7.4.3. Molecular Hydrogen

Increased interest in atmospheric H₂ is due to its potential role as an indirect greenhouse gas (Derwent et al., 2001) and expected perturbations of its budget in a prospective ‘hydrogen economy’ (Schultz et al., 2003; Tromp et al., 2003; Warwick et al., 2004). Potential consequences of increased H₂ emissions include a reduction of global oxidizing capacity (presently H₂ constitutes 5 to 10% of the global average OH sink, Schultz et al., 2003) and increased formation of water vapour, which could lead to increased cirrus formation in the troposphere and increased polar stratospheric clouds (PSCs) and additional cooling in the stratosphere, thereby leading to more efficient ozone depletion (Tromp et al., 2003).

Studies of the global tropospheric H₂ budget (see Table 7.8) generally agree on a total source strength of between 70 and 90 Tg(H₂) yr^–1, which is approximately balanced by its sinks. About half of the H₂ is produced in the atmosphere via photolysis of formaldehyde (CH₂O), which itself originates from the oxidation of CH₄ and other volatile organic compounds. The other half stems mostly from the combustion of fossil fuels (e.g., car exhaust) and biomass burning. About 10% of the global H₂ source is due to ocean biochemistry and N fixation in soils. Presently, about 50 Tg(H₂) yr^–1 are produced in the industrial sector, mostly for the petrochemical industry (e.g., refineries) (Lovins, 2003). Evaporative losses of industrial H₂ are generally assumed to be negligible (Zittel and Altmann, 1996). The dominant sink of atmospheric H₂ is deposition with catalytic destruction by soil microorganisms and possibly enzymes (Conrad and Seiler, 1981). The seasonal cycle of observed H₂ concentrations implies an atmospheric lifetime of about 2 years (Novelli et al., 1999; Simmonds et al., 2000; Hauglustaine and Ehhalt, 2002), whereas the lifetime with respect to OH oxidation is 9 to 10 years, which implies that the deposition sink is about three to four times as large as the oxidation. Loss of H₂ to the stratosphere and its subsequent escape to space is negligible for the tropospheric H₂ budget, because the budgets of the troposphere and stratosphere are largely decoupled (Warneck, 1988).

Estimates of H₂ required to fuel a future carbon-free energy system are highly uncertain and depend on the technology as well as the fraction of energy that might be provided by H₂. In the future, H₂ emissions could at most double: the impact on global oxidizing capacity and stratospheric temperatures and ozone concentrations is estimated to be small (Schultz et al., 2003; Warwick et al., 2004). According to Schultz et al. (2003), the side effects of a global H₂ economy could have a stronger impact on global climate and air pollution. Global oxidizing capacity is predominantly controlled by the concentration of NO_x. Large-scale introduction of H₂-powered vehicles would lead to a significant decrease in global NO_x emissions, leading to a reduction in OH of the order of 5 to 10%. Reduced NO_x levels could also significantly reduce tropospheric ozone concentrations in urban areas. Despite the expected large-scale use of natural gas for H₂ production, the impact of a H₂ economy on the global CH₄ budget is likely to be small, except for the feedback between reduced oxidizing capacity (via NO_x reduction) and CH₄ lifetime.