Oh Ch4 H2o Ch3

Using estimates of global OH levels based on analysis of global concentrations and emissions of the industrial chemical CH3CQ3 leads to an estimated methane loss rate due to OH of around 500 Tg y-1 (Prather et al. 2001; Prinn et al. 2001). Note that because CH4 is a significant global sink for OH, the levels of OH decrease when CH4 increases, yielding a longer lifetime for methane. About 40 Tg y-1 of CH4 is destroyed in the stratosphere, and 20—40 Tg y-1 currently accumulates in the atmosphere (producing the observed trend). The remaining 20—40 Tg y-1 sink needed to balance the total source is probably methanotrophs in soils.

Although these numbers give the impression that the CH4 budget is well understood, it is not. The estimates of individual CH4 source strengths are very uncertain (up to a factor of two), and the reasons for the general decrease in the CH4 trend in recent decades and the large year-to-year variations in this trend continue to be debated (see the section "Unsolved Problems").

Nitrous Oxide

Nitrous oxide is important both as a source of NO, which destroys ozone in the stratosphere, and as a potent greenhouse gas like CH4. It has risen quite steadily in the atmosphere at about 0.23 percent y-1 (3.5 TgN y-1) since 1978 (Prinn et al. 2000).

The major sources of N2O involve nitrogen bacteria in soils (tropical, temperate) and the ocean (open, coastal). Additional sources involve nylon and nitric acid production, cattle and their feedlots, and atmospheric oxidation of NH3. Like CH4, the magnitudes of these individual sources are very uncertain. Best estimates of the total of these sources by various authors are around 15-17 TgN y-1 (Prather et al. 2001).

The major sink for N2O is photodissociation, and, to a much lesser extent, reaction with O(1D), in the stratosphere:

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