Long-term observation of mass-independent oxygen isotope anomaly in stratospheric CO2

Anomalous oxygen isotopic compositions of stratospheric CO2 were first reported in 1989, although their detailed behavior in the middle atmosphere is still open question. We collected 60 stratospheric air samples over Sanriku, Japan from 1991 to 2004 and Kiruna, Sweden in 1997. Using these accumulated air samples, we performed long-term observations of triple oxygen isotope compositions of stratospheric CO2. It is the first simultaneous observation in the stratosphere of both relations between ?17OCO2 and [N2O], and ?18OCO2 and ?17OCO2, accurately. Observations confirmed simultaneous linear correlations between ?18OCO2, ?17OCO2, and [N2O] within [N2O]>50 ppbv: the correlations faded away along with decreasing [N2O] from 50 ppbv. For dividing observation results by [N2O]=50 ppbv, the divided datasets show that 1) both ?17OCO2 and ?18OCO2 are conservative parameters within the N2O-rich division, 2) the slope of least squares regression on ?18O??17O plot for the N2O-rich division is significantly steeper than that of the N2O-depleted one, and 3) the N2O-depleted division shows a discrepancy with recent model calculations, suggesting unconsidered isotope fractionation processes on complicated oxygen interactions in the CO2-O3-O2 system in the upper stratosphere and mesosphere.