Dynamic evaluation of the CMAQv5.0 modeling system: Assessing the model’s ability to simulate ozone changes due to NOx emission reductions

Abstract

Regional air quality models are frequently used for regulatory applications to predict changes in air quality due to changes in emissions or changes in meteorology. Dynamic model evaluation is thus an important step in establishing credibility in the model predicted pollutant response. Beginning in 2003, the U.S. Environmental Protection Agency required substantial reductions in NOx emissions from power plants in the eastern U.S. which resulted in a decrease in ozone concentrations at monitoring sites in this region. This observable change in air quality has been used previously as a case study for dynamic evaluation of the Community Multiscale Air Quality (CMAQ) modeling system. Evaluation studies of previous CMAQ versions have shown that the model predicted decrease in ozone from 2002 to 2005 is less than the observed ozone change in this region. In this study, summertime simulations of 2002 and 2005 were conducted using the CMAQ version 5.0 modeling system released in 2012 that included several model updates aimed at improving the model predicted response, including updates to important model inputs. “Cross” simulations were conducted to separate the modeled impact of the emissions changes on ozone concentrations from the changes attributable to differences in meteorology across these years. Results show the change in the upper end of the ozone distribution explained by emission reductions is similar in magnitude to the change in ozone due to changes in meteorology across these years. Overall, the observed ozone decrease in the eastern U.S. continues to be underestimated by the model at both urban and rural monitoring sites.