Abstract
Chemical transport models are used for federal compliance demonstrations when areas are out of attainment, but there is no guidance for choosing a chemical mechanism. With the 2024 change of the annual PM(2.5) standard and the prevalence of multiday wintertime inversion episodes in the western U.S., understanding the wintertime performance of chemical transport models is important. This study explores the impact of chemical mechanism choice on the Community Multiscale Air Quality (CMAQ) model performance for PM(2.5) and implications for attainment demonstration in inversion-prone areas in the western United States. Total and speciated PM(2.5) observations were used to evaluate wintertime CMAQ simulations using four chemical mechanisms. The study evaluated intermechanism differences in total and secondary PM(2.5) and the impact of meteorology at sites with observed multiday temperature inversions. Model performance for total PM(2.5) was similar across chemical mechanisms, but intermechanism differences for total and secondary PM(2.5) were exacerbated during inversion periods, suggesting that modeled chemistry contributes to the model bias. Results suggest that nitrate, ammonium, and organic carbon are secondary species for which model results do not agree or perform to standard evaluation metrics in scientific literature. These findings show a need for mechanistic investigations of the causes of these differences.