How to determine aerosol model value for i.atcorr in GRASS?

The i.atcorr module in GRASS requires aerosol model data to perform atmospheric correction using the 6S algorithm.

It seems like I need a value from one of these parameters to continue:

• Estimate of the meteorological parameter visibility v [km], or


• The aerosol optical depth (AOD) at 550nm

Is there an easy (and free) way to get such data?

**EDIT (new findings):

It seems that a NASA website called Space-Based Measurements of Ozone and Air Quality in the Ultraviolet and Visible got the free aerosol model data. It is organized by date and by the instruments that collected the data. Basically:

• OMI (Ozone Monitoring Instrument) is for data dated July 2004 onward


• TOMS Earth Probe is for data dated 7/25/96 - 12/31/05



• TOMS Nimbus-7 is for data dated 11/1/78 - 5/6/93

I'm processing a Landsat scene dated in year 2000-2001, so Toms Earth Probe data is the way to go. But as stated by @Tobi in an answer below, TOMS Earth Probe data is actually an index, and unitless (not really AOD at 550nm which I want in the 6S algorithm).

But I've stumbled upon this called A Long-Term Record of Aerosol Optical Depth from TOMS Observations and Comparison to AERONET Measurements, and they hinted how the data in TOMS observation can still be used.

I need more time to learn how to do this. Welcome any further input, always..

Answer

Nasa's Aeronet provides free AOD data from sensors around the world.