Trends in exceedances of the ozone air quality standard in the continentalUnited States, 1980-1998

In 1997, the United States National Ambient Air Quality Standard (NAAQS) fo r ozone was revised from a l-h average of 0.12 parts per million (ppm) to a n 8-h average of 0.08 ppm. Analysis of ozone data for the ensemble of the c ontiguous United States and for the period 1980-1998 shows that the average number of summer days per year in exceedance of the new standard is in the range 8-24 in the Northeast and in Texas, and 12-73 in southern California . The probability of exceedance increases with temperature and exceeds 20% in the Northeast for daily maximum temperatures above 305 K. We present the results of several different approaches to analyzing the long-term trends in the old and new standards over the continental United States from 1980 t o 1998. Daily temperature data are used to resolve meteorological variabili ty and isolate the effects of changes in anthropogenic emissions. Significa nt negative trends are found in the Northeast urban corridor, in the Los An geles Basin and on the western bank of Lake Michigan. Temperature segregati on enhances the detection of negative trends. Positive trends occur at isol ated sites, mostly in the Southeast; a strong positive trend is found in Na shville (Tennessee). There is some evidence that, except in the Southwest, air quality improvements from the 1980s to the 1990s have leveled off in th e past decade. (C) 2001 Published by Elsevier Science Ltd.