Increasing trend of primary NO2 exhaust emission fraction in Hong Kong

Abstract

Despite the successful reduction in roadside NO _x levels, no such decrease has been detected in roadside NO₂ concentration in Hong Kong. One underlying cause could be the rising primary NO₂ fraction of the total emission of NO _x . Primary NO₂ 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 (NO₂), and background ozone (O₃) were used to estimate the mean primary NO₂ fraction from vehicle exhausts in Hong Kong. An overall increasing trend was observed for the primary NO₂ fraction (f-NO₂) values in all the three roadside air monitoring sites. The primary NO₂ as a fraction of total NO _x (f-NO₂) increased approximately from 2% in 1998 to 13% in 2008 in Hong Kong. The two particular periods of rising f-NO₂ 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 NO₂. Health benefit or disease burden estimates should be taken into account and updated in the process of policy planning and evaluation.