Integrated data analysis and characterization of photochemical ozone in subtropical Hong Kong

Abstract

To understand the photochemical O₃ pollution at different elevations in mountainous areas, and to provide a conceptual description of O₃ pollution in Hong Kong, a number of field campaigns were undertaken in different locations in Hong Kong and the inland PRD region, which were followed by in-depth data analysis and model simulation. Intensive field measurements were concurrently conducted for the first time at a mountain site (TMS) and an urban site at the foot of the mountain (TW) from September to November 2010. The mixing ratios of air pollutants were greater at TW than those at TMS, except for O₃. The relatively higher levels of O₃ at TMS were attributed to the combined influence of NO titration, vertical meteorological conditions, regional transport and mesoscale circulations. The photochemical O₃ formation at TMS was mostly influenced by VOCs, with measurable influence of NOx, while O₃ production at TW was generally limited by the concentrations of VOCs. By using a photochemical box model coupled with master chemical mechanism (PBM-MCM), the photochemical reactivity at the above two sites were investigated. It was found that slightly higher HO₂ concentrations were found at TMS, while much higher OH concentrations were estimated at TW, suggesting that the HOx cycling processes were different at the two sites due to the differences of O₃ and its precursors. The O₃ formation was dominated by the reaction of HO₂ + NO at the two sites, while O₃ was mainly destroyed by the reactions of OH + NO₂ at TW, and by the O₃ photolysis and the reaction of O₃ + HO₂ at TMS. Furthermore, more O₃ could be produced for each radical generated at TMS. Since VOCs are the most important chemicals contributing to high O₃ production in Hong Kong and the inland PRD region, identification of VOC sources and quantification of source contributions could provide valuable information for the formulation and implementation of O₃ pollution control measures. A new reactivity-based approach, combining the Positive Matrix Factorization (PMF) model with an observation-based model (OBM), was developed in this study. A new parameter, i.e., Relative Incremental Reactivity (RIR) weighted value, considering both the emissions and reactivity of VOCs, was used to evaluate the contributions of VOC sources and the major species to O₃ formation. In the inland PRD region, ten VOC sources were identified, while seven sources were identified in Hong Kong. Among all the sources, vehicular-and solvent-related emissions contributed significantly to ambient VOCs. In addition, the RIR-weighted values indicated that the O₃ formation in inland PRD and Hong Kong was controlled by a small number of VOC species in specific sources. Sensitivity analysis on the basis of relative O₃ reduction efficiency (RORE) indicated that the O₃ reduction was the most effective when the identified VOC sources and the major species from these sources were cut by certain percentages. Finally, to formulate and implement effective control strategies for O₃ pollution, a conceptual model was developed for the first time in Hong Kong based on the integrated data analysis at Tung Chung (TC) in Hong Kong between 2005 and 2010. By comparing meteorological parameters between O₃ and non-O₃ episode days, it was found that high temperatures, strong solar radiation, low wind speeds and relative humidity, northeasterly and/or northwesterly prevailing winds were favorable for the O₃ formation, while tropical cyclones were most conducive to the occurrence of O₃ episodes. Backward trajectories simulation and graphical illustration of O₃ pollution suggested that super-regional and regional transport were other factors that contributed to high O₃ levels in Hong Kong. The photochemical O₃ formation, generally VOC-limited in Hong Kong, was controlled by a small number of VOCs, which were mainly from solvent usage and vehicular emissions. Overall, the results of this study suggested that mesoscale circulations had a significant influence on the distributions of air pollutants in mountainous areas in Hong Kong, and the cycling processes among radicals, the production and destruction of O₃ were different at the mountain and urban sites due to the different levels of O₃ and its precursors. It is recommended that before the formulation and implementation of VOC control strategies, the abundance and reactivity of each VOC in each source should be considered. This study has provided an alternative way to more efficiently alleviate O₃ pollution by controlling specific VOCs in certain VOC sources and highlighted the importance of monitoring these VOCs.