Atmospheric Environment 40 (2006) 2423–2435 Modeling regional sulfate during the BRAVO study: Part 2. Emission sensitivity simulations and source apportionment Michael G. Barna à , Bret A. Schichtel, Kristi A. Gebhart, William C. Malm Air Resources Division, National Park Service, Fort Collins, CO 80523-1375, USA Received 3 June 2005; received in revised form 14 June 2005; accepted 21 December 2005 Abstract 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 (SO 2 ) 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  10 6 tons SO 2 yr À1 ), Texas (1.0  10 6 tons SO 2 yr À1 ), Mexico (2.5  10 6 tons SO 2 yr À1 ), and the western US (1.8  10 6 tons SO 2 yr À1 ). 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 225km southeast of BBNP (Carbo´ n 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%). Carbo´n 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. Published by Elsevier Ltd. Keywords: Sulfate; Modeling; Source apportionment; BRAVO; REMSAD 1. Introduction The Big Bend Regional Aerosol and Visibility and Observational Study (BRAVO) was an inves- tigation of the composition and contributors to haze at Texas’ Big Bend National Park (BBNP). Ob- servations collected from the IMPROVE monitor- ing network indicate that fine particulate sulfate (aerodynamic diametero2.5 mm) is the single largest contributor to haze within the park, and is responsible for approximately half of the light extinction attributed to fine particles (Malm et al., 2004). This is consistent with findings from the BRAVO field measurement program, in which ARTICLE IN PRESS www.elsevier.com/locate/atmosenv 1352-2310/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.atmosenv.2005.12.038 à Corresponding author. Tel.: +1 970 491 8692; fax: +1 970 491 8598. E-mail address: barna@cira.colostate.edu (M.G. Barna).