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

Abstract

The atmospheric Ar/N 2 ratio is expected to undergo very slight variations due to exchanges of Ar and N 2 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/N 2 , O 2 /N 2 and CO 2 /N 2 ratios. The system achieves a short-term precision in Ar/N 2 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/N 2 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/N 2 based on the TM2 tracer transport model in combination with air-sea Ar and N 2 fluxes derived from climatological air-sea heat fluxes. The possibility is suggested that Ar/N 2 ratios may be detectably enriched near the ground by gravimetric or thermal fractionation under conditions of strong surface inversions.

Development of a new high precision continuous measuring system for atmospheric O₂/N₂ and Ar/N₂ and its application to the observation in Tsukuba, Japan

“…On the other hand, peak-to-peak amplitudes of the seasonal d(Ar/N 2 ) cycle reported in past studies are in a range of 6Á35 per meg (Battle et al, 2003;Keeling et al, 2004;Blaine, 2005;Cassar et al, 2008); these are significantly smaller than those of the seasonal d(O 2 /N 2 ) cycles. Since Keeling et al (2004) estimated the long-term change of atmospheric d(Ar/N 2 ) to be only about 2.6 per meg due to an ocean warming of about 10 )10 22 J, roughly corresponding to an increase in heat content in the top 0Á700 m layer during 1990Á2005 (Levitus et al, 2009) (http:// www.nodc.noaa.gov/OC5/3M_HEAT_CONTENT/).…”

“…The molecular mass of (Bender et al, 1994;Leuenberger et al, 2000;Ishidoya et al, 2003;Keeling et al, 2004). To examine this effect, we repeat number of times an experiment in which a very small and different amount of pure CO 2 is added through SV3 to an air-based CO 2 standard air flowing in the sample-air line shown in Fig.…”

“…Blaine, 2005;Blaine et al, 2006;Sturm et al, 2006). As a result, only a few studies have succeeded in observing the seasonal cycles of the atmospheric Ar/N 2 ratio at surface stations (Battle et al, 2003;Keeling et al, 2004;Blaine, 2005;Cassar et al, 2008 where the subscripts 'sample' and 'standard' indicate the O 2 /N 2 (Ar/N 2 ) ratios of the sample and the standard air, respectively. Because O 2 (Ar) constitutes 20.946 (0.934)% of air by volume (Nicolet, 1960;Machta and Hughes, 1970) , respectively (e.g.…”

“…This requires a precision of 910 per meg or better to detect a significant seasonal cycle and a long-term trend in d(Ar/N 2 ), the latter of which has not been observed so far. In analysing flask air samples of atmospheric d(Ar/N 2 ), Battle et al (2003) and Keeling et al (2004) achieved a precision of about 910 per meg using a mass spectrometer with a customised dual inlet system (Finnigan Delta Plus XL) and a single capillary mass spectrometer (ISO-PRIME), respectively. Keeling et al (2004) and Blaine (2005) also developed a continuous measurement system for atmospheric d(O 2 /N 2 ) and d(Ar/N 2 ) using ISOPRIME and achieved more precise measurements of d(Ar/N 2 ) (92Á3 per meg for 1 hour mean values).…”

“…Since Keeling et al (2004) estimated the long-term change of atmospheric d(Ar/N 2 ) to be only about 2.6 per meg due to an ocean warming of about 10 )10 22 J, roughly corresponding to an increase in heat content in the top 0Á700 m layer during 1990Á2005 (Levitus et al, 2009) (http:// www.nodc.noaa.gov/OC5/3M_HEAT_CONTENT/). This requires a precision of 910 per meg or better to detect a significant seasonal cycle and a long-term trend in d(Ar/N 2 ), the latter of which has not been observed so far.…”