A Two-Pronged Attack on the Problem of Aerosol Climate Forcing

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Essentially all mechanisms by which anthropogenic aerosol is thought to affect climate operate by changing the Earth's albedo (i.e. earth system's reflectivity for solar radiation). High resolution albedo products from satellites are becoming available at the same time that aerosol-sensing satellites are offering an improved ability to delineate the global distribution of anthropogenic aerosol. This suggests that a two-pronged approach to better understanding of aerosol climate forcing may be fruitful - one focused on cause (the global distribution and properties of anthropogenic aerosol), the other on effect (the space/time variability of albedo.) The principle investigators (PIs) are pursuing and integrating these two objectives.Regarding the first objective, the PIs are performing a global reassessment of the sub-micrometer aerosol particle size fraction contribution to aerosol optical depth, building on previous studies by their group. This size fraction represents most of the anthropogenic aerosol. Accurate satellite retrieval of this size fraction is key to empirical assessment of the global distribution of anthropogenic aerosol. Earlier, in collaboration with National Aeronautics and Space Administration (NASA) scientists, they compared satellite to in-situ estimates of aerosol particle loading in this size range. Key activities in the current project include (i) inter-comparing remote retrievals of aerosol characteristics from multiple satellite instruments and ground-based sun photometers, (ii) diagnosis of discrepancies using vertical distribution data from airborne and spaceborne lidar, (iii) global reassessment of submicrometer-size aerosol loading based on analysis of the wavelength dependence of optical depth, and (iv) assimilation of aerosol observations into a chemical transport model in coordination with a separate, NSF-funded, research project for improved estimates of the global distribution of anthropogenic aerosol. Regarding the second objective, the PIs are examining albedo variability at a variety of scales using a new observational tool. They are using downward-looking lidar (airborne and spaceborne) that allows the atmospheric causes of albedo variation to be diagnosed. They are examining the prevalence of states intermediate between traditionally-defined "cloud" and "aerosol" and seeking to detect and quantify the direct and indirect (through impact on haze and cloud properties) influence of anthropogenic aerosol on regional-mean albedo.Broader impacts of the research include its integration with teaching and outreach activities through the University of Washington Department of Atmospheric Sciences. It involves training a post-doctoral scientist and a graduate student. The research is closely coordinated with that of several national laboratories - e.g., National Oceanographic and Atmospheric Administration (NOAA) Pacific Marine Environment Laboratory, NOAA Earth Systems Research Laboratory, NASA Langley Research Center, NASA Goddard Space Flight Center, and NASA Ames Research Center. Improved knowledge of climate forcings derived from this project is essential to improved understanding of climate variability and change.