| Particulates that form the atmospheric aerosol play a significant role in determining Earth's climate, both directly through scattering and absorption of solar radiation, and indirectly by impacting cloud formation processes. They are present in a large range of sizes, concentrations, chemical compositions, and compositional inhomogeneities, complicating our ability to quantify the aerosol's direct and indirect effects.;This work investigates the dependence of aerosol light scattering on relative humidity (RH) and chemical composition, and the effect of organic material on scattering enhancement and implied particle growth characteristics. It utilizes measurements carried out in the Elk Mountain/Laramie Aerosol Characterization Experiment (EMLACE), in which observations were made at both a clean high-altitude mid-continental site (Elk Mountain, WY) in summer (2006), and in a small urban environment (Laramie, WY) during both summer (2005) and winter (2006). High time resolution measurements of scattering extinction from both dry and humidified PM1 aerosols were obtained together with time resolved measurements of PM1 volatile aerosol chemical composition and mass loading.;Variabilities in particle chemical composition and optical behavior with location and season are documented. Scattering extinction is positively correlated with aerosol mass loading. Enhanced scattering, due to water uptake by the particles at high RH, is described by a scattering enhancement factor (f(RH)), which is found to be smaller with increasing mass fraction of organic material. Observed f(RH) values are interpreted in terms of mass weighted contributions (f(RHhigh)Inorg and f(RHhigh)Org) from the particulate inorganic and organic material at high RH. Analyses show that whereas f(RHhigh)Inorg varies continually, depending on the physical and chemical properties of the aerosol, the experimental data can, in general, be adequately interpreted using a value of 1.0 for f(RH high)Org (though values up to 1.1 are occasionally indicated), suggesting that organic material does not significantly promote increased scattering at high RH.;Numerical values of f(RHhigh)Inorg and f(RH high)Org were used to develop an empirical parameterization of changes in aerosol optical properties with increasing RH, based solely on variations in particulate chemical composition. This parameterization successfully reconstructs variations in f(RH) for the observed ambient aerosols over the entire range of RH values employed in the experiments. |
