Minute by minute aerosol optical depth data (AOD) have been collected at Wagga Wagga by the Bureau of Meteorology since 2001. (AOD is a measure of the attenuation of a beam of solar radiation by aerosols – small particles – as it traverses the Earth’s atmosphere.) The radiometer makes measurements at four wavelengths: 412, 500, 610 and 778 nm.

During January and February 2003 a number of large bushfires in southeast Australia emitted significant amounts of smoke. Hourly and daily average values of AOD during this period were variable, both day-to-day, and during the day, as shown in Figure 1, with hourly averages exceeding 1.0 on some days. In addition, visibility during the bushfire period was reduced to less than 1 km on some days, while the typical value of visibility is in the range 25-30 km for smoke-free periods.

The Ångstrom exponent, a gives a qualitative idea of aerosol size, with smaller values corresponding to large particle sizes (in general), and vice versa. Figure 2 shows that the hourly and daily mean values of a for the bushfire period are also variable, both day-to-day, and during the day. This reflects the varying amount of fine mode particles arriving in the area, as a result of changes in wind speed and direction.

a values computed from shorter wavelengths are more sensitive to fine mode particles than a values computed from longer wavelengths. One explanation for this relates to smoke aging. a often decreases as the AOD increases in smoky situations, due to the growth in fine mode particles, resulting from smoke aging processes – coagulation and condensation. Optical depth increases as the particles become more efficient light scatterers, even though aerosol mass may not have increased.

Fig. 3: Scatter plot of hourly average of AOD vs. both a_412-500 and a_610-778