Mass-47 of Atmospheric CO2: Tropospheric Seasonal Variations and Stratospheric Anomalies

CO2 concentration in air and its δ13C and δ18O values are used to constrain fluxes to and from the atmosphere. However, the large number of sources and sinks prevent these from fully constraining the budget. Molecules containing two rare isotopes potentially carry additional information. We examined CO2 having mass 47, mostly 13C18O16O, in ambient air from Pasadena, CA, and in stratospheric air. We report data using Δ_{47, mass 47 anomaly, defined as the deviation of R47 from that expected for a random distribution of isotopologues. At thermodynamic equilibrium, CO2 closely approaches the random distribution at very high temperatures whereas at low temperatures bonds between 13C and 18O preferentially form to produce positive Δ47. Most low temperature sources of tropospheric CO2 involve isotopic exchange with water that should drive CO2 toward thermodynamic equilibrium and are likely to result in Δ47 values reflecting the exchange temperature. Such processes are expected to lead to seasonal variations with maximum winter values of 0.99‰ and minimum summer values of 0.92‰, based on averaged temperatures in Pasadena, CA. These values should then be decreased slightly by CO2 from combustion sources, but the seasonal range should be little changed. During 2004, measured Δ47 was 0.76‰ in winter, increased to 0.87‰ in summer, gradually decreased to 0.82‰ in autumn and increased again to 0.93‰ in June 2005. Summer values approached equilibrium at the local mean temperature (22° C, 0.94‰) but winter values were always lower than predicted (16° C, 0.97‰). Δ47 pattern during 2004 was approximately synchronized with that for δ13C, suggesting vegetation control for both tracers. Importantly, the observed variations were significantly greater than predicted at equilibrium, and the pattern was opposite of that predicted, indicating that at least one of the major fluxes does not reflect equilibrium values. We hypothesize that exchange of atmospheric CO2 with leaf water during photosynthesis drives Δ47 toward equilibrium values in summer and that respiration produces CO2 with values lower than equilibrium and draws the atmospheric value down in winter when photosynthetic activity is low. Stratospheric CO2 is enriched in both 18O and 17O by isotopic exchange with ozone via O(1D). This reaction creates 17O anomalies (Δ17O) in stratospheric CO2, which are reset at the surface by exchange with water and can be used to constrain gross photosynthetic fluxes. We find that stratospheric CO2 samples with high Δ17O values (2.6 and 5.9‰) also had exceptionally high Δ47 values (1.28 to 1.61‰), suggesting that stratospheric mass-47 anomalies, like 17O anomalies, are produced by reaction with O(1D). If this is the mechanism responsible, however, photochemical modeling shows that there must be a non-negligible effect of 13C in isotope exchange between CO2 and O(1D) (e.g., perhaps due to the lower zero point energy of 13C-18O bonds, such that decomposition of CO3* preferentially produces 13C18O16O, thus increasing Δ47). In summary, since Δ47 in atmospheric CO2 is controlled by factors other than those controlling δ13C and δ18O, it potentially provides useful additional information for deconvolving fluxes based on atmospheric records.