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Importance of halogen chemistry in dimethylsulfide oxidation
Citation:
Sarwar, G., D. Kang, K. Fahey, K. Foley, R. Mathur, AND B. Gantt. Importance of halogen chemistry in dimethylsulfide oxidation. 2018 Joint International Conf. on ABaCAS & CMAS-Asia-Pacific, Beijing, CHINA, May 21 - 23, 2018.
Impact/Purpose:
The National Exposure Research Laboratory (NERL) Computational Exposure Division (CED) develops and evaluates data, decision-support tools, and models to be applied to media-specific or receptor-specific problem areas. CED uses modeling-based approaches to characterize exposures, evaluate fate and transport, and support environmental diagnostics/forensics with input from multiple data sources. It also develops media- and receptor-specific models, process models, and decision support tools for use both within and outside of EPA.
Description:
Anthropogenic sources emit sulfur dioxide (SO2) into the atmosphere which is oxidized by gas- and aqueous-phase chemical reactions to form sulfate (SO_4^(2-)). Dimethylsulfide (DMS) present in sea-water can be emitted into the atmosphere which can then react with atmospheric oxidants to produce SO2 leading to SO_4^(2-)formation. In this study, we examine the importance of halogen chemistry in DMS oxidation using the hemispheric Community Multiscale Air Quality (CMAQ) model. The current CMAQ model, however, does not include DMS chemistry. We combine the Carbon Bond chemical mechanism with atmospheric bromine, chlorine, iodine, and DMS chemistry and incorporate it into the hemispheric CMAQ model. The model includes DMS oxidation by hydroxyl, nitrate, chlorine, chlorine monoxide, bromine monoxide, and iodine monoxide radicals. We calculate DMS emissions using oceanic climatological DMS concentrations and the total resistance to gas-transfer at the air/sea interface. We use anthropogenic emissions from the Emissions Database for Global Atmospheric Research (EDGAR) and biogenic emissions from the Global Emissions InitiAtive (GEIA). We performed CMAQ simulations without and with the DMS chemistry over the Northern Hemisphere for a year using meteorological fields obtained from the Weather Research and Forecasting model. The model uses a horizontal resolution of 108-km and 44 vertical layers. The model without the DMS chemistry predicts higher concentrations of SO2 and SO_4^(2-)over land compared to the low concentrations over seawater. Including the DMS chemistry substantially increases SO2 and SO_4^(2-)concentrations over seawater and many coastal areas. It increases mean surface SO2 by 91% and mean SO_4^(2-)by 25% over the seawater. Our results suggest that non-halogen radical initiated chemical pathways and the halogen radical initiated pathways are responsible for ~77% and ~23% of the total DMS oxidation, respectively. The bromine monoxide initiated pathway oxidizes ~12% and the chlorine radical initiated pathway oxidizes ~10% of the DMS in our model. We present a detailed analysis of the model results and a comparison of model predictions with available observed data.
