Northern Ada County

PM10 State Implementation Plan

Maintenance Plan

Ten-Year Update

State of Idaho 

Department of Environmental Quality

February 2013
Printed on recycled paper, DEQ June 2012, 

PID TVMB, CA code 41300. Costs associated with 

this publication are available from the State of Idaho 

Department of Environmental Quality in accordance 

with Section 60-202, Idaho Code.
Northern Ada County

PM10 State Implementation Plan

Maintenance Plan

Ten-Year Update

February 2013

Prepared by

Idaho Department of Environmental Quality

Boise Regional Office

1445 North Orchard

Boise, Idaho 83706
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Northern Ada County PM10 Maintenance Plan

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Table of Contents

1 Introduction and History.......................................................................................................... 1

2 Northern Ada County PM10 Maintenance Area Description................................................... 2

3 Requirements for Maintenance Plan Update ........................................................................... 3

3.1 NAAQS Attainment Verification ..................................................................................... 3

3.2 SIP Approval .................................................................................................................... 3

3.3 Permanent and Enforceable Improvements in Air Quality .............................................. 3

3.4 Full Approval of the PM10 Maintenance Plan .................................................................. 3

3.5 Maintenance Plan Elements ............................................................................................. 4

4 Air Quality Monitoring—Verification of Attainment ............................................................. 4

4.1 PM10 Monitoring .............................................................................................................. 4

4.2 Quality Assurance Program.............................................................................................. 4

4.3 Continued Attainment of the PM10 Standard.................................................................... 4

4.4 Continued Air Monitoring and Verification of Attainment ............................................. 4

5 Emissions Inventory ................................................................................................................ 6

5.1 Base Year (2008) Annual and Episode Emissions Inventory .......................................... 7

5.1.1 Area Sources................................................................................................................ 9

5.1.2 On-Road Mobile Sources .......................................................................................... 10

5.1.3 Non-Road Mobile Sources ........................................................................................ 11

5.1.4 Point Sources............................................................................................................. 11

5.2 Projected Inventories...................................................................................................... 12

5.3 Motor Vehicle Emissions Budget................................................................................... 13

5.3.1 MVEB for PM10......................................................................................................... 13

5.3.2 MVEB for NOx and VOC.......................................................................................... 14

5.3.3 MVEB Conclusion..................................................................................................... 14

6 Maintenance Demonstration.................................................................................................. 15

6.1 Air Quality Modeling Approach .................................................................................... 15

6.2 Attainment Demonstration ............................................................................................. 16

6.2.1 Annual Average ......................................................................................................... 16

6.2.2 Nonwinter High Crustal Mass Scenario .................................................................... 17

6.2.3 Nonwinter High Carbon Scenario ............................................................................. 17

6.2.4 High Winter Scenario ................................................................................................ 18

6.2.5 Stagnant Winter Scenario .......................................................................................... 19

6.3 Modeling Summary........................................................................................................ 20

7 Permanent and Enforceable Improvement in Air Quality ..................................................... 21

7.1 Control Measures............................................................................................................ 21
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7.1.1 Air Quality Index Program........................................................................................ 21

7.1.2 Residential Wood Burning Program.......................................................................... 21

7.1.3 Open Burning Ban Program ...................................................................................... 21

7.1.4 Stationary Source Control Measure........................................................................... 22

8 Contingency Plan................................................................................................................... 22

8.1 Tracking.......................................................................................................................... 23

8.2 Triggering....................................................................................................................... 23

8.3 Potential Contingency Measures.................................................................................... 23

9 Summary................................................................................................................................ 24

9.1 Attainment Verification.................................................................................................. 24

9.2 Full Approval of the State Implementation Plan under Section 110.............................. 24

9.3 Permanent and Enforceable Improvement in Air Quality.............................................. 24

9.4 Maintenance Plan Elements ........................................................................................... 24

9.5 Subsequent Maintenance Plan Revisions....................................................................... 24

References..................................................................................................................................... 25

Appendices.................................................................................................................................... 27
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List of Figures

Figure 1. Northern Ada County PM10 Maintenance Area .............................................................. 2

Figure 2. PM10 annual emissions in the four source categories (to the nearest ton)....................... 9

Figure 3. PM10 paved road dust compared to all other area sources............................................. 10

Figure 4. Predicted annual average PM10 concentration (in micrograms per cubic meter). ......... 16

Figure 5. Predicted 24-hour average PM10 levels (in micrograms per cubic meter) for high 

crustal mass scenario in nonwinter seasons.................................................................... 17

Figure 6. Predicted 24-hour average PM10 levels (in micrograms per cubic meters) for high 

carbon scenario in nonwinter seasons. ........................................................................... 18

Figure 7. Predicted 24-hour average PM10 levels (in micrograms per cubic meters) for high 

winter scenario................................................................................................................ 19

Figure 8. Predicted 24-hour average PM10 levels (in micrograms per cubic meter) for 

stagnation scenario in winter season. ............................................................................. 20

List of Tables 

Table 1. Highest certified 24-hour average PM10 values measured at the Boise fire station. ........ 5

Table 2. Annual average PM10 concentrations measured at Boise Fire Station #5. ....................... 6

Table 3. Annual emissions by source category—2008................................................................... 7

Table 4. Emission for winter seasons—2008. ................................................................................ 8

Table 5. Emission for non-winter seasons—2008. ......................................................................... 8

Table 6. Ada County paved road dust emissions.......................................................................... 10

Table 7. Annual emissions by source category—2015................................................................. 12

Table 8. Annual emissions by source category—2023................................................................. 12

Table 9. PM10 motor vehicle emissions budget. ........................................................................... 14

Table 10. NOx and VOC motor vehicle emissions budget. .......................................................... 14

Table 11. Predicted annual average PM10 levels. ......................................................................... 16

Table 12. Predicted 24-hour average PM10 levels for high crustal mass scenario in nonwinter 

seasons............................................................................................................................ 17

Table 13. Predicted 24-hour average PM10 levels for high carbon scenario in nonwinter 

seasons............................................................................................................................ 18

Table 14. Predicted 24-hour average PM10 levels for high winter scenario. ................................ 18

Table 15. Predicted 24-hour average PM10 levels for stagnation scenario in winter season. ....... 19
Northern Ada County PM10 Maintenance Plan

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1 Introduction and History

The Northern Ada County Maintenance Area was identified as an area of concern for PM10 with 

the promulgation of the PM10 National Ambient Air Quality Standards (NAAQS) in 1987 and 

was formally designated as a moderate PM10 nonattainment area upon passage of the 1990 Clean 

Air Act.

1

Idaho developed a state implementation plan (SIP) and submitted it to the United States 

Environmental Protection Agency (EPA) in November 1991, later submitting revisions in 

December 1994 and July 1995. EPA gave final approval to the Northern Ada County PM10 SIP 

in May 1996.

EPA revised the PM10 NAAQS in 1997, and Idaho demonstrated to EPA’s satisfaction that it 

complied with the new standard. As a result, EPA rescinded the applicability of the PM10

NAAQS in Northern Ada County on March 12, 1999. Shortly thereafter, litigation at the national 

level vacated the new PM10 NAAQS. Subsequent litigation in Idaho to have EPA restore the 

federal standards for northern Ada County resulted in a settlement agreement that required Idaho 

to submit a PM10 maintenance SIP by September 30, 2002, and for EPA to take final action 

within one year. The Northern Ada County PM10 SIP Maintenance Plan and Redesignation 

Request (2002 maintenance plan) was submitted to EPA in September 2002. EPA approved the 

plan in September 2003, restoring northern Ada County to attainment status for PM10

(Environ 2002).

The control strategy in the 2002 maintenance plan includes the measures in the approved 1991 

attainment SIP and additional contingency measures. The key 1991 SIP measures revised and 

enhanced in the 1994 and 1995 supplemental submissions and the 2002 maintenance plan are 

residential wood burning and open burning programs. Both consist of voluntary and mandatory

burn bans in the various communities and unincorporated areas of northern Ada County during 

periods of high PM10 levels.

