Natural halogens buffer tropospheric ozone in a changing climate

Reactive atmospheric halogens destroy tropospheric ozone (O₃), an air pollutant and greenhouse gas. The primary source of natural halogens is emissions from marine phytoplankton and algae, as well as abiotic sources from ocean and tropospheric chemistry, but how their fluxes will change under climate warming, and the resulting impacts on O₃, are not well known. Here, we use an Earth system model to estimate that natural halogens deplete approximately 13% of tropospheric O₃ in the present-day climate. Despite increased levels of natural halogens through the twenty-first century, this fraction remains stable due to compensation from hemispheric, regional and vertical heterogeneity in tropospheric O₃ loss. Notably, this halogen-driven O₃ buffering is projected to be greatest over polluted and populated regions, due mainly to iodine chemistry, with important implications for air quality.