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Title: Application of novel methods for the measurement of acidic ultrafine particles in the atmosphere
Authors: Lu, Haoxian
Degree: Ph.D.
Issue Date: 2021
Abstract: The adverse effects of acidic ultrafine particles (AUFPs) have been widely recognized in scientific communities. AUFPs pollution is closely correlated with total mortality, morbidity and hospital admissions for respiratory diseases. In addition to health issues, AUFPs have impacts on climate, visibility and secondary organic aerosol (SOA) production. However, AUFPs have been rarely measured in the past due to the limitations of reliable measurement technique. Moreover, the existing methods for measuring AUFPs through atomic force microscope (AFM) scanning have several drawbacks such as offline, complicated and time-consuming. There is a need to acquire more AUFPs data, and refine/revise the previous method and develop a new method to achieve semi-automatic measurement of AUFPs without using AFM. Therefore, in the thesis, extensive measurements of AUFPs were firstly conducted in different land-use areas and cities in China using the previous method to better understand the pollution of AUFPs. After the samplings, the disadvantage of the previous method was highlighted and emphasized (i.e., the usage of AFM). To achieve the identification and quantification of AUFPs without using an AFM, a unique method was proposed to remove the non-acidic particles and retain the acidic particles on the surface. With the help of the aforementioned method, a novel system was then developed for semi-automatic measurement of AUFPs in the atmosphere by establishing the relationship between mass of deposited particles and the frequency change detected by the quartz crystal microbalance (QCM). To explore the AUFPs pollution, six measurements were conducted in the roadside, urban and rural areas in Hong Kong, and the urban area in Shanghai between 2017 and 2020 using the previous diffusion sampler (DS)+AFM method. The concentrations of AUFPs and UFPs, and the proportions of AUFPs in UFPs were obtained with the aid of AFM. The concentration of UFPs was the highest at the roadside site, followed by that at the urban site and the rural site, while the proportion of AUFPs in UFPs showed a contrary trend, i.e., rural > urban > roadside. The difference, on one hand, might indicate potential transformation of AUFPs from non-acidic UFPs through condensation of acidic vapor on the surface of non-acidic particles and/or heterogeneous reaction of acidic vapor with non-acidic particles during the transport and aging of air masses, and on the other hand, suggested the minor contribution of anthropogenic sources to the emission of AUFPs. The levels of UFPs ((1.21±0.49) ×104/cm3) (mean ± standard deviation (SD)) and AUFPs ((0.27±0.19) ×104/cm3) in urban Shanghai were lower than those ((1.48±0.64) ×104 and (0.40±0.27) ×104/cm3, respectively) in Hong Kong (p < 0.05). Moreover, the size distributions of AUFPs and UFPs together with the proportions of AUFPs in UFPs in different size bins were investigated. The sizes of UFPs and AUFPs both followed the normal distribution. The proportion of AUFPs in UFPs was peaked in the size range of 35-50 nm in roadside area, while in urban area it was characterized by a hysteretic peak (50- 75 nm), probably suggesting the aggregation of AUFPs with non-acidic UFPs during the transport from source areas to receptor areas. In rural area, the peak was observed in the size range of 5-10 nm, which might indicate the stimulation of new particle formation (NPF) with the AUFPs as seeds that were not easy to be aggregated by other low-concentration preexisting particles in a relatively clean environment. Furthermore, in the urban areas of Hong Kong and Shanghai, no significant difference was found for the geometric mean diameters (GMDs) of UFPs and AUFPs (p > 0.05), suggesting similar emission sources and/or chemical formation mechanisms of UFPs and AUFPs in these two cities. At last, the sulfuric acid proxy (Qsa) was positively correlated with the proportions of AUFPs in UFPs (R2=0.71) but not well correlated with the AUFPs levels (R2=0.17). The results suggested the important roles of both Qsa and preexisting particles in AUFPs formation. At high Qsa level, AUFPs were favorably formed through heterogeneous reaction of sulfuric acid vapor with non-acidic UFPs and/or condensation of sulfuric acid vapor on non-acidic UFPs, which led to high proportion of AUFPs in UFPs. However, the AUFPs level is not necessarily high if the concentration of preexisting particles is low even though sulfuric acid vapor is sufficient (i.e., high Qsa). Due to the significant reduction of sulfuric dioxide (SO2) in China during the last decade, the pollution of AUFPs in urban areas was alleviated with the evidence of lower AUFPs concentrations and proportions of AUFPs in UFPs. Specifically, significant reductions in SO2 levels were observed in Hong Kong and Shanghai. The reduction in Hong Kong was attributed to the increasingly stringent standards of low sulfur fuel oil (LSFO) used in vehicles and the elimination of old diesel vehicles, while the combustion of low sulfur coal in industries and power plants was the important cause for the decrease in SO2 emissions in Shanghai, in addition to the use of LSFO. Nevertheless, this DS+AFM method was high-cost and time-consuming.
