Author: Yao, Dawen
Title: Characteristics, sources and formation mechanisms of organic aerosols in the atmosphere of Hong Kong
Advisors: Guo, Hai (CEE)
Degree: Ph.D.
Year: 2023
Subject: Atmospheric aerosols -- China -- Hong Kong
Air -- Pollution -- China -- Hong Kong
Atmospheric chemistry -- China -- Hong Kong
Particles -- Environmental aspects
Hong Kong Polytechnic University -- Dissertations
Department: Department of Civil and Environmental Engineering
Pages: 156 pages : color illustrations
Language: English
Abstract: Due to rapid industrialization and urbanization in recent years, particulate matter (PM) pollution has become an annoying environmental problem in megacities like Hong Kong. Organic aerosols (OA), comprising a broad spectrum of compounds with different volatilities and properties, are the major components of ambient PM. OAs in the atmosphere contain primary OA (POA) directly emitted from anthropogenic and natural sources, and secondary OA (SOA) formed through chemical evolutions of organic compounds. Due to dynamic changes in emission profiles, POAs vary significantly across time and locations. In addition to various precursors in the atmosphere, SOA formation is influenced by many factors, such as atmospheric oxidative capacity or underlying aqueous processes. Hence, it is of great significance to quantify the contributions of primary emissions and secondary formation to OAs, and to further understand the formation mechanisms and evolution processes of OAs at different locations in the atmosphere. This thesis aims (1) to investigate the variations and composition of non-refractory sub-micron particulate matter (NR-PM1) and OAs in the roadside and urban background environments in Hong Kong, (2) to understand the OA sources and their contributions to OAs in various micro-environments, (3) to probe the potential formation and evolution of OAs through various precursors in different environments, and (4) to compare changes in aerosol chemistry over different years.
To understand the characteristics, sources and evolution processes of atmospheric organic aerosols in the roadside environment, a sampling campaign was conducted at an urban roadside site in Hong Kong from November to December in 2017 using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) to monitor compositions of NR-PM1. The results showed that organic matter in the roadside environment contributed the most to NR-PM1 with an average proportion of 57.7 ± 0.2%. The aerosol size distributions of bulk composition of NR-PM1 and ions indicated that the proportion of primary organic particles caused by the intensive emissions of primary organics at this site was larger. Positive matrix factorization (PMF) analysis was applied to the measurement data and four OA sources were identified, which included a hydrocarbon-like OA (HOA) factor, a cooking organic aerosol (COA) factor, and two oxygenated OA (OOA) factors with different oxidation levels, i.e., less oxidized OOA (LO-OOA) and more oxidized OOA (MO-OOA). Surprisingly, the proportion of primary source of COA was higher than that of another primary source of HOA (p<0.01) even at this roadside site with heavy traffic fleet. Moreover, the contribution of MO-OOA was the highest among all the four factors, suggesting a high degree of oxidation and/or a high regional background in the roadside environment. The COA concentration reached the highest value at 20:00 with an average of 8.2 ± 0.8 μg/m3. Especially on Sundays, the average maximum COA concentration was 16.1 ± 3.0 μg/m3. Comparing the results with previous studies at the roadside sites in Hong Kong, the COA levels were consistently higher than HOA, regardless of season and sampling location. All these indicated that cooking emissions were more dominant than vehicle emissions in urban Hong Kong, even in high-traffic areas. To explore the evolution of cooking OA sources, the ratio of 𝐶3𝐻3𝑂+ / 𝐶3𝐻5𝑂+ was used to represent the degree of COA oxidation in the atmosphere. The average ratio of 𝐶3𝐻3𝑂+ /𝐶3𝐻5𝑂+ and the inverse pattern with Ox (O3 + NO2) during daytime hours suggested that COA underwent some degree of oxidation when transported to the site. The findings implied that cooking activities are a significant source of organic aerosols in Hong Kong, even on busy roads. Control measures should focus on both cooking and traffic emissions in Hong Kong.
