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Overview & User's Guide

Introduction

There are several NASA remote sensing aerosol products available for research and public use.  This users guide will focus on the set of aerosol products created using the Dark Target (DT) algorithm(s).  The DT products have been in use since shortly after the launch of the Terra MODIS sensor in 1999.

This guide will focus on the DT products, beginning with MODIS and later expanding to include other products, and their proper usage but will not go into detail on the DT algorithm. 

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

A comprehensive description of the MODIS DT algorithm, the Algorithm Theoretical Basis Document (ATBD), is available in the ATBD section of this website.

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 MODIS 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  aerosol and atmosphere product names provide information about the product type, content and resolution.

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 stil in their initial release which is collection 1.

Level 2 Aerosol Products

MODIS and VIIRS geophysical products (as opposed to raw data) from a single overpass of the satellite sensor are referred to as Level 2 products. The Level 2 products are made available in files of five minute segments of a single satellite overpass and are referred to as granules. Both the Terra and Aqua satellites have a 16 day orbital cycle.  Granules with the same time designation which are 16 days apart should be observing the same geographic location.

Level 2 Aerosol Products Summary

Product Name Sensor Resolution Product Level Size/Time
MOD04_L2 Terra 10 Km   2  Granule/5 minute
MYD04_L2 Aqua 10 Km   2  Granule/5 minute
MOD04_3K Terra   3 Km   2  Granule/5 minute
MYD04_3K Aqua   3 Km   2  Granule/5 minute

The designation as a 10 km or 3 km aerosol product refers to the resolution of the product (not the sensor!) at the center of the satellite swath.  These products include retrieval parameters over land and ocean. The 10 km product will include data from the Deep Blue retrieval algorithm and a merged dark target-deep blue product.   For more information on the merged product please see our FAQ.  The 3 km product contains only dark target (not deep blue) retrievals over both land and ocean.    

MODIS 10 km Aerosol HDF file names have the following naming convention:

Terra file:   MOD04_L2.AYYYYDDD.HHMM.CCC.YEARDAYHRMNSC.hdf

Aqua file:   MYD04_L2.AYYYYDDD.HHMM.CCC.YEARDAYHRMNSC.hdf

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 (006 for C6, 061 for C6.1), and YEARDAYHRMNSC represents when the file was processed.   3 Km products will follow the same rules but will start with MOD04_3K or MYD04_3K.

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 geophysical global product composites from multiple satellite overpasses are Level 3 products.  Level 3 products also contain parameters relating to cloud properties, water vapor and atmospheric profiles.  

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

Because these 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

Product Name Sensor Resolution Product Level Size/Time
MOD08_D3 Terra  1 Degree  3  Global / 1 Day
MYD08_D3 Aqua  1 Degree  3  Global / 1 Day
MOD08_E3 Terra  1 Degree  3  Global / 8 Day
MYD08_E3 Aqua  1 Degree  3  Global / 8 Day
MOD08_M3 Terra  1 Degree  3  Global / Monthly
MYD08_M3 Aqua  1 Degree  3  Global / Monthly