Optical Properties of Aerosols and Clouds: The Software Package OPAC

TLDR

Atmospheric aerosols are always mixtures of different components. OPAC is a software package that computes optical properties for any mixture of basic aerosol and cloud components and can calculate optical depths from exponential height profiles. It stores microphysical and optical data for six water clouds, three ice clouds, and ten aerosol types in ASCII files, calculates extinction, scattering, absorption, single‑scattering albedo, asymmetry parameter, and phase function from size distributions and refractive indices assuming spherical particles for aerosols and droplets and hexagonal columns for cirrus, and provides derived properties such as mass extinction and Angstrom coefficients across 61 wavelengths (0.25–40 µm) and up to eight relative humidity values. OPAC readily delivers optical properties over the solar and terrestrial spectral range and offers default mixtures and height profiles, while allowing users to customize them for specific cases.

Abstract

The software package OPAC (Optical Properties of Aerosols and Clouds) is described. It easily provides optical properties in the solar and terrestrial spectral range of atmospheric particulate matter. Microphysical and optical properties of six water clouds, three ice clouds, and 10 aerosol components, which are considered as typical cases, are stored as ASCII files. The optical properties are the extinction, scattering, and absorption coefficients, the single scattering albedo, the asymmetry parameter, and the phase function. They 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. Data 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 Angstrom coefficients. Real aerosol in the atmosphere always is a mixture of different components. Thus, in OPAC it is made possible to get 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.