Impact of traffic emissions on ozone formation in Hong Kong

Abstract

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 (O<inf>3</inf>) pollution. This study examined the impact of traffic emissions from ships, vehicles, and aircraft on O<inf>3</inf> levels in Hong Kong using the WRF-Chem model. Currently, traffic sources accounted for about 60 % of NO<inf>x</inf> and 40 % of VOCs — the key precursors to O<inf>3</inf> formation in Hong Kong. During the daytime, eliminating traffic emissions led to a reduction in O<inf>3</inf> concentration as a result of decreased photochemical production. However, at night, O<inf>3</inf> concentrations increased due to weakened NO<inf>x</inf> titration. On average, the effect of reducing traffic emissions on O<inf>3</inf> levels in Hong Kong varied depending on both the quantity and the ratio of NO<inf>x</inf> to VOCs. Among the traffic sources, ships were significant contributors, responsible for 35 % of NO<inf>x</inf> 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 O<inf>3</inf> concentration. Conversely, when the NO<inf>x</inf> to VOCs ratio was low, as with vehicles (21 % NO<inf>x</inf> and 19 % VOCs), or when both NO<inf>x</inf> and VOCs emissions were minimal, as with aircraft (6 % NO<inf>x</inf> and 2 % VOCs), Hong Kong remained in a VOC-limited regime, and removing these emissions led to a 0.2 % increase in O<inf>3</inf> concentration. Furthermore, significant changes in O<inf>3</inf> 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.