NASA’s ARSET site has a very nice overview of imagery and a summary table/chart of where to obtain images.  You can find that information here: 

ARSET Satellite Imagery.   You can also find some informaiton on our site in the links page under "Imagery Links"

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.