A new technique for obtaining aerosol size distributions with applications to estimates of aerosol properties

Abstract

The Big Bend Regional Aerosol Visibility and Observation study (BRAVO) was conducted from July to October 1999. The park is located in a remote region of southwest Texas on the border with Mexico but still has some of the poorest visibility of any Class 1 monitored area in the western United States. The park is frequently influenced by air masses carrying pollutants from Mexico and eastern Texas. Continuous physical, optical and chemical aerosol measurements were performed in an effort to understand the sources and contributions to haze in the park. Dry aerosol size distributions were measured over the size range of 0.05 < Dp < 20 μm. Three instruments were used to cover this range, each having a different measurement technique. A new method was developed to align the size distributions in the instrument overlap regions, allowing for the retrieval of aerosol real refractive index and effective density as well as size parameters for accumulation and coarse particle modes. The combined size distributions were used to calculate particle light scattering coefficients (bsp) for the total distribution and to investigate periods with significant fractional contributions by accumulation and coarse mode particles to total bsp. Calculated and measured bsp from a nephelometer were highly correlated and agreed within the uncertainties of the measurements and uncertainties due to particle losses in the instrument inlets. The agreement between measured and calculated light scattering coefficients supported the accuracy of the size distributions and refractive indices retrieved with the alignment method. Aerosol chemical composition measured daily during BRAVO suggested that the fine aerosol ionic composition was generally acidic with sulfate and ammonium as the dominant species. During certain periods fine organic carbon and soil fractional contributions to fine aerosol mass concentrations were significant. Retrieved refractive index and density were compared with computed values using these fine (Dp < 2.5 μm) and coarse (2.5 < Dp < 10 μm) aerosol compositions. These estimates agreed well, with average retrieved and computed refractive indices of m = 1.566 ± 0.012 and m = 1.56 ± 0.02, respectively. The average retrieved effective density was ρe = 1.56 ± 0.12 g cm-3 and included the effects of dynamic shape factor. The average computed fine bulk density was ρe = 1.85 ± 0.14 g cm-3. Finally, surface-based measurements of aerosol optical properties were related to properties derived from ground-based remote sensing from the USDA UVB radiometer network site located in the park, approximately 30 miles from the BRAVO site. A correlation in aerosol optical depth of R2 = 0.691 was observed between the two methods, and they agreed within 40 %. Ångstrom wavelength exponents were computed to characterize the spectral variations of aerosol optical depth over the wavelength range of 415 - 860 nm. Variations in Ångstrom exponents were observed in both estimates and corresponded to changing aerosol properties as seen in the composition and size distribution data sets. A correlation of R = 0.755 was observed between the in-situ and remote sensing methods of estimating Ångstrom exponents, and the estimates agreed within 30 %. These results were encouraging because they provided insight into the relationships between surface-based measurements of aerosol physical and chemical properties and optical properties retrieved by remote sensing techniques.