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BACKGROUND INFORMATION DOCUMENT 

ADDITIONAL OPTIONS CONSIDERED FOR CLASSIFICATION OF 

NONATTAINMENT AREAS UNDER THE 2008 OZONE NAAQS 

Docket ID No. EPA-HQ-OAR-2010-0885 

U.S. ENVIRONMENTAL PROTECTION AGENCY 

OFFICE OF AIR AND RADIATION 

OFFICE OF AIR QUALITY PLANNING AND STANDARDS 

RESEARCH TRIANGLE PARK, NC 

January 2012 
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INTRODUCTION

In the notice of proposed rulemaking for classification of the 2008 8-hour ozone National 

Ambient Air Quality Standards (NAAQS), EPA is proposing a set of classification thresholds for 

the standard level of 0.075 ppm. The thresholds are based on the same general methodology for 

adjusting the classification thresholds in Table 1 of the Clean Air Act (CAA) to account for an 8-

hour standard that was used for the 1997 ozone NAAQS. That methodology was: Determine the 

percentage by which each classification threshold in Table 1 of CAA section 181 exceeds the 

0.12 ppm 1-hour ozone standard and set the 8-hour threshold value at the same percentage above 

the level of the 8-hour ozone standard. For example, the threshold separating Marginal and 

Moderate areas in Table 1 is 15 percent above the 1-hour standard, so for an 8-hour standard of 

0.075 ppm, we would set the Moderate area lower threshold value at 15 percent above the 0.075 

standard (at 0.086 ppm). The other classification thresholds would be adjusted in a similar 

manner. 

 The EPA also considered a number of other options for adjusting the classification table 

for the 2008 standard. A number of other alternatives were suggested in comments on that 

proposed implementation rule for the 1997 standards and we re-opened the comment period on 

that rule to take comment on several alternative classification approaches.1

 In the final Phase 1 

rule for the 1997 standard, EPA noted the rationales for the alternative methods that were 

recommended for that standard were either not as consistent with the methodology of the Clean 

Air Act’s own table or would have undesirable consequences, and therefore were not ultimately 

adopted. Several other concepts have been suggested since then. This paper describes several 

alternative classification approaches and, where appropriate, provides example results of 

applying the scheme to a set of hypothetical nonattainment areas derived from 2010 ozone 

design values.2

 

1

 Federal Register of October 21, 2003 (68 FR 60054).

2 Background Information Document - Development of Hypothetical Nonattainment Areas for 

Illustrating Proposed Classification Thresholds for Areas Designated Nonattainment for the 2008 

0.075 ppm 8-hour Ozone National Ambient Air Quality Standard. January 2012 draft, Docket 

ID No. EPA-HQ-OAR-2010-0885. 
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ADDITIONAL CONCEPTS CONSIDERED

1. Ratio of Thresholds Method 

A number of commenters on the implementation rule for the 1997 ozone NAAQS 

recommended that EPA consider a classification approach that would place more areas in higher 

classifications in order to provide maximum timeframes for areas to attain. In light of these 

circumstances, EPA also considered a method for translating the CAA’s classification table such 

that more areas would be placed in higher classifications, which we refer to as the “ratio of 

thresholds” method. This method uses actual air quality data from areas that we anticipate will be 

designated nonattainment to establish the lower threshold for extreme areas (i.e., selecting the 

design value from one or more areas with the highest ozone levels as the threshold for the 

extreme area classification) and then establishes the other thresholds based on ratios of the 

Extreme threshold and the values in Table 1 of the CAA. This method for classification of 

nonattainment areas is based on Table 1 in the CAA for a 2008 ozone NAAQS. In adjusting the 

design value threshold levels for each classification, this method accounts for the change in the 

standard and also for the relative amount of reduction in ozone concentration needed to attain the 

standard. This method is based on the principle that a classification table for the 2008 ozone 

NAAQS should account for not only the level of ozone classification threshold Design values 

above the level of the standard, but also the absolute spread of design values that we actually 

measure in the air at the time we establish the classification table. Doing so would classify areas 

based upon how much reduction in ozone concentrations an area needs to achieve attainment 

relative to the areas with the highest design value. 

