The Aerosol Problem: At the Intersection of Chemistry, Dynamics, Radiation and Climate

The recently completed Indian Ocean Experiment revealed several important insights into the role of aerosols, mainly because it had an interdisciplinary team of experimentalists and modelers in chemistry, dynamics, radiation and climate. INDOEX revealed the great Indo-Asian haze, which spread over an entire ocean basin and much of Asia. The aerosols within this thick haze consisted of a mix of inorganic and organic species with black carbon clusters, which were transported thousands of kilometers, in part because of the hydrophobic nature of soot , and due to the peculiar meteorology of the region extending the life time of particles to week or longer. The soot, some of which were coated with organics and inorganics, had a significant impact on the radiative forcing of the region. It decreased the UV and visible radiation by 10 to 20%; decreased the solar heating of the ocean mixed layer by 10 to 40 W m^-2. These reductions were accompanied by a large solar heating of the lower troposphere, which in turn was shown to perturb the ITCZ and tropical precipitation, thus linking local to regional and global climate. The polluted clouds had more drops and smaller effective radii and as a result brighter. However, the cooling effect of brighter clouds was nearly offset by the large positive forcing of aerosols in cloudy regions.

The most important findings, however, were: accurate insights into the chemical composition of aerosols resulted in accurate simulation of radiation fluxes without invoking mysterious absorbers in the atmosphere. Next, the experiment clearly demonstrated that such interdisciplinary observations, if done reliably and integrated with models, can given insights into fundamental questions in global change involving the anthropogenic aerosol climate forcing. INDOEX however raised several outstanding questions involving: the origin of organic aerosols and their influence on cloud formation and radiative transfer in clouds; trans-continental nature of aerosols; long range transport of soot and its impact on ice clouds, to name a few.