Sun photometer sky radiance measurements can be inverted to produce aerosol characteristics such as size distribution, single scattering albedo, phase functions, and the complex index of refraction. Sun photometers measure the radiance at four or more wavelengths using almucantar, hybrid, or principal plane sky scan scenarios.
The almucantar sky scan scenario measures radiance at azimuthal angles relative to the Sun. The principal plane sky scans measure radiance at scattering angles in the solar principal plane away from the Sun. The hybrid sky scan has an angular scanning distribution similar to the almucantar for large solar zenith angles and the hybrid scan approaches the scanning geometry of a principal plane for small solar zenith angles. The hybrid sky scan provides symmetric sky scanning making it less prone to cloud contamination as the mostly asymmetric principal plane scan. As a result, the hybrid sky scan retrievals allow for inversion products to be available closer to the polar orbiting overpass times for validation and provide more detail on the diurnal change of aerosol characteristics when used with the almucantar retrievals.
For at least single-scattering approximation, sky radiances in the almucantar are not sensitive to aerosol vertical variations; however, the spectral MERRA-2 aerosol extinction and ozone profiles are used to estimate the vertical distribution for each hybrid retrieval. The principal plane sky scans are not currently retrieved. The angular distribution of almucantar and hybrid radiance data in combination with aerosol optical depth measurements and estimations of land and water surface reflectance are inverted to estimate aerosol characteristics.
Version 3 inversion data are provided as one model product with additional parameters such as sphericity, radiative forcing and spectral flux. Version 3 inversions is released as Levels 1.5 and 2.0, based on the level of aerosol optical depth (AOD) and other almucantar and hybrid quality checks.
Data may be viewed and downloaded using the links in the left menu bar under Aerosol Inversions. The Version 3 almucantar and hybrid inversion products use the same quality assurance criteria as Version 2 almucantars, except for the hybrid retreivals, the last symmetric scattering angle bin range is ≥75 degrees and this bin needs to have at least 2 symmetric angles.
Version 3 inversion description document is currently under development, which will describe important changes to input data sets, scalar to vector computations, and additional retrieval products including sensitivity to input uncertainties. Please contact the AERONET staff if you have further questions. In general, Version 3 products are similar to Version 2 data products and the following documents can be used as a guide:
+ Version 2 Inversion Product Descriptions (PDF)
+ Version 2 Quality Assurance Criteria or Full Version (PDF)
Version 3 radiance data are normalized using the extraterrestrial solar irradiance spectra using data established jointly by the University of Colorado's Laboratory for Atmospheric and Space Physics (LASP) and the Naval Research Laboratory (NRL) (Coddington et al., 2016).