Measurement of changes in atmospheric Ar/N₂ ratio using a rapid-switching, single-capillary mass spectrometer system

The atmospheric Ar/N₂ ratio is expected to undergo very slight variations due to exchanges of Ar and N₂ across the air—sea interface, driven by ocean solubility changes. Observations of these variations may provide useful constraints on large-scale fluxes of heat across the air—sea interface. A mass spectrometer system is described that incorporates a magnet with a wide exit face, allowing a large mass spread, and incorporates an inlet with rapid (5 s) switching of sources gases through a single capillary, thus achieving high precision in the comparison of sample and reference gases. The system allows simultaneous measurement of Ar/N₂, O2/N₂ and CO₂/N₂ ratios. The system achieves a short-term precision in Ar/N₂ of 10 per meg for a 10 s integration, which can be averaged to achieve an internal precision of a few per meg in the comparison of reference gases. Results for Ar/N₂ are reported from flasks samples collected from nine stations in a north-to-south global network over about a 1 yr period. The imprecision on an individual flask, as estimated from replicate agreement, is ±11 per meg. This imprecision is dominated by real variability between samples at the time of analysis. Seasonal cycles are marginally resolved at the extra-tropical stations with amplitudes of 5 to 15 per meg. Annual-mean values are constant between stations to within ±5 per meg. The results are compared with a numerical simulation of the cycles and gradients in Ar/N₂ based on the TM2 tracer transport model in combination with air—sea Ar and N₂ fluxes derived from climatological air—sea heat fluxes. The possibility is suggested that Ar/N₂ ratios may be detectably enriched near the ground by gravimetric or thermal fractionation under conditions of strong surface inversions.