%Effect of Emissions Control on the Long-Term Frequency Distribution of Regional Ozone Concentrations

Abstract

Photochemical airshed models that simulate the pollutant transport and atmospheric chemical reaction processes leading to ozone formation now can be exercised for years at a time, permitting a thorough evaluation of the extent to which urban and regional ozone concentrations can be controlled. The Los Angeles ozone problem serves as a prototype for severe photochemical smog problems elsewhere. In southern California, the occurrence of peak 1-h average ozone concentrations above 0.12 ppm can be reduced to approximately 20 days per year through control of organic vapor and oxides of nitrogen emissions. Calculations show that the number of days per year with 1-h average O3 concentrations above 0.12 ppm approaches zero more quickly in response to controls than is the case for the number of days with lower but more persistent ozone concentrations; as a result, more than 60 days per year will exceed the new U.S. Federal ozone standard set in 1997 at a level of 0.08 ppm over an 8-h averaging time, even at very stringent levels of emission control. The days with the highest observed ozone concentrations are not necessarily the hardest days to bring below the air quality standards.