The LTER/NASA Sun Photometer collaboration is part of the larger NASA AERONET sun photometer project at NASA/GSFC.  The automated robotic instruments gather data to determine amounts of aerosols, dust and water vapor in the atmosphere.  Sun photometers have been in placed at more than 10 LTER sites on a partial or permanent basis starting in 1993. 

Access to data and further information regarding sunphotometers can be found at the NASA AERONET homepage at NASA/Goddard.
The LTER/NASA Sunphotometer collaboration is closely associated with the atmospheric correction project/collaboration:  http://www.lternet.edu/technology/nasa/atmcor
Further information regarding LTER sunphotometers can be found at the Sevilleta (SEV) LTER sun photometer homepage