DIRECT OBSERVATIONS OF AEROSOL RADIATIVE FORCING OVER THE TROPICAL INDIAN-OCEAN DURING THE JANUARY-FEBRUARY 1996 PRE-INDOEX CRUISE

Simultaneous measurements of aerosol optical depth, size distribution, and incoming solar radiation flux were conducted with spectral and br oadband radiometers over the coastal Indian region, Arabian Sea, and I ndian Ocean in January-February 1996. Columnar aerosol optical depth, delta a, at visible wavelengths was found to be 0.2-0.5 over the Arabi an Sea and <0.1 over the equatorial Indian Ocean. Aerosol mass concent ration decreased from about 80 mu g/m(3) near the coast to just a few mu g/m(3) over the interior ocean. The sub-micron-size particles showe d more than an order of magnitude increase in number concentration nea r the coast versus the interior ocean. This large gradient in particle concentration was consistent with a corresponding large increase in t he Sun-photometer-derived Angstrom exponent, which increased from 0.2 over the Indian Ocean to about 1.4 near the coast. The change in surfa ce-reaching solar flux with delta a was obtained for both the direct a nd the global solar flux in the visible spectral region. The solar-zen ith-angle-normalized global and diffuse fluxes vary almost linearly wi th normalized delta a. The direct visible (<780 nm) solar flux decreas es by about 42 +/- 4 Wm(-2) and the diffuse sky radiation increases by about 30 +/- 3 Wm(-2) with every 0.1 increase in delta a, for solar z enith angles smaller than 60 degrees. For the same extinction optical depth the radiative forcing of the coastal aerosols is larger than the open ocean aerosol forcing by a factor of 2 or larger.