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

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Keeling, Ralph F. ; Blaine, Tegan ; Paplawsky, Bill ; Katz, Laura ; Atwood, Chris ; Brockwell, Tim (2011)

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.DOI: 10.1111/j.1600-0889.2004.00117.x
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