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

Abstract

The physical, chemical and radiative properties of aerosols are investigated over the tropical Indian Ocean during the first field phase (FFP) of the international Indian Ocean Experiment. The FFP was conducted during February 20 to March 31, 1998. The results shown here are from the Kaashidhoo Climate Observatory (KCO), a new surface observatory established on the tiny island of Kaashidhoo (4.965°N, 73.466°E) in the Republic of Maldives. From simultaneous measurements of aerosol physical, chemical, and radiative properties and the vertical structure from lidar, we have developed an aerosol model which, in conjunction with a Monte Carlo radiative transfer model, successfully explains (within a few percent) the observed solar radiative fluxes at the surface and at the top of the atmosphere. This agreement demonstrates the fundamental importance of measuring aerosol physical and chemical properties for modeling radiative fluxes. KCO, during the northeast monsoon period considered here, is downwind of the Indian subcontinent and undergoes variations in the aerosol visible optical depth τ_ν from ~0.1 to 0.4, with a monthly mean of ~0.2. Lidar data suggest that the aerosol is confined largely to the first 3 kms. Sulfate and ammonium contribute ~29% to τ_ν; sea-salt and nitrate contributes ~17%; mineral dust contributes ~15%; and the inferred soot, organics, and fly ash contribute 11%, 20%, and 8% respectively. We estimate that anthropogenic sources may contribute as much as 65% to the observed τ_ν. We consider both an externally and an internally mixed aerosol model with very little difference between the two in the computed radiative forcing. The observed scattering coefficients are in the upper range of those reported for other oceanic regions, the single-scattering albedos are as low as 0.9, and the Angstrom wavelength exponents of ~1.2 indicate the abundance of submicron aerosols. In summary, the data and the model confirm the large impact of anthropogenic sources. The surface global 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 increase by as much as 15 W m^-2, thus indicating that anthropogenic aerosols are having a large impact on the tropical Indian Ocean.