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Note to readers: The report was completed in May 1997, prior to EPA announcing the new 8-hour ozone standard in July 1997. Although the final standard was modified slightly from the December 1996 proposed standard, the conclusions reached in this study are applicable to the final form of the new 8-hour ozone standard. A paper summarizing the research was published in the June 1998 issue of the peer-reviewed Journal, Environmental Science &Technology (Lefohn et al., 32(11):276A-282A). Please see publications listing for further information.

Assessing the Attainment of EPA's Proposed Primary
8-Hour Average Ozone Standard

Allen S. Lefohn, A.S.L. & Associates
111 North Last Chance Gulch, Helena, MT 59601

Douglas S. Shadwick, ManTech Environmental Technology, Inc.
Research Triangle Park, NC 27709

Stephen D. Ziman, Chevron Research and Technology
Richmond, CA 94804

Prepared for

American Petroleum Institute
1220 L St., N.W.
Washington, D.C. 20005

May 8, 1997



Ozone and related pollutants have long been recognized, in both clinical and epidemiological research, as affecting public health. In accordance with sections 108 and 109 of the Clean Air Act, the U.S. Environmental Protection Agency reviewed the air quality criteria and National Ambient Air Quality Standards (NAAQS) for ozone (O3). Based on its review, in December 1996, the EPA proposed to change both the human health (primary) and secondary (welfare/vegetation) standards for O3 (U.S. EPA, 1996a).

Currently the existing primary and secondary standards for O3 are each set at a level of 0.12 ppm, with a 1-hour averaging time and a 1-expected-exceedance form. The standards are attained when the expected number of days per calendar year with maximum hourly average concentrations above 0.12 ppm is equal to or less than 1, averaged over 3 years (i.e., no more than 3 exceedances in 3 years) (as determined by 40 CFR Part 50, Appendix H). Current emissions control strategies are focused on reducing the highest hourly average concentrations. One of the proposed revisions announced by the Agency for O3 is a new 8-hour primary standard set at 0.08 parts per million (ppm) with the third highest daily maximum concentration averaged over 3 years. Based on rounding conventions, the Agency is considering two possible versions of the primary 8-hour 0.08 ppm concentration-based standard. One option that EPA is taking comment on is to use a 0.081 ppm threshold (hereafter referred to as 081CB3); the favored option by the Agency is to use a 0.085 ppm threshold (hereafter referred to as 085CB3).

As of January 1997, under the current 1-hour O3 standard of 0.12 ppm, there were 66 areas and 273 counties designated by the U.S. EPA as nonattainment. For the period 1993-1995, 46 areas violated the current 1-h standard. Based on its analysis of 1993-1995 monitoring data, the EPA identified 335 counties that would violate the 085CB3 primary standard (Figure 1). No estimate was provided either for the number of areas represented by these counties or the number of areas and counties that would violate the 081CB3 primary standard.

For developing an appropriate emission control strategy for meeting the proposed 8-hour standard, it is important to identify the unique patterns of hourly average concentrations that make up the 8-hour violations and investigate the importance of mid-range concentrations (i.e., 0.06-0.09 ppm) in defining these violations. For those sites whose 8-hour violation is defined primarily by hourly average concentrations below 0.09 ppm, control strategies will have to change from focusing on the higher hourly average concentrations to the mid-level (i.e., 0.06 to 0.09 ppm) hourly average values (Lefohn, 1997). Given the potential importance of the mid-level concentrations, it is important to investigate whether the rate of decline of the mid-level hourly average concentrations is similar to the rate experienced by the high hourly average concentrations.

ROM emission control simulations (U.S. EPA, 1985) have shown that mid-level hourly average concentrations decline at a slower rate than the high-level values. Roselle and Schere (1995) have presented similar evidence. The increased resistance as one attempts to reduce the mid-level hourly average concentrations is similar to the resistance experienced when a gas is compressed by a piston. The resistance is initially low, but the resistance increases as the piston compresses. The implication of the "piston" effect is that sites whose 8-hour daily maximum violation is influenced by mid-level hourly average concentrations may have a difficult time attaining the proposed 8-hour standard.

To investigate the possible importance of the "piston" effect, this report identifies, for the period 1993-1995, the number of sites that violated the proposed 8-hour 0.08 ppm (085CB3) O3 standard. A subset of violating sites was identified in which the mid-level hourly average concentrations played an important role in defining the violation. Using data from the EPA's Aerometric Information Retrieval System (AIRS), for trending O3 sites, we quantified the rate of decline (at 0.01 ppm increments) of the hourly average O3 concentrations at the high-level (greater than or equal to 0.09 ppm), mid-level (0.05-0.09 ppm), and low-level (< 0.05 ppm) ranges.

To investigate the relationship between the rate of decline of the hourly concentrations and the 8-hour average concentrations for those sites that experienced trends, we compared the strength of the hourly trend with the strength of the trends of the (1) annual 8-hour third highest concentration, and (2) running 3-year average of the 8-hour third highest value.


The number of potential violating areas for the proposed 8-hour primary standard was quantified and those violating sites that experience 4 or more hourly average concentrations in the mid-range (between 0.06 and 0.09 ppm) that define the 8-hour violations were identified. For the period 1993-1995, 71% of the areas that violate the 8-hour 081CB3 option are heavily influenced by mid-level hourly average concentrations (i.e., < 0.09 ppm); 55% of the areas are heavily influenced by mid-level hourly average concentrations (i.e., < 0.09 ppm) for the 085CB3 form of the standard. If one uses the list of counties that the EPA has identified as violating the 085CB3 standard, 52% (88 of 169 areas) of all potential nonattainment areas are heavily influenced by mid-level hourly average concentrations (i.e., < 0.09 ppm). The remaining violating areas have sites that currently experience hourly average concentrations during the 8-hour period four or more times greater than or equal to 0.09 ppm. Almost all the design value sites either are presently influenced by mid-level hourly average concentrations or will be influenced in the future.

With almost all the design value sites that violate the proposed primary standard, either currently influenced by mid-level concentrations or potentially influenced in the future (after reducing the higher values into the mid-range), it is important to explore the efficiency of reducing the mid-level values. For the 82 trending sites we investigated, 77% of the sites indicate a greater resistance to reducing the hourly average concentrations in the mid-range values (i.e., 0.06-0.09 ppm) than in the hourly average concentrations above 0.09 ppm.

Of the 82 monitoring sites that showed trends, almost all experienced a stronger trending for the hourly concentrations than the 8-hour annual third highest 8-hour daily maximum values or the two forms of the running third highest 8-hour daily maximum concentration averaged over 3-years. This indicates that for the 82 monitoring sites, the rate of decline is slower for the 8-hour average concentrations than the hourly values. Our results indicate that, for most sites that violate the proposed 8-hour primary standard, the attainment of the proposed 8-hour primary standard may be extremely difficult.


Code of Federal Regulations (1991). National primary and secondary ambient air quality standards. C.F.R. 40:§50.

Roselle, S.J. and K.L. Schere. (1995). Modeled response of photochemical oxidants to systematic reductions in anthropogenic volatile organic compound and NOx emissions. Journal of Geophysical Research. 100 (D11):22929-22941.

U.S. Environmental Protection Agency (1985). Application of the first-generation regional oxidant model to an assessment of the effects of proposed 1987 emissions reductions on episodic ozone levels in the northeastern United States. Atmospheric Science Research Laboratory, Office of Air Quality and Planning, Research Triangle Park, NC.

U.S. Environmental Protection Agency (1996a) National Ambient Air Quality Standards for Ozone: Proposed Decision. Federal Register 61, Number 241, 65715-65750. December 13, 1996.

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