Role of factors controlling diurnal variation of cold-season formaldehyde during Satellite Integrated Joint Monitoring of Air Quality 2021 campaign

Formaldehyde (HCHO), a major carbonyl compound in urban air, poses health risks due to its carcinogenic properties. However, the role of FT-PBL exchange in HCHO and the importance of vertical exchange on diurnal variations in HCHO remain unclear. This study investigated the diurnal variability of HCHO in Seoul's planetary boundary layer (PBL) during cold. We examined the impacts of (1) the exchange between the free troposphere (FT) and PBL, (2) primary emissions and secondary HCHO production, (3) heterogeneous nitrous acid production, and (4) dry deposition. Variations in the relationship of VOC precursors between FT and PBL resulted in up to 119.4 % difference in the daily average HCHO. Uncertainties in VOCs and HCHO emissions led to daily average HCHO variabilities of 64.5 % and 56.8 %, respectively. Morning HCHO was sensitive to primary HCHO emissions, whereas daytime HCHO was sensitive to VOC emissions and FT-PBL exchange. Heterogeneous production/dry deposition increased/decreased HCHO levels to a much lesser extent (9.0 % and 8.0 %, respectively). The contribution of VOC precursor emissions was superior to that of primary HCHO emissions, and the important species/classes of VOC precursors were ordered as olefins, toluene, xylene, and ethylene. However, when considering the impact of cold climates on primary emissions from vehicles, the proportion increased to a level comparable to that of secondary HCHO production. These findings highlight the need for accurate VOC and HCHO emission data and vertical profiles to understand diurnal HCHO variations and guide strategies to reduce VOC emissions and mitigate HCHO and O₃ in Seoul.