Light scattering characteristics of aerosols at ambient and as a function of relative humidity: Part II - A comparison of measured scattering and aerosol concentrations using statistical models

The eastern United States national parks experience some of the worst visib ility conditions in the nation. To study these conditions, the Southeastern Aerosol and Visibility Study (SEAVS) was undertaken to characterize the si ze-dependent composition, thermodynamic properties, and optical characteris tics of the ambient atmospheric particles. It is a cooperative three-year s tudy that is sponsored by the National Park Service and the Electric Power Research Institute and its member utilities. The field portion of the study was carried out from July 15 to August 25, 1995. The study design, instrumental configuration, and estimation of aerosol typ es from particle measurements is presented in a companion paper. In the com panion paper, we compare measurements of scattering at ambient conditions a nd as functions of relative humidity to theoretical predictions of scatteri ng. In this paper, we make similar comparisons, but using statistical techn iques. Statistically derived specific scattering associated with sulfates s uggest that a reasonable estimate of sulfate scattering can be arrived at b y assuming nominal dry specific scattering and treating the aerosols as an external mixture with ammoniation of sulfate accounted for and by the use o f Tang's growth curves to predict water absorption. However, the regression s suggest that the sulfate scattering may be underestimated by about 10%. R egression coefficients on organics, to within the statistical uncertainty o f the model, suggest that a reasonable estimate of organic scattering is ab out 4.0 m(2)/g. A new analysis technique is presented, which does not rely on comparing mea sured to model estimates of scattering to evoke an understanding of ambient aerosol growth properties, but rather relies on measurements of scattering as a function of relative humidity to develop actual estimates of f(RH) cu rves. The estimates of the study average f(RH) curve for sulfates compares favorably with the theoretical f(RH) curve for ammonium bisulfate, which is in turn consistent with the study average sulfate ammoniation correspondin g to a molar ratio of NH4/SO4 of approximately one. The f(RH) curve for org anics is not significantly different from one, suggesting that organics are weakly to nonhygroscopic.