Atmospheric methane sinks

Shallcross et al. (Chapter 11, this volume) describe how CH4 Earth's atmosphere.

describe how CH4 is removed from the

The dominant route of removal is by oxidation by OH. This reaction is very important in the atmosphere as it not only limits the radiative forcing potential of CH4 but also contributes to the production of peroxy radicals, which can subsequently lead to the formation of ozone. CH4 oxidation therefore affects the oxidizing capacity of the lower atmosphere.

The reaction of CH4 with OH is:

Although this initial reaction produces methyl (CH3) radicals and water (H2O), the complete reaction involves a number of further reactions and leads both to increased water vapour and to production of CO2. Another indirect effect of the reaction between CH4 and OH is that it can magnify the effects of other pollutants, with increased CH4 in the atmosphere meaning fewer OH radicals and so less oxidizing power in the atmosphere as a whole.

Some additional CH4, ~40 Tg/year, is accounted for in the same way, but in our stratosphere. There are also several relatively minor sinks for CH4, including chemical oxidation by chlorine in the atmosphere and in the surface waters of our seas.

Overall the direct human impact on the atmospheric destruction of CH4 is relatively minor. However, our emissions of other atmospheric pollutants, such as nitrogen oxide (NOx) gases, may reduce the levels of OH radicals in our atmosphere, thereby prolonging the lifetime of CH4 in our atmosphere.

Because of the difficulty of directly measuring OH radical concentrations in the atmosphere (it is not yet possible to measure OH on a global scale, hence it is not possible to know with certainty what is the global average OH), models are used to calculate global OH concentrations. In constructing such models it becomes apparent that the atmospheric chemistry and physics leading to the formation and removal of OH are very complicated and include many feedbacks. Hence, understanding and predicting the rate of removal of CH4 from the atmosphere is not as simple as it might first appear.

Shallcross et al. (Chapter 11, this volume) describe how changes in the isotopic ratios of 13CH4/12CH4 and 12CH3D/12CH4 allow quantification of the various possible routes of removal and explain the fragility of the Earth's greenhouse: changes in the abundance of OH will necessarily result in changes to the radiative forcing due to CH4.

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