Emission reductions were mandated in the Clean Air Act Amendments of 1990 with the expectation that they would result in major reductions in the concentrations of atmospherically transported pollutants. This paper uses generalized additive modelsto characterize the form and magnitude of changes in ground-level ozone concentrations (weekly average, daily maximum 1-h average, and daily maximum 8-h average) adjusted for concomitant meteorological conditions at 35 rural sites in the eastern UnitedStates from 1989 to 1995. At all sites and for all three measures, ozone concentrations declined over the period by about 5 (based on median percent change) for the two daily measures and about 7 for the weekly average. These declines were statistically significant (0.05 level) at 18, 17, and 21 sites (for the daily maximum 1-h, daily 8-h maximum, and weekly averages, respectively), and the median percent total change at these sites for all three measures was approximately -9. All sites were evaluatedwith respect to the old and new National Ambient Air Quality Standard for ozone. Under the new standard (based on the daily maximum 8-h average), 23 sites would not have met the standard for at least one triennial evaluation period as compared to sevensites under the old standard (based on the daily maximum 1-h average).
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