Aerosol Optical Depth (AOD) (or Aerosol Optical Thickness) is the measure of aerosols (particles suspended in the air) distributed with in the column of air from instrument to top of the atmosphere or from space-borne sensor to surface. AOD is the degree to which particles in air (aerosols) prevent the transmission of light by absorption and scattering of sun light and is defined as integrated extinction coefficient over a vertical atmospheric column of unit cross-section. As Aerosols prevent the transmission of light by absorption and scattering of sun light therefore reduces the visibility.

⋄ AOD < 0.1 represents "clean atmosphere" (clear blue sky, bright sun and maximum visibility).

⋄ 0.1 < AOD < 0.5 represents "moderately hazy atmosphere" (moderate pollution and visibility).

⋄ As AOD increases to greater than 0.5, distribution of particles in the air become so dense that sun is obscured (very low visibility, highly polluted atmosphere).

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