Collaborative Research: Hygroscopic Properties of Aerosol Organics

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Aerosols contribute to the Earth's radiation balance directly by absorbing and scattering light and indirectly by nucleating cloud droplets which increase planetary albedo. Both effects depend on hygroscopic growth, but the contribution of organics to aerosol hygroscopicity is not well understood--limiting efforts to model those effects. This study will address several important questions regarding aerosol hygroscopicity through field measurements, laboratory experiments, and modeling: (1) What is the contribution of water-soluble organic carbon (WSOC) to hygroscopic growth under sub- and super-saturated conditions?; (2) What are the chemical and physical properties of WSOC?; and (3) How well can Köhler theory and thermodynamic equilibrium modeling account for observed hydroscopic growth and cloud condensation nucleus (CCN) activity of mixed inorganic and organic aerosols? To answer these questions, aerosol samples will be collected at the Desert Research Institute's high-elevation Storm Peak Laboratory (SPL) during summer. Detailed inorganic and organic composition of the samples will be measured with state-of-the-art analytical techniques to identify heretofore unspeciated high molecular weight WSOC. Water extracts of the samples and isolated WSOC will be re-aerosolized to directly measure their contribution to hygroscopic growth and CCN activity. Measured growth factors and CCN activity will be reconciled using Köhler theory and structure-based thermodynamic activity models. The results of this work will improve ability to simulate hygroscopic growth for aerosols of mixed inorganic and organic composition. The explicit treatment of high molecular weight WSOC compounds produced in this project will be incorporated into the on-line version of the E-AIM thermodynamic equilibrium model for use by researchers and students worldwide. Ultimately, the project's results will provide information that can be incorporated into large-scale climate models to better predict direct and indirect aerosol radiative effects, such as hygroscopic-growth curves for water soluble organics and parameterizations of Köhler theory for aerosols of different inorganic and organic composition. The research will be incorporated into student projects as part of the Geoscience Research at Storm Peak (GRASP) activity, which engages students from underrepresented groups. During the winter of 2010, SPL will also host atmospheric science field courses for several universities. The project will allow the students taking these courses to work together with project researchers during the field deployment. Graduate undergraduate students from the involved research groups participate in laboratory experiments, data analysis, and publication of the results.