Seasonal Variations in Aerosol Chemical and Physical Properties in Sydney, Australia.

In recent years it has become evident that size-resolved chemical composition of atmospheric aerosols is important in determining optical properties such as refractive index, scattering and absorption coefficients, extinction and hygroscopicity. These properties affect the way radiation is scattered and absorbed as it passes through the atmosphere, and thus are important for the calculation of aerosol radiative forcing and "atmospheric correction" of satellite images, as well as local air quality and visibility. Two years of measurements of aerosol optical thickness made at UNSW, Sydney, starting in December 1995, have been examined to identify trends and seasonal cycles. Regression analysis showed a clear annual cycle, with amplitude approximately half the mean, with the highest aerosol loading occurring during the summer months. We have also found a very clear annual cycle for the Angstrom coefficient, with larger particles in the autumn. These cycles are consistent with seasonal variations in local aerosol chemical composition identified by other workers. PM2.5 and PM10 samples were collected at four sites during the four seasons of 2003. Two of the sites are urban, and two are in a rural area just south of the Sydney Basin. Simultaneous nephelometer measurements were made. Elemental composition was determined using Ion Beam Analysis and selected samples have also been analysed by Scanning Electron Microscopy. Seasonal and spatial variations are seen in the chemical composition of 24-hour aerosol samples. Differences were also noted between PM2.5 and PM10 composition at particular sites. Principal components analysis has been applied to identify source factors in the elemental chemistry. The chemical composition data has been used to determine the refractive index of aerosols at the different sites, and to identify variations with size fraction. This provides an optical model of aerosols in the Sydney region.