Shifts of Formation Regimes and Increases of Atmospheric Oxidation Led to Ozone Increase in North China Plain and Yangtze River Delta From 2016 to 2019

China is confronting severe ozone (O-3) pollution although particulate matter reduced significantly, causing damages to public health and ecological systems. Here we utilized the comprehensive methods of ground-level observations, satellite data, and source-oriented chemical transport model to interpret O-3 variations throughout China from 2016 to 2019. A remarkably worsened trend of O-3 levels has been found both by observation and simulation in these years. Our results showed that the remarkable O-3 elevation was found in the North China Plain (NCP) and Yangtze River Delta (YRD) (maximum daily 8 hr average O-3 similar to 60 ppb) with an annual increasing rate of 10%. In addition, O-3 formation regimes also changed in the NCP, where 4.9% grids shifted from VOC-limited to transition regimes and 9.6% grids shifted from transition to NOx-limited regimes. The elevation of MDA8 O-3 was mainly attributed to the enhanced atmospheric oxidation capacity (AOC) in above regions. Particularly, the increasing rates of OH and HO2 radicals (major oxidants) in NCP and YRD reached similar to 15% and similar to 5%, respectively. The comprehensive study of long-term O-3 changes, formation regimes, and AOC based on a multimethod approach should be considered when designing O-3 control policies. Plain Language Summary Tropospheric Ozone (O-3) is a key secondary pollutant in the atmosphere, which is harmful to human health and the ecosystem. Although strict air pollution control policies have been implemented in China recently, the O-3 pollution remains serious nevertheless the reduced precursor such as HCHO and NO2. While field measurements and model simulations have been used to investigate O-3 variations, the roles of formation regimes, atmospheric oxidation capacity (AOC), and source apportionment of O-3 are still unclear. To address this knowledge gap, we utilized a multi-method approach, including satellite and ground-level observations as well as improved source-oriented simulations, to investigate the variations of O-3 and its associated precursors. Our findings revealed that from 2016 to 2019, the annual increasing ratio of O-3 was up to 10% in the North China Plain (NCP). The elevation of O-3 levels was caused by shifts in O-3 formation regimes and increases in AOC, which were associated with meteorological variations and anthropogenic emission reductions.