Introduction

Ranging in size from nanometers (nm) to tens of micrometers (μm), atmospheric particulate matter known as aerosols impact the Earth’s radiative budget, climate change, hydrological processes, and the global carbon, nitrogen and sulfur cycles.  To understand the wide-ranging effects of aerosol, it is necessary to measure the aerosol characteristics globally with high spatial and temporal resolution. The polar-orbiting MODerate resolution Imaging Spectrometer (MODIS-Salmonson et al 1989) on board NASA Terra (1999 - ) and Aqua (2002 - ) sensor was developed, in-part, for the capability of observing global aerosol.  The family of Dark-Target algorithms, discussed here, was originally developed to take advantage of MODIS’s ability to observe aerosols.  In fact, because MODIS has been flying for so long, the aerosol record is now over 20 years. However, with the imminent de-orbiting of both Terra and Aqua missions before 2025, means that such a global data record could be in danger of ending.  Fortunately, DT is versatile, and can be run on any sensor with appropriate coverage and sampling in spectral, spatial, and temporal domains. Hence it has been ported to VIIRS (on Suomi-NPP and now the JPSS series of polar orbiting satellites) in low earth orbit (LEO), as well as to ABI (on GOES-R), AHI (on Himawari), and other sensors now in geostationary (GEO) orbit. It has also been ported to run on higher-spatial resolution sensors (e.g., eMAS), to study aerosol processes near sources or near clouds.

The DT aerosol algorithm is actually two independent algorithms, one for deriving aerosols over land (DT-L) and the second for aerosols over ocean (DT-O).  The general science and methodology of both flavors were conceived and developed before the Terra launch and described in depth in Kaufman, et al. (1997b), Tanré, et al. (1997) and ATBD-96. The theoretical basis of DT-O is largely the same as pre-launch.  DT-L has had more significant changes, first described by Levy et al. (2007a,b) and ATBD-09.

The basic aerosol products from DT include total spectral ‘aerosol optical depth’ (AOD or t) and ‘Fine aerosol-Model Weighting’ (FMW or h). In the literature, the concept of t is sometimes referred as ‘aerosol optical thickness’ (AOT), but AOD is preferred for this document. The concept of FMW is also variously defined, however, here h refers to the fractional contribution of fine (small sized particles) to the total t, and is reported at a particular wavelength (0.55 mm). Each retrieval of AOD + FMW product is also associated with Quality Assurance/Confidence (QAC) and additional products representing diagnostic and derived quantities.