During the environmental impact and subsequent modeling stage of the 2002 maintenance plan 

development process, the Idaho Department of Environmental Quality (DEQ) identified 

13 facilities with potentials to emit well in excess of their actual emissions. In accordance with 

the “Rules for the Control of Air Pollution in Idaho” (IDAPA 58.01.01.401.03), DEQ issued 

permits reducing allowable emissions from these facilities. In addition to these 13 point sources, 

the Amalgamated Sugar Company (TASCO) in Canyon County was shown to potentially 

contribute to PM10 exceedances in both Ada and Canyon Counties. To address this problem, 

DEQ issued a Tier II operating permit that required the company to reduce emissions sufficiently 

to address these air quality concerns.

Contingency measures in the 2002 maintenance plan include additional controls in the wood 

burning program and a reduction of fugitive road dust through a road sweeping program 

designed to reduce particulate emissions by prioritizing road sweeping for certain problem areas.

Additional potential contingency measures include requiring material transport load covering, 

eliminating mud track-out onto paved roads, reducing uncontrolled outdoor burning, expanding 

the existing vehicle inspection and maintenance program in Ada County, prohibiting new 

 

1

PM10 is particulate matter with an aerodynamic diameter less than or equal to a nominal 10 micrometers.
Northern Ada County PM10 Maintenance Plan

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unpaved roadways or parking lots through local ordinances, and including clean burning 

woodstoves in mandatory burn-ban situations.

During 1986–1991, seven recorded PM10 values exceeded the 24-hour standard (150 micrograms 

per cubic meter [μg/m3]) in the Northern Ada County Maintenance Area. With one exception, 

the exceedances were all measured in winter months (January). The highest measured 24-hour 

PM10 value was 314 μg/m3 measured at the downtown Boise fire station in January 1986. With 

the exception of an agricultural-influenced exceedance in 1997, no exceedances were recorded 

from 1992 through 2002. From 2003 through 2008, the highest measured 24-hour PM10 value 

was 97 μg/m3 measured at the downtown Boise fire station. No exceedances were recorded 

during this time period.

Based on preliminary data, two exceedances were recorded on February 15 and 16, 2011 with a 

24-hour average of 183 and 156 μg/m3respectively. DEQ is currently evaluating this 

exceedance. Sustained winds and extremely dry winter conditions contributed significantly to the 

event. Thus, the 3-year average number of 24-hour exceedances was less than one per year. 

Based upon monitoring data, the area has attained the 24-hour PM10 NAAQS. The now revoked 

annual PM10 standard of 50 μg/m3 was never violated in northern Ada County.

Monitoring data show that northern Ada County has moved away from the preponderance of 

high peak values occurring in the winter season (i.e., PM season) to lower peaks, distributed 

through four distinct scenarios, which are discussed in section 6.

This maintenance plan fulfills the requirements for the northern Ada County maintenance plan 

update for the second 10-year maintenance period. The existing control measures in effect will

remain unchanged. However, DEQ will request that the permit requirements for the facilities 

currently included in the Idaho SIP be removed. This request will demonstrate that the permit 

program is an adequate existing control measure for industrial sources. See specifically, IDAPA 

58.01.01.401.03 (Appendix A).

2 Northern Ada County PM10 Maintenance Area Description

The Northern Ada County Maintenance Area includes the portion of Ada County north of the 

Boise baseline (43 degrees north latitude) as shown in Figure 1.

Figure 1. Northern Ada County PM10 Maintenance Area (indicated by gray shaded area). 
Northern Ada County PM10 Maintenance Plan

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South of the maintenance area, the land is largely unpopulated and is primarily managed by 

federal agencies such as the Bureau of Land Management and Department of Defense (the Idaho 

National Guard). To the east and north, the Boise front blocks movement of air and pollution 

between Ada County and neighboring Boise, Elmore, and Gem Counties. Canyon County is 

located on the western edge of the maintenance area. The legal description for the maintenance 

area can be found in Appendix B.

3 Requirements for Maintenance Plan Update

Clean Air Act §175A and related provisions establish the criteria that must be satisfied for a 

maintenance plan update:

 Attainment of the NAAQS for PM10

 Full approval of the SIP under §110

 Demonstration that air quality improvement is due to permanent and enforceable 

emission reductions

 Full approval of the PM10 maintenance plan

 Fulfillment of all applicable §110 (maintenance plan) requirements

3.1 NAAQS Attainment Verification

A maintenance area must meet the applicable NAAQS. Attainment of the NAAQS for PM10 in 

northern Ada County is discussed in section 4, “Air Quality Monitoring—Verification of 

Attainment.”

3.2 SIP Approval

EPA must have fully approved the applicable SIP for the area pursuant to §110(k) of the Clean 

Air Act. DEQ submitted a PM10 SIP in November 1991, with revisions in December 1994 and 

July 1995. EPA gave final approval to the northern Ada County SIP in May 1996. A PM10

maintenance plan and redesignation request was submitted to EPA in September 2002. EPA 

approved the plan in September 2003, restoring northern Ada County to attainment status for 

PM10.

Section 110 requirements were addressed by the northern Ada County PM10 maintenance plan 

and requested redesignation to attainment status—adopted by DEQ on September 26, 2002, and 

approved by EPA effective September 2003—and subsequent infrastructure SIP submittals.

3.3 Permanent and Enforceable Improvements in Air Quality

Permanent and enforceable reductions in emissions and improved ambient PM10 concentrations 

in the northern Ada County area are discussed in section 7.

3.4 Full Approval of the PM10 Maintenance Plan

DEQ is submitting this plan to EPA for approval, as required.
Northern Ada County PM10 Maintenance Plan

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3.5 Maintenance Plan Elements

Section 175A of the Clean Air Act requires DEQ to submit a revision to the original PM10

maintenance plan eight years after redesignation that demonstrates maintenance of the air quality 

standard for an additional 10-year period. This revision modifies the original PM10 maintenance 

plan and includes the following maintenance plan requirements:

 Section 4: Continued Air Monitoring and Verification of Attainment

 Section 5: Emissions Inventory

 Section 6: Maintenance Demonstration

 Section 7: Permanent and Enforceable Controls

 Section 8: Contingency Plan

4 Air Quality Monitoring—Verification of Attainment 

4.1 PM10 Monitoring

PM10 ambient air monitoring in northern Ada County consists of one monitor located at the 

downtown Boise fire station (i.e., Fire Station #5 at 16th and Front Streets in downtown Boise). 

This site is operated by DEQ. The monitor is a tapered element oscillating microbalance 

(TEOM) monitor, which provides continuous, real-time direct measurement of PM10

concentrations. The TEOM monitor is part of the approved DEQ monitoring network and is 

operated in accordance with 40 CFR Part 58. DEQ will continue to operate an appropriate PM10

air quality monitor in accordance with 40 CFR Part 58 to verify continued attainment of the 

PM10 NAAQS. Annual review of the monitoring network will continue.

4.2 Quality Assurance Program

PM10 monitoring data for the Northern Ada County Maintenance Area have been collected and 

quality assured in accordance with 40 CFR Part 58, Appendix A; EPA’s Quality Assurance 

Handbook for Air Pollution Measurement Systems (EPA 2008); DEQ’s standard operating

procedures; and DEQ’s Quality Assurance Project Plan for the State of Idaho Ambient Air 

Quality Monitoring Program (DEQ 2009).

4.3 Continued Attainment of the PM10 Standard

The 24-hour PM10 NAAQS, which is 150μg/m3PM10 in ambient air (based on 24-hour 

averaging), is not to be exceeded more than once per year on average over 3 years.

Table 1 demonstrates that northern Ada County continues to attain the 24-hour NAAQS for 

PM10.

4.4 Continued Air Monitoring and Verification of Attainment

Ambient PM10 concentrations continue to be below regulatory standards. With monitoring data 

used as the triggering mechanism, DEQ will continue to closely track PM10 concentrations and 
Northern Ada County PM10 Maintenance Plan

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verify continued attainment via the approved monitoring network operated in accordance with 

40 CFR Part 58.

Table 1. Highest certified 24-hour average PM10 values measured at the Boise fire station.

