Satellite Characterization of Urban Aerosols: A13B-0889 Satellite Characterization of Urban Aerosols: Importance of Including Hygroscopicity and Mixing State in the Retrieval Algorithms Jun Wang and Scot T. Martin email: junwang@fas.harvard.ued Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA, 02138 RESULTS THE FACT Ambient sulfate and black carbon particles are usually internally mixed. Sulfate particles are hygroscopic, and their optical properties vary with relative humidity (RH) Satellite retrievals of urban aerosol optical thickness and size provide unique datasets for studying air quality and human impact on climate. A. RH and mixing state effect on phase function (P) and single scattering albedo () P in backscattering decreases as RH increases. For same RH, internal mixing gives larger  than external mixing. THE PROBLEM State-of-the-art retrieval algorithms widely omit variable hygroscopicity and mixing state. How does this premise affect the retrieval accuracy is uncertain? METHODOLOGY B. RH and mixing state effect on effective radius (reff) and Angstrom exponent () We assume urban aerosols consist of black carbon, sulfate and insoluble compounds. We then examine the urban aerosol optical properties for two scenarios (shown as cartoon in below) in the context of variable RH and mass ratios among different aerosol species. For same RH, external mixing gives smaller reff, and larger . These difference decreases as RH or mass ratio of insoluble compounds increases. Scenario 1 Scenario 2 Relative Humidity Mass and volume are conserved Black Carbon Sulfate Insoluble Particle external mixing internal mixing C. Impact of RH and aerosol mixing sate on satellite retrievals of  and reff Assuming external mixing and 70% RH in retrieval algorithm gives / = ±20%. overestimate of reff by ±0.2μm depending on ambient RH. We use the calculated optical properties and the following principals to analyze the uncertainty in retrieval of aerosol optical thickness () and effective radius (reff): ACKNOWLEDGEMENT μ and μ0: solar zenith and viewing zenith angle; : wavelength; Kext: extinction coefficient; : reflectance NSF ATM-0317583 and the NOAA Climate & Global Change Postdoctoral Fellowship under the administration of the UCAR visiting scientist program (for J.W.).