Better Quantification of the Recent Unexpected Emission of CFC-11

CFC-11 and CFC-12 have caused the largest destruction of stratospheric ozone and production of these substances was phased-out by the Montreal Protocol. Nonetheless, a new emission of CFC-11 was recently reported and estimated to be 13±5 gigagrams per year since 2012. Here we better quantify this estimate by providing new estimates of both the corrected emission time series inferred from surface measurements and the emission time series expected from reported production. The unexpected emission is the difference between these estimates. The inferred emission time series is corrected using a new analysis of the dynamical contribution to the inferred emission. The dynamical contribution is largely due to interannual variations in stratosphere-troposphere transport, which we explore using a range of model runs using CESM-WACCM. We find no evidence of large dynamical contributions on timescales longer than 3-5 years. The expected emission time series is estimated using bottom up techniques to estimate the emission from reported production. These analyses provide a larger estimate of the unexpected emission than reported earlier. In addition, evidence of significant unexpected emission of CFC-12 is presented. Finally, a range of scenarios of past and future CFC-11 and CFC-12 time series that are consistent with the above analysis are presented and used to estimate the delay in ozone recovery.