[Vertical Distribution of Surface Formaldehyde in the Pearl River Delta Urban Area Based on Observations at the Canton Tower]

To investigate the vertical distribution of atmospheric formaldehyde in the Pearl River Delta (PRD) urban area, simultaneous measurements were performed at three heights on Canton Tower for the first time. Carbonyls including formaldehyde were sampled with 2,4-dinitrophenylhydrazin (DNPH) at noon for 32 days in autumn of 2018, and then analyzed using high-performance liquid chromatography (HPLC). Average mass concentrations of formaldehyde at ground level, 118 m, and 488 m sites at Canton Tower were (5.10±1.93), (6.61±2.84), and (5.33±2.55) μg·m^-3, respectively. The measured formaldehyde was positively correlated with atmospheric oxidant O_x at the three sites (R 0.65-0.75), indicating that photochemical formation is an important source for urban formaldehyde in PRD. Three different profiles were found for formaldehyde vertical distribution during the measurements. The most frequently observed one showed a higher value at 118 m while lower ones at ground level and 488 m, occurring when the boundary layer is in moderate convection state with high photochemical reactivity. The 118 m layer may be also influenced by transported high-chimney emissions from industries in suburban areas. Vertical columns of formaldehyde were also calculated according to its vertical profile. The average value was (11.23±4.80)×10¹⁵ molecules·cm^-2, 19% lower than that from satellite retrieval, while in the same magnitude as values reported in reference papers.