- The following text is adapted from (Hess et al., 1998) and the OPAC site -
The software package OPAC (Hess et al., 1998) (Optical Properties of Aerosols and Clouds) provides optical properties in the solar and terrestrial spectral range of atmospheric particulate matter. OPAC is based on a dataset of microphysical and optical properties of six water clouds, three ice clouds, and 10 aerosol components, which are considered as typical cases. Since real aerosols in the atmosphere always are a mixture of different components, OPAC allows calculating the optical properties of any mixtures of the basic components and to calculate optical depths on the base of exponential aerosol height profiles. Typical mixtures of aerosol components as well as typical height profiles are proposed as default values, but mixtures and profiles for the description of individual cases may also be achieved simply.
The optical properties (i.e., the extinction, scattering, and absorption coefficients, the single scattering albedo, the asymmetry parameter, and the phase function) are given for up to 61 wavelengths between 0.25 and 40 μm and up to eight values of the relative humidity. The software package also allows calculation of derived optical properties like mass extinction coefficients and Ångström coefficients. These optical properties are calculated on the basis of the microphysical data (size distribution and spectral refractive index) under the assumption of spherical particles in case of aerosols and cloud droplets and assuming hexagonal columns in case of cirrus clouds. In the new version of OPAC database (Koepke et al., 2015), the optical properties of the mineral particles are modeled describing the particles as spheroids with size dependent aspect ratio distributions
ALG v1.2 supports the coupling of OPAC version 3.1 and version 4.0b (only for Mie spherical particles) with MODTRAN5. The use of OPAC v. 4.0b for non-spherical mineral particles and its coupling with MODTRAN6 is on-going.
Hess, M., Koepke, P., & Schult, I. (1998), "Optical properties of aerosols and clous: the software package OPAC." Bulletin of the Americal Meteorologic Soctiety. Vol. 79, No. 5, pp. 831‒ 844.
Koepke, P., Gasteiger, J., & Hess (2015), "Technical Note: Optical properties of desert aerosol with non-spherical mineral particles data incorporated to OPAC." Atmos. Chem. Phys.. Vol. 15, No. 5, pp. 5947‒ 5946.