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CHAPTER 4: CONTROL STRATEGIES AND EMISSIONS REDUCTIONS

Overview

In order to estimate the costs and benefits of alternative ozone standards, the EPA has 

analyzed hypothetical control strategies that areas across the country might employ to attain 

alternative revised primary ozone standards of 70, 65, and 60 ppb. This chapter documents the 

emission control measures EPA applied to simulate attainment with these alternative ozone 

standards and the projected emission reductions associated with the measures.

This chapter is organized into four sections. Section 4.1 provides a summary of the steps 

used to conduct the control strategy analysis. Section 4.2 describes the emission reductions by 

sector in analyzing the baseline controls to meet the current (75 ppb) ozone standard. Section 4.3 

discusses control measures and emission reductions applied as part of the alternative standard 

analyses. Section 4.4 lists the key limitations and uncertainties associated with the control 

strategy analysis. And finally, Section 4.5 lists the references for the chapter. 

For the purposes of this discussion, it will be helpful to define some terminology. These 

definitions are specific to this analysis:

 Base Case - Emissions projected to the year 2025 reflecting current state and federal 

programs1. This does not include control programs specifically for the purpose of 

attaining the current ozone standard (75 ppb).

 Baseline - For all areas of the U.S. except California, the base case plus additional 

emissions reductions needed to reach attainment of the current ozone standard (75 

ppb) as well as emissions resulting from the Clean Power Plan (U.S. EPA, 2014b). 

Several areas in California are not required to meet the existing standard by 2025 and 

may not be required to meet a revised standard until 2038, thus we conducted 

analyses of attainment in California for post-2025. We explain the baseline treatment 

for California in more detail later in this chapter.

 Alternative Standard Analysis - Emissions reductions and associated hypothetical 

controls needed to reach attainment of the alternative standards. These reductions and 

controls are incremental to the baseline. 

 Design Value - A metric that is compared to the level of the National Ambient Air 

Quality Standard (NAAQS) to determine compliance. Design values are typically 

 

1 A complete list of programs included in the 2025 base case emissions is included in the Technical Support 

Document: Preparation of Emissions Inventories for the Version 6.1, 2011 Emissions Modeling Platform (U.S. 

EPA, 2014d).
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used to classify nonattainment areas, assess progress towards meeting the NAAQS, 

and develop control strategies. The design value for the 8-hour ozone standard is 

calculated as the 3-year average of the 4th highest 8-hour daily maximum 

concentration recorded at each monitoring site.

The EPA analyzed the impact that additional emissions control measures, across 

numerous sectors, would have on predicted ambient ozone concentrations incremental to the 

baseline. These control measures are based on information available at the time of this analysis, 

and include primarily end of pipe controls. Additional emission abatement strategies such as 

fuel switching, energy efficiency, and process changes may also be employed to reach emission 

reduction targets. The potential impact of some of these types of strategies is discussed in 

Chapter 7. Note that we did not conduct this analysis incremental to controls applied as part of 

previous NAAQS analyses (e.g., NOx or PM2.5 because the data and modeling on which these 

previous analyses were based are now considered outdated and are not compatible with the 

current ozone NAAQS analysis. In addition, there were no incremental NOx controls applied in 

the PM2.5 NAAQS and therefore there would be little to no impact on the controls selected to 

meet the alternative ozone standards analyzed. Thus, the analysis for the alternative standards 

focuses specifically on incremental improvements beyond the current standard and other existing 

and proposed rules. The selection of control strategies is based on a least cost approach selecting 

from those controls for which we have adequate information on costs, effectiveness, and 

applicability. The hypothetical control strategies presented in this RIA represent illustrative 

options for emissions reductions that achieve national attainment of the alternative standards. 

The hypothetical control strategies are not recommendations or requirements for how a revised 

ozone standard should be implemented, and states will make all final decisions regarding 

implementation strategies for a revised ozone NAAQS.

The NOx control measures dataset for non-electric generating unit (EGU) point sources 

applied in these control strategies reflects a number of revisions that EPA made since the 

completion of the previous ozone NAAQS RIA. These changes include:

 Removal of incorrect links between control measures and SCCs,

 Updates to cost equations where more recent data was available to improve their 

accuracy,
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 Inclusion of information that has recently become available concerning known and 

emerging technologies for reducing NOx emissions, and

 Revising costs and control efficiencies from control measures in the dataset based on 

recently obtained information from industry and multi-jurisdictional organizations 

(e.g., Ozone Transport Commissions and Lake Michigan Air Directors Consortium).

