Oxygenated VOCs as significant but varied contributors to VOC emissions from vehicles

Abstract. Vehicular emission is an important source for volatile organic compounds (VOCs) in urban and downwind regions. In this study, we conducted a chassis dynamometer study to investigate VOC emissions from vehicles using gasoline, diesel, and liquefied petroleum gas (LPG) as fuel. Time-resolved VOC emissions from vehicles are chemically characterized by a proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) with high frequency. Our results show that emission factors of VOCs generally decrease with the improvement of emission standard for gasoline vehicles, whereas variations of emission factors for diesel vehicles with emission standards are more diverse. Mass spectra analysis of PTR-ToF-MS suggest that cold start significantly influence VOCs emission of gasoline vehicles, while the influences are less important for diesel vehicles. Large differences of VOC emissions between gasoline and diesel vehicles are observed with emission factors of most VOC species from diesel vehicles were higher than gasoline vehicles, especially for most oxygenated volatile organic compounds (OVOCs) and heavier aromatics. These results indicate quantification of heavier species by PTR-ToF-MS may be important in characterization of vehicular exhausts. Our results suggest that VOC pairs (e.g. C14 aromatics/toluene ratio) could potentially provide good indicators for distinguishing emissions from gasoline and diesel vehicles. The fractions of OVOCs in total VOC emissions are determined by combining measurements of hydrocarbons from canisters and online observations of PTR-ToF-MS. We show that OVOCs contribute 7.7 % ± 6.2 % of gasoline vehicles of the total VOC emissions, while the fractions are significantly higher for diesel vehicles (40–77 %), highlighting the importance to detect these OVOC species in diesel emissions. Our study demonstrated that the large number of OVOC species measured by PTR-ToF-MS are important in characterization of VOC emissions from vehicles.