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.
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.