Quantifying the Primary Emissions and Photochemical Formation of Isocyanic Acid Downwind of Oil Sands Operations

Abstract

Isocyanic acid (HNCO) is a known toxic species and yet the relative importance of primary and secondary sources to regional HNCO and population exposure remains unclear. Off-road diesel fuel combustion has previously been suggested to be an important regional source of HNCO, which implies that major industrial facilities such as the oil sands (OS), which consume large quantities of diesel fuel, can be sources of HNCO. The OS emissions of nontraditional toxic species such as HNCO have not been assessed. Here, airborne measurements of HNCO were used to estimate primary and secondary HNCO for the oil sands. Approximately 6.2 ± 1.1 kg hr was emitted from off-road diesel activities within oil sands facilities, and an additional 116-186 kg hr formed from the photochemical oxidation of diesel exhaust. Together, the primary and secondary HNCO from OS operations represent a significant anthropogenic HNCO source in Canada. The secondary HNCO downwind of the OS was enhanced by up to a factor of 20 relative to its primary emission, an enhancement factor significantly greater than previously estimated from laboratory studies. Incorporating HNCO emissions and formation into a regional model demonstrated that the HNCO levels in Fort McMurray (∼10-70 km downwind of the OS) are controlled by OS emissions; > 50% of the monthly mean HNCO arose from the OS. While the mean HNCO levels in Fort McMurray are predicted to be below the 1000 pptv level associated with potential negative health impacts, (∼25 pptv in August-September), an order of magnitude increase in concentration is predicted (250-600 pptv) when the town is directly impacted by OS plumes. The results here highlight the importance of obtaining at-source HNCO emission factors and advancing the understanding of secondary HNCO formation mechanisms, to assess and improve HNCO population exposure predictions.

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

“…In ambient measurements, HNCO correlated with markers of vehicle exhaust including benzene, toluene, and black carbon (BC). 30,72 Further, from the diurnal cycle observed in HNCO measurements in Toronto, two separate sources for HNCO were identied based on time of day. 30 First, concentrations of HNCO generally increased each morning due to morning rush hour traffic.…”

“…These values agree closely to the values obtained for diesel vehicles in Wentzell et al (2013) 30 and gasoline vehicles in Brady et al, 33 but much lower than dynamometer measurements by Suarez-Bertoa et al, 37 Link et al, 34 and Jathar et al 35 Further evidence of summer and winter emission differences are provided by Suarez-Bertoa et al who found that on average in their eet (both gasoline and diesel vehicles) that vehicles operated at À7 C had emission ratios more than four times greater than when operated at 23 C. 37 In the second mobile study, Liggio et al conducted a regional aircra study in the Athabasca oil sands in northern Alberta, Canada, where bitumen deposits are extracted and rened to crude oil resulting in heavy industrial activity. 72 In the study, HNCO was measured using a time-of-ight CIMS over 22 ights. Elevated HNCO concentrations were attributed to diesel exhaust through correlation with black carbon levels.…”

“…In addition, the aging of diesel exhaust was studied in the Athabasca oil sands. 72 Plumes downwind of oil sands sites were examined using a top down emission rate retrieval algorithm to lter out primary emissions. Indeed, photooxidation of diesel exhaust accounted for the production of 116-186 kg HNCO per h or approximately 1.5 tons HNCO per day.…”

“…51,52 Regional HNCO emissions have also been modelled with respect to areas downwind from the Athabasca oil sands in Alberta, Canada. 72 Using the Canadian regional air quality model (GEM-MACH v2.0), mean HNCO concentrations of 0.025 ppbv and 0.052 ppbv were predicted for Fort McMurray, 70 km away from oil sands industrial activity, and Edmonton, an urban city 575 km from the oil sands, respectively. 72 Emissions of HNCO were separated into two categories: emissions from primary sources and emissions from secondary production.…”

“…72 Using the Canadian regional air quality model (GEM-MACH v2.0), mean HNCO concentrations of 0.025 ppbv and 0.052 ppbv were predicted for Fort McMurray, 70 km away from oil sands industrial activity, and Edmonton, an urban city 575 km from the oil sands, respectively. 72 Emissions of HNCO were separated into two categories: emissions from primary sources and emissions from secondary production. Contributions from primary oil sands emissions were determined through source specic emission factors of 0.0028, 2.3, and 2.9 mmol HNCO per mol of CO for gasoline, on-road diesel, and off-road diesel emissions, respectively.…”