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

Unambiguous observational evidence for a decrease in Antarctic ozone depletion attributable to the Montreal Protocol and its amendments remains elusive more than 20 years after ozone depleting substance (ODS) emissions ceased. Satellite data analyses identified a global decrease in the upper stratospheric HCl, where it represents nearly all inorganic chlorine (Cl_y), more than a decade ago. But in the Antarctic lower stratosphere, both the detection of ozone recovery and its attribution to the Montreal Protocol remain challenging because year-to-year variations in ozone depletion are largely driven by temperature rather than chlorine variations, and there are no Cl_y observations to verify a decrease.

We use Aura Microwave Limb Sounder (MLS) measurements of O₃, HCl, and N₂O to demonstrate that the inorganic chlorine (Cl_y) 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 Cl_y. MLS N₂O measurements are used to identify HCl (Cl_y) changes independent of dynamical variability and provide a reference for quantifying the Cl_y 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 Cl_y each September using N₂O, the N₂O-Cl_y tracer-tracer relationship, and the Cl_y decline rate determined from HCl. Vortex Cl_y has interannual variability that is nearly an order of magnitude larger than the annual mean Cl_y decline (~25 ppt). After filtering out high frequency variability in the O₃ loss and Cl_y time series, we find that Cl_y and O₃ loss have both declined from 2005-2016. Ozone depletion and Cl_y show matching periods of decline (2005-2010), stability (2010-2013), and increase (2013-2016). The observed sensitivity of O₃ depletion to changing Cl_y 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 Cl_y.