| Summary: | 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 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 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. © 2000 American Chemical Society. Received for review August 12, 1999. Revised manuscript received February 4, 2000. Accepted February 29, 2000. We thank Mark Basset, Joe Cassmassi, Kevin Durkee, Xinqiu Zhang, and Satoru Mitsutami of the South Coast Air Quality Management District for their advice on meteorological and emissions data acquisition and processing and the DoD HPC Center Arctic Region Supercomputing Center for a grant of computer time. Funding was provided by the Caltech Center for Air Quality Analysis and the Palace Knight program of the U.S. Air Force. The views expressed are those of the authors and do not necessarily represent those of the U.S. EPA.
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