Interstate Transport State 

Implementation Plan for 2010 NO2

and 2010 SO2

State of Idaho 

Department of Environmental Quality

December 2015
Printed on recycled paper, DEQ, December 2015, 

PID SSIP, CA 81992. Costs associated with this 

publication are available from the State of Idaho 

Department of Environmental Quality in accordance 

with Section 60-202, Idaho Code.
Interstate Transport SIP for 2010 NO2 and SO2

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Table of Contents

Introduction..................................................................................................................................... 1

Background..................................................................................................................................... 1

Nitrogen Dioxide ............................................................................................................................ 1

Sulfur Dioxide................................................................................................................................. 3

Appendix A. Public Comment and Hearing Material..................................................................... 8

List of Tables

Table 1. Areas previously designated nonattainment for SO2 annual NAAQS and 24-hour 

NAAQS. ........................................................................................................................... 4

Table 2 Top 10 SO2 emitters in Idaho (2014 data). ........................................................................ 4

Table 3. Design values for Idaho SO2 monitors, 2014. .................................................................. 6

Table 4. Design values for Idaho and surrounding states, 2011–2013. .......................................... 7

List of Figures

Figure 1. Modeled NO2 design values, 2009–2011. ....................................................................... 3

Figure 2. Top five SO2 point sources and current SO2 monitors in Idaho...................................... 5

Figure 3. Modeled background SO2 background concentrations for Idaho.................................... 6
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Introduction

The interstate transport provision in the Clean Air Act (CAA) section 110(a)(2)(D)(i) (i.e., the 

“good neighbor” provision) requires each state to submit a state implementation plan (SIP) that 

prohibits emissions that will have certain adverse air quality effects in other states. This SIP 

submittal, along with other components of an infrastructure SIP, is due within 3 years of the US

Environmental Protection Agency (EPA) promulgating a new or revised National Ambient Air 

Quality Standard (NAAQS). On January 22, 2010, EPA promulgated a revised NAAQS for 

nitrogen dioxide (NO2) and on June 3, 2010, EPA promulgated a revised sulfur dioxide (SO2) 

standard. Idaho submitted NO2 and SO2 infrastructure SIPs (minus the elements addressed in this 

submittal) on September 16, 2013. Due to uncertainty surrounding the effect of pending court 

cases in other states, Idaho did not include an analysis addressing all interstate transport 

provisions. Idaho’s NO2 and SO2 infrastructure SIPs were approved by EPA on September 10, 

2014 (79 FR 46707). This SIP submittal addresses Idaho’s remaining infrastructure SIP 

obligations under section 110(a)(2)(D)(i) for SO2 and NO2.

The SIP was made available for public comment from November 20 through December 21, 

2015. A public hearing was held on December 21 and noticed 30 days in advance. No comments 

were received during the public involvement process. The legal proof of publication in the 

Idaho Statesman, hearing notice, and copy of the Idaho Department of Environmental Quality’s

news release are provided in Appendix A. 

Background

CAA section 110(a)(2)(D)(i) identifies four distinct “prongs” related to the impacts of air 

pollutants transported across state lines. For a new or revised NAAQS, CAA requires each SIP 

contain adequate provisions prohibiting any source or other type of emissions activity within the 

state from emitting air pollutants that result in the following:

1. Contribute significantly to nonattainment of the applicable NAAQS in any other state.

2. Interfere with maintenance of the applicable NAAQS in any other state.

3. Interfere with measures required to be included in the applicable SIP for any other 

state to prevent significant deterioration of air quality.

4. Interfere with measures required to be included in the applicable SIP for any other 

state to protect visibility.

This analysis focuses on states that are contiguous to Idaho: Washington, Oregon, Nevada, Utah, 

Wyoming, and Montana. Since both NO2 and SO2 will most likely either disperse in the 

atmosphere or chemically react to form a secondary pollutant within a few miles of a source, 

impacts beyond the contiguous states are not considered.

Nitrogen Dioxide

EPA first set NO2 standards in 1971 for both a primary standard (to protect health) and a 

secondary standard (to protect the public welfare) at 53 parts per billion (ppb), averaged 

annually. EPA reviewed the standards in 1985 and 1996, and after each review, decided to retain 

the standards. In 2005, EPA began another review resulting in the January 22, 2010, rulemaking
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that established an additional primary NO2 standard at 100 ppb, averaged over 1 hour 

(75 FR 6474).

