We measured aerosol chemical, physical and optical properties at Mount Gibbes, North Carolina during November 1997 and July 1998 in an effort to determine whether aerosols were responsible for observed climatic cooling. This work is preceding in three phases. The field measurement program was the first phase. The second phase, currently underway, is the characterization of aerosol properties and the identification of the processes controlling these properties. The third phase will extend our detailed measurements to the atmospheric column using measurements of column integrated aerosol properties and will incorporate our results in regional climate models.

Total aerosol mass and number concentrations were higher during July compared to November. Aerosol number size distributions were shifted to larger particles during July relative to November; consistent with the size distributions of sulfate, nitrate and ammonium. Aerosol light scattering increases by more than 10 times from November to July, while light absorption increased by 5 times. Consequently, the single scattering albedo was higher during July than November, indicating greater aerosol cooling during between the seasons. However , Ozone , an important oxidant, increased by nearly a factor 2 during July; suggesting more active atmospheric chemical processing of gas phase species on to particles during July than November.