between 6:00 a.m. and 9:00 a.m., 

suggesting a regularly occurring direct 

PM10 source, such as reentrained road 

dust from the morning commute. As 

morning temperatures rise, so does 

wind speed as wind direction changes 

from south to north dispersing the spike 

in morning PM10 concentrations. The 

PM10 concentrations continue to fall 

through the afternoon and reach their 

lowest points between 10:00 a.m. and 

4:00 p.m. The morning and afternoon 

increases in ambient temperature and 

wind speed can be attributed to the 

heating portion of a diurnal heating and 

cooling cycle where heated air flows 

from lower elevations in the north to the 

higher elevations in the south. 

On the 25 days, the meteorological 

and ambient concentration data also 

provide an explanation for regularly 

occurring increases in PM10

concentrations during the evening 

hours. As sunset approaches and night 

falls, the diurnal cooling cycle begins. 

Ambient temperatures drop and lower 

elevation air masses no longer rise with 

convection, causing wind speed to drop 

and wind direction to be variable. As 

temperatures continue to drop after 

sunset, wind speeds drop and cold air 

masses flow downslope from higher 

elevations, causing wind direction to 

shift from a variable/northerly direction 

to a southerly direction. A pronounced 

spike in PM10 concentration is then 


emcdonald on DSK67QTVN1PROD with PROPOSALS2
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47Completed in 2002 by A.W. Ellis, the final 

report is available through The Southwest Center 

for Environmental Research and Policy at http:// 

scerpfiles.org/cont_mgt/doc_files/A-02-2.pdf. 

48For the estimated values providing the basis for 

the conceptual model’s 25 exceedance day values 

discussed in this paragraph, see Table 12 in ‘‘Clean 

Air Act, Section 179B Attainment Determination for 

the Nogales PM10 Nonattainment Area’’ in 

Appendix A of the Nogales 2012 Plan. 

49For the complete discussion of coarse versus 

fine particulate matter on all exceedance days, see 

Section 4.4 and Table 8 in ‘‘Analysis of Ambient 

PM10 Levels, Topography, and Meteorological Data 

in Nogales, Arizona: 2007–2009’’ in Appendix D of 

the Nogales 2012 Plan. 

observed beginning between 4:00 p.m. 

and 6:00 p.m.; roughly corresponding 

with the evening commute hours. 

Concentrations remain high for several 

hours into the evening and gradually 

begin to decrease as midnight 

approaches. The highest concentrations 

of PM10 occur in these evening hours 

when reentrained dust from unpaved 

and paved roads may be captured by 

cold air flows moving south to north 

from higher to lower elevations (later in 

the discussion this phenomenon is 

referred to as ‘‘downslope air flows’’). 

Also, home heating combustion may 

add a component to the evening PM10

load and also be captured in the evening 

southerly and downslope air flows from 

Nogales, Mexico into Nogales, Arizona. 

This pattern of exceedances is usually 

observed during times when the general 

weather pattern allows for stagnation 

and a relatively still air mass subject to 

movement by the diurnal cooling and 

heating cycle. At other times of the year, 

frontal systems move through often 

enough and with enough energy to 

prevent a stagnant air mass in the 

Ambos Nogales region and this diurnal 

heating and cooling cycle exerts less 

influence on the local meteorology. 

The conceptual model the State has 

presented to explain the exceedances in 

the Nogales NA is consistent with the 

study by Arizona State University, 

‘‘Atmospheric, Hydroclimatic, and 

Anthropogenic Causes of Fugitive Dust 

in the Nogales, Arizona-Nogales, Sonora 

Airshed.’’ 47 In this study—based on a 

regression analysis of 815 daily PM10

observations at Nogales, Arizona, and 

457 daily PM10 observations at Nogales, 

Mexico, and other information—the 

authors conclude that stagnant 

atmospheric conditions over a large 

scale (i.e., a stagnant synoptic 

atmosphere) is the most important factor 

in predicting high daily PM10

concentrations. 

For the 25 similar days examined by 

ADEQ, the ambient PM10 concentration 

attributed to the southerly wind 

direction quadrant always exceeds the 

150 mg/m3 level, in most cases 

markedly.48 Conversely, the ambient 

concentration attributed to the ‘‘all other 

wind direction’’ quadrants never 

exceeds the 150 mg/m3 level. Across all 

25 days, the average of the hourly 

monitored PM10 concentration values 

for the hours with a southerly wind 

direction ranges from 163 to 369 mg/m3

for each of the days, with an average 

value across the 25 days of 264 mg/m3. 

In comparison, the average of the hourly 

concentration values for all other wind 

direction quadrants ranges from 38 to 

148 mg/m3 for each of the days, with an 

average value across the 25 days of 80 

mg/m3. This suggests that emissions 

sources to the south in Mexico are 

contributing significantly to those 

hourly ambient concentrations and the 

resulting 24-hour average 

concentrations. 

In sum, for 25 of the 29 exceedance 

days, the State provided a conceptual 

model explaining how exceedances of 

the PM10 NAAQS occur in the Nogales 

NA. Moreover, for all of these 25 days, 

the origin and contribution of PM10 to 

exceedances of the standard at the 

Nogales, Arizona Post Office monitor 

has a very large southerly component. 

Given the wind direction, the proximity 

of the monitor to the border, and the 

comparison of the magnitude of 

emissions on either side of the border, 

the majority of the emissions that result 

in these 25 exceedances most likely 

originate from the Nogales, Mexico side 

of the international border. 

Analysis of Four Days Differing From 

Conceptual Model: January 1, 2007; 

January 26, 2008; May 22, 2008; and, 

January 1, 2009. The conceptual model 

of Mexican influence on Nogales NA 

PM10 concentrations described above 

fits the observations on 25 of the 29 

exceedance days in 2007–2009. The 

State identified four specific exceedance 

days that differ in one or more ways 

from the 25-day conceptual model of 

PM10 exceedances in the Nogales NA: 

January 1, 2007, May 22, 2008, January 

26, 2008, and January 1, 2009. See Table 

8 for more information. 

TABLE 8—24-HOUR PM10 CONCENTRATION (μG/M3) AND HOURLY CONCENTRATION AVERAGES (μG/M3) DISAGGREGATED

BY SOUTHERLY WIND DIRECTION QUADRANT FOR EXCEEDANCE DAYS DIFFERING FROM CONCEPTUAL MODEL

Date 24-hour 

concentration 

Southerly wind quadrant 

(135 to 224 degrees) 

average concentration 

All other wind direction 

(225 to 134 degrees) 

average concentration 

January 1, 2007 ..................................... 210 199 (15 of 24 values) ............................. 231 (9 of 24 values).

January 26, 2008 ................................... 204 257 (7 of 24 values) ............................... 182 (17 of 24 values). 

May 22, 2008 ......................................... 217 217 (24 of 24 values) ............................. No Observed Values. 

January 1, 2009 ..................................... 238 323 (14 of 24 values) ............................. 119 (10 of 24 values). 

