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Potential Impacts of two SO2 oxidation pathways on regional sulfate concentrations: acqueous-hase oxidation by NO2 and gas-phase oxidation by Stabilized Criegee Intermediates
Sarwar, G., K. Fahey, R. Kwok, R. Gilliam, S. Roselle, R. Mathur, X. Jian, J. Yu, AND W. Carter. Potential Impacts of two SO2 oxidation pathways on regional sulfate concentrations: acqueous-hase oxidation by NO2 and gas-phase oxidation by Stabilized Criegee Intermediates. ATMOSPHERIC ENVIRONMENT. Elsevier Science Ltd, New York, NY, 68(4):186-197, (2013).
The National Exposure Research Laboratory′s (NERL′s) Atmospheric Modeling and Analysis Division (AMAD) conducts research in support of EPA′s mission to protect human health and the environment. AMAD′s research program is engaged in developing and evaluating predictive atmospheric models on all spatial and temporal scales for forecasting the Nation′s air quality and for assessing changes in air quality and air pollutant exposures, as affected by changes in ecosystem management and regulatory decisions. AMAD is responsible for providing a sound scientific and technical basis for regulatory policies based on air quality models to improve ambient air quality. The models developed by AMAD are being used by EPA, NOAA, and the air pollution community in understanding and forecasting not only the magnitude of the air pollution problem, but also in developing emission control policies and regulations for air quality improvements.
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.
URLs/Downloads:Potential Impacts of two SO2 oxidation pathways on regional sulfate concentrations: acqueous-hase oxidation by NO2 and gas-phase oxidation by Stabilized Criegee Intermediates (PDF,NA pp, 5620 KB, about PDF)
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Record Details:Record Type: DOCUMENT (JOURNAL/PEER REVIEWED JOURNAL)
Organization:U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
NATIONAL EXPOSURE RESEARCH LABORATORY
ATMOSPHERIC MODELING AND ANALYSIS DIVISION