Decline in Antarctic Ozone Depletion and Lower Stratospheric Chlorine Determined From Aura Microwave Limb Sounder Observations

Attribution of Antarctic ozone recovery to the Montreal protocol requires evidence that (1) Antarctic chlorine levels are declining and (2) there is a reduction in ozone depletion in response to a chlorine decline. We use Aura Microwave Limb Sounder measurements of O 3 , HCl, and N 2 O to demonstrate that inorganic chlorine (Cl y ) from 2013 to 2016 was 223 ± 93 parts per trillion lower in the Antarctic lower stratosphere than from 2004 to 2007 and that column ozone depletion declined in response. The mean Cl y decline rate, ~0.8%/yr, agrees with the expected rate based on chlorofluorocarbon lifetimes. N 2 O measurements are crucial for identifying changes in stratospheric Cl y loading independent of dynamical variability. From 2005 to 2016, the ozone depletion and Cl y time series show matching periods of decline, stability, and increase. The observed sensitivity of O 3 depletion to changing Cl y agrees with the sensitivity simulated by the Global Modeling Initiative chemistry transport model integrated with Modern Era Retrospective Analysis for Research and Applications 2 meteorology.