Exploring the potential of combining column‐integrated atmospheric polarization with airborne in situ size distribution measurements for the retrieval of an aerosol model: A case study of a biomass burning plume during SAFARI 2000

Abstract

Ground‐based columnar and airborne in situ measurements of aerosol optical properties acquired during the Southern African Regional Science Initiative (SAFARI 2000) in August–September 2000 are analyzed to retrieve the aerosol model of a haze layer affected by long‐range transport of biomass burning emissions. One case study is considered. A columnar value of the aerosol polarized phase function Qmeasp(Θ) and of the aerosol single scattering albedo ω0, both at 870 nm, are retrieved from measurements acquired by a ground‐based Sun/sky photometer, assuming that the surface albedo is 0.3. The maximum value of the polarized phase function is 0.37 ± 0.02 at a scattering angle of 70°, ω0 is 0.80 ± 0.05. The in situ particle size distribution is measured in a vertical profile over the ground‐based site by an airborne optical particle counter. Because the size distribution integrated over the column is inconsistent with the polarized phase function, aerosol concentration of the 0.25 μm mode is reduced by a factor of 7.5. Taking into account that the estimation of particle size depends on particle refractive index, it is found that the radius of absorbing particles cannot be larger than 0.15 μm for reproducing Qmeasp(Θ), suggesting external mixture of absorbing particles smaller than 0.15 μm with nonabsorbing particles larger than 0.15 μm. The imaginary part of the effective refractive index is estimated to be (0.09 ± 0.03)i. Comparing Ångström exponent obtained from Sun/sky photometer extinction measurements and the Ångström exponent calculated for the in situ measured aerosol size distribution acquired in eleven vertical profiles allows us to conclude that in most considered cases, the mixture of absorbing with nonabsorbing particles is external with a radius limit at around 0.15 μm.