Atmospheric History of H2 Over the Past Century Reconstructed From South Pole Firn Air

Molecular hydrogen (H-2) is an abundant and reactive constituent of Earth's atmosphere, with links to climate and air quality. Anthropogenic emissions of H-2 are expected to rise as the use of H-2 as an energy source increases. Documenting past variations in atmospheric H-2 will help to validate current understanding of the global H-2 budget. The modern instrumental record begins in the 1980s; there is little information about atmospheric H-2 prior to that time. Here, we use firn air measurements from a 2001 South Pole campaign to reconstruct atmospheric H-2 levels over the 20th century. Inversion of the measurements indicates that H-2 over South Pole has increased from 350-540 ppb from 1910-2000. A biogeochemical box model indicates that the atmospheric burden of H-2 increased by 37% over that time. The rise in H-2 is consistent with increasing H-2 emissions from fossil fuel combustion and increasing atmospheric production from methane oxidation. Plain Language Summary Hydrogen (H-2) is an atmospheric trace gas with both natural and anthropogenic sources. In the atmosphere, photochemical reactions of H-2 can contribute to air pollution and influence climate. Future use of hydrogen as a possible replacement for fossil fuels will likely lead to increased emissions and atmospheric levels. Here we use measurements of hydrogen in South Pole firn air to reconstruct atmospheric hydrogen levels over the 20th century. We find that atmospheric H-2 increased by about 40% over that time. This increase is consistent with estimates of emissions of hydrogen and hydrogen precursors from fossil fuel burning.