The major sink for CH4 is the OH radical throughout the troposphere and stratosphere (lowest 25 km of the atmosphere). Chlorine and O(aD) atoms play an important role in the removal of CH4 in the mid and upper stratosphere. A simple loss rate analysis confirms that this is the case qualitatively, and inspection of the ratios of 13CH4/12CH4 and 12CH3D/12CH4 allows one to quantify the contributions more precisely. Changes in the hydroxyl radical concentration over the last 20 years are estimated and future trends in the three major sinks are discussed.
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