Aerosol characteristics and aerosol radiative forcing over Maitri, Antarctica

During the 20th Indian Antarctic expedition conducted in January-February 2001, a detailed study on the aerosol spectral optical depth, mass concentration and size-distribution along with columnar ozone and water-vapour concentrations was made from the Indian station, Maitri (70.77degreesS, 11.73degreesE). A low aerosol optical depth of about 0.03 at 400 nm wavelength and a dry aerosol mass concentration of about 7 mug/m(3) for the PM10 particles are found for this anthropogenically least-affected continent on the earth. The aerosol size-distribution reveals that about 63% of the total aerosol mass' comes from particles of size greater than 1 mum, which are of mainly natural origin. Average columnar ozone and total precipitable water-vapour content during the observation period were found to be 271.6 DU and 0.147 cm respectively, and the observed day-to-day variations are explained using air back-trajectory analysis. Estimation of aerosol radiative forcing over Maitri reveals a positive forcing of 0.95 W/m(2) at the top of the atmosphere and -0.83 W/m(2) at the surface. Using model calculations, it is shown that these forcing values can have large annual variation both in magnitude and sign due to variation in the sun-earth geometry, typical of a polar region, even if we assume a constant aerosol amount throughout the year.