LAUSR

Abstract Organic acids play important roles in the atmosphere by contributing to formation of secondary organic aerosol (SOA), cloud condensation nuclei (CCN), and rainwater acidity. Vehicle emissions can be an important source of organic acids in urban environments. In this study, we carried out chassis dynamometer tests to measure C1-C5 carboxylic acids, hydrogen cyanide (HCN), and isocyanic acid (HNCO) from gasoline, diesel, and liquefied petroleum gas (LPG) vehicles using an iodide-adduct time-of-flight chemical ionization mass spectrometer (I- ToF-CIMS). We present fuel-based and mileage-based emission factors base on 31 vehicles under different stages of emission standards. Our measurements show that emission factors of carboxylic acids from diesel vehicles are much higher than those from gasoline vehicles. The determined emission factors of formic acid from diesel vehicles (130 ± 134 mg/kgfuel) are nearly 800 times higher than those from gasoline vehicles (0.16 ± 0.18 mg/kgfuel), which suggests that fuel type is a key metrics affecting emission factors of carboxylic acids from motor vehicles. We quantified the emission factors of toxic species HCN (gasoline: 10.4 ± 17.5 mg/kgfuel, diesel: 12.5 ± 7.49 mg/kgfuel) and HNCO (gasoline: 0.09 ± 0.25 mg/kgfuel, diesel: 2.26 ± 8.94 mg/kgfuel). Although the main source of organic acids is likely to be related to secondary production mechanisms on global scales according to previous research, our results highlight that primary emissions from vehicles may contribute significantly to local and finer regional scales. Emission factors of organic acids obtained in this study can be used to constrain the budget of organic acids in urban environments.