Study on oxygenated volatile organic compounds (OVOCs) formation and impact in Hong Kong : a combined field study and chamber simulation

Abstract

As an important constituent of volatile organic compounds (VOCs), oxygenated volatile organic compounds (OVOCs) participate directly in photochemical reactions due to their high reactivity. In addition to primary emission, OVOCs are generated as typical products of the oxidation of VOCs by hydroxyl radicals (OH), ozone (O3), nitrate radicals (NO3), and chlorine radicals (Cl) in the atmosphere. Furthermore, OVOCs are the precursors of O3, peroxy acetyl nitrate (PAN), and secondary organic aerosols (SOA), and they play a non-negligible role in the atmosphere. A study that included a field campaign and laboratory experimentation was conducted to investigate the formation and impact of OVOCs in Hong Kong. The field measurement study was carried out from August to October 2018 at a rural coastal site (Hok Tsui site) in Hong Kong. VOC and OVOC species were monitored continuously with proton transfer reaction quadrupole mass spectrometry (PTR-QMS), and the concentrations of OVOCs were found to be higher than those reported in previous studies in rural areas. Diurnal variations in the VOC concentrations were observed to be influenced by photochemical reactions. The amount of O3 formation was estimated via the maximum incremental reactivity model, and the top five contributors were isoprene (13.46 μg/m3), methyl ethyl ketone (12.74 μg/m3), xylene (8.52 μg/m3), acetaldehyde (8.22 μg/m3), and acrolein (4.32 μg/m3), which suggests that OVOCs were the dominant species with the potential for O3 formation. Five major VOC sources were identified with the positive matrix factorization method, including (1) biomass burning (63.7%), (2) ship-related emissions (13.5%), (3) secondary formation (9.2%), (4) industry-related and vehicle-related sources (8.1%), and (5) biogenic emissions (5.5%). The positive matrix factorization results showed that the Hok Tsui site was strongly influenced by both the urban plumes from the Guangdong–Hong Kong–Macao Greater Bay Area/Pearl River Delta region and by oceanic emissions. Because the results of the field study indicated that OVOCs and isoprene were critical in Hong Kong, a new environmental chamber was constructed to better understand the relationship between the formation of OVOCs and isoprene. The chamber consists of a 6-m3 Teflon film inside a stainless-steel enclosure with controllable temperature and relative humidity, and organic compounds with m/z values between 31 and 400 were identified by proton transfer reaction-time-of-flight-mass spectrometry (PTR-ToF-MS) and high-resolution time-of-flight-chemical ionization mass spectrometer (ToF-CIMS). The chamber can thus be used to investigate and simulate the gaseous chemical reactions and secondary aerosol formation after obtaining the characterization of performance and yields satisfactory results. Furthermore, nocturnal oxidation of isoprene by OH radicals, O3, and NO3 radicals was investigated under various concentrations of NO2. Methyl vinyl ketone plus methacrolein and formaldehyde were the major OVOC products of isoprene oxidation detected by PTR-ToF-MS. It was interesting to note that C5H10N2O8 (isoprene dihydroxy dinitrates), a secondary generation product, was the most abundant compound among the highly oxygenated products detected by high-resolution ToF–CIMS. As indicated by the isoprene oxidation experiments, the oxidation processes driven by OH radicals, O3, and NO3 radicals, and the combined oxidation by OH + NO3 radicals, existed concurrently in the reaction system, and the pathways were affected by the NO2 concentration. In addition, as the NO2 concentration increased from 1 ppb to 800 ppb, the yield of SOA grew 150-fold. The highly oxygenated compounds detected in this reaction system could explain more than 90% of SOA formation. In addition, the carbon balance in the oxidation of isoprene varied from 37.2% to 60.4% as the NO2 concentration changed, indicating that other products that were not detected in this study still account for a large portion. According to the results of this field campaign and laboratory experiments to evaluate the formation and impact of OVOCs based on the situation in Hong Kong, OVOCs were the dominant species and exerted a significant influence on O3 formation. Meanwhile, the contribution of isoprene oxidation to the formation of OVOCs was examined and analyzed, and the pathways were found to be influenced by the NO2 concentration.