Stephen D. Ziman

Understanding the effectiveness of precursor reductions in lowering 8-hr ozone concentrations.

Abstract 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 (NOx ) 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. NOx 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 NOx 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.

Understanding the Effectiveness of Precursor Reductions in Lowering 8-Hr Ozone Concentrations—Part II. The Eastern United States

Abstract Analyses of ozone (O3) measurements in conjunction with photochemical modeling were used to assess the feasibility of attaining the federal 8-hr O3 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 O3 concentrations. VOC emissions reductions alone had only a modest impact on modeled peak 8-hr O3 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, O3 production efficiency increases, which accounts for the less than proportional response of calculated 8-hr O3 levels. Such increases in O3 production efficiency also were noted in previous modeling work for central California. O3 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 O3 levels. The findings help to explain differences in historical trends in 1- and 8-hr O3 levels and have serious implications for the feasibility of attaining the 8-hr O3 standard in several areas of the eastern United States.

The Application of Exposure-Based Criteria in Developing Alternative Primary Ambient Ozone Standards

The form of the primary National Ambient Air Quality Standard (NAAQS) for ozone has been a matter of some concern in recent years. The Environmental Protection Agency is reviewing the ozone NAAQS and has proposed a set of alternative standards that address some deficiencies. However, the proposed alternatives are all based upon the ozone concentrations occurring at the peak monitor, which may not be representative of the air quality and population exposure throughout a nonattainment area. Representativeness could be improved by converting the form of the standard from an exceedance-based metric (with x allowable exceedances per year, on average) to one based upon exposure. Three approaches to defining an exposure-based standard are developed here. Data from the San Francisco Bay area are used to illustrate the proposed standards.

South Central Coast Cooperative Aerometric Monitoring Program — An Integrated Atmospheric Pollutant Monitoring Program for the California Outer Continental Shelf (OCS)

In January 1984, the California Air Resources Board (CARB); EPA Region IX; the Minerals Management Services, Pacific Outer Continental Shelf Group; the counties of Ventura. Santa Barbara, and San Luis Obispo; and the Western Oil and Gas Association, a trade association composed of petroleum companies with operations in California, agreed to carry out the South Central Coast Cooperative Aerometric Monitoring Program (SCCCAMP). These groups joined together to develop a plan to collect a meteorological and air quality data base suitable for evaluating the predictive performance of a regional photochemical grid model in estimating the onshore ozone impacts of air emissions from offshore development and production operations in the Santa Barbara Channel and the southern Santa Maria Basin. The region is shown in Figure 1. SCCCAMP presently comprises a data collection program; no modeling activities are included in the Memorandum of Understanding governing the program.