Aerosol optical depth at Cape Grim, Tasmania, 1986-1999

Abstract

The aerosol optical depth at four wavelengths (368, 500, 778, and 868 nm) has been measured automatically at Cape Grim since 1986. The site, on the northwest tip of Tasmania, Australia, was chosen to be representative of much of the Southern Ocean. Fourteen years of measurement have been calibrated and analyzed. The data have been filtered so that only measurements made under onshore wind conditions are considered. The major feature observed in the record is the eruption of Mount Pinatubo, which resulted in the aerosol optical depth at 500 nm rising to 0.2-0.3. If the period of high stratospheric aerosol is ignored, the resulting record shows an average aerosol optical depth near 0.04 for all four wavelengths. In the winter months the observed optical depth correlates well with wind speed. This correlation indicates, that, as an annual average, 40% of the observed aerosol optical depth is due to wind-generated aerosol. The annual cycle of the aerosol optical depth can be understood in terms of the impact of photochemically generated aerosol in the summer months, and an injection of midtropospheric aerosol in September, presumably due to biomass burning.