Primary and Secondary Emissions from a Modern Fleet of City Buses

LIYUAN ZHOU, Qianyun Liu, Christian M. Salvador, Michael Le Breton, Mattias Hallquist, Jian Zhen Yu, Chak K. Chan, Åsa M. Hallquist, King Abdullah University of Science and Technology

     Abstract Number: 272
     Working Group: Urban Aerosols

Abstract
The potential impact of transitioning from conventional fossil fuel to a non-fossil fuel vehicle fleet was investigated by measuring primary emissions via extractive sampling of bus plumes and assessing secondary mass formation using a Gothenburg Potential Aerosol Mass (Go:PAM) reactor from 76 in-use transit buses. Online chemical characterization of gaseous and particle emissions from these buses was conducted using a chemical ionization mass spectrometry (CIMS) with acetate as the reagent ion, coupled with a filter inlet for gases and aerosols (FIGAERO). A significant reduction (48-98%) in fresh particle emissions was observed in buses utilizing compressed natural gas (CNG), biodiesels like rapeseed methyl ester (RME) and hydrotreated vegetable oil (HVO), as well as hybrid-electric HVO (HVO_HEV), compared to diesel (DSL) buses. However, secondary particle formation from photooxidation of emissions was substantial across all fuel types. The median ratio of particle mass emission factors of aged to fresh emissions increased in the following order: DSL buses at 4.0, HVO buses at 6.7, HVO_HEV buses at 10.5, RME buses at 10.8, and CNG buses at 84. Of the compounds that can be identified by CIMS, fresh gaseous emissions from all Euro V/EEV buses, regardless of fuel type, were dominated by nitrogen-containing compounds such as nitrous acid (HONO), nitric acid (HNO₃), and isocyanic acid (HNCO), alongside small monoacids (C₁-C₃). Notably, nitrogen-containing compounds were significantly reduced in Euro VI buses equipped with more advanced emission control technologies. Secondary gaseous organic acids correlated strongly with gaseous HNO₃ signals (R²= 0.85-0.99) in Go:PAM, but their moderate to weak correlations with post-photooxidation secondary particle mass suggest they are not reliable tracers for secondary organic aerosol formation from bus exhaust. Our study highlights that non-regulated compounds and secondary pollutant formation, not currently addressed in legislation, are crucial considerations in the evaluation of environmental impacts of future fuel and engine technology shifts.