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- By avmehta Date 2020-09-22 20:22
    I am  trying to use all available optical sensor data in addition to MODIS [i.e. Landsat OLI, Sentinel 2 MSI,  Sentinel 3 OLCI ] for a Chesapeake Bay water quality monitoring program.
         I have been testing various l2gen options for OLI.  When  aer_opt  is set to -2,  the ch-a statistics differ substantially from  cases when aer_opt is -1.  I am not sure how to choose  aer_opt for this processing.  I have not tried any other options.
         Another question, especially for OLI, MSI, and MODIS is about using SWIR for  aer_short and aer_long. There are  more missing pixels  when SWIR bands are used - compared to default but the ch-a statistics are not too different.
          Are there rules on how to decide these options?  Particularly, aer_opt.
Thank you.
- By seanbailey Date 2020-09-22 22:04
There are no rules, but some options are more (or less) applicable depending upon the specific needs of the user.  That said, setting aer_opt=-1 does not enable the NIR correction (as is done with -2 ...and -3, which is effectively the same as -2 now), and should not be used over waters for which the black pixel assumption is violated.   The waters of the Chesapeake definitely violate that assumption. 

As for using  the SWIR bands, Werdell et al., (2010, details some of the limitations of the SWIR channels on MODIS for atmospheric correction....with a particular focus on the Chesapeake.   It boils down to an issue of inadequate signal to noise.   The  OLI and MSI instruments are focused on land applications and so the SNR is rather low and the bandpasses rather wide for ocean color applications,  but they can be of use if these limitations are understood and accounted for when these data are used. 

- By avmehta Date 2020-09-24 02:24
Hello Sean,
  Thanks so much for the explanation.  Is my understanding correct that no matter what aer_opt I use (or use -99 for no correction) Rhos values will not be affected by it?
Thanks again for your help.
- By seanbailey Date 2020-09-24 12:52
If all you want is rhos, you can set atmocor=0, since no aerosol subtraction is performed for that yes, the result would be the same regardless of the aer_opt setting
The processing will be faster with atmocor=0, but if you need any products that require Rrs, you cannot use this setting (Rrs requires aerosol subtraction)

- By avmehta Date 2020-09-24 13:11
Thank you very much Sean.
- By avmehta Date 2020-09-30 20:14
  I just want to confirm that even when I use aer_opt=-99, Rhos will include correction for gases. Or do I have to specify this?
Thanks much.
- By seanbailey Date 2020-09-30 20:17
rhos is rhos.  period.  aer_opt only affects the aerosol subtraction, it has no impact on gaseous species corrections.


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