Near-global distribution of CO isotopic fractionation in the Earth's atmosphere

The first near-global (-85 degrees to 85 degrees) measurements of the isotopic fractionation of (CO)-C-13 relative to (CO)-C-12 have been obtained from 5 to 90 km using the ACE-FTS (Atmospheric Chemistry Experiment-Fourier Transform Spectrometer). These observations have been compared to predictions from WACCM (Whole Atmosphere Community Climate Model). The highest positive fractionation (i.e. relatively more (CO)-C-13) values of over 100 parts per thousand are observed in the lower thermosphere during winter in both hemispheres, whereas the highest negative fractionation (i.e. relatively more (CO)-C-12) is observed in the mesosphere in the summer at high latitudes (due to the highly fractionating effect that UV light has on CO2) and year round in the tropics. Agreement between measurements and model results is generally good at high altitude, although ACE shows a stronger fractionation effect from CO2 photolysis than predicted by WACCM. In the lower atmosphere, agreement is qualitatively good, although there is a distinct discrepancy at 40 km in all seasons, which is likely a retrieval artifact. (C) 2015 Elsevier Inc. All rights reserved.