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Mineral Dust Aerosol

Aerosols are small particulates (solid or liquid) suspended in the atmosphere. Their sizes typically range from a few nanometres to 10 micrometres or more, and their sources, reactions and properties are extremely varied and complex. Most aerosols act to cool the Earth by scattering some solar radiation back to space, and also increasing cloud brightness and (possibly) lifetimes. However, soot (from incomplete combustion) actually warms the planet by absorbing solar radiation. Mineral dust aerosol (from deserts etc) is a conundrum, as it is both an efficient absorber and scatterer of radiation. Thus we are not even sure if it warms or cools the Earth.

A large part of Australia is desert or semi-arid area, and is the greatest contributor of mineral aerosol in the Southern Hemisphere with 5% contribution to the total annual global dust emission. Australian desert aerosol is highly absorbing in the blue wavelength region (Qin and Mitchell, 2009), so that we might expect Australian mineral dust aerosol to be relatively more warming. This is reflected in the fact that Australian’s deserts are reddish, in contrast to the Sahara, for example, which is more yellow. This difference is a reflection of mineralogy, particularly in relation to iron oxides such as hematite. Australian dust aerosol is the main terrestrial source to the open and remote ocean.

Lake Eyre Basin

The Lake Eyre Basin (LEB) is the largest closed drainage system in the least-developed arid and semi-arid zone in the heart of Australia. Lake Eyre is the lowest point in the basin, and all the rivers in the basin flow towards it (although much of the time no water actually reaches the Lake). This region covers around 1.2 million km2 with different sedimentary environments. The deserts that have formed in the LEB constitute the largest airborne dust source region on the Australian continent and southern hemisphere. Studies have found that there is a significant contribution to airborne dust in this region from floodplains, ephemeral lakes and dry lake beds. Mitchell et al. (2010) found that dust activity over the Lake Eyre Basin has increased from 2003-2007, by contrast with the period 1997-2002, most likely as a result of the recent, prolonged drought.

In addition to mineral dust, there are a number of other significant aerosol sources, either within, or close to, the Lake Eyre Basin. Dry salt lakes, of which Lake Eyre is the largest, are a source of NaCl (and other salts), potentially exacerbating salinity problems in inland Australia. At the northern end of the LEB, biomass burning aerosol from the winter/spring fires in northern Australia can be a significant part of the aerosol mix. Marine advection can bring sea salt, as well as biogenic compounds to the region. Mt Isa, on the northern edge of the LEB, is one of the world’s largest point sources of SO2. Finally, although vegetation is sparse, biogenic compounds may also contribute to the total aerosol mix within the LEB.

Figure 2: Lake Eyre Basin map.