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What aerosol properties do we want to measure?

AOD

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).[1]

Animation from SeaWiFS showing annual variation of global AOD.

This map from the SeaWiFS Deep Blue Version 4 dataset shows the annual cycle of AOD. Intense, darker colors indicate more aerosols. Areas with persistent snow/ice/cloud cover or polar night are shown in grey.

Ångström Exponent

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.

A histogram showing the relationship between the Ångström exponent, AOD, and visible light wavelengths.

The Ångström exponent is a measure of the wavelength-dependence of AOD, which is related to aerosol particle size. For most aerosol types, AOD is higher at shorter wavelengths (e.g. blue light) than longer (e.g. red light).

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.[3]

The equation for finding spectral dependence of an aerosol’s AOD with the Ångström exponent.

The equation for finding spectral dependence of an aerosol’s AOD with the Ångström exponent. (Equation image provided by NASA GES DISC)

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.[2] 

A diagram showing how PM2.5 aerosols are significantly smaller than PM10 aerosols, which are significantly smaller than the diameter of a single grain of sand.

Visualization of the size of PM10 and PM2.5 particles relative to grains of sand. Emitted mineral dust particles are typically much smaller than the grains of sand you see on the beach.

SSA

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.[1]

Aerosol Measuring Instruments

Read more about the instruments that measure aerosols here.