Increasing trend of primary NO₂exhaust emission fraction in Hong Kong

Despite the successful reduction in roadside NOxlevels, no such decrease has been detected in roadside NO2concentration in Hong Kong. One underlying cause could be the rising primary NO2fraction of the total emission of NOx. Primary NO2can be particularly detrimental to Hong Kong because a large fraction of the population are exposed to the traffic-related primary pollutants in the street canyons formed by congested high-rise buildings. In this study, hourly mean concentration data for roadside nitrogen oxides (NOx), nitrogen dioxide (NO2), and background ozone (O3) were used to estimate the mean primary NO2fraction from vehicle exhausts in Hong Kong. An overall increasing trend was observed for the primary NO2fraction (f-NO2) values in all the three roadside air monitoring sites. The primary NO2as a fraction of total NOx(f-NO2) increased approximately from 2% in 1998 to 13% in 2008 in Hong Kong. The two particular periods of rising f-NO2coincided with the two implementation periods of the diesel retrofit programs for the light-duty vehicles and heavy-duty vehicles. Future vehicle emission control strategies should target not only total NOxbut also primary NO2. Health benefit or disease burden estimates should be taken into account and updated in the process of policy planning and evaluation.