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The twin Moderate resolution Imaging Spectroradiometer (MODIS) sensors havebeen flying on Terra since 2000 and Aqua since 2002, creating an extensivedata set of global Earth observations. Here, we introduce the Collection 6(C6) algorithm to retrieve aerosol optical depth (AOD) and aerosol sizeparameters from MODIS-observed spectral reflectance. While not a majoroverhaul from the previous Collection 5 (C5) version, there are enoughchanges that there are significant impacts to the products and theirinterpretation. The C6 aerosol data set will be created from three separateretrieval algorithms that operate over different surface types. These arethe two "Dark Target" (DT) algorithms for retrieving (1) over ocean (darkin visible and longer wavelengths) and (2) over vegetated/dark-soiled land(dark in the visible), plus the "Deep Blue" (DB) algorithm developedoriginally for retrieving (3) over desert/arid land (bright in the visible).Here, we focus on DT-ocean and DT-land (#1 and #2). We have updatedassumptions for central wavelengths, Rayleigh optical depths and gas(H2O, O3, CO2, etc.) absorption corrections, while relaxingthe solar zenith angle limit (up to ≤ 84°) to increasepoleward coverage. For DT-land, we have updated the cloud mask to allowheavy smoke retrievals, fine-tuned the assignments for aerosol type asfunction of season/location, corrected bugs in the Quality Assurance (QA)logic, and added diagnostic parameters such topographic altitude. ForDT-ocean, improvements include a revised cloud mask for thin-cirrusdetection, inclusion of wind speed dependence on the surface reflectance,updates to logic of QA Confidence flag (QAC) assignment, and additions ofimportant diagnostic information. At the same time, we quantified how"upstream" changes to instrument calibration, land/sea masking and cloudmasking will also impact the statistics of global AOD, and affect Terra andAqua differently. For Aqua, all changes will result in reduced global AOD(by 0.02) over ocean and increased AOD (by 0.02) over land, along withchanges in spatial coverage. We compared preliminary data to surface-basedsun photometer data, and show that C6 should improve upon C5. C6 will includea merged DT/DB product over semi-arid land surfaces for reduced-gap coverageand better visualization, and new information about clouds in the aerosolfield. Responding to the needs of the air quality community, in addition tothe standard 10 km product, C6 will include a global (DT-land and DT-ocean)aerosol product at 3 km resolution

The Collection 6 MODIS aerosol products over land and ocean