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component of biogeochemical processes on land, in the air, and in water. Changes in the oxygen concentration have been routinely used in biogeochemical studies.

Carbon dioxide is an important constituent of interest in the field of air pollution, from both local and global perspectives. Its anthropogenic sources include combustion of fossil fuels and depletion of rain forests. The background concentration of carbon dioxide in the northern hemisphere has risen from approximately 310 ppm in the middle of the 20th century to 369 ppm in 2000.1 This situation is connected to the greenhouse effect and has become an important issue beyond national boundaries. Moreover, enormous amounts of CO are generated by incomplete combustion of carbonaceous fuels such as wood, coal, gasoline, and natural gas. From the human health point of view, the current U.S. Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) for CO is 50 ppm.2 The exposure standard recommended by the U.S. National Institute for Occupational Safety and Health (NIOSH) is 35 ppm with a ceiling value of 200 ppm. The U.S. Environmental Protection Agency (EPA) has set an ambient air quality standard of 9 ppm, averaged over an 8-h period, and 35 ppm for 1 h, not to be exceeded more than once a year. The American Conference of Governmental and Industrial Hygienists (ACGIH) has set a threshold limit value for CO of 25 ppm.3

Methane is the simplest of the hydrocarbon molecules. While it is colorless, odorless, nontoxic, and rather inert, it is one of the greenhouse gases and is estimated to contribute 15% of the greenhouse effect.4'5 Its atmospheric concentration is increasing, even though its rate of increase has declined.6 Naturally occurring methane can be classified as either a thermogenic or biogenic process. The former process refers to the thermal degradation of organic matter at depth within sedimentary basins and is commonly associated with coal and accumulations of petroleum and natural gas. Biogenic methane is produced under anaerobic, near-surface conditions by microbial degradation of organic matter, such as by ruminant animals7 and in rice paddies.8 Terrestrial wetlands are another important source of methane. Approximately 20 to 40% (120 to 200 Tg/year) of total methane emission comes from these wetland areas.9 On the other hand, anthropogenic methane can be derived from natural gas pipeline leakages, oil and gas wells, sewer pipes and septic systems, burial compost, landfill sites, spilled petroleum, and so on.

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