Atmos. Chem. Phys. https://doi.org/10.5194/acp-19-14387-2019 (2019)

The impact of ozone depends on where it is. In the lowest atmospheric layer (the troposphere), ozone is a pollutant and greenhouse gas. But in the overlying stratosphere, it is naturally abundant and absorbs harmful solar radiation.

Tropospheric ozone concentrations are governed by precursor emissions at the surface and by delivery from above, which occurs when the stratosphere ‘folds’ and dips into the underlying troposphere. How climate change will affect the frequency of this folding — and in turn stratosphere-to-troposphere transport — is important for projecting lower-atmosphere ozone abundance, as this mechanism is independent of ground-level pollution.

Dimitris Akritidis at Aristotle University of Thessaloniki, Greece, and co-authors in Greece and Germany used a global climate model to project robust changes in folding frequency under climate warming, particularly in the mid-latitudes. They find that stratospheric ozone recovery and regional increases in folding frequency enhance stratosphere-to-troposphere ozone transport, especially for hotspots such as the summertime Mediterranean and Middle East. Importantly, tropospheric ozone concentrations are slated to grow despite near-surface decreases in precursor emissions.