New determination of the 3He mixing ratio in the Earth's lower atmosphere from an international tritium intercomparison exercise

The light isotope of helium, 3He, is essentially a primordial substance entrapped within the Earth's interior during the formation of the planet. It is released into the atmosphere by volcanic/magmatic activity, and eventually escapes to outer space. 3He is also produced by the radioactive beta-decay of tritium. Hence, measurements of 3He can be used to derive the concentration of tritium. The so-called “3He ingrowth” method uses a mass spectrometer to detect the amount of 3He that accumulates in a sample during a given period of storage. The 3He measurements are classically calibrated against an air standard. The method thus relies on the accurate knowledge of the atmospheric mixing ratio of 3He. This value is based on mass spectrometric measurements with gravimetrically or volumetrically prepared 3He standard mixtures. Here, we apply the 3He ingrowth method in reverse, using a solution of tritiated water prepared for an international comparison of tritium activity measurements to precisely determine the 3He mixing ratio of our air standard. The measured atmospheric mixing ratio of 3He, based on a series of ten measurements, is [3He] = 7.12 ± 0.06 ppt. This value is between 1% and 2% lower than previous determinations reported in the literature. However, all results remain statistically consistent.