Carbon monoxide, tropospheric sinks

Thus the net effect of dissociating nitrogen dioxide is neutral. Net production of tropospheric ozone occurs as a result of other reactions that convert NO into NO2 without destroying ozone. There are many such reactions, most of which involve the photooxidation of chemicals like carbon monoxide, methane and other hydrocarbons. Since these are produced by traffic and industrial processes, ozone production is a feature of polluted regions, and ozone itself is considered a pollutant at low levels of the atmosphere where it is detrimental to human and other life forms. Sinks of ozone include photodissociation and reactions with OH and HO2 (as in the stratosphere) and deposition. 

It is assumed that the majority of carbon monoxide is removed from the atmosphere by these reactions. Seiler (1974) hypothesizes that the yearly CO loss in the troposphere due to [3.5] and [3.7], is (1940-5000) x 1061 yr-1 The corresponding figure for the stratosphere is estimated to be 110 x 106 t yr-1 (see Table 7). In contrast, Warneck (1974) speculates that the global atmospheric strength of this sink is much smaller than the Seiler s figure. Finally, according to the calculations of Ehhalt and Schmidt (1978) about (1500-2900) x 1061C02 is produced yearly from CH4 by reaction steps [3.4], [3.2], [3.5] and [3.7]. On the basis of these data for the schematic representation of the atmospheric pathways of carbon a value of 2800 x 106 t yr 1 expressed in C02 will be accepted (see Fig. 8, p. 46) for this sink term. 

A fairly general treatment of trace gases in the troposphere is based on the concept of the tropospheric reservoir introduced in Section 1.6. The abundance of most trace gases in the troposphere is determined by a balance between the supply of material to the atmosphere (sources) and its removal via chemical and biochemical transformation processes (sinks). The concept of a tropospheric reservoir with well-delineated boundaries then defines the mass content of any specific substance in, its mass flux through, and its residence time in the reservoir. For quantitative considerations it is necessary to identify the most important production and removal processes, to determine the associated yields, and to set up a detailed account of sources versus sinks. In the present chapter, these concepts are applied to the trace gases methane, carbon monoxide, and hydrogen. Initially, it will be useful to discuss a steady-state reservoir model and the importance of tropospheric OH radicals in the oxidation of methane and many other trace gases. 

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