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Other Indirect Greenhouse Gases - Carbon monoxide

Carbon monoxide (CO) is only a very weak direct greenhouse gas, but has important indirect effects on global warming. Carbon monoxide reacts with hydroxyl (OH) radicals in the atmosphere, reducing their abundance. As OH radicals help to reduce the lifetimes of strong greenhouse gases, like methane, carbon monoxide indirectly increases the global warming potential of these gases.

Carbon monoxide in the atmosphere can also lead to the formation of the tropospheric greenhouse gas 'ozone'. Atmospheric concentrations of carbon monoxide vary widely around the world and throughout the year, ranging from as low as 30 parts per billion up to around 200 parts per billion. Concentrations increased during the 20th century, but there are some signs that concentrations dropped slightly in the 1990s due to widespread use of catalytic convertors, with their lower carbon monoxide emissions, in cars.

Aside from man-made sources, a great deal of carbon monoxide comes from the chemical oxidation of methane and other hydrocarbons in our atmosphere. Additional natural sources include emission from vegetation and the world's oceans. By far the largest sink for carbon monoxide is its reaction with OH in the atmosphere, as noted previously. However, a small but significant amount is also lost from the atmosphere through deposition on the ground.

Human Impact

Today more than half of carbon monoxide emissions are man-made. The highest concentrations of carbon monoxide tend to occur close to areas of high human population. On a global scale, this has meant that the more densely populated northern hemisphere has higher concentrations of carbon monoxide than the southern hemisphere. Biomass burning and fossil fuel use are the main sources of man-made carbon monoxide emissions.

Potential for control

As with many direct and indirect greenhouse gases, reductions in carbon monoxide emissions can most effectively be made through direct reductions in fossil fuel use. There is some evidence that the widespread use of catalytic convertors in cars has significantly reduced carbon monoxide emissions from this source. However, such reductions must be balanced against the increased emissions of the greenhouse gases carbon dioxide and nitrous oxide which often result from a switch to catalytic convertors.

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