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Prof. Bothun,

In answering your question, methane is removed from the atmosphere by
reacting with atmospheric trace species such as OH, O(1D), and Cl. The
atmospheric lifetime of methane varies with time and location, depending on
the abundance of the other reactants. In general, within the altitudes
ranging from surface to troposphere, the removal of methane is dominated by
the reaction with OH radical, and yes, the lifetime of methane with respect
to the OH attack is about 10 years on the global average basis. (the
lifetime can be calculated by using 1/k[OH], where k is the rate constant
for OH + CH4, which has been measure to be 4e(-15) cm3 molecule-1 s-1 under
average tropospheric temperature of 277 K, and [OH] (~8e5 molecule/cm3) is
the concentration of the OH radical in the atmosphere. There is a more
sophisticated way of doing this in the modeling community, but the estimate
here should not be too far away). In the stratosphere, methane will be
mainly removed by both OH and Cl species, and as a result the stratospheric
methane liftime will be slightly shorter, say about 7~9 years.

Measurements have shown a declining atmospheric methane growth rate since
early 1980s. The decline of the growth rate in the 30-90N semi-hemisphere
was 2-3 times more rapid than in the other semi-hemisphere. The cause of
this global decline in methane growth is not entirely clear, but could be
related to changes in emissions from fossil fuel (particularly natural gas)
in the former Soviet Union and from biomass burning in the tropics. I
don't think the "threshold effect" theory would explain the methane growth
rate decline. Since the atmospheric lifetime of methane is mainly
determined by the OH abundance, unless there is a dramatic increase in OH
concentration or another unknown methane destruction mechanism, the growth
rate will be mainly governed by the emission from the surface.