Other Greenhouse Gases Methane Absorption and Sinks
After carbon dioxide and water, methane, CH4, is the next most important greenhouse gas. A methane molecule contains four C—H bonds. Although C—H bond-stretching vibrations occur well outside the thermal 1R region,
HCH bond-angle-bending vibrations absorb at 7.7 fim, near the edge of the thermal ir window; consequently, atmospheric methane absorbs ir in this region.
In contrast to the century-long lifetime of carbon dioxide emissions, molecules of methane in air have an average lifetime of only about a decade. As discussed in Chapters 3 and 5, the dominant sink for atmospheric methane, accounting for almost 90% of its loss from air, is its reaction with molecules of hydroxyl, OH, the very reactive gas present in air in tiny concentration:
CH4 + OH-> CH3 + HzO
This reaction is the first step of a sequence that transforms methane ultimately to CO and then to C02 (see Chapter 5 for details).
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