These revisions made the NOx control measures dataset for non-EGUs more accurate, 

defensible, and up to date. These improvements in this dataset will improve our control strategy 

and cost analyses not only for this RIA, but also for other potential rulemakings where control of 

NOx from non-EGUs is an important concern.

4.1 Control Strategy Analysis Steps

The primary year of analysis for analyzing the incremental costs and benefits of meeting 

a revised ozone standard is 2025. The analysis year was chosen because most areas of the U.S. 

will be required to meet a revised ozone standard by 2025. In estimating the incremental costs 

and benefits of potential alternative standards, we recognize that there are several areas that are 

not required to meet the existing ozone standard by 2025. The Clean Air Act allows areas with 

more significant air quality problems to take additional time to reach the existing standard. 

Several areas in California are not required to meet the existing standard by 2025 and may not be 

required to meet a revised standard until 2038. Depending on how areas in California are 

eventually designated, some areas may have attainment dates earlier than 2038, but for simplicity 

we are not distinguishing unique attainment years for different locations within California. To 

reflect these differences in required attainment dates, we conducted analyses of attainment in 

California for the period post-2025. 

While our goal for the California analysis was to reflect 2038, we were not able to project 

emissions and air quality beyond 2025 for California. However, we were able to adjust baseline 

air quality to reflect mobile source emissions reductions for California that would occur between 

2025 and 2030; these emissions reductions were the result of state and federal mobile source 

regulations expected to be fully implemented by 2030. For ease of discussion throughout the 

analyses we refer to the time periods for potential attainment in California and in other areas of 

the U.S. as post-2025 and 2025, respectively. Because we estimate incremental emissions 
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reductions, costs, and benefits for these two distinct time periods, it is not appropriate to add the 

estimates together or to directly compare the estimates.

To conduct the control strategy analyses, we require information on (i) control costs, (ii) 

control effectiveness in terms of NOx or VOC emissions reduced, (iii) the sensitivity of ozone 

design values to the NOx and VOC emissions reductions, and (iv) design value targets for each 

area. For the air quality modeling, the EPA prepared one control scenario for an alternative 

standard level of 70 ppb (Step 2 below) because we did not expect to have sufficient known 

controls for all locations to reach attainment for all of the alternative standards analyzed. The 

control scenario is not really designed to model how areas reach an alternative standard level of 

70 ppb, instead it sets up a process for developing, and applying, a list of potentially available 

known controls ordered by cost. To develop the a sufficient amount of controls for the 65 and 60 

ppb alternative standard levels, we also worked to identify known controls in areas or regions 

expected to contribute to nonattainment for these alternative standards.

The following steps were taken by the EPA to analyze the impacts and costs of the 

control scenario incremental to the base case air quality modeling:

1. Identify geographic areas in the U.S. projected to exceed the alternative standard of 

70 ppb in the year 2025 in the base case air quality modeling.

2. Develop a hypothetical control scenario for these areas and generate a control case 

2025 emissions inventory for all areas except California; for California develop a 

control case post-2025 emissions inventory.

3. Perform air quality modeling to assess the air quality impacts of the hypothetical 

control scenario. Additionally, perform a series of emissions sensitivity simulations to 

develop average ozone response to across-the-board NOx and VOC emissions 

reductions in different areas (see Chapter 3).

4. Calculate the portion of the hypothetical control scenario emission reductions that are 

attributed to meeting the baseline. Estimate any additional emissions reductions 

beyond the known controls that are needed to meet the current standard based on 

average ozone response factors. These are the baseline emission reductions.

5. Estimate the additional emissions reductions incremental to the baseline that are 

needed to meet the alternative standards of 70, 65, and 60 ppb. Costs of controls 

incremental to (i.e., over and above) the baseline reductions are attributed to the costs 

of meeting the alternative standards. These emissions reductions can come from 

specific known controls or emission reductions needed beyond known controls, also 

referred to as unknown controls. Potential controls may be categorized as unknown 

because these needed emissions reductions come from sectors for which we have not 
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sufficiently explored emissions abatement opportunities or sectors that might require 

non-traditional abatement through measures like energy efficiency or process 

changes.

The following sections will discuss in more detail the analysis steps presented above.

