Study of photochemical ozone pollution in Hong Kong

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Study of photochemical ozone pollution in Hong Kong

 

Author: So, King-lung
Title: Study of photochemical ozone pollution in Hong Kong
Degree: Ph.D.
Year: 2007
Subject: Hong Kong Polytechnic University -- Dissertations.
Ozone -- Environmental aspects -- China -- Hong Kong.
Air -- Pollution -- China -- Hong Kong.
Department: Dept. of Civil and Structural Engineering
Pages: 222 leaves : ill. ; 31 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2094020
URI: http://theses.lib.polyu.edu.hk/handle/200/2212
Abstract: The Pearl River Delta (PRD) of China has experienced serious air pollution along with the astonishing economic and industrial developments in the past two decades. Large emissions of pollutants like respirable suspended particulates, sulphur dioxides (SO2), nitrogen oxides (NOx) and volatile organic compounds (VOC), coupled with the subtropical climate, have resulted in photochemical ozone (O3) pollution and visibility degradation in the region, including Hong Kong (HK) which is situated in the southern part of the PRD. Photochemical theory and overseas experience show that control of O3 pollution is a very challenging undertaking because of the non-linear dependency of O3 on its precursors, VOC and NOx. Such O3-O3 precursors' relationship is often site specific and the O3 migration strategy is effective only should there be an in-depth understanding of the sources, patterns, and formation of photochemical O3 and its precursors, as well as their relationship in the region. However, as the information on the above aspects is very limited in the PRD (including HK), efforts to formulate an effective O3 control strategy has been hindered. This research is therefore aimed at filling our knowledge gap and improving our understanding of the spatio-temporal distribution of O3 and O3 precursors in HK, the roles of O3 precursors in O3 production, and the fast O3 chemistry in the subtropical region. Measurement data from Environmental Protection Department (EPD) of HK and by researchers of the Hong Kong Polytechnic University were analyzed using various statistical and modelling tools. The results are summarized in the following sections. O3 and regularly monitored gaseous air-quality data collected from June 1999 to May 2000 at three EPD's sites (rural, urban and suburban) were analyzed in order to obtain the information on the seasonal and spatial distribution of O3 and the relationship with other gas pollutants. The highest average O3 level was found at a rural site (Tap Mun) in northeastern HK which had increased levels of carbon monoxide (CO) and SO2 (relative to NOx) indicating frequent influences of regional air masses from the inland region. The lowest concentration of O3 was found at the urban site (Tsuen Wan). The southwestern suburban site (Tung Chung) showed intermediate O3 levels on average, and analysis of CO/NOx and SO2/NOx ratios suggests this site was impacted by both local and regional pollution. Principal component analysis further elucidated that the O3 variation in the northeastern rural area was largely determined by regional chemical and transport processes, while the O3 variability in the southwestern suburban and urban sites were more influenced by local emissions. High O3 episodes were also found to correlate with an enhanced ratio of SO2 to NOx, further suggesting the influence of regional emissions from the PRD. The results indicated that it is imperative to consider regional scale emissions and dynamic transport in managing O3 pollution in HK. VOC data collected by the EPD at the rural, urban and roadside sites from November 2000 to October 2001 and September 2002 to August 2003 were examined for their spatial distribution, seasonal variation, source-receptor relationships and photochemical reactivity. As expected, the roadside site showed the highest VOC level owing to the vehicular emissions and the rural site was characterized by the aged air mass with the lowest VOC level. Ratios of VOC with different photochemical reactivities indicated the mix of emission sources and increased atmospheric processing from urban to rural areas. Ratios of VOC to NOx were about 2-3 in the urban areas and 10 at the rural site, suggesting that the formation of O3 in HK is generally sensitive to VOC. Among the VOC, toluene was the most abundant species on a carbon basis and also a leading contributor to O3 production, especially in the urban areas. Isoprene, despite having a low mass concentration compared to other VOC, had a high reactivity value and was a key precursor contributing to O3 formation especially in the rural area. To further quantify the roles of O3 precursors in the O3 formation, an observation-based model (OBM) and measurement indicators method were employed using the data collected in autumn 2002 and summer 2004 at two suburban/rural sites in HK (Tai O) and the PRD (Wan Qing Sha), respectively. Photochemical O3 production was calculated to be limited by VOC at both sites. Nitric oxide (NO) tended to suppress the O3 formation, and CO was found to be negligible in the O3 formation. Among different VOC, reactive aromatic compounds such as toluene and xylenes dominated the O3 production in most cases, but naturally emitted isoprene could be important on warm days. This result is consistent with that from previous analysis of urban datasets. Model results revealed that O3 formation at Tai O was most sensitive to stationary hydrocarbons but a stronger influence of mobile hydrocarbons was found at Wan Qing Sha. The results were in line with the corresponding emission patterns in HK and the PRD. The OBM, which is a time-dependent photochemical model using the Carbon Bond (IV) mechanism, was used to infer the O3 chemistry and elucidate the processes (in-situ production versus transport) that control the variation of O3 at the sites. About half of the O3 episodes at Tai O were indicative of an impact from long-range transport but the O3 episodes at Wan Qing Sha were mainly attributable to local production. Photochemistry of two O3 episodes was further investigated in detail. It shows that in-situ O3 production accounted for most of the observed increase in O3 concentrations at the PRD site (1-hour O3 = 178 ppbv) indicating the importance of local sources, while for the case of HK (1-hour O3 = 203 ppbv), local production accounted for only 50% suggesting the importance of transport of regional pollution in this case. In both cases, conversion of NO to NO2 by HO2 was the major reaction for photochemical O3 production. The photochemical O3 loss rate was dominated by the reaction between NO2 and OH at Tai O while the reaction of O3 and HO2 was the major O3 destruction pathway at Wan Qing Sha. The O3 production efficiency and partition of reactive nitrogen compounds were also estimated with the calculated concentrations of NO2, PAN, HNO3 and measured values of NO and NOy during the two O3 episodes. The results showed that three to four O3 molecules were formed per NOx molecule oxidized while PAN and HNO3 were the most significant reactive nitrogen compounds in the afternoon.

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