Under the 1-hour ozone NAAQS, Extreme areas needed to reduce ozone levels by about3

0.160 ppm (0.280 ppm – 0.120 ppm) or more to lower their design value to or below the 

commensurate level of the standard. In contrast, 1-hour Marginal areas only needed to reduce 

their DV by at most 0.017 ppm (0.137 ppm - 0.120 ppm) to achieve the same results. This is 

only 10.6 percent of the minimum reduction needed (0.160 ppm) for an Extreme area. Moderate 

areas needed at most 24.3 percent (100 * (0.159 ppm - 0.12 ppm) / 0.160 ppm) reduction in 

ozone levels of the 0.160 ppm minimum reduction needed by an Extreme area. 

 Under this alternative approach, a ratio would be calculated using the following: 

[the difference between a specific classification threshold and the lowest threshold (Marginal) in 

CAA Table 1] 

divided by 

[the difference between the highest threshold (the bottom of the Extreme classification) and the 

lowest threshold (Marginal) in CAA Table 1] 

 

3 The rounding technique allowed a 0.124 DV to meet the NAAQS. This approach is based on 

the design values listed in the CAA design value thresholds for 1-hour ozone areas. 
4

 The ratio would then be used in conjunction with the 2008 ozone NAAQS. For a 

standard of 0.075 ppm, we know the bottom threshold of Marginal will be the lowest violating 

DV (0.076 ppm). The highest 8-hour design value recorded for the period 2008-2010 is in Los 

Angeles: 0.112 ppm. 

Under this method, a ratio would be calculated using the following ratio: 

[the lower threshold of each classification - the Marginal lower threshold (0.121 ppm)] 

divided by 

[the Extreme threshold (0.280 ppm) - the Marginal lower threshold (0.121 ppm)] 

This ratio would then be applied to the 0.075 ppm 8-hour standard, using current air 

quality data, to determine the upper range for the classification (the bottom threshold for the 

Extreme classification). For the purposes of the example, we presented the 8-hour thresholds for 

each classification that result from this option using Extreme threshold of 0.112 ppm. 

Area class CAA design value 

thresholds 1-hour ozone 

ppm (Col A) 

Percent above 

1-hour ozone 

NAAQS 

Ratio of range of (class 

threshold to min) to 

(range of max to min 

thresholds) 

(Col B) 

8-hr thresholds 

(ppm) 

0.076 + Col B * 

(0.112 - 0.076) 

Marginal 

from 0.121 NA 0.076 

up to1 0.138 15 0.107 0.080 

Moderate 

From 0.138 15 0.107 0.080 

up to1 0.160 33.333 0.245 0.085 

Serious 

From 0.160 33.333 0.245 0.085 

up to1 0.180 50 0.371 0.089 

Severe-15 

From 0.180 50 0.371 0.089 

up to1 0.190 58.333 0.434 0.092 

Severe-17 From 0.190 58.333 0.434 0.092 

up to1 0.280 133.333 1.000 0.112 

Extreme equal to 

or above 0.280 133.333 1.000 0.112 

1 but not including 

EPA is not proposing this method, in part, because the results would vary depending on 

the years of data used for the design values in calculating the ratios in the 8-hour thresholds 

design values. 

 
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2. Adjustment of the percentages above the standard used to set the classification 

thresholds. 

One observer suggested that the percentage above the standard be reduced by the average 

ratio of 8-hour design values to 1-hour design values that are recorded. The observer suggested 

that this ratio is generally 0.7. In actuality, an analysis of 8-hour and 1-hour design values from 

all ozone monitors with both values from the year 2005 (based on 2003-2005 data) yields an 

average of 0.78 rather than 0.7. The 2010 design values (based on 2008-2010 data) yield an 

average of 0.80. Using 2010 design values for classifications and assuming hypothetical 

nonattainment areas, if the threshold percentages were indeed reduced by 0.80, only 1 area 

would be classified different than under the proposed classification method. One area in 

California would be classified as serious rather than as Moderate. We anticipate that a number 

of the major metropolitan areas (including Philadelphia, New York, Washington (DC), 

Charlotte) that were classified Moderate under the 1997 standard and that were not able to 

demonstrate attainment by their maximum attainment date would be classified as Marginal 

under either the Percent-Above-Standard method we are proposing or under this approach. 

