Abstract
Abstract. The atmospheric He/N2 ratio is expected to increase due to the emission of He associated with fossil fuels and is expected to also vary in both space and time due to gravitational separation in the stratosphere. These signals may be useful indicators of fossil fuel exploitation and variability in stratospheric circulation, but direct measurements of He/N2 ratio are lacking on all timescales. Here we present a high-precision custom inlet system for mass spectrometers that continuously stabilizes the flow of gas during sample–standard comparison and removes all non-noble gases from the gas stream. This enables unprecedented accuracy in measurement of relative changes in the helium mole fraction, which can be directly related to the 4He/N2 ratio using supplementary measurements of O2/N2, Ar/N2 and CO2. Repeat measurements of the same combination of high-pressure tanks using our inlet system achieves a He/N2 reproducibility of ∼ 10 per meg (i.e., 0.001 %) in 6–8 h analyses. This compares to interannual changes of gravitational enrichment at ∼ 35 km in the midlatitude stratosphere of order 300–400 per meg and an annual tropospheric increase from human fossil fuel activity of less than ∼ 30 per meg yr−1 (bounded by previous work on helium isotopes). The gettering and flow-stabilizing inlet may also be used for the analysis of other noble-gas isotopes and could resolve previously unobserved seasonal cycles in Kr/N2 and Xe/N2.
Highlights
The atmospheric mole fraction of helium in dry air is typically ∼ 5.24 ppm (Glückauf, 1944), with an isotopic abundance of 4He about 106 times greater than 3He
A mass scan of air introduced through the gettering and flowstabilizing inlet system revealed that N2 and O2 are almost completely removed from the air by the online getter (Fig. 4)
He is effectively preconcentrated in the gas mixture. 40Ar ions with one or more charges yield the largest beams in the scan followed by 36Ar, with H2 evolving from the hot metal in the getter oven
Summary
The atmospheric mole fraction of helium in dry air is typically ∼ 5.24 ppm (Glückauf, 1944), with an isotopic abundance of 4He about 106 times greater than 3He. On geological timescales, the natural concentration of 4He in the atmosphere is set by a balance of 4He loss to space and 4He release from the Earth’s crust, where it is produced by radioactive decay of uranium and thorium (Kockarts, 1973; PiersonWickmann et al, 2001; Sano et al, 2013; Torgersen, 1989; Zartman et al, 1961). Gravitational separation is only partially counteracted by the large-scale stratospheric circulation and mixing, which tends to homogenize the atmosphere. Variability in stratospheric circulation and stratosphere–troposphere exchange (STE) could impact the degree of fractionation and cause additional interannual changes in the stratospheric and, to a much lesser extent, the tropospheric abundance of 4He
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