In many cities worldwide, modern fleets have been introduced to reduce gaseous and particle emissions from city buses. To date, most emission studies are limited to a few vehicles, making a statistically significant assessment of control options difficult, especially under real-world driving conditions. The potential effect of changing to a non-fossil fuel vehicle fleet was investigated by measuring primary EF and secondary gas and particle formation downstream a Gothenburg Potential Aerosol Mass (Go:PAM) reactor. Exhaust emissions of 234 individual city buses were measured under real-world stop-and-go traffic conditions at a bus stop in Gothenburg, Sweden. The buses comprised models fulfilling Euro III-VI and EEV (Enhanced Environmentally Friendly Vehicle) standards with diffe...[ Read more ]

In many cities worldwide, modern fleets have been introduced to reduce gaseous and particle emissions from city buses. To date, most emission studies are limited to a few vehicles, making a statistically significant assessment of control options difficult, especially under real-world driving conditions. The potential effect of changing to a non-fossil fuel vehicle fleet was investigated by measuring primary EF and secondary gas and particle formation downstream a Gothenburg Potential Aerosol Mass (Go:PAM) reactor. Exhaust emissions of 234 individual city buses were measured under real-world stop-and-go traffic conditions at a bus stop in Gothenburg, Sweden. The buses comprised models fulfilling Euro III-VI and EEV (Enhanced Environmentally Friendly Vehicle) standards with different engine technologies, fuels, and exhaust after-treatment systems, and also included hybrid-electric buses (HEV). Both gaseous (NOx, CO, HC, and SO₂) and size-resolved particle number (PN) and mass (PM) emission factors (EF) were calculated for vehicles using compressed natural gas (CNG), diesel (DSL), Rapeseed Methyl Ester (RME) and Hydro-treated Vegetable Oil (HVO) equipped with various after-treatment technologies, e.g., diesel particulate filter (DPF), selective catalytic reduction (SCR) and exhaust gas recirculation (EGR) systems. The highest median EF_PN was obtained from Euro V_HEV-HVO-SCR buses when their combustion engines were used though 53 % of their accelerations were below detection limits indicating the use of their electrical engine. The highest ^MdEF_PM was obtained from the Euro V-DSL-SCR buses and the lowest from EEV-CNG buses (below detection threshold) and Euro VI_HEV-HVO- SCR+EGR+DPF buses. The highest ^MdEF_NOx was obtained from the Euro V-RME-SCR and Euro V_HEV-HVO-SCR buses, and the lowest from CNG buses and Euro VI_HEV-HVO-SCR+EGR+DPF buses. Hybrid buses can give higher PN emissions compared to traditional diesel engines, likely due to downsized combustion engines. Overall, the EEV-CNG buses performed the best regarding both the ^MdEF and low contribution to the high emitters.

Regarding the particle phase, replacing diesel by biodiesel and biogas fuel reduced the fresh EF of PM significantly (by a factor of 2.7 and 5, respectively). However, secondary particle formation resulting from exhaust aging was generally significant for all the fuel types tested, suggesting an essential nonfuel dependent source. Regarding the gas phase, EFs of glycolic acid, glyoxylic acid, dihydroxy acetic acid, lactic acid, pyruvic acid, and malonic acid varied from 0.8-6.3 mg kg-fuel^-1 and were well correlated with EF of HNO₃ (r²= 0.65-0.97) from the aged emissions.

The results suggest that hybrid buses can give higher PN emissions compared to traditional diesel engines. Replacing diesel by biodiesel fuel reduced PM EFs significantly but increased NOx EFs. A small proportion of the buses contributed significantly to the total emissions. The results also highlighted that the potential for forming secondary mass should be considered in future fuel shifts since the environmental impact is different when only considering the primary emissions. It is also of great significance to include both primary and secondary anthropogenic sources in budgets of low molecular weight organic acids. The inclusion of secondary pollutant sources in emissions regulation may lead to a more accurate portrayal of the potential impact of mobile sources or emissions control procedures on regional air quality.