Technical note: Use of an atmospheric simulation chamber to investigate the effect of different engine conditions on unregulated VOC-IVOC diesel exhaust emissions
Authors: Pereira Kelly L. , Dunmore Rachel , Whitehead James , Alfarra M. Rami , Allan James D. , Alam Mohammed S. , Harrison Roy M. , McFiggans Gordon , Hamilton Jacqueline F. 
Published: August 2018

Abstract. Diesel exhaust emissions were introduced into an atmospheric simulation
chamber and measured using thermal desorption (TD) comprehensive
two-dimensional gas chromatography coupled to a flame ionisation detector
(GC × GC-FID). An extensive set of measurements were performed to
investigate the effect of different engine conditions (i.e. load, speed,
“driving scenarios”) and emission control devices (with or without diesel
oxidative catalyst, DOC) on the composition and abundance of unregulated
exhaust gas emissions from a light-duty diesel engine, fuelled with ultra-low
sulfur diesel (ULSD). A range of exhaust dilution ratios were investigated
(range = 1 : 60 to 1 : 1158), simulating the chemical and physical
transformations of the exhaust gas from near to downwind of an emission
source. In total, 16 individual and 8 groups of compounds (aliphatics and
single-ring aromatics) were measured in the exhaust gas ranging from volatile
to intermediate volatility (VOC-IVOC), providing both detailed chemical
speciation and groupings of compounds based on their structure and
functionality. Measured VOC-IVOC emission rates displayed excellent
reproducibility from replicate experiments using similar exhaust dilution
ratios. However, at the extremes of the investigated exhaust dilution ratios
(comparison of 1 : 60 and 1 : 1158), measured VOC-IVOC emission rates
displayed some disagreement owing to poor reproducibility and highlighted the
importance of replicate sample measurements. The investigated DOC was found
to remove 43±10 % (arithmetic mean ± experimental
uncertainty) of the total speciated VOC-IVOC (∑SpVOC-IVOC) emissions.
The compound class-dependant removal efficiencies for the investigated DOC
were 39±12 % and 83±3 % for the aliphatics and
single-ring aromatics, respectively. The DOC aliphatic removal efficiency
generally decreased with increasing carbon chain length. The ∑SpVOC-IVOC emission rates varied significantly with different engine
conditions, ranging from 70 to 9268 mg kg−1 (milligrams of mass
emitted per kilogram of fuel burnt). ∑SpVOC-IVOC emission rates
generally decreased with increasing engine load and temperature, and to a
lesser degree, engine speed. The exhaust gas composition changed considerably
as a result of two influencing factors: engine combustion and DOC hydrocarbon
(HC) removal efficiency. Increased engine combustion efficiency resulted in a
greater percentage contribution of the C7 to C12 n-alkanes to the
∑SpVOC-IVOC emission rate. Conversely, increased DOC HC removal
efficiency resulted in a greater percentage contribution of the C7 to
C12 branched aliphatics to the ∑SpVOC-IVOC emission rate. At low
engine temperatures (<150 ∘C, below the working temperature of
the DOC), the contribution of n-alkanes in the exhaust gas increased with
increasing combustion efficiency and may be important in urban environments,
as n-alkanes are more efficient at producing secondary organic aerosol
(SOA) than their branched counterparts. At very high engine temperatures
(maximum applied engine speed and load, engine
temperature = 700 ∘C), the n-alkane contribution increased by
a factor of 1.6 times greater than that observed in the cold-start experiment
(most similar to unburnt fuel) and may suggest liquid-fuel-based estimates of
SOA yields may be inconsistent with exhaust SOA yields, particularly at high
engine speeds and loads (i.e. high engine temperatures). Emission rates were
found to be 65 times greater from a cold-start experiment than at maximum
applied engine speed and load. To our knowledge, this is the first study
which uses an atmospheric simulation chamber to separate the effects of the
DOC and combustion efficiency on the exhaust gas composition.