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Title: Quantitative measurement of acidic ultrafine particles and observations of new particle formation in the atmosphere of Hong Kong
Authors: Wang, Dawei
Advisors: Guo, Hai (CEE)
Keywords: Air -- Pollution -- Measurement.
Atmospheric aerosols -- China -- Hong Kong.
Issue Date: 2015
Publisher: The Hong Kong Polytechnic University
Abstract: The number concentration and size-resolved properties of acidic ultrafine particles (AUFPs) have been observed to more closely associate with adverse health effects than do indices of total particulate mass. However, no reliable measurement techniques are currently available to quantify the number concentration and the size distribution of ambient AUFPs. In this study, a method with the use of iron nanofilm detectors for enumeration and size measurement of acid aerosols was developed and refined. An electrostatic precipitator (ESP) was improved and a diffusion sampler (DS) was designed as short-term and long-term collection system respectively with iron nanofilm detectors to measure the number concentration and size distribution of airborne AUFPs. Field campaigns were undertaken in rural and urban environments in Hong Kong to validate the method. Then the number concentration and size distributions of airborne AUFPs were quantified, the formation and growth processes of airborne AUFPs at these two different sites were investigated. The main contents and results are summarized as follows: (1) A method with the use of iron nanofilm detectors for enumeration and size measurement of acid aerosols was developed and refined. Standard sulfuric acid (H₂SO₄) or ammonium hydrogen sulfate (NH₄HSO₄) droplets and sulfuric acidcoated particles were generated and deposited on the detectors causing reaction spots. The dimensions of the reaction spots were examined with Atomic Force Microscopy (AFM) to establish the correlations between the diameter of the particle and the size of the reaction spot. To validate this method, a field measurement was conducted from 06 September to 30 November, 2010 at Mt.Tai Mo Shan, a rural site in Hong Kong. Scanning mobility particle sizer (SMPS) and electrostatic precipitator (ESP) were utilized as the collection system. A commercially available online measurement system, i.e., SMPS+CPC (condensation particle counter) was simultaneously used in the field measurement. The results indicated that the particle number (PN) concentrations obtained from the AFM scanning of the exposed detectors via SMPS+ESP collection system were comparable to those derived from the SMPS + CPC measurements (p > 0.05). The average geometric mean diameter of particles measured by the SMPS + CPC and the detectors scanned by the AFM was 52.3 ± 6.9 nm and 51.9 ± 3.1 nm, respectively, showing good agreement. It is suggested that the iron nanofilm detectors could be a reliable tool for the measurement and analysis of acidic particles in the atmosphere. (2) Besides the ESP, a diffusion sampler (DS) with iron nanofilm detectors was designed for long-term collection of airborne AUFPs. The developed DS was made of stainless steel with a flat and rectangular channel with 1.0 mm height, 50 mm width and 500 mm length. The iron nanofilm detectors were deployed on rectangular recesses inside the sampler at three different locations along the length of the channel to collect the ultrafine particles. The exposed detectors were then scanned using an AFM to numerate and distinguish the AUFPs from the non-acidic UFPs. Prior to sampling, the semi-empirical equations for the diffusive deposition efficiency of particles at the different detector locations in the sampler were obtained based on theoretical diffusive mechanism and modified by the experimental data. After calibration, the DS + AFM method and SMPS+CPC system were simultaneously used in a one-month field measurement conducted from 22 December 2010 to 20 January 2011 at an urban site. Both methods showed very good agreement in terms of total PN concentration and size. Therefore, it is reasonable to assume that the number concentration and size distribution of acidic particles estimated by the DS+AFM method were reliable.
(3) At the rural site, the potential formation and growth processes of new particles were investigated. New particle formation (NPF) events were found on 12 out of 35 days with the estimated formation rate J5.5 from 0.97 to 10.2 cm⁻³ s⁻¹, and the average growth rates from 1.5 to 8.4 nm h⁻¹. The events usually began at 10:00~11:00 local time characterized by the occurrence of a nucleation mode with a peak diameter of 6~10 nm. Solar radiation, wind speed, sulfur dioxide (SO₂) and ozone (O₃) concentrations were on average higher, whereas temperature, relative humidity and nitrogen oxide (NOx) concentration were lower on NPF days than on non-NPF days. Back trajectory analysis suggested that in majority of the NPF event days, the air masses originated from the northwest to northeast directions. The observed associations of the estimated formation rate J5.5 and the growth rate GR5.5-25 with gaseous sulfuric acid and volatile organic compounds (VOCs) suggested the critical roles of sulfuric acid and biogenic VOCs (e.g., αpinene and βpinene) in the new particle formation. (4) At the urban site, the potential causes and factors influencing acidic particle number concentrations were investigated. Northeastern monsoon prevailed during the sampling period. Apart from PN peaks appeared in traffic rush hours (i.e., 08:00 - 09:00 and 17:00 - 18:00), a distinct peak of PN concentrations in the afternoon (11:00 - 16:00) was observed during the sampling period. Concurrent measurement data of PSD, O₃ and SA concentrations revealed that the afternoon peaks observed were likely due to NPF via photochemical reactions. These NPF events were frequently observed under clean and dry weather in Hong Kong. Besides the NPF events, we also found four nucleation mode particle burst events, typically with increased number concentrations of nucleation mode particles (Nnuc) without growth to larger size particles. These burst events were generally accompanied by high-level primary air pollutants, i.e., sulfur dioxide (SO₂), nitrogen oxide (NOx) and carbon monoxide (CO), low SR and high CS conditions. The very different characteristics of the burst events from those of the NPF events indicated that these nucleation mode particle burst events were not caused by the photochemical reactions, but by the primary emission from the local combustion source(s). Overall, this study developed a novel method to quantify the number concentration of acidic ultrafine particles in the air, and improved the understanding of the sources and formation of acidic ultrafine particles in the rural and urban environments. These results, besides their scientific values, will have direct implications for the local and regional control efforts for reducing acidic particle pollution.
Description: PolyU Library Call No.: [THS] LG51 .H577P CEE 2015 WangD
xii, 193 pages :color illustrations
Rights: All rights reserved.
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