An ultra-pure gas system for the CLOUD experiment at CERN

The Cosmics Leaving Outdoor Droplets (CLOUD) experiment aims to recreate atmospheric conditions inside a large chamber where the mechanisms for the formation of aerosol and cloud are studied. Aerosol and cloud are recognized as representing the largest uncertainty in the current understanding of climate change together with the influence of the cosmic rays. The CLOUD chamber, a 26 m^3 cylinder, has been built to the highest technical standard of cleanliness. A key aspect of the CLOUD experiment is the gas system. It is made of several units with specific function and it was built following the highest cleanliness standard as the chamber. A detailed description of the gas system units is given in the present contribution. Data taking periods are scheduled two times per year and important physics results have been already obtained by the CLOUD collaboration.