The new Collection 6.1 (C61) MOD04_L2 product is an improved version based on algorithm changes in Dark Target (DT) Aerosol retrieval over urban areas and uncertainty estimates for Deep Blue (DB) Aerosol retrievals.

The MODIS level-2 atmospheric aerosol product provides retrieved ambient aerosol optical properties, quality assurance, and other parameters, globally over ocean and land. In Collection 5, and earlier collections, there was only one aerosol product (MOD04_L2) at 10km (at nadir) spatial resolution. Starting from C6, the Dark Target (DT) Aerosol algorithm team provided a new 3 km spatial resolution product (MOD04_3k) intended for the air quality community.

For more information visit the MODIS Atmosphere website at:
https://modis-atmos.gsfc.nasa.gov/products/aerosol

And, for C6.1 changes and updates, visit:
https://modis-atmosphere.gsfc.nasa.gov/documentation/collection-61