Particle emissions of Euro VI, EEV and retrofitted EEV city buses in real traffic

Exhaust emissions from traffic significantly affect urban air quality. In this study, in-traffic emissions of diesel-fueled city buses meeting enhanced environmentally friendly vehicle (EEV) and Euro VI emission limits and the effects of retrofitting of EEV buses were studied on-road by chasing the buses with a mobile laboratory in the Helsinki region, Finland. The average emission factors of particle number (PN), particle mass (PM₁) and black carbon mass (BC) were 0.86·10¹⁵ 1/kg_fuel, 0.20 g/kg_fuel and 0.10 g/kg_fuel, respectively, for EEV buses. For Euro VI buses, the emissions were below 0.5·10¹⁵ 1/kg_fuel (PN), 0.07 g/kg_fuel (PM₁) and 0.02 g/kg_fuel (BC), and the exhaust plume concentrations of these pollutants were close to the background concentrations. The emission factors of PM₁ and BC of retrofitted EEV buses were at the level of Euro VI buses, but their particle number emissions varied significantly. On average, the EEV buses were observed to emit the largest amounts of nanocluster aerosol (NCA) (i.e., the particles with size between 1.3 and 3 nm). High NCA emissions were linked with high PN emissions. In general, results demonstrate that advanced exhaust aftertreatment systems reduce emissions of larger soot particles but not small nucleation mode particles in all cases.