Investigating Diesel Engines as an Atmospheric Source of Isocyanic Acid in Urban Areas

Abstract

<p><strong>Abstract.</strong> Isocyanic acid (HNCO), an acidic gas found in tobacco smoke, urban environments and biomass burning-affected regions, has been linked to adverse health outcomes. Gasoline- and diesel-powered engines and biomass burning are known to emit HNCO and hypothesized to emit precursors such as amides that can photochemically react to produce HNCO in the atmosphere. Increasingly, diesel engines in developed countries like the United States are required to use Selective Catalytic Reduction (SCR) systems to reduce tailpipe emissions of oxides of nitrogen. SCR chemistry is known to produce HNCO as an intermediate product, and SCR systems have been implicated as a atmospheric source of HNCO. In this work, we measure HNCO emissions from an SCR system-equipped diesel engine and, in combination with earlier data, use a three-dimensional chemical transport model (CTM) to simulate the ambient concentrations and source/pathway contributions to HNCO in an urban environment. Engine tests were conducted at three different engine loads, using two different fuels and at multiple operating points. HNCO was measured using an acetate chemical ionization mass spectrometer. The diesel engine was found to emit primary HNCO (3–90 mg kg-fuel<sup>−1</sup>) but we did not find any evidence that the SCR system or other aftertreatment devices (i.e., oxidation catalyst and particle filter) produced or enhanced HNCO emissions. The CTM predictions compared well with the only available observational data sets for HNCO in urban areas but under-predicted the contribution from secondary processes. The comparison implied that diesel-powered engines were the largest source of HNCO in urban areas. The CTM also predicted that daily-averaged concentrations of HNCO reached a maximum of ~ 110 pptv but were an order of magnitude lower than the 1 ppbv level that could be associated with physiological effects in humans. Precursor contributions from other combustion sources (gasoline and biomass burning) and wintertime conditions could enhance HNCO concentrations but need to be explored in future work.</p>

Observations of Isocyanate, Amide, Nitrate, and Nitro Compounds From an Anthropogenic Biomass Burning Event Using a ToF‐CIMS

“…Emissions of nitrogen‐containing VOCs such as HCN and HNCO are known to be highly variable depending on the type and origin of the fuel. Increasing the nitrogen content of a biogenic fuel type by 1% can increase the emission of nitrogen‐containing VOCs by 2–6% (Coggon et al, ), and the inclusion of a diesel oxidation catalyst to a diesel engine can increase the HNCO NEMR by a factor of 30 (Jathar et al, ). Total isocyanate concentrations measured after burning various plastics can vary by 3 orders of magnitude depending on the precursor fuel nitrogen content, and accordingly, plastics containing no nitrogen do not produce any isocyanates (Blomqvist et al, ).…”

“…Isocyanic acid (HNCO) is another highly toxic, long‐lived gas (lifetime of days to decades; Borduas et al, ) emitted from BB with similar anthropogenic and biogenic sources as HCN. Urban sources of HNCO are attributed to primary activity such as automotive emission (Jathar et al, ), residential heating (BB) (Woodward‐Massey et al, ), and industrial processes, for example, from brick kiln emissions (Sarkar et al, ). A secondary source of HNCO is amide oxidation (e.g., Borduas et al, ), which has been observed at a suburban site in Mohali, India (Chandra & Sinha, ), and in an urban environment in Pasadena, California (Roberts et al, ).…”

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

“…Emissions of HNCO into the atmosphere have been quantied from a variety of anthropogenic and natural sources through laboratory experiments and eld measurements. The primary emission sources include biomass burning, 19,20,22,[26][27][28][29] fossil fuel combustion (coal, gasoline, and diesel), 19,[30][31][32][33][34][35][36][37] and cigarette smoke. 19,38,39 The secondary production of HNCO, through the oxidation of amines and amides is also a potential source.…”

“…74 In these systems, urea thermally decomposes into NH 3 and HNCO. 35,49,75 The reaction proceeds as follows (R7): 35 (R7) HNCO is then hydrolysed to produce NH 3 and CO 2 (R9). NH 3 acts as the reducing agent for NO and NO 2 yielding N 2 and H 2 O.…”

Isocyanates and hydrogen cyanide in fumes from heated proteins and protein‐rich foods

“…6 VOCs are ubiquitous in our environment and are emitted or generated from many different sources. 12 ICA is a strong organic acid that, together with other pollutants, has been found in fire smoke, 13,14 in diesel emissions, 15,16 and in emissions from heated products that contain urea-formaldehyde resins as binders. 7 Monoisocyanates such as isocyanic acid (ICA) and methyl isocyanate (MIC) are one type of VOCs that could be generated during heat degradation or combustion of nitrogen-containing products.…”