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

Arlene Fiore, Daniel J. Jacob, Hongyu Liu, Robert M. Yantosca, T. Duncan Fairlie, Qinbin Li
J. Geophys. Res., 108, 4787, doi:10.1029/2003JD003855, 2003.


The U.S. Environmental Protection Agency (EPA) presently uses a 40 ppbv background ozone (O3) 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 < 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 O3 at high-altitude sites, our results indicate that these events are rare and would not compromise the O3 air quality standard. We find that the O3 background is generally 15-35 ppbv, with some incidences of 40-50 ppbv in the west in spring at high-elevation sites (> 2km). 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 concentrations. 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.

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