Modeling Sulfur Concentrations and Depositions in the United States during ANATEX
Glenn D. Rolph, Roland R. Draxler, and Rosa G. de Pena
Atmospheric Environment, Vol. 26A, No. 1, pp. 73-93, 1992
Abstract - The Hybrid Single-Particle Lagrangian Integrated Trajectory (HY-SPLIT) long-range transport, personal-computer-based model was incorporated with a non-linear chemistry module that includes gas- and aqueous-phase oxidation of sulfur dioxide (SO2) and dry and wet removal of SO2 and sulfate (SO4) particles. The model considers multiple area and point sources (~500) and calculates simultaneously 24-h averages of several species: air concentrations of SO2 and SO4 particles and SO4 wet deposition on each grid cell of the meteorological domain. Three-month averages of modeled air concentrations of SO2 and SO4 were compared with measurements from five National Oceanic and Atmospheric Administration Air Resources Laboratory dry deposition sites. The model tended to under-predict air concentrations of SO2 (ratios of model-predicted to measure concentrations between 0.6 and 1.2) and over-predict air concentrations of SO4 (ratios between 1.0 and 1.6). Modeled SO4 wet deposition values were calculated, averaged for the 3 months, and compared with the measured values at 137 combined UAPSP, MAP3S, and NADP precipitation chemistry sites. For the majority of these sites, the agreement was good: the rations between modeled and measure depositions were between 0.5 and 1.6. However, a consistent under-prediction was observed across the southern and extreme eastern U.S. This under-prediction was primarily attributed to the proximity of the edge of the model domain to the eastern and southern U.S., which caused trajectories from the continent that would have curved back into the southeastern U.S. to terminate prematurely. When SO2 emissions were reduced by 50% over the entire model domain, approximately 50-55% reductions were observed in SO2 air concentrations over the entire model domain and at all levels. However, sulfate air concentrations and depositions were only reduced 35-50% due to non-linear reactions involving SO2. When emissions of SO2 were reduced by 50% only in the Ohio Valley, reductions in SO2 and SO4 air concentrations to the north and south of the Ohio Valley were generally less than 15%. These small reductions are attributed to local emissions of SO2 and long-range transport of SO4 from other regions not affected by the reduction in emissions. SO4 wet deposition, however, was reduced by 15 to 23 % to the north of the Ohio Valley, indicating some long-range transport from the Ohio Valley. In the Ohio Valley, reductions of SO2 air concentrations were between 10 and 42%, depending on the proximity to major sources, and reductions of SO4 air concentrations were between 10 and 30%. However, SO4 wet deposition was reduced less than 10%, consistent with the assumption that its primary contribution is from long-range transport.
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