Premonsoon aerosol mass loadings and size distributions over the Arabian Sea and the tropical Indian Ocean

Abstract

From the cruise expeditions conducted onboard the Oceanographic Research Vessel Sagar Kanya during 1996-2000 over the Arabian Sea (AS) and the tropical Indian Ocean (TIO), aerosol mass concentrations and size distributions are measured in situ in the radius range of 0.025-12.5 μm and are analyzed. The monthly and yearly mean variabilities and trends in aerosol characteristics during the NE winter monsoon (December-April) over these oceanic regions are studied. The effect of relative humidity (RH) on the measured mass concentrations is calculated and the scaling factors are applied to bring the mass concentrations to 50% RH. The 5-year mean mass variations show that the aerosol mass concentrations over Coastal India (CI) in the coarse, accumulation, and nucleation modes are higher than those measured over the AS and the TIO. The coarse mode is found to contribute about 20% over CI, the AS, and the TIO. Over all the three regions, the accumulation mode contribution is more or less similar and lies in the range of 42-48%. The nucleation mode aerosols are found to contribute about 32% over CI, 39% over the AS, and 30% over the TIO. The monthly mean accumulation and nucleation mass concentrations over CI and the AS are found to increase from winter to summer as has been seen over land. The winter variability trends show that the aerosol mass concentrations on an average increase by about 22% yr^-1, 15% yr^-1, and 42% yr^-1 over CI, the AS, and the TIO. Examination of the relation between aerosol mass concentrations and wind speeds reveals that the wind index over the AS and the Indian Ocean is higher than most of the other oceanic regions of the world and the background aerosol mass concentration is quite high, which indicates a strong influence from the continent. Lognormal fits made to the aerosol size distribution indicate that the CI, the AS, and the Indian Ocean region measurements could be fitted with three modes. The size distributions are found to peak at ~0.03, 0.1, and 1.5 μm. The widths of the size distributions show little variations, indicating that while the influx and production of aerosols influenced mainly by meteorological conditions have changed, the physical processes responsible for the shape of the distribution have remained the same over the 5-year period. The number densities obtained over Indian Ocean region are found to be the highest ever measured when compared with other oceanic regions. The size distributions in the nucleation mode have more than an order of magnitude higher number densities when compared with maritime models. The total aerosol number density (N) and the effective radius (r_eff) of the aerosol size distributions track the variations of mass concentration and number density distributions. "Effective" refractive indices are suggested for the three regions based on a comparison of the measured and estimated extinction coefficients from the aerosol mass data. Over the AS, two distinct groups of comparisons between measured and estimated extinction coefficients indicate that the sources of aerosols are different. Seven-day back trajectory analyses show that for one group of values the air masses traveled a long way from Saudi Arabia and Iran/Iraq before reaching the location of measurement, while for the other the air mass originates from Pakistan and passes the Indian Coast of Gujarat through the Thar desert and Rann of Kutch. The measured and calculated single scattering albedo (ω) values are found to compare well within one standard deviation for all the three regions and are in the range of 0.83-0.88, 0.87-0.93, and 0.88-0.99 for CI, AS, and Indian Ocean, respectively. The good agreement between the measured and the calculated ω values indicates that suggested refractive indices for the different oceanic regions are valid.