Atmospheric aerosols (small particles) are among the most heterogeneous of the Earth’s atmospheric components. Primary aerosols are injected directly into the atmosphere (e.g. desert dust, sea spray, smoke particles), while secondary aerosols are produced by gas-to-particle conversion of precursor gases such as SO₂ and biogenic compounds. Each may be the result of either natural or anthropogenic processes. Once in the atmosphere they may undergo further processing, especially within cloud droplets, leading to further changes in characteristics. Hence it is not just the concentrations, or column ‘loadings’, of aerosols, but also the properties of the aerosol populations themselves that vary in space and time.

In recent years it has become evident that knowledge of the size-resolved chemical composition of atmospheric aerosols is important in determining optical properties such as refractive index, scattering and absorption coefficients, extinction and hygroscopic growth. 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.
Samples were collected at four sites (University of New South Wales, Campbelltown, Berrima and Moss Vale) during the summer, autumn, winter and spring of 2003. These samples were collected using a PM2.5 (‘particulate matter’ less than 2.5 microns) and a PM10 sampler. This allows us to determine the properties of both the fine mode (less than 2.5 microns) and coarse mode (2.5 to 10 microns). The samples have been analysed using Ion Beam Analysis (IBA) to provide elemental composition, and selected samples have also been analysed by Scanning Electron Microscopy (SEM).

The IBA results show seasonal differences within sites and between sites, as well as differences, in at least some cases, between PM2.5 and PM10-PM2.5 composition at particular sites. The figure on the left shows results for 7 key elements (elemental carbon, sodium, chlorine, sulfur, aluminium, silicon and iron) at all sites and both modes, for the summer season, while the figure on the right shows the results at UNSW for all seasons. The most significant difference shown in this data is the fact that elemental carbon, the major contributor to aerosol absorption of sunlight, is overwhelmingly concentrated in the fine mode. We now propose to follow up this important finding with a new sampler which will allow considerably improved size resolution.

Taleb Hallal and Gail Box