Characteristics of Cloud-Nucleating Aerosols and Their Effects on Cloud Microphysical and Radiative Properties in INDOEX

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This project is aimed at understanding how aerosol particles affect cloud radiative properties through changes in droplet number concentration and size (the so-called "indirect" effect). It will employ observations of the physical and chemical properties of aerosols obtained with airborne instruments during the Indian Ocean Experiment (INDOEX). Optical particle counters, cloud condensation nucleus spectrometers, and electron microscope analysis are the principal means of analyzing the aerosol properties. Characteristics of particles sampled both below and above clouds will be compared with residual nuclei extracted from cloud droplets to assess which particles act as cloud condensation nuclei. In particular, the relative importance of sulfate, sea-salt, soil-dust, and elemental carbon will be evaluated. The critical supersaturation spectra of the ambient particles and residual nuclei will be compared as well. Through a series of case studies, the principal investigator will determine whether in-cloud aerosol follow an expected simple adiabatic relationship, that is, the particles with the lowest critical supersaturation preferentially nucleate cloud droplets. Air trajectories and aerosol chemical measurements will be used to determine the specific role of anthropogenic aerosols in affecting cloud properties. Variations in thermodynamic, aerosol, microphysical, and cloud radiative parameters will be studied via multivariate techniques in order to ascertain parameter dependence and to help separate aerosol effects from dynamic effects on clouds. Support for this project is shared by the Atmospheric Chemistry and Physical Meteorology Programs and the Water and Energy: Atmospheric, Vegetative and Earth Interactions (WEAVE) Initiative of the Global Change Program.