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 (Oinline-formula₃) precursors are expected to alter the temporal and spatial distributions of tropospheric Oinline-formula₃. In this study, we quantify changes in the tropospheric Oinline-formula₃ budget and in the spatiotemporal distribution of surface Oinline-formula₃ 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 Oinline-formula₃ and its precursors. The results reveal that the global tropospheric Oinline-formula₃ 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 Oinline-formula₃ in the troposphere is expected to increase from 69 to 77 Tg. The mean lifetime of tropospheric Oinline-formula₃ is expected to increase by 2 d (inline-formula∼10 %). Changes in climate under a net-zero pathway are relatively small and only lead to small increases in tropospheric Oinline-formula₃. Over eastern China, surface Oinline-formula₃ increases in winter due to the weakened titration of Oinline-formula₃ by NO associated with reduced anthropogenic NO emissions and due to enhanced stratospheric input. In summer, surface Oinline-formula₃ decreases by more than 30 ppbv, and peak concentrations shift from July to May. Local contributions from anthropogenic emissions to surface Oinline-formula₃ 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 Oinline-formula₃, especially in summer (inline-formula∼40 %). This enhanced contribution is mainly due to the increased Oinline-formula₃ production efficiency under lower anthropogenic precursor emissions. Over eastern China, local anthropogenic contributions decrease from 50 % to 30 %. The decreases in surface Oinline-formula₃ are strongly beneficial and are more than sufficient to counteract the increases in surface Oinline-formula₃ 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 Oinline-formula₃ pollution over east Asia.