Impact of NO_x reduction on long-term surface ozone pollution in roadside and suburban Hong Kong: Field measurements and model simulations

Continuous measurements of ozone (O₃) and nitrogen oxides (NO_x = NO + NO₂) were conducted from 2007 to 2019 in Hong Kong in order to evaluate the effectiveness of control strategies for NO_x emission from diesel commercial vehicles (DCV). DCV control programs were periodically applied in three phases starting from 2007, 2010 and 2014. It was found that NO and NO₂ levels decreased during the study period but more dramatically after the implementation of DCV Phase III than pre-DCV Phase III. Source apportionment analysis confirmed that the ambient NO and NO₂ in Hong Kong attributed to the regulated DCV emissions in Phase III reduced at rates of 5.1–14.4 ppbv/yr in roadside environment and 1.6–3.1 ppbv/yr in suburban area. Despite overall NO_x reduction, increased NO₂/NO_x ratios were recorded during the study period possibly due to the application of diesel particulate filter (DPF) in DCVs. However, after introducing DCV Phase III, observed O₃ values experienced more dramatic increasing trends in most areas of Hong Kong than pre-DCV Phase III. Model simulations revealed that O₃ production rate kept increasing and turned to be less sensitive to NO_x from 2014 to 2019. On the roadside, net O₃ production rate was more than doubled during 2014–2019 owing to NO_x reduction. Moreover, the levels of oxidants (OH, HO₂ and RO₂) were 1.5–5 times those before 2014. In suburban environment, NO_x reduction also facilitated O₃ production and radical cycling, but made smaller contributions than those on the roadside during 2014–2019. This study unraveled that NO_x reductions benefited from DCV regulations caused increase in surface O₃ and fueled O₃ photochemistry in various environments. More stringent control measures on emissions of VOCs, especially those with high OH reactivity, might help to better mitigate O₃ pollution.