H2 in Antarctic firn air: Atmospheric reconstructions and implications for anthropogenic emissions

The atmospheric history of molecular hydrogen (H₂) from 1852 to 2003 was reconstructed from measurements of firn air collected at Megadunes, Antarctica. The reconstruction shows that H₂ levels in the southern hemisphere were roughly constant near 330 parts per billion (ppb; nmol H₂ mol⁻¹ air) during the mid to late 1800s. Over the twentieth century, H₂ levels rose by about 70% to 550 ppb. The reconstruction shows good agreement with the H₂ atmospheric history based on firn air measurements from the South Pole. The broad trends in atmospheric H₂ over the twentieth century can be explained by increased methane oxidation and anthropogenic emissions. The H₂ 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 H₂ 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 H₂ levels is consistent with late twentieth-century flask air measurements from high southern latitudes. An additional unknown source of H₂ is needed to explain twentieth-century trends in atmospheric H₂ and to resolve the discrepancy between bottom-up and top-down estimates of the anthropogenic source term. The firn air–based atmospheric history of H₂ provides a baseline from which to assess human impact on the H₂ cycle over the last 150 y and validate models that will be used to project future trends in atmospheric composition as H₂ becomes a more common energy source.

Molecular hydrogen (H₂) is an abundant and reactive constituent of Earth’s atmosphere. The utilization of H₂ as an energy source emits no carbon to the atmosphere if produced from water using renewables, and increasing adoption of H₂ as a substitute for fossil fuels is likely (1). As the H₂ energy sector expands, anthropogenic emissions are expected to increase due to leakage. Projecting the effects of increasing anthropogenic emissions requires a comprehensive understanding of the biogeochemical cycle of H₂. Reconstructing the paleoatmospheric levels of H₂ contributes to that understanding by establishing the baseline for quantifying anthropogenic emissions since the industrial revolution.Presently, the average atmospheric abundance of H₂ is 530 parts per billion (ppb; nmol H₂ mol⁻¹ air), and the atmospheric lifetime is about 2 y (2). The budget of H₂ is complex, including both natural and anthropogenic terms. Globally, the largest source of H₂ is the photolysis of formaldehyde, formed by the atmospheric oxidation of methane and nonmethane hydrocarbons (NMHCs). Other major sources include direct emissions from fossil fuel combustion and biomass burning. N₂ fixation both on land and in the ocean is a small additional source. The major sink for atmospheric H₂ is uptake by soil microbes, with oxidation by OH accounting for the remaining losses. H₂ levels are higher by about 3% in the southern hemisphere than in the northern hemisphere due to the hemispheric asymmetry of the soil sink (2–5).Atmospheric H₂ levels impact Earth’s radiative budget and air quality. H₂ serves as a sink for the OH radical, increasing the atmospheric lifetime of radiatively important trace gases like methane. Additionally, the reaction of H₂ with OH results in the catalytic production of O₃ in the troposphere (2, 6–8). Oxidation of H₂ by OH in the stratosphere leads to increased concentrations of HO₂ and water vapor. The increase in these species will have indirect radiative effects due to losses of ozone and alterations to the distribution of polar stratospheric clouds. Increased stratospheric concentrations of water vapor will also have direct radiative effects via increased infrared absorption (7, 9–12).The modern instrumental record of tropospheric H₂ abundance began in the late 1980s (2, 13). Mid-twentieth century studies reported a wide range (400 to 2,000 ppb), which likely reflects analytical issues and/or influence from urban pollutants (14–16). The surface flask air measurements of Khalil and Rasmussen (13), the National Oceanic and Atmospheric Administration Global Monitoring Laboratory (NOAA/GML; refs. 2, 17), and the Commonwealth Scientific and Industrial Research Organisation (CSIRO; ref. 18) show atmospheric levels of H₂ of 510 to 550 ppb during the late 20th and early 21st centuries at background sites around the world. In situ measurements from Cape Grim, Tasmania, and Mace Head, Ireland, made by the Advanced Global Atmospheric Gases Experiment (AGAGE) show similar levels of atmospheric H₂ during the same time period (19). To date, there have been two published firn air studies of the historical trends of atmospheric H₂. Petrenko et al. (20) reconstructed northern hemisphere H₂ from Greenland firn air measurements. Their results show an increase in atmospheric H₂ levels from 450 to 520 ppb during the 1960s to a peak near 550 ppb during the late 1980s or early 1990s, then a recent decline to about 515 ppb in 2010. The peak and recent decline are inconsistent with modern flask measurements that show roughly constant H₂ levels during the 1990s (2, 17, 19). The authors note that the firn air model used for the reconstruction does not include pore close-off fractionation of H₂, and it is possible that the inferred peak is a modeling artifact (20, 21). Firn air measurements of H₂ from the South Pole showed that atmospheric H₂ increased from 350 ppb to 550 ppb over the twentieth century in the high southern latitudes (22). This reconstruction does not include a late twentieth century peak or decline, and the reconstructed levels of H₂ are consistent with available modern flask measurements beginning in the late 1980s. The increase in atmospheric H₂ is primarily attributed to increasing anthropogenic emissions and increasing atmospheric concentrations of methane over the twentieth century.Here we report H₂ measurements in firn air samples collected from the Megadunes site (80.78 °S, 124.49 °E, Alt: 2,283 m) in central Antarctica during January of 2004 and analyzed at NOAA/GML in April 2004 (23, 24). Megadunes is a complex site, with very low annual accumulation rates and an unusually large amplitude of surface topography. The oldest firn air ever recovered, with a mean CO₂ age of 140 y, was sampled during this campaign (23). A firn air model is used to reconstruct southern-hemisphere atmospheric H₂ levels since the 1850s. The reconstruction is compared to the existing South Pole firn air reconstruction (22), and implications of the reconstructed atmospheric history for the global budget of H₂ are discussed.