The Role of Natural Halogens in Global Tropospheric Ozone Chemistry and Budget Under Different 21st Century Climate Scenarios

Abstract 

Tropospheric ozone (O₃) is an important greenhouse gas and a surface pollutant. The future evolution of3OE 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 O₃budget over the 21st century following Representative Concentration Pathway (RCP) 6.0 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 vapor 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 global3OE 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 offuture tropospheric ozone.

Key Points

-Water vapor and iodine are key drivers of future tropospheric ozone destruction

-Natural halogens reduce 30–35 Tg of tropospheric ozone burden throughout the 21st century regardless of the Representative Concentration Pathway scenario considered

-Enhanced halogen-driven surface ozone loss will determine the effectiveness of future policies on air quality