I understand that MODIS Terra has 3 chlorophyll products viz. Product 14 Chlor_MODIS, Product 26 Chlor_a_2 and Product 27 chloro_a_3 which have been  calibrated until Jan 2004. In contrast, AQUA has two products viz. MOD 19 and MOD 21. I would like to know how these two products differ and their relationship vis a vis the three Terra products. Also when I go to the Level 3 browser I am able to load monthly SMI images of chlorophyll onto my computer. Which product am I loading - 19 or 21? This question also applies to the ftp site from where I can also download monthly chlorophyll from Aqua. Am I wrong in assuming that chlorophyll products from Aqua are 19 and 21 for I do not see this nomenclature even when I try to order Aqua chl products from the DAAC site. Forgive a confused me!

The default chlorophyll product for Aqua processing is based on the OC3 algorithm.  It is equivalent to what you have known as chlor_a_2, and it is closely related to the SeaWiFS default chlorophyll (OC4).  The OC3 product is the only chlorophyll product currently being generated for Aqua. 

We don't use the same nomenclature because we don't use the same code or file formats. 

The Aqua products currently available from the DAAC are based on an early calibration which is known to show significant temporal drift.  The Aqua products available from the OceanColor Web superecede those at the DAAC, and any new or improved Aqua ocean color products will only be distributed through the OceanColorWeb for the foreseeable future.

The Terra products available from the DAAC are currently the best available.  At some future date Terra production for ocean color will likely resume, with initial distribution via the OceanColorWeb using the same product suite and file formats of the new Aqua products.

The final product suite for Terra and Aqua ocean color products is likely to evolve as a result of community feedback and recently established algorithm working groups. See http://oceancolor.gsfc.nasa.gov/forum/oceancolor/topic_show.pl?tid=72 for more details.