9612891 Clarke The Indian Ocean Experiment (INDOEX) is an international field experiment with participation from France, Germany, India, the Netherlands, and the United States. The main effort under the U.S. component focuses on assessing the role of sulfates and other continental aerosols in global radiative forcing and is being supported jointly by the Department of Energy and the National Science Foundation. In addition, complementary studies involving investigations of a variety of related chemical and radiative processes will be undertaken by scientists from other federal agencies and other international partners. The composite observing system consists of a wide range of platforms, from ships to aircraft, as well as surface-based and balloon measuring systems, and satellite data. The main part of the three year effort will be a four-month intensive field phase which begins January 1999. The ultimate goal of this program is to obtain a better understanding of the decadal and longer time scale climate forcing. The equatorial Indian Ocean during the northeast winter monsoon season provides a unique natural laboratory for this experiment. The experimental area is probably the only place in the world where an air mass which has been modified by intense sources of continental aerosols, anthropogenic trace species, and their reaction products (e.g., sulfates and ozone) from the northern hemisphere comes into contact with the pristine air of the southern hemisphere, which has been transported via cross equatorial monsoonal flow, at the Intertropical Convergence Zone. The juxtaposition of these two very distinct air masses will allow a thorough examination of both the direct and indirect effects of the aerosols on the radiative properties of the atmosphere. Dr. Clarke's role in INDOEX is to carry out a suite of aerosol microphysical and optical measurements aboard the NCAR C-130 aircraft which are designed to provide fundamental information on aerosol properties. These data are needed to help evaluate the role of aerosols in both direct and indirect climate forcing. Specialized instrumentation will provide rapid assessment of aerosol physio-chemistry and of aerosol processes, including nucleation, as well as provide a continuous characterization of the aerosol field during the aircraft missions. This combination of instruments has been used to distinguish between marine, continentally polluted, continental dusts, and "clean regions of the troposphere characterized by low aerosol mass and but high concentrations of "new" nuclei. These instruments also will be used obtain information on the aerosol size distribution and optical signatures. An important aspect of the project are the validation and inter-comparison activities that will be carried out in conjunction with the other INDOEX PIs.