Technical Note: Optical properties of desert aerosol with non-spherical mineral particles: data incorporated to OPAC

Mineral particles, in general, are not spheres and so the assumption of spherical particles, instead of more real- istic shapes, has significant effects on modeled optical prop- erties and therefore on remote-sensing procedures for desert aerosol and the derived radiative forcing. Thus, in a new ver- sion of the database OPAC (Optical Properties of Aerosols and Clouds; Hess et al., 1998), the optical properties of the mineral particles are modeled describing the particles as spheroids with size dependent aspect ratio distributions, but with the size distributions and the spectral refractive in- dices not changed against the previous version of OPAC. The spheroid assumption is known to substantially improve the scattering functions but pays regard to the limited knowledge on particle shapes in an actual case. The relative deviations of the optical properties of non-spherical mineral particles from those of spherical particles are for the phase function in the solar spectral range up toC60 % at scattering angles of about 130 and up to 60 % in the backscatter region, but less than 2 % for the asymmetry parameter. The deviations are gener- ally small in the thermal infrared and for optical properties that are independent of the scattering angle. The improved version of OPAC (4.0) is freely available at www.rascin.net.