Data Source: Air Quality System database; and, Table 4.2 in Nogales 2012 Plan. 

The State examined each of these 

days in further detail to evaluate the 

influences on the high ambient PM10

values that occurred on those days and 

to determine whether the four 

remaining exceedance days—January 1, 

2007, January 26, 2008, May 22, 2008, 

and January 1, 2009—should be 

assigned to the category of exceedance 

days having a significant contribution 

from emission sources originating from 

the Nogales, Mexico side of the 

international border. The State’s 

analysis is summarized below. 

January 1, 2007 Exceedance Day 

Review. Considering the January 1, 2007 

exceedance day, it differs from the 

conceptual model average exceedance 

day in the timing and distribution of 

observed ambient PM10 values and high 

PM2.5 component most likely caused by 

a combustion source.49 The PM10: PM2.5

ratio for January 1, 2007 is the lowest in 

the 29-day sample (1.49 to 1). What 

differs in the case of the January 1, 2007 

exceedance is that the 270 degree wind 

direction quadrants contain enough 

high values to contribute 

disproportionately to the overall 24- 

hour average concentration. Although 

more detailed and different field studies 

might prove otherwise, with the 

information available, the State’s 

analysis is inconclusive as to whether 

this exceedance is attributable to a 

disproportionate international 


emcdonald on DSK67QTVN1PROD with PROPOSALS2
38414 Federal Register / Vol. 77, No. 124 / Wednesday, June 27, 2012 / Proposed Rules 

50For a detailed review of the January 1, 2007 

exceedance day, see Section 4.2.1 of ‘‘Clean Air Act, 

Section 179B Attainment Determination for the 

Nogales PM10 Nonattainment Area’’ in Appendix A 

of the Nogales 2012 Plan. 

51For a detailed review of the January 26, 2008 

exceedance day, see Section 4.2.2 of ‘‘Clean Air Act, 

Section 179B Attainment Determination for the 

Nogales PM10 Nonattainment Area’’ in Appendix A 

of the Nogales 2012 Plan. 

52See Figure 6 in ‘‘Clean Air Act, Section 179B 

Attainment Determination for the Nogales PM10

Nonattainment Area’’ in Appendix A of the Nogales 

2012 Plan. 

53For a detailed review of the May 22, 2008 

exceedance day, see Section 4.2.3 of ‘‘Clean Air Act, 

Section 179B Attainment Determination for the 

Nogales PM10 Nonattainment Area’’ in Appendix A 

of the Nogales 2012 Plan. 

54For a detailed review of the January 1, 2009 

exceedance day, see Section 4.2.4 of ‘‘Clean Air Act, 

Section 179B Attainment Determination for the 

Nogales PM10 Nonattainment Area’’ in Appendix A 

of the Nogales 2012 Plan. 

contribution and the Nogales NA would 

not have exceeded the 24-hour PM10

standard but for Mexican emissions.50

January 26, 2008 Exceedance Day 

Review. The State’s review of the 

January 26, 2008 exceedance day 

suggests that this day is most like the 

conceptual model average exceedance 

day in the timing and distribution of 

observed ambient PM10 values. While 

the southerly wind direction quadrant 

contains enough high values to 

contribute disproportionately to the 

overall 24-hour average concentration, 

there are enough remaining high values 

in the 17 of 24 hourly observations from 

the 270 degree wind direction quadrants 

to be above the 150 mg/m3 level. Again, 

while specifically designed field studies 

might help clarify the relative 

contributions to this exceedance, with 

the information available, the State’s 

analysis is inconclusive as to whether 

this exceedance is attributable to a 

disproportionate international 

contribution and the Nogales NA would 

not have exceeded the 24-hour PM10

standard but for Mexican emissions.51

May 22, 2008 Exceedance Day 

Review. The May 22, 2008 exceedance 

day is wholly different from the State’s 

conceptual model exceedance day given 

the relative high wind speeds, a 17 mph 

high observation, and higher than usual 

coarse PM component likely from 

disturbed surfaces.52 The PM10:PM2.5

ratio for May 22, 2008 is the highest in 

the 29-day sample (10.96 to 1), well 

beyond the sample average of 6.24 to 1. 

As with total PM10 emissions, emissions 

of coarse PM (e.g., unpaved roads) are 

higher from Nogales, Mexico, than they 

are from the Nogales NA. The wind 

direction is from a southerly quadrant in 

all hourly observations. See Table 8. 

Given this information, we concur that 

the day should be placed with the 25 

other exceedance days in the conceptual 

model, because it is likely that the 

sources of PM10 causing the exceedance 

originated from the Nogales, Mexico 

side of the international border.53

January 1, 2009 Exceedance Day 

Review. Like the January 1, 2007 

exceedance, the January 1, 2009 

exceedance day is different from the 

conceptual model exceedance day in the 

timing and distribution of observed 

ambient PM10 values and high PM2.5

component most likely caused by a 

combustion source. As with total PM10

emissions, emissions of fine PM (e.g., 

combustion sources) are higher from 

Nogales, Mexico, than they are from the 

Nogales NA. For example, a comparison 

of the 2008 Nogales Municipality, 

Mexico and Nogales NA emissions 

inventories for the residential 

woodburning source category shows 176 

tpy compared to 24 tpy, respectively 

(see Tables 2 and 6, above). The key 

factor for assigning this day is the 

contribution of high hourly ambient 

concentrations with a southerly wind 

direction quadrant compared to the 

remaining 270 degree wind direction 

quadrants. See Table 8. Consequently, 

we concur that the day should be placed 

with the 25 other exceedance days in 

the conceptual model, because it is 

likely that the sources of PM10 causing 

the exceedance originated from the 

Nogales, Mexico side of the 

international border.54

To summarize, the State concludes 

that two exceedance days, May 22, 2008 

and January 1, 2009, should be 

categorized with the 25 exceedance days 

where the State found that there was a 

high likelihood of a large contribution of 

PM10 from sources on the Nogales, 

Mexico side of the international border 

such that the Nogales NA would likely 

have attained the PM10 standard but for 

emissions from Mexico. The two 

remaining exceedance days, January 1, 

2007 and January 26, 2008, have 

contributions from PM10 sources on the 

Nogales NA side of the international 

border such that it cannot be 

determined that there is a similarly high 

likelihood that the Nogales NA would 

not have exceeded the PM10 standard 

but for PM10 emissions originating from 

the Mexican side of the international 

border. Therefore, according to this 

daily analysis, the State found that at 

least 27 of 29 exceedances of the PM10

NAAQS observed in the Nogales NA 

during 2007–2009 can be attributed 

primarily to sources of PM10 from across 

the international border. Based on these 

two exceedances and on data 

completeness and every day sampling 

for the 2007–2009 timeframe, the State 

calculated a maximum expected annual 

exceedance rate of 0.7 exceedances per 

year. 

