The MODIS 3 km Aerosol Product monitors the ambient aerosol optical properties (e.g., optical thickness and size distribution), mass concentration, look-up-table-derived reflected and transmitted fluxes, as well as quality assurance and other ancillary parameters, over the oceans globally and over a portion of the continents.
With Collection 5 and earlier collections, only one aerosol product existed (MOD04_L2), at 10 km resolution, because its original aim was for climate studies, and the 10 km resolution worked well for global analyses. However, the air quality community had been requesting higher resolution data for quite some time to help resolve urban and neighborhood air quality. Thus, in Collection 6, the Dark Target (DT) aerosol algorithm is used to create a new 3 km aerosol product (MOD04_3K).
The MOD04_3K product is based on the same algorithm and using the same Look Up Tables as the standard Dark Target aerosol product. Because of finer resolution, subtle differences are made in selecting pixels for retrieval and in determining QA. The only differences between the existing 10 km algorithm and the new 3 km algorithm are:
- the size of the pixel arrays defining each retrieval box (6x6 retrieval boxes of 36 pixels at 0.5 km resolution for 3 km algorithm as opposed to 20x20 retrieval boxes used for 10km product)
- the minimum percentage of "good" pixels required for a retrieval (a minimum of 5 pixels over ocean and 6 pixels over land, instead of a minimum of 10 pixels over ocean or 12 pixels over land)
- the 10km algorithm attempts a "poor quality" retrieval when the number of available pixels after screening is small. The 3 km algoritm will not attempt a retrieval under these circumstances. This means that with the 3 km product, there will be no retrievals with a QA of 1 o 2 over land.
Other than these differences, everything else in the two products is the same. For additional details on the differences see the MODIS Atmospheres web site page on Collection 6.1 Updates.
Preliminary analysis indicates that the 3 km product tends to be noiser than the 10 km product. Comparisons of the global mean AOD from the two products shows that the 3 km AOD is 0.01 to 0.02 higher over land. Over ocean, it was found that there was more very low AOD with the 3 km product.