Elucidating real-world vehicle emission factors from mobile measurements over a large metropolitan region: a focus on isocyanic acid, hydrogen cyanide, and black carbon

Abstract

A mobile laboratory equipped with state-of-the-art gaseous and particulate instrumentation was deployed across the Greater Toronto Area (GTA) during two seasons. A high-resolution time-of-flight chemical ionization mass spectrometer (HR-TOF-CIMS) measured isocyanic acid (HNCO) and hydrogen cyanide (HCN), and a high-sensitivity laserinduced 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 (BB) and secondary photochemistry are weakest. Plume-based and time-based algorithms were developed to calculate fleet-average vehicle emission factors (EFs); 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-, and 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 (LDV). Our fleet-average EF for BC (median: 25 mg kg −1 fuel ; interquartile range, IQR: 10-76 mg kg −1 fuel ) 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 kg −1 fuel , IQR: 1.4-4.2 mg kg −1 fuel ) and HCN (median: 0.52 mg kg −1 fuel , IQR: 0.32-0.88 mg kg −1 fuel ). 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. 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.Published by Copernicus Publications on behalf of the European Geosciences Union.

Isocyanic acid (HNCO) and its fate in the atmosphere: a review

“…This instrument also uses so ionisation through proton transfer but from hydronium ions (H 3 O + ) and can detect HNCO mixing ratios from low ppbv up to 1000s of ppbv. [23][24][25][51][52][53] Online CIMS techniques with acetate and iodide reagent ions and PTR-MS instruments have facilitated atmospheric measurements of HNCO providing observations from eld campaigns from 10 locations across three continents (Pasadena, California; 19 Erie, Colorado; 26,54 La Jolla, California; 55 Fort Collins, Colorado; 26 Toronto, Ontario; 30,56 Calgary, Alberta; 50 the marine boundary layer in the Canadian Arctic Archipelago; 57 Mohali, India; 51 Kathmandu Valley, Nepal; 52 and Manchester, UK 58 ) ( Table 1). The number of ambient measurements continues to grow, yet long term datasets are still missing (Fig.…”

“…Wren et al (2018) conducted a mobile study using a CIMS operated in a mobile lab to measure real time HNCO and HCN mixing ratios of on-road vehicles. 56 They found that on-road vehicles were clear emitters of HNCO to the atmosphere, with higher emissions observed in winter than in summer. Algorithms were used to calculate both plume-based and timed-based emission factors from the combined diesel and gasoline emissions.…”

“…In the Alberta Oil Sands region, the diesel emissions are dominated by large industrial engines compared with relatively smaller commercial and freight trucks on Toronto highways, which may explain the higher emission factors compared to Wentzell et al (2013) 30 and Wren et al (2018). 56 Additionally, because the plane intercepted the vehicle exhaust much further downwind of the source that the on-road mobile lab, there was more time for secondary production of HNCO to occur.…”

Integrated urban services: Experience from four cities on different continents

“…The observational network was designed to provide environmental conditions at the sport venues sites and to capture meteorological patterns specific to Toronto, such as lake-breeze front tracking (as they can lead to high-impact weather or modify local conditions rapidly, e.g., Mariani et al, 2018), heat stress conditions (monitored by the Wet-Bulb Globe temperature -WBGT-index obtained with black globe thermometers measuring the radiant temperature, e.g., Herdt et al, 2018) or atmospheric pollution (e.g., using vehicle traverse sampling, Wren et al, 2018). Substantial time (years in some cases) was needed to obtain land use licenses or leases required for siting fixed stations.…”

Advances in science and applications of air pollution monitoring: A case study on oil sands monitoring targeting ecosystem protection

“…They are both products of secondary formation in the atmosphere but are also directly emitted. The transportation sector is an important source of HNCO (Brady et al 2014;Wentzell et al 2013;Wren et al 2018), and emission rates were estimated from the aircraft measurements to be 2.2 ± 0.8 kg hr −1 from SUN, followed by that from the SML facility of 1.5 ± 0.5 kg hr −1 . Figures S3.2a and S3.2b show that by tracking a specific integrated plume downwind, increases in HNCO and LMWOAs can be observed (Liggio et al 2017).…”