Short-term variations in the ¹³C/¹²C ratio of CO as a measure of Cl activation during tropospheric ozone depletion events in the Arctic

Analyses of the CO stable isotopic composition in surface air samples collected at high northern latitudes during Arctic spring reveal significant depletions in the ¹³C content of atmospheric CO, which coincide with episodes of tropospheric O₃ loss. This isotope signal can be explained by production of small amounts of highly ¹³C-depleted CO, formed in Cl-initiated oxidation of CH₄ due to the presence of enhanced Cl levels during O₃ depletion events. Using the recently measured fractionation factor for the CH₄ + Cl reaction, the observed ¹³C depletion is employed to estimate the time-integrated amount of Cl radicals encountered by the O₃ depleted air mass. The results support findings obtained from measurements of hydrocarbon mixing ratios during O₃ depletion events. The ¹³C content of atmospheric CO can thus be used as a sensitive indicator for enhanced Cl radical levels in the atmosphere.