The atmospheric cycle of methane


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    The National Center for Atmospheric Research is sponsored by the National Science Foundation.


The atmospheric distribution of CH4 is rather uniform zonally and vertically but exhibits a slight gradient with latitude in the troposphere; in the stratosphere it shows a definite decrease with altitude. The average volume mixing ratio in the troposphere is 1.4 ppm which corresponds to a total amount of 4 × 1015 g of CH4 present in the atmosphere. Most is of recent biologic origin. C14 analyses show that no more than 20% is released by fossil sources. The various ecosystems producing CH4 are discussed and the total annual production is estimated to lie between 5.5 × 1014 g/yr and 11 × 1014 g/yr. The corresponding turnover times for atmospheric CH4 range from 4 to 7 yr. The destruction of CH4 takes place mainly in the troposphere, most probably through the reaction CH4 + OH → CH3 + H2O. About 10% of the CH4 is destroyed in the stratosphere. The CH4 cycle is strongly coupled to the H2 and CO cycles and contributes also on the order of 1% to the atmospheric carbon cycle.