An Agorithm Theoretical Basis Document (ATBD) is a fairly complete description of the algorithm(s) used to create the satellite product(s) under discussion. As such it is intended for an advanced audience. A more simplified discussion and explanation of the dark target products and how they are created can be found in the algorithm and product pages of our website. The content of this ATBD has been modified from previous versions to provide information on the generalized version of the NASA Dark Target Aerosol (dark target) algorithm currently in use, rather than focusing on the MODIS sensor for which it was originally developed. This approach has become necessary as the algorithm has been applied to an increasing number of sensors that now include LEO (MODIS and VIIRS) and GEO (ABI and AHI) instruments and the EMAS airborne simulator. Although we will be describing a generic retrieval we will provide sensor specific information as needed throughout the ATBD.
The dark target algorithm retrieves the aerosol optical depth (AOD) over vegetated land, as well as proxies for aerosol size (e.g. Fine-Weighting or Ångström Exponent) over ocean. These aerosol products are primarily intended for radiative budget and climate applications, but are also relevant for hydrological, oceanographic and air quality applications. The combined ocean/land algorithm takes advantage of the wide spectral range of the sensors where it has been applied. Generally 500 m pixels are used for cloud screening and 1 Km pixels are used for aerosol retrievals. LEO sensors can obtain global coverage on a daily basis (with small gaps near the equator for MODIS), and GEO sensors provide data every 10 minutes.
A prior version of this Algorithm Theoretical Basis Document (ATBD-16) describes the Collection 6 (C6) version of the “Dark-Target” (DT) algorithms for retrieving aerosol properties over global land and ocean, as applied to spectral reflectance observed by Moderate resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Terra and Aqua satellites.
Although the “Deep Blue” (DB) retrieval (Hsu et al., 2006, Hsu et al., 2013, Sayer et al., 2013) is a component of the MODIS and VIIRS products, they are not detailed in this ATBD.