Helium is produced by radioactive decay of uranium and thorium in the Earth’s crust and co-released during human extraction and usage of fossil fuels. This should lead to an atmospheric build-up of helium but so far direct observations have not detected a change in helium.
I will present a record of changes in the atmospheric helium-to-nitrogen ratio (δ4He/N2) which is sensitive primarily to atmospheric helium variability because N2 is essentially stable. Using a novel mass spectrometric technique, we analyzed 46 archived air samples from three different locations (La Jolla [U.S.A.], Trinidad head [U.S.A.], and Cape Grim [Australia]) and find a 1.9 per mil increase in δ4He/N2 from 1974 to 2020. This corresponds to an annual increase of 39 ± 3 billion mol of 4He per year.
Taken together with previous observations of a near constant atmospheric helium-3/helium-4 (3He/4He) ratio, the increase in 4He implies a comparable increase in atmospheric 3He. Currently known sources of 3He are insufficient to explain the implied 3He trend, which is notable because 3He is an important, yet very scarce, fuel resource for nuclear fusion reactors.
Picture: Benni Birner in front of some of the air samples he used in his work. Copyright Eric Jepsen at SIO.