P. Koepke, J. Gasteiger, and M. Hess (2015)

P. Koepke, J. Gasteiger, and M. Hess (2015): Technical Note: Optical properties of desert aerosol with non-spherical mineral particles: data incorporated to OPAC, Atmos. Chem. Phys., 15, 5947-5956, doi:10.5194/acp-15-5947-2015 (PDF)


Abstract

Mineral particles, in general, are not spheres and so the
assumption of spherical particles, instead of more realistic shapes, has
significant effects on modeled optical properties and therefore on
remote-sensing procedures for desert aerosol and the derived radiative
forcing. Thus, in a new version 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 indices 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 to +60% 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 generally 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.