Long-term trend in NO2 and NOx levels and their emission ratio in relation to road traffic activities in East Asia

Long-term trend in emission ratio of NO2 to NOx was analyzed in relation to traffic activities using ambient monitoring data (such as NO, NO2, NOx, and O3) collected from 7 urban roadside (U-RS) and 7 urban background (U-BG) locations in Seoul, Korea over a 14-year period (1996–2009). In general, the temporal trend of NO2 concentrations is less distinctive than that of NOx, whether being downward or upward. However, if their differences are checked statistically, only a few cases were significant. The mean emission ratio of NO2 to NOx values varied slightly across the 7 U-RS sites during the study period, ranging from 0.11 to 0.19. In general, there was no significant annual trend in the relative fraction of NO2 in NOx emission (f(NO2)) at U-RS, with an exceptional downward trend at one site (slope of −0.008 y−1). On the other hand, diurnal variations of f(NO2) were likely affected by driving conditions and fuel type of vehicles. The overall results of this study suggest that NOx emission control strategies such as diesel particulate filter (DPF) or diesel oxidation catalyst (DOC) should have direct influences on f(NO2) values at U-RS sites.