Spatiotemporal Variations of Airborne Formaldehyde (HCHO) in Hong Kong (HK) and Guangzhou (GZ): Insights from Offline and Online Measurements

Abstract

Formaldehyde (HCHO) is a key atmospheric pollutantwith significant health risks and plays a strong role in ozone (O₃) formation. Despite its importance, diurnal and seasonal characteristics of airborne HCHO remain underexplored in Hong Kong (HK) and Guangzhou (GZ). This study investigates the spatiotemporal variations of HCHO using offline and online measurements, including (1) a 3.5-year offline cartridge sampling campaign across three sites in HK (i.e., rural CD stie, new town TC site, and roadside MK site) from October 2019 to July 2023, and (2) continuous online monitoring in new town university campus sites for 1.5 years in HK (i.e., HKUST campus) from September 2023 to January 2024 and 9 months in GZ (i.e., HKUST-GZ campus) from April 2024 to January 2025. Results show that HCHO concentrations were 1.71 ± 1.17 ppb at rural CD site, 2.96 ± 1.38 ppb at new town TC site, and 6.18 ± 6.43 ppb at roadside MK site, as revealed by offline cartridge measurements during October 2019 to July 2023. Online measurements recorded HCHO concentrations of 2.50 ± 1.09 ppb at HKUST and 3.03 ± 1.48 ppb at HKUST-GZ from April 2024 to January 2025.

By combining both offline and online measurements, we found that elevated HCHO levels occurred in autumn and winter, while low levels were observed in spring. Fourier Series analysis on the offline measurements modeled the seasonal variations into significant annual cycles (p<0.05), with the background cycle peaking in late November and reaching minimum in May, as well as site-wise variations depending on local anthropogenic activities. Diurnal HCHO of the two online sites rose from morning, peaked around noon, and declined in the evening, aligning with solar irradiance and temperature. Evidence from Pearson correlation analysis and Multiple Linear Regression suggested photochemical production was the major HCHO contributor. GZ showed stronger peak fluctuations of diel HCHO levels and better alignment with O₃, especially in summer, indicating greater in-situ formation, while HKUST showed weaker diurnal increments and a lag in O₃ rise compared to HCHO, particularly in winter, as a result of regional transport with air masses from the north. Backward trajectory analyses indicated industrial cities in the Pearl River Delta (PRD) such as GZ were major contributors of the HCHO levels thus the key regions for air quality control. HCHO exhibited the highest contribution of ozone formation potential (OFP) among cartridge carbonyls. The high correlation between the OFP and production rate of peroxyl radicals from online HCHO with O₃ levels suggests HCHO was highly likely to contribute to O₃ formation, and Formaldehyde – NO2 ratios demonstrated that GZ was in a more VOC-limited regime than HK, thereby underscoring the importance of extended HCHO monitoring for O₃ control strategies. Health risk assessments revealed significant carcinogenic risks at all sites, while non-carcinogenic risks were below safety thresholds yet not negligible. These findings highlight the value of integrated long-term monitoring of HCHO, and provide valuable insights for informed regional HCHO and O₃ air quality management in the Greater Bay Area (GBA) of China.

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