Thus, this method would not appear to result in classifications that would allow areas more time 

to attain the 2008 ozone standard than would be provided under the percent-above standard 

method. 

 The EPA is not proposing this option because the results would vary based on the years 

of analysis used to derive the thresholds, and also because the method does not generally result 

in classifications different than the more straight-forward approach we are proposing. 

3. Base the classifications on the amount of time each individual area is projected to need 

to attain the NAAQS or on the relative amount of emission reductions needed to attain the 

NAAQS. 

This possible method has the advantage of basing the classification system on the 

projected difficulty of attainment of the standard. However, it has a number of drawbacks. The 

CAA’s classification system, as enacted in 1990, is applied virtually without judgment—areas 

are classified “by operation of law” based on solid physical data—their ozone design values. The 

only judgment provided under the Act is the provision under 181(a)(4) that allows for 

reclassification upward or downward if an area’s design value is within 5 percent of the next 

higher or lower threshold. Allowing additional judgment to enter into the classification process 

would negate any “operation of law” perception or action. In addition to appearing inconsistent 

with the approach contemplated in the Act, the very fact that a predictive method is used that 

relies on judgment concerning projected emissions, meteorology, and a model that does not 

depict actual reality would require significant data gathering and judgment in setting the 

classification which would be time consuming, complex, and likely contentious and using such 

an approach could delay the designation process. 
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4. Set the classification thresholds such that each classification range is a fixed ppm 

amount (such as 0.005 ppm). 

The EPA is not soliciting comment on this option because EPA believes this method is 

entirely arbitrary and not consistent with the methodology employed in the CAA’s classification 

system. 

5. Retention of classification under 1-hour ozone standard if requested by State . 

 Because of the concern at the time of the Phase 1 Rule that some areas should be in 

higher classifications because they need more time to attain, EPA also considered another 

method for classifying areas that was based on a comment received on the 2003 proposal. Under 

this method, an area that had a higher classification for the 1-hour ozone standard could choose 

to receive either that higher classification for the 2008 8-hour standard or the classification that it 

would receive under the revised classification table. Applying that concept here, we could use 

the current 1997 ozone standard as setting the classification that the State could request. Thus, if 

a state requested the higher 1997 classification for the area, the higher classification would apply 

to the entire nonattainment area regardless of whether all or only a portion of the 2008 8-hour 

area had the higher classification for the 1997 8-hour standard. Areas that do not contain a 

former 1997 8-hour nonattainment area or contain a 1997 8-hour standard nonattainment area 

that had a higher classification would receive the classification from the revised classification 

table under the percent above standard proposal. 

The EPA is not proposing this approach in part because while it might reflect past 

difficulties encountered by an area, it does not necessarily account for an area’s current ability to 

achieve the 2008 8-hour 0.075 ppm standard. 

6. Method that would rely on the same proportion of areas in each classification as in the 

original 1991 classifications. 

 In a letter of August 26, 2008 (see Appendix A), the Texas Commission on 

Environmental Quality (TCEQ) suggested another alternative method for classification that they 

feel would better represent the degree of difficulty that areas faced in lowering ozone 

concentrations. Their method and rationale is presented in detail in their letter, but in short it 

relies on placing areas into classifications such that the same proportion of areas would be in 

each of the five classifications for the 2008 ozone standard that were in those five classifications 

when 1-hour areas were classified by operation of law in 1991. 

