Natural halogens buffer tropospheric ozone in a changing climate

An Earth system model estimates that natural halogens, of marine biotic and abiotic origin, remove about 13% of present-day global tropospheric O-3. Projections suggest this ratio is stable through 2100, with high spatial heterogeneity, despite increasing natural halogens. Reactive atmospheric halogens destroy tropospheric ozone (O-3), 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 O-3, are not well known. Here, we use an Earth system model to estimate that natural halogens deplete approximately 13% of tropospheric O-3 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 O-3 loss. Notably, this halogen-driven O-3 buffering is projected to be greatest over polluted and populated regions, due mainly to iodine chemistry, with important implications for air quality.