Global and regional validation of the Collection 6 MODIS dark target aerosol products, and comparison to Collection 5

The MODIS Collection 6 (C6) dark targets aerosol algorithms include several updates, including multiple wind speed look up tables over ocean and improved sensor calibration. We analyze the entirety of the MODIS-Aqua aerosol record against AERONET to characterize uncertainty in the products, and relate the new collection to the well-characterized Collection 5 (C5) products to understand specific improvements. Over land, ~70% of high quality AOD retrievals at 0.55 μm are within the C5 expected error bounds, which is comparable to C5; however, a slight overestimation of AOD at low optical depths and a slight underestimation at high optical depths that was observed in C5 has been eliminated in C6. The highest agreement with AERONET occurs in the Eastern U.S. and Europe. Regions with large surface reflectance, such as the Western U.S., or higher aerosol loading, including much of Africa and South America, remain a challenge. Over ocean, the inclusion of wind speed in the surface characterization has removed a wind speed dependant bias, and globally, ~63% of high quality AOD retrievals at 0.55 μm are within the C5 expected error bounds. The dust outflow regions off the coast of Africa show the poorest agreement with AERONET. The aerosol products validate acceptably for science, though users should be aware of some regional biases we present in this work.