(ii) Hourly Analysis to Demonstrate 

Attainment But For International 

Sources of PM10 Emissions 

In a second analysis, the State 

classified each hourly PM10

concentration value from the 29 

exceedance days based on the likely 

influence of emissions from Mexico and 

then recalculated the 24-hour average 

concentration that would have occurred 

but for international transport of PM10

emissions from Nogales, Mexico. An 

hourly concentration was classified as 

influenced by international transport if 

it met one of four criteria, or decision 

rules, related to hourly observations of 

wind direction, wind speed, and 

temperature change: 

(1) Hours with sustained (more than 

one hour consecutively) southerly 

winds greater than 4.5 mph (2 meters/ 

second (m/s)), suggesting the primary 

influence of wind-blown PM10 from 

across the international border; 

(2) hours with southerly winds or air 

flow and decreasing or stable 

temperatures preceded by or followed 

by hours with similar conditions, 

suggesting sustained downslope air 

flows from higher elevations south of 

the international border; 

(3) any hour preceded by and 

followed by hours with southerly wind 

or air flow and decreasing or stable 

temperatures, suggesting continued 

influence of downslope air flow from 

higher elevations south of the 

international border; and, 

(4) surface wind speed less than or 

equal to 1.1 mph (0.5 m/s), preceded by 

or followed by hours with similar 

conditions, suggesting sustained air 

mass stagnation where PM10 emissions 

suspended in previous hours remain 

suspended in the stagnant air mass. 

The first decision rule identifies periods 

consistent with sustained high winds 

from the south carrying wind-blown 

PM10, as discussed earlier concerning 

the May 22, 2008 exceedance day. The 

second and third decision rules identify 

periods influenced by downslope wind 

flow conditions described in the 

conceptual model as usually occurring 

in the late afternoon and evening and 

transporting PM10 from higher 

elevations in Nogales, Mexico to lower 

elevations in the Nogales NA. The 

fourth decision rule identifies periods of 

sustained air mass stagnation usually 

found in the late night and early 

morning hours after the early evening 

downslope wind or air flow has ebbed 

and before sunrise, after which wind 

speeds begin to increase from their 

overnight low values. 


emcdonald on DSK67QTVN1PROD with PROPOSALS2
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55The observed concentrations and 

meteorological data for each hour of each 

exceedance day, the classification based on the 

criteria listed above, and the re-calculation of the 

estimated 24-hour average concentrations but for 

international transport are provided in Section 3.7 

of ‘‘Analysis of Ambient PM10 Levels, Topography, 

and Meteorological Data in Nogales, Arizona: 2007– 

2009’’ in Appendix D of the Nogales 2012 Plan. 

Using the low estimate of total 

Nogales Municipality, Mexico PM10

emissions, the analysis of emissions 

inventories discussed earlier showed 

that U.S. sources are responsible for a 

maximum of 36 percent of PM10

emissions in the Ambos Nogales region; 

see Table 9. Conversely, using the high 

estimate of total Nogales Municipality, 

Mexico emissions, U.S. sources are 

responsible for a minimum of 17 to 18 

percent of PM10 emissions in the Ambos 

Nogales region in 2008 and 2011, 

respectively. 

TABLE 9—2008 AND 2011 TOTAL PM10 EMISSION INVENTORIES: NOGALES NA, ARIZONA AND NOGALES MUNICIPALITY, 

MEXICO

[Low estimate, tons per year] 

2008 2011 Percent 

Nogales NA, Arizona ....................................................................................................... 1,524 1,521 36 

Nogales Municipality, Mexico .......................................................................................... 2,713 2,757 64 

Total Ambos Nogales Region .................................................................................. 4,237 4,278 100 

Source: Tables 6–7 from ‘‘Clean Air Act, Section 179B Attainment Determination for the Nogales, Arizona PM10 Nonattainment Area’’ in Appendix A of the Nogales 2012 Plan. 

Therefore, for each hour that meets 

one of the four criteria listed above, 

instead of assuming that the 

concentration is entirely due to Mexican 

sources, a more conservative 

assumption is that up to 36 percent of 

the hourly concentrations may be due to 

contributions from U.S. emission 

sources. Therefore, in this next step, the 

observed hourly concentrations were 

weighted by 0.36 for each hour that 

meets any one of the four criteria listed 

above and used this weighted 

concentration to estimate the 24-hour 

average concentration that would have 

occurred in the Nogales NA but for 

international transport from Mexico. 

To show the effects of each decision 

rule, an estimated 24-hour 

concentration was calculated after the 

application of Rule 1, Rules 2 and 3, 

Rules 1–3, and Rules 1–4. The results 

are summarized below.55

• The application of Rule 1 only 

removes one day, May 22, 2008, leaving 

28 days showing a concentration value 

greater than 150 mg/m3. 

• The application of Rules 2 and 3 

removes 27 days, leaving January 1, 

2007 and January 26, 2008 showing a 

concentration value greater than 150 mg/ 

m3; 196 mg/m3 and 244 mg/m3, 

respectively. 

• The application of Rules 1, 2, and 

3 again removes 27 days, leaving 

January 1, 2007 and January 26, 2008 

showing a concentration value greater 

than 150 mg/m3; 196 mg/m3 and 244 mg/ 

m3, respectively. 

• The application of Rules 1, 2, 3, and 

4 removes 29 days, leaving no estimated 

days with a value greater than 150 mg/ 

m3. The highest 24-hour average 

concentration estimated was 107 mg/m3. 

In sum, based on this analysis 

apportioning hourly concentration data 

using the four criteria to produce an 

estimated 24-hour average concentration 

but for international emissions, no 

exceedance days would have been 

expected to occur in the Nogales NA, 

but for transport from Mexico. 

Considering the relatively large 

differences in emissions inventories 

between the Nogales NA and Nogales 

Municipality, Mexico and the 

meteorology described by the 

conceptual model, it is likely that 

observed pollution during southerly 

downslope wind flows originating from 

Nogales, Mexico also contributed to 

observed pollution during following 

hours of sustained stagnation. With the 

wind direction varying under low wind 

speeds and stable temperatures, it 

remains possible, however, that a 

greater proportion of PM10 pollution 

during hours of sustained stagnation 

may be coming from U.S. sources. 

Therefore, a slightly more conservative 

approach would be to relax the decision 

rules by not considering sustained 

stagnation (Rule 4) and assign PM10

levels during these hours entirely to the 

Nogales NA. Consequently, when 

considering Mexican influence to only 

occur under conditions of relative high 

wind speeds (Rule 1) and sustained 

downslope wind flows from the south 

(Rules 2 and 3), two exceedance days 

would have been expected to occur but 

for international transport: January 1, 

2007 and January 26, 2008. Given the 

finding that no more than two 

exceedance days would have occurred 

applying criteria one through three, as 

determined by this hourly analysis of 

concentration data, the maximum 

expected number of annual exceedances 

is 0.7. 

