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With the coming of the “14th Five-Year Plan,” the coordinated control of particulate matter with an aerodynamic diameter no greater than 2.5 μm (PM2.5) and O3has become a major issue of air pollution prevention and control in China. The stereoscopic monitoring of regional PM2.5and O3and their precursors is crucial to achieve coordinated control. However, current monitoring networks are currently inadequate for monitoring the vertical profiles of both PM2.5and O3simultaneously and support air quality control. The University of Science and Technology of China (USTC) has established a nationwide ground-based hyperspectral stereoscopic remote sensing network based on multi-axis differential optical absorption spectroscopy (MAX-DOAS) since 2015. This monitoring network provides a significant opportunity for the regional coordinated control of PM2.5and O3in China. One-year vertical profiles of aerosol, NO2and HCHO monitored from four MAX-DOAS stations installed in four megacities (Beijing, Shanghai, Shenzhen, and Chongqing) were used to characterize their vertical distribution differences in four key regions, Jing–Jin–Ji (JJJ), Yangtze River Delta (YRD), Pearl River Delta (PRD), and Sichuan Basin (SB), respectively. The normalized and yearly averaged aerosol vertical profiles below 400 m in JJJ and PRD exhibit a box shape and a Gaussian shape, respectively, and both show exponential shapes in YRD and SB. The NO2vertical profiles in four regions all exhibit exponential shapes because of vehicle emissions. The shape of the HCHO vertical profile in JJJ and PRD was Gaussian, whereas an exponential shape was shown in YRD and SB. Moreover, a regional transport event occurred at an altitude of 600–1000 m was monitored in the southwest–northeast pathway of the North China Plain (NCP) by five MAX-DOAS stations (Shijiazhuang (SJZ), Wangdu (WD), Nancheng (NC), Chinese Academy of Meteorological Sciences (CAMS), and University of Chinese Academy of Sciences (UCAS)) belonging to the above network. The aerosol optical depths (AOD) in these five stations decreased in the order of SJZ > WD > NC > CAMS > UCAS. The short-distance regional transport of NO2in the 700–900 m layer was monitored between WD and NC. As an important precursor of secondary aerosol, the peak of NO2air mass in WD and NC all occurred 1 h earlier than that of aerosol. This was also observed for the short-distance regional transport of HCHO in the 700–900 m layer between NC and CAMS, which potentially affected the O3concentration in Beijing. Finally, CAMS was selected as a typical site to determine the O3–NOx–volatile organic compounds (VOCs) sensitivities in vertical space. We found the production of O3changed from predominantly VOCs-limited conditions to mainly mixed VOCs–NOx-limited condition from the 0–100 m layer to the 200–300 m layer. In addition, the downward transport of O3could contribute to the increase of ground surface O3concentration. This ground-based hyperspectral stereoscopic remote sensing network provide a promising strategy to support management of PM2.5and O3and their precursors and conduct attribution of sources.

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 NO2and 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 NO2VMRs (HCHOsec/NO2). The secondary sources in ambient HCHO participate in the photochemical reactions directly, and the FNRseccan 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 O3versus the normalized secondary HCHO or NO2mixing ratios for different FNRsecranges, 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. Diagnosing the O3formation regimes at different altitudes is essential to the regional control of tropospheric O3pollution. This study utilized the vertical profiles of NO2and 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 NO2VMRs (FNRsec) was more favorable for indicating O3formation sensitivities. During the observation, The FNRsecranging 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 O3formation 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 NOxemissions and limiting the regional transportation of NOxfrom nearby cities becomes more significant. The FNRsecvertical profiles were used to indicate the O3formation 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 O3formation 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) The FNRsecvertical profiles were used to indicate the O3formation sensitivities in different height layers For the ground layer, the VOC‐limited, transitional, and NOx‐limited regimes accounted for 40.66%, 32.42%, and 26.92%, respectively Vertically, the O3formation 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)