Hydrogen, tropospheric sources

Cox (246) estimates the concentration of HN02 to be 109 molec cm"3 in the daytime natural troposphere. The photolysis of HN02 may be an important source of OH in the troposphere, since HN02 absorbs the sun s radiation above 3000 A. The reactions of OH with hydrocarbons (either hydrogen abstraction from paraffins or addition to the double bond in olefins) in the troposphere are known to be the initial steps for photochemical smog formation [see Section VIII-2, p. 333],... 

Nitrogen oxides (NOx= N02 and nitrogen monoxide NO) sources are mainly emitting NO into the troposphere. Thai, NO may be converted to N02 by reaction with hydrogen peroxy radical (H02) or with higher peroxy radicals (R02), produced from hydrocarbon oxidation. 

Biogenic Sulfur Emissions from the Ocean. The ocean is a source of many reduced sulfur compounds to the atmosphere. These include dimethylsulfide (DMS) (2.4.51. carbon disulfide (CS2) (28). hydrogen sulfide (H2S) (291. carbonyl sulfide (OCS) (30.311. and methyl mercaptan (CH3SH) ( ). The oxidation of DMS leads to sulfate formation. CS2 and OCS are relatively unreactive in the troposphere and are transported to the stratosphere where they undergo photochemical oxidation (22). Marine H2S and CH3SH probably contribute to sulfate formation over the remote oceans, yet the sea-air transfer of these compounds is only a few percent that of DMS (2). 

Conversely, generating hydrogen from sustainable sources would reduce emissions of carbon monoxide and NOx, with a consequent fall in tropospheric ozone levels. This would improve air quality in many regions of the world. Furthermore, C02 emissions would be reduced, thereby slowing the global warming trend. 

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. 

Tropospheric HNO3 is formed by several routes reaction (25) is the major source, with hydrogen abstraction by HNO3 (reaction (28)) and hydrolysis of (reaction (29)) as the minor sources... 

With the substantial Increase In rate-constant data for the reaction of OH radicals with organic and Inorganic compounds in the last few years has come the general acceptance of the Importance of the OH radical In both tropospheric (1-18) and stratospheric (4, 12, 50-55) chemistry. In the lower atmosphere, where photodlssoclatlon of H2O Is not possible, the Initial direct sources of OH radicals are from the reaction of 0( D) atoms, formed from the photodlssoclatlon (X 310 nm) of O3 (56-59), with hydrogen-containing species ... 

The formation of hydrogen peroxide by photolysis of natural waters is discussed in Chapter 6. It is also formed by illumination of some sands and semiconductor oxides (Kormann et al., 1988 see also Section 6.E.3). Other sources of H2O2 include formation in the gas phase of the troposphere by the self-termination (dismutation) reaction of OOH and the autooxidation of reduced transition metals such as iron (Equation 4.4). The formation and fate of H2O2 in the atmosphere has been reviewed (Gunz and Hoffmann, 1990 Sakugawa et al., 1990). 

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). 

---