An alternative class of pollutants of considerable current interest are the long-lived globally mixed gases principally responsible for climate change and stratospheric ozone depletion: long-lived greenhouse gases (LLGHGs) such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fall into the former category, whereas chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) make up the latter (although most CFCs/HCFCs are also potent greenhouse gases on a molecule-for-molecule basis). These species are measured at a number of global monitoring stations, which operate under the authority of international programs such as the Advanced Global Atmospheric Watch program, and various other national programs. A number of manned/semiautonomous stations make measurements of these gases in situ, and flask samples are also collected at other stations and returned to base laboratories for periodic analysis – an approach only possible for the nonreactive, long-lived species under consideration.