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 (3 km, 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.
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.