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

[1] The U. S. Environmental Protection Agency (EPA) presently uses a 40 ppbv background O-3 level as a baseline in its O-3 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 O-3 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 O-3 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 O-3 concentrations, including the springtime high-O-3 events, with only a minor stratospheric contribution (always < 20 ppbv). We conclude that the previous interpretations of a stratospheric source for these events underestimated the role of regional and hemispheric pollution. While stratospheric intrusions might occasionally elevate surface O-3 at high-altitude sites, our results indicate that these events are rare and would not compromise the O-3 air quality standard. We find that the O-3 background is generally 15 - 35 ppbv, with some incidences of 40 - 50 ppbv in the west in spring at high-elevation sites (> 2 km). It declines from spring to summer and further decreases during O-3 pollution episodes. The 40 ppbv background assumed by EPA thus actually underestimates the risk associated with O-3 during polluted conditions. A better definition would represent background as a function of season, altitude, and total surface O-3 concentration. Natural O-3 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 O-3 above 40 ppbv) in the United States.