A number of research groups from France (LGGE, LSCE and LaMP), Italy (ISAC), Germany (TROPOS), Sweden (SU – ITM) and Switzerland (PSI) have combined their efforts to support the initiative led by the Universidad Mayor de San Andres to install unique infrastructure for documenting the variability of the atmospheric composition at Mt. Chalcaltaya (5240 MSL). The US NASA GSFC has joined this group of institutions helping with implementation of a fully functional lidar system in Cota-Cota, at only 22 km from Chacaltaya. For this region, information on the atmospheric composition is largely unknown due to the lack of observations. Measurements at the station started in December 2011 and include monitoring of gases (CO2, CO and O3), characterization of aerosol particle properties (absorption and scattering coefficients, number size distribution, chemical composition, and small ions) as well as radiation and meteorology. For the first time, an atmospheric observatory provides data on the composition of the free troposphere in the tropics in the southern hemisphere on a seasonal to annual basis. The CHC station is part of the WMO-GAW network.