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 r2=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. Ångstrom 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 Ångstrom exponents corresponded to changing aerosol properties as seen in the ground-based composition and size distribution data sets. The average Ångstrom 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 Ångstrom exponents was 0.20 ( 12%) and on average the estimates were highly correlated (r2=0.89). Aerosol optical depths and ambient surface aerosol extinction coefficients were positively correlated with PM2.5 sulfate mass concentrations (r2=0.68 and r2=0.79, respectively), suggesting that sulfate aerosols were a major contributor to visibility degradation and column aerosol loading during the study.