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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.
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Atmospheric History of H 2 Over the Past Century Reconstructed From South Pole Firn Air
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.
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- Award ID(s):
- 1907974
- PAR ID:
- 10386835
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 47
- Issue:
- 14
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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