AERDT_L2_VIIRS_NOAA21 - VIIRS/NOAA21 Dark Target Aerosol 6-Min L2 Swath 6 km

The NOAA-21 Visible Infrared Imaging Radiometer Suite (VIIRS) NASA Standard Level-2 (L2) Dark Target (DT) Aerosol product provides satellite-derived measurements of Aerosol Optical Thickness (AOT) and aerosol properties over land and ocean. It delivers spectral AOT measurements and their size parameters over oceans every 6 minutes globally.

This aerosol product uses the same dark target algorithm originally developed for the Terra and Aqua missions' Moderate Imaging Spectroradiometer instruments. It includes two separate and distinct algorithms:

• One retrieves aerosol information over ocean (which appears dark in visible and longer wavelengths).
• The second retrieves aerosol data over vegetated and dark-soiled land (which appears dark in visible wavelengths).

This orbit-level product (Short-name: AERDT_L2_VIIRS_NOAA21) has an at-nadir resolution of 6 km x 6 km and progressively increases away from nadir due to the sensor's scanning geometry and Earth's curvature. The product uses 8 x 8 native VIIRS moderate-resolution (M-band) pixels that nominally have approximately 750 m horizontal pixel size at nadir. Therefore, the Level-2 Dark Target Aerosol Optical Thickness data product incorporates 64 (750 m) pixels over a 6-minute acquisition.

As of December 2025, Version 2.1 is the first collection of the NOAA-21 VIIRS L2 Dark Target Aerosol product and contains the following updates relative to the earlier v2.0 SNPP and NOAA20 product incarnations:

• New Data: New AOT retrievals from the NOAA-21 VIIRS instrument observations.
• Data Quality Fixes: Corrected write error in Mean_Reflectance_Land and STD_Reflectance_Land.
• Enhanced Dust Detection: New Dust_Flag_Ocean Science Data Sets (SDS) and additional retrieval model for dust detection.
• Improved Cloud Filtering: Multiple updates to cloud mask over ocean to prevent cloud-contaminated retrievals.
• Cross Platform Calibration: Cross-calibration coefficients for SNPP, NOAA-20, and NOAA-21.
• Surface Reflectance Updates: Revised surface reflectance parameterization and urban correction.
• Updated Meteorological Data: Transitioned Global Modeling and Assimilation Office (GMAO) ancillary data from Global Earth Observing System Forward Processing for Instrument Teams (GEOS-FPIT) to its successor, Global Earth Observing System Instrument Teams (GEOS-IT).
• Quality Assurance Fix: Fixed a bug that incorrectly included land Aerosol Optical Depth (AOD) values and replaced these values with Quality Assurance values of 1 and 2 in the Optical_Depth_Land_And_Ocean dataset.

The Level-2 netCDF product's geophysical data contains 38 Science Data Set (SDS) layers:

1. Aerosol_Cldmask_Land_Ocean
2. Aerosol_Cloud_Fraction_Land
3. Aerosol_Cloud_Fraction_Ocean
4. Aerosol_Type_Land
5. Angstrom_Exponent_1_Ocean
6. Angstrom_Exponent_2_Ocean
7. Asymmetry_Factor_Average_Ocean
8. Average_Cloud_Pixel_Distance_Land_Ocean
9. Backscattering_Ratio_Average_Ocean
10. Cloud_Pixel_Distance_Land_Ocean
11. Corrected_Optical_Depth_Land
12. Dust_Flag_Ocean
13. Effective_Optical_Depth_Average_Ocean
14. Effective_Radius_Ocean
15. Error_Flag_Land_And_Ocean
16. Fitting_Error_Land
17. Image_Optical_Depth_Land_And_Ocean
18. Land_Ocean_Quality_Flag
19. Land_Sea_Flag
20. Least_Squares_Error_Ocean
21. Mass_Concentration_Land
22. Mass_Concentration_Ocean
23. Mean_Reflectance_Land
24. Mean_Reflectance_Ocean
25. Number_Pixels_Used_Land
26. Number_Pixels_Used_Ocean
27. Optical_Depth_By_Models_Ocean
28. Optical_Depth_Land_And_Ocean
29. Optical_Depth_Large_Average_Ocean
30. Optical_Depth_Ratio_Small_Land
31. Optical_Depth_Ratio_Small_Ocean_0p55micron
32. Optical_Depth_Small_Average_Ocean
33. PSML003_Ocean
34. STD_Reflectance_Land
35. STD_Reflectance_Ocean
36. Surface_Reflectance_Land
37. Topographic_Altitude_Land
38. Wind_Speed_GMAO_Ocean

The geolocation data group includes Glint Angle, Geodetic Latitude and Longitude, Scattering Angle, Sensor Azimuth and Zenith Angles, and Solar Azimuth and Zenith Angles.

The Dark Target Aerosol Product User Guide provides additional information regarding these SDS parameters.