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 a t 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 of the aerosol partic les in that region. These experiments were conducted over 8 two-day periods during February 11-26, 1999, using filter-based samplers and cascade impac tors operated at ambient relative humidity which was in the range of 80-89% relative humidity over 83% of the period sampled. Fine particle concentrat ions (D-alpha<1.8 mum) averaged 17.7(+/-0.22) mug m(-3) and varied between 8.4(+/-0.33)-24.7(+/-0.21) mug m(-3) over the period studied. Sulfate ion a nd carbonaceous aerosols are the largest contributors to the fine particle mass concentration, accounting for 33-37% and 26-27% of the fine mass, resp ectively. Calcium carbonate contributes 3% of the mass measured on the impa ctor 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 und etermined fine particle material stands at 28-30%. Black elemental carbon p articles contribute 6-11% of the fine particle mass concentration and domin ate light absorption in the atmosphere at KCO (A. Eldering et al., unpublis hed manuscript, 2001). These fine particle concentrations are comparable 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 downwi nd of the Indian subcontinent.