3. Proposed Action on the Nogales 

Nonattainment Area Section 179B 

Analysis and Demonstration of 

Attainment but for International Sources 

of PM10 Emissions 

We propose to approve Arizona’s 

section 179B analysis and 

demonstration of attainment but for 

international sources of PM10 emissions. 

After meeting the PM10 NAAQS from 

1994–1997, an increasing number of 

exceedances occurred in the Nogales 

NA. While population in the Nogales 

NA has grown slightly since 1995, the 

Nogales Municipality population has 

increased 65 percent, such that in 2010, 

90 percent of the Ambos Nogales 

regional population is the Nogales 

Municipality, Mexico area. This 

difference in relative population and 

population growth over time supports 

the inference that a much larger 

proportion of PM10 in the Nogales NA 

comes from emissions sources on the 

Nogales, Mexico side of the 

international border. 

A comparison of 2008 and 2011 

emission inventories between the 

Nogales Municipality and the Nogales 

NA shows that pollution loads may 

differ by a ratio of 1.8–4.6 to one on a 

south-to-north basis relative to the 

international border. The Nogales NA 

contributes 17 to 36 percent of PM10

emissions in the Ambos Nogales region, 

depending on the emissions inventory 

estimate chosen for the Nogales 

Municipality, Mexico. Conversely, the 

Nogales Municipality, Mexico 

contributes 83 to 64 percent of PM10

emissions in the Ambos Nogales region. 

In its review of the ambient PM10

data, meteorological data, and through 

its analyses, Arizona found that the 

Ambos Nogales area’s meteorology and 

topography influence the observed 

exceedances of PM10 NAAQS and there 

is a definite south-to-north directional 

component to the ambient air quality 


emcdonald on DSK67QTVN1PROD with PROPOSALS2
38416 Federal Register / Vol. 77, No. 124 / Wednesday, June 27, 2012 / Proposed Rules 

56To put a rate of two miles of paving per year 

into context, we note that, by 1993, there remained 

approximately 10 miles of unpaved public roads 

within the city of Nogales. 

57For perspective on the county’s rate of paving/ 

chip sealing of unpaved roads, we note that as of 

2011 there were approximately 40 to 50 miles of 

unpaved roads remaining in the unincorporated 

area of the Nogales NA. 

58See 1993 Nogales PM10 Plan, pages 31 and 46. 

data underlying the exceedances of the 

PM10 NAAQS. Finally, daily and hourly 

analyses of the most recent three years 

of quality assured and State certified 

ambient PM10 and meteorological data 

from 2007–2009 show that no more than 

two, and likely none, of the 29 

exceedances would have occurred in the 

Nogales NA, but for PM10 emissions 

from Mexico. 

Based on these two exceedances, data 

completeness, and every day sampling 

for the 2007–2009 timeframe, the 

calculated maximum expected annual 

exceedance rate is 0.7 exceedances per 

year. The standard we use to 

demonstrate attainment of the PM10

NAAQS, ‘‘but for’’ international 

emissions, is that the expected number 

of days per calendar year with a 24-hour 

average concentration above 150 mg/m3

must be equal to or less than one. To 

conclude, we propose to determine that 

Arizona has met this standard and to 

approve their section 179B Analysis and 

demonstration of attainment but for 

international emissions for the Nogales 

NA. 

Even if a nonattainment area would 

have attained the PM10 NAAQS but for 

international transport of emissions 

from outside the U.S., section 179B still 

requires the area to meet the statutory 

requirements for a nonattainment plan. 

Section 179B suspends the obligation to 

provide an attainment demonstration 

showing actual attainment of the 

NAAQS, but a nonattainment area still 

has to meet basic requirements such as 

RACM/RACT, RFP and contingency 

measures. We will discuss how the 2012 

Nogales PM10 Plan addressed these 

requirements in the following sections 

of this proposed rule. 

C. Reasonably Available Control 

Measures (RACM)/Reasonably Available 

Control Technology (RACT) and 

Adopted Control Strategy 

1. Requirement for RACM/RACT 

CAA section 172(c)(1) requires that an 

attainment plan ‘‘provide for the 

implementation of all reasonably 

available control measures as 

expeditiously as practicable (including 

such reductions in emissions from 

existing sources in the area as may be 

obtained through the adoption, at a 

minimum, of reasonably available 

control technology), and shall provide 

for attainment of the national primary 

ambient air quality standards.’’ EPA 

defines RACM as measures that a State 

finds are both reasonably available and 

contribute to attainment as 

expeditiously as practicable in its 

nonattainment area. See also the 

General Preamble, 57 FR 13560; (April 

16, 1992). 

The General Preamble also discusses 

the moderate area PM10 requirements for 

RACM/RACT at section 189(a)(1)(C). As 

a starting point, a State should review 

the list of available control measures 

provided with the General Preamble and 

provide a reasoned judgment for 

rejecting any of these available control 

measures. A State may show that one or 

more control measures are unreasonable 

because emissions from those sources 

are insignificant within the 

nonattainment area; as such, those 

control measures would not be 

considered RACM for the nonattainment 

area. Any remaining control measures 

from the General Preamble list should 

then be evaluated for reasonableness 

according to their technological 

feasibility and cost of control. See 57 FR 

13540–13541; (April 16, 1992). 

The 1994 General Preamble 

Addendum also discusses the 

requirements for RACM as applied to 

nonattainment areas affected by 

international transport. In international 

border areas, ‘‘RACM/RACT must be 

implemented to the extent necessary to 

demonstrate attainment by the 

applicable attainment date if emissions 

emanating from outside the U.S. were 

not included in the analysis.’’ As set 

forth in section 179B(a)(2), a State’s 

moderate area PM10 plan must be 

‘‘adequate’’ to attain and maintain the 

PM10 NAAQS, but for emissions from 

outside the U.S. Therefore, nothing in 

section 179B relieves a State from the 

requirement to address and implement 

RACM. Nonetheless, States are not 

required to implement control measures 

that go beyond what the plan 

demonstrates would otherwise be 

adequate for attainment and 

maintenance of the PM10 NAAQS but 

for emissions from outside the U.S. See 

59 FR 42001; (August 16, 1994). For a 

nonattainment area making a showing 

under section 179B, the area is required 

to implement RACM/RACT sufficient to 

attain the standard by the applicable 

attainment date, but for emissions from 

outside the U.S., and to maintain the 

level of emissions from U.S. sources 

sufficient to provide for continued 

attainment of the NAAQS, but for the 

emissions from outside the U.S. 

2. RACM/RACT in the Nogales 

Nonattainment Area 

For the Nogales 2012 Plan, ADEQ 

reviewed the RACM/RACT 

demonstration from the 1993 Nogales 

PM10 Plan in light of the updated 

emissions inventories and section 179B 

demonstration and concluded that no 

additional RACM beyond that already 

implemented is required. In support of 

this conclusion, ADEQ describes the 

status of implementation of the RACM 

adopted as part of the 1993 Nogales 

PM10 Plan. Based on our review of both 

the 1993 plan and the current 2012 

plan, and for the reasons given below, 

we agree with ADEQ’s conclusion that 

no further RACM is required. 

