The merged Dark Target/Deep Blue Aerosol Optical Depth layer provides a more global, synoptic view of aerosol optical depth over land and ocean. This layer is created from three algorithms: two “Dark Target” (DT) algorithms for retrieving (1) over ocean (dark in visible and longer wavelengths) and (2) over vegetated/dark-soiled land (dark in the visible) and the Deep Blue (DB) algorithm, originally developed for retrieving (3) over desert/arid land (bright in the visible wavelengths). Which algorithm is used for a particular location on the Earth depends on its surface cover.
The MODIS Merged DT/DB Aerosol Optical Depth (Land and Ocean) layer is available from both the Terra (MOD04_L2) and Aqua (MYD04_L2) satellites for daytime overpasses. The sensor/algorithm resolution is 10 km at nadir, the imagery resolution is 2 km at nadir, and the temporal resolution is daily. Resolution is much coarser out toward the edge of the swath.
References: MODIS Atmosphere - Aerosol (04_L2); NASA Earth Observations - Aerosol Optical Thickness; MODIS Atmosphere - ATBD MOD04, C005; The Collection 6 MODIS aerosol products over land and ocean. Levy, R. et al. Atmos. Meas. Tech., 6, 2989–3034, 2013, doi:10.5194/amt-6-2989-2013.; MODIS Dark Target website; MODIS Deep Blue website.
Aerosol Optical Depth (AOD) (or Aerosol Optical Thickness) indicates the level at which particles in the air (aerosols) prevent light from traveling through the atmosphere. Aerosols scatter and absorb incoming sunlight, which reduces visibility. From an observer on the ground, an AOD of less than 0.1 is “clean” - characteristic of clear blue sky, bright sun and maximum visibility. As AOD increases to 0.5, 1.0, and greater than 3.0, aerosols become so dense that sun is obscured. Sources of aerosols include pollution from factories, smoke from fires, dust from dust storms, sea salt, and volcanic ash and smog. Aerosols compromise human health when inhaled by people, particularly those with asthma or other respiratory illnesses. Aerosols also have an effect on the weather and climate by cooling or warming the earth, helping or preventing clouds from forming. Since aerosols are difficult to identify when they occur over different types of land surfaces and ocean surfaces, Worldview provides several different types of imagery layers to assist in the identification.