Levels of helium-4 in the Earth’s atmosphere have been increasing since at least 1974, according to a study by researchers at the University of California, San Diego, US that resolves a longstanding anomaly in atmospheric science.
Helium-4, the most abundant isotope of helium, is produced by radioactive decay in the Earth’s crust and accumulates in small amounts in natural gas. When the gas is extracted and burned, the helium is released into the atmosphere. Theoretical calculations have therefore suggested that atmospheric levels of helium-4 must have been increasing for decades due to fossil fuel usage. Until now, however, convincing experimental evidence for such a build-up had been lacking.
Helium mole fraction measurements
In the latest work, researchers led by Benjamin Birner of Scripps Institution of Oceanography developed a new and precise technique to measure the atmospheric ratio of helium-4 (4He) to nitrogen (N2). They studied 46 air samples acquired between 1974 and 2020 and found that the 4He concentration increased at an average rate of 39 +/-3 billion moles per year.
“Since nitrogen levels in the atmosphere are steady, changes in the 4He/N2 ratio indicate variability in the atmospheric helium content,” Birner explains. “We determined this ratio using a novel mass spectrometric technique that measures the helium mole fraction in the air with unprecedented precision.”
The new observations also suggest that atmospheric levels of another isotope of helium, 3He, have also increased. “While we didn’t measure 3He, only 4He/N2, some previous studies of the atmospheric helium isotope ratio (3He/4He) found no apparent trend,” Birner says. “Together with our observation of a clear increase in 4He, a stable isotopic ratio implies a corresponding build-up of 3He and 4He.”
Birner adds that the increase in 3He remains largely unexplained and warrants further research – especially since this isotope is very rare and has several important applications, including as a fuel for nuclear fusion. “To give an idea of how precious it is, people have even thought about mining it on the Moon,” he says.
The inferred increase in atmospheric 3He also greatly exceeds estimates of anthropogenic emissions from natural gas, nuclear weapons and nuclear power generation, suggesting potential problems with previous isotope measurements or an incorrect assessment of known sources, he says. “By improving the measurement of atmospheric helium, we hope to develop an indicator of regional to global natural gas usage that could be used in air quality studies of cities,” he tells Physics World.
The researchers say they will now be verifying the atmospheric helium isotope trend from the high-fidelity samples they used in this study. “Another avenue we would like to pursue is to make continuous helium measurements, to better understand local emission sources in cities and from these, verify reported fossil fuel emissions,” Birner concludes.
The present work is detailed in Nature Geoscience.