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Abstract Reconstructions of paleoatmospheric H₂using polar firn air and ice cores would lead to a better understanding of the H₂biogeochemical cycle and how it is influenced by climate change and human activity. In this study, the permeability, diffusivity, and solubility of H₂are determined experimentally in ice Ih at temperatures relevant to polar ice sheets (199–253 K). The experimental data are used in conjunction with simplified diffusion models to assess the implications for: (a) Diffusion of H₂from pressurized closed bubbles to open pores in polar firn, (b) diffusive smoothing of H₂gradients in the ice sheet, and (c) post‐coring diffusive losses of H₂from ice core samples. The results indicate that diffusive equilibrium between open and closed pores is likely achieved in the firn lock‐in zone. Diffusive smoothing of atmospheric variations is significant and should be accounted for in atmospheric reconstructions on millennial time scales. Diffusive losses from a bubbly ice sample are sufficiently slow that samples may be meaningfully analyzed for H₂after storage on the order of a year. These results suggest that the mobility of H₂in ice should not preclude the reconstruction of paleoatmospheric H₂from firn air and ice cores. 

Abstract Molecular hydrogen (H₂) is an abundant and reactive constituent of Earth's atmosphere, with links to climate and air quality. Anthropogenic emissions of H₂are expected to rise as the use of H₂as an energy source increases. Documenting past variations in atmospheric H₂will help to validate current understanding of the global H₂budget. The modern instrumental record begins in the 1980s; there is little information about atmospheric H₂prior to that time. Here, we use firn air measurements from a 2001 South Pole campaign to reconstruct atmospheric H₂levels over the 20th century. Inversion of the measurements indicates that H₂over South Pole has increased from 350–540 ppb from 1910–2000. A biogeochemical box model indicates that the atmospheric burden of H₂increased by 37% over that time. The rise in H₂is consistent with increasing H₂emissions from fossil fuel combustion and increasing atmospheric production from methane oxidation. 

The atmospheric history of molecular hydrogen (H 2 ) from 1852 to 2003 was reconstructed from measurements of firn air collected at Megadunes, Antarctica. The reconstruction shows that H 2 levels in the southern hemisphere were roughly constant near 330 parts per billion (ppb; nmol H 2 mol −1 air) during the mid to late 1800s. Over the twentieth century, H 2 levels rose by about 70% to 550 ppb. The reconstruction shows good agreement with the H 2 atmospheric history based on firn air measurements from the South Pole. The broad trends in atmospheric H 2 over the twentieth century can be explained by increased methane oxidation and anthropogenic emissions. The H 2 rise shows no evidence of deceleration during the last quarter of the twentieth century despite an expected reduction in automotive emissions following more stringent regulations. During the late twentieth century, atmospheric CO levels decreased due to a reduction in automotive emissions. It is surprising that atmospheric H 2 did not respond similarly as automotive exhaust is thought to be the dominant source of anthropogenic H 2. The monotonic late twentieth century rise in H 2 levels is consistent with late twentieth-century flask air measurements from high southern latitudes. An additional unknown source of H 2 is needed to explain twentieth-century trends in atmospheric H 2 and to resolve the discrepancy between bottom-up and top-down estimates of the anthropogenic source term. The firn air–based atmospheric history of H 2 provides a baseline from which to assess human impact on the H 2 cycle over the last 150 y and validate models that will be used to project future trends in atmospheric composition as H 2 becomes a more common energy source.