Comparisons of aerosol optical properties derived from Sun photometry to estimates inferred from surface measurements in Big Bend National Park, Texas

As a part of the Big Bend Regional Aerosol and Visibility Observational Study (BRAVO, July-October 1999), aerosol physical, optical and chemical properties were measured continuously in an effort to characterize visibility in Big Bend National Park. Surface-based estimates of aerosol extinction coefficients derived from size distribution data were compared to aerosol optical properties retrieved independently from ground-based remote sensing measurements from the United States Department of Agriculture (USDA) UVB radiometer network site. Comparisons suggested that for the majority of the study, surface visibility was a good indicator of the column aerosol optical depth, with a correlation coefficient of r(2) = 0.77. The average (and one standard deviation) aerosol optical depth from the USDA network was 0.13 +/- 0.06 at a wavelength of 500 nm. Angstrom wavelength exponents were computed for both data sets to characterize the spectral variations of aerosol optical properties over the wavelength range of 415-860 nm. Variations in Angstrom exponents corresponded to changing aerosol properties as seen in the ground-based composition and size distribution data sets. The average Angstrom exponent from in situ measurements was 1.5 +/- 0.4 compared to column estimates of 1.3 +/- 0.4. The root mean square of the difference between the estimates of Angstrom exponents was 0.20 (similar to12%) and on average the estimates were highly correlated (r(2) = 0.89). Aerosol optical depths and ambient surface aerosol extinction coefficients were positively correlated with PM2.5 sulfate mass concentrations (r(2) = 0.68 and r(2) = 0.79, respectively), suggesting that sulfate aerosols were a major contributor to visibility degradation and column aerosol loading during the study. (C) 2004 Elsevier Ltd. All rights reserved.