Arctic halogens reduce ozone in the northern mid-­latitudes

While the dominant role of halogens in Arctic ozone loss during spring has been widely studied in the last decades, the impact of sea-­ice halogens on surface ozone abundance over the northern hemisphere (NH) mid-­latitudes remains unquantified. Here, we use a state-­of-­the-­art global chemistry-­climate model including polar halogens (Cl, Br, and I), which reproduces Arctic ozone seasonality, to show that Arctic sea-­ice halo- gens reduce surface ozone in the NH mid-­latitudes (47°N to 60°N) by ~11% during spring. This background ozone reduction follows the southward export of ozone-­poor and halogen-­rich air masses from the Arctic through polar front intrusions toward lower latitudes, reducing the springtime tropospheric ozone column within the NH mid-­latitudes by ~4%. Our results also show that the present-­day influence of Arctic halogens on surface ozone destruction is comparatively smaller than in preindustrial times driven by changes in the chemical interplay between anthropogenic pollution and natural halogens. We conclude that the impact of Arctic sea-­ice halogens on NH mid-­latitude ozone abundance should be incorporated into global models to improve the representation of ozone seasonality.