1 

 

Technical Support Document for 

EPA’s Proposed Action on the 

State of Montana’s 

2010 Infrastructure State Implementation Plan 

(Transport Portion) for the 

2006 24-hour Fine Particulate Matter (PM2.5) 

National Ambient Air Quality Standard 

Air Program 

U.S. EPA Region 8 

May 2013 

 
2 

For information on this technical support document, please contact: 

Adam Clark 

Office of Partnerships and Regulatory Assistance 

U.S. Environmental Protection Agency – Region 8 

1595 Wynkoop St. 

Denver, Colorado 80202 

(303) 312-7104 

clark.adam@epa.gov

 
3 

Table of Contents 

I. Interstate Transport and Montana 

A. Clean Air Act Section 110(a)(2)(D)(i)(I) Requirements for the 2006 Fine 

Particulate Standards

 B. Summary of Transport Assessment 

 1. Transport to Nonattainment Receptors in Western States 

 2. Transport to Maintenance Receptors in Western States 

3. Transport to Nonattainment and Maintenance Receptors in Eastern States 

 C. Conclusion 

II. State of Montana’s Assessment of Transport 

A. Summary of State of Montana’s Assessment of Transport 

B. EPA’s Evaluation of the State of Montana’s Assessment of Transport 

III. Transport Assessment Methodology 

A. Discussion of Nonattainment and Maintenance Receptor Selection Methodology 

B. Factors Considered in the Transport Analysis for Nonattainment and Maintenance 

Receptors 

IV. Transport Assessment for Nonattainment Receptors 

A. Introduction 

 B. Transport to Nearby States 

 1. Idaho 

 C. Transport to Other Western States – Utah, Washington, Oregon, Nevada and 

California 

 1. Utah 

 2. Washington 

 3. Oregon 

 4. Nevada 

5. California 

D. Transport to Eastern States 

V. Transport Assessment for Maintenance Receptors 

A. Introduction 

 B. Transport to Western States 

1. Utah 

 2. California 

3. Arizona 

 C. Transport to Eastern States 

 
4 

Appendices 

Appendix A – Nonattainment and Maintenance Receptors List 

1. List of Nonattainment and Maintenance Receptors 

Appendix B – Nonattainment Receptors 

1. EPA Region 9 States – California and Nevada 

2. EPA Region 8 States – Utah 

3. EPA Region 10 States – Idaho, Oregon, and Washington 

 

Appendix C – Maintenance Receptors 

 1. EPA Region 9 States – California, Arizona 

2. EPA Region 8 States – Utah 

Appendix D – PM2.5 Data – IMPROVE Data for Utah, Nevada, Montana, Idaho, Wyoming and 

California/Oregon Border Sites for the Years 2006-2010 

 1. Map of IMPROVE Monitoring Sites 

 2. Utah – IMPROVE Data for the Years 2006-2010 

 

 3. Nevada – IMPROVE Data for the Years 2006-2010 

 4. California/Oregon Border – IMPROVE Data for the Years 2006-2010 

 5. Montana, Idaho and Wyoming – IMPROVE Monitor Values for the Years 2006-2010 
1 

I. Interstate Transport and Montana 

I.A. Clean Air Act Section 110(a)(2)(D)(i)(I) Requirements for the 2006 Fine Particulate

Standards

Section 110(a)(2)(D)(i)(I) of the Clean Air Act (CAA) requires each state to prohibit emissions 

that contribute significantly to nonattainment in, or interfere with maintenance by, any other state 

with respect to any primary or secondary national ambient air quality standard (NAAQS). The 

CAA requires states to submit plan provisions to address this requirement, among others, within 

three years after EPA’s promulgation of any new or revised NAAQS, or within such shorter 

period as EPA may prescribe. CAA section 110(a)(1). For purposes of implementing the 2006 

NAAQS for fine particulate matter (PM2.5), states were required to submit state implementation 

plan (SIP) provisions satisfying the requirements related to interstate transport in CAA section 

110(a)(2)(D)(i)(I) (among other section 110(a) requirements) by September 21, 2009. The rule 

establishing the revised PM2.5 NAAQS was signed by the Administrator and publically 

disseminated on September 21, 2006. Because EPA did not prescribe a shorter period for section 

110(a) infrastructure SIP submittals for these NAAQS, these submittals were due on September 

21, 2009, three years from the September 21, 2006 signature date pursuant to section 110(a)(1) 

of the CAA. See 42 U.S.C. 7410(a)(1). 

On February 10, 2010, the Montana Department of Environmental Quality (MDEQ) submitted a 

letter certifying that Montana’s SIP is adequate to implement the 2006 PM2.5 NAAQS for the 

“infrastructure” requirements of CAA section 110(a)(2), including the requirements of CAA 

section 110(a)(2)(D)(i)(I).1 Based on our evaluation of MDEQ’s submittals and additional 

technical information, as discussed further below, we are proposing to partially approve and 

partially disapprove the 2010 SIP with respect to the requirements of CAA section 

110(a)(2)(D)(i)(I) for the 2006 24-hour PM2.5 NAAQS. Based on our independent evaluation of 

relevant technical information, however, we are also proposing to conclude that the applicable 

implementation plan for Montana is adequate to address the requirements of CAA section 

110(a)(2)(D)(i)(I) for the 2006 24-hour PM2.5 NAAQS and that additional control measures in 

Montana are not necessary for this purpose because emissions from Montana sources do not 

contribute significantly to nonattainment or interfere with maintenance of these standards in any 

other state. 

I.B. Summary of Transport Assessment 

I.B.1. Transport to Nonattainment Receptors in Western States 

EPA reviewed the state of Montana’s PM2.5 submission and additional technical information to 

evaluate the potential for Montana emissions to contribute significantly to nonattainment of the 

2006 PM2.5 NAAQS at specified monitoring sites in the western United States. EPA first 

identified as “nonattainment receptors” all monitoring sites in the western states that had 

recorded PM2.5 design values above 35 micrograms per cubic meter (μg/m3), the level of the 

 

1

 MDEQ’s certification letter, dated February 10, 2010 is included in the docket for this action. 
2 

2006 24-hour PM2.5 NAAQS, during the years 2008-2010.2 See section III below (discussing 

EPA’s methodology for selection of nonattainment receptors, and providing a list and map of 

western receptors). 

Because geographic distance is a relevant factor in the assessment of potential pollution 

transport, EPA focused its review on information related to potential transport of PM2.5 

pollution from Montana to the only state bordering Montana that has nonattainment receptors: 

Idaho. EPA believes that the following factors support a finding that emissions from Montana do 

not significantly contribute to nonattainment of the 2006 24-hour PM2.5 NAAQS in Idaho: (1) 

technical information indicating that elevated PM2.5 levels at nonattainment receptors are 

predominantly caused by local emission sources, and (2) air quality data indicating that regional 

background levels of PM2.5 are generally low during the time periods of elevated PM2.5 at these 

receptors. 

EPA also evaluated potential PM2.5 transport to nonattainment receptors in the more distant 

western states of Oregon, Washington, Utah, Nevada, and California. EPA believes that the 

following factors support a finding that emissions from Montana do not significantly contribute 

to nonattainment of the 2006 24-hour PM2.5 NAAQS in any of these states: (1) the distance 

from the state of Montana to the nonattainment receptors in these states, and (2) technical 

information indicating that elevated PM2.5 levels at nonattainment receptors in these states are 

predominantly caused by local emission sources. 

We discuss these evaluations in more detail below. 

I.B.2. Transport to Maintenance Receptors in Western States 

EPA reviewed the state of Montana’s submission and additional technical information to 

evaluate the potential for Montana emissions to interfere with maintenance of the 2006 24-hour 

PM2.5 standards at specified monitoring sites in the western U.S. EPA first identified as 

“maintenance receptors” all monitoring sites in the western states that had recorded PM2.5 

design values above the level of the 2006 24-hour PM2.5 NAAQS (35 μg/m3) during the 2006-

2008 and/or 2007-2009 periods but below this standard during the 2008-2010 period.3

See

section III below (discussing EPA’s methodology for selection of maintenance receptors). All of 

the maintenance receptors in the western states are located in California, Utah and Arizona. EPA 

thus evaluated the potential for transported Montana emissions to interfere with maintenance of 

the NAAQS at receptor locations in each of these states. EPA believes that the following factors 

support a finding that emissions from Montana do not interfere with maintenance of the 2006 24-

hour PM2.5 NAAQS in these states: (1) technical information indicating that elevated PM2.5 

levels at maintenance receptors are predominantly caused by local emission sources; (2) in the 

case of California and Arizona maintenance receptors, distance. 

