Modeling regional sulfate during the BRAVO study: Part 2. Emission sensitivity simulations and source apportionment

The Regional Modeling System for Aerosols and Deposition (REMSAD) was used to estimate a sulfate source apportionment as part of the Big Bend Regional Aerosol and Visibility Observational Study (BRAVO). Fine particulate sulfate was the main component of haze measured at Big Bend National Park (BBNP) during the BRAVO field measurement campaign (July–October 1999). The sulfate apportionment was estimated using a suite of emission sensitivity simulations in which sulfur dioxide (SO2) emission rates were modified. Four large regional emission source areas were evaluated with respect to their contributions to estimated sulfate at BBNP: the eastern US (14.0×106 tons SO2 yr−1), Texas (1.0×106 tons SO2 yr−1), Mexico (2.5×106 tons SO2 yr−1), and the western US (1.8×106 tons SO2 yr−1). In addition to these large regional sources, several sub-regions were considered (e.g., northeastern Texas), as well as a large coal-fired power plant in Mexico located approximately 225 km southeast of BBNP (Carbón I/II). During the four-month study period, it was estimated that the eastern US was the largest contributor to sulfate at BBNP (42%), followed by Mexico (23%), Texas (16%), and the western US (9%). Carbón I/II accounted for more than half of the Mexican contribution of sulfate to BBNP. Boundary conditions, which were specified by a global model, contributed 7%. The sum of the sulfate concentrations resulting from the emission sensitivity simulations were within a few percent of the sulfate concentration estimated from the original base emissions simulation, indicating that sulfate predictions responded approximately linearly to gross changes in sulfur dioxide emissions. The source apportionment results presented here are one estimate in a suite of estimates that were considered for the BRAVO study, and do not represent the final BRAVO sulfate source apportionment for BBNP.