Natural halogens buffer tropospheric ozone in a changing climate

10 pags., 6 figs.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.This study has received funding from the European Research Council Executive Agency under the European Union’s Horizon 2020 Research and Innovation programme (Project ‘ERC-2016-COG 726349 CLIMAHAL’). R.H. is supported by a NERC Independent Research Fellowship (NE/N014375/1). CAM-Chem is a component of the Community Earth System Model (CESM), which is supported by the NSF and the Office of Science of the US Department of Energy. Computing resources were provided by NCAR’s Climate Simulation Laboratory, which is sponsored by the NSF and other agencies. Computing resources, support and data storage were provided and are maintained by the Computational and Information System Laboratory from the National Center for Atmospheric Research (CISL).Peer reviewe