Impact of traffic emissions on ozone formation in Hong Kong

Due to its high population density and extensive commercial activity, traffic emissions are the primary source of anthropogenic pollutants in Hong Kong, significantly contributing to ozone (O₃) pollution. This study examined the impact of traffic emissions from ships, vehicles, and aircraft on O₃ levels in Hong Kong using the WRF-Chem model. Currently, traffic sources accounted for about 60 % of NO_x and 40 % of VOCs — the key precursors to O₃ formation in Hong Kong. During the daytime, eliminating traffic emissions led to a reduction in O₃ concentration as a result of decreased photochemical production. However, at night, O₃ concentrations increased due to weakened NO_x titration. On average, the effect of reducing traffic emissions on O₃ levels in Hong Kong varied depending on both the quantity and the ratio of NO_x to VOCs. Among the traffic sources, ships were significant contributors, responsible for 35 % of NO_x and 15 % of VOCs. Eliminating ship emissions shifted Hong Kong from a VOC-limited regime to a transitional regime, resulting in a 0.6 % decrease in O₃ concentration. Conversely, when the NO_x to VOCs ratio was low, as with vehicles (21 % NO_x and 19 % VOCs), or when both NO_x and VOCs emissions were minimal, as with aircraft (6 % NO_x and 2 % VOCs), Hong Kong remained in a VOC-limited regime, and removing these emissions led to a 0.2 % increase in O₃ concentration. Furthermore, significant changes in O₃ concentrations typically occurred downwind of the emission centers under prevailing northeasterly winds. These findings support the dual goals of improving air quality and combating climate change in Hong Kong, and they may be applicable to other coastal cities worldwide.