4.2 Baseline Control Strategy

Establishing the baseline allows us to estimate the incremental costs and benefits of 

attaining the alternative standards. Three steps were used to develop the baseline. First, we 

estimated 2025 base case emissions and air quality, reflecting “on the books” regulations (see 

Section 3.1.3 Emissions Inventories for a discussion of the rules included in the Base Case for 

this analysis).2 Second, we accounted for changes in ozone predicted to occur due to one 

potential approach for implementing the Clean Power Plan. Third, we identified additional 

controls that could be applied to demonstrate attainment of the current ozone standard of 75 ppb. 

Additional control measures were used in three sectors to meet the current ozone standard 

in establishing the baseline:3 Non-Electric Generating Unit Point Sources (Non-EGUs), NonPoint (Area) Sources, and Nonroad Mobile Sources. See Table 4-1 for a summary of controls 

applied in the baseline analysis. There were several areas in California that did not reach 

attainment of the current standard with known controls. For these geographic areas, we estimated 

the additional emissions reductions needed beyond those achieved by identified known controls 

for NOx and VOC to attain the current standard.

 

2 Among others factors, the baseline for this analysis is also affected by the choice of the future year -- a year farther 

into the future allows for more time for federal measures to work and to attain. This baseline is also affected by the 

air quality starting point, potentially reducing the amount of emissions reductions required for attainment – this 

analysis started from a standard of 75 ppb, where the analysis in 2008 started from a standard of 84 ppb. In addition, 

we have identified additional “known” controls to apply in this analysis, controls which are less expensive per ton 

than unknown controls.

3

In establishing the baseline, the U.S. EPA selected a set of cost-effective controls to simulate attainment of the 

current ozone standard. These control sets are hypothetical as states will ultimately determine controls as part of 

the SIP process.
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Table 4-1. Controls Applied for the Alternative Standard Analyses Control Strategy

Sector NOx VOC

Non-EGU Point LEC (Low Emission Combustion) Solvent Recovery System

SCR (Selective Catalytic Reduction) Work Practices, and Material 

Reformulation/Substitution

SNCR (Selective Non-Catalytic Reduction) Low-VOC materials Coatings and AddOn Controls

NSCR (Non-Selective Catalytic Reduction) Low VOC Adhesives and Improved 

Application Methods

LNB (Low NOx Burner Technology) Permanent Total Enclosure (PTE)

LNB + SCR Solvent Substitution, Non-Atomized 

Resin Application Methods

LNB + SNCR Petroleum Wastewater Treatment 

Controls

OXY-Firing Incineration (Thermal, Catalytic, etc) to 

Reduce VOC Emissions

Biosolid Injection Technology

LNB + Flue Gas Recirculation

LNB + Over Fire Air

Ignition Retard

Natural Gas Reburn

Ultra LNB

NonPoint NSCR (Non-Selective Catalytic Reduction) Process Modification to Reduce Fugitive 

VOC Emissions

LEC (Low Emission Combustion) Reformulation to Reduce VOC Content

LNB (Low NOx Burner Technology) Incineration (Thermal, Catalytic, etc) to 

Reduce VOC Emissions

LNB Water Heaters Low Pressure/Vacuum (LPV) Relief 

Valves in Gasoline Storage Tanks

Reduced Solvent Utilization

Gas Recovery in Landfills

Nonroad Diesel Retrofits & Engine Rebuilds

A map of the country is presented in Figure 41, which shows the counties projected to 

exceed the current ozone standard of 75 ppb in the 2025 base case scenario. This includes 8 

projected exceeding counties in California and 3 exceeding counties in Texas. NOx control 
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measures were applied in these 11 counties in the baseline analysis to meet the current ozone

standard. In addition, NOx control measures were applied to 40 California counties and 52 Texas 

counties adjacent to exceeding counties in order to address transport coming from these adjacent 

counties. A map of the areas where control measures were applied to demonstrate attainment of 

the current standard and establish the baseline is presented in Figure 4-2.

