The role of natural halogens in global tropospheric ozone chemistry and budget under different 21st century climate scenarios

Abstract

Tropospheric ozone (03) is an important greenhouse gas and a surface pollutant. The future evolution of O3 abundances and chemical processing are uncertain due to a changing climate, socioeconomic developments, and missing chemistry in global models. Here, we use an Earth System Model with natural halogen chemistry to investigate the changes in the O3 budget over the 21st century following RCP6.0 and RCP8.5 climate scenarios. Our results indicate that the global tropospheric O3 net chemical change (NCC, chemical gross production minus destruction) will decrease -50%, notwithstanding increasing or decreasing trends in ozone production and loss. However, a wide range of surface NCC variations (from -60% to 150%) are projected over polluted regions with stringent abatements in O3 precursor emissions. Water vapour and iodine are found to be key drivers of future tropospheric O3 destruction, while the largest changes in O3 production are determined by the future evolution of peroxy radicals. We show that natural halogens, currently not considered in climate models, significantly impact on the present-day and future global O3 burden reducing - 30-35 Tg (11-15%) of tropospheric ozone throughout the 21st century regardless of the RCP scenario considered. This highlights the importance of including natural halogen chemistry in climate model projections of future tropospheric ozone.