Year Highest 24-Hour Average PM10 Values

(μg/m3)

2000 95

2001 95

2002 150

2003 88

2004 70

2005 89

2006 97

2007 88

2008 92

2009

2010

2011

118

95

183

Note: Data for which exceptional event exclusion requests will be submitted is included. 

These data include some extreme natural conditions causing concentrations above the PM10

standard. The higher levels on these days were caused by dust storms. Because of the small 

number of these extreme events, these higher values do not affect compliance with the NAAQS

nor significantly affect the overall trend. The 183 μg/m3 value (and a 156 μg/m3value the 

following day) in 2011 have been flagged by DEQ as extreme or exceptional events. DEQ is in 

the process of preparing the exceptional event exclusion concurrence requests to the EPA for

these two days in 2011. Once granted, the removal of this monitoring data from consideration 

would result in a considerably lower design concentration (71 μg/m3 for 2009-2011). Even with 

the inclusion of these two days, DEQ remains in compliance with the PM10 NAAQS. The 

modeling conducted for this plan was completed prior to the 2011 data becoming official. The 

data from 2011, if included, even with two days that exceeded the daily standard would result in 

an increase of the design concentration of 1 μg/m3. Even if this data was included, maintenance 

would still be demonstrated. If the EPA concurs on these two exceptional events, as expected, 

the exclusion of that data would result in a 3-year design concentration of 71 μg/m3, which is 19 

μg/m3lower than the design concentration used in DEQ’s rollback modeling. Thus, by including 

this data, an extremely conservative approach is being used by DEQ.
Northern Ada County PM10 Maintenance Plan

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Table 2 shows the annual average PM10 concentrations for 2005-2011. While the annual 

standard has been revoked by EPA, the annual averages are shown for informational purposes

and for consistency with previous plans. The data demonstrate that the PM10 annual average has 

been stable and that northern Ada County continues to be well below the revoked annual PM10 

NAAQS of 50 μg/m3.

Table 2. Annual average PM10 concentrations measured at Boise Fire Station #5.

Year Annual Average PM10 Concentrations

(μg/m3)

2005 22

2006 25

2007 25

2008 23

2009 22

2010

2011

18

20

Note: All exceptional event-flagged days are included in these averages.

Based on the effectiveness of the existing control measures, the instances of high PM10 days have 

changed from predominantly wintertime peaks to more dispersed elevated values at different 

times of year based on four distinct scenarios. Those four scenarios—winter stagnation, high 

winter, non-winter high carbon, and non-winter high crustal mass are discussed in detail in 

Appendix C, Speciated Linear PM10 Roll-Forward Modeling Report.

5 Emissions Inventory

Environ International Corporation (Environ) and ERG, under contract with DEQ, completed the 

2008, 2015, and 2023 Emissions Inventories for the Treasure Valley Airshed (ERG/Environ 

inventory). A modified ERG/Environ inventory is included as Appendix D. The ERG/Environ 

inventory was designed to support this 10-year maintenance plan update, the previously 

submitted carbon monoxide limited maintenance plan, and Treasure Valley airshed planning and 

management needs for the future; as such, it is a multi-pollutant inventory. Two sections of the 

original 2010 ERG/Environ inventory have been superseded to account for the most currently 

available information. Section 4 has been replaced to account for the adoption of the MOVES 

model for motor vehicle emissions. This is explained in more detail in the next paragraph. The 

second change was to replace the Treasure Valley Road Dust Study which was used by 

ERG/Environ in the 2010 EI for paved road dust. This study was replaced by the January 2011

AP-42 Compilation of Air Pollutant Emission Factors (EPA 2011b, section 13.2.1). Additional

information explaining and supporting this change is provided in Appendix E, Section 3. For 

purposes of this plan, average winter season (PM season) means from November 1 through 

February 29 (2008 was a leap year), and summer season (ozone season) refers to April 1 through 

October 31. The month of March is not considered to be part of either the high PM season, or of 

the ozone season, and as such, is not used in seasonal analyses.

The ERG/Environ inventory used the Mobile6 model, the only EPA-approved model available at 

the time to calculate emissions due to on-road mobile sources. However, as this document was 
Northern Ada County PM10 Maintenance Plan

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being drafted, the Community Planning Association of Southwest Idaho (COMPASS), which is 

the designated Treasure Valley metropolitan planning organization, was expected to be required 

to begin using the newly released MOVES2010a (MOVES) model for transportation conformity 

determinations beginning in March 2012 (40 CFR 93.111(b)(1)). This date was later changed by 

EPA to 2013, well after the modeling had begun. DEQ, with input from COMPASS, has 

completed additional modeling that supplants portions of the ERG/Environ inventory. Input from 

COMPASS included projected growth rates, land use planning, vehicle miles traveled (VMT)

estimates, road classification, and travel demand modeling outputs. Total carbon was speciated 

by DEQ using speciation profiles from the Receptor Model Source Composition Library

(EPA 1984). The mobile emissions were re-modeled using the MOVES2010a model (MOVES),

and DEQ re-estimated the road dust using the new AP-42 method (EPA 2011). 

MOVES is known to result in significantly higher estimates for PM10 and nitrogen oxides (NOx)

emissions. For this reason, DEQ is using MOVES to develop the motor vehicle emissions budget 

(MVEB) and will ask for an adequacy determination to allow the MVEB in this plan be used to 

demonstrate conformity as soon as the adequacy determination is completed and the MVEB 

approved.

5.1 Base Year (2008) Annual and Episode Emissions Inventory

The ERG/Environ inventory includes Ada County in its entirety as opposed to just the Northern 

Ada County Maintenance Area. Using the entire county is a more conservative approach that 

meets the maintenance plan requirements while supporting the “airshed management” approach 

used by DEQ.

The inventory is divided into five categories of sources: point, area, on-road mobile, non-road 

mobile, and biogenic. These categories will be discussed in more detail below. Table 3

summarizes the 2008 annual PM10 emissions for the maintenance area; Table 4 summarizes the 

winter season average daily emissions; and Table 5 summarizes the non-winter season average 

daily emissions.

Table 3. Annual emissions by source category—2008.

Source 

Type

NOx SO2 VOC PM10 PM2.5 TC

(tons per year)

Point 356 66 268 169 143 77

Area 921 27 12,963 19,555 3,552 6,522

On-road 9,775 67 4,182 413 330 384

Non-road 2,895 90 2,031 258 245 227

Biogenic 202 0 12,802 0 0 0

Total 14,149 250 32,247 20,395 4,270 7,210

Note: nitrogen oxides (NOx); sulfur dioxide (SO2); volatile organic compounds (VOC); an aerodynamic diameter less 

than or equal to a nominal 10 micrometers (PM10); particulate matter with a diameter 2.5 micrometers or less (PM2.5); 

total carbon (TC)

Note: Source type categories may not sum exactly to the total emissions listed due to rounding. 
Northern Ada County PM10 Maintenance Plan

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Table 4. Emission for winter seasons—2008.

Source 

Type

NOx SO2 VOC PM10 PM2.5 TC 

(tons per day)

Point 1.0 0.2 0.7 0.5 0.4 0.2

Area 4.4 0.1 39.1 67.6 15.1 19.9

On-road 24.0 0.2 12.3 1.2 1.0 1.1

Non-road 5.6 0.2 4.4 0.4 0.4 0.7

Biogenic 0.3 0.0 5.8 0.0 0.0 0.0

Total 35.2 0.7 62.3 69.7 16.9 21.9

Note: nitrogen oxides (NOx); sulfur dioxide (SO2); volatile organic compounds (VOC); an aerodynamic diameter less 

than or equal to a nominal 10 micrometers (PM10); particulate matter with a diameter 2.5 micrometers or less (PM2.5); 

total carbon (TC)

Note: Source type categories may not sum exactly to the total emissions listed due to rounding. 

Table 5. Emission for non-winter seasons—2008.

