Ground-level ozone (O.sub.3) pollution has been steadily getting worse in most parts of eastern China during the past 5 years. The non-linearity of O.sub.3 formation with its precursors like nitrogen oxides (NO.sub.x = NO + NO.sub.2) and volatile organic compounds (VOCs) are complicating effective O.sub.3 abatement plans. The diagnosis from space-based observations, i.e. the ratio of formaldehyde (HCHO) columns to tropospheric NO.sub.2 columns (HCHO / NO.sub.2 ), has previously been proved to be highly consistent with our current understanding of surface O.sub.3 chemistry. HCHO / NO.sub.2 ratio thresholds distinguishing O.sub.3 formation sensitivity depend on regions and O.sub.3 chemistry interactions with aerosol. To shed more light on the current O.sub.3 formation sensitivity over China, we have derived HCHO / NO.sub.2 ratio thresholds by directly connecting satellite-based HCHO / NO.sub.2 observations and ground-based O.sub.3 measurements over the major Chinese cities in this study. We find that a VOC-limited regime occurs for HCHO / NO.sub.2 2.3, and a NO.sub.x -limited regime occurs for HCHO / NO.sub.2 4.2. The HCHO / NO.sub.2 between 2.3 and 4.2 reflects the transition between the two regimes. Our method shows that the O.sub.3 formation sensitivity tends to be VOC-limited over urban areas and NO.sub.x -limited over rural and remote areas in China. We find that there is a shift in some cities from the VOC-limited regime to the transitional regime that is associated with a rapid drop in anthropogenic NO.sub.x emissions, owing to the widely applied rigorous emission control strategies between 2016 and 2019. This detected spatial expansion of the transitional regime is supported by rising surface O.sub.3 concentrations. The enhanced O.sub.3 concentrations in urban areas during the COVID-19 lockdown in China indicate that a protocol with simultaneous anthropogenic NO.sub.x emissions and VOC emissions controls is essential for O.sub.3 abatement plans.