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Aerosol optical depth (AOD) is an assessment of the amount of visible and infrared light aerosols scatter or absorb in a column of the atmosphere, and is sometimes referred to as aerosol optical thickness (AOT). AOD is a unitless measure. From an observer on the ground, an AOD of less than 0.1 is characteristic of a clear sky, bright Sun, and maximum visibility. As AOD increases to 0.5, 1.0, and greater than 3.0, aerosols become so dense that the Sun is obscured. 

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ARSET Trainings on Health and Air Quality
The Applied Remote Sensing Training (ARSET) Program provides hands on online and in-person trainings that cover remote sensing applications for monitoring health and air quality.
Dark Target and Deep Blue data can be interactively visualized using NASA Worldview
The merged Dark Target/Deep Blue AOD layer provides a more global, synoptic view of AOD over land and ocean
The non-aerosol signal of surface reflectance needs to be separated from the aerosol signal to accurately obtain AOD. Scientists have developed two algorithms for MODIS data to account for these effects: Dark Target and Deep Blue. In the latest dataset collection, these two algorithms have been merged, using the highest quality for each.
View the MODIS Aqua/Terra Combined Algorithm Aerosol Optical Depth Layer
Catalog of Aerosol Optical Depth Data
This is a screenshot from NASA Worldview showing the merged Dark Target/Deep Blue aerosol optical depth layer.

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