Northern Ada County Air Quality Maintenance Area 

Second 10-Year Carbon Monoxide Limited 

Maintenance Plan 

February 2011 

Prepared by: 

Idaho Department of Environmental Quality 

1445 North Orchard, Boise, Idaho 83706-1255 
Table of Contents 

1. Introduction.............................................................................................................................. 2 

2. Background - Nonattainment Designation and Redesignation to Attainment .................. 2 

3. Maintenance Area Description............................................................................................... 2 

4. Requirement for Second Maintenance Plan.......................................................................... 3 

5. Limited Maintenance Plan Option......................................................................................... 3 

5.1 Attainment Year Emissions Inventory............................................................................. 3 

A. Point Sources..................................................................................................................... 4 

B. Area Sources...................................................................................................................... 4 

C. On Road Mobile Sources .................................................................................................. 4 

D. Non-Road Mobile Sources................................................................................................ 5 

5.2 Maintenance Demonstration.............................................................................................. 5 

A. Air Quality Criteria .......................................................................................................... 5 

B. Maintenance Plan Control Measures.............................................................................. 6 

5.3 Monitoring Network /Verification of Continued Attainment........................................ 8 

5.4 Contingency Plan ............................................................................................................... 8 

A. Contingency Measure Triggering Protocol...................................................................... 8 

B. Contingency Measures...................................................................................................... 8 

5.5 Conformity.......................................................................................................................... 8 

A. Transportation Conformity ............................................................................................... 9 

B. General Conformity......................................................................................................... 10 

6. Conclusion .............................................................................................................................. 10 

Appendix A. Legal Description for Ada County CO Maintenance Area 

Appendix B. References 

Appendix C. Emissions Inventory 

Appendix D. U.S. EPA Air Quality System Quick Look Report 

Appendix E. Ada County Vehicle Inspection and Maintenance Program 

Appendix F. Rules Establishing State Authority 

Appendix G. Public Comment, Public Hearing and Response

Tables

Table 1 – 2008 Ada County Carbon Monoxide Emissions...........................................4 

Table 2 – 2nd Max 8-Hour Monitored CO Values......................................................6 

Figures

Figure 1 – Map of Northern Ada County CO Maintenance Area...................................2 

Figure 2 – Northern Ada County Carbon Monoxide Trend..........................................5 

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Northern Ada County Second 10-Year Limited Maintenance Plan 

1. Introduction 

The Clean Air Act requires that an area redesignated from nonattainment to maintenance submit 

to the Environmental Protection Agency (EPA) a plan for maintaining the National Ambient Air 

Quality Standards (NAAQS) for a second 10-year period. This Northern Ada County limited 

maintenance plan was prepared by the Idaho Department of Environmental Quality (DEQ) for 

inclusion into the Idaho State Implementation Plan (SIP) and will serve as the second 10-year 

maintenance plan for the Northern Ada County carbon monoxide (CO) maintenance area. The 

plan demonstrates that Northern Ada County will maintain the CO standard for a second 10-year 

period following the expiration of the first plan. 

2. Background - Nonattainment Designation and Redesignation to Attainment 

Northern Ada County (Boise), Idaho, was designated nonattainment for CO and classified as 

“not classified” upon enactment of the Clean Air Act (CAA) Amendments in 1990 based on 

violations of the 8-hour CO NAAQS. In January 2002, DEQ submitted a Limited Maintenance 

Plan and Request for Redesignation to Attainment for the Northern Ada County Carbon 

Monoxide Not-Classified Nonattainment Area to the EPA to support redesignation of the area to 

attainment status. The plan was approved in December 2002 and the area reclassified to 

attainment for CO1

. 

CO levels in Ada County have dropped since the 1970s due initially to the Vehicle Inspection 

and Maintenance (I/M) program and subsequently to federal vehicle emissions standards. In 

addition, efforts to increase the use of alternative transportation, as well as improvements in 

traffic flow in downtown Boise, have reduced congestion and minimized opportunities for CO 

pollution to build up in hotspot areas. The last exceedance of the NAAQS for CO in Northern 

Ada County was in January 1991. No violations (more than one exceedance of the standard in a 

single year) have been recorded since 1986. 

