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. In the range of (0.2 < NDVI <0.3), land if only one of the two algorithms produces a high QA value, that value is used in the product. If both retrievals produce a high QA value, the dark target and deep blue values will be averaged.
The advantage of the merged product is its ease of use to obtain the greatest global coverage of AOD.
There are two potential risks in using this product:
1) The values where dark target and deep blue are averaged may be less accurate than either the deep blue or dark target areas.
2) The default assumptions that dark target will always be the superior product where NDVI > 0.3 and that deep blue will always be the superior product where NDVI < 0.2 may not always hold true.