First, we note that, based on the 

emissions inventories from the 1993 and 

2012 plans, entrainment of PM10 by 

vehicle travel over unpaved surfaces, 

primarily roads, remains the most 

significant source of PM10 emissions 

generated within the Nogales NA, and 

while PM10 emissions from this source 

are certainly lower than they would 

have been without additional paving, 

they still account for more than 50 

percent of the overall PM10 inventory in 

the Nogales NA. 

In the late 1980s, ADEQ, Santa Cruz 

County, and the city of Nogales 

recognized the importance of PM10

emissions from entrainment by vehicle 

travel over unpaved surfaces. To reduce 

such emissions, the city of Nogales 

undertook a program to pave the 

unpaved roads in the city, paving an 

average of two miles of unpaved roads 

per year from 1989 through 1992,56 to 

chip-seal the city’s equipment yard, and 

to pave the unpaved parking areas of 

Memorial Park and Neighborhood 

Center. Over this same period, within 

the unincorporated area of the Nogales 

NA, Santa Cruz County undertook a 

program to chip-seal unpaved county 

roads and chip-sealed approximately 2– 

3 miles of previously unpaved roads per 

year.57

Through the 1993 Nogales PM10 Plan, 

the city of Nogales committed to paving 

the remainder of its unpaved streets by 

1998, and Santa Cruz County committed 

to chip-seal at least one mile of unpaved 

road per year over 1993 and 1994 within 

the Nogales NA.58

The 1993 Nogales PM10 Plan also 

cited diesel-powered truck idling at two 

ports of entry (DeConcini and Mariposa) 

along the U.S. Mexico border in Nogales 

as a source of PM10 emissions within the 

Nogales NA and identified the reduction 

of idling time by such trucks as a RACM 

for implementation by the U.S. Customs 

Service. In response, the U.S. Customs 

Service committed to complete certain 


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59See 1993 Nogales PM10 Plan, page 30. 

60The estimated 24-hour average concentrations 

but for international transport for the 29 exceedance 

days are provided in Section 3.7 of ‘‘Analysis of 

Ambient PM10 Levels, Topography, and 

Meteorological Data in Nogales, Arizona: 2007– 

2009’’ in Appendix D of the Nogales 2012 Plan. 

61Memorandum, G.T. Helms, Chief, Ozone/ 

Carbon Monoxide Programs Branch to Air Directors, 

‘‘Contingency Measures for Ozone and Carbon 

Monoxide Redesignations,’’ June 1, 1992. 

capital improvements, including the 

addition of four north-bound lanes at 

the DeConcini Port of Entry (central 

business district within Nogales) and 

three north-bound lanes at the Mariposa 

Port of Entry (west of the central 

business district). 

Third, in the late 1980s and early 

1990s, the dragging of the unpaved 

border road by the U.S. Border Patrol (to 

detect fresh footprints) was considered 

another source of PM10 emissions 

contributing to ambient PM10

concentrations in Nogales. The 1993 

Nogales PM10 Plan does not identify 

RACM for this source. However, the 

1993 Nogales PM10 Plan notes that, in 

1992, the U.S. Border Patrol 

discontinued the practice of dragging a 

1.5-mile stretch of border road within 

the Nogales NA.59 The Border Patrol 

discontinued the practice over this 

stretch of road because it was 

ineffective. The road was also wired for 

movement sensors to detect human 

movement. These changes reduced this 

source of PM10 emissions within the 

Nogales NA. 

By the end of 1994, which was the 

applicable attainment date for the 

Nogales PM10 nonattainment area, the 

city of Nogales had paved an additional 

four miles of unpaved roads (beyond 

that completed through 1992); Santa 

Cruz County had paved an additional 

four miles of South River Road; and the 

U.S. Customs Service had completed the 

capital improvements described above 

at the DeConcini and Mariposa Ports of 

Entry. Together, these measures, in 

addition to those PM10-reducing 

measures completed in the late 1980s 

and early 1990s and certain other 

measures implemented outside of the 

SIP process (i.e., the discontinuance of 

dragging the border road), were 

sufficient to reduce PM10 concentrations 

in the Nogales NA such that maximum 

24-hour PM10 concentrations decreased 

from greater than 200 mg/m3 in the late 

1980s to less than 120 mg/m3 by 1994. 

Based on the data collected during the 

1992–1994 period, EPA determined that 

the Nogales area had attained the PM10

standard by the 1994 area’s statutory 

attainment date. See 76 FR 1532; 

(January 11, 2011). Thus, the measures 

implemented by the city of Nogales, 

Santa Cruz County, and U.S. Customs 

Service provided for attainment by the 

applicable attainment date and thereby 

met the RACM requirement. The 

Nogales 2012 Plan did not include the 

RACM commitments contained in the 

1993 Nogales PM10 Plan but, given their 

prior completion and permanent nature, 

we do not believe that the commitments 

need be made a part of the SIP. 

EPA does recognize that violations of 

the PM10 standard began to occur once 

again in Nogales beginning in 1998 and 

that such violations continue to the 

present, but, based on the section 179B 

demonstration contained in the 2012 

Nogales Plan, and evaluated in section 

IV.B herein, we do not believe that 

additional RACM are required to be 

implemented within the Nogales NA 

because we believe that the violations 

that have occurred since 1998 would 

not have occurred but for emissions 

from Mexico. 

Our conclusion in this regard 

recognizes that PM10 emissions in 

various important PM10 source 

categories are affected by changes in 

population, and whereas the population 

in the Nogales NA increased by 

approximately 5,000 persons during the 

20-year period from 1990 to 2010, the 

population in Nogales, Mexico 

increased by approximately 118,000 

persons during that same period. 

Moreover, the passage of the North 

American Fair Trade Agreement 

(NAFTA) in 1994 has continued to fuel 

the already high level of industrial 

(Maquiladoras) development on the 

Mexican side of the border. Most 

significantly, however, we note ADEQ’s 

detailed evaluation, as part of the 

section 179B demonstration, of the 29 

exceedances measured during the 2007– 

2009 period and determination that the 

highest 24-hour PM10 concentration in 

Nogales, but for emissions from Mexico, 

was 107 mg/m3, i.e., well below the 150 

mg/m3 standard.60 ADEQ’s section 179B 

demonstration, which we are proposing 

to approve, thus provides support for 

the conclusion that the violations that 

have occurred since 1998 would not 

have occurred but for the emissions 

from Mexico and thus no additional 

RACM need be implemented within the 

Nogales NA. 

D. Reasonable Further Progress 

Demonstration and Contingency 

Measures in the Nogales Nonattainment 

Area 

1. Reasonable Further Progress 

CAA section 172(c)(2) requires that 

plans for nonattainment areas shall 

provide for reasonable further progress 

(RFP). RFP is defined in section 171(1) 

as ‘‘such annual incremental reductions 

in emissions of the relevant air pollutant 

as are required by this part or may 

reasonably be required by the 

Administrator for the purpose of 

ensuring attainment of the applicable 

[NAAQS] by the applicable date.’’ 