The EPA is not proposing this option, in part, because the basis for the method relies on 

the situation that existed in 1990, which is not necessarily similar to the situation faced by 

nonattainment areas under the 2008 standard. Also, the method would require the areas to be 

ranked in ascending design values. Many hypothetical nonattainment areas at the lower design 

values have the same design value. Since the areas with the same design value should be placed 

in the same classification, the same proportion of hypothetical areas in each of the five 
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classifications could not be achieved. This method to determine threshold cut points would 

require more judgment in setting the classification than the CAA contemplated. 

7. Method that would rely on the range of classification thresholds and the range of design 

values. 

 The EPA also considered another method for classification of nonattainment areas under 

the 2008 8-hour ozone NAAQS that is based on Table 1 in the CAA that would account not only 

for the change in the standard, but also the spread of design values similar to the method used 

originally to develop the classification table in the CAA. This method is based on the principle 

that a classification table for the 2008 ozone standard should account for not only the relative 

ozone design values above the level of the standard, but also the absolute spread of design values 

higher than the standard that we actually measure in the air at the time we establish the 

classification table. Doing so would grade areas based upon how much reduction in ozone 

concentrations an area needs to achieve attainment relative to the worst possible area. 

 Under the 1-hour ozone NAAQS, Extreme areas needed to reduce ozone levels by 0.160 

ppm (0.280 ppm – 0.120 ppm) or more to get the DV at or below the commensurate level of the 

standard. The 1-hour DV, available to Congress at the time of CAA drafting, for the South Coast 

for the 1988-1990 period was 0.340 ppm, Thus, the actual reduction level needed for South 

Coast’s then-presumptive Extreme area to reach attainment turned out to be (0.340 ppm - 0.12 

ppm = 0.220 ppm.4

) In contrast, 1-hour Marginal areas only needed to reduce their DV by at 

most 0.017 ppm (0.137 ppm - 0.12 ppm) to achieve the same results. This is only 10.6 percent of 

the minimum needed (0.160 ppm) for an Extreme area. Moderate areas needed at most 24.3 

percent (100 * (0.159 ppm - 0.12 ppm) / .0160 ppm) reduction in ozone levels of the 0.160 ppm 

minimum reduction needed by an Extreme area. 

 Under this alternative, a ratio would be calculated using the following: 

[the difference between a specific classification threshold and the lowest threshold (Marginal) in 

CAA Table 1] 

divided by 

[the difference between the highest DV measured at the time of setting the classification table 

and the lowest threshold (Marginal) in CAA Table 1] 

 The ratio would then be used in conjunction with the 2008 ozone standard. For that 

standard, we know the bottom threshold of Marginal will be the lowest violating DV (0.076 

ppm). The highest 8-hour DV recorded for the period 2008-2010 data is in Los Angeles: 0.112 

ppm. 

 

4 Clean Air Act Amendments of 1990. Report of the Committee on Energy and Commerce. 

U.S. House of Representatives on H.R. 3030 together with Additional, Supplemental, and 

Dissenting Views. May 17, 1990. 101st Congress, 2d Session. Rept. 101-490 part 1. Page 230.
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The resulting table would be as follows: 

Area class CAA DV thresholds 1-hour ozone ppm Translated 8-hour DV ppm 

Marginal 

from 0.121 0.076 

up to1 0.138 0.079 

Moderate 

From 0.138 0.079 

up to1 0.160 0.082 

Serious 

From 0.160 0.082 

up to1 0.180 0.086 

Severe2 From 0.180 0.086 

up to1 0.280 0.102 

Extreme equal to or above 0.280 0.102 

1

 but not including 

2

 Section 181(a)(2) of the CAA provides that, in the case of a Severe area with a DV between 0.190 and 0.280 ppm, 

the attainment date shall be 17 years (in lieu of 15 years) after November 15, 1990 The comparable range using this 

classification method is 0.087 ppm up to but not including 0.102 ppm. 

 Note that this method is similar to, but different from the “Ratio of Thresholds” method. 