Idaho addressed prongs 3 and 4 in its September 16, 2013, NO2 infrastructure SIP submittal. 

Prong 4 (visibility impairment) was addressed in the Idaho Regional Haze SIP, which was 

approved by EPA on June 9, 2011 (76 FR 33651). This SIP submittal addresses the requirements 

of CAA section 110(a)(2)(D)(i)(I) for revised 2010 NO2 NAAQS for the first two prongs listed 

above. 

On January 20, 2012, EPA designated all areas of the country as “unclassifiable/attainment” for 

the 2010 NO2 NAAQS (77 FR 9532). The available air quality data show that all monitored 

areas in the country meet the 2010 NO2 NAAQS for 2008–2010. No state or tribe recommended 

an area be designated “nonattainment.” As listed in EPA’s NO2 Design Values report 

(http://www.epa.gov/airtrends/values.html), only one maintenance area exists for the prior 

annual NO2 NAAQS (Los Angeles, California). With no nonattainment or maintenance areas in 

surrounding states, Idaho does not significantly contribute to nonattainment or maintenance of 

these NAAQS in any of the contiguous states.

As further evidence that Idaho’s NO2 emissions do not impact neighboring states, Figure 1

shows the estimated 2009–2011 1-hour NO2 design value background concentrations for Idaho 

(http://lar.wsu.edu/nw-airquest/lookup_faq.html). The design values are well below the NAAQS, 

and areas of increased NO2 concentrations are localized in nature.

Conclusion—Based on the information, Idaho has met the CAA requirements for prongs 1 and 

4.
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Figure 1. Modeled NO2 design values, 2009–2011.

Sulfur Dioxide

Primary standards for SO2 were first set in 1971, at 140 ppb averaged over a 24-hour period, not 

to be exceeded more than once per year, and 30-ppb annual arithmetic mean. In 1996, EPA 

subsequently reviewed the primary standards, and after each review, decided to retain the

standards. More recently, on June 2, 2010, EPA promulgated a revised primary SO2 standard at 

75 ppb, based on a 3-year average of the annual 99th percentile of 1-hour daily maximum 

concentrations (75 FR 35520).

For the revised 2010 SO2 standard, Idaho addressed interstate transport prongs 3 and 4 in its 

September 16, 2013, SO2 infrastructure SIP submittal. The Idaho Regional Haze SIP also 

addressed prong 4, and the SIP was approved by EPA on June 9, 2011 (76 FR 33651). The 

current SIP submittal addresses prongs 1 and 2 of CAA section 110(a)(2)(D)(i)(I) for the revised 

2010 SO2 NAAQS. 

Unlike NO2, a nonattainment area for the revised SO2 NAAQS exists in an adjacent state. 

Billings, Montana, was declared nonattainment for the 1-hour SO2 standard in 2013

(77 FR 9532). In addition, areas in neighboring states were declared nonattainment for the earlier 

SO2 standards (Table 1).
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Table 1. Areas previously designated nonattainment for SO2 annual NAAQS and 24-hour NAAQS.

Designated Area State Designation Status

East Helena Montana Nonattainment

Laurel Montana Nonattainment

Central Steptoe Valley Nevada Maintenance

Salt Lake County Utah Nonattainment

Tooele County (part) Utah Nonattainment

To determine whether SO2 emissions from facilities in Idaho could negatively impact any of 

these maintenance or nonattainment areas, the largest SO2 emission sources were identified from 

our 2014 emissions inventory (Table 2). Since SO2 will most likely either disperse in the 

atmosphere or chemically react to form a secondary pollutant within a few miles of the source, 

only large pollutant sources in proximity to the state boundary would be expected to significantly 

contribute to or interfere with air quality in adjacent states. Only one of the facilities shown in 

Table 2 (Clearwater Paper Corporation) is within 5 miles of the Idaho border. The Clearwater 

Paper Corporation’s emissions are relatively small (~42 tons in 2014), and the facility is over 

400 miles from Billings, Montana, and hundreds of miles from the older 

nonattainment/maintenance areas listed in Table 1.

Table 2 Top 10 SO2 emitters in Idaho (2014 data).