To construct the post-2025 baseline, we included mobile source NOx and VOC emissions 

changes that are projected to occur in California from 2025 - 2030 as a result of California’s 

current mobile source control programs and projected changes in vehicle miles traveled and 

nonroad activity levels. These changes were included because they would result in emission 

reductions that would contribute toward attainment of the current ozone standard. No emission 

projections were available for other sectors for this time period, and no mobile source emissions 

projections were available beyond 2030. Additionally, VOC controls were applied in California 

counties highlighted in Figure 4-2. Even with the above mentioned controls, some areas in 

California did not reach attainment with known controls. For these areas, we estimated the 

additional NOx emission reductions needed beyond identified known controls to attain the 

standard. 
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Figure 4-1. Counties Projected to Exceed the Baseline Level of the Current Ozone 

Standard (75 ppb) in 2025 Base Case 
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Figure 4-2. Counties Where Emissions Reductions Were Applied to Demonstrate 

Attainment of the Current Standard for the Baseline Analysis

Tables 4-2 and 4-3 summarize the NOx and VOC emission reductions needed to 

demonstrate attainment of the current ozone standard (75 ppb). 
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Table 4-2. Summary of Emission Reductions by Sector for Known Controls Applied to 

Demonstrate Attainment of the Current Standard for the 2025 Baseline - U.S., 

except California (1,000 tons/year)a

Geographic Area b Emissions Sector NOx VOC

East EGU - -

Non-EGU Point 18 -

Nonpoint 26 -

Nonroad 0.83 -

Onroad - -

Total 45 -

West EGU - -

Non-EGU Point - -

Nonpoint - -

Nonroad - -

Onroad - -

Total - -

a Estimates are rounded to two significant figures. 

b For the control strategy and cost analysis, “East” includes the Northeast, Midwest, and Central regions, and “West” 

includes the Southwest region. See Chapter 3 for a description of these regions.

Table 4-3. Summary of Emission Reductions (Known and Unknown Controls) Applied to 

Demonstrate Attainment in California for the post-2025 Baseline (1,000 

tons/year)a

Emissions Sector NOx VOC

Known Controls EGU - -

Non-EGU Point 14 0.61

Nonpoint 14 47

Nonroad 4.2 -

Onroad - -

Total 31 48

Unknown Controls All 160 -

Total 190 48

a Emission reduction estimates are rounded to two significant figures. 

The 2025 baseline for this analysis presents one scenario of future year air quality based 

upon specific control measures, additional emission reductions beyond known controls, 

promulgated federal rules such as Tier 3, and specific years of initial values for air quality 

monitoring and emissions data. This analysis presents one illustrative strategy relying on the 

identified federal measures and other strategies that states may employ. States may ultimately 
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employ other strategies and/or other federal rules may be adopted that would also help in 

achieving attainment with the current standard. 

4.3 Alternative Standard Analyses

After identifying the controls in the baseline scenario, additional controls needed to meet 

the alternative standards were identified in four sectors: Electric Generating Units (EGUs), NonElectric Generating Unit Point Sources (Non-EGUs), Non-Point (Area) Sources, and Nonroad 

Mobile Sources. Onroad mobile source controls were not applied because they are largely 

addressed in existing rules such as the recent Tier 3 rule. Controls applied for the alternative 

standard analyses were the same as were applied for the baseline analysis (see Table 4-1 for a 

summary of controls applied in the baseline) with the addition of Selective Catalytic Reduction 

applied to EGUs. Other than the addition of EGU controls, the primary difference between the 

controls applied for the alternative standards versus the baseline was the geographic areas to 

which they were applied.

The EPA performed a national scale air quality modeling analysis to estimate ozone 

concentrations for the future base case year of 2025. To accomplish this, we modeled multiple 

emissions cases for 2025, including the 2025 base case and twelve 2025 emissions sensitivity 

simulations. The twelve emissions sensitivity simulations were used to develop ozone sensitivity 

factors (ppb/ton) from the modeled response of ozone to changes in NOx and VOC emissions 

from various sources and locations. These ozone sensitivity factors were then used to determine 

the amount of emissions reductions needed to reach the 2025 baseline and evaluate potential 

alternative standard levels of 70, 65, and 60 ppb incremental to the baseline. As mentioned 

previously, only a subset of known controls were included in the modeled control scenarios. 

Therefore, any additional emissions reductions may include both known and unknown controls. 

In areas of the country outside of California, Texas, and the northeast, total emissions reductions 

needed beyond the baseline were based entirely on response factors, i.e., not based on the 

hypothetical control scenario used in the air quality modeling. 

Figure 4-3 shows the counties projected to exceed the alternative standards analyzed for 

the 2025 baseline for areas other than California. For the 70 ppb scenario, emissions reductions 

were required for monitors in the Central and Northeast regions (see Chapter 3, Figure 3-3 for a 
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depiction of the regions). For the 65 and 60 ppb scenarios, emissions reductions were applied in 

all regions with projected baseline DVs above these levels. For the 60 ppb scenario, additional 

VOC tons were identified in Chicago because some sites in that area experienced NOx

disbenefits meaning that the regional NOx reductions resulted in ozone DV increases from below 

60 ppb to above 60 ppb. Therefore it was not possible to identify a scenario in which regional 

NOx reductions and maximum known VOC controls alone resulted in all Midwest monitors 

meeting a 60 ppb standard. An iterative approach was used determine a combination of NOx and 

VOC emissions reductions that would not lead to over-control at either the NOx-limited monitor 

with the highest design value or the VOC-limited monitor with the highest design value. 