EPA is not proposing this method in part because the results would vary depending on the years 

of data used for the design values in calculating the ratios in both of the columns above (CAA 

DV thresholds 1-hour ozone ppm, and the translated 8-hour design values). This amount of 

variability does not appear consistent with the CAA’s concept of classification “by operation of 

law.” 

8. Method that would rely on exceedances of the NAAQS as well as design values. 

In a letter of May 26, 2009 (see Appendix B), the California Air Resources Board 

(CARB) suggested another alternative method for classification that would rely on exceedances 

of the NAAQS as well as design values. The California Air Resources Board (CARB) staff 

provided a recommended classification method that relies not only on ozone design values, but 

also the number of exceedances of the level of the 8-hour ozone standard. The rationale for this 

method rests in part on the CAA’s provision in section 181(a)(4) that allows reclassification 

downward of an area within 90 days after designation if an area’s design value is within 5 

percent of a classification’s lower threshold and reclassification upward if the area’s design value 

is within 5 percent of a classification’s upper threshold. One of the criteria for consideration in 

allowing such reclassification is the number of exceedances of the standard in the area. While 

the Act does allow consideration of exceedances for the reclassification provision, the Act’s 

basic classification table does not account for the number of exceedances. Thus, EPA believes 

that the classification method based on threshold levels that EPA is proposing is more closely 

tied to the structure in Table 1 in the CAA. 
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9. Administrator’s Discretion To Reclassify Areas Under 5 Percent Bump-Up Authority 

Concept.

EPA considered an approach that would automatically apply an upward classification to 

any hypothetical nonattainment area with an initial design value within 5 percent of the upper 

threshold of their initial classification. This approach could be applied to any final classification 

scheme for purposes of initial area designations. The Administrator has discretion to use section 

181(a)(4) to place areas that have design values that are within 5 percent of the upper threshold 

of their initial classification into the next higher classification. The CAA sets forth criteria for 

EPA to consider in applying the 5 percent classification adjustment: (1) the number of 

exceedances of the national primary ambient air quality standard for ozone in the area, (2) the 

level of pollution transport between the area and other affected areas, including both intrastate 

and interstate transport, (3) the mix of sources in the area, and (4) the mix of air pollutants in the 

area. The statute does not limit EPA to considering only those factors. 

EPA’s past practice in applying the 5 percent classification adjustment was described in 

the preamble to the designations and classifications for the 1997 ozone standard and in the 

rulemaking in which EPA applied the 5 percent adjustment provision to several areas under the 

1997 ozone standard.5

 Although that exercise of the section 181(a)(4) provision was only done 

for cases that were requested by the States and only as downward reclassifications, EPA could, if 

we followed this approach, apply upward reclassifications in the absence of state requests and 

based on available information that would strongly suggest that the initial classification for an 

area likely would not provide sufficient time for the area to attain. In fact, in the April 30, 2004 

designation and classification rulemaking, we said, “EPA will consider bumping up areas subject 

to the five percent provision on our own initiative where there is evidence that an area is unlikely 

to attain within the period allowed by their classification. In making this determination, EPA 

would consider criteria similar to that listed above (adjusted to consider bump ups rather than 

bump downs) regarding discontinuity, attainment, emissions reduction and trends.” However, 

EPA did not exercise that discretion on its own initiative at that time. 

Based on hypothetical nonattainment areas using 2010 design values and a standard of 

0.075 ppm, approximately 9 Marginal areas, 2 Moderate areas and 1 Serious area would 

potentially qualify under the Percent-Above-Standard method (see Appendix C) for an upward 

classification adjustment. However, we believe that to codify the 5 percent upward classification 

provision as an automatically executing adjustment (i.e., so that it would occur “by operation of 

law/rule”), would require EPA to conclude that the CAA criteria listed above have been 

assessed, and/or articulate some other overriding criteria, without knowing exactly which 

potential nonattainment areas would be affected. In the absence of advance certainty about 

which areas would be affected, EPA believes that for initial area designations for the 2008 

NAAQS that it would be more appropriate to use the section 181(a)(4) provision on a case-bycase basis (rather than automatically) using information relevant to each affected area. 

