From the description at http://grass.osgeo.org/grass64/manuals/i.atcorr.html you can enter a value for AOD at 550 nm, which you can get from MODIS. It does not seem you can make that value variable if you consider a large area, though.

You could also try the online version of 6S:

http://6s.ltdri.org/cgi-bin/atmos.cgi?ID=97.103.196.25.ZYBGpZuRfM

Dear Chris,

I try to use S6 online version, but whatever I input,I received no result. (just see blank when I click on the result link).

Please find the input file in this attachment