Importance of Chemical Mechanism Complexity for Simulating Ozone under the Changing Chemical Regimes in 2020 and Beyond

Abstract:

In 2020, mobile emissions of nitrogen oxides (NO_x) and volatile organic compounds (VOCs) have declined due to decreased travel caused by shutdowns and other policy decisions implemented to reduce the spread of COVID-19. These changes in emissions cause a shift in the chemical regime that we expect will occur in the future as electrification of motor vehicles increases. Thus, 2020 is an opportunity to evaluate how well 3D models and reduced chemical mechanisms within these 3D models represent these changing chemical regimes. Here, we use WRF-chem (Weather Research and Forecasting model coupled with chemistry) to evaluate how well different chemical mechanisms of varying complexity (e.g., RACM-ESRL, MOZCART-T1, MOZCART-T2, and updated MOZCART-T2 with improved chemistry for VOCs from volatile chemical products) simulate ozone under two different emissions scenarios: business as usual and reductions in mobile emissions due to COVID-19. First, we compare the WRF-chem results against a comprehensive suite of measurements of ozone and ozone precursors collected by the NOAA Chemical Sciences Laboratory (CSL) during the spring and summer of 2020 in Boulder, CO and evaluate the performance of each chemical mechanism. Second, we use WRF-chem to evaluate how different chemical mechanisms of varying complexity respond to changes in mobile emissions throughout the US. This analysis improves our understanding of how much chemical complexity is needed in 3D models to accurately predict air quality in the future where NO_x levels and VOC mixtures will likely differ from the present.