|Title:||Ozone pollution around a coastal region of South China Sea : ozone photochemistry and long-term ozone-precursor relationships|
|Advisors:||Guo, Hai (CEE)|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Air -- Pollution -- South China Sea
|Department:||Department of Civil and Environmental Engineering|
|Pages:||219 pages : color illustrations|
|Abstract:||Marine atmosphere is usually considered to be a clean environment, but the study in this thesis indicates that the near-coast waters of the South China Sea (SCS) suffer from even worse air quality than coastal cities. The analyses were based on concurrent field measurements of target air pollutants and meteorological parameters conducted at a suburban site (Tung Chung, TC) and a nearby marine site (Wan Shan Island, WSI) from August to November 2013. The observations showed that the levels of primary air pollutants were significantly lower at WSI than those at TC, while the ozone (O3) value was greater at WSI. Higher O3 levels at WSI were attributed to the weaker NO titration and higher O3 production rate because of stronger oxidative capacity of the atmosphere. However, O3 episodes (defined as the days with maximum hourly average O3 value exceeding 100 ppbv, China's Grade II Standard) were concurrently observed at both sites under certain meteorological conditions, such as tropical cyclones, continental anticyclones and sea-land breezes (SLBs). Driven by these synoptic systems and mesoscale recirculation, the interaction between continental and marine air masses profoundly changed the atmospheric composition and subsequently influenced the formation and redistribution of O3 in the coastal areas. When continental air intruded into marine atmosphere, the O3 pollution was magnified over the SCS, and the elevated O3 (>100 ppbv) could overspread the sea boundary layer 8 times the area of Hong Kong. During the intrusion, O3 episode occurred at WSI and the mixing ratios of O3 precursors (such as volatile organic compounds (VOCs), nitrogen oxides (NOx=NO+NO2) and carbon monoxide (CO)) increased significantly compared to those non-episode days. Additional knowledge was gained when a photochemical box model incorporating the near-explicit Master Chemical Mechanism (PBM-MCM) was applied to further investigate O3 photochemistry, in terms of O3-precursor relationship, atmospheric photochemical reactivity and O3 production. The simulation results revealed that, from non-O3 episode days to episode days at WSI: 1) O3 production changed from both VOC and NOx-limited (transition regime) to VOC-limited; 2) OH radicals increased and photochemical reaction cycling processes accelerated; and 3) both O3 production and destruction rates increased significantly, resulting in an elevated net O3 production over the SCS. Moreover, to further discover the photochemical evolutions during the intrusion of continent air to the maritime atmosphere, a photochemical trajectory model coupled with the near-explicit Master Chemical Mechanism (PTM-MCM) was adopted, and the photochemical processes of air masses during the transport from TC to WSI were investigated. The modeling results revealed that: 1) during the transport of air masses from TC to WSI, both VOC and NOx decreased in the morning while O3 increased significantly, mainly due to rapid chemical reactions with elevated radicals over the SCS; 2) the elevated radicals over the SCS were attributable to the fact that higher NOx at TC consumed more radicals, whereas the concentration of radicals increased from TC to WSI because of NOx dilution and destruction; 3) the photochemical cycling of radicals accelerated subsequently, leading to high O3 mixing ratios over the SCS. Furthermore, based on the speciated VOC source profiles of the emission inventory used in the PTM-MCM, the contributions of six VOC sources, i.e. gasoline vehicle exhaust, diesel vehicle exhaust, gasoline evaporation and liquefied petroleum gas (LPG) usage, solvent usage, biomass and coal burning, and biogenic emissions, to maritime O3 formation were evaluated. The results suggested that gasoline vehicle exhaust and solvent usage largely contributed the O3 formation over the SCS (about 5.2 and 3.8 ppbv, respectively).|
Lastly, as the center of the coastal area in the Pearl River Delta (PRD) region, Hong Kong has suffered from severe O3 pollution in the past decades, which was impacted not only by heavy continental emissions but also by the land-sea interaction. Hence, there is an urgent need to solve the O3 pollution problem in this coastal metropolis. Over the past ten years (2005-2014), ground-level O3 in Hong Kong has consistently increased in all seasons except winter, despite the yearly reduction of its precursors (i.e., NOx, VOCs and CO). To explain the contradictory phenomenon, an observation-based photochemical box model coupled with CB05 mechanism (PBM-CB05) was applied to understand the influence of both locally-produced O3 and regional transport. The simulation of locally-produced O3 showed an increasing trend in spring, a decreasing trend in autumn and no changes in summer and winter. The O3 increase in spring was caused by the net effect of more rapid decrease of NO titration and unchanged total VOC (TVOC) reactivity despite decreased TVOC mixing ratios, while the decreased local O3 formation in autumn was mainly due to the reduction of aromatic VOC mixing ratios and the TVOC reactivity, and much slower decrease of NO titration. However, the decreased in-situ O3 formation in autumn was overridden by the regional contribution, resulting in elevated O3 observations. Furthermore, the PBM-derived relative incremental reactivity indicated that the O3 formation was VOC-limited in all seasons, and the long-term O3 formation was more sensitive to VOCs and less to NOx and CO in the past 10 years. In addition, the PBM results indicated that the contributions of aromatics to O3 formation decreased in all seasons of these years, particularly in autumn, likely due to effective control of solvent-related sources. In contrast, the contributions of alkenes increased, suggesting a continuing need to reduce traffic emissions. Overall, the findings in this thesis provided updated information on photochemical O3 pollution and its impact on the coastal region of SCS, filled up the knowledge gap of O3 photochemistry over the SCS, and advanced our knowledge on the long-term O3-precursor relationships in the coastal area. The findings are applicable to similar mesoscale environments around the world where the maritime atmosphere is potentially influenced by severe continental air pollution.
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