3. Maintenance Area Description 

The Northern Ada County Maintenance Area consists of only the portion of Ada County that is 

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

Figure 1 – Map of Northern Ada County CO Maintenance Area (Shaded Area). 

 

1

 Approval and Promulgation of Air Quality Implementation Plans; State of Idaho; Northern Ada County Carbon 

Monoxide Redesignation to Attainment. 67 Fed Reg. 65713-65718, October 28, 2002. Available at: 

http://yosemite.epa.gov/r10/airpage.nsf/283d45bd5bb068e68825650f0064cdc2/e2ab2cc6df433b8688256b2f00800ff

8/$FILE/67%20FR%2065713.pdf 

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South of the baseline the land is largely unpopulated and is primarily managed by federal 

agencies such as the Bureau of Land Management and 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 is populated and has 

sources of carbon monoxide. 

The legal description for the maintenance area can be found in Appendix A.

4. Requirement for Second Maintenance Plan 

Section 175A of the CAA Amendments requires redesignated areas to submit a second maintenance 

plan to EPA eight years after the first plan has been approved. The second plan must demonstrate 

continued compliance with the NAAQS during the 10-year period following expiration of the first 

plan. As with the first Northern Ada County CO 10-Year Limited Maintenance Plan, this second plan 

qualifies for the limited maintenance plan option in accordance with the October 6, 1995, EPA 

memorandum, “Limited Maintenance Plan Option for Nonclassifiable CO Nonattainment Areas,” 

from Joseph W. Paisie, Group Leader, Integrated Policy and Strategies Group (hereafter “the Paisie 

Memo”; see Appendix B – References Cited). Specific criteria and compliance methods are 

discussed in the following section. 

5. Limited Maintenance Plan Option 

As required by the limited maintenance plan option, the core provisions to be included in a 

limited maintenance plan for CO nonclassifiable areas are an attainment inventory, a 

maintenance demonstration, continued operation of a monitoring network, a contingency plan, 

and conformity determinations. 

5.1 Attainment Year Emissions Inventory 

In accordance with the Paisie Memo, Idaho is advised to provide an emissions inventory that 

“should represent emissions during the time period associated with the monitoring data showing 

attainment. The inventory should be based on actual “typical winter day” emissions of CO.” 

A comprehensive emissions inventory has been completed for carbon monoxide for 2008. 

The emissions inventory includes Ada County in its entirety, as opposed to just the Northern Ada 

County Maintenance Area. This is a more conservative approach that meets the maintenance 

plan requirements, while supporting the “airshed management” approach used by DEQ. The 

emissions inventory is attached as Appendix C. Table 1 shows the 2008 annual as well as 

average winter day carbon monoxide emissions for the maintenance area. The average winter 

day season used was calculated from November 1 through February 29, 2008 (2008 was a leap 

year). 

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Table 1. 2008 Ada County Carbon Monoxide Emissions – Annual and Average Winter 

Day Emissions (2008, 2015, and 2023 Emissions Inventories for the Treasure Valley Airshed) 

Source 2008 Annual Emissions 2008 Average Winter Day 

Emissions 

Tons Per Year Percent of Total Tons Per Day Percent of Total 

Point Sources 198.5 .2 0.5 .2

Major Point Sources 

w/in 25 miles 

862.4 1.1 4.2 1.6

Area Sources 7,715.1 9.5 49.7 18.8

On-Road Mobile 45,574.2 55.9 146.1 55.2

Non-Road Mobile 24,870.5 30.5 62.5 23.6

Total Anthropogenic 79,220.7 263.0 

Biogenic 2,246.5 2.8 1.5 .6

Total 81,467.2 264.5

A. Point Sources 

The emissions inventory developed for Ada County is a multi-pollutant inventory. To capture all 

pollutants and all facilities, facilities were divided into point or area sources based on emissions 

of all pollutants. If a facility exceeds a defined point for any pollutant, it is included as a point 

source for all pollutants. The Treasure Valley Emissions Inventory defines point sources as any 

facility emitting greater than any of the following thresholds: 5 tons per year of PM10, PM2.5, 

SOx, or NH3; 10 tons per year of volatile organic compounds; or 25 tons per year of NOx or CO. 

