Seasonal characteristics of atmospheric formaldehyde (HCHO) in a coastal city of southeast China: Formation mechanism and photochemical effects

Abstract. Formaldehyde (HCHO) is a vital reactive carbonyl compound, which plays a critical role in the atmospheric oxidation capacity (AOC), radical chemistry, and O₃ formation. Yet, the majority of the current studies on HCHO photochemical mechanism in coastal areas remain scarce, thus limiting the full understanding of potential atmospheric impacts with limited influence from marine sources. Here, field campaigns were conducted at a typical urban site in southeast China to reveal the characteristics and potential source of ambient HCHO, as well as its impact on photochemistry, during spring and autumn of 2021. The result showed that the HCHO mixing ratios were 2.94±1.28 ppbv and 3.19±1.41 ppbv in spring and autumn, respectively. Secondary formation made the largest contributions to HCHO (49 % in spring and 46 % in autumn), followed by vehicle exhaust (25 % and 20 %) and biogenic emission (18 % and 24 %) in this study. Furthermore, in order to identity the impact of HCHO on photochemistry process, the formation pathways and key precursors (alkenes and aromatics) of secondary HCHO were furtherly investigated based on Observation-Based Model (OBM). The net HCHO production rate in autumn (−0.40±0.70 ppbv h⁻¹) was lower than that in spring (0.10±0.37 ppbv h⁻¹), due to the increase in HCHO loss rate under the intense solar radiation and relatively low precursor levels to limited HCHO secondary formation. Disabling HCHO mechanism decreased the abundance of OH (25 % in spring and 16 % in autumn), HO₂ (45 %, 40 %), and RO₂ (26 %, 19 %). Meanwhile, the net O₃ production rates dropped by 32 % in spring and 29 % in autumn, which were mainly dominated by the reduction of radical propagation efficiencies. The analysis of HCHO potential sources, formation pathways, and impacts on O₃ formation provided significant insights into photochemical mechanisms and pollution control in coastal areas.