| Author: | Yuan, Qi |
| Title: | Transformation of aromatic hydrocarbons and formation of oxygenated volatile organic compounds at a regional background site in the PRD region : insights from field study to chamber experiments |
| Advisors: | Lee, Shuncheng (CEE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Hydrocarbons -- Environmental aspects Volatile organic compounds -- Environmental aspects Atmospheric chemistry Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxii, 198 pages : color illustrations |
| Language: | English |
| Abstract: | Atmospheric oxidation of aromatic hydrocarbons produces secondary organic compounds including oxygenated VOCs (OVOCs) via multiple reaction steps and pathways, reflecting significant chemical complexity relevant to multi-phase oxidation and phase-partitioning. Nighttime chemistry was considered less important to SOA formation from aromatics, due to its low reaction rate coefficient with nitrate (NO3) radicals. However, OVOCs and other intermediates from aromatic oxidation can be quickly oxidized by NO3 in dark conditions. In the Pearl River Delta (PRD) region, the meteorological conditions including high temperature, high humidity, strong solar radiation, and pollutants frequently transported from northerly and easterly urban areas led to high ozone formation and atmospheric reactivity. Hence, we comprehensively assessed the transformation of aromatic hydrocarbons and OVOC formation in the composite air pollution conditions at a background site in the PRD region with field observations and chamber experiments. VOC measurement with proton transfer reaction-time of the flight- mass spectrometry (PTR-TOF-MS) was conducted at Hok Tsui, a regional background site of Hong Kong in autumn 2020. Coupled with trajectory cluster analysis and positive matrix factorization (PMF) modeling, the pollution pattern, emission sources, and evolution processes of VOCs, and their impact on ozone formation and atmospheric reactivity were discussed. Primary anthropogenic sources and secondary formation of OVOCs are the two largest contributors to VOCs at Hok Tsui. The secondary OVOCs were mainly from anthropogenic sources during the transportation of air masses. Specifically, different degrees of oxidation processes of aromatic hydrocarbons and the OVOCs were found during the transportation of air masses. These OVOCs contributed largely to the ozone formation potential and atmospheric reactivity. These results highlight that the significant variation of pollution characteristics of VOCs in the transport and chemical oxidation processes in different air masses have a great impact on the atmospheric oxidation capacity. To further investigate the transformation processes of aromatics and OVOCs, we built a new environment chamber to conduct the oxidation and ageing experiments. The chamber was characterized and tested with toluene photooxidation experiments. The chamber system is characterized to provide reliable results of experiments with good temperature and humidity stability, high mixing efficiency, low contamination of background air, small wall loss of gases and particles, and moderate light intensity compared with other chambers and ambient environment. The yields of OVOCs and SOA showed good agreement with other studies and model results. The ageing processes of toluene photooxidation products were observed with higher toluene and OH radicals, which were caused by faster consumption of the intermediate products. A potential aerosol mass oxidation flow reactor (PAM OFR) was coupled with the chamber to investigate the ageing processes of secondary products from toluene photooxidation. Gas-phase secondary products during photooxidation and ageing processes were monitored by PTR-TOF-MS and time-of-flight chemical ionization mass spectrometry (TOF-CIMS). Size distribution and mass concentration of SOA were monitored by scanning mobility particle sizer (SMPS). Effects of preexisting oxidation products, NOx levels, relative humidity (RH) in ageing stage and OH exposure in the photooxidation stage on the ageing processes were investigated. New particle formation was found in the dark ageing processes when the toluene SOA from photooxidation was filtered, whereas gaseous mixtures including OVOCs and peroxyl radicals (RO2) were the main precursors of the ageing processes. A large proportion of nitrogen-containing products were produced from the ageing processes by reactions of previously formed RO2 with NO/NO2 and NO3-initiated reactions of aromatic ring-retaining products. Under high NOx and low RH conditions, the RO2+NO/NO2 reactions were facilitated to form more nitrogen-containing products, while the formation of ring-scission carbonyls and hydroperoxides was enhanced under low NOx and high RH conditions. High OH exposure during the photooxidation processes promoted the transformation of early-generation products into more oxygenated and fragmented products, thus resulting in more aged products during the dark ageing processes. The overall results from this thesis highlight the influence of ageing processes on gaseous oxidation products from aromatics under composite air pollution conditions on a regional scale. The ageing of gaseous pollutants during the transport of air from urban polluted areas to the background environment was observed at this regional background site. The further chamber studies provide a deep understanding of how and to what extent the composite pollution conditions influence the transformation of aromatics, and the formation and transformation of OVOCs and other gaseous organic products. |
| Rights: | All rights reserved |
| Access: | open access |
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