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BIGGER efforts are needed to curb emissions of carbon monoxide, atmospheric
chemists warn this week. They say that the pollutant is adding to global warming
by extending the lifetime of greenhouse gases, especially methane. The finding
will add impetus to calls for treaties such as the Kyoto Protocol to include
controls on pollutants that may have indirect climatic effects.

Vehicles, industrial plants and burning vegetation spew out carbon monoxide
and other toxic chemicals. For instance, the amount of carbon monoxide in the
air has tripled since pre-industrial times. Now Oliver Wild of Frontier Research
System for Global Change in Yokohama, Japan, and colleagues have investigated
the effects of these pollutants on global warming.

Wild and his team found that carbon monoxide destroys large amounts of the
hydroxyl radicals that occur naturally in the atmosphere, where they break down
methane and other pollutants. So reducing emissions of carbon monoxide benefits
the climate as well as health.

But other toxic chemicals pose a dilemma for pollution regulators, Wild has
found. Reducing emissions of nitrogen oxides (NOx) could have the surprise
effect of boosting global warming. Although NOx promotes the formation of ozone,
which is a greenhouse gas, it also leads to the production of hydroxyl, a
greenhouse-gas guzzler.

If pollution regulators stick with their policy of being tougher on NOx than
on carbon monoxide, they may end up killing off a key cooling effect and 鈥渢ip
the radiative balance in favour of warming鈥�, says Wild. 鈥淲arming from additional
ozone lasts little more than a year, but the cooling from less methane is still
visible 40 to 50 years later.鈥�

However, there is another twist. At high levels of pollution some of the
reactions involving NOx go into reverse. 鈥淏eyond certain threshold values [for
NOx], hydroxyl can decrease catastrophically,鈥� says Sasha Madronich at the US
government鈥檚 National Center for Atmospheric Research in Boulder, Colorado,
(see Global Environment Supplement with this issue, p 9).
This means we need to be careful how we reduce pollution, says Wild,
and pay attention to the details of atmospheric chemistry.