Therefore, facilities were included in the point source category for CO if they exceeded the limit 

for any pollutant. 

To be consistent with the approved December 2002 maintenance plan and included emissions 

inventory, facilities that emit in excess of 100 tons per year of CO and are within a 25-mile 

radius of the maintenance area, but are not within the maintenance area boundaries, are included 

in the CO inventory for Northern Ada County. These facilities are included in a separate line 

item in Table 1. This adds an additional 862.4 tons per year or 1.1% of the total to the CO 

emissions for the maintenance area. In total, point sources accounted for 1,060.9 tons per year or 

1.3% of the CO emissions. 

B. 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.” For the purposes of this emission inventory, area 

sources are defined as those sources emitting annual emissions less than the point source 

thresholds. Area sources accounted for 7,715.1 tons per year or 9.5% of the annual total. 

C. On Road Mobile Sources 

Motor vehicles remain the largest carbon monoxide source in the maintenance area, accounting 

for 55.9% of the annual total and 55.2% of the average winter day emissions. On-road mobile 

source emissions were estimated using vehicle miles traveled (VMT) data and emissions factors 

from EPA’s MOBILE6 vehicle model using outputs from the Community Planning Association 

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(COMPASS) transportation demand model (TDM). COMPASS is the metropolitan planning 

organization (MPO) for Northern Ada County and the Treasure Valley. 

D. Non-Road Mobile Sources 

Non-road mobile sources encompass 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, locomotives, and 

aircraft. Non-road mobile sources accounted for 30.5% of the annual and 23.6% of the average 

winter day carbon monoxide emissions. 

5.2 Maintenance Demonstration 

In accordance with the Paisie Memo “The maintenance demonstration requirement is considered 

to be satisfied for nonclassifiable areas if the monitoring data show that the area is meeting the 

air quality criteria for limited maintenance areas (7.65 ppm or 85% of the CO NAAQS......The 

EPA believes if the area begins the maintenance period at or below 85 percent of exceedances 

levels, the air quality along with the continued applicability of PSD requirements, any control 

measures already in the SIP, and Federal measures, should provide adequate assurance of 

maintenance over the initial 10-year maintenance period.” All of these criteria have been met in 

the Northern Ada County carbon monoxide maintenance area. 

A. Air Quality Criteria 

The last exceedance of the NAAQS for CO was in January 1991; however, no violation of the 

NAAQS occurred at that time. No violations (more than one exceedance of the standard in a 

single year) have been recorded since 1986. Current CO levels are well below the 8-Hour 

NAAQS of 9 ppm, with a 2008 design value of 2.9, or 32.2% of the NAAQS. Additionally, as 

illustrated in Figure 2, the maintenance area has shown a generally declining trend in the ambient 

eight-hour concentrations over the past several years. 

Northern Ada County Highest and Second Highest EightHour Average Carbon Monoxide Concentration

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

CO - Highest

Co 2nd Highest

Figure 2 – Northern Ada County Carbon Monoxide Trend 

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Design value is based on the second highest non-overlapping running eight-hour ambient carbon 

monoxide concentrations measured in eight consecutive quarters (two years of data). The method 

for calculating design values is presented in the June 18, 1990, EPA memorandum “Ozone and 

Carbon Monoxide Design Value Calculations,” from William G. Laxton, former director of the 

OAQPS Technical Support Division to Regional Air Directors (see Appendix B – References 

Cited). 2007 and 2008 data are used for this maintenance plan. These years are representative of 

current emissions levels. Design values for the last five years are presented in Table 2. 

