Aerosol size distributions and visibility estimates during the Big Bend regional aerosol and visibility observational (BRAVO) study

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 0.05<D_p<20 μm. 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 0.26±0.04 and 3.4±0.8 μm, 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 0.026±0.016 km⁻¹. Computed dry b_sp values were highly correlated with measured values (r²=0.97). Real-time sulfate measurements were correlated with accumulation mode volume concentrations (r²=0.89) and computed dry light scattering coefficients (r²=0.86), suggesting sulfate aerosols were the dominant contributor to visibility degradation in the park.