Variability in surface ozone background over the United States: Implications for air quality policy

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Abstract
The U.S. Environmental Protection Agency (EPA) presently uses a 40 ppbv background O3 level as a baseline in its O3 risk assessments. This background is defined as those concentrations that would exist in the absence of North American emissions. Lefohn et al. [2001] have argued that frequent occurrences of O3 concentrations above 50–60 ppbv at remote northern U.S. sites in spring are of stratospheric origin, challenging the EPA background estimate and implying that the current O3 standard (84 ppbv, 8‐hour average) may be unattainable. We show that a 3‐D global model of tropospheric chemistry reproduces much of the observed variability in U.S. surface O3 concentrations, including the springtime high‐O3 events, with only a minor stratospheric contribution (always 2 km). It declines from spring to summer and further decreases during O3 pollution episodes. The 40 ppbv background assumed by EPA thus actually underestimates the risk associated with O3 during polluted conditions. A better definition would represent background as a function of season, altitude, and total surface O3 concentration. Natural O3 levels are typically 10–25 ppbv and never exceed 40 ppbv. International controls to reduce the hemispheric pollution background would facilitate compliance with an AOT40‐type standard (cumulative exposure to O3 above 40 ppbv) in the United States.