Introduction

There are several NASA remote sensing aerosol products available for research and public use.  These User's Guides focus on the set of aerosol products created using the Dark Target (DT) algorithm(s).  The DT product suite began with the "first light" of the MODIS sensor (on Terra) in 2000.  

This following guides focus on the DT products, beginning with MODIS, and later expanding to include other sensors. They focus on casual usage of the products, but do not go into major detail, nor details about the DT algorithm. More comprehensive descriptions of the DT algorithm, known as the Algorithm Theoretical Basis Document (ATBD), is available in the ATBD section of this website.

In addition to MODIS on Terra, DT products are available representing MODIS on Aqua since 2002 and on VIIRS on Suomi-NPP and NOAA-20 since 2012 and 2017.  VIIRS on NOAA-21 (data record beginning in 2022) should be available soon.  So-called standard (or "continuity") products for MODIS (currently known as Collection 6.1) and VIIRS (Version 2.0) are available for public use.  

A pdf version User's Guide for MODIS and VIIRS can be downloaded here. 

DT can be run on third-generation multi-spectral geostationary (GEO) imagers such as ABI (on NOAA's GOES-R series) and AHI (on Japan's Himawari series).   As a demonstration in support of NASA's MEaSUREs (2017) program, the DT algorithm was adapted to provide a set of "consistent" products on MODIS, VIIRS, ABI and AHI for the period 2019-2022.  These are known as "XAERDT" to differentiate from set of continuity products from MODIS and VIIRS only. 

A pdf version User's Guide for GEO-LEO (XAERDT) can be downloaded here. 

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All satellite remote sensing aerosol products must contend with the problem of separating the aerosol signal from the surface signal observed by the sensor.  The DT algorithm relies on the phenomena that aerosols over a dark surface target will generally brighten the observed scene in order to separate these signals.

Where the surface is bright, such as over ocean glint or desert areas, the DT algorithm will not create a product retrieval. NASA also provides a MODIS Deep Blue (DB) product which is more successful in providing aerosol information over bright surfaces. There is a combined DT-DB land product which attempts to merge the best retrievals from both products. The merged product will also be discussed in this guide.  DB will only be discussed in this guide in terms of its availability in the merged product. For more information on DB please visit their website.

There is also a higher resolution MODIS aerosol product the Multi-Angle Implementation of Atmospheric Correction product (MAIAC).  MAIAC will not be covered in this guide. For more information look here.

Aerosol & Atmosphere Products

MODIS and VIIRS atmosphere products are archived and can be obtained at either LAADS DAAC or NASA'S Earth data site. These are the primary sites for searching through and downloading files. Product files are stored in HDF format. The Earth Data Site has an online tutorial. A downloadable tutorial for LAADS DAAC and many remote sensing resources may be available on the NASA ARSET website. Another easy way to visually search and find files for individual event is to use the NASA Worldview site.  Worldview has an online tutorial.

In addition to the information available in this guide the MODIS-atmos product pages provide an excellent overview as well as in-depth information on not only the MODIS and VIIRS aerosol product but also the water vapor, cloud, cloud mask and aerosol profile products.  New and intermediate users of NASA atmosphere products would do well to familiarize themselves with this content  available there as well as bookmark it for reference.

Product Collections

NASA satellite product developers are constantly working to evaluate, update and improve their products.  From time to time after significant changes are made to the algorithm and/or instrument calibration the entire data set for a single or related group of instrument products will be reprocessed using the new algorithm and/or calibration. The set of data products created using the same set of algorithms and calibration coefficients is referred to as a collection. MODIS Collection 6 (C6) was released in 2016 for all atmosphere products.  A smaller update was later applied and, as of this writing the current MODIS collection is Collection 6.1 (C6.1).  Generally, only the most recent data set is available to the public for use.   If older data collections are required one should inquire with the NASA LAADS DAAC. VIIRS products are organized differently, with the current DT products are known as Version 2 (V2).  

