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.
Temporal Coverage: 24 February 2000 - present (Terra); 3 July 2002 - present (Aqua)
The Aerosol Optical Depth layer is useful for studying aerosol optical depth over the ocean as well as vegetation/dark-soiled land, which are both dark in the visible wavelengths. It is produced using 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). Note: The algorithm is not applied over sunglint ocean regions or bright (desert) land regions, therefore gaps will occur at those locations.
The MODIS Aerosol Optical Depth layer uses the Optical Depth Land And Ocean parameter 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.
Temporal Coverage: 24 February 2000 - present (Terra); 3 July 2002 - present (Aqua)
The 3 km Aerosol Optical Depth layer was designed to provide a higher-resolution view to meet some users’ needs. Caution should be exercised in urban regions, as there are known issues over urban landscapes. This layer is created from 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).
The MODIS Aerosol Optical Depth (3km, Land and Ocean) layer is available from both the Terra (MOD04_3K) and Aqua (MYD04_3K) satellites for day time overpasses. The sensor/algorithm resolution is 3 km at nadir, imagery resolution is 2 km at nadir, and the temporal resolution is daily. Resolution is much coarser out toward the edge of the swath.
Temporal Coverage: 24 February 2000 - present (Terra); 3 July 2002 - present (Aqua)
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.
Temporal Coverage: 24 February 2000 - present (Terra); 3 July 2002 - present (Aqua)
The Deep Blue Aerosol Optical Depth layer is useful for studying aerosol optical depth over land surfaces. This layer is created from the Deep Blue (DB) algorithm, originally developed for retrieving over desert/arid land (bright in the visible wavelengths) where Dark Target approaches fail.
The MODIS Deep Blue Aerosol Optical Depth (Land) 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, imagery resolution is 2 km at nadir, and the temporal resolution is daily. Resolution is much coarser out toward the edge of the swath.
Temporal Coverage: 24 February 2000 - present (Terra); 3 July 2002 - present (Aqua)
The MODIS Deep Blue Aerosol Ångström Exponent layer can be used to provide additional information related to the aerosol particle size over land. This layer is created from the Deep Blue (DB) algorithm, originally developed for retrieving over desert/arid land (bright in the visible wavelengths). The Ångström exponent provides additional information on the particle size (larger the exponent, the smaller the particle size). Values < 1 suggest optical dominance of coarse particles (e.g. dust) and values > 1 suggest optical dominance of fine particles (e.g. smoke).
The MODIS Deep Blue Aerosol Aerosol Ångström Exponent (Land) 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, imagery resolution is 2 km at nadir, and the temporal resolution is daily. Resolution is much coarser out toward the edge of the swath.
Temporal Coverage: 24 February 2000 - present (Terra); 3 July 2002 - present (Aqua)
The MODIS Deep Blue Aerosol Ångström Exponent layer can be used to provide additional information on the aerosol particle size over ocean. This layer is created from the Dark Target (DT) algorithm that retrieves over ocean (dark in visible and longer wavelengths). The Ångström exponent provides additional information on the particle size (larger the exponent, the smaller the particle size). Values < 1 suggest optical dominance of coarse particles (e.g. dust) and values > 1 suggest optical dominance of fine particles (e.g. smoke).
The MODIS Dark Target Aerosol Aerosol Ångström Exponent (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, 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 www.atmos-meas-tech.net/6/2989/2013/ doi:10.5194/amt-6-2989-2013.; MODIS Dark Target website; MODIS Deep Blue website.