Carbon monoxide (CO) is a colorless, odorless, tasteless, and highly toxic gas composed of an atom of carbon and an atom of oxygen chemically bonded together. It is useful in the production of a wide variety of chemicals in various industries, including the automotive, construction, agrochemical, cosmetics, pharmaceutical, plastics, and textile industries. Environmental CO is primarily produced from the incomplete combustion of carbon-containing materials.

Anthropogenic (human) sources of CO include incomplete combustion of fossil fuels in internal combustion engines, from which it is released in automobile exhaust; industrial plant exhaust, including exhaust from industry oxidation of hydrocarbons; cigarette smoke; burning of biomass; and various fuel-burning household appliances, including wood-burning stoves, water heaters, clothes dryers, furnaces, fireplaces, generators, and space heaters. CO released in automobile exhaust accounts for about 60 percent of all U.S. CO emissions. Moreover, such automobile exhaust can represent up to about 95 percent of all CO emissions in U.S. cities. Natural sources of CO include coal mines, forest fires, volcanoes, vegetation, soil (including water-saturated areas such as wetlands), the ocean, and atmospheric oxidation of hydrocarbons.

In the United States, CO is considered to be the leading cause of death from poisoning. CO is toxic in that it interferes with delivery of oxygen in the body. Normally, oxygen binds to a blood protein called hemoglobin, which then transports the oxygen throughout the body. CO has a higher affinity for hemoglobin than does oxygen. Therefore, when CO is inhaled, it binds to hemoglobin, displacing oxygen or preventing it from binding and thereby preventing the hemoglobin from delivering the oxygen to the cells that need it.

CO affects global warming through its ability, either directly or indirectly, to increase the levels of other gases in the atmosphere. Such other gases, including carbon dioxide (CO2), methane (CH4), and ozone (O3), directly affect global warming. After the Earth is heated by the Sun, some terrestrial heat normally leaves the planet, escaping into outer space. This process allows the Earth to maintain a constant temperature, rather than growing steadily warmer as more solar energy impinges upon it. CO2, methane, and ozone have the ability to trap the terrestrial heat attempting to leave the Earth, thereby preventing this heat from escaping into outer space. Although such gases, known as greenhouse gases (GHGs), play a role in maintaining a stable and moderate temperature on Earth, an excess of GHGs results in a terrestrial buildup of heat and an elevation of the Earth's temperature. The atmospheric concentrations of many GHGs have significantly increased since the advent of industrialization, around 1750.

Although CO is not a GHG, it acts directly or indirectly to increase the levels of Earth's GHGs--including methane, tropospheric ozone (the troposphere is the lowest portion of the Earth's atmosphere), and CO2--by participating in various chemical reactions in the atmosphere. For example, CO indirectly affects the levels of methane and tropospheric ozone by reacting with the hydroxyl radical. The hydroxyl radical is a reactive molecule, consisting of an oxygen atom chemically bonded to a hydrogen atom, that is responsible for decreasing the levels of many atmospheric pollutants, including methane and tropospheric ozone. When atmospheric carbon monoxide reacts with the hydroxyl radical, it decreases the amount of this radical available to react with and remove methane and tropospheric ozone. As a result, methane and ozone build up in the atmosphere.

CO also indirectly affects the levels of tropospheric ozone by its involvement in reactions producing substances that can generate tropospheric ozone. For example, when CO reacts with the hydroxyl radical, one of the products formed in a series of reactions is the hydroperoxyl radical (a reactive molecule consisting of an atom of hydrogen and two atoms of oxygen). The hydroperoxyl radical can participate in reactions that form tropospheric ozone. Finally, CO directly affects the levels of atmospheric CO2 through its reaction with the hydroxyl radical. When CO reacts with the hydroxyl radical in the atmosphere, it forms CO2. CO2 is one of the most potent GHGs.

Bibliography:

1)         Houghton, John. Global Warming: The Complete Briefing. 4th ed. New York: Cambridge University Press, 2009.

2)         Intergovernmental Panel on Climate Change. Climate Change, 2007--Synthesis Report: Contribution of Working Groups I, II, and III to the Fourth Assessment Report of the Intergovernmental Panel on ClimateChange. Edited by the Core Writing Team, Rajendra K. Pachauri, and Andy Reisinger. Geneva, Switzerland: 2008.

3)         Kroschwitz, Jacqueline I., and Arza Seidel, eds. Kirk-Othmer Encyclopedia of Chemical Technology. 5th ed. Hoboken, N.J.: Wiley-Interscience, 2004-2007.

4)         Van Ham, J., et al., eds. Non-CO2 Greenhouse Gases: Scientific Understanding, Control, and Implementation. Amsterdam, the Netherlands: Kluwer Academic, 2000.

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