We developed a simple measurement system for delta17O in nanomole quantities of CO2 using continuous flow isotope ratio mass spectrometry (CF-IRMS). The analytical system consisted of a sample injection system, a helium-purged CO2 purification line, a capillary GC, a combustion unit, and CF-IRMS. A unique feature of the system is that we use molecular CO2 to determine the isotopic compositions including delta17O. The delta17O of CO2 in a sample is calculated from the mass ratios of both 45/44 and 46/44 of two different kinds of CO2, which have been purified quantitatively from different aliquots of a sample. While one aliquot (rCO2) flows into IRMS directly, the other (eCO2) flows through a CuO unit (900 degrees C) prior to injection into IRMS, to exchange oxygen atoms in the sample CO2 molecules with those in CuO for which we can assume Delta17O = 0. In our system, we introduce both rCO2 and eCO2 alternately to IRMS repeatedly by using an automatic multianalytical system to improve analytical precision statistically. The standard deviation of 0.35 per thousand for Delta17O can be realized using as little as 8.7 nmol CO2 in a approximately 3-h analysis. Based on this system, we have quantified delta17O in the stratospheric CO2 over Japan.

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 CO 2 and the N 2 O mixing ratio. The maximum 17 O CO 2 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 δ 18 O CO 2 -δ 17 O CO 2 plot (r 2 >0.95) shows a slope of 1.63±0.10, 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 17 O CO 2 and δ 18 O CO 2 with the N 2 O mixing ratio, we quantified triple oxygen-isotope fluxes of CO 2 to the troposphere as +48‰ GtC/yr ( 17 O CO 2 ) and +38‰ GtC/yr (δ 18 O CO 2 ) with ∼30% uncertainty. Comparing recent model results and observations, underestimation of the three isotope slope and the maximum 17 O CO 2 value in the model were clarified, suggesting a smaller O 2 photolysis contribution than that of the model. Simultaneous observations of δ 18 O CO 2 , δ 17 O CO 2 , and N 2 O mixing ratios can elucidate triple oxygen isotopes in CO 2 and clarify complex interactions among physical, chemical, and photochemical processes occurring in the middle atmosphere.

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 CO 2 and the N 2 O mixing ratio. The maximum 17 O CO 2 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 δ 18 O CO 2-δ 17 O CO 2 plot (r 2 >0.95) shows a slope of 1.63±0.10, 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 17 O CO 2 and δ 18 O CO 2 with the N 2 O mixing ratio, we quantified triple oxygen-isotope fluxes of CO 2 to the troposphere as +48‰ GtC/yr (17 O CO 2) and +38‰ GtC/yr (δ 18 O CO 2) with ∼30% uncertainty. Comparing recent model results and observations , underestimation of the three isotope slope and the maximum 17 O CO 2 value in the model were clarified, suggesting a smaller O 2 photolysis contribution than that of the model. Simultaneous observations of δ 18 O CO 2 , δ 17 O CO 2 , and N 2 O mixing ratios can elucidate triple oxygen isotopes in CO 2 and clarify complex interactions among physical, chemical , and photochemical processes occurring in the middle atmosphere.