Angular scattering of the Gobi Desert aerosol and its influence on radiative forcing

The volume scattering function and the scattering coefficient of the aerosol originating from the Gobi Desert and the Chinese Loess Plateau have been measured in Kyoto using a polar nephelometer and a three wavelength integrating nephelometer. An unambiguous identification of the desert aerosol was possible both by back trajectories and by its specific optical properties. Despite the larger size of the desert aerosol particles a considerable light scattering occurs at angles between 40 and 140°. This is caused by the irregular shape of the particles. Using the measured scattering phase function, the effect of the desert aerosol particles on the radiative forcing was estimated. Even for an optically thin layer the negative forcing is comparable to the positive forcing by the green house gases. When assuming the particles to be spherical the forcing is by far less, since the resonance effects occurring with the spherical particles cause less side scattering (up to a factor of 3). Therefore, it is essential to use the appropriate scattering phase function.