The ELT project is currently under way in Europe and North America. Astronomical sites depend critically on sky transparency and on aerosol loadings. A quantitative survey of aerosol optical properties at candidate ELT sites is an essential part of the site selection process. There are basically two methods to characterize aerosol properties: ground based measurements and satellite measurements. In this paper we will establish a full climatology of two sites very close to each other, but at a difference of 2300m in altitude: Izaña and Santa Cruz located in the Canary Islands. Both have sun photometers from the AERONET network. We also use the aerosol index determined from TOMS satellite data to determine how aerosol optical properties vary with altitude. We establish a correlation between the TOMS index and the aerosol optical thickness in both sites. Aerosol optical properties show very good correlation between Izaña and Santa Cruz. As a result we establish a set of relationships helpful to characterize sites at elevated altitude from data of neighbouring sites at low altitude.
