Most instruments measure the quantity and distribution of aerosols with the unit “Aerosol Optical Depth (AOD)”. This unit represents the amount of light aerosols scatter or absorb, and can also be referred to as Aerosol Optical Thickness (AOT).
The Ångström Exponent (α or AE) measures how AOD changes through the spectrum of visible light. AE describes how Aerosol Optical Depth (AOD) generally trends downward as wavelength increases. This property causes aerosols to stand out more against bright backgrounds at low wavelengths such as 412 nm, the band used by Deep Blue. AE can also be used to determine the aerosol type, as coarse aerosols tend to have a low AE while small aerosols tend to have a higher AE. Liquid and ice clouds, as well as desert dust, have an AE around 0. Smoke and sulfates have high wavelength dependencies, with AE between 1 and 2, and their AOD drops dramatically towards the red spectrum. Mixed aerosols tend to have an AE in the 0.5 - 1.5 range. Thus, AE can be used to differentiate between aerosol types, allowing the algorithm to make more accurate assumptions about the aerosol’s properties, resulting in better retrievals.
The Ångström Exponent (denoted as AE or α) provides information about the size of aerosol particles, and is a measure of the spectral dependence of the aerosol’s AOD. Values less than one suggest a dominance of coarse particles, while values greater than one suggest dominance of fine particles. Put simply, larger AE values mean smaller particle size.
PM2.5 and PM10
In order to measure the size of aerosols, scientists use terms such as PM2.5 and PM10. PM stands for Particulate Matter, another term for aerosols. The number represents the diameter of the aerosol in micrometers. PM10 are therefore coarse dust particles, and can be from 2.5 to 10 micrometers in diameter. PM2.5 are fine particles, and 2.5 micrometers in diameter or smaller.
Another measured aspect of aerosols are their “Single Scattering Albedo” (SSA), or the fraction of light that is scattered compared to the total optical depth (extinction). A value of 1 indicates that an aerosol only scatters light, and values closer to 0 indicate that an aerosol mostly absorbs light. Most aerosols have an SSA in the range 0.7 to 1.
Aerosol Measuring Instruments
Read more about the instruments that measure aerosols here.