Table 2. 2nd Max, 8-Hour Monitored CO Values 

Year 2nd Max Value 

2004 2.4 ppm 

2005 2.2 ppm 

2006 2.1 ppm 

2007 1.6 ppm 

2008 2.9 ppm 

Source: U.S. EPA Air Quality System Quick Look Report – see Appendix D. 

 

As shown in Table 2, the highest 2nd Max value of 2.9 ppm is well below the 7.65 ppm threshold 

established for the limited maintenance plan option and significantly below the eight-hour 

NAAQS of 9 ppm. 

B. Maintenance Plan Control Measures 

The control measures included in the initial Northern Ada County maintenance plan will be 

continued in the second 10-year maintenance plan. 

1. Federal Motor Vehicle Emissions Control Program 

A significant reduction in carbon monoxide emissions can be attributed to the federal motor 

vehicle emissions control program. Emissions standards for motor vehicles are established under 

Title II of the Clean Air Act. As newer vehicles replace older, more polluting vehicles, these 

standards will continue to have an increasing role in reducing carbon monoxide emissions. This 

program is federally enforced. 

2. Controls on Stationary Sources 

Idaho has a federally approved New Source Review Program, including a Prevention of 

Significant Deterioration program. Major sources that are new or are undergoing major 

modifications will continue to be required to undergo Prevention of Significant Deterioration 

review, which includes requirements for Best Available Control Technology. New minor 

sources or sources undergoing minor modifications must obtain a Permit to Construct, which 

includes provisions to ensure protection of the NAAQS. 

 

3. Ada County Vehicle Inspection and Maintenance (I/M) Program 

Vehicles registered in Ada County are required to pass an emissions test or face revocation of 

registration. 

Under local ordinances emissions testing is mandatory in Ada County, the cities of Boise, 

Meridian, Garden City, and Eagle. Modifications enacted between July 2000 and January 2001 

have made the I/M Program even more effective. Changes include: 

• Lowering the carbon monoxide cut-point, or allowable percentage 

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• Adding a standard for hydrocarbon emissions 

• Modifying the waiver policy so initial waivers can be issued only after at least $300 in 

repair work has been done on the vehicle 

• Adding diesel vehicles to the list of vehicles required to pass an emissions test 

• Expanding the test from idle only to a two-speed test conducted at both idle and at 2,500 

revolutions per minute 

• Eliminating the test and repair option for test stations and their owners. 

• Stations are no longer permitted to offer repair services to correct emissions. 

These improvements were not required by the Air Quality Improvement Plan. 

Although the I/M Program is currently operated under local ordinances, the Idaho State 

Constitution authorizes the legislature to impose rules relating to motor vehicles. 

State legislation passed by the 2008 legislature will impact implementation of I/M programs in 

Idaho in the future. Idaho Code § 39-116B mandates a vehicle emission testing program, or a 

program that results in equivalent motor vehicle emission reductions if the following triggers are 

met: the design value of any criteria pollutant exceeds 85% of a NAAQS and motor vehicles 

constitute one of the top two sources of that pollutant. Based on the ozone design value 

exceeding 85% of the NAAQS, and motor vehicles being one of the top two sources of ozone 

precursors, these triggers have been met in the Treasure Valley and as a result, an emission 

testing or equivalent program is now mandated in Ada County by state law, in addition to the 

local ordinances previously relied upon to mandate the program. Also as a result of the 2008 

state law, an I/M program requiring biennial testing was implemented in adjacent Canyon 

County in June 2010. While not federally mandated or included as a control measure in the State 

Implementation Plan (SIP), this program is expected to provide additional air quality benefits for 

the Northern Ada County Maintenance Area. 

