Physical, chemical and optical properties of Australian desert dust aerosols

Abstract

Atmospheric aerosols interact with incoming solar radiation, and to a lesser extent with outgoing longwave radiation, in a number of ways. Aerosols scatter incoming sunlight, with a fraction being scattered back to space. This process is known as the direct aerosol effect, and is partially offsetting the warming from increases in greenhouse gases. Some aerosols, depending on their refractive index, and hence on their chemistry, absorb sunlight. This can result in not only a planetary warming (rather than a cooling), but also changes in the atmospheric temperature profile, and hence vertical stability, with potential hydrological consequences. Desert dust (mineral aerosol) is one of the major natural aerosol types, as recognized by the IPCC. However, at the time of the Third Assessment it was not possible to even be sure of the sign of desert dust forcing - i.e. warming or cooling. This is because mineral aerosols are mildly absorbing, a reflection of their variable iron content. Unlike many Northern Hemisphere deserts, which are yellow, Australian deserts are predominantly red, indicative of a different mineralogy. We have established a field station at Birdsville, in SW Queensland. Among other instrumentation, we operated aerosol samplers during a field campaign in November 2006. Using a twelve-stage MOUDI sampler, we collected size-resolved aerosols and subjected them to ion-beam analysis to determine the elemental composition. Samples were also collected in a single stage LowVol sampler. These samples were also sent for IBA as a cross check, but also for electron microscopy to study both shape and size, and the mineralogy of individual particles. Preliminary results of these analyses are presented. After further analysis we plan to construct size-resolved refractive indices, to determine the key optical parameters of Australian desert dust, and hence its role in climate forcing.