  In this work we analyze one year observation of an Aeronet (GSFC-NASA Aerosol
Robotic Network) sun-photometer installed on January 11, 2011 in CASLEO and
being operational up to date. The main goal of placing the instrument in this
location is to characterize the aerosol loads of this astronomical complex
which is close and has the same sky characteristics of El Leoncito (31deg
43.33' South - 69deg 15.93' West, 2552 m ASL) one of the southern candidate
site for Cherenkov Telescope Array (CTA). The low aerosol optical depth (AOD)
annual mean of 0.038 measured at 500 nm shows exceptional clear sky quality.
Data is compared with the measurements being done at Mauna Loa (19deg 32.34'
North, 55deg 34.68' West, 3397 m ASL), where Aeronet reference instruments are
being re-calibrated two to four times per year. Long term MODIS observations
are studied, showing that the site is far enough to biomass burning transport
regions to be affected by its influence.
