Aerosol Size Distribution, Complex Refractive Index and Spherical Fraction Retrieved from Airborne In Situ Angular Light Scattering and Absorption Measurements

Aerosol models summarizing the optical properties of suspended particles of a particular type are necessary to simulate radiative transfer through the atmosphere in climate simulations and remote sensing retrievals. In this work, we derive a set of aerosol models from a synergistic retrieval of airborne in situ scattering measurements made by the Polarized Imaging Nephelometer (PI-Neph) as well as absorption measurements made by the Particle Soot Absorption Photometer (PSAP). The data utilized include phase functions, polarized phase functions and absorption coefficients measured during the SEAC4RS and DC3 field campaigns. The Generalized Retrieval of Aerosol and Surface Properties (GRASP) is applied to these measurements in order to obtain size distributions, complex refractive indices and spherical fractions that are optically representative of the original measurements. A classification scheme, that is derived from ancillary data but correlates strongly with the optical properties, is then used to categorize the corresponding retrievals. The retrieved size distributions of all fine mode dominated aerosols were found to be remarkably similar among a range of aerosol types, including biomass burning, biogenic as well as urban and industrial emissions. There were, however, consistent differences between the angular scattering patterns of the biomass burning samples and the other fine mode dominated types. The GRASP retrieval predominantly attributed these differences to elevated real and imaginary refractive indices in the biomass burning samples. The retrieval also revealed significant proportions of coarse mode particles in some samples, especially those acquired around the base of convective systems during the DC3 campaign. These coarse mode particles frequently had reduced spherical fractions, elevated real refractive indices and concentrations that depended strongly on the underlying surface features suggesting a dust-like composition. Lastly, the possibility of developing new aerosol optical models based on GRASP retrievals of synergistic remote sensing observations will be explored.