Investigating vertical distributions and photochemical indications of formaldehyde, glyoxal, and NO2 from MAX-DOAS observations in four typical cities of China

Formaldehyde (HCHO), glyoxal (CHOCHO), and nitrogen dioxide (NO₂) are crucial in atmospheric photochemical processes at both surface and elevated altitudes. This study presents synchronous multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of the vertical distributions of summertime HCHO, CHOCHO and NO₂ in four representative megacities within the Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD), Sichuan Basin (SB), and Pearl River Delta (PRD) regions of China. The vertical distributions of HCHO and CHOCHO tended to occur at higher altitudes compared to NO₂, influenced by both primary emissions near the ground and photochemical oxidation processes at elevated altitudes. Source separation regression analysis using the CO-CHOCHO trace pair identified secondary formation as the predominant source of ambient HCHO. In urban areas, the ratio of CHOCHO to secondary HCHO (R_GFsec) serves as a more reliable metric at ground level for diagnosing VOC precursor sources, excluding the interference of primary and background HCHO. The increase in R_GF values at higher altitudes highlights the relative contribution of VOCs favoring CHOCHO production. Moreover, four indicators (e.g. FNR, FNR_sec, GNR, and MNR) were utilized to characterize O₃ formation sensitivity at different altitudes. The range of FNR, FNR_sec, GNR, and MNR marking the O₃ formation sensitivity regime varies regionally, highlighting the need for localized assessments. The VOC-limited regime dominated at the ground level, whereas the contribution of the NO_x-limited regime increased with altitude. Therefore, a comprehensive control strategy addressing both VOC and NO_x emissions across different altitudes is essential for effectively mitigating photochemical pollution in urban areas of China.