An updated version of the Statewide Air Pollution Research Center (SAPRC) chemical mechanism (SAPRC07C) was implemented into the Community Multiscale Air Quality (CMAQ) version 4.6. CMAQ simulations using SAPRC07C and the previously released version, SAPRC99, were performed and compared for an episode during JulyAugust, 2000. Ozone (O 3 ) predictions of the SAPRC07C simulation are generally lower than those of the SAPRC99 simulation in the key areas of central and southern California, especially in areas where modeled concentrations are greater than the federal 8-hr O 3 standard of 75 parts per billion (ppb) and/or when the volatile organic compound (VOC)/nitrogen oxides (NO x ) ratio is less than 13. The relative changes of ozone production efficiency (OPE) against the VOC/NO x ratio at 46 sites indicate that the OPE is reduced in SAPRC07C compared with SAPRC99 at most sites by as much as approximately 22%. The SAPRC99 and SAPRC07C mechanisms respond similarly to 20% reductions in anthropogenic VOC emissions. The response of the mechanisms to 20% NO x emissions reductions can be grouped into three cases. In case 1, in which both mechanisms show a decrease in daily maximum 8-hr O 3 concentration with decreasing NO x emissions, the O 3 decrease in SAPRC07C is smaller. In case 2, in which both mechanisms show an increase in O 3 with decreasing NO x emissions, the O 3 increase is larger in SAPRC07C. In case 3, SAPRC07C simulates an increase in O 3 in response to reduced NO x emissions whereas SAPRC99 simulates a decrease in O 3 for the same region. As a result, the areas where NO x controls would be disbeneficial are spatially expanded in SAPRC07C. Although the results presented here are valuable for understanding differences in predictions and model response for SAPRC99 and SAPRC07C, the study did not evaluate the impact of mechanism differences in the context of the U.S. Environmental Protection Agency's guidance for using numerical models in demonstrating air quality attainment. Therefore, additional study is required to evaluate the full regulatory implications of upgrading air quality models to SAPRC07.
INTRODUCTIONRegional chemical transport models (CTMs) are widely used in air quality applications for forecasting ozone (O 3 ) and particulate matter (PM) pollution events 1,2 and for developing emission control strategies for reducing pollution to protect human health. 3 CTM simulations are the only reliable method for estimating the response of pollutant concentrations to changes in emissions and meteorology over regional domains. Therefore air quality management agencies use CTM-derived estimates of concentration responses to emission reductions in developing State Implementation Plans for reducing O 3 and PM pollution to comply with the National Ambient Air Quality Standards (NAAQS). 4,5 The U.S. Environmental Protection Agency's (EPA) Community Multiscale Air Quality (CMAQ) model 6 is routinely used for regulatory applications in California.A major component of a CTM is the gas-phase chemical mecha...