Measurement of Light Extinction and Scattering by Aerosols for Deriving the Wavelength and Humidity Dependence of Absorption and Single Scatter Albedo

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The goal of this project is to make in situ measurements of the light absorption coefficient for ambient aerosols as a function of relative humidity (RH) by measuring the difference between the coefficients of light extinction and light scattering. Measurements will be made at three wavelengths in the visible range (ca. 450, 550 and 700 nm) and over a range of relative humidities. A three-wavelength optical extinction cell capable of measuring the extinction coefficient of ambient aerosol particles for use in integrated field experiments will be developed, based on an existing prototype. An integrating nephelometer will provide the light scattering coefficient. A humidity control system will be incorporated with a second optical extinction cell and the nephelometer. A complete system consisting of extinction cells, integrating nephelometers, filter-based absorption photometers and necessary supporting equipment for RH control, data acquisition, etc. will be assembled and tested. Subsequently, field measurements will be carried out at three field sites selected to have significant light absorbing carbon concentration and fraction in the aerosol. The sites tentatively selected are: Seattle, WA, Bondville, IL, and a site in Thailand. At the latter two sites the investigators will collaborate with larger, intensive field campaigns to take advantage of the physical, chemical and optical aerosol measurements made by other research groups. By gathering detailed in situ data on "black carbon"-containing aerosols, this work will provide more accurate information on the optical properties of these very important aerosols. These data are needed for models of the radiation budget of the earth-atmosphere system. The research will be integrated with teaching and outreach activities at the University of Washington and the Joint Institute for the Study of the Atmosphere and Oceans. A post- doctoral scientist will be supported through this project.