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What is aerosol fine mode fraction?

Atmospheric aerosols generally have a bimodal distribution.  The smaller particles are referred to as the fine mode or accumulation mode aerosols. These particles have radii between 0.1 and 0.25 microns. The larger particles comprise the coarse mode. These particles generally have radii between 1.0 and 2.5 microns.  The aerosol fine mode fraction is the proportion of fine mode aerosols to the total. This is an optical measurement of the proportion by volume.

Is there a difference in Terra vs Aqua AOD?

Although sensor and algorithm design are identical between the two sensors, over time the sensors have decayed at different rates.  The rate of decay of Terra has been greater than Aqua. Consequently there is a small offset between Terra and Aqua AOD (.015 in Collection 5). Additionally, there is a decreasing trend of global land AOD in Terra Collection 5 (Levy et al., 2010), which is not observed in Aqua. This has been traced to a calibration issue (e.g. Wang et al., 2012), and is expected to be corrected in Collection 6.