Optical, physical and chemical characteristics of Australian Desert dust aerosols: results from a field experiment

Abstract

Abstract. Mineral dust is one of the major components of the world's aerosol mix, having a number of impacts within the Earth system. However, the climate forcing impact of mineral dust is currently poorly constrained, with even its sign uncertain. As Australian deserts are more reddish than those in the northern hemisphere, it is important to better understand the physical, chemical and optical properties of this important aerosol. We have investigated the properties of Australian desert dust at a site in SW Queensland, which is strongly influenced by both dust and biomass burning aerosol. Three years of ground-based monitoring of spectral optical thickness has provided a statistical picture of gross aerosol properties. In November 2006 we undertook a field campaign which collected 4 sets of size-resolved aerosol samples for laboratory analysis – both ion beam analysis and ion chromatography. The aerosol optical depth data showed a weak seasonal cycle with an annual mean of 0.06±0.03. The Angstrom coefficient showed a stronger cycle, indicating the influence of the winter-spring burning season in Australia's north. Size distribution inversions showed a bimodal character, with the coarse mode assumed to be mineral dust, and the fine mode a mixture of biomass burning and marine biogenic material. Ion Beam Analysis was used to determine the elemental composition of all filter samples, although elemental ratios were considered the most reliable output. Scatter plots showed that Fe, Al and Ti were well correlated with Si, and Co reasonably well correlated, with the Fe/Si ratio higher than the crustal average, as expected. Scatter plots for Ca, Mn and K against Si showed clear evidence of a second population, which in some cases could be identified with a particular sample day or size fraction. Ion Chromatography was used to quantify water soluble ions for 2 of our sample sets, showing the importance of marine influences on both fine (biogenic) and coarse (sea salt) modes.

Preparing Landsat Image Time Series (LITS) for Monitoring Changes in Vegetation Phenology in Queensland, Australia

“…The value of 0.05 was chosen as available aerosol robotic network (AERONET) sun-photometer data over the Australian continent showed that AOD remains under 0.1 in most of the cases http://aeronet.gsfc.nasa.gov/cgi-bin/bamgomas_interactive. For example, about 80% of the AERONET data from Birdsville, Southwest Queensland were less than 0.1 [43].…”

“…The atmospheric correction routine applied in this study assumed that the AOD at 550 nm in the study area remained 0.05. Published reports [43,51] and the data available from AERONET (http://aeronet.gsfc.nasa.gov/) webpage for different sites in Australia showed that it is reasonable to assume an AOD of 0.05 in most of the areas in Australia except the Northern tropics. The almost similar prediction accuracy of September and June images ( Table 4) also indicated that the assumption of a fixed AOD of 0.05 was appropriate in this context.…”