Future tropospheric ozone budget and distribution over East Asia under a Net Zero scenario

Abstract. Under future net zero emission policies, reductions in emissions of ozone (O₃) precursors are expected to alter the temporal and spatial distribution of tropospheric O₃. In this study, we quantify changes in the tropospheric O₃ budget, spatiotemporal distribution of surface O₃ in East Asia and the contributions from regional emissions, intercontinental transport and climate change between the present day and 2060 under a net zero scenario, using the NCAR Community Earth System Model (CESM) with online tagging of O₃ and its precursors. The results reveal that the global tropospheric O₃ burden is likely to decrease by more than 20 %, from 316 Tg in present day to 247 Tg in 2060, under a net zero scenario. The burden of stratospheric O₃ in the troposphere is expected to increase from 69 to 77 Tg. The mean lifetime of tropospheric O₃ increases by 2 days, ~10 %. Changes in climate under a net zero pathway are relatively small, and only lead to small increases in tropospheric O₃. Over East China, surface O₃ increases in winter, due to the weakened titration of O₃ by NO associated with reduced anthropogenic NO emissions, and to enhanced stratospheric input. In summer, surface O₃ decreases by more than 30 ppbv, and peak concentrations shift from July to May. Local contributions from anthropogenic emissions to surface O₃ over East Asia are highest in summer, but drop substantially, from 30 % to 14 %, under a net zero scenario. The contribution of biogenic sources is enhanced, and forms the dominant contributor to future surface O₃, especially in summer, ~40 %. This enhanced contribution is mainly due to the increased O₃ production efficiency under lower anthropogenic precursor emissions. Over Eastern China, local anthropogenic contributions decrease from 50 % to 30 %. The decreases in surface O₃ are strongly beneficial, and are more than sufficient to counteract the increases in surface O₃ observed in China over recent years. This study thus highlights the important co-benefits of net zero policies that target climate change in addressing surface O₃ pollution over East Asia.