 

2

 Because the Clean Air Interstate Rule (CAIR) did not cover states in the western United States, these data are not 

significantly impacted by the remanded CAIR and thus could be considered in this analysis. In contrast, recent air 

quality data in the eastern, midwestern and southern states are significantly impacted by reductions associated with 

CAIR and because the Transport Rule was developed to replace CAIR, EPA could not consider reductions 

associated with the CAIR in the base case transport analysis for those states. See 76 FR at 48223-24. 

3

 As noted above, EPA can consider data from these years for states in the western United States as such data is not 

impacted by the remanded CAIR. 
3 

We discuss these evaluations in more detail below. 

I.B.3. Transport to Nonattainment and Maintenance Receptors in Eastern States 

EPA has also considered potential PM2.5 transport from Montana emission sources to the 

nearest potential nonattainment or maintenance receptors located in the eastern states within the 

domain of the modeling analysis for the Cross State Air Pollution Rule (“CSAPR,” also referred 

to as the “Transport Rule”) (76 FR 48208). The nearest nonattainment and/or maintenance 

receptor identified in the 2012 CSAPR modeling analysis is in Milwaukee, Wisconsin. The 

CSAPR modeling did not analyze the contribution of Montana sources to Milwaukee, 

Wisconsin. Thus, EPA analyzed the magnitude of Montana’s sulfur dioxide (SO2) and nitrogen 

oxides (NOx) emissions relative to states modeled in the CSAPR and found not to significantly 

contribute to nonattainment at that receptor. EPA also considered the distance of Montana from 

Milwaukee, Wisconsin relative to states whose contribution to that area was modeled in the 

CSAPR. EPA believes that the following factors support a finding that emissions from Montana 

sources do not significantly contribute to nonattainment or interfere with maintenance of the 

2006 24-hour PM2.5 NAAQS at this location: (1) the relatively small magnitude of the emission 

inventory of PM2.5 precursors (SO2 and NOx) in Montana combined with (2) the relatively long 

distance between the nearest portion of the state of Montana to the Milwaukee, Wisconsin 

receptor.4 EPA believes that these factors also support a finding that emissions from Montana 

sources do not contribute significantly to nonattainment or interfere with maintenance of the 

2006 24-hour PM2.5 NAAQS at any of the other CSAPR receptors, all of which are farther from 

Montana than Milwaukee, Wisconsin. 

I.C. Conclusion 

EPA’s review of available technical information and air quality data supports the conclusion in 

Montana’s PM2.5 transport analysis that emissions of direct PM2.5 and PM2.5 precursors from 

the state of Montana do not significantly contribute to nonattainment in, or interfere with 

maintenance by, any other state with respect to the 2006 24-hour PM2.5 standards. 

Based on our evaluation of technical information, as discussed further below, EPA proposes to 

conclude that emissions of direct PM2.5 and PM2.5 precursors from the state of Montana do not 

significantly contribute to nonattainment or interfere with maintenance of the 2006 24-hour 

PM2.5 standards in any other state and that CAA section 110(a)(2)(D)(i)(I) therefore does not 

require Montana to adopt additional controls to prohibit such impacts. 

II. State of Montana’s Assessment of Transport 

 

4

 Emission inventory data used for the CSAPR modeling analyses indicate that SO2 emissions in Montana are about 

22% of SO2 emissions in North Dakota, and that NOx emissions in Montana are about 73% of NOx emissions in 

North Dakota. Thus, the conclusion that North Dakota does not significantly contribute to nonattainment in 

Milwaukee, Wisconsin, which is also closer to North Dakota than to Montana, demonstrates that emissions from 

Montana are even less likely to significantly contribute to nonattainment in Milwaukee. See “Emissions Inventory 

Final Rule TSD,” Technical Support Document (TSD) for the final Transport Rule, Docket ID No. EPA-HQ-OAR2009-0491, June 28, 2011, at Tables 7-1 and 7-2. 
4 

II.A. Summary of State of Montana’s Assessment of Transport 

Montana’s PM2.5 transport analysis in the February 10, 2010 submission relies almost solely on 

EPA’s decision not to model contribution from Montana to other states for the Clean Air 

Interstate Rule (“CAIR”, 70 FR 25162, May 12, 2005). We do not consider this adequate 

analysis, in part because CAIR did not address the 2006 PM2.5 NAAQS. EPA’s 2011 CSAPR 

similarly did not include Montana amongst the states whose emissions were modeled for 

transportation purposes, and CSAPR did address the 2006 PM2.5 NAAQS. However, EPA’s 

decision in both CAIR and CSAPR to focus primarily on states in the Eastern and Central 

portion of the U.S. does not excuse those states in the Western U.S. from conducting a 

transportation analysis. MDEQ’s certification letter contains no technical analysis of potential 

interstate transport or any other support for the State’s conclusion that the existing Montana SIP 

satisfies the requirements of CAA section 110(a)(2)(D)(i)(I) for the 2006 24-hour PM2.5 

NAAQS. We propose to disapprove the 110(a)(2)(D)(i)(I) portion of Montana’s 2010 2006 

PM2.5 certification, in part, on these bases. 

As explained further below, however, EPA conducted an evaluation of potential transport of 

PM2.5 emissions from Montana using technical information consistent with the methodologies 

and analyses developed in the CSAPR in response to the judicial remand of the 2005 “Clean Air 

Interstate Rule.”5

 Based on our assessment of relevant technical information, including the 

characteristics of the formation of elevated PM2.5 at nonattainment and maintenance receptors in 

neighboring states, ambient PM2.5 levels at monitoring sites representative of regional 

background in nearby states, and meteorological and topographical information, we agree with 

the State’s conclusion in the 2010 submission that the existing Montana SIP satisfies the 

requirements of CAA section 110(a)(2)(D)(i)(I) for the 2006 PM2.5 NAAQS. Accordingly, we 

propose to approve the element D(i)(I) of Montana’s 2010 2006 PM2.5 submission, in part, 

based on our finding that emissions of direct PM2.5 and PM2.5 precursors from the state of 

Montana do not significantly contribute to nonattainment in, or interfere with maintenance by, 

any other state with respect to the 2006 24-hour PM2.5 standards and that CAA section 

110(a)(2)(D)(i)(I) therefore does not require additional controls in Montana to prohibit such 

impacts. 

Although the Clean Air Act obligates EPA to promulgate a Federal Implementation Plan (FIP) to 

correct a SIP deficiency associated with any final disapproval (see CAA 110(c)), we propose to 

conclude that any FIP obligation resulting from finalization of the partial disapproval would be 

satisfied by our determination that there is no deficiency in the SIP to correct. Finalization of 

this proposed disapproval also would not require any further action on the part of the State of 

Montana because EPA has determined that the SIP is adequate to meet the requirements of CAA 

section 110(a)(2)(D)(i)(I). 

II.B. EPA’s Evaluation of the State of Montana’s Assessment of Transport 

As stated, we do not consider Montana’s 2006 PM2.5 interstate transport analysis adequate. 

Therefore, we conducted our own technical evaluation to determine Montana’s potential 

 

5 CSAPR was vacated by the District of Columbia Circuit Court on August 21, 2012, and CAIR was reinstated until 

EPA corrects the alleged deficiencies described in the Court’s majority opinion in support of vacating CSAPR. 
5 

interstate transport contribution for this NAAQS. We conducted this evaluation using 

information consistent with the methodologies and analyses developed in the CSAPR despite 

that the rule was vacated on August 21, 2012, because the methodologies and technical 

information used for the purposes of this action were not among the portions of the CSAPR that 

were vacated. Our proposed action takes into account relevant technical information, including 

the characteristics of the formation of elevated PM2.5 at nonattainment and maintenance 

receptors in neighboring states, and ambient PM2.5 levels at monitoring sites representative of 

regional background in nearby states. This information is discussed further below. 

