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

Continuous measurements of ozone (O 3) and nitrogen oxides (NO x = NO + NO 2) 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 NO 2 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 NO 2 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 NO 2 /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 O 3 values experienced more dramatic increasing trends in most areas of Hong Kong than pre-DCV Phase III. Model simulations revealed that O 3 production rate kept increasing and turned to be less sensitive to NO x from 2014 to 2019. On the roadside, net O 3 production rate was more than doubled during 2014–2019 owing to NO x reduction. Moreover, the levels of oxidants (OH, HO 2 and RO 2) were 1.5–5 times those before 2014. In suburban environment, NO x reduction also facilitated O 3 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 O 3 and fueled O 3 photochemistry in various environments. More stringent control measures on emissions of VOCs, especially those with high OH reactivity, might help to better mitigate O 3 pollution. [Display omitted] • DCV control programs, especially Phase III led to NO x decrease in urban Hong Kong. • O 3 production rate increased and became less sensitive to NO x after DCV Phase III. • Roadside O 3 production rate was more than doubled owing to NO x reduction. • More stringent control measures on VOC emissions are suggested for O 3 mitigation. [ABSTRACT FROM AUTHOR]