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 N2O 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, similar to 0.8%/yr, agrees with the expected rate based on chlorofluorocarbon lifetimes. N2O 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. Plain Language Summary The Antarctic ozone hole is healing slowly because levels of the man-made chemicals causing the hole have long lifetimes. We use Microwave Limb Sounder (MLS) satellite data to measure O-3 over Antarctica at the beginning of winter and then compare it to O-3 near the end of winter to calculate depletion. During this period, nearly all O-3 change is due to depletion. MLS also measures HCl, and when ozone levels are very low, nearly all the reactive chlorine species (Cl-y) are converted to HCl. Cl-y varies a lot from year to year from atmospheric motions. Fortunately, MLS measures nitrous oxide (N2O), a long-lived gas that also varies with the motions. Using the ratio of Cl-y to N2O, we find that there is less chlorine now than 9 years ago and that Cl-y has decreased on average about 25 parts per trillion/yr (0.8%/yr). The O-3 depletion we calculate from MLS data responds to changes in the Cly levels, and the ratio of the change in ozone loss to the change in Cl-y matches model calculations. All of this is evidence that the Montreal Protocol is working-the Cl-y is decreasing in the Antarctic stratosphere and the ozone destruction is decreasing along with it.