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Title: The acidity and basicity of the atmosphere in Hong Kong
Other Titles: Xianggang da qi ceng de suan jian xing
Authors: Lee, Sze Chung (李思聰)
Department: Dept. of Biology and Chemistry
Degree: Doctor of Philosophy
Issue Date: 2006
Publisher: City University of Hong Kong
Subjects: Atmosphere -- Research -- China -- Hong Kong
Notes: 1 v. (various pagings) : ill. (some col.), col. maps ; 30 cm.
CityU Call Number: QC869.4.C5 L43 2006
Includes bibliographical references.
Thesis (Ph.D.)--City University of Hong Kong, 2006
Type: Thesis
Abstract: The aims of this thesis are to study the contributions to the acidity and basicity in the atmosphere of Hong Kong, and in doing so, to develop new analytical methods. In the Introduction, a summary is given of the current air pollution; environmental setting; climate and the meteorological factors which affect it Hong Kong. The study of bulk deposition acidity focuses upon summer rain events from July to August in 2004 and is presented in Chapter 2. Following a request to assess acid rain in Lamma Island (LI), three sampling sites (LI); Cyberport (Cy) and the City University of Hong Kong (CityU)) were chosen to investigate the horizontal variation of the ionic composition of rainwater. It was found that the acidity of bulk deposition in Hong Kong has greatly increased from 1996, and the volume-weighted (VW) mean pH values were 4.29 at LI, 4.70 at Cy and 4.20 at CityU respectively. There was no simple correlation between the amount of rainfall or the logarithm of local windspeed [p(local windspeed)] and the acidity of rainwater. Sulphate was found to be the most abundant anionic species at all sampling sites. Sulphate and nitrate ions were nearly tripled in concentration compared with the reported results at CityU in 1996. Ammonium and hydrogen were the dominant cationic species at LI and CityU. The ionic concentrations (in μeq l-1) are in the descending order: SO4 2- > H+ and NH4 + > NO3 - and Cl- > Ca2+ > Na+ > Mg2+ > K+ at LI and CityU while SO4 2- > Ca2+ >H+ and NH4 + > NO3 - and Cl- > Na+ > Mg2+ > K+ at Cy. This spatial variation was due to the emissions from point sources and anthropogenic sources nearby. Backward trajectory and pressure map analysis showed that acidic episodes mainly occurred under the influence of the approaching cyclone type of weather system. A comparison with the pH of rainwater in other Asian countries from previous studies is also made. For the first time, measurements of ambient NH3 in Hong Kong were performed using different sampling methods including sampling by impingers, honeycomb denuders, Ogawa© passive samplers and by a real-time gas autoanalyzer. Each method has potential advantages for certain situations, and together with their limitations, this is discussed in Chapter 3. The results of optimizing different parameters of the impinger sampling and denuder sampling and the results and discussion of real site measurements of each method are also presented in Chapter 4. The collection efficiency of both impinger and denuder sampling was not good enough, ranging from 17.7 % to 68.4 % and 22 % to 18 %, respectively. The low efficiency of impinging sampling is suggested to be contributed to through the carry-over of particulate matter with aerodynamic particle size lower than 5 μm in the sampling air stream. The development of a “back-pressure” difference among the sampling train is suggested to be another reason that the impinger train was unable to collect the ambient ammonia (NH3) effectively. Boric, citric and phosphorous acid honeycomb denuder coatings were examined for their collection efficiencies and these three coatings were found to be more or less the same with lowest blank level being for boric acid. The particle size was found to be one of the factors which affected the collection efficiency of the honeycomb denuder sampling system by sampling with and without PM2.5 impactor. The simplicity of Ogawa® passive sampling facilitated the spatial representation study of the ambient NH3 levels in Hong Kong while the autoanalyzer was suitable for real-time measurements of the NH3 in the present study. In general, both ambient NH3 and NH4+ particulate ion levels were found to be low (2.3 ± 2.2 μg m-3 and 3.1 ± 2.0 μg m-3, at CityU by denuder sampling, respectively) in Hong Kong, with higher levels (4.7 – 9.5 μg m-3) at roadside with livestock sites (8.9 – 19.3 μg m-3). The relatively lower ambient concentrations at the urban site (CityU) and sewage treatment plant indicated that the emission sources nearby were the major factor which affected the ambient NH3 level locally. Higher NH3 levels were found in on-road passive samplings. This indicated that vehicle emission of NH3 is an important NH3 source in Hong Kong and this conclusion is reinforced by the observation of the diurnal variation of NH3. Synchronized real-time measurements of NH3 together with NO, NO2 and NOx were performed using chemiluminescence autoanalyzers. Episodes of NH3 and NOx (i.e. concentrations exceeding the mean + 2 standard deviations) were greatly affected by the type of synoptic weather system in Hong Kong. The episodes frequently occurred under the influence of the approaching cyclone. For the first time, the relationship of the acidic precursor gas NO with thunderstorm lightning has been investigated in Hong Kong at remote and urban sites. Data of the stroke frequencies of (intra-cloud) IC and cloud-to-ground (CG) lightning were compared with NO ambient levels at various HKEPD air monitoring sites and at CityU. The NO levels showed an anthropogenic diurnal trend with negligible contributions from lightning events. This behavior in a megacity (10 million population) contrasts with that reported for remote sites where the major production of NO is natural. In summary, the acidity of Hong Kong rainwater is getting more pronounced whereas the alkaline gas NH3 is much less important than in northern China. Most of NH3 is found in the form of NH4+ ions in particulate matter.
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