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concern to Congress; to federal, state, and local regulatory agencies; to industry, and to the public.

It is important that areas not in attainment of the NAAQS be able to track their progress in reducing ozone. A downward trend in ozone concentrations over several years might suggest that emission controls are having the desired effect, whereas the absence of such a trend might suggest the need for additional controls. However, analyses of ambient-air-monitoring data for ozone in major U.S. cities indicate that. the number of days on which ozone concentrations exceed the NAAQS 1-hour average concentration of 120 parts per billion (ppb) varies widely from one year to the next (Stoeckenius, 1990). EPA's principal statistical measure of ozone trends, the composite average of second-highest daily maximum 1-hour concentrations in a given year, also varies considerably from year to year. Furthermore, there is no discernible downward trend (EPA, 1991a). A substantial portion of this variability is attributable not to year-to-year changes in precursor emissions, but to natural fluctuations in the weather. Meteorologically induced variability makes it difficult to identify underlying trends in ozone concentrations that could result from changes in the amount, type, and geographical distribution of precursor emissions. This chapter outlines the existing form of the ozone NAAQS, summarizes studies on ozone trends, evaluates recent efforts to screen meteorological influences on ozone trends, and highlights research needs for more robust indicators of progress in reducing ozone concentrations.

National Ambient Air Quality Standard For Ozone

To guide in monitoring and controlling ambient concentrations of ozone, EPA has developed ambient air quality standards for ozone. The national form of the ozone NAAQS is explicitly defined in the U.S. Code of Federal Regulations (40 CFR 50.9(a)), in part:

The [ozone] standard is attained when the expected number of days per calendar year with a maximum hourly average concentration above 0.12 part[s] per million (235 µg/m3) is equal to or less than 1, as determined by Appendix H [of the regulations].

Appendix H provides a method to account for incomplete monitoring data and allows the number of days exceeding the standard each year to be averaged over the past 3 calendar years. An area is in compliance as long as this average remains less than or equal to one (McCurdy and Atherton, 1990).