Source 

Type

NOx SO2 VOC PM10 PM2.5 TC 

(tons per day)

Point 1.0 0.2 0.7 0.5 0.4 0.2

Area 1.4 0.0 33.7 43.5 6.6 12.3

On-road 28.5 0.2 10.9 1.1 0.9 1.0

Non-road 10.1 0.3 7.2 0.9 0.9 0.8

Biogenic 0.7 0.0 55.6 0.0 0.0 0.0

Total 41.7 0.7 108.1 46.1 8.7 14.3

Note: nitrogen oxides (NOx); sulfur dioxide (SO2); volatile organic compounds (VOC); an aerodynamic diameter less 

than or equal to a nominal 10 micrometers (PM10); particulate matter with a diameter 2.5 micrometers or less (PM2.5); 

total carbon (TC)

Note: Source type categories may not sum exactly to the total emissions listed due to rounding. 
Northern Ada County PM10 Maintenance Plan

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Total annual PM10 emissions are predominantly due to area sources, which account for 19,555

tons per year (96%) of the total annual PM10 emissions (Table 3; Figure 2).

Figure 2. PM10 annual emissions in the four source categories (to the nearest ton).

Four source categories are summarized below, including changes due to use of the MOVES 

model. Biogenic emissions are not discussed.

5.1.1 Area Sources

In the Handbook for Criteria Pollutant Inventory Development: A Beginner’s Guide for Point 

and Area Sources, the EPA describes area sources as “facilities or activities whose individual 

emissions do not qualify them as point sources. Area sources represent numerous facilities or 

activities that individually release small amounts of a given pollutant, but collectively they can 

release significant amounts of a pollutant” (EPA 1999). For the purposes of this emissions

inventory, area sources are defined as those sources emitting annual emissions less than the point 

source thresholds (see section 5.1.4).

Specific categories of area sources include, but are not limited to, residential wood burning, 

agricultural and open burning, industrial fuel combustion, paved and unpaved road dust, 

commercial cooking, auto body refinishing, and construction. Residential wood burning was 

found to be the primary contributor to the original violations of the PM10 NAAQS, accounting 

for 22 wintertime tons per day (tpd). The 2008 EI, attributes 13.4 tpd to residential wood smoke, 

a reduction of 39% even with the county population almost doubling. This indicates that the 

residential wood burning program has been very successful at reducing PM10 emissions.

169

19555

413

258

2008 PM10 Annual Emissions by Source 

Category (Tons Per Year)

Point

Area

On-Road

Non-Road
Northern Ada County PM10 Maintenance Plan

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For base year 2008, the largest contributor of PM10 emissions is paved road dust, which accounts 

for 36.8% of the annual PM10 emissions and 49.9% of the winter daily emissions (Table 6).

Table 6. Ada County paved road dust emissions.

Paved Road Dust Emission

(% of Total PM10)

Total PM10 Emission

Year 2008 2015 2023 2008 2015 2023

Annual 

(tons/year)

7,501

(36.8%)

9,164

(42.1%)

13,243

(51.2%)

20,395 21,756 25875

Non-winter 

daily (tons/day)

10

(22.5%)

12

(26.1%)

18

(33.7%)

46 48 53

Winter daily 

(tons/day)

35

(49.9%)

43

(56.3%)

62

(64.8%)

70 77 96

The 2008 PM10 contribution from road dust for Ada County was 7,501 tons per year (tpy). 

Unpaved road dust accounted for 966 tpy. In total, road dust in Ada County contributed 8,467 

tpy, or 41.5% of the total PM10 emissions in 2008. (Figure 3).

Figure 3. PM10 road dust compared to all other sources.

5.1.2 On-Road Mobile Sources

On-road mobile source emissions were calculated using the MOVES model, which is expected to 

result in significantly higher PM10 and NOx emissions than the previous Mobile6 model. The 

MOVES model calculated annual PM10 on-road direct mobile emissions of 413 tons per year

with 1.2 tons per day during the winter season. In addition, secondary aerosol precursor 

emissions estimated by the MOVES model included 9,775 tons per year NOx, 67 tons per year

sulfur dioxide (SO2), and 4,182 tons per year volatile organic compounds (VOCs).

7,501

966

11,928

2008 Total PM10 Road Dust Vs. Other 

Sources (Tons Per Year)

Paved Road Dust

Unpaved Road Dust

All Other Sources
Northern Ada County PM10 Maintenance Plan

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5.1.3 Non-Road Mobile Sources

Non-road mobile sources include a wide variety of equipment that either move under their own 

power or are capable of being moved from site to site. Non-road mobile equipment sources not 

licensed or certified as highway vehicles are defined as those that move or are moved within a 

12-month period and are covered under EPA's emissions regulations as non-road mobile sources.

The three types of non-road mobile sources are non-road equipment, such as construction and 

recreational off-road vehicles; locomotives; and aircraft. Non-road mobile sources accounted for 

258 tons per year PM10, 2,895 tons per year NOx, 90 tons per year SO2, and 4,182 tons per year

VOC in the 2008 ERG/Environ inventory.

5.1.4 Point Sources

To capture all pollutants and all facilities located within the maintenance area for the 

ERG/Environ inventory, facilities were divided into point or area sources based on the emission 

levels of all criteria pollutants. If a facility exceeded a defined point for any pollutant, it was

included as a point source. The ERG/Environ inventory defines point sources as any facility 

emitting greater than any of the following thresholds: 

 5 tons per year of PM10, PM2.5, SO2, or ammonia (NH3)

 10 tons per year of VOCs

 25 tons per year of NOx or CO 

Facilities were included in the point source category for PM10 if they exceeded the limit for any 

pollutant. Sources with annual emissions below these levels for all criteria pollutants were

included in the area source inventory. The universe of permitted point sources in northern Ada 

County is larger than the point sources included in the ERG/Environ inventory.

The total 2008 annual point source emissions inventory results were 169 tons per year of PM10

emissions with 0.45 tons per day emitted in the PM season and 0.46 tons per day emitted in Ada 

County in the ozone season. Point sources also contribute 356 tons per year NOx, 66 tons per 

year SO2, and 268 tons per year VOC.

Previous modeling indicated that emissions from the TASCO facility in Canyon County, outside 

of the maintenance area were “shown to potentially contribute to PM10 exceedances in Canyon 

County”, however neither the photochemical model nor the CMB modeling indicated any

significant impact in Ada County. Nevertheless TASCO’s permit-allowable emission levels were 

addressed in the Tier II operating permit issued as part of the control measures in the 2002 SIP 

“to insure that TASCO impacts in Ada County are at acceptable levels.” Pertinent permit 

requirements are included in Idaho’s SIP at 40 CFR 52.670. 

In view of the 2002 concerns about potential Canyon County PM10 exceedances due to TASCO, 

it should be noted that TASCO’s 2008 PM10 emissions (219 μg/m3) represent a 35% reduction 

over the 1999 base year emission in the 2002 SIP. In addition, the Nampa, Idaho average 2nd

high PM10 value for 2009-2011is 85 μg/m3at the Nampa Fire Station, approximately 2 miles 

from TASCO. Therefore, TASCO emissions, as limited by the Tier II operating permit, appear 

to have effectively eliminated the threat to the PM10 NAAQS in Canyon County.
Northern Ada County PM10 Maintenance Plan

12

5.2 Projected Inventories

The methodologies used in the future year (2015 and 2023) emissions projections are discussed 

in section 7 of the ERG/Environ inventory (Appendix D). Future year projections for on-road

MOVES emissions modeling and paved road dust emissions are included in Appendix E, 

Development of the Base- and Future-Year Mobile Source Emission Inventories for the Treasure 

Valley, Idaho.

Total PM10 emissions in Ada County are projected to increase through 2023. Most of the 

increase in emissions can be attributed to an increase in paved road dust, which increases in 

proportion to the increase in VMT. Despite this expected increase, modeling shows that the PM10

standard will be protected. As such, the modeled concentrations are expected to be high 

(i.e., conservative) compared to actual expected concentrations, for reasons summarized in 

Section6.3, second paragraph.

NOx and VOCs from on-road sources, important in secondary aerosol formation, are expected to 

continue decreasing over the life of the maintenance plan as more efficient vehicles continue to 

replace older, higher emitting vehicles. Table 7 and Table 8 show the annual emissions by 

source category for 2015 and 2023, respectively.

