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

Abstract

Abstract. 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.