| Author: | Yang, Jin |
| Title: | Roles of volatile organic compound speciation in winter ozone photochemistry, long-term ozone trends, and secondary carbonyl formation in a Northwestern Chinese industrial city |
| Advisors: | Guo, Hai (CEE) |
| Degree: | Ph.D. |
| Year: | 2026 |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | 245 pages : color illustrations |
| Language: | English |
| Abstract: | Volatile organic compounds (VOCs) are key precursors in the formation of secondary air pollutants, such as ozone (O₃) and carbonyls. Among VOCs, alkenes are particularly reactive and play a dominant role in secondary pollution formation, especially in industrial cities like Lanzhou, where high alkene concentrations are largely attributed to petrochemical activities. The unique VOC profile in these cities, characterized by alkene-rich emissions, raises important questions about their impact on urban air quality and secondary pollution formation. This thesis addresses these issues through three main studies: (1) investigating the mechanisms of wintertime O₃ formation in Lanzhou, (2) analyzing the long-term trends of O₃ in Lanzhou, and (3) exploring the formation and sources of secondary carbonyl compounds across five major Chinese cities. By integrating field measurements with advanced modelling, this work aims to elucidate the chemical processes, temporal variations, long-term trends, and source contributions of O₃ pollution driven by VOCs in northwestern Chinese industrial cities, thereby providing a scientific basis for more effective air quality management strategies. The intensive field measurements in January 2018 revealed four O₃ episode days with maximum hourly concentrations exceeding 100 ppbv and peaking at 121 ppbv, despite cold temperatures and weak solar radiation. The average daytime concentration of total volatile organic compounds (VOCs) reached 153.4 ± 19.0 ppbv, with alkenes, mainly from the local petrochemical industry, comprising 82.3 ± 13.1 ppbv. Using a photochemical box model coupled with the Master Chemical Mechanism, it was found that the low temperatures and weak sunlight had minimal effect on VOC reactivity with OH radicals. Instead, the ozonolysis of alkenes generated Criegee intermediates, which rapidly decomposed into substantial ROx radicals (OH, HO₂, and RO₂) even without sunlight. This radical production led to the oxidation of VOCs, with alkene ozonolysis ultimately contributing to nearly 90% of O₃ formation during these episodes. The mechanism was not active at night due to the NO titration effect. The study further showed that a reduction of alkenes by about 28.6% or NOₓ by 27.7% in the early afternoon could significantly mitigate wintertime O₃ pollution. These findings challenge the traditional view that strong solar radiation is necessary for high O₃ production and highlight the importance of dark reactions in wintertime photochemistry. The results also suggest that alkene ozonolysis can be a major source, rather than a sink, of O₃ in heavily industrialized regions. Except for the uniquely high O₃ concentration in winter, this research found that while concentrations of primary air pollutants such as PM2.5, CO, SO₂, and NO₂ have declined significantly in recent years due to effective emission control measures, O₃ pollution in summer remains a persistent challenge. Although NO₂ and CO concentrations declined steadily, the effectiveness of VOC control was limited after the COVID-19 lockdown. Model simulations revealed that O₃ formation in Lanzhou transitioned from a VOC-limited regime to a transitional regime from 2019 to 2023, largely driven by reductions in NOₓ. The observed increase in O₃ levels during this period was mainly attributable to the decrease in NOₓ concentrations. As the O₃ formation regime shifted from VOC-limited to a VOC-NOₓ co-limited regime, it became evident that effective O₃ control now requires simultaneous management of both NOₓ and VOC emissions. Alkenes were the primary contributors to O₃ formation throughout the decade, accounting for around 50–63% of the total VOC contribution in Lanzhou. There was a notable contribution from reactive alkenes such as trans/cis-2-butene, ethene, and propene to O₃ formation. Source apportionment identified vehicle emissions, followed by petrochemical industry and solvent usage, as the major contributors to O₃ formation. These findings highlight the importance of targeted VOC controls, focusing not only on alkenes but also on specific aromatic compounds, to effectively mitigate O₃ pollution. In addition to the significant impact of alkenes and aromatics on O₃, we also found the impact of carbonyls on O₃, by investigating the photochemical characteristics, sources, and joint O₃-carbonyl control strategies of formaldehyde, acetaldehyde, and acetone. These three carbonyls made up over 85 % of the total observed carbonyls, with the highest concentrations found in Beijing and Wuhan, indicating the impact of regional secondary pollution. Observations showed that Beijing (9.6 ± 0.1 ppbv) and Wuhan (5.2 ± 0.1 ppbv) exhibited the highest formaldehyde concentrations, followed by Shanghai, Chengdu, and Lanzhou. Model simulations revealed that anthropogenic alkenes accounted for over 50 % of formaldehyde formation in Beijing and Lanzhou, while isoprene contributed 10–20 % in other cities. Other VOC species contributed 1–2 % each, highlighting the complexity of VOC sources for formaldehyde production. In contrast, acetaldehyde and acetone were mainly produced from propene and α/β-pinene, respectively (about 50 % each), with the top ten VOCs accounting for around 90% of their formation. Furthermore, carbonyls were quantified for their role in O₃ formation across the cities, with the highest contribution observed in Beijing (33 %) and the lowest in Lanzhou (6 %). Comparison of their isopleths suggested that O₃ reduction strategies can also effectively reduce carbonyl concentrations. Finally, source apportionment combined with chemical mechanism analysis traced the sources of primary and secondary carbonyls, showing that vehicle emissions rich in alkenes were the largest contributors to secondary aldehyde formation in all five cities. Including their primary contributions, vehicles accounted for 40–50% of total aldehydes, followed by solvent usage. For acetone, biogenic sources dominated over anthropogenic ones. These findings deepen our understanding of carbonyl photochemistry and inform possible future mitigation strategies. In summary, this thesis provides new insights into the mechanisms of O₃ and carbonyl pollution in urban China, especially in Lanzhou. The findings highlight the importance of considering both traditional and non-traditional photochemical pathways, the need for targeted precursor controls, and the value of coordinated strategies for managing multiple secondary pollutants. The results fill important knowledge gaps in understanding O₃ and carbonyl pollution in Lanzhou. These findings offer a scientific basis for refining air quality management policies and can inform similar strategies in other cities facing persistent O₃ and carbonyl challenges. To ensure a comprehensive understanding and broader applicability of the findings, this thesis adopts a stepwise research framework. It begins with an in-depth case study of wintertime O₃ formation mechanisms in Lanzhou, followed by a long-term trend analysis in the same city, and ultimately expands to a multi-city comparative study of secondary carbonyl compounds. This progressive approach not only addresses the unique challenges faced in Lanzhou, but also facilitates the identification of common patterns and region-specific features across major Chinese cities. |
| Rights: | All rights reserved |
| Access: | open access |
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