Abstract
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.
Highlights
The optical properties of aerosol particles are the basis for modeling their direct radiative forcing (Lacis and Mishchenko, 1995; Haywood and Boucher, 2000; Yi et al, 2011) and correspondingly for their effect on climate (McCormick and Ludwig, 1967; Myhre et al, 2013)
This paper presents an improvement of OPAC, by modifying the shape of mineral particles
Examples are presented for the deviations between optical properties caused by mineral particles that are assumed as spheres and those assumed as spheroids with the aspect ratio distributions after Kandler C
Summary
The optical properties of aerosol particles are the basis for modeling their direct radiative forcing (Lacis and Mishchenko, 1995; Haywood and Boucher, 2000; Yi et al, 2011) and correspondingly for their effect on climate (McCormick and Ludwig, 1967; Myhre et al, 2013). The optical properties of aerosol particles in general are modeled using the size distribution and the spectral refractive indices of the particles. The assumption has commonly been made that the particles are spheres using Mie theory (Mie, 1908). This has different reasons: on the one hand, the assumption of spherical particles is reasonable in many cases, especially for water-soluble aerosol types under typical meteorological conditions with relative humidity higher than 50 %. Even if the particle shape were available, the problem remains that modeling of non-spherical particles would be complex and time consuming (Mishchenko et al, 2000; Kahnert, 2003). The use of Mie theory is often a good assumption (or the only possible assumption) and it has been used in OPAC
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