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Abstract. Tropospheric helium variations are tightly linked to CO2 due to the co-emission of He and CO2 from natural-gasburning. Recently, Birner et al. (2022a) showed that the global consumption of natural gas has measurably increased the He content of theatmosphere. Like CO2, He is also predicted to exhibit complex spatial and temporal variability on shorter timescales, butmeasurements of these short-term variations are lacking. Here, we present the development of an improved gas delivery and purification system for thesemi-continuous mass spectrometric measurement of the atmospheric He-to-nitrogen ratio (He/N2). The method replaces the chemicalgetter used previously by Birner et al. (2021, 2022a) to preconcentrate He in an air stream with a cryogenic trap which can be more simplyregenerated by heating and which improves the precision of the measurement to 22 per meg (i.e., 0.022 ‰) in10 min (1σ). Using this “cryo-enrichment” method, we measured the He/N2 ratios in ambient air at La Jolla (California,USA) over 5 weeks in 2022. During this period, He/N2 was strongly correlated with atmospheric CO2 concentrations, as expectedfrom anthropogenic emissions, with a diurnal cycle of 450–500 per meg (max–min) caused by the sea–land breeze pattern of local winds,which modulates the influence of local pollution sources. 

Abstract. The atmospheric He/N2 ratio is expected to increase due to the emission of He associated with fossil fuels and isexpected to also vary in both space and time due to gravitational separationin the stratosphere. These signals may be useful indicators of fossil fuelexploitation and variability in stratospheric circulation, but directmeasurements of He/N2 ratio are lacking on all timescales. Here wepresent a high-precision custom inlet system for mass spectrometers thatcontinuously stabilizes the flow of gas during sample–standard comparisonand removes all non-noble gases from the gas stream. This enablesunprecedented accuracy in measurement of relative changes in the helium molefraction, which can be directly related to the 4He/N2 ratio usingsupplementary measurements of O2/N2, Ar/N2 and CO2.Repeat measurements of the same combination of high-pressure tanks using ourinlet system achieves a He/N2 reproducibility of∼ 10 per meg (i.e., 0.001 %) in 6–8 h analyses. This compares to interannual changesof gravitational enrichment at ∼ 35 km in the midlatitudestratosphere of order 300–400 per meg and an annual tropospheric increasefrom human fossil fuel activity of less than ∼ 30 per meg yr−1 (bounded by previous work on helium isotopes). The gettering andflow-stabilizing inlet may also be used for the analysis of other noble-gasisotopes and could resolve previously unobserved seasonal cycles inKr/N2 and Xe/N2.