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 (O ) and nitrogen oxides (NO = NO + NO ) were conducted from 2007 to 2019 in Hong Kong in order to evaluate the effectiveness of control strategies for NO 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 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 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 reduction, increased NO /NO 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 values experienced more dramatic increasing trends in most areas of Hong Kong than pre-DCV Phase III. Model simulations revealed that O production rate kept increasing and turned to be less sensitive to NO from 2014 to 2019. On the roadside, net O production rate was more than doubled during 2014-2019 owing to NO reduction. Moreover, the levels of oxidants (OH, HO and RO ) were 1.5-5 times those before 2014. In suburban environment, NO reduction also facilitated O production and radical cycling, but made smaller contributions than those on the roadside during 2014-2019. This study unraveled that NO reductions benefited from DCV regulations caused increase in surface O and fueled O photochemistry in various environments. More stringent control measures on emissions of VOCs, especially those with high OH reactivity, might help to better mitigate O pollution.