Atmospheric particle size and composition measurements to support light extinction calculations over the Indian Ocean

The size distribution and chemical composition of the atmospheric aerosol at the Kaashidhoo Climate Observatory (KCO) in the Republic of Maldives was determined during the winter northeast monsoon season to aid in determining the light scattering and light absorption properties ofthe aerosol particles in that region. These xperiments were conducted over 8 two-day periods during February 11-26, 1999, using filter-based samplers and cascade impactors operated atambient relative humidity which was in the range of 80-89% relative humidity over 83% of the period sampled. Fine particle concentrations (Da<l.8 gm) averaged 17.7(+0.22) gg m -3 and varied between 8.4(+0.33) -24.7(+0.21) [xg m -3 over the period studied. Sulfate ion and carbonaceous aerosols are the largest contributors to the fine particle mass concentration, accounting for 33-37% and 26-27% of the fine mass, respectively. Calcium carbonate contributes 3% of the mass measured on the impactor stages. Ammonium, nitrate, and chloride ion account for 7-9, 1, and 0-1% of the fine particle mass, respectively. The residual mass of as yet undetermined fine particle material stands at 28-30%. Black elemental carbon particles contribute 6-11% of the fine particle mass concentration and dominate light absorption in the atrfiosphere at KCO (A. Eldering et al., unpublished manuscript, 2001). These fine particle concentrations arecomparable to those found in major cities in the United States and are surprisingly high for a remote location such as the Maldive Islands, which is located downwind of the Indian subcontinent.