Because of the regional approach we used for locations other than Texas, we were not able to 

geographically fine tune the control strategies, and thus there is some uncertainty in the actual 

amounts of emissions reductions estimated for attaining the alternative standards.

Figure 4-3. Projected Ozone Design Values in the 2025 Baseline Scenario
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Figure 4-4 shows the counties projected to exceed the alternative standards analyzed for 

the post-2025 baseline analysis for California. For the California post-2025 alternative standard 

analyses, all known controls were applied in the baseline so incremental reductions are from 

unknown controls.

Figure 4-4. Projected Ozone Design Values in the post-2025 Baseline Scenario

4.3.1 Identifying Known Controls Needed to Meet the Alternative Standards

For the 2025 alternative control strategy analyses of 70, 65 and 60 ppb, known NOx

controls for four sectors were used: EGUs, non-EGU point, nonpoint, and nonroad mobile 

sources. In a smaller number of geographic areas, VOC controls were applied to non-EGU point 

and nonpoint sources. 
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For Texas, reductions were first applied to NOx sources in the areas surrounding Dallas 

and Houston. Additional reductions then were applied to VOC sources in the area surrounding 

Houston, to NOx sources in other parts of Texas, and to sources in the surrounding states. For the 

Northeast, Midwest, and Southwest areas of the country, reductions were applied to NOx sources 

within the regions. For regions where additional reductions were needed, controls were applied 

to VOC sources in urban areas with the highest ozone design values in the region.

4

 

For California, all known controls were applied in the baseline analysis so there were no 

known controls available to apply toward the incremental emissions reductions needed for the 

alternative analysis levels for post-2025. Maps of the areas where control measures were applied 

to demonstrate attainment of the alternative analysis levels are presented in Figures 4-5 and 4-6. 

Note that we do not account for between region transport of ozone, and therefore, especially for 

the 65 and 60 ppb alternative standard levels, we may be overstating the amount of emissions 

reductions needed for attainment.

 

4 Texas, California, and the northeast were included in the hypothetical control scenario for an alternative standard 

of 70 ppb. Other regions were modeled as part of the 2025 emissions sensitivity simulations. 
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Figure 4-5. Counties Where Emissions Reductions Were Applied to Demonstrate 

Attainment with a 70 ppb Ozone Standard in the 2025 Analysis
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Figure 4-6. Counties Where Emissions Reductions Were Applied to Demonstrate 

Attainment with 65 and 60 ppb Ozone Standards in the 2025 Analyses

Table 4-4 shows the number of exceeding counties and the number of adjacent counties 

to which controls were applied for the alternative standards. For a complete list of geographic 

areas for the alternative standards see Appendix 4.A. 
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Table 4-4. Number of Counties with Exceedances and Number of Additional Counties 

Where Reductions Were Applied for the 2025 Alternative Standards Analyses -

U.S., except California

Alternative Standard Number of Counties with 

Exceedances

Number of Additional Counties Where Reductions 

Were Applied

70 ppb 15 479

65 ppb 115 1,925

60 ppb 289 1,791a

a Number of additional counties where reductions are applied declined for 60 ppb analysis because the 

number of overall counties in the analysis remained the same while the number of exceeding counties increased.

Tables 4-5 through 4-7 show the emissions reductions from known controls for the 

alternative standards analyzed. No exceedances were projected for the West region, outside of 

California, for the 70 ppb alternative standard. For the lower alternative standards of 65 and 60 

ppb, similar controls were applied as were used in the 70 ppb analysis, but the geographic area in 

which they were applied increased. The largest emission reductions were in the non-EGU point 

source and nonpoint sectors. For details regarding emission reductions by control measure see 

Appendix 4.A

Table 4-5. Summary of Emission Reductions by Sector for Known Controls Applied to 

