On the Role of Heterogeneous Chemistry in Ozone Depletion and Recovery

We demonstrate that identification of stratospheric ozone changes attributable to ozone depleting substances and actions taken under the Montreal Protocol requires evaluation of confounding influences from volcanic eruptions. Using a state-of-the-art chemistry-climate model, we show that increased stratospheric aerosol loading from volcanic eruptions after 2004 impeded the rate of ozone recovery post-2000. In contrast, eruptions increased ozone loss rates over the depletion era from 1980 to 1998. We also present calculations without any aerosol chemistry to isolate contributions from gas-phase chemistry alone. This study reinforces the need for accurate information regarding stratospheric aerosol loading when modeling ozone changes, particularly for the challenging task of accurately identifying the early signs of ozone healing distinct from other sources of variability. Plain Language Summary This study examines the impact of volcanic eruptions on ozone depletion and recovery. Volcanic eruptions increase stratospheric aerosols and thereby enhance ozone depletion. Model simulations indicate that several large eruptions increased the rate of ozone depletion from 1979 to 1998, while a series of moderate eruptions after 2004 slowed the rate of ozone recovery from 1999 to 2014. The Montreal Protocol's effectiveness in restoring the ozone layer can be expected to emerge most clearly whenever volcanically quiescent periods occur in the future.