Satellite characterization of urban aerosols: Importance of including hygroscopicity and mixing state in the retrieval algorithms

This model study examines the sensitivity of the calculated optical properties of urban aerosols to (1) hygroscopicity and (2) internal or external mixing state, and it further investigates the associated implications for the accuracy of satellite retrievals of aerosol optical thickness (t) and aerosol effective radius (reff). State-of-the-art retrieval algorithms widely omit variable hygroscopicity and mixing state. For the study described herein, the modeled urban aerosols are composed of water-soluble sulfates and water-insoluble black carbon (BC) in the fine mode and of water-insoluble compounds in the coarse mode. The calculations show that external compared to internal mixing of black carbon and sulfate not only significantly affects the single-scattering albedo but also alters the diagnostic relationship of the Angstrom exponent (a) to the aerosol effective radius. The implication is that over a dark surface of visible reflectance less than 0.1, satellite retrievals of urban aerosols having a BC/sulfate mass ratio of 5% can differ in t and reff by as much as 60% and 0.2 mm, respectively, depending upon the retrieval algorithm’s assumptions regarding hygroscopicity and mixing state. For surface reflectances greater than 0.1 or BC/sulfate mass ratios larger than 5%, the retrieval bias, including the possibility of unphysical retrievals, increases further. The calculations also show that hygroscopic growth at elevated relative humidity increases the single-scattering albedo of urban aerosols, decreases their backscattering, and as a consequence reduces the influence of mixing state on t and reff. These results suggest that current operational retrieval algorithms lead to a possibly systematic underestimate of aerosol optical thickness when ambient BC/sulfate aerosols are internally mixed at mass ratios greater than 3%. This study’s recommendation is that aerosol retrieval algorithms, when applied to urban aerosols, incorporate in situ knowledge of relative humidity, mixing state, and BC/sulfate mass ratios, either from ground-based measurements or by auxiliary use of chemical transport models.