Methane is an important greenhouse gas that is emitted at the Earth's surface through biological activity in swamps, marshes, rice paddies, lakes and oceans. It is also a product in emissions from agriculture, animal husbandry, mining, landfills and volcanoes. Tropospheric methane concentrations have increased from about 0.70 ppmv pre-1980 to 1.75 ppmv in 1998 (IPCC 2001). About half of the current emissions are of anthropogenic origin. Methane is removed from the atmosphere primarily through oxidation by tropospheric OH and subsequent deposition of its products at the Earth's surface, and in the case of its constituent hydrogen (both atomic and molecular forms) by escape to space at the top of the atmosphere. Methane's photochemistry is thus important as it affects stratospheric water-vapour levels, molecular hydrogen and CO levels.
Methane oxidation is initiated by its reaction with OH and follows a sequence of reactions that lead to the production of formaldehyde, CH2O, that is photolysed to CO and molecular hydrogen. One important path is
Other possible paths are given in Appendix B, Table B.8. Reaction of CH3O2 with NO produces NO2 that photolyses to produce atomic oxygen as a net effect and is thus an important source of ozone in the lower stratosphere (Crutzen 1971). Methane is thus destroyed in the lower atmosphere by oxidation, as shown in Fig. 7.13. It then undergoes a second major decrease above 80 km due to photolysis and reaction with O(XD).
The atmospheric H2 has been examined for many years with detailed measurements made by many workers. The measurements reported by Novelli et al. (1999) give a global mean H2 mixing ratio within about 1% of 530 ppbv, with an
CH4 + OH ^ CH3 +H2O, CH3 + O2 + M ^ CH3O2 + M, CH3O2 + HO2 ^ CH3OOH + O2, ch3ooh + oh ^ CH2O + OH + h2o
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