An understanding of the effect of aerosols on biologically- and photochemically-active UV radiation reaching the Earth's surface is important for many ongoing climate, biophysical, and air pollution studies. In particular, estimates of the UV characteristics of the most common Australian aerosols will be valuable inputs to UV Index
forecasts, air quality studies, and assessments of the impact of regional environmental changes. Based on MODIS fire maps and MISR aerosol property retrievals, we have analyzed the climatological distributions of Australian dust and smoke particles and have identified sites where collocated ground-based UV-B and ozone measurements were available during episodes of relatively high aerosol activity. Since at least June 2003, overhead ozone and surface UV spectra (285-450 nm) have been measured routinely at Darwin and Alice Springs in Australia by the Australian Bureau of Meteorology (BoM). Using collocated AERONET sunphotometer measurements at Darwin and collocated BoM sunphotometer measurements at Darwin and Alice Springs, we identified several episodes of relatively high aerosol activity that could be used to study the effects of dust and smoke on the UV-B solar irradiance at the Earth's surface. To assess smoke effect we compared the measured UV irradiances at Darwin with irradiancies simulated with the LibRadtran radiative transfer model for aerosol-free conditions. We found that for otherwise similar atmospheric conditions, aerosols reduced the UVB irradiance by 50% near the fire source and up to 15% downwind. We also found the effect of smoke particles to be 5 to 10% larger in the UV-B part of the spectrum. For the selected period at Darwin, changes in the aerosol loadings gave larger variations in the surface UV irradiances than previously reported changes seen in the ozone column. We are continuing similar investigations for the Alice Springs site to assess spectral differences between smoke and dust aerosols.
During the period August 2001 to February 2003 global solar UV measurements were performed with a high resolution spectroradiometer at Melbourne Australia (37.7°S, 144.9°E). From these measurements total column ozone amounts were derived and compared with those of a nearby Dobson spectrophotometer. The effects of cloud cover, solar zenith angle and the choice of various parameters used for the total ozone retrieval were examined. The comparison showed that mean difference between the two instruments was either -0.3% or +1.8% depending on the choice of parameters used in the total ozone retrieval. The standard deviation of the differences was 2.7% in both cases. It was observed that the presence of smoke haze increased the difference between the instruments and some preliminary work on reducing this effect is presented.
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