Understanding the effectiveness of precursor reductions in lowering 8-hr ozone concentrations - Part II. The eastern United States

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 (NOx) 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 NOx 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 NOx 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 NOx 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.