Table 7. Annual emissions by source category—2015.

Source 

Type

NOx SO2 VOC PM10 PM2.5 TC 

(tons per year)

Point 355.6 65.7 268.1 169.1 142.6 68.5

Area 900.0 23.6 14,551.2 21,107.4 3,651.0 7,806.1

On-road 5,856.6 33.4 2,939.7 283.0 193.0 246.0

Non-road 1,979.8 28.2 1,480.7 196.9 185.8 172.7

Biogenic 202.3 0.0 12,802.5 0.0 0.0 0.0

Total 9,294.3 150.9 32,042.2 21,756.4 4,172.4 8,293.3

Note: nitrogen oxides (NOx); sulfur dioxide (SO2); volatile organic compounds (VOC); an aerodynamic diameter less 

than or equal to a nominal 10 micrometers (PM10); particulate matter with a diameter 2.5 micrometers or less (PM2.5); 

total carbon (TC)

Note: Source type categories may not sum exactly to the total emissions listed due to rounding. 

Table 8. Annual emissions by source category—2023.

Source 

Type

NOx SO2 VOC PM10 PM2.5 TC 

(tons per year)

Point 391.2 72.3 294.9 186.0 156.8 75.3

Area 951.9 23.8 18,062.8 25,267.1 4,072.7 9,775.5

On-road 4,306.4 42.0 2,396.8 285.2 157.1 228.6

Non-road 1,355.0 33.8 1,407.9 135.8 125.9 119.9

Biogenic 202.3 0.0 12,802.5 0.0 0.0 0.0

Total 7,206.7 171.9 34,964.8 25,874.1 4,512.5 10,199.2

Note: nitrogen oxides (NOx); sulfur dioxide (SO2); volatile organic compounds (VOC); an aerodynamic diameter less 

than or equal to a nominal 10 micrometers (PM10); particulate matter with a diameter 2.5 micrometers or less (PM2.5); 

total carbon (TC)

Note: Source type categories may not sum exactly to the total emissions listed due to rounding. 
Northern Ada County PM10 Maintenance Plan

13

5.3 Motor Vehicle Emissions Budget

Clean Air Act §176 requires any activity that is federally supported or permitted to conform with 

approved air quality SIPs. To help guide the transportation process, Clean Air Act §108(e) 

compelled the EPA administrator and secretary of transportation to develop a “continuous 

transportation-air quality planning” process and guidance on developing and implementing

transportation and other measures necessary to demonstrate and maintain attainment of NAAQS. 

This effort led to the “conformity” rules found at 40 CFR 93.100–93.129. 

The intent of the conformity rules is to synchronize the air quality planning process with 

transportation plans developed by metropolitan planning organizations (MPOs) and other 

transportation organizations to ensure air quality standards are met. Simply put, transportation 

plans must “conform” to air quality plans and show that transportation projects using federal 

funds or deemed to be “regionally significant” in air quality nonattainment and maintenance 

areas do not contribute to a degradation of air quality.

COMPASS serves as the MPO for Ada County. COMPASS develops a regional long-range 

transportation plan (Communities in Motion) that identifies needed transportation projects that 

can be funded within the life of the plan (20 years and beyond). Only funded projects can be 

evaluated for air quality conformity.

Near-term transportation projects are included in the transportation improvement program (TIP).

The TIP is a short-range (3–5 year) budget of transportation projects that lists all projects for 

which federal funds are anticipated, along with non-federally funded projects that are regionally 

significant. Projects in the TIP must be consistent with the regional long-range transportation 

plan. 

To reconcile long-range transportation plans and TIPs with air quality implementation plans, the 

conformity rules at 40 CFR 93.118 require these plans be consistent with the MVEB, which is 

part of the air quality SIP. This PM10 maintenance plan covers the time period of 2013 through 

2023. Since the regional long-range transportation plan has at least a 20-year horizon and 

updated plans will be developed in 2014, 2018, and 2022, the MVEB needs to cover 

transportation projects through the 2040s. For this plan, DEQ has chosen to establish MVEBs for 

the base year of 2008, an intermediary year of 2015, and an ending year of 2023, which will be 

applied to transportation plans with horizon years extending to 2050. 

5.3.1 MVEB for PM10 

The PM10 MVEB covers fugitive dust from paved and unpaved roads and direct PM10 vehicle 

emissions (due to exhaust, tire wear, and break wear). Construction emissions from 

transportation projects were determined not to be significant and are not included in the MVEB 

for conformity purposes. EPA’s MOVES model was used to develop vehicle emissions estimates 

for the MVEB. The road dust and vehicle emissions were estimated for the time periods 2008–

2015 and 2015–2023. To ensure the 2023 MVEB is sufficient to cover the horizon year of future 

transportation plans, the motor vehicle emissions in the last period (2023–2050) are forecasted 

2050 emissions based only on VMT increases from 2015 to 2050. PM10 emissions for the MVEB 

years 2015 and 2023 were increased by a 10% safety margin as allowed by the conformity rule at 

40 CFR 93.124(a). Since PM10 emissions are primarily influenced by VMT and the VMT is 

trending upward, the emissions are also projected to increase over time. Therefore, the largest 
Northern Ada County PM10 Maintenance Plan

14

emissions for PM10 are in the last year of each of the time periods listed above. The conformity 

rule states that “emissions in years for which no motor vehicle emissions budget are specifically 

established must be equal to the motor vehicle emission budget(s) established for the most recent 

prior year” (40 CFR 93.118(b)(1)(ii)). Table 9 highlights the PM10 MVEB for each planning 

period.

Table 9. PM10 motor vehicle emissions budget.

Time Period PM10 MVEB

(tons per day)

2008 31.0

2015 42.9

2023 60.1

Note: The PM10 emissions were taken from the “Development of the Base- and Future-Year Mobile Source Emission 

Inventories for the Treasure Valley, Idaho” (Appendix E). They include the annual tail pipe emissions (Table 10), 

annual paved road dust emissions (Table 13), and the unpaved road dust emissions (Table 16) in Appendix E. The 

annual emissions were divided by 365 days to calculate tons per day and for the 2008–2015 and 2015–2023 periods, 

increased by a 10% safety margin.

5.3.2 MVEB for NOx and VOC

The conformity rule requires an MVEB for VOCs and NOx as precursors to PM10 when they are 

significant contributors (40 CFR 93.109(b)(2)(iii)). Sulfur dioxide (SO2) is an insignificant 

contributor to the secondary aerosol formation and is not included. Although NH3 contributes to 

secondary aerosol formation, the region is “ammonia rich,” so the very small mobile source NH3

emissions are also not considered in MVEBs. Accordingly, DEQ is establishing NOx and VOC 

emissions budgets for the base year 2008 as well as 2015 and 2023. The forecasted NOx and 

VOC emissions are expected to decline over time even though the MVEB increases (see Table 

10 footnote). Similar to the PM10 MVEB, a safety margin of 10% was added to the 2008–2015 

and 2015–2023 planning periods (Table 10). 

Table 10. NOx and VOC motor vehicle emissions budget.

Time Period NOx MVEB VOC MVEB

2008 29.5 12.6

2015 29.5 12.6

2023 34.2 17.2

Note: From Table 23 in the “Speciated Linear PM10 Roll-Forward Modeling Report” (Appendix C). As discussed in that 

report, the model does not treat nonlinear chemical processes, so NOx and VOC were held constant for modeling 

purposes even though they are generally declining over time. However, since the out year of 2050 was calculated by 

multiplying the projected 2050 vehicle miles traveled by the projected 2015 vehicle fleet emission rates, this causes 

an increase in NOx and VOC because the fleet emission rates are held constant and emissions reductions expected 

from a newer fleet are not included. The emissions from this conservative approach were included in the PM10 rollforward modeling run to demonstrate adequacy of the MVEB.

