Ångström exponent is the name of the exponent in the formula that is usually used to describe the dependency of the aerosol optical thickness, or aerosol extinction coefficient on wavelength.
Depending on particle size distribution, the spectral dependence of the aerosol optical thickness is given approximately by
where
For measurements of optical thickness
The Ångström exponent is inversely related to the average size of the particles in the aerosol: the smaller the particles, the larger the exponent. Thus, Ångström exponent is a useful quantity to assess the particle size of atmospheric aerosols or clouds, and the wavelength dependence of the aerosol/cloud optical properties. For example, cloud droplet, usually with large sizes and thus very smaller Ångström exponent (nearly zero), is spectrally neutral, which means, e.g., the optical depth does not change with wavelength. This exponent is now routinely estimated by analyzing radiation measurements acquired on Earth Observation platforms, such as AErosol RObotic NETwork, or AERONET.