Demonstrate Nationwide Attainment with a 70 ppb Ozone Standard in 2025, 

except California (1,000 tons/year)a

Geographic 

Area

Emissions 

Sector

Baseline Emissions Emission Reductions

NOx VOC NOx VOC

East EGU 884 25 -

Non-EGU Point 1,485 210 0.98

Nonpoint 1,487 260 54

Nonroad 1,235 5 -

Onroad 1,135 - -

Total 6,226 490 55

West EGU 159 - -

Non-EGU Point 226 - -

Nonpoint 193 - -

Nonroad 219 - -

Onroad 197 - -

Total 1,025 - -

a Emission reduction estimates are rounded to two significant figures. 
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Table 4-6. Summary of Emission Reductions by Sector for Known Controls Applied to 

Demonstrate Nationwide Attainment with a 65 ppb Ozone Standard in 2025 -

except California (1,000 tons/year)a

Geographic Area Emissions Sector NOx VOC

East EGU 170 -

Non-EGU Point 410 3.6

Nonpoint 420 95

Nonroad 12 -

Total 1,000 99

West EGU 36 -

Non-EGU Point 38 0.47

Nonpoint 37 6.6

Nonroad 1.3 -

Total 110 7

a Emission reduction estimates are rounded to two significant figures.

Table 4-7. Summary of Emission Reductions by Sector for Known Controls Applied to 

Demonstrate Nationwide Attainment with a 60 ppb Ozone Standard in 2025 -

except California (1,000 tons/year)a

Geographic Area Emissions Sector NOx VOC

East EGU 170 -

Non-EGU Point 410 4.2

Nonpoint 420 99

Nonroad 12 -

Total 1,000 100

West EGU 62 -

Non-EGU Point 48 0.47

Nonpoint 39 6.6

Nonroad 1.3 -

Total 150 7

a Emission reduction estimates are rounded to two significant figures. 

4.3.2 Known Control Measures Analyzed 

Known control measures were applied to electric generating units (EGU), non-EGU 

point, nonpoint (area), and nonroad mobile sources for demonstration of attainment with the 

current and alternative standards. The applied control measures were identified using the EPA’s 

Control Strategy Tool (CoST) (U.S. EPA, 2014c), Integrated Planning Model (IPM), and 

NONROAD Model. CoST models emissions reductions and engineering costs associated with 

control strategies applied to point, area, and mobile sources of air pollutant emissions by 

matching control measures to emissions sources using algorithms such as "maximum emissions 
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reduction", "least cost", and "apply measures in series". For this analysis, we applied the 

maximum emissions reduction algorithm. These controls are described further in Appendix 4.A. 

Specific controls were applied in the air quality modeling only for a portion of the analysis (for 

California, Texas, and the northeast, areas projected to exceed an alternative standard level of 70 

ppb). A majority of the emission reductions needed were identified using the ozone sensitivity 

factors developed from the twelve emissions sensitivity simulations (see Chapter 3, Section 3.1

for a discussion of the development of the ozone sensitivity factors).

Nonpoint and nonroad mobile source emissions data are generated at the county level, 

and therefore controls for these emissions sectors were applied at the county level. EGU and 

non-EGU point source controls are applied to individual point sources. Control measures were 

applied to point and nonpoint sources of NOx, including: industrial boilers, commercial and 

institutional boilers, reciprocating internal combustion engines in the oil and gas industry and 

other industries, glass manufacturing furnaces, and cement kilns. The analysis for nonroad 

mobile sources applied NOx controls to diesel engines.

In a portion of the geographic areas where NOx controls were applied, the EPA also 

applied control measures to sources of VOC including surface coating, solvents, and fuel storage 

tanks. VOC reductions were analyzed in the urban areas with the highest ozone design values in 

each region: northern and southern California; Denver in the Southwest; Houston and Dallas in 

the Central region; Chicago, Detroit, and Louisville in the Midwest; New York, Baltimore, and 

Pittsburgh in the Northeast. Even among these areas, in some cases NOx reductions necessary to 

bring the very highest urban areas into attainment made the VOC reductions unnecessary in other 

high ozone urban areas within the region. 

To more accurately depict available controls, the EPA employed a decision rule in which 

controls were not applied to any non-EGU or nonpoint sources with less than 25 tons/year of 

emissions per pollutant. This decision rule is more inclusive of sources than the rule we 

employed in the previous O3 and PM2.5 NAAQS RIAs where we applied a minimum of 50 

tons/year for each pollutant. We modified the decision rule for this NAAQS analysis to 

recognize the potential for emissions reductions in the large number of sources emitting in the 

25-50 tons/year range in order to devise control strategies for full, or closer to full, attainment. 
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Historically, the reason for not applying controls to sources emitting less than 25 tons/year has 

been that many point sources with emissions below this level already have controls in place. For 

the analysis, we applied best engineering judgement to apply controls to select sources.