III. Transport Assessment Methodology 

III.A. Discussion of Nonattainment and Maintenance Receptor Selection Methodology 

On September 25, 2009, EPA issued a guidance memorandum that provides recommendations to 

states for making submissions to meet the requirements of CAA section 110(a)(2)(D)(i) for the 

2006 24-hour PM2.5 standards. See Memorandum from William T. Harnett, Director, Air 

Quality Policy Division, to Regional Air Division Directors, Regions I-X, “Guidance on SIP 

Elements Required Under Sections 110(a)(1) and (2) for the 2006 24-Hour Fine Particle (PM2.5) 

National Ambient Air Quality Standards (NAAQS),” September 25, 2009 (“2006 PM2.5 

NAAQS Infrastructure Guidance” or “Guidance” ).6

 The Guidance recommends that each state 

explain in its section 110(a)(2)(D)(i)(I) SIP whether or not its emissions contribute significantly 

to nonattainment or interfere with maintenance of the NAAQS in any other state, and to address 

any such impacts. See Guidance at 3, 4. The Guidance states as follows: 

The state's conclusion must be supported by an adequate technical analysis. Information 

to support the state's determination with respect to significant contribution to 

nonattainment might include, but is not limited to, information concerning emissions in 

the state, meteorological conditions in the state and the potentially impacted states, 

monitored ambient concentrations in the state and the potentially impacted states, the 

distance to the nearest area that is not attaining the NAAQS in another state, and air 

quality modeling. The EPA believes that it would be appropriate for states to make this 

assessment by considering the impact of current or future emissions on nearby 

nonattainment areas, and evaluating the air quality impact and potential mitigation 

strategies.7

To determine whether the CAA section 110(a)(2)(D)(i)(I) requirement is satisfied, EPA must 

determine whether a state’s emissions contribute significantly to nonattainment or interfere with 

maintenance in downwind areas. If this factual finding is in the negative, then section 

110(a)(2)(D)(i)(I) does not require any changes to a state’s SIP. If, however, the evaluation 

reveals that emissions from sources within the state do contribute significantly to nonattainment 

or interfere with maintenance in other states, then the state must adopt substantive provisions to 

eliminate those emissions. The state could achieve any required reductions through traditional 

command and control programs, or at its own election, through participation in a cap and trade 

program. Consistent with EPA's approach in the 1998 NOx SIP call, the 2005 CAIR, and the 

 

6

 Available at: http://www.epa.gov/ttn/caaa/t1/memoranda/20090925_harnett_pm25_sip_110a12.pdf 

7

 Guidance at 3. 
6 

2011 CSAPR,8 EPA is evaluating these impacts with respect to specific monitors identified as 

having nonattainment and/or maintenance problems, which we refer to as “receptors.”9 EPA 

notes that no single piece of information is by itself dispositive of the issue. Instead, the total 

weight of all the evidence taken together is used to evaluate significant contributions to 

nonattainment or interference with maintenance of the 2006 24-hour PM2.5 NAAQS in another 

state. 

A receptor may be a downwind location that is currently violating the PM2.5 NAAQS, has 

violated in the past, is projected to violate in the future, or may have trouble maintaining the 

NAAQS in the future. Nonattainment and maintenance receptors can be identified using future 

year modeling, recent ambient data, or both. 

If appropriate future year modeling is not available,10 nonattainment and maintenance receptors 

can be identified using recent ambient air quality monitoring data. For this action, we evaluated 

the three most recent consecutive 3-year periods for which complete, quality-assured and 

certified air quality data were available (covering a 5-year period) to calculate “design values”11

for those periods. Because EPA uses the most recent 3-year design value period for purposes of 

promulgating air quality designations and redesignations, it is logical to use the most recent 3-

year period to identify “nonattainment” receptors. However, previous violations of the NAAQS 

are indicative of an air quality problem that may return in the future. The year to year variability 

of emissions and/or meteorology may cause an area to have trouble maintaining the NAAQS in 

the future. Therefore, we have identified “maintenance” receptors by examining previous years 

of data. This methodology is conceptually similar (although not identical) to the methodology 

that EPA used in the CSAPR to identify “nonattainment” and “maintenance” receptors. 

We applied the following methodology: 

Similar to the CSAPR analysis, we examined ambient data which spanned a 5-year period. This 

allows examination of year to year variability. The three most recent consecutive three-year 

design value periods are 2006-2008, 2007-2009, and 2008-2010.12 

1) Nonattainment receptors are those sites that are violating the NAAQS in the most recent 

3-year period.13

 

 

8

See NOx SIP Call, 63 FR 57371 (October 27, 1998); CAIR, 70 FR 25172 (May 12, 2005); and CSAPR, 76 FR 

48208 (August 8, 2011). 

9

 Air quality designations are not relevant to the methodology for identifying these “receptors.” 

10 Available modeling data should be evaluated for the intended purpose. If modeling data is used to establish 

nonattainment and/or maintenance receptors, it should be for an appropriate future year (within the time frame of 

downwind attainment dates) and model performance should be shown to be acceptable for the upwind and 

downwind areas being considered. 

11 “Design value” means the “calculated concentration according to the applicable appendix of [40 CFR part 50] for 

the highest site in an attainment or nonattainment area.” 40 CFR 58.1 (definitions). 

12 As noted above, EPA can consider data from these years for states in the western United States as such data is not 

impacted by the remanded CAIR. 

13 When evaluating recent design values, it is not necessary to average the design values together to create a fiveyear weighted average. Weighted average data is only necessary when projecting modeling data to the future, since 

modeled projections are generally based on only a single year (or less) of modeled meteorology. 
7 

2) Maintenance receptors are those receptors that show attainment in the most recent 3-year 

design value period (2008-2010) but violated the NAAQS in at least one of the previous 

two periods (2006-2008 and/or 2007-2009).14

 

Ambient data included the three most recent official design value periods available at the time of 

the analysis. The official design values are posted by EPA’s Office of Air Quality Planning and 

Standards (OAQPS) on the Air Trends website and are available at: 

http://www.epa.gov/airtrends/values.html. A list of receptors selected for this interstate transport 

analysis is shown below in Figure III.A.1., and the corresponding map of receptors is shown 

below in Figure III.A.2. Receptors with PM2.5 design values above the PM2.5 standard of 

35μg/m3

 for the years 2008 through 2010 are listed as nonattainment receptors. Receptors with 

PM2.5 design values at or below the PM2.5 standard of 35μg/m3 for 2008-2010 but above the 

standard for at least one of the previous 3-year periods (2006-2008 or 2007-2009) are listed as 

maintenance receptors. Based on this methodology, New Mexico, Colorado, and Wyoming do 

not contain any nonattainment or maintenance receptors. 

To evaluate the potential for significant contribution to nonattainment in other states, EPA first 

evaluated potential transport to the nearest states with identified nonattainment receptors. The 

only state bordering Montana with nonattainment receptors is Idaho. EPA then evaluated 

potential transport to other western states with nonattainment receptors located farther away, i.e.,

Utah, Oregon, Washington Nevada, and California. To evaluate the potential for interference 

with maintenance in other states, EPA evaluated potential transport to maintenance receptors in 

Utah, Arizona and California, which are the only western states with identified maintenance 

receptors. 

 

 

