Characterisation of episodic aerosol types over the Australian continent

Abstract

Abstract. Classification of Australian continental aerosol types resulting from episodes of enhanced source activity, such as smoke plumes and dust outbreaks, is carried out via cluster analysis of optical properties obtained from inversion of sky radiance distributions at Australian aerosol ground stations using data obtained over the last decade. The cluster analysis distinguishes four significant classes, which are identified on the basis of their optical properties and provenance as determined by satellite imagery and backtrajectory analysis. The four classes are identified respectively as aged smoke, fresh smoke, coarse dust and a superabsorptive aerosol. While the first three classes show similarities with comparable aerosol types identified elsewhere, the super-absorptive aerosol has no obvious foreign prototype. The class identified as coarse dust shows a prominent depression in single scattering albedo in the blue spectral region due to absorption by hematite, which is shown to be more abundant in central Australian dust relative to the "dust belt"of the Northern Hemisphere. The super-absorptive class is distinctive in view of its very low single scattering albedo (∼0.7 at 500 nm) and variable enhanced absorption at 440 nm. The strong absorption by this aerosol requires a high black carbon content while the enhanced blue-band absorption may derive from organic compounds emitted during the burning of specific vegetation types. This aerosol exerts a positive radiative forcing at the top of atmosphere (TOA), with a large deposition of energy in the atmosphere per unit aerosol optical depth. This contrasts to the other three classes where the TOA forcing is negative. Optical properties of the four types will be used to improve the representation of Australian conCorrespondence to: R. M. Mitchell (ross.mitchell@csiro.au) tinental aerosol in climate models, and to enhance the accuracy of satellite-based aerosol retrievals over Australia.

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

“…There is good consistency -although based on just a few sample sets -between MOUDI-derived size distributions and AERONET distributions. These distributions are also consistent with the generic Australian size distributions obtained by Qin and Mitchell (2009).…”

“…In particular, the comparison shown in Fig. 7c suggests that the AERONET inversion leads to an artificially steep decline on the large particle side of the coarse mode peak, an issue discussed by Qin and Mitchell (2009). Given these caveats, and the errors associated with both approaches, we consider the agreement to be as good as could be expected.…”

“…In Fig. 6c we show Qin and Mitchell's (2009) size distributions for their aged smoke ("class0"), and coarse dust ("class2") classes (with appropriate normalization). The similarity with the fine and coarse modes, respectively, of the distributions shown in Fig.…”

“…On 8 October 2007 the fine mode peak was dominant, and the air mass back trajectory during this day shows advection from the north, where hot spot satellite imagery confirmed that biomass burning was occurring (as expected for this time of year). Qin and Mitchell (2009) have performed a cluster analysis of AERONET retrievals from a number of Australian sites (3 in northern Australia subject to biomass burning, 2 desert sites, plus others), including Birdsville. This yielded 4 classes which they identify as aged smoke, fresh smoke, coarse dust, and a super-absorptive class of undetermined nature.…”

Inferring absorbing organic carbon content from AERONET data

“…There are also elevated concentrations retrieved in the Top End of the Northern Territory of Australia in September that is within the burning season (e.g Luhar et al, 2008). Qin and Mitchell (2009) classified a large part of the AERONET data as smoke in this region during the burning season, but also with class they called "super-absorptive". This class was distinct with very low single scattering albedo (0.7 at 500 nm) and enhanced absorption at 440 nm.…”