In order to develop a method without using AFM, one possible way is to use an online microbalance (i.e., quartz crystal microbalance (QCM)), which is able to obtain the correlations between the mass of deposited particles and the frequency changes of the QCM. Moreover, if the above idea works, a method that can differentially remove non-acidic particles and retain acidic particles on a surface is a necessity. Subsequently, the acidic particles can be simply quantified based on the frequency change of a QCM. Therefore, in the study, three methods were attempted for differential removal of non-targeted nanoparticles on the surface, including air jet, nanobubble and ultrasonic methods. Acidic particles were taken as the targeted particles while non-acidic particles were regarded as non-targeted particles. Results showed that, regardless of methods, acidic particles were retained on the surface due to the strong particle-surface interaction. The air jet treatment and nanobubble treatment were not able to completely remove non-acidic particles from the surface with the removal efficiency of 5.1% ± 3.4% and 89.3% ± 4.1%, respectively, while the non-acidic particles were entirely removed in the ultrasonic treatment. Ethanol rather than deionized (DI) water was the proper solution in the ultrasonic treatment to avoid contamination. In conclusion, ultrasonic by ethanol was fully efficient for differential removal of non-acidic particles on the surface. The principle of differential removal of particle is the differences of particle-surface interaction force between non-acidic particles (i.e., physically attached particles) and acidic particles (i.e., chemically-formed particles). Non-acidic particles are removed from the surface through cavitation to form bubbles in the gap between a non-acidic particle and the surface in the ultrasonic treatment. In contrast, the space between an acidic particle and the surface is filled by the reaction, and thus bubbles cannot enter the crevice to remove the acidic particle. The developed method is useful for aerosol research, especially for AUFPs. By combining the differential removal method, a convenient, rapid and accurate measurement system was developed for semi-automatic measurement of AUFPs in the atmosphere through integrating a DS with three QCMs, namely a QCM+DS system. The QCM detectors were coated with a nano-film of metal (metal-QCM detectors) and then placed inside the DS at three sampling spots for collection and detection of ultrafine particles (UFPs). The frequency changes obtained from the metal-QCM detectors were converted into the weights of deposited particles and used to determine the proportions of AUFPs in UFPs through the differential removal process of non-AUFP particles. Prior to sampling, the sensitive response of the QCM system and collection efficiencies of the QCM+DS system were calibrated using standard acidic and non-acidic particles. Reactions between the AUFPs and nano-film of metal were guaranteed by confirming much lower than one-layer deposition of particles on the detectors based on theoretical calculation and experimental results. Eventually, the QCM+DS system was validated in a field measurement by comparing the results with those obtained from the previously developed method and a commercial measurement system, i.e., Scanning Mobility Particle Sizer (SMPS). All the three methods showed good agreements in measuring AUFPs and UFPs concentrations, indicating the reliability of the QCM+DS system for the quantification of ambient UFPs and AUFPs.
Subjects: Air -- Pollution
Particles -- Environmental aspects
Hong Kong Polytechnic University -- Dissertations
Pages: 145 pages : color illustrations
Appears in Collections:Thesis

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