Evolution of the optical properties of biomass-burning aerosol during the 2003 southeast Australian bushfires

During January and February 2003, drought conditions led to major bushfires across southeast Australia, causing considerable damage. We have examined aerosol optical depth (AOD) data recorded by a sunphotometer at Wagga Wagga. Although this site lies to the northeast of the fires, periodic changes in wind direction brought smoke plumes over our instrument (AOD in excess of 1.0), sometimes via circuitous routes. By examining the hourly AOD spectra and, specifically, the Aangstrom exponent derived from our two shortest wavelengths, we have observed clear evidence of a shift in the peak radius of the fine mode, most likely as a result of particle coagulation. Selected data sets were inverted to obtain the aerosol size distribution, confirming this conclusion. This was particularly clear on 25 January, when the wind changed during the day so that the afternoon observations were of smoke that had traveled on a more circuitous route to the north of Wagga Wagga before returning.