We use Aura Microwave Limb Sounder (MLS) measurements of O3, HCl, and N2O to demonstrate that the inorganic chlorine (Cly) in the Antarctic lower stratosphere has declined from 2004-2016. When MLS measures extremely low O3 in late October, co-located MLS HCl measurements are very nearly equal to total Cly. MLS N2O measurements are used to identify HCl (Cly) changes independent of dynamical variability and provide a reference for quantifying the Cly decline rate. The decline rate determined, ~0.8%/yr, agrees with the expected rate based on chlorofluorocarbon lifetimes.
For each winter 2005-2016, we calculate O3 depletion between early July and mid-September using vortex-averaged MLS partial column O3. This time period is optimal for detecting a change in depletion due to changing chlorine because dynamically-driven O3 variability is relatively low. We infer the vortex-averaged Cly each September using N2O, the N2O-Cly tracer-tracer relationship, and the Cly decline rate determined from HCl. Vortex Cly has interannual variability that is nearly an order of magnitude larger than the annual mean Cly decline (~25 ppt). After filtering out high frequency variability in the O3 loss and Cly time series, we find that Cly and O3 loss have both declined from 2005-2016. Ozone depletion and Cly show matching periods of decline (2005-2010), stability (2010-2013), and increase (2013-2016). The observed sensitivity of O3 depletion to changing Cly agrees with the sensitivity simulated by the Global Modeling Initiative chemistry transport model integrated with MERRA2 meteorology. MLS observations provide substantial evidence that the Montreal Protocol is working to reduce O3 depletion by decreasing Cly.