14 Consistent with the Transport Rule methodology, EPA considers all nonattainment receptors to also be 

maintenance receptors, though not vice-versa. See 75 FR 45247. 
8 

Table III.A.1. List of Western Counties with Daily PM2.5 

Nonattainment or Maintenance Receptors 

State County AIRS ID Design Value 

2006-08 

Design Value 

2007-09 

Design Value 

2008-10 

Receptor 

Type 

Arizona Pinal 040213013 48 NA 40 NA 31 A Maintenance 

Arizona Santa Cruz 040230004 40 NA 31 A 32 A Maintenance 

California Alameda 060010007 36 NA 34 A 30 A Maintenance 

California Butte 060070002 69 NA 59 NA 51 NA Nonattainment 

California Fresno 060190008 58 NA 60 NA 54 NA Nonattainment 

California Imperial 060250005 36 NA 34 inc-a 32 inc-a Maintenance 

California Inyo 060271003 34 inc-a 38 NA 36 NA Nonattainment 

California Kern 060290016 70 NA 70 NA 65 NA Nonattainment 

California Kings 060310004 52 NA 53 NA 49 NA Nonattainment 

California Los Angeles 060370002 41 NA 42 NA 38 NA Nonattainment* 

California Merced 060472510 50 NA 51 NA 45 NA Nonattainment 

California Orange 060590007 38 NA 37 NA 30 A Maintenance 

California Riverside 060658005 53 NA 49 NA 41 NA Nonattainment* 

California Sacramento 060670006 54 NA 49 NA 40 NA Nonattainment* 

California San Bernardino 060712002 52 NA 48 NA 37 NA Nonattainment* 

California San Diego 060731011 9 inc-a 9 inc-a 43 NA Nonattainment 

California San Joaquin 060771002 51 NA 50 NA 44 NA Nonattainment 

California Santa Clara 060850005 36 NA 34 A 30 A Maintenance 

California Solano 060950004 36 NA 36 NA 31 A Maintenance 

California Stanislaus 060990006 67 inc-na 60 inc-na 55 NA Nonattainment 

California Sutter 061010003 47 NA 42 NA 36 NA Nonattainment 

California Tulare 061072002 57 NA 59 NA 51 NA Nonattainment 

Idaho Franklin 160410001 27 inc-a 31 inc-a 46 NA Nonattainment 

Idaho Shoshone 160790017 34 A 34 A 36 NA Nonattainment 

Montana Silver Bow 300930005 32 A 32 A 38 NA Nonattainment 

Nevada Washoe 320310016 35 A 39 NA 38 NA Nonattainment 

Oregon Klamath 410350004 46 NA 47 NA 45 NA Nonattainment 

Oregon Lake 410370001 41 inc-na 41 NA 38 NA Nonattainment 

Oregon Lane 410392013 40 NA 41 NA 38 NA Nonattainment 

Utah Box Elder 490030003 35 A 37 NA 42 NA Nonattainment 

Utah Cache 490050004 36 NA 40 NA 43 NA Nonattainment 

Utah Davis 490110004 35 A 38 NA 38 NA Nonattainment 

Utah Salt Lake 490350003 45 NA 48 NA 45 NA Nonattainment* 

Utah Utah 490494001 44 NA 50 NA 41 NA Nonattainment* 

Utah Weber 490570002 36 NA 41 NA 38 NA Nonattainment* 

Washington Pierce 530530029 44 NA 46 NA 38 NA Nonattainment 

Washington Yakima 530770009 36 inc-na 37 NA 37 NA Nonattainment 

NA = Not attaining; A = attaining; inc-na = incomplete/not attaining; inc-a = incomplete/attaining 

* = This county contains both nonattainment and maintenance receptors. See Appendix A of this TSD for more 

details. 

 
9 

Figure III.A.2: Map of Daily PM2.5 Nonattainment and Maintenance Receptors

III.B. Factors Considered in Transport Analysis for Nonattainment and Maintenance 

Receptors. 

Consistent with the recommendations in the Guidance, EPA reviewed emission inventory data, 

ambient monitoring data, and meteorological conditions within the state of Montana. In 

addition, EPA reviewed PM2.5 monitoring data in areas representative of western regional 

background levels of PM2.5 in selected remote monitoring sites in the west. Finally, EPA 

reviewed technical information related to each of the nonattainment and maintenance receptors 

in the potentially impacted states, including meteorological conditions and ambient PM2.5 

monitoring data. 

Emission inventories compiled for the CSAPR modeling analysis provide the following amounts 

of NOx and SO2 emissions for Montana in 2012: 97,575 tons of NOx, and 25,621 tons of SO2.15

The monitored ambient PM2.5 concentrations for nonattainment and maintenance receptors in 

the state of Montana are shown above in Figure III.A.1, “List of Daily PM2.5 Nonattainment and 

Maintenance Receptors.” 

 

15 See “Emissions Inventory Final Rule TSD,” Technical Support Document (TSD) for the final Transport Rule, 

Docket ID No. EPA-HQ-OAR-2009-0491. June 28, 2011, as Tables 7-1 and 7-2. 
10 

EPA then reviewed PM2.5 monitoring data from relatively remote ambient air monitoring sites 

that EPA believes are representative of regional background PM2.5 levels in the western United 

States. EPA reviewed five years of data collected at the IMPROVE16 monitoring network at 

locations in Utah (Bryce Canyon, Zion, and Capitol Reef National Parks), Idaho (Craters of the 

Moon Wilderness Area, Sawtooth Wilderness Area), Nevada (Jarbidge Wilderness Area, Great 

Basin National Park), Montana (Sula Peak, Cabinet Mountains Wilderness area, Gate of the 

Mountains Wilderness Area), Wyoming (Bridger Wilderness, North Absaroka Wilderness, 

Yellowstone National Park) and at the California-Oregon border (Lava Beds National Monument 

and Crater Lake National Park). Ambient PM2.5 monitoring data, as well as chemical species 

data (ammonium nitrate, ammonium sulfate, elemental carbon, organic carbon, sea salt, and soil) 

is available for each of these monitoring sites.17 We provide monitored PM2.5 data from each of 

these monitoring sites during the 2006-2010 period, for each day when measurements occurred, 

as well as a map of the site locations in Appendix D of this TSD. This provides information 

about regional background levels of PM2.5 in the states surrounding Montana. 

Most of the elevated levels of PM2.5 in the western states are measured during the winter 

months. For example, of the 17 days with measured values above the standard in 2010 in Salt 

Lake City, eleven days were in January (with levels up to 76μg/m3

 ), and four days were in 

December (with levels up to 53μg/m3 ). The remaining two days above the standard were March 

30 and April 27. 

The PM2.5 data for IMPROVE monitoring sites outside of Salt Lake City indicate low 

background PM2.5 levels during the winter months for each of the years from 2006 through 

2010. For example, the PM2.5 values for January and December of 2010 to the North 

(Yellowstone National Park, WY, Craters of the Moon Wilderness Area, ID) West (Great Basin 

National Park, NV, Jarbridge Wilderness Area, NV) and South (Bryce Canyon National Park, 

UT) of Salt Lake City are generally below 4μg/m3

.

18

In the winter, when the measured PM2.5 values in Salt Lake City are as high as 76 μg/m3, the 

PM2.5 values in remote areas are very low (below 3 μg/m3), indicating that most of the PM in 

the urban area is locally generated. Because the ambient PM2.5 at these locations appears to be 

from local sources, it is reasonable to conclude that emissions from Montana do not have a 

significant impact on nonattainment in Salt Lake City. In contrast, an analysis of the CSAPR 

2012 nonattainment and maintenance receptors shows that a large fraction of the daily PM2.5 

concentrations in Eastern areas are from upwind states. For the 41 CSAPR daily PM2.5 

 

16 The Interagency Monitoring of Protected Visual Environments (IMPROVE) program is a cooperative 

measurement effort governed by a steering committee composed of representatives from Federal and regional-state 

organizations. The IMPROVE monitoring program was established in 1985 to aid the creation of Federal and State 

implementation plans for the protection of visibility in Class I areas. The objectives of IMPROVE include 

establishing current aerosol conditions in mandatory class I areas; and identifying chemical species and emission 

sources responsible for existing man-made visibility impairment. 

http://vista.cira.colostate.edu/improve/Overview/IMPROVEProgram_files/frame.htm 

17 See site above, as well as http://views.cira.colostate.edu/web/. 

18 Id. The Craters of the Moon Wilderness Area monitor had the highest wintertime value of the monitors, about 6 

μg/m3, in January 2007. There were two other instances of monitors in these areas reading between 4 and 6 μg/m3 

during the winter months between 2006 and 2010. 
11 

nonattainment and maintenance receptors, the modeled upwind sulfate plus nitrate contribution 

to total PM2.5 ranged from a low of 20% to a high of 53% with a median value of 37%.19 Since 

CSAPR only tracked the contributions of sulfate and nitrate, these values represent the minimum 

percentage of upwind contribution to the receptors. Accounting for transport of additional 

primary PM2.5 species would only increase the upwind percentage and therefore decrease the 

percentage of PM2.5 due to local sources. 

In addition to reviewing background monitoring data which indicates that the elevated PM2.5 

levels are not likely due to regional transport, EPA reviewed technical information, such as 

meteorological conditions and monitored ambient data, related to each of the areas with 

nonattainment and maintenance receptors in the potentially impacted states to determine whether 

local emissions cause or contribute to the elevated PM2.5 levels at these locations. We discuss 

our evaluation for each of the areas with receptors in the following sections. 

