Atmospheric Environment 36 (2002) 5043–5055 Aerosol size distributions and visibility estimates during the Big Bend regional aerosol and visibility observational (BRAVO) study J.L.Hand a,1 ,S.M.Kreidenweis a, *,D.EliSherman a ,J.L.CollettJr. a ,S.V.Hering b , D.E. Day c , W.C. Malm c a Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523, USA b Aerosol Dynamics, Inc., Berkeley, CA 94710, USA c Cooperative Institute for Research in the Atmosphere (CIRA), Colorado State University, Fort Collins, CO 80523, USA Received 15 March 2002; accepted 3 July 2002 Abstract The Big Bend Regional Aerosol and Visibility Observational (BRAVO) study was conducted in Big Bend National Park in 1999. The park is located in a remote region of southwest Texas but has some of the poorest visibility of any Class 1 monitored area in the western US. The park is frequently influenced by air masses carrying emissions from Mexico and eastern Texas. Continuous physical, optical and chemical aerosol measurements were performed in an effort to understand the sources of and contributions to haze in the park. As part of this characterization, dry aerosol size distributions were measured over the size range of 0:05oD p o20 mm. Three instruments with different measurement techniques were used to cover this range. Complete size distributions were obtained from all of the instruments in terms of a common measure of geometric size using a new technique. Size parameters for accumulation and coarse particle modes were computed and demonstrate periods when coarse mode volume concentrations were significant, especially during suspected Saharan dust episodes in July and August. Study average (and one standard deviation) geometric volume mean diameters for the accumulation and coarse particle modes were 0.2670.04 and 3.470.8 mm, respectively. Dry light scattering coefficients (b sp ) were computed using measured size distributions and demonstrated periods when contributions to b sp from coarse particles were significant. The study average computed b sp was 0.02670.016km 1 . Computed dry b sp values were highly correlated with measured values (r 2 ¼ 0:97). Real-time sulfate measurements were correlated with accumulation mode volume concentrations (r 2 ¼ 0:89) and computed dry light scattering coefficients (r 2 ¼ 0:86), suggesting sulfate aerosols were the dominant contributor to visibility degradation in the park. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Particle size distributions; Aerosol monitoring; Aerosol optical properties; Remote continental aerosol particles; Visibility 1. Introduction The Big Bend Regional Aerosol and Visibility Observational (BRAVO) study was conducted from July to October 1999. The park is located along the Rio Grande River on the border of Mexico and Texas in the Chihuahan Desert. The study was motivated by historical data demonstrating that Big Bend National *Corresponding author. E-mail address: soniak@aerosol.atmos.colostate.edu (S.M. Kreidenweis). 1 Now at National Center for Atmospheric Research, Boulder, CO 80305, USA. 1352-2310/02/$-see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S1352-2310(02)00568-X