5.3.3 MVEB Conclusion

Before the MVEB can be used for conformity purposes it must demonstrate “adequacy.” For 

EPA to determine the MVEB is adequate, the conformity rule requires that MVEB emissions 

must be considered with all other emissions sources and be consistent with the applicable 

maintenance plan (40 CFR 93.118(e)(4)(iv)). Modeling must demonstrate that all emissions
Northern Ada County PM10 Maintenance Plan

15

sources combined (point sources, area sources, and mobile sources) will not cause or contribute 

to a violation of the PM10 NAAQS. As required by the conformity rule, the MVEBs were 

included with all other emissions in modeling, which demonstrated that future concentrations 

were below the PM10 NAAQS. For a complete discussion on the modeling demonstration, see 

Appendix C, Speciated Linear PM10 Roll-Forward Modeling Report.

6 Maintenance Demonstration

DEQ has completed the Speciated Linear PM10 Roll-Forward Modeling Report, which is 

included as Appendix C.

6.1 Air Quality Modeling Approach

Linear roll-forward modeling is a relatively simple technique for evaluating the effect of 

emission reductions on ambient concentrations of air pollutants. The model assumes that ambient 

concentrations above some regional background level are proportional to the estimated emissions 

of the local sources. By reducing the size of one or more of the local sources, the resulting 

reduction in ambient concentrations can be estimated. Since PM10 is composed of several 

different components (i.e., geologic material, carbon mass, ammonium nitrate, and ammonium 

sulfate), roll-forward calculations must be performed on each of these species individually to 

more accurately evaluate different emissions reductions or control options.

Although the roll-forward model does not analyze the chemical reactions and dispersion of the 

pollutants and it provides neither spatial nor temporal information for pollutant concentrations, it 

does provide a low-cost and relatively reliable approach to estimate the pollutant levels for 

worst-case meteorology, annual average, and seasonal average conditions. The roll-forward 

model can be used to safely estimate whether the PM10 concentrations can be maintained below 

the NAAQS in future years.

Since the 3-year average 24-hour PM10 values have been significantly lower than the PM10

NAAQS (150 μg/m3) over the last two decades in northern Ada County, DEQ has determined,

and EPA has agreed, that the roll-forward model is an appropriate method to demonstrate

compliance in future years.

Four speciation profiles and an annual profile were created for the following scenarios:

wintertime “stagnation” (severe stagnation events, wet conditions); wintertime “high-winter” 

(drier winter stagnation conditions); high carbon mass events; high crustal mass events in 

nonwinter seasons; and annual average. 

The data required to conduct roll-forward modeling included monitored PM10 data, base-year 

and projected future-years emissions data, and background PM10 data. All data were speciated for 

major PM10 components: geological mass (crustal mass), carbon mass, sulfate mass (ammonium 

sulfate), and nitrate mass (ammonium nitrate). Additionally, the emissions inventory included the 

precursor emissions (NOx, SO2, and NH3) for the secondary aerosols.

Data from 2007–2009 were used for the demonstration, as they were the most complete data set 

(including ambient concentrations, background, and speciation) available at the time of this 
Northern Ada County PM10 Maintenance Plan

16

analysis. Some additional data that have since become available are also discussed. The newer 

data do not change the conclusions reached in the completed analysis.

6.2 Attainment Demonstration

The model has demonstrated that the PM10 levels in northern Ada County will remain well under 

the NAAQS in future years, despite the increase of predicted county-level road dust emission 

rates. Tables 11–15 and Figures 4–8 show the relative importance of the major species at present 

and in future years. Three species—crustal mass, total carbon mass, and nitrate mass—will 

continue to play important roles in future years. In all scenario types and all time periods, the 

contribution of sulfate remains low. The sulfate contribution is decreasing even though the total 

amount remains the same since total PM10 is increasing. The annual average scenario and other 

four scenarios are described below. 

6.2.1 Annual Average 

In the case of annual average PM10 concentrations, Table 11 and Figure 4 show that crustal mass, 

which is mainly due to road dust, is primarily responsible for the increase in total PM10

concentrations, while the other species remain virtually unchanged.

Table 11. Predicted annual average PM10 levels.

Year

Sulfate 

Mass

Nitrate 

Mass

Total 

Carbon

Crustal 

Mass Unknown PM10Totala

(μg/m3)

2008 1.1 1.2 3.8 16.9 0.4 23.3

2015 1.1 1.2 4.2 17.9 0.4 24.7

2023 1.1 1.2 5.0 20.2 0.4 27.9

a

For comparison to the now-revoked NAAQS annual PM10 standard of 50 μg/m3.

Figure 4. Predicted annual average PM10 concentration (in micrograms per cubic meter).

0

5

10

15

20

25

30

Sulfate

Mass

Nitrate

Mass

Total

Carbon

Crustal

Mass

Unknown PM10

Predicted Annual PM10

2008

2015

2023
Northern Ada County PM10 Maintenance Plan

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6.2.2 Nonwinter High Crustal Mass Scenario

In the nonwinter high crustal mass scenario, road dust (i.e., crustal mass) is the primary 

contributor (Table 12; Figure 5). Similar to the annual average scenario, crustal mass increases 

with time while the other contributors change very little.

Table 12. Predicted 24-hour average PM10 levels for high crustal mass scenario in nonwinter 

seasons.

Year

Sulfate 

Mass

Nitrate 

Mass

Total 

Carbon

Crustal 

Mass Unknown PM10Totala

(μg/m3)

2008 3.2 0.0 8.6 77.2 1.0 90.0

2015 3.2 0.0 9.7 80.1 1.1 94.1

2023 3.2 0.0 11.7 89.1 1.2 105.1

a

For comparison to the NAAQS 24-hour PM10 standard of 150 μg/m3.

Figure 5. Predicted 24-hour average PM10 levels (in micrograms per cubic meter) for high crustal 

mass scenario in nonwinter seasons.

6.2.3 Nonwinter High Carbon Scenario

In this scenario, crustal mass (i.e., road dust) is still the predominant contributor, but high levels 

of carbon mass indicate the presence of wildfires in the area. High carbon scenario days have

lower PM10 concentrations than PM2.5, also indicating the presence of fires. Table 13 and Figure 

6 show both carbon and crustal mass make significant contributions to PM10.

0

20

40

60

80

100

120

Sulfate Mass Nitrate Mass Total Carbon Crustal Mass Unknown PM10

Predicted PM10

Nonwinter High Crustal Scenario

2008

2015

2023
Northern Ada County PM10 Maintenance Plan

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Table 13. Predicted 24-hour average PM10 levels for high carbon scenario in nonwinter seasons.

Year

Sulfate 

Mass

Nitrate 

Mass

Total 

Carbon

Crustal 

Mass Unknown PM10Totala

(μg/m3)

2008 2.8 1.5 29.7 54.8 1.2 90.0

2015 2.8 1.5 34.1 56.8 1.3 96.5

2023 2.8 1.5 41.5 62.8 1.5 110.1

a

For comparison to the NAAQS 24-hour PM10 standard of 150 μg/m3.

Figure 6. Predicted 24-hour average PM10 levels (in micrograms per cubic meters) for high carbon

scenario in nonwinter seasons.

6.2.4 High Winter Scenario

The high winter scenario, while also stagnant, differs from the stagnant winter scenario due to 

relatively warmer and drier conditions. Table 14 and Figure 7 show that carbon mass plays the 

most important role, followed by crustal mass; the importance of nitrate decreases with time as a 

result of tightening federal motor vehicle standards.

Table 14. Predicted 24-hour average PM10 levels for high winter scenario.

Year

Sulfate 

Mass

Nitrate 

Mass

Total 

Carbon

Crustal 

Mass Unknown PM10 Totala

(μg/m3)

2008 4.8 20.5 34.5 28.3 1.9 90.0

2015 4.8 20.5 37.8 32.4 2.0 97.6

2023 4.8 20.5 45.9 42.0 2.4 115.7

a

For comparison to the NAAQS 24-hour PM10 standard of 150 μg/m3.

0

20

40

60

80

100

120

Sulfate

Mass

Nitrate

Mass

Total

Carbon

Crustal

Mass

Unknown PM10

Predicted PM10

Nonwinter High Carbon Scenario

2008

2015

2023
Northern Ada County PM10 Maintenance Plan

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Figure 7. Predicted 24-hour average PM10 levels (in micrograms per cubic meters) for high winter 

scenario.

