Investigating diesel engines as an atmospheric source of isocyanic acid in urban areas
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 th...
Main Authors: | , , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2017-07-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | https://www.atmos-chem-phys.net/17/8959/2017/acp-17-8959-2017.pdf |
Summary: | 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 (3–90 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
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 ∼ 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. |
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ISSN: | 1680-7316 1680-7324 |