Hydrogen, tropospheric sinks

Only a small percentage of the chlorine released by photolysis of CFCs is present in the active forms as Cl or CIO, however. Most of it is bound up in reservoir compounds such as hydrogen chloride and chlorine nitrate, formed respectively by hydrogen abstraction (equation 10) from methane and addition (equation 11) to nitrogen dioxide. Slow transport of these reservoir species across the tropopause, followed by dissolution in tropospheric water and subsequent rain-out, provide sink processes for stratospheric chlorine. 

Molecular hydrogen is assumed to be well mixed in the troposphere, with a mixing ratio of 0.4 to 0.6 ppm [Junge (128) and Scholz, Ehhalt, Heidt, and Martell (219)]. Koyama (142) found that swamps and paddies are very small natural sources. Levy (153) proposed both an atmospheric source (photodissociation of formaldehyde) and an atmospheric sink (oxidation by hydroxyl radical). From daily average number densities for the hydroxyl radical and a daily average hydrogen production rate,... 

The rate of ozone production is critically dependent on the availability of odd hydrogen radicals (defined by Kleinman (1986) as the sum of OH, HO2, and RO2) and in particular by the OH radical. The OH radical is important because reaction sequences that lead to either the production or removal of many tropospheric pollutants are also initiated by reactions involving OH. In particular, the ozone production sequence is initiated by the reaction of OH with CO (reaction (1)) and hydrocarbons (reaction (2)). The split into NO -sensitive and VOC-sensitive regimes, discussed below, is also closely associated with sources and sinks of radicals. 

We can thus conclude that the spring maximum cannot be explained either by the annual variation of source intensity at the Earth s surface or by the variation of the quantity of precipitation. It has been postulated (E. Meszaros, 1974a) that this maximum is due to the oxidation effects of tropospheric ozone, the concentration of which also has a maximum during the spring (see Fig. 13). Ozone oxidizes S02 and N02 in atmospheric liquid water (see Subsection 5.3.2) which leads to the lowering of the pH. The increase in the concentration of hydrogen ions promotes the absorption of ammonia gas from the air, as well as the transformation of insoluble mineral components (e.g. calcium carbonate) into water-soluble materials. If this speculation is correct, this process provides a non-negligible ozone sink in the... 

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. 

The troposphere has an estimated 155 Tg of hydrogen gas (H2), with approximately a two-year lifetime (Chapter 2.8.2.10). Many sources of hydrogen gas and a few major sinks account for this relatively short lifetime. The main pathway in the production of hydrogen atoms in the air is the methane (CH4) conversion by the OH radical and subsequent photolysis of formaldehyde (HCHO) see reactions (5.42) to (5.48). This process accounts for about 26 Tg H yr (Novelli et al. 1999). 

For halocarbons that contain a carbon-hydrogen bond, reaction with tropospheric OH becomes important and their atmospheric lifetimes become dependent on the relative rates of the OH-halocar-bon reaction and the global concentration and distribution of OH. Compovmds for which OH reaction is the predominant sink include HCFCs, HFCs, CH3CCI3, CH3CI, and CH3Br. 

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