IV. Transport Assessment for Nonattainment Receptors 

IV.A. Introduction 

Based on our methodology for selecting nonattainment receptors in Section III.A, above 

(Discussion of Nonattainment and Maintenance Receptor Selection Methodology), EPA has 

identified nonattainment receptors in seven western states: Utah, Nevada, California, Oregon, 

Washington, Idaho, and Montana. EPA has evaluated technical information regarding transport 

for each of the areas with nonattainment receptors identified above, following the methodology 

discussed in Section III. B. Transport to the state which borders Montana and has nonattainment 

receptors, Idaho, is addressed first. Transport to the more distant western states with 

nonattainment receptors (Utah, Washington, Oregon, Nevada and California) is then addressed. 

Finally, transport to eastern states is addressed. 

IV.B. Transport to Nearby States 

IV.B.1. Idaho 

EPA believes that the factors indicating that the elevated PM2.5 levels at the two Idaho 

nonattainment receptors are the result of locally generated emissions support our finding that 

emissions from Montana do not contribute to nonattainment of the 24-hour PM2.5 standard at 

the nonattainment receptors in the state of Idaho. 

Franklin County/Cache County

Portions of Franklin County, Idaho and Cache County, Utah are designated as part of the Cache 

Valley 2006 PM2.5, two-state, nonattainment area. Limited chemical speciation data in the 

Cache Valley indicates the PM2.5 problem is dominated by ammonium nitrate aerosol. Transport 

of PM2.5 and precursors from the rural regions at the domain periphery is very small and most of 

 

19 See Memorandum to File, Brian Timin, EPA, Office of Air Quality Planning and Standards, “CSAPR Daily 

PM2.5 Upwind Contribution,” July 2012.
12 

the observed Wasatch Front and Cache Valley PM2.5 is formed in-situ (and trapped) via fresh 

locally derived emission sources and secondary chemistry. 

Local heating emissions from woodstoves, emissions from agricultural activities, and mobile 

emissions may contribute to violations of the PM2.5 NAAQS during stable weather events 

associated with extremely strong inversions as they are trapped in the Cache Valley due to 

temperature inversions, low wind, and local topography. Because of the local nature of the 

exceedances in the Cache Valley, we find that emissions from Montana do not significantly 

contribute to nonattainment of the 24-hour PM2.5 standard at receptors in this area.

Shoshone County (Pinehurst)

Information from the state of Idaho indicates that emissions from woodstoves contribute to 

primary PM2.5 that violates the standard during stable weather events associated with strong 

inversions. These emissions and the related effects are limited to the city of Pinehurst airshed, as 

they are trapped there due to temperature inversions, low wind, and local topography. 

Additionally, the IMPROVE PM2.5 monitoring data for the Cabinet Mountains Wilderness Area 

and Sula Peak, which are on the Western edge of Montana to the Northeast and Southeast of the 

Pinehurst monitor, show very low levels of PM2.5 (below 5μg/m3) from November through 

February for the years 2007 through 2010. Because all exceedances of the standard in Pinehurst 

occurred during the winter months of these years, this is further evidence that the PM2.5 issue in 

Pinehurst is driven by local emissions rather than by transported emissions from Montana. The 

Montana/Idaho border is also very mountainous, further decreasing the likelihood of transport 

from Montana sources to the Pinehurst monitor. 

The local nature of the PM2.5 levels during the time periods when the standard is exceeded, the 

low levels of background PM2.5 during those time periods, and the mountainous intervening 

topography between Montana and the Pinehurst monitor support our finding that emissions from 

the state of Montana do not contribute significantly to nonattainment of the PM2.5 standard at 

the Pinehurst nonattainment receptor. 

IV. C. Transport to Other Western States – Utah, Washington, Oregon, Nevada and 

California, 

IV.C.1. Utah20

 

As noted in section III.B., the background monitoring sites to the north, south and west of the 

Utah nonattainment receptors show very low levels of emissions (below 4 μg/m3

)

21 during the 

winter months when most exceedances in the Utah nonattainment areas occur.22 These low 

background PM2.5 levels indicate that exceedances in these nonattainment areas are the result of 

local emissions rather than transport. Below, we provide additional information which further 

 

20 For discussion of the Idaho/Utah – Franklin County/Cache County area, see section IV.B.1 Idaho of this TSD. 

21 There were three instances of these monitors reading between 4 and 6 μg/m3 during the winter months between 

2006 and 2010. 

22 See Appendix E at E.1. 
13 

indicates that elevated PM2.5 levels in the Salt Lake City and Provo 2006 PM2.5 nonattainment 

areas are driven by local emissions. All of this evidence indicates that emissions from the state of 

Montana do not contribute significantly to nonattainment of the PM2.5 standard at these 

nonattainment receptors. 

Box Elder, Davis, Salt Lake, Tooele, and Weber Counties (Greater Salt Lake Area)

All of Davis and Salt Lake counties, and portions of Box Elder, Tooele and Weber counties are 

designated as part of the Salt Lake City (SLC) 2006 PM2.5 nonattainment area (NAA). The SLC 

NAA is situated in a valley bordered on the west by the Stansbury Mountains,23 Promontory 

Mountains and Great Salt Lake, and on the east by the Wasatch Front. During inversions, the 

NAA becomes a closed airshed (save for potential transport between the Salt Lake City and 

Provo NAAs) featuring stagnant air masses and light winds which typically oscillate back and 

forth within the airshed. As the airshed is essentially capped during inversions, PM2.5 is formed 

in-situ (and trapped) via fresh locally derived emission sources and secondary chemistry.24 

Speciation data from wintertime PM2.5 exceedance days from 2004-2006 showed the average 

composition of PM2.5 in Salt Lake City is 58% ammonium nitrate, 31% carbon and organic 

compounds, 8% ammonium sulfate, and 2% crustal. Several factors, including significant 

mobile, point, and area sources contribute to primarily and secondarily formed PM2.5 that 

violate the standard during stable weather events associated with strong inversions. 

Utah County (Provo-Orem)

A large portion of Utah County is designated as the Provo 2006 PM2.5 NAA. The Provo NAA is 

situated in the Utah Valley. Like the other low-lying valleys just west of the Wasatch Front, the 

topography of the Utah Valley makes it prone to temperature inversions. During inversions, the 

Provo NAA features stagnant air masses and calm to light winds. As the airshed is essentially 

capped, PM2.5 is formed in-situ (and trapped) via fresh locally derived emission sources and 

secondary chemistry.25 Transport of air masses between the Salt Lake City and Provo NAAs can 

occur during inversions, as there is a 4.75-mile gap in the mountains separating these airsheds 

that is below the approximate average inversion height for these areas.26 

Speciation data from wintertime PM2.5 exceedance days from 2004-2006 showed the average 

composition of PM2.5 in Provo is 71% ammonium nitrate, with 21% carbon and organic 

compounds, 8% ammonium sulfate, and 2% crustal. Several factors, including significant 

amounts of emissions from mobile, point, and area sources, contribute to ambient levels of 

primarily and secondarily formed PM2.5 that violate the standard during stable weather events 

associated with strong inversions. 

 

23 The Quqirrh Mountains lie between the Wasatch Front and the Stansbury Mountains. However, EPA determined 

that some areas located west of the Quqirrh Mountains should still be considered part of the SLC nonattainment 

area. 

24 2006 24-Hour PM2.5 NAAQS Designations (74 FR 58688, November 13, 2009), Technical Support Document, 

December 2008, Index 583-669 at 40. 

25 Id.

26 Id. at 41. 
14 

IV.C.2. Washington 

Given the factors that indicate the elevated PM2.5 levels at these locations are caused by local 

emissions, the mountainous topography between Washington receptors and Montana, and the 

general west to east wind direction in the western US, we believe it is reasonable to conclude that 

emissions from Montana do not significantly contribute to nonattainment of the 24-hour PM2.5 

standard at either of the two nonattainment receptor locations in the state of Washington. 

Pierce County (Tacoma)

The state of Washington’s analysis indicates that wood smoke associated with home heating is the 

largest source of PM2.5 in the Tacoma-Pierce County nonattainment area. Surface measurements 

at multiple sites suggest that the only design values which violate the 2006 PM2.5 NAAQS are in 

the core of the nonattainment area, within about 3 kilometers (km) of the South L St. monitor. 

Beyond about 3-5km, the design values drop and are well below the NAAQS. 

