Future tropospheric ozone budget and distribution over East Asia under a net-zero scenario

Under future net-zero emission policies, reductions in emissions of ozone (O₃) precursors are expected to alter the temporal and spatial distributions of tropospheric O₃. In this study, we quantify changes in the tropospheric O₃ budget and in the spatiotemporal distribution of surface O₃ in east Asia and the contributions of 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 the 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 is expected to increase by 2 d (∼ 10 %). Changes in climate under a net-zero pathway are relatively small and only lead to small increases in tropospheric O₃. Over eastern China, surface O₃ increases in winter due to the weakened titration of O₃ by NO associated with reduced anthropogenic NO emissions and due 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 NO 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.