The Nogales 2012 Plan cites EPA’s 

determination that the area attained the 

PM10 standard by the applicable 

attainment date as affirming that RFP 

requirements have been met. We agree 

that the RFP requirement was met in the 

Nogales NA by 1994 through the various 

paving projects and other measures 

implemented by the city of Nogales, 

Santa Cruz County, and U.S. Customs 

Service because the measures in fact 

provided the incremental reductions 

needed by the area to attain by the 

applicable attainment date (1994). In 

addition, for the same reasons that no 

additional RACM need be implemented 

in the Nogales NA, notwithstanding the 

advent of violations of the PM10

standard once again in 1998, we believe 

that no additional RFP demonstration 

must be submitted by ADEQ for this 

area. 

2. Contingency Measures 

Regarding contingency measures, 

under CAA section 172(c)(9), all 

attainment plans must include 

contingency measures to be 

implemented if an area fails to meet RFP 

(RFP contingency measures) and 

contingency measures to be 

implemented if an area fails to attain the 

PM10 NAAQS by the applicable 

attainment date (attainment contingency 

measures). These contingency measures 

must be fully adopted rules or control 

measures that are ready to be 

implemented quickly without 

significant additional action by the 

State. They must also be measures not 

relied on in the plan to demonstrate RFP 

or attainment and should provide SIPcreditable emissions reductions 

equivalent to one year of RFP. Finally, 

the SIP should contain trigger 

mechanisms for the contingency 

measures and specify a schedule for 

their implementation. 

EPA guidance also provides that 

contingency measures could be 

implemented early, i.e., prior to the 

milestone or attainment date.61

Consistent with this policy, states are 

allowed to use excess reductions from 

already adopted measures to meet the 

CAA section 172(c)(9) contingency 

measure requirement. This is because 

the purpose of contingency measures is 

to provide extra reductions that are not 


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62See Appendix E.4 of the Nogales 2012 Plan for 

aerial photography used in implementation review. 

63See Appendix E of the Nogales 2012 Plan for 

the Technical Support Document concerning the 

calculation of these emission reduction estimates. 

64Correspondence from Juan Guerra, City 

Engineer, City of Nogales, Arizona to James Wagner, 

ADEQ; April 11, 2012; see Appendix F.3 of Nogales 

2012 Plan. 

65See appendix E.4 of the Nogales 2012 Plan for 

aerial photography used in implementation review. 

66See Appendix E.2 of the Nogales 2012 Plan for 

supporting information from Santa Cruz County 

concerning paving/chip-sealing projects completed 

by the County. 

relied on for RFP or attainment that will 

provide for continued progress while 

the plan is being revised to fully address 

the failure to meet the required 

milestone or failure to meet the standard 

by the applicable attainment date. 

Nothing in the CAA precludes a State 

from implementing such measures 

before they are triggered. This approach 

has been approved in numerous SIPs. 

See 62 FR 15844; (April 3, 1997), 

(approval of the Indiana portion of the 

Chicago area 15 percent Rate of Progress 

plan); 66 FR 30811; (June 8, 2001), 

(proposed approval of the Rhode Island 

post-1996 ROP plan); and 66 FR 586 and 

66 FR 634; (January 3, 2001), (approval 

of the Massachusetts and Connecticut 1- 

hour ozone attainment demonstrations). 

In the only adjudicated challenge to this 

approach, the court upheld it. See 

Louisiana Environmental Action 

Network v. EPA, 382 F.3d 575 (5th Cir. 

2004). The Nogales 2012 Plan points to 

the paving projects that have been 

implemented since 1994 as meeting the 

contingency measure requirement for 

the Nogales NA and as the justification 

for not including any additional 

contingency measures in the 2012 

Nogales Plan. In assessing the extent of 

road paving in the Nogales NA, ADEQ 

consulted with officials in the city of 

Nogales and Santa Cruz County to 

determine the extent of road paving 

since 1992, when the Nogales NA began 

to record ambient PM10 levels below the 

NAAQS. 

As noted above, in the 1993 Nogales 

PM10 Plan, the city of Nogales 

committed to paving all public roads in 

the city by 1998. For the purposes of the 

Nogales 2012 Plan, ADEQ reviewed the 

status of implementation of the city’s 

paving program, and using aerial 

photography, ADEQ identified 11 

unpaved roads that were paved between 

1993 and 1996 totaling 8.4 miles.62

Among these 11 roads, ADEQ could 

locate traffic data for only nine of them 

(totaling 7.7 miles) from which to 

estimate the associated reduction in 

PM10 emissions. Based on the control 

effectiveness of paving and available 

traffic data, ADEQ estimated that paving 

of the nine roads between 1993 and 

1996 reduced PM10 emissions by 

approximately 80 tons per year. See 

Table 5.3 from the Nogales 2012 Plan.63

Assuming that half that reduction 

occurred after 1994, the resulting 

reduction that was surplus to the 

attainment needs for the Nogales NA 

was approximately 40 tons per year, 

although the actual reduction was 

greater than 40 tons per year because 

two specific roadways that were paved 

(but for which no traffic data was 

available) were not included in the 

calculation. ADEQ also checked on the 

status of the paving program with 

officials from the city of Nogales who 

reported that all of the unpaved public 

roads in Nogales have been paved and 

accepted into the City’s Street 

Maintenance Program.64

In a similar implementation review 

using aerial photography and data 

provided by Santa Cruz County, ADEQ 

estimated that Santa Cruz County 

paved/chip-sealed 40 miles of unpaved 

roads between 1994 and 2001 and an 

additional 40 miles of unpaved roads 

between 2002 and 2008. Traffic data 

was available, however, for only 

approximately 10 miles of the total 80 

miles of paving/chip-sealing in the postattainment era, but ADEQ estimates that 

paving/chip-sealing this subset of the 

larger amount reduced PM10 emissions 

in the Nogales NA by approximately 110 

tons per year. See Table 5.4 in the 2012 

Nogales Plan.65 66 Overall, Santa Cruz 

County and ADEQ provided different 

estimates of the number and extent of 

paved/chip-sealed roads and unpaved 

roads in the unincorporated area of the 

Nogales NA, but both sets of estimates 

indicate that more than 70 percent of 

the roads in the unincorporated area 

within the Nogales NA are paved/chipsealed at the present time. 

Based on our review of the data 

collected by ADEQ and presented in the 

Nogales 2012 Plan, we agree with ADEQ 

that post-1994 paving projects in the 

Nogales NA have provided PM10

emissions reductions beyond those 

relied upon by RFP or attainment and 

have also served to ensure that 

emissions generated within the Nogales 

NA do not cause a violation of the PM10

standard. The city of Nogales and Santa 

Cruz County did not wait until a 

triggering event to implement the 

paving projects but continued the 

paving programs that began in the late 

1980s and that helped the Nogales NA 

attain the standard by the applicable 

attainment date (1994). These projects 

have provided significant PM10

emissions reductions, i.e., greater than 

150 tons per year if all of the unpaved 

roads that were paved/chip-sealed were 

included, beyond that required for 

attainment by the applicable attainment 

date. 

