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Basic Definitions

On this page we will attempt to provide some simplified definitions of commonly used aerosol terms.

Aerosol Optical Depth (AOD) symbolized by τ - an optical measure of light extinction in the total atmospheric column.  This is used as an indication of total aerosol amount in the column.  This is a unitless value.  The value is wavelength dependent.

Aerosol Optical Thickness (AOT) - often used interchangeably with AOD.

Particle Size Distrtibution - aerosols in the optically active size ranges are best represented as a bimodal distribution.  The aerosol size distribution can be represented as a volume or number distribution.


                                 Volume particle size distribution showing a typical division between fine mode and coarse mode aerosols

Fine Mode Aerosol - aerosols in the size range from .1 to 1 micron in radius.  These are usually produced by a combustion process such as biomass burning and industrial or auto pollution.  As such they are often considered to be man-made (antrhopogenic) in origin.

                                   These are sometimes referred to as accumulation mode aerosols. These particles have the largest impact on health issues, Earth's radiation budget, cloud processes and climate.

Coarse Mode Aerosol - aerosols in the size range from 1 to 10 microns in radius.  These are typically produced by a wind or erosion process such as sea salt, soil or desert dust.  They are considered to be produced by natural processes.

Fine Fraction - a simple ration of the volume of fine particles to the total volume of particles.  Values range from 0 - 1.

Fine AOD - the fraction of light extinction due to particles in the fine mode.  This is equal to Total AOD x Fine Fraction.

PM 2.5 - particles of less than 2.5 µm aerodynamic diameter. These can penetrate deeply into the lungs.  The PM 2.5 mass measured at ground level is therefore an important indicator for human health impacts and has become the international standard for evaluation of air quality.