Characteristics and Source Apportionment of Formaldehyde in Hong Kong During Autumn 2024

Abstract

Over the past decade, Hong Kong has experienced a gradual increase in ambient ozone (O₃) levels. Understanding the characteristics and sources of formaldehyde (HCHO) is crucial for reducing volatile organic compound (VOC) emissions and addressing O₃ pollution. In this study, HCHO and VOCs were measured at the HKUST supersite from 15th October to 17th November 2024. The average concentrations of HCHO and total VOCs (TVOC) during the study period were 3.89 ± 1.61 ppb and 17.98 ± 14.37 ppb, respectively. Among all measured carbonyls, HCHO emerged as the largest contributor to O₃ formation potential (OFP), hydroxyl radical (OH) reactivity, and radical production rates, suggesting the need to control HCHO emissions. On O₃-episode days, the average concentrations of HCHO, TVOC, and O₃ were 1.10 ppb, 5.59 ppb, and 17.86 ppb higher than on non-episode days, respectively. The OFP, OH reactivity and radical production rates of HCHO were all higher during O₃-epsiode days, which conveys the need to identify the sources of HCHO in order to reduce O₃-pollution.

Given HCHO’s significant role in regional atmospheric chemistry, we further investigated its sources using three independent source apportionment methods: principal component analysis–multi-linear regression (PCA-MLR), positive matrix factorisation (PMF), and photochemical age-based parameterisation (PCAP). Results indicated that secondary formation was the predominant source of HCHO (23.9–56.0%), followed by primary emissions from solvent use and vehicles (8.0–24.8%), biogenic emissions (13.0–27.6%), and, to a lesser extent, biomass burning (2.2-8.0%) and oceanic sources (1.4-9.9%). During O₃-episode days, higher HCHO (4.55 ppb) was attributed to the higher rates of secondary formation, increased biogenic emissions and the transport of biomass burning pollutants, caused by the increase in photochemical activity and increased influences from local emissions. Whereas on non-episode days, lower HCHO (3.45 ppb) was influenced by reduced photochemical activity and the transport of coastal air masses bringing cleaner air to HK. Therefore, more emphasis should be placed on reducing regional biomass burning sources and the precursors of secondary HCHO, in order to better control VOC emissions and tackle O₃ pollution in HK.

Further intercomparison of the three source apportionment methods showed good agreement in overall HCHO concentrations simulated by PCA-MLR and PMF (R² = 0.894). PCAP also demonstrated reasonable agreement with PCA-MLR (R² = 0.797) and PMF (R² = 0.736) for overall HCHO concentrations, but agreement was poor for secondary sources (R² = 0.023) due to differences in model rationale and limitations. Overall, we recommend applying all three source apportionment methods to the same VOC dataset, and overlapping conclusions can be valuable in identifying the main sources of HCHO and further guide VOC pollution reduction in Hong Kong.

Date of Award	2025
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Zhe WANG (Supervisor)