Effectiveness of Emission Controls on Atmospheric Oxidation and Air Pollutant Concentrations: Uncertainties due to Chemical Mechanisms and Inventories

Abstract. In this study, three photochemical mechanisms of varying complexity from the Statewide Air Pollution Research Center (SAPRC) family and two widely used anthropogenic emission inventories are employed to quantify the discrepancies in the predicted effectiveness of nitrogen oxides (NO_x) and volatile organic compound (VOC) emission controls on ozone (O₃), secondary inorganic aerosols (SIA), and hydroxyl (OH) and nitrate (NO₃) radicals using the Community Multiscale Air Quality (CMAQ) model. For maximum daily average 8-hour O₃ (O₃-8 h), relative reductions predicted using different emission inventory and mechanism combinations are consistent for up to 80 % NO_x or VOC reductions, with maximum differences of approximately 15 %. For secondary inorganic aerosols (SIA), while the predicted relative changes in their daily average concentrations due to NO_x reductions are quite similar, very large differences of up to 30 % occur for VOC reductions. Sometimes even the direction of change (i.e., increase or decrease) is different. For the oxidants OH and NO₃ radicals, the uncertainties in the relative changes due to emission changes are even larger among different inventory-mechanism combinations, sometimes by as much as 200 %. Our results suggest that while the O₃-8 h responses to emission changes are not sensitive to the choice of chemical mechanism and emission inventories, using a single model and mechanism to evaluate the effectiveness of emission controls on SIA and atmospheric oxidation capacity may have large errors. For these species, the evaluation of the control strategies may require an ensemble approach with multiple inventories and mechanisms.