Characterization and Evaluation of Long-term Air Quality Across Urban Hong Kong

Air pollution is a serious environmental issue and has attracted much attention owing to the rapid urbanization and industrialization worldwide. Hong Kong, one of the well-developed cities located in the southeast of the Pearl River Delta (PRD) region, struggling with frequent air pollution episodes because of the intensive land use, dense daily anthropogenic activities, and enormous vehicle emissions. Therefore, a 10-year evaluation of air pollutants across roadside, urban and background sites was conducted to analyze the variations of air quality and contamination in Hong Kong. The continuous decrease of annual averaged concentrations of traffic-related NO₂, SO₂, CO, PM_2.5 and PM₁₀ and numbers of days with severe pollution conditions validated the efficiency of the series of air pollution control schemes implemented by the Hong Kong government. However, the concentration of O₃ at roadside and urban stations increased by 135% ± 25% and 37% ± 18% from 2011 to 2020, respectively, meanwhile the highest 8-hour averaged O₃ concentration was observed as 294 μg/m³ at background station in 2020, which pointed out the worsening ozone pollution in Hong Kong. To further investigate typical precursors of ozone and secondary organic aerosols, the intensive measurements of VOCs and OVOCs were conducted at both urban roadside station with heavy traffic and coastal station in suburban area to study the chemical compositions and emission patterns of ambient VOCs and OVOCs thoroughly. Results from those two campaigns showed significant differences across Hong Kong. Acetone, formaldehyde, and acetaldehyde were the most abundant species at roadside environment, while formaldehyde, methanol and acetone were the major species in suburban atmosphere. The major contributors to ozone formation were formaldehyde (89.64 μg/m³) at roadside and isoprene (13.46 μg/m³) at suburban among the measured VOCs in Hong Kong. In addition, the source apportionment results from positive matrix factorization (PMF) analysis showed that the sampling site at the southeastern tip of Hong Kong was strongly influenced by urban plumes from the PRD region and by oceanic emissions as well.