A model for the natural and anthropogenic aerosols over the tropical Indian Ocean derived from Indian Ocean Experiment data

The physical, chemical and radiative properties of aerosols are investigate d over the tropical Indian Ocean during the first field phase (FFP) of the international Indian Ocean Experiment. The FFP was conducted during Februar y 20 to March 31, 1998. The results shown here are from the Kaashidhoo Clim ate Observatory (KCO), a new surface observatory established on the tiny is land of Kaashidhoo (4.965 degrees N, 73.466 degrees E) in the Republic of M aldives. From simultaneous measurements of aerosol physical, chemical, and radiative properties and the vertical structure from lidar, we have develop ed an aerosol model which, in conjunction with a Monte Carlo radiative tran sfer model, successfully explains (within a few percent) the observed solar radiative fluxes at the surface and at the top of the atmosphere. This agr eement demonstrates the fundamental importance of measuring aerosol physica l and chemical properties for modeling radiative fluxes. KCO, during the no rtheast monsoon period considered here, is downwind of the Indian subcontin ent and undergoes variations in the aerosol visible optical depth tau(v) fr om similar to 0.1 to 0.4, with a monthly mean of similar to 0.2. Lidar data suggest that the aerosol is confined largely to the first 3 kms. Sulfate a nd ammonium contribute similar to 29% to tau(v); sea-salt and nitrate contr ibutes similar to 17%; mineral dust contributes similar to 15%; and the inf erred soot, organics, and fly ash contribute 11%, 20%, and 8% respectively. We estimate that anthropogenic sources may contribute as much as 65% to th e observed tau(v). We consider both an externally and an internally mixed a erosol model with very little difference between the two in the computed ra diative forcing. The observed scattering coefficients are in the upper rang e of those reported for other oceanic regions, the single-scattering albedo s are as low as 0.9, and the Angstrom wavelength exponents of similar to 1. 2. indicate the abundance of submicron aerosols. In summary, the data and t he model confirm the large impact of anthropogenic sources. The surface glo bal fluxes (for overhead Sun) decrease by as much as 50 to 80 W m(-2) owing to the presence of the aerosols, and the top of the atmosphere fluxes incr ease by as much as 15 W m(-2), thus indicating that anthropogenic aerosols are having a large impact on the tropical Indian Ocean.