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. Roberts41Department of Mechanical Engineering, Colorado State University, Fort Collins, CO, USA, 80523 2Department of Chemistry, Colorado State University, Fort Collins, CO, USA, 8052 3Department of Civil and Environmental Engineering, University of California Davis, Davis, CA, USA, 95616 4NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, CO, USA, 80305 5Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA, 80305
Received: 20 Jan 2017 – Accepted for review: 21 Feb 2017 – Discussion started: 22 Feb 2017
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 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−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 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.
Jathar, S. H., Heppding, C., Link, M. F., Farmer, D. K., Akherati, A., Kleeman, M. J., de Gouw, J. A., Veres, P. R., and Roberts, J. M.: Investigating Diesel Engines as an Atmospheric Source of Isocyanic
Acid in Urban Areas, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-52, in review, 2017.