Level 2 Aerosol Products (MODIS and VIIRS standard)

MODIS and VIIRS -retrieved geophysical products (as opposed to the observations) derived "along-orbit" of the satellite sensor are referred to as Level 2 (L2) products. The Level 2 products are made available in files of 5-minute (6-minute) for MODIS (for VIIRS) segments known as granules. MODIS standard (C6.1) product files are collectively known as MxD04 where the x=O for Terra and x=Y for Aqua.  MODIS products are at two different spatial resolutions, one at 10 km × 10 km (nominal at nadir) known as MxD04_L2 and the other at 3 km × 3 km known as MxD04_3K. VIIRS (V2) standard products are denoted as AERDT_L2_VIIRS_SNPP and AERDT_L2_VIIRS_N20, and are nominally at 6 × 6 km. 

The designation as a 10 km or 3 km (or 6 km) aerosol product refers to the resolution of the product (not the sensor!) along the nadir of the satellite swath.  These products include retrieval parameters over land and ocean. The 10 km MODIS product (only) also includes retrievals from the Deep Blue (DB) algorithm as well as a merged DT/DB product.   For more information on the merged product please see our FAQ.  The 3 km MODIS products and 6 km VIIRS products, contains only DT retrievals over both land and ocean.    

MODIS standard products (C6.1) are in HDF4 (.hdf) format, and have the following naming convention:

      MxD04_KK.AYYYYDDD.HHMM.CCC.YEARDAYHRMNSC.hdf

where x=O or Y for Terra or Aqua, KK = L2 or 3K for 10km or 3km, YYYY, DDD and HHMM are the four digit year, three digit Julian day, and time of day (hours and minutes) in UTC, CCC is the collection (061 for the current C6.1), and YEARDAYHRMNSC represents when the file was processed.   

VIIRS data are in NetCDF4 (.nc) format and are named as:         

    AERDT_L2_VIIRS_PLAT.AYYYYDDD.HHMM.VVV.YEARDAYHRMNSC.nc

where PLAT = SNPP, N20 or N21, VVV is the Version number (currently = 002), and the with YYYYDD.HHMM and YEARDAYHRMNSC the same as for MODIS. 

The MOD04 Level 2 product contains data from Dark Target  (DT) and Deep Blue (DB) land algorithms as well as a merged DT - DB product.  The DB product is described on the Deep Blue website.  The merged product parameters are described on our Merged Products page.

Level 3 Atmosphere Products (MODIS and VIIRS standard)

Statistics derived from aggregations of satellite-derived geophysical L2 products are known as Level 3 (L3) data.  For MODIS, the standard (C6.1) Level 3 product is derived on a 1° × 1° latitude/longitude grid, and provided at Daily, Eight-Day, and Monthly aggregations. The product files are collectively known as MxD08_D3, MxD08_E3, and MxD08_M3, respectively.  For MODIS, they also include aggregations related to Deep Blue aerosol products as well as parameters related to other atmospheric retrievals including cloud properties, water vapor and atmospheric profiles.  Like their associated Level 2 products, MxD08 are reported in HDF4 (.hdf) format. 

Because these are composite products they contain a variety of statistical summaries.

Statistics for a given measurement might include:

• Simple (mean, minimum, maximum, standard deviation) statistics
• Parameters of normal and log-normal distributions
• Fraction of pixels that satisfy some condition (e.g. cloudy, clear)
• Histograms of the quantity within each grid box
• Histograms of the confidence placed in each measurement
• Histograms and/or regressions derived from comparing one science parameter to another, statistics may be computed for a subset that satisfies some condition

Level 3 Dark-Target based products are not yet available for VIIRS, but are expected to be available during 2024.  The Level 3 (L3) VIIRS product, known as AERDT_L3, looks similar to MxD08 in that aggregations are performed on a 1°x1° grid.  However, unlike MODIS, the VIIRS DT Aerosol product is provided only at daily (AERDT_D3) and monthly (AERDT_M3) resolutions. Also unlike MODIS, the VIIRS DT product does not include variables from any other retrieval (Deep Blue aerosol, clouds, water vapor, nor atmospheric profiles).  Finally, AERDT_L3 is provided in NetCDF4 (.nc) format. 

Because L3 data are composite products we strongly caution users to have a full understanding of the product content before attempting to use these products in research investigations!  A much more detailed explanation of the level 3 atmosphere products can be found on the MODIS-atmos website.  Links are provided in the following table.

Level 3 Atmosphere Products Summary