Facility ID Facility Name Pollutant Emissions

(tons)

02700010 Amalgamated Sugar (Tasco—Nampa) SO2 1,585.9

08300001 Amalgamated Sugar (Tasco—Twin Falls) SO2 1,081.9

07700006 JR Simplot Company—Don Siding SO2 794.9

02900001 P4 Production LLC (TV Facility) SO2 580.5

02900003 Nu-West Industries, Inc. (Agrium Conda Phosphate Operations) SO2 332.4

06700001 Amalgamated Sugar (Tasco—Paul) SO2 248.3

01900025 Busch Ag Resources Inc.—Malt Plant SO2 42.6

06900001 Clearwater Paper Corporation—PPD & CPD SO2 42.2

02700009 JR Simplot Company Food Group—Caldwell SO2 26.8

Idaho’s SO2 monitoring network can also be used to characterize the SO2 emissions of three of 

the four largest SO2 emitters listed in Table 2 and shown in Figure 2.
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Figure 2. Top five SO2 point sources and current SO2 monitors in Idaho.

In eastern Idaho (Pocatello and Soda Springs), source-oriented monitors report the highest 

ambient SO2 concentrations surrounding these facilities. The Meridian monitor is more of a 

background monitor but should reflect any significant emissions from the largest SO2 emitter in 

Idaho (TASCO in Nampa). Note: TASCO’s Nampa facility is in the process of converting a coal 

boiler to natural gas, which should result in a significant decrease in SO2 emissions from this 

facility. As shown in Table 3, all of the Idaho monitors have 2014 3-year design values that are 

below the 75-ppb SO2 NAAQS.
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Table 3. Design values for Idaho SO2 monitors, 2014.

Site County/AIRS ID

99th Percentile—Highest Daily Maximum

1-Hour Average (ppb) 3-Year Design 

Value (ppb)

2012 2013 2014

Pocatello STP Bannock 73 40 38 51

Soda Springs Caribou 35 31 23 30

Meridian 

St. Luke’s

Ada 6 11 5 7

As further evidence that Idaho’s SO2 emissions do not impact neighboring states, Figure 3 shows 

the estimated 2009–2011 1-hour SO2 modeled design value background concentrations for the 

state. This map was generated by Washington State University researchers using the AIRPACT3 

model (http://lar.wsu.edu/nw-airquest/lookup_faq.html). The design values are well below the 

NAAQS value of 75 ppb, and areas of increased SO2 concentrations are localized in nature.

Figure 3. Modeled background SO2 background concentrations for Idaho.

The Idaho Department of Environmental Quality reviewed EPA’s Clean Air Status and Trends 

Network (CASTNET) monitoring data (http://epa.gov/castnet/javaweb/index.html). CASTNET 

is a national air quality monitoring network designed to provide data to assess trends in air 

quality, atmospheric deposition, and ecological effects due to changes in air pollutant emissions. 

CASTNET provides long-term air quality monitoring in rural areas to determine trends in 

regional atmospheric nitrogen, sulfur, and ozone concentrations and deposition fluxes of sulfur 

and nitrogen pollutants to evaluate the effectiveness of national and regional air pollution control 

programs. The SO2 data for the western United States show low background SO2 levels 
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throughout the year. Both average weekly and seasonal SO2 concentrations were low (<6 ppb) 

indicating that the regional SO2 background concentrations are relatively low, which in turn 

implies that the bulk of the SO2 in urban receptor areas is locally generated and not a regional or 

transport problem.

Finally, as listed in EPA’s 2013 SO2 Design Values report 

(http://www.epa.gov/airtrends/values.html), the valid 3-year design values for SO2 for 2011–

2013 indicate that except for Yellowstone County, Montana (area declared nonattainment), no 

other areas in the surrounding states have significant SO2 issues (Table 4).

Table 4. Design values for Idaho and surrounding states, 2011–2013.

State County Valid 3-Year Design 

Value

Idaho Caribou 40

Montana Richland 4

Montana Yellowstone 64

Nevada Clark 8

Nevada Washoe 6

Oregon Multnomah 6

Utah Davis 9

Utah Salt Lake 21

Utah Salt Lake 20

Utah Salt Lake 7

Washington Clallam 1

Wyoming Campbell 38

Wyoming Laramie 6

Wyoming Sweetwater 18

Wyoming Weston 12

Conclusion—Based on the analysis of monitoring and modeling data provided above, Idaho’s

SO2 emissions do not result in the following:

1. Contribute significantly to nonattainment of the applicable NAAQS in any other state.

2. Interfere with maintenance of the applicable NAAQS in any other state.
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Appendix A. Public Comment and Hearing Material
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