Both of these variables are merged land and ocean AOD at 0.55 µm from the dark target algorithms. The “Optical_Depth_Land_And_Ocean” SDS only contains high quality pixels (QA=3 over land, QA > 0 over ocean). The “Image_Optical_Depth_Land_And_Ocean” SDS contains all valid (non-fill) pixels; this product can be used to trace aerosol plumes and make aerosol maps, but should not be used for quantitative study.

Ocean and land products use completely different algorithms. For any dark target retrieval algorithm we must accurately account for surface effects and build correct models of the aerosols.  Due to the complexities of the land surface it is more difficult for the land retrieval to account for surface effects. In addition aerosol models are fixed for the land product based on season and location.  The ocean product selects coarse and fine mode models dynamically.

These two factors account for several differences in the two products.

Quality (QA) flags are an indicator of the algorithm team’s assessment of the quality of the data.  QA flag values range from 0 – 3 where 0 is lowest quality. We advise using only highest quality data (QA=3) for land products.  For ocean products we advise using anything above QA zero.  In collection 6 data this information is included in the SDS “Land_Ocean_Quality_Flag”.