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Analyses of ambient measured ozone data were used in conjunction with the application of photochemical modeling to determine the technical feasibility of attaining the federal 8-hr ozone standard in central California. Various combinations of volatile organic compound (VOC) and oxides of nitrogen (NO x ) emission reductions were effective in lowering modeled peak 1-hr ozone concentrations. However, VOC emissions reductions were found to have only a modest impact on modeled peak 8-hr ozone concentrations. NO x emission reductions generally lowered 8-hr ozone concentrations, but their effectiveness was partially or, in some cases, wholly offset by the increase in the number of NO cycles and, hence, in the ozone produced per NO. As a result, substantial NO x emission reductions-70 to 90%-were required to reduce peak 8-hr ozone concentrations to the level of the standard throughout the modeling domain. These modeling results provide a possible physical explanation for recent analyses that have reported more prominent trends in peak 1-hr ozone levels than in peak 8-hr ozone concentrations or in occurrences of mid-level (60 -90 parts per billion by volume) ozone concentrations. The findings also have serious implications for the feasibility of attaining the 8-hr ozone standard in central California. Further efforts are needed to clarify the applicability of the modeling results to the full set of days with ozone levels exceeding the 8-hr ozone standard, as well as their applicability to other geographical areas. INTRODUCTIONRecent empirical and modeling analyses indicate that attainment of the new federal 8-hr 0.08 parts per million by volume (ppmv; 84 parts per billion by volume, ppbv) ozone standard may be very difficult at many locations because the occurrences of high (Ͼ0.09 ppmv) hourly average concentrations have decreased faster than those of mid-level (0.06 -0.09 ppmv) hourly average concentrations. 1 Moreover, statistically significant downward ozone trends in the United States over the period 1980 -1995 were clustered in three metropolitan areas, Los Angeles, New York, and Chicago, with less than 15% of rural sites exhibiting significant trends. 2,3 Some locations exhibiting significant downward trends in peak hourly maximum ozone have not shown trends in the annual fourthhighest 8-hr maximum, 2 and the trend in the composite average fourth-highest 8-hr maximum is less than the trend in the 1-hr max. 2,4 A physical explanation for these various differential ozone trends has not yet been advanced.Different emission control strategies may be required for attainment of the 8-hr ozone standard than those that have successfully reduced peak hourly ozone concentrations, and very large reductions of anthropogenic emissions might be needed. To determine the technical feasibility of attaining the 8-hr standard, and to identify the best precursor reduction strategies for moving toward attainment, it is necessary to understand the physical and chemical mechanisms underlying the observed ozone trends and to use ...

Analyses of ozone (O 3 ) measurements in conjunction with photochemical modeling were used to assess the feasibility of attaining the federal 8-hr O 3 standard in the eastern United States. Various combinations of volatile organic compound (VOC) and oxides of nitrogen (NO x ) emission reductions were effective in lowering modeled peak 1-hr O 3 concentrations. VOC emissions reductions alone had only a modest impact on modeled peak 8-hr O 3 concentrations. Anthropogenic NO x emissions reductions of 46 -86% of 1996 base case values were needed to reach the level of the 8-hr standard in some areas. As NO x emissions are reduced, O 3 production efficiency increases, which accounts for the less than proportional response of calculated 8-hr O 3 levels. Such increases in O 3 production efficiency also were noted in previous modeling work for central California. O 3 production in some urban core areas, such as New York City and Chicago, IL, was found to be VOC-limited. In these areas, moderate NO x emissions reductions may be accompanied by increases in peak 8-hr O 3 levels. The findings help to explain differences in historical trends in 1-and 8-hr O 3 levels and have serious implications for the feasibility of attaining the 8-hr O 3 standard in several areas of the eastern United States. INTRODUCTIONRecent empirical and modeling analyses indicate that attainment of the federal 8-hr 0.08 parts per million by volume (ppmv; 84 parts per billion by volume, ppbv) ozone (O 3 ) standard may be very difficult at many locations because the occurrences of high (Ͼ0.09 ppmv) hourly average concentrations have decreased faster than those of mid-level (0.06 -0.09 ppmv) hourly average concentrations. 1 Moreover, statistically significant downward O 3 trends in the United States over the period 1980 -1995 were clustered in three metropolitan areas, Los Angeles, New York, and Chicago, with less than 15% of rural sites exhibiting significant trends. 2,3 Some locations exhibiting significant downward trends in peak hourly maximum O 3 have not shown trends in the annual fourth-highest 8-hr maximum, 2 and the trend in the composite average fourth-highest 8-hr maximum is less than the trend in the 1-hr maximum. 2,4 These reported trends appear consistent with the U.S. Environmental Protection Agency's (EPA) most recent trends analysis. 5 In previous work, analyses of ambient O 3 data and photochemical modeling were used to examine the technical feasibility of attaining the federal 8-hr O 3 standard in central California. 6,7 Various combinations of volatile organic compound (VOC) and oxides of nitrogen (NO x ) emission reductions have been effective in lowering modeled peak 1-hr O 3 concentrations. VOC emission reductions have had only a modest impact on modeled peak 8-hr O 3 concentrations. NO x emission reductions generally lowered 8-hr O 3 concentrations, but their