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frost ppr Ryerson_Science_2001

frost ppr Ryerson_Science_2001 - REPORTS Observations of...

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Observations of Ozone Formation in Power Plant Plumes and Implications for Ozone Control Strategies T. B. Ryerson, 1 * ² M. Trainer, 1 * J. S. Holloway, 1,2 D. D. Parrish, 1 L. G. Huey, 3 D. T. Sueper, 1,2 G. J. Frost, 1,2 S. G. Donnelly, 4 S. Schauffler, 4 E. L. Atlas, 4 W. C. Kuster, 1 P. D. Goldan, 1 G. Hu ¨bler, 1,2 J. F. Meagher, 1 F. C. Fehsenfeld 1,2 Data taken in aircraft transects of emissions plumes from rural U.S. coal-fired power plants were used to confirm and quantify the nonlinear dependence of tropospheric ozone formation on plume NO x (NO plus NO 2 ) concentration, which is determined by plant NO x emission rate and atmospheric dispersion. The ambient availability of reactive volatile organic compounds, principally biogenic isoprene, was also found to modulate ozone production rate and yield in these rural plumes. Differences of a factor of 2 or greater in plume ozone formation rates and yields as a function of NO x and volatile organic compound concentrations were consistently observed. These large differences suggest that consideration of power plant NO x emission rates and geographic locations in current and future U.S. ozone control strategies could substantially enhance the efficacy of NO x reductions from these sources. Ozone is the most abundant tropospheric ox- idant and an important component of photo- chemical pollution. Elevated concentrations of near-surface ozone that principally occur during the summer months have been shown to be harmful to human health and damaging to vegetation ( 1 ). Regulation of ozone precur- sor emissions under the U.S. Clean Air Act of 1970 and its subsequent amendments has been partially successful in reducing human exposure, but many areas of the country are still subject to episodes of high ambient ozone levels ( 2 ). Tropospheric ozone is formed by photo- chemical reactions involving volatile organic compounds (VOCs) and the oxides of nitro- gen NO and NO 2 ; a simplified scheme is shown in Fig. 1A ( 3, 4 ). Early ozone man- agement strategies emphasized reductions of anthropogenic emissions of VOCs, such as those emitted in automobile exhaust. These strategies have successfully reduced peak ozone concentrations over time in cities where anthropogenic emissions dominate the ambient VOC mixture (e.g., Los Angeles) ( 5 ). In the eastern United States, however, large emissions of very reactive VOCs from biogenic sources [e.g., isoprene from oak trees (Fig. 1B)] have been shown to contrib- ute substantially to ozone formation in both rural and urban areas ( 6, 7 ). Controls on anthropogenic NO x in addition to anthropo- genic VOCs are now thought to be needed to reduce ozone in regions characterized by strong biogenic VOC sources ( 1 ) (e.g., Atlanta). Anthropogenic NO x emissions in the United States are estimated to total 22.2 Tg (as NO 2 ) per year, with 53% from transpor- tation, 25% from electric utility power plants, 12% from commercial fuel combustion, and the balance emitted from miscellaneous in- dustrial processes ( 2 ). Early efforts to reduce anthropogenic NO x were focused on the larg- est fraction, that from motor vehicles. Al-
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