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This study analyzed the differences in ozone (O3) sensitivity in four different urban areas in China from February 2019 to January 2020 based on data on various near-surface pollutants from passive multi-axis differential optical absorption spectroscopy (MAX-DOAS) sites and nearby China National Environmental Monitoring Center (CNEMC) sites. Across the four cities, the nitrogen dioxide (NO2) and formaldehyde (HCHO) concentrations varied seasonally. Xianghe consistently displayed the lowest NO2 levels, suggesting reduced emissions compared to other cities. Guangzhou, a city with a robust economy and a high level of vehicle ownership, exhibited higher concentrations in spring. Summer brought elevated HCHO levels in Guangzhou, Xianghe, and Shenyang due to intensified photochemical processes. Autumn and winter showed higher HCHO concentrations in Guangzhou and Xianghe compared to Lanzhou and Shenyang. Overall, Guangzhou recorded the highest annual averages, due to its developed economy, while Xianghe's lower NO2 levels were offset by the elevated HCHO due to higher O3 values. The analysis delved into primary and secondary HCHO sources across seasons and used carbon monoxide (CO) and O3 data. Xianghe showcased the dominance of secondary sources in summer and autumn, while Lanzhou was characterized by primary dominance throughout the year. Shenyang mirrored Xianghe's evolution due to industrial emissions. In Guangzhou, due to the high levels of vehicular traffic and sunlight conditions, secondary sources predominantly influenced HCHO concentrations. These findings highlight the interplay between primary and secondary emissions in diverse urban settings. This study explored O3 sensitivity variations across seasons. Xianghe exhibited a balanced distribution among volatile organic compound (VOC)-limited conditions, nitrogen oxide (NOx)-limited conditions, and transitional influences. Lanzhou was mainly affected by VOC-limited conditions in winter and NOx-limited conditions in other seasons. Shenyang's sensitivity varied with the seasons and was primarily influenced by transitions between VOCs and NOx in autumn and NOx-limited conditions otherwise. Guangzhou experienced varied influences. During periods of high O3 pollution, all regions were affected by NOx-limited conditions, indicating the necessity of NOx monitoring in these areas, especially during summer in all regions and during autumn in Xianghe and Guangzhou. [ABSTRACT FROM AUTHOR]

Diagnosing the vertical distributions of ozone formation sensitivities within the boundary layers is the prerequisite for formulating proper ozone control strategies. From 1 January 2020, to 20 August 2021, a multi‐function MAX‐DOAS instrument was set up in Guangzhou to detect the NO2 and HCHO vertical profiles. The ozone formation sensitivities in different height layers were diagnosed using the FNRsec, which was defined as the ratio of secondary HCHO to NO2 VMRs (HCHOsec/NO2). The secondary sources in ambient HCHO participate in the photochemical reactions directly, and the FNRsec can indicate the ozone formation sensitivities more accurately. Therefore, the secondary source was separated from ambient HCHO via a multi‐linear regression method, and its relative contribution was approximately 66.9 ± 6.3%. The sensitivity thresholds of ozone formation were determined by the slope of the linear regression analysis for O3 versus the normalized secondary HCHO or NO2 mixing ratios for different FNRsec ranges, which were 0.22 and 0.36. For the ground layer, the VOC‐limited, transitional, and NOx‐limited regimes accounted for 40.66%, 32.42%, and 26.92%, respectively. Diurnally, the ground layer was usually in the VOC‐limited regime in morning, but in the transitional regime in noon and afternoon. Vertically, the ozone formation sensitivity changed with altitude from VOC‐limited (0.02–0.22 km) to transitional (0.22–0.42 km) to NOx‐limited (0.42–2.02 km). These findings can help formulate proper ozone control strategies in time according to the varying local conditions. Plain Language Summary: Diagnosing the O3 formation regimes at different altitudes is essential to the regional control of tropospheric O3 pollution. This study utilized the vertical profiles of NO2 and HCHO detected by MAX‐DOAS to diagnose the vertical distributions of ozone formation sensitivities in Guangzhou. The secondary sources in ambient HCHO participated in the photochemical reaction directly. Hence the ratio of secondary HCHO to NO2 VMRs (FNRsec) was more favorable for indicating O3 formation sensitivities. During the observation, The FNRsec ranging from 0.22 to 0.36 indicated a transitional regime, and FNRsec < 0.22 and FNRsec > 0.36 represented VOC‐limited and NOx‐limited regimes, respectively. For the ground layer, the VOC‐limited, transitional, and NOx‐limited regimes accounted for 40.66%, 32. 42%, and 26.92%, respectively. Diurnally, the ground layer was usually in the VOC‐limited regime in morning, but in the transitional regime in noon and afternoon. Vertically, the O3 formation sensitivity changed with altitude from VOC‐limited (0.02–0.22 km) to transitional (0.22–0.42 km) to NOx‐limited (0.42–2.02 km). Therefore, the policymakers should pay more attention to controlling emissions of VOC (for example, HCHO and its precursors for the near‐ground layers. For the higher layers, reducing the local NOx emissions and limiting the regional transportation of NOx from nearby cities becomes more significant. Key Points: The FNRsec vertical profiles were used to indicate the O3 formation sensitivities in different height layersFor the ground layer, the VOC‐limited, transitional, and NOx‐limited regimes accounted for 40.66%, 32.42%, and 26.92%, respectivelyVertically, the O3 formation sensitivity changed with increasing altitude from VOC‐limited (0.02–0.22 km) to transitional (0.22–0.42 km) to NOx‐limited (0.42–2.02 km) [ABSTRACT FROM AUTHOR]