Short-term variability of atmospheric helium revealed through a cryo-enrichment method

Abstract. Tropospheric helium variations are tightly linked to CO₂ due to the co-emission of He and CO₂ from natural gas burning. Recently Birner et al. (2022) showed that the global consumption of natural gas has measurably increased the He content of the atmosphere. Like CO₂, He is also predicted to exhibit complex spatial and temporal variability on shorter time scales, but measurements of these short-term variations are lacking. Here we present the development of an improved gas delivery and purification system for the semi-continuous mass spectrometric measurement of the atmospheric He-to-nitrogen ratio (He/N₂). The method replaces the chemical getter used previously by Birner et al. (2021, 2022) to preconcentrate He in an air stream with a cryogenic trap which can be more simply regenerated by heating and which improves the precision of the measurement to 22 per meg in 10 minutes (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 expected from 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.