We consider such ‘‘early’’ 

implementation of contingency 

measures to be acceptable in this 

instance because the associated 

emissions reductions provide extra 

reductions that are not relied upon for 

RFP or attainment and that provide 

extra assurance that no violations would 

occur in the Nogales NA but for 

emissions from Mexico. The 

effectiveness of implementation of the 

contingency measures is supported by 

the conclusion in ADEQ’s section 179B 

demonstration that estimates that the 

highest 24-hour PM10 concentration in 

Nogales, but for emissions from Mexico, 

during the 2007–2009 period was 107 

mg/m3, i.e., well below the 150 mg/m3

standard. Therefore, we conclude that 

implementation of the post-1994 paving 

projects in the Nogales NA meets the 

contingency measure requirement of 

section 172(c)(9). 

E. Motor Vehicle Emissions Budgets for 

Transportation Conformity 

1. Requirements for Transportation 

Conformity 

Transportation conformity is required 

by section 176(c) of the CAA. Actions 

involving Federal Highway 

Administration (FHWA) or Federal 

Transit Administration (FTA) funding 

or approval are subject to the EPA’s 

transportation conformity rule, codified 

at 40 CFR part 93, subpart A. Our 

transportation conformity rule requires 

that transportation plans, programs, and 

projects developed by Metropolitan 

Planning Organizations (MPOs) in 

nonattainment and maintenance areas 

conform to SIPs and establishes the 

criteria and procedures for determining 

whether or not they do so. Conformity 

to the SIP means that transportation 

activities will not cause or contribute to 

new air quality violations, worsen 

existing violations, or delay timely 

attainment of the national ambient air 

quality standards or any interim 

milestone. 

Control strategy SIP submittals (such 

as RFP and attainment SIP submittals) 

must specify the maximum emissions of 

transportation-related emissions from 

existing and planned highway and 

transit systems allowed in the 

appropriate years, i.e., the motor vehicle 

emissions budgets (MVEB or 

‘‘budgets’’). The submittal must also 

demonstrate that these transportationrelated emissions levels, when 

considered with emissions from all 


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67 40 CFR 93.118(e)(4)(iv) requires motor vehicle 

emissions budget(s), when considered together with 

all other emissions sources, to be consistent with 

applicable requirements for reasonable further 

progress, attainment, or maintenance (whichever is 

relevant to the given implementation plan 

submission). 

other sources, are consistent with RFP 

or attainment of the NAAQS, whichever 

is applicable. MPOs cannot use the 

budgets and the U.S. Department of 

Transportation (USDOT) cannot 

approve a Regional Transportation Plan 

(RTP) or Transportation Improvement 

Program (TIP) conformity analysis using 

the budgets until EPA had made an 

affirmative adequacy finding based on a 

preliminary review of the SIP. MPOs 

must use budgets in a submitted but not 

yet approved SIP, after EPA has 

determined that the budgets are 

adequate. For EPA to find these 

emissions levels or ‘‘budgets’’ adequate 

and/or approvable, the submittal must 

meet the conformity adequacy 

provisions of 40 CFR 93.118(e)(4) and 

(5). Also, motor vehicle emissions 

budgets cannot be approved until EPA 

completes a detailed review of the entire 

SIP and determines that the SIP and the 

budgets will achieve their intended 

purpose (i.e., RFP, attainment or 

maintenance). For more information on 

the transportation conformity 

requirement and applicable policies on 

budgets, please visit our transportation 

conformity Web site at: http:// 

www.epa.gov/otaq/stateresources/ 

transconf/index.htm. 

PM10 attainment and RFP plans 

should identify budgets for direct PM10

and PM10 attainment plan precursors. 

Direct PM10 budgets should include 

PM10 motor vehicle emissions from 

tailpipe, brake wear, and tire wear. 

States must also consider whether 

reentrained paved and unpaved road 

dust or highway and transit 

construction dust are significant 

contributors and should be included in 

the direct PM10 budget. (See 40 CFR 

93.102(b) and 93.122(e) and the 

conformity rule preamble at 69 FR 

40004, 40031–40036; (July 1, 2004)). 

The applicability of emission trading 

between conformity budgets for 

conformity purposes is described in 40 

CFR 93.124(c). 

2. Motor Vehicle Emissions Budget for 

the Nogales Nonattainment Area 

Usually, States are required to consult 

with local metropolitan planning 

organizations (MPOs) when developing 

a MVEB. The Nogales NA does not have 

an MPO. To develop the MVEB, ADEQ 

consulted with EPA and the Arizona 

Department of Transportation (ADOT). 

The Federal Highway Administration’s 

Highway Statistics statewide series data 

on Arizona shows a decline in vehicle 

miles traveled (VMT) between 2007 and 

2008, and no change in VMT between 

2008 and 2009. Emission inventory 

estimates for 2011 show a slight 

decrease in VMT. This trend is 

consistent with economic conditions. 

As discussed earlier in this proposed 

rule, the section 179B demonstration 

shows attainment of the PM10 standard 

in the Nogales NA, but for emissions 

from Mexico. The section 179B 

demonstration, proposed for approval 

herein, relies on a detailed analysis of 

PM10 exceedances that occurred during 

a specific three-year period (2007– 

2009), but assuming the 2007–2009 

period is representative of the postattainment date (1994) period, the 

conclusion that no violations would 

occur in Nogales but for emissions from 

Mexico can be applied throughout the 

post-attainment period. As such, there 

are several different years which are 

consistent with the applicable 

requirements for reasonable further 

progress and attainment, and which 

could be used for development of a 

MVEB.67 The State chose 2011 as the 

year for the MVEB. The MVEB was 

determined using information from the 

emissions inventories described in 

Chapter 3 and included in Appendix B 

of the Nogales 2012 Plan. 

The State’s estimated MVEB for the 

Nogales NA includes PM10 emissions 

from all on-road vehicle emissions 

source, and reentrained fugitive dust 

from unpaved and paved roads. EPA’s 

current MOVES (MOVES2010a) 

emissions model for on-road mobile 

sources was used to estimate the onroad motor vehicle portion of the 2011 

MVEB. MOVES estimates tailpipe 

emissions from cars, trucks, 

motorcycles, buses, as well as brake and 

tire wear. Secondary PM10 derived from 

PM10 precursors are not identified as 

sources of PM10 contributing to 

exceedances of the PM10 NAAQS in the 

Nogales NA, either in the emissions 

inventories or in the plan, in general. 

Fugitive emissions from paved and 

unpaved roads are affected by the 

number of VMT, silt volume on paved 

roads, and other local factors. Emissions 

estimates for these source categories 

were based on data obtained from State 

and federal agencies for the 2008 NEI. 

