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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 17, issue 14 | Copyright
Atmos. Chem. Phys., 17, 8959-8970, 2017
https://doi.org/10.5194/acp-17-8959-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 26 Jul 2017

Research article | 26 Jul 2017

Investigating diesel engines as an atmospheric source of isocyanic acid in urban areas

Shantanu H. Jathar1, Christopher Heppding1, Michael F. Link2, Delphine K. Farmer2, Ali Akherati1, Michael J. Kleeman3, Joost A. de Gouw4,5, Patrick R. Veres4,5, and James M. Roberts4 Shantanu H. Jathar et al.
  • 1Department of Mechanical Engineering, Colorado State University, Fort Collins, CO 80523, USA
  • 2Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA
  • 3Department of Civil and Environmental Engineering, University of California Davis, Davis, CA 95616, USA
  • 4NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, CO 80305, USA
  • 5Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80305, USA

Abstract. 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 an 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 (3–90mgkgfuel−1) 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 datasets for HNCO in urban areas but underpredicted 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 ∼110pptv but were an order of magnitude lower than the 1ppbv 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.

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Our work makes novel emissions measurements of isocyanic acid, a toxic gas, from a modern-day diesel engine and finds that diesel engines emit isocyanic acid but the emissions control devices do not enhance or destroy the isocyanic acid. Air quality model calculations suggest that diesel engines are possibly important sources of isocyanic acid in urban environments although the isocyanic acid levels are ten times lower than levels linked to adverse human health effects.
Our work makes novel emissions measurements of isocyanic acid, a toxic gas, from a modern-day...
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