Yakima County (Yakima)

EPA Region 10’s Positive Matrix Factor receptor modeling identified 10 factors contributing to 

elevated wintertime PM2.5 including wood smoke (24%), nitrate rich (24%), gasoline vehicles 

(16%), organic pyrolysis rich (9%), sulfate rich (7%), and smaller contributions from other 

identified factors (iron rich, sea salt, soil, zinc rich, and urban unidentified). 

IV.C.3. Oregon 

The available data for each of the three areas with nonattainment receptors, discussed below, 

indicate that the cause of elevated PM2.5 levels is local. Given the factors that indicate the 

elevated PM2.5 levels at these locations are caused by local emissions, together with the 

mountainous intervening topography and general west to east wind direction in the western U.S., 

we believe it is reasonable to conclude that emissions from Montana sources do not significantly 

contribute to nonattainment of the 2006 24-hour PM2.5 standard at these receptor locations. 

Klamath County (Klamath Falls)

 

A portion of Klamath County is designated nonattainment for the 2006 PM2.5 NAAQS. Klamath 

Falls is a relatively small urban community located in and supporting a vast rural area in southcentral Oregon. 

The state of Oregon’s analysis includes a 2005 emission inventory that shows that wood stove 

emissions are the most significant source of emissions in Klamath Falls. Additional air quality 

monitoring data provide supporting evidence that residential wood combustion emissions are the 

primary cause of exceedance days. Data show that exceedances occur during winter months, at a 

time when woodstoves are contributing to smoke in the area. Exceedances occurred from 

December through January, corresponding directly with the winter wood heating season, 

indicating the exceedances (except for flagged wildfire events) are a seasonal occurrence. 
15 

Speciation data analysis indicates most of the PM2.5 comes from elemental and organic carbon, 

typical indicators of combustion activities such as wood burning. Meteorological data from 

Klamath Falls show that low wind speeds dominate during violating days, and the Klamath Falls 

area experiences frequent winter-time nocturnal inversions, when PM pollution can build up over 

time. Diurnal concentrations of PM2.5 are observed by hourly light scatter data that correspond 

closely with night-time use of woodstoves. Night-time inversions coupled with low wind speeds 

strongly suggest that pollution in the Klamath Falls area is a localized effect heavily influenced 

by the use of woodstoves. 

EPA Region 10’s Positive Matrix Factor receptor modeling identified the 6 factors contributing 

to elevated wintertime PM2.5 including wood smoke (65%), organic pyrolysis rich (23%), and 

smaller contributions by other identified factors (nitrate rich, soil, sulfate rich, and urban 

unidentified). 

Lane County (Oakridge)

A portion of Lane County is designated nonattainment for the 2006 PM2.5 NAAQS. 

EPA’s technical analysis points to residential home heating, using wood burning appliances, as 

the main contributors to fine particle pollution in this area. EPA Region 10’s Positive Matrix 

Factor receptor modeling identified seven factors which contribute to elevated wintertime PM2.5 

including wood smoke (76%), organic pyrolysis rich (15%), as well as smaller contributions by 

other identified factors (gasoline vehicles, iron rich, sulfate rich, sea salt, and soil). 

PM2.5 monitoring data from the Crater Lake National Park and Lava Beds IMPROVE 

monitoring sites27 are representative of regional background PM2.5 in southern Oregon. 

Because PM2.5 levels at these monitors are consistently low (below 3μg/m3) in the wintertime, 

this indicates that PM2.5 levels at this receptor are driven by local sources. 

Lake County (Lakeview)

EPA Region 10’s Positive Matrix Factor receptor modeling identified seven factors contributing 

to PM2.5 including wood smoke (67%), organic pyrolysis rich (13%), nitrate rich (7%), soil 

(5%), and smaller contributions by other identified factors (sulfate rich and urban unidentified). 

PM2.5 monitoring data from the Crater Lake National Park and Lava Beds IMPROVE 

monitoring sites28 are representative of regional background PM2.5 in southern Oregon, and are 

consistently low (below 3μg/m3) in the wintertime. This information indicates that PM2.5 levels 

at this monitor are driven by local sources. 

 

27 The Interagency Monitoring of Protected Visual Environments (IMPROVE) program is a cooperative 

measurement effort governed by a steering committee composed of representatives from Federal and regional-state 

organizations. The IMPROVE monitoring program was established in 1985 to aid the creation of Federal and State 

implementation plans for the protection of visibility in Class I areas. The objectives of IMPROVE include 

establishing current aerosol conditions in mandatory class I areas; and identifying chemical species and emission 

sources responsible for existing man-made visibility impairment. 

28 Id.
16 

IV.C.4. Nevada 

PM2.5 formation at the nonattainment receptor in Nevada is discussed in detail in Appendix B 

and is summarized below. 

Elevated PM2.5 levels in the Reno area occur during wintertime stagnation episodes. The highest 

values are generally measured in January and December.29 Wildfires in June and July of 2008 

had a large impact on PM2.5 concentrations in this area; however, the Washoe County Health 

District has flagged the affected data as exceptional events.30

 

The local nature of the PM2.5 levels during the time period when the standard is exceeded31 and 

the distance between the Reno receptor and Montana support our finding that emissions from the 

state of Montana do not contribute significantly to nonattainment of the PM2.5 standard at this 

nonattainment receptor. 

IV.C.5. California 

Transport to nonattainment receptors in California is discussed in detail in Appendix B and is 

summarized below. 

Northern California – Butte, Sacramento, and Sutter Counties

In Northern California, monitored exceedances of the PM2.5 standard appear to result from 

wintertime temperature inversions in which low winds persist for several days in an area where 

the complex topography traps localized emissions. The speciation data from high concentration 

days indicates that as much as 75% of the concentrations are due to organic carbon, which also 

indicates that residential wood burning emissions are the dominant source.32 Given the factors 

that indicate the elevated PM2.5 levels at these locations are caused by local emissions and the 

distance of the area from Montana, it is reasonable to conclude that emissions from Montana do 

not significantly contribute to nonattainment of the 2006 24-hour PM2.5 standard at these 

receptors. 

Central California – Fresno, Kern, Kings, Merced, San Joaquin, Stanislaus, and Tulare Counties

In Central California, monitored exceedances of the PM2.5 standard appear to result from 

wintertime temperature inversions in which low winds persist for several days in an area where 

the complex topography traps localized emissions. The speciation data from high concentration 

days indicate high levels of secondary nitrate. On some days elevated levels of organic carbon 

indicate that residential wood burning also contributes to elevated levels of PM2.5. 

 

29 Washoe County, Nevada Air Quality Trends (2002-2011). 

http://www.co.washoe.nv.us/repository/files/4/Washoe-County-Air-QualityTrends-2002-2011.pdf 

30 Washoe County, Nevada Air Quality Trends (2002-2011). 

http://www.co.washoe.nv.us/repository/files/4/Washoe-County-Air-QualityTrends-2002-2011.pdf 

31 See Appendix B of this TSD. 

32 ARB Almanac website (2006) http://www.arb.ca.gov/ei/maps/statemap/cntymap.htm) 
17 

Given the factors that indicate the elevated PM2.5 levels at these locations are caused by local 

emissions, together with mountainous intervening topography and the distance of the area from 

Montana, we believe it is reasonable to conclude that emissions from Montana sources do not 

significantly contribute to nonattainment of the 2006 24-hour PM2.5 standard at these receptor 

locations. 

Southern California – Los Angeles, Riverside, and San Bernardino Counties

PM2.5 in Southern California is essentially a combustion-generated pollutant and is associated 

with the volume of traffic and numbers of sources (both point and area) located in the region. It 

is important to note that the areas with the highest concentrations of PM2.5 are directly 

downwind of an area with major ammonia sources associated with dairies and poultry farming. 

Eastern California – Inyo County

Elevated levels of PM2.5 in Inyo County are measured at a single monitor, the Keeler monitor, 

which is located adjacent to the Owens Lake Bed. The Owens Lake Bed is surrounded by steep 

terrain. EPA believes that given the proximity of the monitor to a large local source of particulate 

matter and the large distance and mountainous topography separating this area from Montana, it 

is reasonable to conclude that emissions from Montana sources do not significantly contribute to 

nonattainment of the 2006 24-hour PM2.5 standard at this monitor. 

