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Abstract: We examine the potential impacts of two additional sulfate production pathways using the Community Multiscale Air Quality modeling system. First we evaluate the impact of the aqueous-phase oxidation of S(IV) by nitrogen dioxide using two published rate constants, differing by 1–2 orders of magnitude. The reaction with alternate high and low rate constants enhances monthly mean wintertime sulfate by 4–20% and 0.4–1.2% respectively. The reaction does not significantly impact summertime sulfate. The higher sulfate predictions in winter compare better with the observed data as the model tends to underpredict sulfate concentrations both in winter and summer. We also investigate the potential impact of the gas-phase oxidation of sulfur dioxide by the Stabilized Criegee Intermediate (SCI) using a recently measured rate constant for its reaction with sulfur dioxide. Model results indicate that the gas-phase oxidation of sulfur dioxide by the SCI does not significantly affect sulfate concentrations due to the competing reaction of the SCI with water vapor. The current estimate of the rate constant for the SCI reaction with water vapor is too high for the SCI reaction with sulfur dioxide to significantly affect sulfate production. However, a sensitivity analysis using a lower rate constant for the water vapor reaction suggests that the SCI reaction with sulfur dioxide could potentially enhance sulfate production in the model. Further study is needed to accurately measure the rate constants of the aqueous-phase oxidation of S(IV) by nitrogen dioxide and the gas-phase reaction of the SCI with water vapor. [Copyright &y& Elsevier]

Abstract: Impact of the excited nitrogen dioxide (NO2 ∗) chemistry on air quality in the U.S. is examined using the Community Multiscale Air Quality (CMAQ) model for a summer month. Model simulations were conducted with and without the NO2 ∗ chemistry. The largest impact of the NO2 ∗ chemistry in the eastern U.S. occurred in the northeast and in the western U.S. occurred in Los Angeles. While the single largest daily maximum 8-h ozone (O3) increased by 9 ppbv in eastern U.S. and 6 ppbv in western U.S., increases on most days were much lower. No appreciable change in model performance statistics for surface-level O3 predictions relative to measurements is noted between simulations with and without the NO2 ∗ chemistry. Based on model calculations using current estimates of tropospheric emission burden, the NO2 ∗ chemistry can increase the monthly mean daytime hydroxyl radicals (OH) and nitrous acid (HONO) by a maximum of 28% and 100 pptv, respectively. [Copyright &y& Elsevier]