Level 2 data are Geophysical parameter data. For MODIS this is distributed in 5 minute pieces referred to as granules. Level 3 processing produces Earth-gridded geophysical parameter data, which have been averaged, gridded, or otherwise rectified or composited in time and space. For more information on the overall structure of MODIS files please see the FAQ on the Ladsweb site.
The merged product is a single SDS (AOD_550_Dark_Target_Deep_Blue_Combined) comprised of only high quality dark target (QA=3 over land, QA > 0 over ocean) and deep blue (QA 2 & 3) data to produce a global 10 Km product. Over ocean this product uses only dark target retrievals. Over land, monthly NDVI maps are used to assign which retrieval will fill the merged SDS. Over bright surfaces (NDVI < 0.2) the deep blue value is selected. Over dark vegetated surfaces (NDVI > 0.3) the dark target value is selected.
For advanced users it is best to know the details of the validation of each product and the locations and seasons where each product has the best performance. Additional information can be found here: http://onlinelibrary.wiley.com/doi/10.1002/2014JD022453/abstract
The ocean algorithm attempts to match calculated reflectances to measured reflectances. To do this the ocean algorithm selects all combinations of the four fine mode and five coarse mode aerosol models and varies the amount of each to find solutions within pre-defined error limits. All solutions within the error limits are used to calculate an “average” value. The solution with the lowest error is used for the “best” value. We recommend that users select the “average” value for ocean products.
MODIS data is in HDF 4 format. HDF files contain both data and metadata so there is no need to download separate data and data location files. Most programming languages and software programs will have the capability to read MODIS HDF files.
A Tutorial for reading HDF4 files in python, C, FORTRAN, IDL, and Matlab are available here from Lille University.
MODIS Aerosol HDF file names have this 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 four digit year, three digit Julian day, and time of day in UTC, CCC is the collection (005 for Collection 5, 051 for Collection 5.1 and 006 for Collection 6), and YEARDAYHRMNSC represents when the file was processed.
The MOD04 and MYD04 files include many parameters, each stored as a Scientific Data Set (SDS) within the file.
The ladsweb FAQ http://modaps.nascom.nasa.gov/services/faq/ provides a tremendous amount of information about MODIS files, file structure etc. including several methods to obtain large numbers of files.
MODIS data for atmospheric products is available from several NASA websites. The primary location is the ladsweb site http://ladsweb.nascom.nasa.gov/
Within ladsweb there are several ways to access the files depending on your needs. A sample exercise that will guide you through most of these is available on our educational and reference materials page.