Abstract
Abstract. Stratospheric and upper tropospheric air samples were collected during 1994–2004 over Sanriku, Japan and in 1997 over Kiruna, Sweden. Using these archived air samples, we determined the triple oxygen-isotope composition of stratospheric CO2 and the N2O mixing ratio. The maximum Δ17OCO2 value of +12.2‰, resembling that observed previously in the mesosphere at 60 km height, was found in the middle stratosphere over Kiruna at 25.6 km height, suggesting that upper stratospheric and mesospheric air descended to the middle stratosphere through strong downward advection. A least-squares regression analysis of our observations on a δ18OCO2−δ17OCO2 plot (r2>0.95) shows a slope of 1.63±pm0.10, which is similar to the reported value of 1.71±0.06, thereby confirming the linearity of three isotope correlation with the slope of 1.6–1.7 in the mid-latitude lower and middle stratosphere. The slope decrease with increasing altitude and a curvy trend in three-isotope correlation reported from previous studies were not statistically significant. Using negative linear correlations of Δ17OCO2 and δ18OCO2 with the N2O mixing ratio, we quantified triple oxygen-isotope fluxes of CO2 to the troposphere as +48‰ GtC/yr (Δ17OCO2) and +38‰ GtC/yr (δ18OCO2) with ~30% uncertainty. Comparing recent model results and observations, underestimation of the three isotope slope and the maximum Δ17OCO2 value in the model were clarified, suggesting a smaller O2 photolysis contribution than that of the model. Simultaneous observations of δ18OCO2, δ17OCO2, and N2O mixing ratios can elucidate triple oxygen isotopes in CO2 and clarify complex interactions among physical, chemical, and photochemical processes occurring in the middle atmosphere.
Highlights
The value of δ17O, (defined as [(17O/16O)sample/ (17O/16O)VSMOW−1]×1000 (‰), where VSMOW is an international oxygen isotope standard) varies during many isotopic fractionation processes, but the relation between δ17O and δ18O values is usually conserved according to the mass-dependent relation of δ17O=0.516×δ18O (Matsuhisa et al, 1978; Santrock et al, 1985)
Mass-independent triple oxygen-isotopic fractionation processes, which engender a non-zero value of 17O (=δ17O−0.516×δ18O), so-called 17O anomaly, have been found in several chemical reactions, such as in the photochemical production of O3 from O2 (Thiemens and Heidenreich, 1983; Mauersberger, 1987)
Anomalously high δ18O values and a positive 17O anomaly have been observed in stratospheric CO2 over Texas and New Mexico (Thiemens et al, 1991), in southern high-latitude regions (Thiemens et al, 1995a), over Sanriku in Japan (Gamo et al, 1989;, 1995; Aoki et al, 2003; Kawagucci et al, 2005), over Kiruna in Sweden (Alexander et al, 2001; Lammerzahl et al, 2002), Aire-sur-l’Adour in France (Lammerzahl et al, 2002), in northern high-latitudinal regions (Boering et al, 2004), and in the upper stratosphere and lower mesosphere over New Mexico (Thiemens et al, 1995b)
Summary
The value of δ17O, (defined as [(17O/16O)sample/ (17O/16O)VSMOW−1]×1000 (‰), where VSMOW is an international oxygen isotope standard) varies during many isotopic fractionation processes, but the relation between δ17O and δ18O values is usually conserved according to the mass-dependent relation of δ17O=0.516×δ18O (Matsuhisa et al, 1978; Santrock et al, 1985). Lammerzahl et al (2002) reported a close linear relation with a slope of 1.71±0.06 (2σ ) in the lower/middle stratosphere air over Aire-sur-l’Adour (44◦ N) and Kiruna (68◦ N); in contrast, Thiemens et al (1995b) reported a slope of 1.18±0.17 for the upper stratosphere/lower mesospheric air over New Mexico (32◦ N) This difference in slope was interpreted as a gradual decrease in slope occurring concomitantly with increasing altitude (Kawagucci et al, 2005; Liang et al, 2007), such a change in slopes has not been observed to date. Oxygen isotope fluxes in CO2 from the stratosphere to the troposphere and triple oxygen isotope fractionation processes in the middle atmosphere are discussed using the comprehensive dataset of triple oxygen isotope composition of stratospheric CO2 and the mixing ratio of N2O
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