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

Abstract

Continuous measurements of ozone (O3) and nitrogen oxides (NOx = NO + NO2) were conducted from 2007 to 2019 in Hong Kong in order to evaluate the effectiveness of control strategies for NOx 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 NOx reduction, increased NO2/NOx 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 NOx from 2014 to 2019. On the roadside, net O3 production rate was more than doubled during 2014–2019 owing to NOx reduction. Moreover, the levels of oxidants (OH, HO2 and RO2) were 1.5–5 times those before 2014. In suburban environment, NOx reduction also facilitated O3 production and radical cycling, but made smaller contributions than those on the roadside during 2014–2019. This study unraveled that NOx 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.