Characteristics of cloud-nucleating aerosols in the Indian Ocean region

During the Indian Ocean Experiment (INDOEX), cloud droplets were collected and evaporated using a counterflow virtual impactor (CVI). The nonvolatile residual particles were then analyzed by various instruments. Physical and chemical properties of the cloud droplets and their residual nuclei were compared with properties of the below-cloud aerosol to evaluate which aerosol particles act as cloud nuclei in different environments, and their effects on cloud microphysics. Four cases, ranging from clean Southern Hemispheric clouds to heavily polluted clouds near India, were analyzed. For the cleaner clouds, droplet concentrations were a much higher fraction of the available particle concentrations than for polluted clouds, but entrainment apparently acted to reduce droplet nupiber concentrations in both regimes. For clean clouds the median critical supersaturation and size of the ambient particles and droplet residual particles were similar. In polluted clouds there were stronger differences between ambient and droplet residual distributions, and particles with lower critical supersaturations were favored as nuclei. Simple model calculations were used to show that polluted clouds are expected to achieve lower water supersaturations than clean clouds; thus only particles with relatively low critical supersaturations are likely to affect clouds in polluted regions. Soluble fractions for the ambient aerosol inferred from the size and cloud condensation nuclei measurements were in general agreement with another study in the region. Droplet residual particles did not necessarily have higher soluble fractions than the ambient aerosol, but did tend to have higher total amounts of soluble material per particle, particularly in the polluted cases.