Pollution, not rice accumulated methane

Published: Monday 15 June 1992

THOUGH a great deal of fuss has been raised in scientific circles over methane production by rice paddies and cattle in the developing world, which is likely to heat up the earth, the atmospheric chemistry of methane is still a grey area. Scientists-are finding that it is quite possible that the increase in methane accumulation in the atimasphere could be partly due to a decrease in methane sinks - the natural cleansing mechanisms in the atmosphere - rather than an increase in methane sources.

Methane is an important green-house gas with a high global warming potential. Its present atmospheric concentration of 1. 72 parts per million by volume (ppmv) is more than double its pre-industrial presence. But the rate of methane accumulation in the atmosphere, unlike that of carbon dioxide, has decreased from about 20 ppbv per year in the late 1970s to about 13 ppbv per year in 1989. While a number of hypotheses have been put forward to explain the decrease, the United Nations' intergovernmental Panel on Climate Change finds none totally satisfactory.

Methane is removed from the atmosphere primarily by its reaction with the hydroxyl ions in the lower atmosphere. The hydroxyl radical is highly reactive and acts as an atmospheric cleanser capable of breaking down a number of pollutants, including methane. Less hydroxyl means more methane. Methane is also removed by soils and through photochemical reactions in the upper atmosphere. Some 340 million tonnes to 500 million tormes of methane are removed when it reacts with the hydroxyl ions, but there is as yet some ambiguity about the rate at which methane reacts with the ions.

Pollution in the atmosphere, especially the presence of nitrogen dioxide which is produced by burning fossil fuels, can lead to a depletion of hydroxyl ions. Recent investigations have revealed that hydroxyl depletion over the northem hemisphere is twice as bad as before and four times as bad as in the less-polluted southern hemisphere. Jim Kao, an atmospheric scientist with the Los Alamos National Laboratory in USA, says the higher depletion over the northern hemisphere is linked to the heavier industrial pollution there.

Methane is oxidised on reaction with the hydroxyl ions in the troposphere. A positive feedback mechanism is set up. The oxidation of methane leads to a decrease in hydroxyl ion levels. This in turn results in slower atmospheric loss of methane and a. longer lifetime. Increased emissions of nitrogen oxides, however, lead to an increase in hydroxyl ions, reducing the lifetime of methane.

The political implications of this study are obvious: factories and not rice paddies bear an increased responsibility for methane pollution.

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