[1] Observations indicate that ozone (O 3 ) concentrations in surface air over the United States in summer contain a 20-45 ppbv background contribution, presumably reflecting transport from outside the North American boundary layer. We use a threedimensional global model of tropospheric chemistry driven by assimilated meteorological observations to investigate the origin of this background and to quantify its contribution to total surface O 3 on both average and highly polluted summer days. The model simulation is evaluated with a suite of surface and aircraft observations over the United States from the summer of 1995. The model reproduces the principal features in the observed distributions of O 3 and its precursors, including frequency distributions of O 3 concentrations and the development of regional high-O 3 episodes in the eastern United States. Comparison of simulations with 1995 versus 1980 global fossil fuel emissions indicates that the model captures the previously observed decrease in the high end of the O 3 probability distribution in surface air over the United States (reflecting reduction of domestic hydrocarbon emissions) and the increase in the low end (reflecting, at least in the model, rising Asian emissions). In the model, background O 3 produced outside of the North American boundary layer contributes an average 25-35 ppbv to afternoon O 3 concentrations in surface air in the western United States. and 15-30 ppbv in the eastern United States during the summer of 1995. This background generally decays to below 15 ppbv during the stagnation conditions conducive to exceedances of the 8-hour 0.08 ppmv (80 ppbv) National Ambient Air Quality Standard (NAAQS) for O 3 . A high background contribution of 25-40 ppbv is found during 9% of these exceedances, reflecting convective mixing of free tropospheric O 3 from aloft, followed by rapid production within the U.S. boundary layer. Anthropogenic emissions in Asia and Europe are found to increase afternoon O 3 concentrations in surface air over the United States by typically 4-7 ppbv, under both average and highly polluted conditions. This enhancement is particularly large (up to 14 ppbv) for O 3 concentrations in the 50-70 ppbv range, and would represent a major concern if the NAAQS were to be tightened.

Sensitivity analysis is a powerful tool for understanding the response of an environmental system (e.g., air quality) or model of that system to both inputs and system parameters. A fast, formal sensitivity analysis method was developed for application to multidimensional, chemicallyactive environmental models. The technique was then implemented in a three-dimensional air quality model and applied to the South Coast Air Basin (SoCAB) of California. Using direct derivatives of the equations governing the evolution of species concentrations, the local sensitivities to a variety of model parameters (e.g., rate constants, dry deposition velocities, wind speed) and inputs (e.g., initial concentrations, ground-level emissions, wind speed) are computed simultaneously. Since the equations governing sensitivity coefficients have a structure similar to that of the pollutant concentrations, the implementation using this technique is straightforward and computationally efficient.

Relationships between ambient levels of selected air pollutants and pediatric asthma exacerbation in Atlanta were studied retrospectively. As a part of this study, temporal and spatial distributions of ambient ozone concentrations in the 20-county Atlanta metropolitan area during the summers of 1993, 1994, and 1995 were assessed. A universal kriging procedure was used for spatial interpolation of aerometric monitoring station data. In this paper, the temporal and spatial distributions of ozone are described, and regulatory and epidemiologic implications are discussed. For the study period, the Atlanta ozone nonattainment area based on the 1-h, exceedance-based standard of 0.12 ppm is estimated to expand-from 56% of the Atlanta MSA by area and 71% by population to 88% by area and 96% by population-under the new 8-h, concentration-based standard of 0.08 ppm. Regarding asthma exacerbation, a 4% increase in pediatric asthma rate per 20-ppb increase in ambient ozone concentration was observed (p-value = 0.001), with ambient ozone level representing a general indicator of air quality due to its correlations with other pollutants. The use of spatial ozone estimates in the epidemiologic analysis demonstrates the need for control of demographic covariates in spatiotemporal assessments of associations of ambient air pollutant concentrations with health outcome.

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