Estimates for Santa Cruz County were 

then apportioned to the Nogales NA 

based on population. The 2011 p.m.10 

motor vehicle emissions budget for the 

Nogales NA was estimated at 1,000.3 

tons per year. See Table 10. 

TABLE 10—2011 NOGALES NA PM10

MOTOR VEHICLE EMISSIONS BUDGET

[Tons] 

Source category PM10

Unpaved Road Dust ..................... 864.9 

Paved Road Dust ......................... 121.4 

On-road Motor Vehicle—Gasoline 2.6 

On-road Motor Vehicle—Diesel .... 11.4 

Total ....................................... 1,000.3 

Source: Table 7.1 of the Nogales 2012 Plan 

and ‘‘2008 and 2011 PM10 Emissions Inventories for the Nogales NA, Santa Cruz County, 

Arizona’’ in Appendix B of the Nogales 2012 

Plan. 

3. Proposed Action on the Motor 

Vehicle Emissions Budget for the 

Nogales Nonattainment Area 

We propose to approve the MVEB for 

the Nogales NA as submitted by ADEQ 

contingent upon ADEQ’s inclusion of 

road construction PM10 in the MVEB. 

Road construction PM10 should be 

included because, as the second largest 

source of PM10 emissions generated 

within the Nogales NA, road 

construction PM10 is a significant 

contributor to the overall Nogales NA 

PM10 inventory. See 40 CFR 93.122(e). 

As revised to include road construction 

PM10, we propose to approve the MVEB 

for three reasons. First, we find that the 

MVEB is derived from a comprehensive, 

accurate, and current emissions 

inventory that we believe meets the 

requirements of section 172(c)(3) of the 

CAA. Second, the MVEB includes all 

on-road sources of PM10 including 

fugitive dust emissions from unpaved 

and paved roads and will include road 

construction PM10, and was estimated 

using the latest motor vehicle emissions 

model available at the time of the 

emissions inventory was composed, the 

MOVES2010a model. Third, the MVEB 

are derived from emissions estimates 

used by ADEQ in the section 179B 

demonstration to show that the Nogales 

area would attain the PM10 standard, but 

for emissions from Mexico. 

VI. EPA’s Proposed Action and Request 

for Comment 

Based on our review, EPA proposes to 

approve this moderate area plan 

submitted by Arizona to attain the PM10

NAAQS for the Nogales nonattainment 

area. Specifically, under CAA section 

110(k)(3), EPA proposes to approve the 

following elements of the Nogales 2012 

p.m.10 attainment plan: 

(1) The 2008 base year and 2011 

emissions inventories as meeting the 

requirements of CAA section 172(c)(3); 

(2) the demonstration of attainment 

but for international emissions as 


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meeting the requirements of CAA 

section 179B(a)(1); 

(3) the implementation of paving 

projects and capital improvement 

projects at the Ports of Entry within the 

Nogales NA prior to the attainment 

deadline (1994) as meeting the RACM/ 

RACT requirements of CAA sections 

172(c)(1), 179B(a)(2), and 189(c)(1)(C); 

(4) the implementation of paving 

projects and capital improvement 

projects at the Ports of Entry to meet the 

RFP demonstration requirement of CAA 

sections 172(c)(2) and 179B(a)(2); 

(5) the implementation of post-1994 

paving projects as meeting the 

contingency measure requirements of 

CAA sections 172(c)(9) and 179B(a)(2); 

and, 

(6) the 2011 attainment year motor 

vehicle emissions budget if revised to 

include road construction PM10, 

because, as revised, it is derived from 

the section 179B demonstration and 

meets the requirements of CAA section 

176(c) and of 40 CFR 93, subpart A. 

Even with our proposed approval of 

Arizona’s demonstration that the 

Nogales NA is attaining the PM10

NAAQS but for international transport 

from Mexico, any final action resulting 

from this proposal would not constitute 

a redesignation to attainment under 

CAA section 107(d)(3) because we have 

not determined that the area has met the 

other CAA requirements for 

redesignation to attainment of the PM10

NAAQS. The classification and 

designation status in 40 CFR part 81 

would remain moderate nonattainment 

for the Nogales NA until such time as 

EPA determines that Arizona has met 

the CAA requirements for redesignating 

the Nogales NA to attainment for the 

PM10 NAAQS. EPA is soliciting public 

comments on the issues discussed in 

this Federal Register Notice. We will 

accept comments from the public on 

this proposal for the 30 days after 

publication of this proposed rule in the 

Federal Register. We will consider these 

comments before taking final action. 

VII. Statutory and Executive Order 

Reviews 

With this action, we propose to 

approve the moderate area PM10 plan 

submitted by Arizona for the Nogales 

NA and, if finalized, this proposed 

action would not impose additional 

requirements beyond those imposed by 

State law or by the CAA. For that 

reason, this proposed action: 

• Is not a ‘‘significant regulatory 

action’’ subject to review by the Office 

of Management and Budget under 

Executive Order 12866 (58 FR 51735, 

October 4, 1993); 

• Does not impose an information 

collection burden under the provisions 

of the Paperwork Reduction Act (44 

U.S.C. 3501 et seq.); 

• Is certified as not having a 

significant economic impact on a 

substantial number of small entities 

under the Regulatory Flexibility Act 

(5 U.S.C. 601 et seq.); 

• 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); 

• Does not have Federalism 

implications as specified in Executive 

Order 13132 (64 FR 43255, August 10, 

1999); 

• Is not an economically significant 

regulatory action based on health or 

safety risks subject to Executive Order 

13045 (62 FR 19885, April 23, 1997); 

• Is not a significant regulatory action 

subject to Executive Order 13211 (66 FR 

28355, May 22, 2001); 

• Is not subject to requirements of 

section 12(d) of the National 

Technology Transfer and Advancement 

Act of 1995 (15 U.S.C. 272 note) because 

application of those requirements would 

be inconsistent with the Clean Air Act; 

and 

• Does not provide EPA with the 

discretionary authority to address 

disproportionate human health or 

environmental effects with practical, 

appropriate, and legally permissible 

methods under Executive Order 12898 

(59 FR 7629, February 16, 1994). 

In addition, this proposed action does 

not have Tribal implications as 

specified by Executive Order 13175 (65 

FR 67249; November 9, 2000), because 

the SIP obligations discussed herein do 

not apply to Indian Tribes and thus will 

not impose substantial direct costs on 

Tribal governments or preempt Tribal 

law. 

List of Subjects in 40 CFR Part 52 

Environmental protection, Air 

pollution control, Intergovernmental 

relations, Particulate matter, Reporting 

and recordkeeping requirements. 

Authority: 42 U.S.C. 7401 et seq. 

Dated: June 20, 2012. 

Jared Blumenfeld, 

Regional Administrator, EPA Region IX. 

[FR Doc. 2012–15544 Filed 6–26–12; 8:45 am] 

BILLING CODE 6560–50–P 


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