Noble gas in the United States alone. For large-scale use, helium is extracted by fractional distillation from natural gas, which can contain up to 7% helium. Neon, argon, krypton, and xenon are obtained from air using the methods of liquefaction of gases, to convert elements to a liquid state, and fractional distillation, to separate mixtures into component parts. Helium is typically produced by separating it from natural gas, and radon is isolated from the radioactive decay of radium compounds. The prices of the noble gases are influenced by their natural abundance, with argon being the cheapest and xenon the most expensive. As
Global Atmosphere Watch warning system for chemical or physical changes in the Earth's atmosphere which could be cause for environmental concern. Such changes might involve a change in ozone, and therefore ultraviolet, levels, levels of greenhouse gases, or precipitation chemistry, the culprit in the world's acid rain woes. The GAW consists of a coordinated system of various components, prominent among which are: More than 65 countries currently host and operate the GAW's global or regional measurement stations. There are also "contributing stations" that furnish additional data. Lately, satellite programmes have also become important to the GAW, providing atmospheric data that complement ground measurements.