Aerosol Optical Depth (AOD)

Aerosol optical depth (AOD) is a dimensionless measure of the extinction of solar radiation by aerosols integrated through a vertical atmospheric column. It quantifies how much direct sunlight is scattered or absorbed by particles such as sea salt, dust, sulfate, and smoke. AOD follows the Beer-Lambert relation, so transmittance equals exp(-AOD/cos(theta)) at solar zenith angle theta. Typical clean marine values run 0.05 to 0.15 at 550 nanometers, rising above 1 in dust or smoke plumes. Sun photometers (AERONET) and satellite sensors (MODIS, VIIRS) retrieve it, and it controls the aerosol direct radiative effect on the surface energy budget.