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Molecular hydrogen in Earth’s atmosphere: reconstructions and interpretation


The biogeochemical cycling of atmospheric hydrogen (H2) is linked to several important aspects of the Earth system including the radiative budget, biomass burning, biogenic hydrocarbon emissions, soil microbial activity, and the oxidative capacity of Earth’s atmosphere. As H2 becomes a more important part of the energy sector, anthropogenic emissions are likely to increase dramatically. Projecting the effects of increasing anthropogenic emissions in a changing climate require a comprehensive understanding of the biogeochemical cycle of H2. Studying past changes in the atmospheric levels of H2 is one way to improve understanding of the cycling of H2 through the Earth system. However, knowledge of past variability in atmospheric H2 is limited.

In this dissertation, atmospheric H2 levels over the last century are reconstructed. We conduct laboratory experiments to make the first measurements of the diffusivity of H2 in ice under conditions relevant to polar ice sheets. Numerical models are used to understand the implications of our results for the mobility of H2 in polar firn and ice. We show that the partial pressure of H2 equilibrates between open and closed pores in the firn column, that significant diffusive smoothing of atmospheric variability should be expected in ice core measurements of

H2 older than a few centuries, and that shallow ice core samples could be analyzed for H2 for up to a year after drilling without confounding diffusive losses to the atmosphere.The results of the diffusion experiment are incorporated into a new firn air model. The firn air model is used to reconstruct atmospheric H2 using measurements from two Antarctic sites and three Greenland sites. The reconstructions show that H2 levels increased by ~60% over Antarctica and ~40% over Greenland during the 20th century. A simplified atmospheric box model is used to demonstrate that the 20th century increase in H2 levels is consistent with increasing anthropogenic emissions and increasing photochemical production. This study is the first comprehensive investigation of past changes to the biogeochemical cycling of H2. Further research is required to confirm the records presented here and to extend the reconstruction of atmospheric H2 levels further back in time in order to analyze the covariance of climate and H2.

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