6.2.5 Stagnant Winter Scenario

A stagnant event is a high PM scenario with a severely stagnant atmosphere and very cold, wet 

conditions, such as snow cover on the ground. These episodes usually last for a week or longer.

Table 15 and Figure 8 show nitrate, carbon, and crustal mass having approximately similar 

contributions near term, but with declining nitrate and increasing crustal mass contributions in 

future years.

Table 15. Predicted 24-hour average PM10 levels for stagnation scenario in winter season.

Year

Sulfate 

Mass

Nitrate 

Mass

Total 

Carbon

Crustal 

Mass Unknown PM10Totala

(μg/m3)

2008 7.1 37.7 19.4 24.5 1.2 90.0

2015 7.1 37.7 21.3 27.8 1.3 95.2

2023 7.1 37.7 25.8 35.6 1.5 107.7

a

For comparison to the NAAQS 24-hour PM10 standard of 150 μg/m3.

0

20

40

60

80

100

120

140

Sulfate

Mass

Nitrate

Mass

Total

Carbon

Crustal

Mass

Unknown PM10

Predicted PM10

High-winter Scenario

2008

2015

2023
Northern Ada County PM10 Maintenance Plan

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Figure 8. Predicted 24-hour average PM10 levels (in micrograms per cubic meter) for stagnation 

scenario in winter season.

6.3 Modeling Summary

The roll-forward modeling has demonstrated that northern Ada County will meet the PM10 24-

hour NAAQS in future years up to 2023 with at least a 23% margin of safety. The model results 

for mobile source emissions conformity also show that the ambient PM10 concentrations will be 

below the NAAQS up to year 2023. Presently, nitrate mass contributes much less to the total 

ambient PM10 compared to the early 1990s. Based on the future year inventories, particularly 

NOx reductions due to federal vehicle emission standards, this trend in declining nitrate aerosol 

will likely continue into the future or at least remain level. Carbon mass will remain an 

important contributor in future years. Sulfate mass makes minimal contributions currently and 

even less in the future. 

Crustal mass will be the important contributor to ambient PM10 in all occasions. The relative 

importance of crustal mass will increase over time due to an increase in vehicle miles travelled. 

The relative importance of nitrate mass is likely to drop with decreases in vehicle NOx 

emissions, or at least remain level. Nitrate mass was conservatively assumed to remain constant, 

rather than decrease as expected due to lower new car NOx emissions in the future. This 

conservative nitrate emissions treatment is intended to allow for potential uncertainty in the 

nitrate formation process (see Appendix C, Section 6.5).

In addition, the model results are conservative since the design value was determined based on 

the relatively high monitored values on days that were often influenced by windblown dust or 

wildfire smoke. It is also important to note that projected future VMT increases with associated 

crustal mass increase will be spread throughout the valley rather than concentrated at the 

downtown Boise area where exceedances have occurred in the past. As a result, future 

concentrations are not expected to increase with the same rate of VMT growth as assumed in this 

simplified roll-forward model based on the downtown monitor. This conclusion is reinforced by 

the steady decline in PM10 levels witnessed over the past decade.

0

20

40

60

80

100

120

Sulfate

Mass

Nitrate

Mass

Total

Carbon

Crustal

Mass

Unknown PM10

Predicted PM10

Winter Stagnation Scenario

2008

2015

2023
Northern Ada County PM10 Maintenance Plan

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7 Permanent and Enforceable Improvement in Air Quality

The Clean Air Act requires each maintenance plan to demonstrate that measures credited with 

bringing an area into attainment are federally enforceable and will continue into the future. The 

residential wood burning and open burning ban program meet this requirement as they are 

enacted and currently being implemented at the respective government levels throughout 

northern Ada County. These programs were approved by EPA as meeting the RACM/RACT 

(reasonably available control measures/reasonably available control technology) requirements 

(see 59 FR 48583) and were found to meet the enforceability requirements of the Clean Air Act.

The maintenance demonstration in section 6 provides documentation that the area will maintain 

PM10 attainment for the foreseeable future as a result of implementing these measures. 

Therefore, the northern Ada County plan meets the Clean Air Act requirement for permanent and 

enforceable control measures.

7.1 Control Measures

The following control measures resulted in the area’s continued attainment of the PM10 NAAQS.

Continued implementation of these measures should ensure future maintenance of the PM10

NAAQS through the year 2023, which is the duration of the maintenance period.

7.1.1 Air Quality Index Program

DEQ operates an Air Quality Index (AQI) Program to support the public information and 

regulatory components of the PM10 SIP. Through this program, DEQ provides information on 

the measured and predicted ambient air pollution levels for the highest measured and highest 

predicted criteria pollutant. Information on voluntary or mandatory open burning bans and

indoor wood burning bans is provided via several formats. AQI information and information on 

burn conditions is posted daily on DEQ’s website, is made available via recorded message 24-

hours per day, and is e-mailed to people who wish to be on a daily or an “alert” list. The alert list 

is only sent on days when the AQI is forecast to be at a level where local no-burn ordinances go 

into effect.

7.1.2 Residential Wood Burning Program

Key elements in the current residential wood-burning program include issuing a voluntary burn 

ban when PM10 is at, or is expected to reach approximately 64 μg/m3and a mandatory burn ban 

when levels reach approximately 100 μg/m3of PM10. Each city in Ada County and 

unincorporated Ada County has an existing ordinance prohibiting the use of woodstoves or 

fireplaces when an air quality alert is issued, as noted above.

A woodstove change-out program is in place, and the cities and counties have ordinances that 

prohibit installation of non-EPA-certified woodstoves. Additionally, Idaho provides tax 

incentives for replacing older, more polluting woodstoves for cleaner-burning EPA-certified 

woodstoves.

7.1.3 Open Burning Ban Program

The open burn ban program includes voluntary bans when PM10 reaches approximately 64 μg/m3

and mandatory bans when the forecast AQI is 60 or greater for any pollutant (i.e., approximately 

74 μg/m3for PM10).
Northern Ada County PM10 Maintenance Plan

22

Additional mandatory burn bans can be implemented by DEQ beginning at the Stage 1 -- Air 

Pollution Forecast and Caution level. The stage one level is the 24-hour NAAQS of 150 μg/m3as 

set in the “Rules for the Control of Air Pollution in Idaho” (IDAPA 58.01.01.550).

7.1.4 Stationary Source Control Measure

In 2002, stationary sources made up a relatively small fraction of the total annual PM10 emissions 

inventory at 3.2%. However, the modeling conducted for the 2002 analysis for future year 

compliance indicated that a subset of existing stationary sources had the potential to interfere 

with maintenance of the NAAQS. Therefore, the 2002 maintenance plan addressed 13 facilities 

located within the maintenance area that had the potential to interfere with maintenance of the 

NAAQS based on their potential or allowable emissions (Environ 2002). DEQ used the authority 

provided in IDAPA 58.01.01.401.03 to require that Tier II permits be issued to these facilities.

These permits established federally enforceable permit conditions that limited the ambient 

impacts due to facility emissions to acceptable levels. Although actual emissions from these 

facilities did not cause or contribute to a violation of the NAAQS, limits on the potential to emit 

were necessary to demonstrate ongoing maintenance of the NAAQS. Currently, 8 of these 

facilities continue to operate in the maintenance area.

In addition to the facilities discussed above, permit conditions for the TASCO facility in Nampa, 

Idaho, which is outside of the maintenance area but within the modeling domain, were included

in the 2002 maintenance plan. A Tier II operating permit was issued to TASCO that required 

reductions in actual, as opposed to potential, PM10 emissions. TASCO emissions are 39% lower 

in the 2008 EI than they were in the 1999 inventory. 

Based on the emissions inventory used for this plan, point sources within the maintenance area, 

accounted for 0.8% of the 2008, 0.8% of the 2015, and 0.7% of the 2023 annual PM10 emissions. 

The previously approved stationary source control measures included in the 2002 maintenance 

plan submittal and codified in 40 C.F.R. § 52.670 will remain in effect. In addition, Idaho’s 

permit program will continue to be the control measure for new and modified sources as well as 

any sources determined to pose a threat to continued maintenance of the PM10 NAAQS. 