 

 

5

 April 30, 2004 (69 FR 23585 at 23863); September 22, 2004 (69 FR 56697). 
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10. Establish a relationship between the 1-hour and 8-hour monitored values or design 

values based on measured air quality data. 

The EPA evaluated this concept in 20036

 and was critical of it at the time based on a high 

degree of “scatter” (relation of individual actual 1-hour to 8-hour values would differ from the 

relationship established from all monitors, counties, or areas). We did not attempt to re-evaluate 

this method for the 2008 NAAQS, which is still an 8-hour form. 

 

 

6 Additional Options Considered for “Proposed Rule to Implement the 8-Hour Ozone National 

Ambient Air Quality Standard.” U.S. Environmental Protection Agency, Office of Air Quality 

Planning and Standards, Research Triangle Park, NC. March 2003. Docket document EPA-HQOAR-2003-0079-0070.
11

 

APPENDIX A 

Method Using Same Proportions as 1991 Classification

Recommendation from the Texas Commission on Environmental Quality 
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17
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19
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APPENDIX B 

Method that Relies on Exceedances of the NAAQS as well as Design Values 

Recommendation from the California Air Resources Board 
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25

 
26
27

 
28

 
29

 
30

 
31

 
32
33

 
34
35
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Appendix C 

8-hour Ozone NAAQS Hypothetical Nonattainment Areas That Could Be Reclassified 

Under CAA Section 181(A)(4) (5% Adjustment Provision). 

Section 181(a)(4) provides that an area may within 90 days after the initial classification 

receive a lower (or higher) classification if its design value is within 5 percent of the low (or 

high) threshold for the initial classification. Although we are not proposing this as a specific 

option, we did look at how many potential Marginal and Moderate areas might have design 

values within 5% of the threshold for the next higher classification. This is based on an 8-hour 

standard of 0.075ppm and the percent-above-standard classification method. This would mean 

that any Marginal area with an 8-hour design value of 0.082 ppm (up to 0.086 ppm) could be 

classified by EPA as Moderate, any Moderate area with an 8-hour design value of 0.095 ppm (up 

to 0.100 ppm) could be classified by EPA as Serious and, any Serious area with an 8-hour design 

value of 0.107 ppm (up to 0.113 ppm) could be classified by EPA as Severe. 

 Under this method, 9 of the hypothetical Marginal nonattainment areas, 2 of the 

hypothetical Moderate nonattainment areas, and 1 of the hypothetical Serious nonattainment 

areas would have design values within 5 percent of the next higher classification. The 

hypothetical Marginal, Moderate, and Serious nonattainment areas, along with their 2010 design 

values, are shown in the following table.

8-Hr Ozone NAAQS Hypothetical Nonattainment Areas That Could Be Reclassified Under 

CAA Section 181(A)(4) (5% Adjustment Provision)

Hypothetical Nonattainment Area 2010 DV (ppm) Classification 

Charlotte-Gastonia-Rock Hill, NC-SC 0.082 Marginal 

Amador and Calaveras Cos (Central Mtn), CA 0.083 Marginal 

Kern Co (Eastern Kern), CA 0.083 Marginal 

Mariposa and Tuolumne Cos (Southern Mtn),CA 0.083 Marginal 

Philadelphia-Wilmin-Atlantic Ci,PA-NJ-MD-DE 0.083 Marginal 

Houston-Galveston-Brazoria, TX 0.084 Marginal 

Nevada Co. (Western Part), CA 0.084 Marginal 

New York-N. New Jersey-Long Island,NY-NJ-CT 0.084 Marginal 

San Luis Obispo-Paso Robles, CA 0.084 Marginal 

 

Riverside Co, (Coachella Valley), CA 0.095 Moderate 

Los Angeles-San Bernardino Cos(W Mojave),CA 0.099 Moderate 

 

Los Angeles South Coast Air Basin, CA 0.112 Serious