SOA formation in the atmosphere is influenced by many factors. To study the formation mechanisms and evolution of OAs in Hong Kong, a sampling campaign was conducted in November 2018 at an urban background site for characterization of secondary air pollution. A HR-ToF-AMS together with other on-line instruments was used to monitor the composition of NR-PM1 and OA molecular makers. Organics constituted the largest fraction (43.8%) of NR-PM1, and 86.5% of the organics was contributed by OOAs. The formation mechanisms of a dominant and more variable component of a less-oxidized OOA (LO-OOA1) and the more-oxidized OOA (MO-OOA) were explored. Based on multilinear regressions with molecular markers of OA (e.g., hydroxybenzoic acids and 2,3-dihydroxy-4-oxopentanoic acid) measured by Thermal-desorption Aerosol Gas-chromatography (TAG), we speculated that anthropogenic organic compounds, especially aromatics, were the most likely precursors of LO-OOA1. The correlations between OOAs and the odd oxygen Ox (Ox = O3 + NO2) were examined to understand the role of photochemical oxidation, and MO-OOA correlated well with Ox. Moreover, the concentration of MO-OOA responded positively to the increase of liquid water content (LWC) in NR-PM1, indicating that the formation of MO-OOA involved photochemical oxidation and aqueous processes at this site. Besides, MO-OOA exhibited the best correlation with malic acid, which can be formed by oxidation of various precursors. Furthermore, it was plausible that LO-OOA1 was further oxidized to MO-OOA through aqueous processes, as indicated by the consistent diurnal variations of MO-OOA to LO-OOA1 ratio and LWC. This study highlights the important roles of anthropogenic emissions and aqueous processes in SOA formation in coastal areas downwind of cities like Hong Kong.
In addition, OA pollution in the atmosphere can be affected by changes in emissions and meteorological conditions. As such, a detailed comparison of aerosol chemistry from different years is indispensable. A comprehensive sampling campaign was carried out at the above urban background site in October – November 2020 to investigate changes in aerosol sources and formation mechanisms. Organics was the most dominant component of all the measured substances with an average percentage (51.0 ± 0.5 %) higher than in 2018 (43.8 ± 0.7 %). The diurnal variations of all bulk compositions in NR-PM1 and secondary air pollutants showed the highest peaks in the same hour around 15:00, indicating intensive photochemical formation of secondary aerosols at this site. In addition, higher levels of Ox observed during the day than in 2018 suggested that there may be stronger photochemical processes in 2020. This was further confirmed by higher O:C ratios during the day in 2020 than in 2018. Furthermore, the diurnal variations of MO-OOA and LO-OOA1 also showed the same peak as Ox at 15:00, and the levels generally increased with the increase of Ox values, implying that the formation of both OOAs was associated with photochemical oxidation. By combining the measured molecular OA markers, we sought to explore the formation mechanisms and precursors of these OOAs. The diel cycles of secondarily-formed malic acid, citramalic acid, 2,3-dihydroxy-4-oxopentanoic acid (DHOPA), hydroxybenzoic acids (OHBAs) and monoterpene SOA tracers (MT-SOA-T) all showed their highest peaks in the afternoon, consistent with OOAs and Ox, indicating that photochemical oxidation played an important role in the formation of SOAs, regardless of the anthropogenic or biogenic origins at this site. By combining LO-OOA1 and MO-OOA with OA markers, we found that biogenic SOA contributed to the formation of MO-OOA. These findings could further enhance our understanding of OAs in urban background areas.
To sum up, this thesis explored the characteristics, sources and formation processes of organic aerosols in the atmosphere of Hong Kong. Differences in variations and formation of OAs in the roadside environment and urban background environment were investigated. The formation mechanisms and evolution processes of OOAs and COA were understood. The possible precursors of OOAs were also identified. The findings deepen the understanding of the origins and chemical evolution of OAs in Hong Kong and provide a scientific basis for controlling OA pollution. The outcomes may be applicable to other similar cities around the world and provide guidance for policymakers.
Rights: All rights reserved
Access: open access

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