The current I/M testing in Ada County will continue, but to ensure consistency with existing or 

future potential I/M programs administered under state law and rules, frequency of testing will 

be adjusted to biennial. The move to biennial testing was addressed in Appendix B1 of the 2002 

approved CO Maintenance Plan for Northern Ada County which stated, “Moving from annual to 

biennial (every other year) testing was planned to begin in July of 2000. The Emissions 

Inventory for the Northern Ada County Carbon Monoxide Limited Maintenance Plan reflects this 

change. The Mobile 5b model used to estimate emissions interprets this change to result in 

slightly increased carbon monoxide emissions, as vehicles with excessive emissions would 

potentially not be repaired as often.” The switch to biennial testing is also supported by the CO 

Emissions Inventory for the Treasure Valley (see Appendix C). 

 

The new testing protocol in effect in Ada County is as follows: 

• Testing is required biennially 

• Vehicles from model year 1981 to 1995 are tested using the tailpipe/exhaust (TSI) 

method 

• Vehicles from model year 1996 and newer are tested using the OBD II method 

• First five model years are exempt from testing 

• All cut-points will remain unchanged 

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See appendix E - Description of Ada County Vehicle Inspection and Maintenance Program for 

links to rules and ordinances related to and requiring emissions testing. 

5.3 Monitoring Network /Verification of Continued Attainment 

In accordance with the Paisie Memo“...the maintenance plan should contain provisions for 

continued operation of an appropriate, EPA-approved air quality monitoring network, in 

accordance with 40 CFR Part 58.” 

Ambient CO concentrations continue to be low, with a current design value of 32% of the 

NAAQS. With monitoring data used as the triggering mechanism, DEQ will continue to closely 

track CO concentrations and to verify continued attainment via the approved monitoring network 

operated in accordance with 40 CFR Part 58. 

5.4 Contingency Plan 

In accordance with the Paisie Memo “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 occurs after redesignation of the area. These contingency measures do not 

have to be fully adopted at the time of redesignation. ... The State should also identify specific 

indicators, or triggers, which will be used to determine when the contingency measures need to 

be implemented.”

A. Contingency Measure Triggering Protocol 

Implementation of carbon monoxide contingency measures will be triggered under the following 

two circumstances: (1) immediately upon any exceedance of the Carbon Monoxide NAAQS, 

based on quality assured data, recorded at any site within the maintenance area or (2) if quality 

assured monitoring data show non-overlapping eight-hour average carbon monoxide 

concentrations of 8.0 ppm on four or more days within a single winter season (November 

through March) within the maintenance area. 

B. Contingency Measures 

If either of these two triggers is activated, DEQ will initiate one of the following contingency 

measures: 

(1) Oxygenated Fuels Program 

DEQ will promulgate a rule through the Board of Environmental Quality requiring all 

gasoline fuels dispensed in the maintenance area to contain ethanol at a minimum of ten 

percent (10%) by volume. 

(2) Alternative EPA-Approved Contingency Measure 

DEQ understands EPA is reviewing the effectiveness of oxygenated fuels as a carbon 

monoxide reduction measure and may provide guidance on more effective alternative 

contingency measures for carbon monoxide reduction. Should a more effective EPAapproved contingency measure become available DEQ will negotiate and implement the 

alternative. 

5.5 Conformity 

In accordance with the Paisie Memo “...In areas with approved limited maintenance plans, 

Federal actions requiring conformity determinations under the transportation conformity rule 

could be considered to satisfy the “budget test” required in sections; 93.118, 93.119 and 93.120 

of the rule. Similarly, in these areas, Federal actions subject to the general conformity rule 

8
could be considered to satisfy the “budget test” specified in section 93.158(a)(5)(i)(A) of the 

rule.”

A. Transportation Conformity 

Although a budget test is not required, EPA has not precluded states and local agencies from 

satisfying other requirements of the conformity process. For transportation conformity, 

components of the conformity process include analysis of hotspots, consultation on applicable 

actions, and implementation of any Transportation Control Measures specified in state and 

federal transportation conformity rules. 