8 Contingency Plan 

Based on the emissions inventory used for this plan, point sources within the maintenance area, 

accounted for 0.8% of the 2008, 0.8% of the 2015, and 0.7% of the 2023 annual PM10 emissions.

The previously approved stationary source control measures included in the 2002 maintenance 

plan submittal and codified in 40 C.F.R. § 52.670 will remain in effect. In addition, Idaho’s 

permit program will continue to be the control measure for and new and modified sources as well 

as any sources determined to pose a threat to continued maintenance of the PM10 NAAQS. 

Section 175A of the Clean Air Act requires that a maintenance plan include contingency 

provisions as necessary to promptly correct any violation of the NAAQS that may occur after 

redesignation to attainment. Areas in attainment are not required to have enacted contingency 

measures, just a list of measures that could be considered for future implementation.
Northern Ada County PM10 Maintenance Plan

23

The contingency plan must ensure that the contingency measures are adopted expeditiously once 

the need is triggered. The elements required of a contingency plan are the tracking plan and 

triggering mechanism to determine if and when contingency measures are needed, the list of 

potential contingency measures, and a description of the process for recommending and 

implementing the contingency measures.

8.1 Tracking 

The tracking plan for the Northern Ada County Maintenance Area will consist of monitoring and 

analyzing the PM10 concentrations. DEQ will continue to operate and maintain the PM10 monitor 

in accordance with 40 CFR Part 58. 

8.2 Triggering

The triggering mechanism for contingency measure implementation will be a violation of the 

PM10 NAAQS. Specifically, the 3-year average number of expected exceedances at a monitoring 

site would need to be greater than 1.0 for a violation to occur.

Contingency plan triggering does not necessarily mandate a SIP revision, nor is the area 

necessarily redesignated to nonattainment. The state will normally have an adequate and 

appropriate period of time to correct the violation by implementing one or more contingency 

measures, as appropriate. In the event that violations continue to occur after measures have been 

implemented, additional contingency measures will be implemented until the violations are 

corrected.

8.3 Potential Contingency Measures

If monitoring data indicate a violation of the PM10 NAAQS, Idaho will examine the data to 

assess the spatial extent, severity, cause, and time period of the episode as well as trends over 

time. Based on this analysis, Idaho will determine which measures to implement. The following 

list of potential contingency measures has been or can be implemented if a violation of the PM10

standard occurs in the future. 

 Adopt local ordinances that require covering all loads of material that may have the 

potential to contribute to particulate matter pollution.

 Adopt local ordinances that require no track-out onto paved roads. 

 Adopt local ordinances that prohibit burning of household garbage.

 Eliminate local permits that allow any kind of uncontrolled outdoor burning not 

specifically allowed under state rule.

 Expand the mandatory burning restrictions to include clean-burning woodstoves during 

air quality alerts.

 Adopt local ordinances that prohibit constructing any unpaved private roads, driveways,

or parking lots.

 Revise street sweeping plan with local highway districts and the Idaho Transportation 

Department based on latest traffic data to prioritize street sweeping efforts to reduce 

fugitive road dust.

 Analyze the impacts from all industrial sources. Develop potential emission reductions, if 

necessary to maintain attainment, in accordance with the rules.
Northern Ada County PM10 Maintenance Plan

24

Contingency measures that address emissions from stationary sources include the DEQ director’s 

compliance order and enforcement authorities and the authority provided by IDAPA 

58.01.01.401.03 to require or revise a permit of any stationary source, at any time, whenever it is 

determined that emission rate reductions are necessary to attain or maintain any ambient air 

quality standard.

In addition to the potential contingency measures discussed above, DEQ may evaluate other 

strategies to address any future violations in the most appropriate and cost-effective manner 

possible. Due to continual changes in the mixture of PM10 sources and evolving technologies to 

understand and control PM10 emissions and precursors, other contingency measures may become 

viable in the future. DEQ will evaluate the need and viability of additional contingency measures 

and will consider future additions to the listed measures, if necessary. 

9 Summary 

This plan for continued attainment of the PM10 NAAQS meets all requirements of a 10-year 

maintenance plan update.

9.1 Attainment Verification

Attainment of the NAAQS for PM10 in northern Ada County was demonstrated in section 4, “Air 

Quality Monitoring—Verification of Attainment.”

9.2 Full Approval of the State Implementation Plan under Section 110

The SIP has been fully approved per section 110, with approval dates included in section 3 of 

this plan. Following a public comment period, DEQ will submit this maintenance plan update to 

EPA for approval. A summary of public participation will be provided in Appendix F. 

9.3 Permanent and Enforceable Improvement in Air Quality

DEQ demonstrated in section 7 that improvements in air quality are based on permanent and 

enforceable emissions reductions.

9.4 Maintenance Plan Elements

The following sections have met the required maintenance plan requirements of section 110.

 Section 5: Emissions Inventory

 Section 6: Maintenance Demonstration

 Section 8: Continued Air Monitoring and Verification of Attainment

 Section 9: Contingency Plan

9.5 Subsequent Maintenance Plan Revisions

This revised maintenance plan provides for continued maintenance of the PM10 NAAQS for an 

additional 10 years beyond the original 10-year period. Consequently, no further maintenance 

plan updates are anticipated.
Northern Ada County PM10 Maintenance Plan

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References

CFR (Code of Federal Regulations). “Ambient Air Quality Surveillance.” 40 CFR Part 58.

CFR (Code of Federal Regulations). “Approval and Promulgation of Implementation Plans.”

40 CFR Part 52.

CFR (Code of Federal Regulations). “Determining Conformity of Federal Actions to State or 

Federal Implementation Plans.” 40 CFR Part 93.

CFR (Code of Federal Regulations). “Requirements for Preparation, Adoption, and Submittal of 

Implementation Plans.” 40 CFR Part 51.

DEQ (Idaho Department of Environmental Quality). 2009. Quality Assurance Project Plan for 

the State of Idaho Ambient Air Quality Monitoring Program. Boise, ID: DEQ. 

Environ. 2002. Northern Ada County PM10 SIP Maintenance Plan and Redesignation Request. 

Novato, CA: Environ. Prepared for the Idaho Department of Environmental Quality. 

EPA (United States Environmental Protection Agency). 1994. Receptor Model Source 

Composition Library. Research Triangle Park, NC: EPA, Office of Air Quality Planning 

and Standards. 

EPA (United States Environmental Protection Agency). 1999. Handbook for Criteria Pollutant 

Inventory Development: A Beginner’s Guide for Point and Area Sources. Research 

Triangle Park, NC: EPA, Office of Air Quality Planning and Standards.

EPA (United States Environmental Protection Agency). 2011. AP 42 Compilation of Air 

Pollutant Emission Factors, Section 13.2.1 Paved Roads.

EPA (United States Environmental Protection Agency). 2008. Quality Assurance Handbook for 

Air Pollution Measurement Systems. Volume II: Ambient Air Quality Monitoring 

Program. Research Triangle Park, NC: EPA, Office of Air Quality Planning and 

Standards.

Idaho Code. 2011. “Idaho Environmental Protection and Health Act.” Idaho Code 39-101 

through 39-130.

IDAPA. 2012. “Rules for the Control of Air Pollution in Idaho.” Idaho Administrative Code. 

IDAPA 58.01.01.

US Congress. 1970. Clean Air Act. 42 USC §7401 et seq. 
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Appendices

Appendix A. Rules Establishing State Authority

Appendix B. Legal Description for Northern Ada County PM10 Maintenance Area

Appendix C. Speciated Linear PM10 Roll-Forward Modeling Report: In Support of the 

Northern Ada County PM10 Maintenance Plan Renewal

Appendix D. 2008, 2015, and 2023 Emissions Inventories for the Treasure Valley Airshed

Appendix E. Development of the Base- and Future-Year Mobile Source Emissions Inventory

for the Treasure Valley, Idaho

Appendix F. Legal Notification of Public Comment Period, Public Comments Received, Public 

Hearing Documents, and Response to Public Comments
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