California/Mexico Border – San Diego County 

The La Posta receptor is classified as a nonattainment receptor, based on a single measured 

PM2.5 value of 114 μg/m3 measured on June 1, 2010. All other measured values of PM2.5 at the 

La Posta monitoring site are very low. On June 1, 2010, the La Posta monitor recorded the 

highest measured value in the entire state of California, and 99% of the PM2.5 values for the rest 

of the State were below 20 μg/m3. 

The level of PM2.5 measured at the La Posta monitor on June 1, 2010 appears to be from local 

sources, which together with complex topography and the distance of the area from Montana 

supports our finding that emissions from Montana sources do not significantly contribute to the 

elevated PM2.5 levels at this location. 

IV.D. Transport to Eastern States 

EPA has also considered potential PM2.5 transport from Montana emission sources to the 

nearest potential nonattainment or maintenance receptors located in the eastern states within the 

domain of the modeling analysis for the CSAPR. The nearest nonattainment and/or maintenance 

receptor identified in the 2012 CSAPR modeling analysis is in Milwaukee, Wisconsin. The 

CSAPR modeling did not analyze the contribution of Montana sources to any CSAPR receptors, 

including the receptor in Milwaukee, Wisconsin. Thus, EPA analyzed the magnitude of 

Montana’s SO2 and NOx emissions relative to states whose contribution to the Milwaukee 

receptor was modeled for the CSAPR. EPA also considered the distance of Montana from 

Milwaukee, Wisconsin relative to states modeled for the CSAPR. EPA believes that the 

following factors support a finding that emissions from Montana sources do not significantly 
18 

contribute to nonattainment or interfere with maintenance of the 2006 24-hour PM2.5 NAAQS at 

this location: (1) the relatively small magnitude of the emission inventory of PM2.5 precursors 

(SO2 and NOx) in Montana combined with (2) the relatively long distance between the nearest 

portion of the state of Montana to the Milwaukee, Wisconsin receptor.33 EPA believes that these 

factors also support a finding that emissions from Montana sources do not contribute 

significantly to nonattainment or interfere with maintenance of the 2006 24-hour PM2.5 NAAQS 

at any of the other CSAPR receptors, all of which are farther from Montana than Milwaukee, 

Wisconsin. 

North Dakota was modeled for CSAPR, and was found below the threshold of significant 

contribution to downwind nonattainment and interference with maintenance for all receptors 

modeled for CSAPR. EPA has evaluated the relative amounts of PM2.5 precursor (SO2 and 

NOx) emissions in Montana compared to the amounts of PM2.5 precursor emissions in North 

Dakota, as modeled in the 2012 CSAPR “base case.” The emission inventories used in the 

CSAPR modeling analyses indicate that SO2 emissions in Montana are about 22% of SO2 

emissions in North Dakota, and NOx emissions in Montana are about 73% of NOx emissions in 

North Dakota. Specifically, the CSAPR “2012 Base Case” inventories provide the following 

statewide emissions inventory data for these two states: 97,575 tons of NOx and 25,621 tons of 

SO2 from Montana emission sources, compared to 133,332 tons of NOx and 118,490 tons of 

SO2 from North Dakota emission sources.34 Even though the relationship between upwind 

emissions and downwind secondary PM2.5 formation is not linear, we would expect the 

Montana PM2.5 contribution to Wisconsin to be less than that of North Dakota and therefore 

well below the 1% threshold. In addition, the distance from the closest part of Montana to 

Milwaukee, WI is over 300 miles further than the distance from the closest part of North Dakota 

to Milwaukee, WI. The distance from Montana to the other eastern receptors is even greater, and 

therefore, the expected impact somewhat smaller. Given all of these factors, EPA believes it is 

reasonable to conclude that emissions of PM2.5 and PM2.5 precursors from Montana sources do 

not contribute significantly to nonattainment or interfere with maintenance of the 2006 24-hour 

PM2.5 NAAQS in any eastern state. 

V. Transport Assessment for Maintenance Receptors 

V.A. Introduction 

Based on our methodology for selecting maintenance receptors in Section III.A, above 

(Discussion of Nonattainment and Maintenance Receptor Selection Methodology), EPA has 

identified maintenance receptors in only three western states: California, Utah, and Arizona. 

Thus, to evaluate the potential for Montana emissions to interfere with maintenance in other 

states, we have evaluated technical information related to the maintenance receptors in all three 

states. 

V.B. Transport to Western States 

V.B.1. Utah 

 

33 See “Emissions Inventory Final Rule TSD,” Technical Support Document (TSD) for the final Transport Rule, 

Docket ID No. EPA-HQ-OAR-2009-0491, June 28, 2011, at Tables 7-1 and 7-2. 

34 Id.
19 

Elevated levels of PM2.5 in the Utah nonattainment areas occur primarily in the winter months.35

The levels of elevated PM2.5 in the Salt Lake City 2006 PM2.5 nonattainment area and the 

Provo 2006 PM2.5 nonattainment area are primarily due to local emissions, as noted in the 

discussion of these nonattainment areas below. Because of the local nature of these exceedances, 

we find that emissions from Montana do not interfere with maintenance of the 24-hour PM2.5 

standard at these receptors. 

Box Elder, Davis, Salt Lake, Tooele, and Weber Counties (Greater Salt Lake Area)

All of Davis and Salt Lake counties, and portions of Box Elder, Tooele, and Weber counties are 

designated as part of the Salt Lake City 2006 PM2.5 NAA. The SLC NAA is situated in a valley 

bordered on the west by the Stansbury Mountains,36 the Promontory Mountains, and the Great 

Salt Lake, and on the east by the Wasatch Front. During inversions, the NAA becomes a closed 

airshed (save for potential transport between the Salt Lake City and Provo NAAs) featuring 

stagnant air masses and light winds which typically oscillate back and forth within the airshed. 

As the airshed is essentially capped during inversions, PM2.5 is formed in-situ (and trapped) via 

fresh locally derived emission sources and secondary chemistry.37

 

Speciation data from wintertime PM2.5 exceedance days from 2004-2006 showed the average 

composition of PM2.5 in Salt Lake City is 58% ammonium nitrate, 31% carbon and organic 

compounds, 8% ammonium sulfate, and 2% crustal. Several factors, including significant 

emissions from mobile, point and area sources, contribute to primary and secondarily formed 

PM2.5 that violate the standard during stable weather events associated with strong inversions. 

Utah County (Provo-Orem)

A large portion of Utah County is designated as the Provo 2006 PM2.5 nonattainment area. The 

Provo NAA is situated in the Utah Valley. Like the other low-lying valleys just west of the 

Wasatch Front, the topography of the Utah Valley makes it prone to temperature inversions. 

During inversions, the Provo NAA features stagnant air masses and calm to light winds. As the 

airshed is essentially capped, PM2.5 is formed in-situ (and trapped) via fresh locally derived 

emission sources and secondary chemistry.38 Transport of air masses between the Salt Lake City

and Provo NAAs can occur during inversions, as there is a 4.75-mile gap in the mountains 

separating these airsheds which is below the approximate average inversion height for these 

areas.39

Speciation data from wintertime PM2.5 exceedance days from 2004-2006 showed the average 

composition of PM2.5 in Provo is 71% ammonium nitrate, with 21% carbon and organic 

compounds, 8% ammonium sulfate, and 2% crustal. Several factors, including significant 

 

35 See Appendix E at E.1. 

36 The Quqirrh Mountains lie between the Wasatch Front and the Stansbury Mountains. However, EPA determined 

that some area located west of the Quqirrh Mountains should still be considered part of the SLC nonattainment area. 

37 2006 24-Hour PM2.5 NAAQS Designations (74 FR 58688, November 13, 2009), Technical Support Document, 

December 2008, Index 583-669 at 40. 

38 Id.

39 Id. at 41. 
20 

mobile, point, and area sources contribute to ambient levels of primary and secondarily formed 

PM2.5 that violate the standard during stable weather events associated with strong inversions. 

V.B.2 California 

Transport to maintenance receptors in California is discussed in detail in Appendix C and 

summarized below. 

