Air chemistry and terrestrial gas emissions: a global perspective

The troposphere is that atmospheric region which occupies the lowest 8-13 km or so, between the Earth’s surface and the tropopause, the boundary with the stratosphere. It contains the bulk of the trace gas burden of the atmosphere and has an active chemistry which removes many of the trace gases and pollutants emitted by both terrestrial processes and by human activities. The hydroxyl radical, OH, plays an important role in cleansing the troposphere by oxidizing trace gases to harmless products or to those more readily removed from the atmospheric circulation. The distribution of hydroxyl radicals defines the oxidizing capacity of the troposphere and is itself controlled by the trace gas composition and hence by terrestrial gas emissions. This review identifies the important roles played by the terrestrial source gases: methane, carbon monoxide, oxides of nitrogen and isoprene in controlling the fast photochemical balance and oxidizing capacity of the troposphere.