MODIS Surface Reflectance Data State QA Aerosol Quantity field meaning is unclear I am trying to compare NDVI values computed from MODIS Surface Reflectance(MOD09) with the NDVI product (MOD13).

MODIS Surface reflectance Collection 5 products include a Data State QA field that I am using to estimate which pixels are best suited for computing NDVI.

A part of this field is labeled Aerosol quantity. The values in this field seem to significantly affect the result of NDVI computation.

The explanation of the possible values of this field in the MOD09 Users' Guide  seems not entirely clear to me.

         | 00 - climatology
aerosol  | 01 - low
quantity | 10 - average
         | 11 - high


The last three values are clear, while the meaning of the climatology value is somewhat non-intuitive.

I have selected a crop field and calculated NDVI for all pixels clear of clouds or cloud shadows during one vegetation period and observed the following:



It seems that results with low aerosol quantity (green points) match well with the results obtained with the complex rating algorithm used in the standard NDVI MOD13Q1 product. While the data with climatology state (orange points) seems far off.

What does the climatology attribute mean and how to treat it?
Most likely, climatology means there was no retrieval at all.

Bayesian retrievals combine information from an a priori with information from measurements.  When the retrieval fails for whatever reason, or the measurement contains insufficient information, instead of reporting no measurement at all, they copy over information from the a priori and use the flag to indicate that the reported "measurement" is in fact simply the climatology.

For details, see optimal estimation, in particular the book by Rodgers:

Clive D. Rodgers (2000). Inverse Methods for Atmospheric Sounding: Theory and Practice. World Scientific.