Implications of constant CFC-11 concentrations for the future ozone layer

Abstract. This investigation is motivated by the results presented by Montzka et al. (2018). They discussed a strong deviation of the assumed emissions of chlorofluorocarbon-11 (CFC-11, CFCl3) in the past 15 years, which indicates a violation of the Montreal Protocol for the protection of the ozone layer. A Chemistry-Climate Model (CCM) study is performed, investigating the consequences of a constant CFC-11 surface mixing ratio for stratospheric ozone: In comparison to a reference simulation (REF-C2), where a decrease of the CFC-11 surface mixing ratio of about 50 % is assumed from the early 2000s to the middle of the century, a sensitivity simulation (SEN-C2-fCFC11) is carried out where after the year 2002 the CFC-11 surface mixing ratio is kept constant until 2050. Differences between these two simulations are shown. These illustrate possible effects on stratospheric ozone. The total column ozone (TCO) in the 2040s (i.e. the years 2041–2050) is in particular affected in both polar regions in winter and spring. At the end of the 2040s maximum discrepancies of TCO are identified with reduced ozone values of up to around 30 Dobson Units (in the order of 10 %) in the SEN simulation. An analysis of the respective partial column ozone (PCO) for the stratosphere indicates that strongest ozone changes are calculated for the polar lower stratosphere, where they are mainly driven by the enhanced stratospheric chlorine content and associated heterogeneous chemical processes. Furthermore, it turns out that the calculated ozone changes, especially in the upper stratosphere, are smaller than expected. In this altitude region the additional ozone depletion due to the catalysis by reactive chlorine is compensated partly by other processes related to enhanced ozone production or reduced ozone loss, for instance from nitrous oxide (NOx).