Nonsphericity of aerosol particles and their contribution to radiative forcing

Abstract

Observations of combined spectral aerosol optical thickness and sky brightness in the almucantar for clear sky conditions indicate a discrepancy between the measured and modeled radiation field in certain air mass situations [maritime polar air (mP) and particular continental subtropic air (cS)], which cannot be closed using the commonly made assumption of sphericity of the aerosol particles or explained by measurement uncertainties. In these cases, mostly associated with flat spectral slopes of the aerosol optical thickness (caused by large particles, e.g. at marine and desert aerosols as well as thin ice clouds) increased side scattering is observed in the sky brightness. A Coupled Inversion Radiation Transfer program (CIRATRA) is used for the determination of climate-relevant aerosol parameters assuming spherical as well as nonspherical particle shapes. The observed increased side scattering for the marine and desert aerosols only can be explained by the use of scattering phase functions produced by nonspherical particles and consequently smaller asymmetry parameters. In these cases the best fit to the measured sky brightnesses (within the observable range of scattering angles (3–150 deg)) could be obtained by the use of the semi-empirical scattering theory of Pollack and Cuzzi 1 with less asymmetric phase functions and smaller asymmetry parameters than Mie theory. Conclusions regarding radiative forcing by aerosols are made for different values of aerosol optical thickness. The cooling effect by aerosols in cases of nonsphericity reaches up to a factor of 2 compared with the cooling of spherical particles.