Investigation into the stratosphere by long-term observations of the triple isotopic compositions of stratospheric CO2

Triple oxygen isotopic composition (δ18O, Δ17O) of stratospheric CO2 is useful as a tracer to study processes of both chemical reactions and air circulations in the stratosphere. In this study, we quantified triple oxygen isotopic compositions of stratospheric CO2 sampled over Japan by using a balloon-borne cryogenic sampler, as well as their temporal variations for past 15 years. We found significant oxygen isotope anomalies in stratospheric CO2, while the temporal and seasonal variation was little in both δ18O and Δ17O. Besides, the values coincide well with those reported for lower stratospheric CO2 sampled over Sweden and France by Laemmerzahl et al. (2002), especially for the relation between δ18O and Δ17O. We conclude that our dataset represents the actual triple oxygen isotopic compositions of CO2 in the lower stratosphere. The anomalous oxygen isotopic compositions of stratospheric CO2 have been explained by oxygen exchanging reaction between CO2 and O3 via O(1D) [Yung et al., 1991]. However, it is difficult to explain the observed triple oxygen isotopic compositions only by the process, because the observed Δ17O values are substantially higher than the expected Δ17O values from each δ18O value, assuming oxygen exchanging reaction between tropospheric CO2 and stratospheric O3, using the reported Δ17O and δ18O values for stratospheric O3 [Mauersberger et al., 2003]. Thus, we conclude that some unknown mass independent fractionation processes by some photochemical reactions in upper stratosphere or mesosphere, such as production and/or decomposition reaction of CO3 complex, for example, must be more responsible for the origin of the anomalous oxygen isotopic compositions of stratospheric CO2. That is to say, the mixing of air masses between upper stratosphere and troposphere must be responsible for the anomalous oxygen isotopic compositions of CO2 in the lower stratosphere, rather than by in-situ photochemical reactions in the lower stratosphere.