Detection and characteristics of volatile organic compounds at suburban Hong Kong using NO+ PTR-MS

Abstract

Hong Kong faces increased surface ozone levels, posing environmental and health concerns. Understanding its major precursor, volatile organic compounds (VOCs), is therefore essential. Proton transfer reaction time-of-flight mass spectrometry (H₃O⁺ PTR-ToF-MS) enables fast online detection of VOCs, but cannot ionize alkanes or resolve carbonyl isomers, both are important pollutants. Here, measurements from NO⁺ PTR-ToF-MS were validated through comparing to H₃O⁺, demonstrating its effectiveness in addressing these limitations and providing better compound specificity. NO⁺ PTR-ToF-MS was then applied in an ambient study in suburban Hong Kong from June to December 2023. The average VOC concentration in summer, autumn and winter was 13.4 ± 10.2 ppb, 15.5 ± 10.8 ppb, and 17.8 ± 10.0 ppb. The most dominant VOC classes were oxygenated VOCs in all seasons, including C_xH_yO₁ (63.5%-69.8%) and C_xH_yO_z>1 (9.3%-13.9%), with acetaldehyde, acetone and ethanol being the most abundant species. OH reactivity and precursor-product correlation indicated increased reactive VOC loadings and atmospheric processing from summer to winter. In summer, stagnation prior to tropical cyclones were major events affecting VOC levels. Saola and Haikui jointly resulted in the highest observed VOC concentration and composition to change. In autumn and winter, backward trajectory suggested regional transport from inland regions brought both primary and secondary pollutants to Hong Kong. Positive Matrix Factorization identified ten VOC sources. Vehicle-related emissions (31.5%-41.6%) and secondary formations (30%-30.4%) were the predominant sources, both contributed significantly to OVOCs. Hydrocarbons including alkanes and aromatics were dominated by different primary sectors. The findings demonstrated the potential of NO⁺ PTR-ToF-MS in complementing current online measurement techniques and allowing for more accurate ambient analysis. Results also highlight the importance of understanding VOC sources and chemistry in different seasons for strategizing tailored management, and encouraged regional collaboration on VOC control to mitigate urban ozone issues.

Date of Award	2024
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Zhe WANG (Supervisor)