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<p>Ground-level ozone (O<span class="inline-formula">₃</span>) pollution has been steadily getting worse in most parts of eastern China during the past 5 years. The non-linearity of O<span class="inline-formula">₃</span> formation with its precursors like nitrogen oxides (NO<span class="inline-formula">_<i>x</i>=</span> NO <span class="inline-formula">+</span> NO<span class="inline-formula">₂</span>) and volatile organic compounds (VOCs) are complicating effective O<span class="inline-formula">₃</span> abatement plans. The diagnosis from space-based observations, i.e. the ratio of formaldehyde (HCHO) columns to tropospheric NO<span class="inline-formula">₂</span> columns (HCHO <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="fb147fccdcf98a9911cf3d26a8f6dc33"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-7253-2021-ie00001.svg" width="8pt" height="14pt" src="acp-21-7253-2021-ie00001.png"/></svg:svg></span></span> NO<span class="inline-formula">₂</span>), has previously been proved to be highly consistent with our current understanding of surface O<span class="inline-formula">₃</span> chemistry. HCHO <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="165b352473919034209a9d51d0eaf41d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-7253-2021-ie00002.svg" width="8pt" height="14pt" src="acp-21-7253-2021-ie00002.png"/></svg:svg></span></span> NO<span class="inline-formula">₂</span> ratio thresholds distinguishing O<span class="inline-formula">₃</span> formation sensitivity depend on regions and O<span class="inline-formula">₃</span> chemistry interactions with aerosol. To shed more light on the current O<span class="inline-formula">₃</span> formation sensitivity over China, we have derived HCHO <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="c6f00d13d95b9183e3e2526db4298e27"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-7253-2021-ie00003.svg" width="8pt" height="14pt" src="acp-21-7253-2021-ie00003.png"/></svg:svg></span></span> NO<span class="inline-formula">₂</span> ratio thresholds by directly connecting satellite-based HCHO <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M18" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="62d2c8208bbdf49afb8db19c9f7b6b50"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-7253-2021-ie00004.svg" width="8pt" height="14pt" src="acp-21-7253-2021-ie00004.png"/></svg:svg></span></span> NO<span class="inline-formula">₂</span> observations and ground-based O<span class="inline-formula">₃</span> measurements over the major Chinese cities in this study. We find that a VOC-limited regime occurs for HCHO <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="493c42bdafda293669646c99f86fe0ce"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-7253-2021-ie00005.svg" width="8pt" height="14pt" src="acp-21-7253-2021-ie00005.png"/></svg:svg></span></span> NO<span class="inline-formula">₂</span> <span class="inline-formula"><i>&lt;</i></span> 2.3, and a NO<span class="inline-formula">_<i>x</i></span>-limited regime occurs for HCHO <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M25" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="64e3733ac81609367f37ca130d7132b9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-7253-2021-ie00006.svg" width="8pt" height="14pt" src="acp-21-7253-2021-ie00006.png"/></svg:svg></span></span> NO<span class="inline-formula">₂</span> <span class="inline-formula"><i>&gt;</i></span> 4.2. The HCHO <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M28" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="6e9042c80619a800f49fe8d9da77f107"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-7253-2021-ie00007.svg" width="8pt" height="14pt" src="acp-21-7253-2021-ie00007.png"/></svg:svg></span></span> NO<span class="inline-formula">₂</span> between 2.3 and 4.2 reflects the transition between the two regimes. Our method shows that the O<span class="inline-formula">₃</span> formation sensitivity tends to be VOC-limited over urban areas and NO<span class="inline-formula">_<i>x</i></span>-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<span class="inline-formula">_<i>x</i></span> 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<span class="inline-formula">₃</span> concentrations. The enhanced O<span class="inline-formula">₃</span> concentrations in urban areas during the COVID-19 lockdown in China indicate that a protocol with simultaneous anthropogenic NO<span class="inline-formula">_<i>x</i></span> emissions and VOC emissions controls is essential for O<span class="inline-formula">₃</span> abatement plans.</p>