Perturbation of Oxidant Cycles in the Troposphere

The global increase of tropospheric O3, CO, and CH4 concentrations, which is expected to continue in the future, is an indication of a fundamental change in the chemical behavior of the troposphere. Many gases, particularly hydrocarbons, are continuously emitted into the atmosphere from natural and anthropogenic sources. A buildup of these gases in the atmosphere is prevented by a self-cleaning mechanism, whereby these substances are slowly combusted photochemicaUy to COz- 

The key molecule responsible for this oxidation process is OH. The reaction chains involved are such that OH is consumed when the concentration of NOx is low. This is the normal condition of most of the unpolluted troposphere. On the basis of the observed increase of CO and CH4, it has heen suggested that global decreases in OH, the primary sink for CH4 and CO, could lead through a feedback mechanism to a further increase in CO and CH4, and that this situation could produce an unstable chemical condition [56]. Injection of large amounts of NOx from biomass burning and other anthropogenic activities may counteract this feedback, because hydrocarbon oxidation in the presence of elevated amounts of NOx creates additional O3 and OH. This counter effect is, however, much more regionally limited because of the much shorter residence time of NOx compared to that of CO and CH4. 

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