Increasing trend of primary NO_2 exhaust emission fraction in Hong Kong

摘要

Despite the successful reduction in roadside NO~* levels, no such decrease has been detected in roadside NO_2 concentration in Hong Kong. One underlying cause could be the rising primary NO_2 fraction of the total emission of NO_x. Primary NO_2 can 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 (NO_x, nitrogen dioxide (NO_2), and background ozone (O_3) were used to estimate the mean primary NO_2 fraction from vehicle exhausts in Hong Kong. An overall increasing trend was observed for the primary NO_2 fraction (f-NO_2) values in all the three roadside air monitoring sites. The primary NO_2 as a fraction of total NO_x (f-NO_2) increased approximately from 2% in 1998 to 13% in 2008 in Hong Kong. The two particular periods of rising f-NO_2 coincided 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 NO_x but also primary NO_2. Health benefit or disease burden estimates should be taken into account and updated in the process of policy planning and evaluation.