Increasing trend in atmosphere helium from 1975 to 2020

Abstract:

The anthropogenic release of crustal helium associated with global fossil fuel extraction is thought to increase atmospheric ⁴He concentrations and thus reduce the atmospheric ³He/⁴He isotope ratio. Although early observational studies indeed found evidence of a human impact on atmospheric helium, these findings were challenged by several recent investigations that detected no significant atmospheric ³He/⁴He trend.

Here we present results from a novel mass spectrometry technique that can constrain relative changes in the atmospheric helium-to-nitrogen (⁴He/N₂) ratio at an unprecedented precision of <50 per meg (1 per meg = 10^-4 %, 1σ) for a single air sample. The new method was used to analyze over 35 air samples that are stored in high pressure gas cylinders at Scripps Institution of Oceanography and span the age range 1975–2020. Gas samples were screened for artifactual fractionation by leakage.

Preliminary results indicate a increase in ⁴He/N₂ of +1.8 per mil from 1975 through 2020, corresponding to an average rate of +40 per meg per year. The record also suggests the possibility of accelerated ⁴He release after 1990. This ties together observations and theoretical predictions of an anthropogenic signature in atmospheric helium and places tight boundaries on the trend. The implied ~1.8 per mil change in the atmospheric ³He/⁴He since 1975 challenges the common view of ambient air as a time-invariant reference gas.