Carbon Monoxide In The Atmosphere

PAUL NOVELLI

Carbon monoxide (CO) is present in trace quantities in the atmosphere. Although first detected in the late 1940s using solar spectroscopic methods,1'2 few measurements of CO were made during the period between the early 1950s and the mid-1960s. However, as chromatographic and related detection techniques were developed, discrete measurements of CO were made in many locations around the world. These provided considerable insight on global tropospheric distributions; most notable among these was the observation that CO concentrations generally decreased from north to south.3-5 The significance of CO in atmospheric chemistry was recognized in 1971 when Levy,6 and McConnell et al.7 proposed a photochemically driven, radical chain reaction linking the tropospheric cycles of methane (CH4), CO, nitric oxide and nitrogen dioxide (NO,), and formaldehyde (CH20), with those of the oxidants ozone (03), the hydroxyl (OH) and hydroperoxyl (H02) radicals. These models describe an atmosphere in which the photolysis of 03 (.hv < 320 nm) leads to the formation of OH, initiating a series of oxidation/ reduction reactions that both produce and destroy CO, CH20, OH and H02.

In much of the background atmosphere the reaction of CO and OH [Eq. (1)] accounts for 90 to 95% of the loss of CO8 and about 75% of the removal of OH.9

While the stoichiometric relationship between CO oxidation and OH loss is dependent upon several possible reaction pathways, the inverse relationship between CO and OH concentrations suggested by Eq. (1) is expected in the background atmosphere.9'10 Not only does the hydroxyl radical regulate the concentration of

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CO, but oxidation at the expense of OH is also the primary removal pathway for many other reduced gases, several of which are radiatively important [e.g., CH4, the hydrogenated chlorofluorocarbons (CFCs)]. Therefore, trends in atmospheric CO levels are expected to have an effect on climate through its role in regulating [OH], which in turn affects the levels of several important greenhouse gases.11

Carbon monoxide impacts both local and regional air quality through its influence on ozone. In areas of relatively high NOx levels (>5 to 10 pmol/mol), such as urban areas or air parcels affected by fossil fuel or biomass burning, H02 produced through CO oxidation enters into a series of photochemical reactions that produce

In the background atmosphere, where [NOJ is often <5 pmol/mol, H02 produced by the oxidation of CO may destroy 03.

As a result, the oxidizing capacity of the lower atmosphere is coupled to the concentrations, distributions, and trends of CO.

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