Effect of emissions control on the long-term frequency distribution of regional ozone concentrations

Photochemical airshed models that simulate the pollutant transport and atmo spheric chemical reaction processes leading to ozone formation now can be e xercised for years at a time, permitting a thorough evaluation of the exten t to which urban and regional ozone concentrations can be controlled. The L os Angeles ozone problem serves as a prototype for severe photochemical smo g problems elsewhere. In southern California, the occurrence of peak 1-h av erage ozone concentrations above 0.12 ppm can be reduced to approximately 2 0 days per year through control of organic vapor and oxides of nitrogen emi ssions. Calculations show that the number of days per year with l-h average O-3 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 pe rsistent 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 c ontrol. The days with the highest observed ozone concentrations are not nec essarily the hardest days to bring below the air quality standards.