Abstract. A mobile laboratory equipped with state-of-the-art gaseous and particulate instrumentation was deployed across the Greater Toronto Area during two seasons. A high-resolution time-of-flight mass spectrometer (HR-TOF-CIMS) measured isocyanic acid (HNCO) and hydrogen cyanide (HCN), and a high-sensitivity laser-induced incandescence (HS-LII) instrument measured black carbon (BC). Results indicate that on-road vehicles are a clear source of HNCO and HCN, and that their impact is more pronounced in the winter, when influences from biomass burning and secondary photochemistry are weakest. Plume-based and time-based algorithms were developed to calculate fleet-average vehicle emission factors (EF); the algorithms were found to yield comparable results, depending on the pollutant identity. With respect to literature EFs for benzene, toluene, C2 benzene (sum of m,p,o-xylenes and ethylbenzene), nitrogen oxides, particle number concentration (PN), and black carbon, the calculated EFs were characteristic of a relatively clean vehicle fleet dominated by light-duty vehicles. Our fleet-average EF for BC (median: 25 mg kgfuel−1, interquartile range: 10–76 mg kgfuel−1) suggests that overall vehicular emissions of BC have decreased over time. However, the distribution of EFs indicates that a small proportion of high-emitters continue to contribute disproportionately to total BC emissions. We report the first fleet-average EF for HNCO (median: 2.3 mg kgfuel−1, interquartile range: 1.4–4.2 mg kgfuel−1) and HCN (median: 0.52 mg kgfuel−1, interquartile range: 0.32–0.88 mg kgfuel−1). The distribution of the estimated EFs provides insight into the real-world variability of HNCO and HCN emissions, and constrains the wide range of literature EFs obtained from prior dynamometer studies. Our results demonstrate that although biomass burning is a dominant source of both air toxics on a national scale, vehicular emissions play an increasingly important role at a local scale, especially in heavily-trafficked urban areas. The impact of vehicle emissions on urban HNCO levels can be expected to be further enhanced if secondary HNCO formation from vehicle exhaust is considered.
Elucidating real-world vehicle emission factors from mobile
measurements over a large metropolitan region: a focus on isocyanic
acid, hydrogen cyanide, and black carbon
