Atmospheric Chemistry Within Global Carbon Cycle

Despite its dominance as a member of the global carbon cycle, carbon dioxide does not play any significant role in atmospheric chemistry. The most abundant carbon species that is an active player in atmospheric chemistry is methane (CH4), which reacts in the troposphere with the hydroxyl radical (OH) to initiate a series of reactions that were shown to play important roles in the global cycles of a number of atmospheric trace species. The seminal work in tropospheric chemistry was published by Hiram Levy in 1971. Levy showed that the hydroxyl radical should exist in sufficient quantities in the troposphere to initiate a sequence of photochemical reactions that both produce and destroy a number of important tropospheric trace gases.

The initial formation of OH comes from the photolysis products of ozone in the troposphere:

5 ATMOSPHERIC CHEMISTRY WITHIN GLOBAL CARBON CYCLE 17

followed by its reaction with water vapor:

Hydroxyl then reacts with methane to form a host of products:

CH3 + 02 + M CH302 + M CH302 + NO -> CH30 + N02 CH30 + 02 ^ CH20 + H02

This sequence, commonly referred to as methane oxidation was hypothesized to be a major source of formaldehyde (CH20) and carbon monoxide (CO). In addition, other radicals such as CH302 (methyl peroxy) and H02 (hydroperoxy) were formed and became important factors in the tropospheric ozone budget. Once formaldehyde (CH20) was formed, it could photolyze in an alternate pathway to produce even more reactive radicals:

Note that either photolysis sequence of CH20 results in the formation of carbon monoxide.

Like C02, methane also absorbs infrared radiation and contributes to global warming. Ice core data show that atmospheric CH4 concentrations remained relatively constant at about half of its present form for thousands of years before beginning to increase about 200 years ago from —0.65 ppmv to —1.8 ppmv (Fig. 7). Methane's atmospheric lifetime is —8 years and its dominant removal mechanism is oxidation by OH.

Methane is produced in oxygen-deficient environments of Earth's surface (swamps, lakes, rice paddies, tundra, boreal marshes, etc.). Methane production in soils and oceans is the end product of a variety of reductive pathways during the decomposition of organic matter. Methane is also released by cattle, termites, and perhaps other insects, whereas coal mining, natural gas losses, and solid-waste burning are important anthropogenic sources. Large amounts of methane are also produced by biomass burning. The global methane budget is given in Table 1.

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Figure 7 Ice for samples of atmospheric methane showing only a slight increase from 950 to 1800, but a sharp rise in concentration over the last 150 years.

One of the important products of methane oxidation is CO, which can also be oxidized by OH to form C02:

Thus, CH4, CO, and C02 are linked together through a series of oxidation processes that take place in the atmosphere; all forms of carbon emitted to the atmosphere eventually become C02.

As can be seen from the above discussion, carbon monoxide is caught in the middle as an intermediate oxidation product. One of the fundamental questions in tropospheric chemistry is to determine how much CO is emitted directly to the atmosphere, relative to the amount that is produced in situ through CH4 oxidation. The primary and only significant sink for CO is removal by OH, which leads to an atmospheric residence on the order of 1 to 2 months. Thus, CO can be used as a useful tracer for atmospheric transport processes that take place on times scales of several days to a week or so.

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