Environment Canada study finds vehicle exhaust a significant source of isocyanic acid; suggests consideration of new emission standard
Researchers at Environment Canada have found that vehicle exhaust is a significant source of isocyanic acid (HNCO), a toxic gaseous acid that is a product of various forms of combustion and a potential health concern. HNCO and its aqueous anion isocyanate (CNO−) have been linked—at exposure levels as low as 1 ppbv (parts-per-billion by volume)—to health issues such as atherosclerosis, cataracts, and rheumatoid arthritis (Roberts 2011).
In a paper published in the ACS journal Environmental Science & Technology, the team found that emission factors for HNCO ranged from 0.69 to 3.96 mg kgfuel–1 and were significantly higher than previous biomass burning emission estimates. Canada-wide, up to 770 tonnes of HNCO may be emitted annually from on-road vehicles, they concluded—likely representing the dominant source of exposure in urban areas.
Automobile exhaust is also a known source of inorganic acids such as nitric (HNO3) and nitrous (HONO) acids. HONO in particular has been shown to be an important source of the OH radical, and once emitted can react with ambient amines leading to the production of carcinogenic nitrosamines. Reported primary fuel based emission factors (EFs) for gaseous acids are few, yet are required as inputs into air quality models to constrain model predictions of organic aerosol, organic aerosol O:C ratios and aqueous phase chemistry.
From a health perspective, recent studies have implicated isocyanic acid as a highly toxic gaseous acid and a potential health concern due to its dissociation at physiological pH....Despite its toxicity, HNCO-specific exposure or air quality standards do not exist, although limits on occupational exposure to methyl isocyanate (the simplest organic isocyanate) have been established in some Canadian provinces and US states.
...Despite its important health implications, the magnitudes of HNCO emissions from all primary sources have not been clearly quantified. Laboratory proxies for real world automobile catalysts have shown that HNCO could be a trace intermediate in automobile catalytic converters during the warm-up phase. It has also been suggested as a direct additive in selective catalytic reduction (SCR) catalyst systems as a method of limiting NOx emissions and has been quantified as a byproduct in urea-based SCR systems. While urea-type SCR system have been commercialized, they are so far only sparingly used on heavy-duty diesel (HDD) vehicles and light-duty trucks, however stricter NOx emissions requirements will likely increase their use. While these are known or potential sources of HNCO, EFs for HNCO (and most other acids) from the transportation sector in particular, do not exist.—Wentzell et al.
In the reported study, Wentzell et al. measured several organic and inorganic acids from light duty diesel exhaust using acetate ion chemical ionization mass spectrometry (Acid-CIMS). They calculated the emission ratios (mg of compound/kg of fuel consumed) for several species, including HNCO.
For the tests, they used a turbo diesel (TDI) engine from a 2001 Volkswagen Jetta equipped with a diesel oxidation catalyst (DOC) and operating on ultra-low sulfur diesel. Measurements were performed from a constant volume sampler (CVS) which diluted the raw exhaust with HEPA filtered room air, by factors ranging from ∼16 to 77 depending on driving mode.
They used four different steady state driving modes were utilized, as determined by a constant catalytic converter temperature. These steady state modes were derived from the average speed and engine torque during transient drive cycles including (1) the US06 Supplemental Federal Test Procedure (US06) representative of aggressive driving; (2) the Highway Fuel Economy Driving Schedule (HWFET) representing normal highway driving conditions; (3) US Federal Test Procedure 75 (FTP75), representing city driving; and (4) the engine operating at idle.
They then calculated the ratio between isocyanic acid and carbon monoxide in the exhaust, and estimated that light-duty diesel vehicles alone emit roughly 6 tons of isocyanic acid per year across Canada.
The initial HNCO estimate did not include potential emissions from gasoline or heavy duty diesel (HDD) engines, both of which far outnumber the LDD vehicles in Canada (<2% of the vehicle fleet), they noted.
They then performed ambient measurements of HNCO in Toronto to determine the extent that vehicle emissions could contribute to urban ambient HNCO concentrations, and to provide an estimate of the total emitted HNCO from vehicles in the Greater Toronto Area (GTA).
Using the ratio of HNCO and benzene (for which there is a tight correlation), and combining that with the known annual on-road benzene emissions for the GTA, they calculated that approximately 23 tonnes of HNCO is emitted into the GTA from the tailpipe of on-road vehicles annually. Applying that on a national scale, they calculated the 770 tonnes of HNCO figure.
In addition to calculating the magnitude of the estimates of HNCO for Canada from vehicle sources, the researchers said that the results also suggested that there is a photochemical source of HNCO, which, when combined with vehicle sources, would mean that these sources are potentially more important than biomass burning sources.
Based on other work on HNCO, the Environment Canada team noted that there are circumstances where HNCO concentrations could be large enough to have a detrimental effect on human health.
Mean mixing ratios measured during the Toronto sampling period were 85 pptv (parts-per-trillion by volume), however spikes as high as 990 pptv were observed during the night-time hours when the boundary layer was lowest, and related to unknown primary sources. Based upon the 1 ppbv threshold calculated to mimic carbamylation in vitro by Roberts et al, there are circumstances where HNCO concentrations could be large enough to have a detrimental effect on human health.
Air quality standards for HNCO do not exist. However, the province of Ontario has established air quality standards for methyl isocyanate (CH3NCO), which like HNCO results in carbamylation reactions in the human body caused by the NCO−functional group. The 24 h average and 30 min point of impingement standards for CH3NCO are 1.0 µg m−3 and 3.0 µg m−3, corresponding to mixing ratios of 0.4 and 1.2 ppbv respectively....The night-time spikes in HNCO are within 20% of the 30 min point of impingement value for the province. These results, in conjunction with the fact that HNCO is likely more soluble at physiological pH than methyl isocyanate, suggests that a HNCO-specific standard should also be considered.—Wentzell et al.
The researchers cautioned that significantly more data is required to fully understand the varied sources, sinks, and impact of HNCO on urban populations. Toxicological studies to assess the effect of prolonged exposure to lower levels of HNCO (several hundred pptv, for days or weeks) are required. Furthermore, additional laboratory experiments are required to quantify the HNCO emissions from common gasoline-powered vehicles and to estimate the affect of urea-SCR diesel exhaust catalysts on urban HNCO emissions given their expected market penetration, they suggested.
Finally, since they made their measurements in the fall, the strength of a potential photochemical source was likely weakened by the decreased hours of sunlight, they suggested.
If a photochemical source exists it adds to the potential for seasonal variations in HNCO emissions (and higher concentrations) with the likely maxima during summertime smog episodes. These results suggest that further ambient measurements of HNCO are also required.—Wentzell et al.
Jeremy J. B. Wentzell, John Liggio, Shao-Meng Li, A. Vlasenko, Ralf Staebler, Gang Lu, Marie-Josée Poitras, Tak Chan, and Jeffrey R. Brook (2013) Measurements of Gas phase Acids in Diesel Exhaust: A Relevant Source of HNCO? Environmental Science & Technology doi: 10.1021/es401127j
James M. Roberts, Patrick R. Veres, Anthony K. Cochran, Carsten Warneke, Ian R. Burling, Robert J. Yokelson, Brian Lerner, Jessica B. Gilman, William C. Kuster, Ray Fall, and Joost de Gouw (2011) Isocyanic acid in the atmosphere and its possible link to smoke-related health effects. PNAS doi: 10.1073/pnas.1103352108