4.3.3 Emissions Reductions beyond Known Controls Needed to Meet the Alternative Standards

There were several areas where known controls did not achieve enough emissions

reductions to attain the alternative standards of 70, 65, and 60 ppb. To complete the analysis, the 

EPA then estimated the additional emissions reductions beyond known controls needed to reach 

attainment, also referred to as unknown controls. For information on the methodology used to 

develop the emission reductions estimates, see Chapter 3. Table 4-8 shows the emissions

reductions needed from unknown controls in 2025 for the U.S., except California, for the 

alternative standards analyzed. Table 4-9 shows the reductions needed from unknown controls 

for California for the post-2025 analysis.

Table 4-8. Summary of Emissions Reductions by Alternative Standard for Unknown 

Controls for 2025 - except California (1,000 tons/year)a

Alternative Standard Region NOx VOC

70 ppbb East

West

150

-

-

-

65 ppbc East

West

750

-

-

-

60 ppbd East

West

1,900

350

41

-

a Estimates are rounded to two significant figures. 

b Unknown controls for the 70 ppb alternative standard are needed in the Northeast and Central regions (see Chapter 

3 for a description of these regions). 

c Unknown controls for the 65 ppb alternative standard are needed in the Northeast, Central, and Midwest regions 

(see Chapter 3 for a description of these regions). 

d Unknown controls for the 60 ppb alternative standard are needed in the Northeast, Central, Midwest, and 

Southwest regions (see Chapter 3 for a description of these regions). 

Table 4-9. Summary of Emissions Reductions by Alternative Level for Unknown Controls 

for post-2025 - California (1,000 tons/year)a

Alternative Standard Region NOx VOC

70 ppb CA 53 -

65 ppb CA 110 -
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60 ppb CA 140 -

a Estimates are rounded to two significant figures. 

4.3.4 Summary of Emissions Reductions Needed to Meet the Alternative Standards

Table 4-10 summarizes the known and unknown emissions reductions needed to meet the 

alternative standard levels in 2025 for the East and West, except California. In the East for 2025, 

the unknown NOx reductions needed as percentage of the total rises from 23 percent to 66 

percent as the alternative standard level decreases from 70 ppb to 60 ppb. Meanwhile, no 

unknown VOC reductions are needed in the East for the 70 ppb and 65 ppb levels. In the West 

(except California) for 2025, unknown NOx reductions are not needed until the 60 ppb level, 

when the unknown tons constitute about 70 percent of the total reductions needed. No unknown 

VOC reductions are needed in the West (except California) for 2025 for any of the alternative 

standard levels.

Table 4-10. Summary of Known and Unknown Emissions Reductions by Alternative 

Standard Levels in 2025, Except California (1,000 tons/year)a

Alternative Standard

Geographic Area Emissions Reductions 70 ppb 65 ppb 60 ppb

East NOx Known 490 1,000 1,000

NOx Unknown 150 750 1,900

% NOx Unknown 23% 43% 66%

VOC Known 55 99 100

VOC Unknown 0 0 41

% VOC Unknown 0% 0% 29%

West NOx Known 0 110 150

NOx Unknown 0 0 350

% NOx Unknown N/A 0% 70%

VOC Known 0 7 7

VOC Unknown 0 0 0

% VOC Unknown N/A 0% 0%

a Estimates are rounded to two significant figures. 

Table 4-11 shows again that there were no known NOx emissions reductions identified 

for meeting the alternative standard levels for post-2025 California and that 100 percent of the 

NOx tons needed were unknown. Meanwhile, no unknown VOC reductions are needed for the 

any of the alternative standard levels for post-025 California.
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Table 4-11. Summary of Known and Unknown Emissions Reductions by Alternative 

Standard Levels for post-2025 - California (1,000 tons/year)a

Alternative Standard

Geographic Area Emissions Reductions 70 ppb 65 ppb 60 ppb

California NOx Known 0 0 0

NOx Unknown 53 110 140

% NOx Unknown 100% 100% 100%

VOC Known 0 0 0

VOC Unknown 0 0 0

% VOC Unknown N/A N/A N/A

a Estimates are rounded to two significant figures. 

