Molecular Hydrogen in the Atmosphere
After methane molecular hydrogen (H2) is the second most abundant reduced gas in the atmosphere, with a globally averaged mixing ratio of about 500 ppb. This value corresponds to a global tropospheric H2inventory of around 150 Tg (1012 g H2). Since hydrogen is believed to play a major role as a "clean" energy carrier in future energy scenarios, it is important to investigate the potential impact of an additional anthropogenic (man made) H2 source with respect to climate interactions. Molecular hydrogen as such does not affect the radiation budget of the atmosphere directly, thus H2 is not regarded as a greenhouse gas. However, H2 plays an important role in air chemistry as it influences the atmospheric oxidation capacity through reaction with the OH radical. Increased atmospheric hydrogen levels will most probably lead to an increased lifetime of many atmospheric constituents including the greenhouse gas methane. A second aspect of H2 concentration changes in the atmosphere pertain its ability to raise the water vapour content in the stratosphere. This in turn influences the radiation budget of the stratosphere (increased water vapour will cool the stratosphere) and is expected to contribute to stratospheric ozone depletion (e.g. ice particle formation).
The recent atmospheric H2 budget is not well established quantitatively. Important sources are combustion processes as well as oxidation of CH4 and non methane hydroCarbons in the atmosphere which themselves are emitted from various anthropogenic and natural sources. The global atmospheric H2 budget is closed by two main sinks: the oxidation of H2 initiated by the reaction with OH radicals and the uptake by soils. The latter is believed to be the dominating part leading to a total atmospheric lifetime of H2 of about two years. The present uncertainties in the budget are large (25%), and it is, therefore, difficult to assess the impacts of potential future changes of the atmospheric hydrogen burden.
In Heidelberg we perform semi-continuous measurements of atmospheric H2 mixing ratios and other related tracers like CO and 222Rn since 2005. These data provide new insight into anthropogenic emissions of H2 (Hammer et al., 2009) as well as soil sink processes (Hammer and Levin, 2009). In addition, grab samples from the High Arctic and from Antarctica are analysed (see figure). Direct soil sink studies for atmospheric H2 were conducted by dedicated chamber experiments at our local Grenzhof Soil-Atmosphere Monitoring Station (schmitt et al., 2009).
Flask measurements of atmospheric H2 mixing ratios in high northern (Alert) and high southern (Neumayer, Antarctica) latitudes. Mixing ratios are higher in Antarctica than in the Arctic pointing to the important role of the continental soil sink in the northern hemisphere.
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