Roadside NO2/NOx and primary NO2 from individual vehicles

Traffic-derived nitrogen oxides (NOx) is a major component of kerbside pollution in large cities and represent an important source of exposure and health risk to urban populations, especially during the use of roadside pave-ments. The ratio of ambient NO2/NOx (xNO(2)) varies widely in urban environments, influenced by relative source strengths, but also oxidation. Primary NO2 emissions have changed over the years, with changes in engine and exhaust controls. Over the years 1998/2021 the NO2 fraction in exhaust (f(NO2)) in Hong Kong has changed from 0.05 in the late 1990s to values similar to 0.2 in the years 2010/2015, and has fallen to similar to 0.08 in recent years. To better understand f(NO2) from individual on-road vehicles, this study monitored NO and NO2 at the kerb in a busy street canyon in Hong Kong, capturing the rapid concentration changes in response to vehicles passing by. The kerbside xNO(2) was much lower (similar to 0.2) than that at the roadside air quality monitoring station (similar to 0.5) whose inlet was elevated some 3 m above the road nearby. Output from pollution sensors was deconvoluted to explore rapidly changing concentrations in the plume segments from individually identifiable vehicles as they passed and enable to estimate of the NO2 fraction in the exhaust for individual vehicle. Each vehicle was assigned to a type, Euro emission class and year of manufacture. The f(NO2) from individual vehicles depends on emission class, increasing from Euro 4 to Euro 6, while f(NO2) decreases as vehicles get older. The estimates of f(NO2) made at the kerb were similar to those derived for the local air quality monitoring station, but not necessarily that of the city as a whole, where f(NO2) varies by a factor of two.