Northern California/San Francisco Bay Area – Alameda, Solano, and Santa Clara County 

Receptors

The receptors in these counties are in the San Francisco Bay airshed. The monitored values of 

PM2.5 that exceed the standard likely reflect localized sources because they occur during 

wintertime temperature inversions with low winds that persist for several days, allowing directly 

emitted and secondary PM2.5 to build up over days. The Bay Area Air Quality Management 

District analyses show that during the winter peak season, wood burning can account for 33 to 

over 80 percent of the PM2.5 concentrations. Ammonium nitrate and elemental carbon from 

diesel sources can also contribute to these high values. 

EPA believes that the factors that indicate the elevated PM2.5 levels at these locations are caused 

by local emissions and the distance of the area from Montana support our finding that emissions 

from Montana do not interfere with maintenance of the 24-hour PM2.5 standard at this receptor. 

Southern California – Orange County (Maintenance Only) and Los Angeles, Riverside, San 

Bernardino Counties 

Given the factors that indicate the elevated PM2.5 levels at these locations are caused by local 

emissions, regional and local flow patterns, and the distance from Southern California to 

Montana emission sources, we believe it is reasonable to conclude that emissions from Montana 

do not interfere with maintenance of the 2006 24-hour PM2.5 standard at this receptor. 

California/Mexico Border – Imperial County 

The high monitored levels of PM2.5 are limited to the Calexico – Ethel Street monitoring site, 

which is located near the Mexican Border. The elevated PM2.5 levels occur primarily in the 

winter months during stagnant conditions, when long distance transport is unlikely. On days of 

elevated PM2.5 at the Calexico Ethel Street monitor, the levels of PM2.5 measured at the other 

monitors in Imperial County, the Brawley and El Centro sites, are lower on all days. The PM2.5 

appears to be from a local source near the California/Mexico Border. These factors support our 

finding that emissions from Montana sources do not interfere with maintenance of the 24-hour 

PM2.5 standard at this receptor. 

 

Given the factors that indicate the elevated PM2.5 levels at these locations are caused by local 

emissions, regional and local flow patterns, and the distance from Southern California to 

Montana emission sources, we believe it is reasonable to conclude that emissions from Montana 

do not interfere with maintenance of the 2006 24-hour PM2.5 standard at this receptor. 
21 

Northern California – Sacramento County

In Northern California, monitored exceedances of the PM2.5 standard likely reflect wintertime 

temperature inversions in which low winds persist for several days in an area where the complex 

topography traps localized emissions. The speciation data from high concentration days indicates 

that the PM2.5 has a high carbon mass, which also indicates that residential wood burning 

emissions are the dominant source.40 Given the factors that indicate the elevated PM2.5 levels at 

these locations are caused by local emissions and the distance of the area from Montana, it is 

reasonable to conclude that emissions from Montana do not interfere with maintenance of the 

2006 24-hour PM2.5 standard at these receptors. 

V.B.3. Arizona 

Pinal County (Cowtown)

 

This receptor is in the West Central Pinal nonattainment area. 

This monitoring site is a middle scale site and is only comparable to the 24-hour NAAQS due to 

its close proximity to nearby sources. Emission inventory data, combined with speciation and 

source apportionment data, point to agricultural activities and cattle feedlots, as well as other 

nearby sources of PM2.5, as primary sources contributing to PM2.5 levels at the Cowtown 

monitor on days with exceedances of the 24-hour PM2.5 NAAQS. 

The Cowtown monitor has a consistent PM2.5/PM10 ratio that is lower than other nearby sites, 

suggesting that the causes of Cowtown PM2.5 concentrations are different than for other 

locations. The correlation of the PM2.5/PM10 ratio with PM2.5 concentrations at the Cowtown 

monitor suggests that PM2.5 and PM10 concentrations at the Cowtown monitoring site are 

generated by the same sources. Also, the Cowtown monitor is the only site in the area with a 

pronounced diurnal pattern, with high PM in the morning and evening hours, further suggesting 

the influence of local sources. Given these factors and the large distance from Arizona to 

Montana emission sources, we believe it is reasonable to conclude that emissions from Montana 

do not interfere with maintenance of the 2006 24-hour PM2.5 standard at this receptor. 

Santa Cruz County (Nogales)

This receptor is in the Nogales nonattainment area.

High PM2.5 values in Nogales generally occur under stagnant, light wind conditions during 

winter, and are associated with southerly flow (i.e., from the direction of Mexico). Nighttime 

drainage flow is typically from south to north along the Nogales Wash, and the narrow valley 

topography can trap air pollutants, especially during the evening hours when the diurnal flow is 

from the south, out of Nogales, Sonora, Mexico. Measurements of PM2.5 composition show that 

the main emission sources are wood burning, food cooking, open burning, dust emissions, and 

tailpipe emissions from on-road and off-road vehicles. Given these factors and the large distance 

 

40 ARB Almanac website (2006) http://www.arb.ca.gov/ei/maps/statemap/cntymap.htm) 
22 

from Arizona to Montana emission sources, we believe it is reasonable to conclude that 

emissions from Montana do not interfere with maintenance of the 2006 24-hour PM2.5 standard 

at this receptor.

V.C. Transport to Eastern States 

EPA has also considered potential PM2.5 transport from Montana emission sources to the 

nearest potential nonattainment or maintenance receptors located in the eastern states within the 

domain of the modeling analysis for the CSAPR. The nearest nonattainment and/or maintenance 

receptor identified in the 2012 CSAPR modeling analysis is in Milwaukee, Wisconsin. The 

CSAPR modeling did not analyze the contribution of Montana sources to any receptors in the 

CSAPR domain, including the receptor in Milwaukee, Wisconsin. Thus, EPA analyzed the 

magnitude of Montana’s SO2 and NOx emissions relative to states whose contribution to the 

Milwaukee receptor was modeled for the CSAPR. EPA also considered the distance of Montana 

from Milwaukee, Wisconsin relative to states found to states with CSAPR-modeled contribution. 

EPA believes that the following factors support a finding that emissions from Montana sources 

do not significantly contribute to nonattainment or interfere with maintenance of the 2006 24-

hour PM2.5 NAAQS at this location: (1) the relatively small magnitude of the emission 

inventory of PM2.5 precursors (SO2 and NOx) in Montana combined with (2) the relatively long 

distance between the nearest portion of the state of Montana to the Milwaukee, Wisconsin 

receptor.41 EPA believes that these factors also support a finding that emissions from Montana 

sources do not contribute significantly to nonattainment or interfere with maintenance of the 

2006 24-hour PM2.5 NAAQS at any of the other CSAPR receptors, all of which are farther from 

Montana than Milwaukee, Wisconsin. 

North Dakota was modeled for the CSAPR, and was found to be below the threshold of 

significant contribution to nonattainment and interference with maintenance for all receptors 

modeled for the CSAPR. EPA has evaluated the relative amounts of PM2.5 precursor (SO2 and 

NOx) emissions in Montana compared to the amounts of PM2.5 precursor emissions in North 

Dakota, as modeled in the 2012 CSAPR “base case.” The emission inventories used in the 

CSAPR modeling analyses indicate that SO2 emissions in Montana are about 22% of SO2 

emissions in North Dakota, and NOx emissions in Montana are about 73% of NOx emissions in 

North Dakota. Specifically, the CSAPR “2012 Base Case” inventories provide the following 

statewide emissions inventory data for these two states: 97,575 tons of NOx and 25,621 tons of 

SO2 from Montana emission sources, compared to 133,332 tons of NOx and 118,490 tons of 

SO2 from North Dakota emission sources.42 Even though the relationship between upwind 

emissions and downwind secondary PM2.5 formation is not linear, we would expect the 

Montana PM2.5 contribution to Wisconsin to be less than that of North Dakota and therefore 

well below the 1% threshold. In addition, the distance from the closest part of Montana to 

Milwaukee, WI is over 300 miles further than the distance from the closest part of North Dakota 

to Milwaukee, WI. The distance from Montana to the other eastern receptors is even greater, and 

therefore, the expected impact somewhat smaller. Given all of these factors, EPA believes it is 

reasonable to conclude that emissions of PM2.5 and PM2.5 precursors from Montana sources do 

 

41 See “Emissions Inventory Final Rule TSD,” Technical Support Document (TSD) for the final Transport Rule, 

Docket ID No. EPA-HQ-OAR-2009-0491, June 28, 2011, at Tables 7-1 and 7-2. 

42 Id.
23 

not contribute significantly to nonattainment or interfere with maintenance of the 2006 24-hour 

PM2.5 NAAQS in any eastern state.