As identified in 40 CFR 93.105, the lead transportation planning agency is responsible for 

demonstrating compliance with transportation conformity requirements. The lead transportation 

agency in Ada County is COMPASS, the metropolitan planning organization. COMPASS will 

oversee transportation conformity determinations of the Interagency Consultation Committee 

(ICC), a group of representatives of state and local transportation agencies, the state air quality 

agency, Department of Transportation (DOT) and EPA, as specified in 40 CFR 93.105 (51.402) 

and the Idaho Transportation Conformity Rule located in the Rules for the Control of Air 

Pollution in Idaho, IDAPA 58.01.01.563 through 574. The ICC is responsible for overseeing 

development of conformity analyses. The Idaho Transportation Conformity Rule was finalized 

on April 12, 2001. This rule adopts by reference portions of 40 CFR Part 93, Subpart A, Section 

100-129 (see Appendix F - Rules Establishing State Authority). 

DEQ will continue use of the build/no-build analysis conducted by COMPASS. This analysis 

will be used for planning purposes, future emission analyses, and as assurance that transportation 

activities will not lead to carbon monoxide emissions that could contribute to exceedances of the 

NAAQS. If the build scenario is larger than no-build, the next test step for determining 

conformity will be to compare the build scenario to 1990 emissions, as allowed by the 

conformity rule under 40 CFR Part 93.119(c)(ii). If the build scenario is below 1990 emissions, 

conformity is demonstrated. 

Regionally significant projects are also subject to conformity analysis. 

The ICC has developed the following definition of a “Regionally Significant” project: 

A transportation project in Ada County, Idaho is designated “Regionally Significant” if: 

(a) the project is for the improvement of either: 

(i) a principal arterial or higher functional classification; or 

(ii) a minor arterial which will have a twenty (20) year projected traffic 

volume of at least 45,000 vehicles a day after completion of the project; 

and 

(b) the project will add at least one new continuous vehicular lane which 

either: 

(i) extends from one intersecting principal or minor arterial to another 

intersecting principal or minor arterial; or 

(ii) in the case of an interstate, extends from the on ramp of one 

interstate interchange to a point beyond the off ramp of the next 

adjacent interstate interchange. 

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10

This definition is based on the federal definition of regional significance, as specified in 40 CFR 

93, and takes into account local features. This definition is a dynamic process that may change as 

conditions change. COMPASS is the designated contact for the most current definition. 

For any transportation plans, programs, or projects that meet these criteria, the ICC will conduct 

an analysis to determine if the action could cause or contribute to an exceedance of the carbon 

monoxide NAAQS. The analysis will be conducted according to procedures specified in 40 CFR 

93.100-182. For any regionally significant plans, programs, or projects that show potential 

carbon monoxide concentrations exceeding the standards, mitigation measures will be required. 

B. General Conformity 

Although a budget test is not required, EPA has not precluded states and local agencies from 

satisfying other requirements of the general conformity process. For any new projects or actions 

identified in 40 CFR 93.150, the federal agency overseeing the project is responsible for 

conducting a conformity analysis. DEQ will provide guidance to the federal agency to ensure 

compliance is achieved. IDAPA 58.01.01.123 authorize DEQ to require responsible agencies to 

provide certified information about the project. Agencies are advised to begin to work with DEQ 

during early stages of planning to determine the expected quantity of carbon monoxide 

emissions. If expected carbon monoxide emissions are greater than or equal to 100 tons per year, 

the deminimus threshold analysis specified in 40 CFR 51.853 must be conducted to determine 

the magnitude of emissions and impact on carbon monoxide concentrations. This analysis must 

be conducted according to procedures specified in 40 CFR 93.158. If necessary, mitigation 

measures must be applied as specified in 40 CFR 93.160. DEQ will review and comment on 

conformity determinations. 

6. Conclusion 

DEQ has shown that the Northern Ada County CO Maintenance Area has stayed well below the 

NAAQS for CO, and that there is little risk of future NAAQS violations. Having met all criteria 

for the Second 10-year maintenance plan, DEQ asks EPA to approve this Second 10-year CO 

Limited Maintenance Plan for the Northern Ada County CO Maintenance Area.