Some hydrocarbons are natural components of the atmosphere. For example, most of the atmospheric methane is produced by anaerobic bacteria in swamps and bogs, and in the stomachs and intestines of cows, sheep, and termites; a much smaller percentage is released by natural gas wells. It has been proposed that many of the hydrocarbons found in the Gulf of Mexico may be due to the marsh plants indigenous to the Gulf Coast. Many plants, especially conifers and citrus, release terpenes into the atmosphere. It has been suggested that the

6-day air pollution alert in the Washington, DC, area in August 1974 was due mainly to the presence of terpenes originating from plants in the Appalachian Mountains. In the absence of any man-made pollution, there would be still parts-per-million concentrations of methane (Table 2-1) and parts-per-billion amounts of ethane, ethylene, acetylene, and propane in the atmosphere.

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