|MadSci Network: Environment & Ecology|
Carbon monoxide is a gas which is very poisonous to people and to most other living creatures. It is produced in significant quantities whenever you have any burning taking place with a slight shortage of air or oxygen.
Carbon monoxide has very little effect "on the atmosphere". It is not a greenhouse gas. It does not accumulate, but is readily degraded. It can reach levels that are dangerously high for people in situations where traffic fumes can accumulate, in poorly ventilated rooms with poorly adjusted gas fires, or in air inhaled from a burning cigarette end. It is generally considered one of the five major primary pollutant classes in episodes of acute urban air pollution.
There is an interesting story about carbon monoxide in the atmosphere. About 30 years ago it was believed that most of the carbon monoxide in the atmosphere came from industry and traffic and domestic fires and incinerators. Calculations were done about the amount of carbon monoxide that would have come from all of these sources. It turned out to be enough to account for the level of carbon monoxide in the atmosphere, if the carbon monoxide lasted for 2-3 years on average before being degraded. It was also discovered that background levels of carbon monoxide varied quite significantly, quite apart from local urban pollution. There was about 3 times as much in the Northern hemisphere as in the Southern, and about 50% more over land than over ocean. Carbon monoxide levels are about 20 parts in 100 million in the Northern hemisphere, and about 7 parts per hundred million in the Southern hemisphere.
At first sight everything seems to fit, but it doesn't really. If carbon monoxide really had an average residence time of 2-3 years, it would not be possible to have such different levels, because there is plenty of time for air currents to mix up the air much more thoroughly. What the uneven distribution meant was that the average residence time was 4-5 months at most, and that meant that the total amount of carbon monoxide being generated was at least 5 times what was being produced by all of the obvious human sources -- traffic, industry, fires, incinerators. And yet the distribution of the carbon monoxide matched the distribution of human population. Levels were highest near areas of large population and industry.
The solution turned out to be a very reactive chemical known as hydroxyl radicals. Hydroxyl radicals are generated from other pollutants, and their concentration is much higher near concentrations of traffic and industry than in wilderness areas. Hydroxyl radicals have many reactions, but one of the important ones is to react with methane in the atmosphere, to initiate a sequence that eventually leads to carbon monoxide. Methane levels in the atmosphere are around 170 parts per hundred million, easily enough to account for all of the carbon monoxide. Interestingly, another important reaction of hydroxyl radicals is to attack carbon monoxide, transforming it into carbon dioxide. This turns out to be the main way that carbon monoxide is destroyed in the atmosphere. The quantities involved are very small compared to natural carbon dioxide levels, so it does not significantly add to any problems caused by carbon dioxide.
Because carbon monoxide is both created and destroyed by hydroxyl radicals, it is unlikely that its concentration could ever build up on a global scale to a level that would cause concern. It can build up to high levels in urban pollution episodes, where it is produced by the other sources -- traffic, industry, fires, incinerators -- that account for about 15-20% of total carbon monoxide production. But because hydroxyl radicals are also produced in these situations, and because hydroxyl radicals destroy carbon monoxide as well as creating it, any build-up is fairly rapidly dissipated.
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