4.4 Limitations and Uncertainties

EPA’s analysis is based on its best judgment for various input assumptions that are 

uncertain. As a general matter, the Agency selects the best available information from 

engineering studies of air pollution controls and has set up what it believes is the most reasonable 

modeling framework for analyzing the cost, emissions changes, and other impacts of regulatory 

controls. However, the estimates of emissions reductions associated with our control strategies 

above are subject to important limitations and uncertainties. In the following, we discuss the

limitations and uncertainties that are most significant.

 Illustrative control strategy: A control strategy is the set of actions that States 

may take to meet a standard, such as which industries should be required to install 

end-of-pipe controls or certain types of equipment and technology. The illustrative 

control strategy analysis in this RIA presents only one potential pathway to 

attainment. The control strategies are not recommendations for how a revised ozone 

standard should be implemented, and States will make all final decisions regarding 

implementation strategies for the revised NAAQS. We do not presume that the 

control strategies presented in this RIA are an exhaustive list of possibilities for 

emissions reductions.

 Emissions Inventories and Air Quality Modeling: These serve as a foundation 

for the projected ozone values, control strategies and costs in this analysis and thus 

limitations and uncertainties for these inputs impact the results, especially for 
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issues such as future year emissions projections and information on controls 

currently in place at sources. Limitations and uncertainties for these inputs are 

discussed in previous chapters devoted to these subject areas. In addition, there are 

factors that affect emissions, such as economic growth and the makeup of the 

economy (e.g., growth in the oil and natural gas sector), that introduce additional 

uncertainty. 

 Projecting level and geographic scope of exceedances: Estimates of the 

geographic areas that would exceed revised alternative levels of the standard in a 

future year, and the level to which those areas would exceed, are approximations 

based on a number of factors. The actual nonattainment determinations that would 

result from a revised standard will likely depend on the consideration of local 

issues, changes in source operations between the time of this analysis and 

implementation of a new standard, and changes in control technology over time.

 Assumptions about the baseline: There is significant uncertainty about the 

illustration of the impact of rules, especially the Clean Power Plan because it is a 

proposal and because it contains significant flexibility for states to determine how 

to choose measures to comply with the standard. 

 Applicability of control measures: The applicability of a control measure to a 

specific source varies depending on a number of process equipment factors such as 

age, design, capacity, fuel, and operating parameters. These can vary considerably 

from source to source and over time. This analysis makes assumptions across broad 

categories of sources nationwide.

 Control measure advances over time: The control measures applied do not reflect 

potential effects of technological change that may be available in future years and 

the effects of “learning by doing” or “learning by researching” are not accounted 

for in the emissions reduction estimates. Thus, all estimates of impacts associated 

with control measures applied reflect our current knowledge, and not projections, 

of the measures’ effectiveness. In our analysis, we do not have the necessary data 

for cumulative output, fuel sales, or emissions reductions for all sectors included in 

order to properly generate control costs that reflect learning-curve impacts or the 
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impacts of technological change. We believe the effect of including these impacts 

would be to lower our estimates of costs for our projected year control strategies. 

 Pollutants to be targeted: Local knowledge of atmospheric chemistry in each 

geographic area may result in a different prioritization of pollutants (VOC and 

NOx) for control. For the baseline in this analysis, we included only promulgated 

or proposed rules, but that there may be additional regulations promulgated in the 

future that reduce NOx or VOC emissions. These regulations could reduce the 

current baseline levels of emissions. 

4.6 References

U.S. Environmental Protection Agency (U.S. EPA). 2014a. Control of Air Pollution from Motor Vehicles: Tier 3 

Motor Vehicle Emission and Fuel Standards. Office of Transportation and Air Quality. Available at 

http://www.epa.gov/otaq/tier3.htm.

U.S. Environmental Protection Agency (U.S. EPA). 2014b. Proposed Carbon Pollution Guidelines for Existing 

Power Plants and Emission Standards for Modified and Reconstructed Power Plants. Available at 

http://www2.epa.gov/sites/production/files/2014-06/documents/20140602ria-clean-power-plan.pdf

U.S. Environmental Protection Agency (U.S. EPA). 2014c. Control Strategy Tool (CoST) Documentation Report. 

Office of Air Quality Planning and Standards, Research Triangle Park, NC. Available at 

http://www.epa.gov/ttnecas1/cost.htm.

U.S. Environmental Protection Agency (2014d) Preparation of Emissions Inventories for the Version 6.1, 2011 

Emissions Modeling Platform (http://www.epa.gov/ttn/chief/emc