Introduction
Urban air pollution is detrimental to human health, adversely effects air
quality, and results in increased morbidity and mortality rates (Han and
Naeher, 2006; Cohen et al., 2005; Prüss-Üstün and Corvalán,
2006). The World Health Organisation attributed 1.34 million premature deaths
to urban air pollution in 2008 (WHO, 2006; Krzyzanowski and Cohen, 2008). Of
these deaths, 1.09 million could have been prevented if the air quality
guidelines had been met (WHO, 2006; Krzyzanowski and Cohen, 2008). Over half
of the world's population now live in urban areas (Prüss-Üstün
and Corvalán, 2006; United Nations, 2014). By
2050, this population is expected to grow to 6.34 billion people, with an
estimated 66 % of the world's population living in urban environments
(Prüss-Üstün and Corvalán, 2006; United Nations, 2014). Road
transport emissions are a dominant source of urban air pollution (DEFRA,
1993; Colvile et al., 2001; HEI, 2010) with common road-traffic pollutants
including gaseous hydrocarbons (including volatile organic compounds, VOCs),
nitrogen oxides (sum of NO+NO2), carbon oxides (CO and
CO2), and particulate matter (PM), with secondary reaction
processes resulting in the formation of ozone and secondary aerosol (WHO,
2006; HEI, 2010). Exposure to road-traffic air pollutants, both primary and
secondary, are of a major health concern (United Nations, 2014; WHO, 2006;
HEI, 2010). Secondary aerosol formation from diesel and gasoline-powered
motor vehicles has received considerable attention in recent years (Gentner
et al., 2017). There is currently considerable debate as to whether diesel or
gasoline powered motor vehicles are more important for secondary organic
aerosol (SOA) formation and which precursors are the most efficient at
forming SOA (Gentner et al., 2017). In Europe, almost half of all new
passenger cars are diesel (49.5 %), with petrol (45.8 %), electric
hybrids (2.1 %), electric (1.5 %), and alternative fuels (1.2 %)
accounting for the remaining fraction (ACEA, 2016). Diesel exhaust emissions
vary considerably with vehicle type, age, operation conditions, fuel,
lubricant oil, and emission control devices, among other factors (HEI, 2010).
Emission regulations of nitrogen oxides, carbon monoxide, PM, and total
hydrocarbon mass has resulted in the reduction of exhaust emissions (HEI,
2010). However, this “blanket approach” for the reduction of total
hydrocarbon mass has, in-part, resulted in few studies investigating the
detailed chemical composition of exhaust emissions with varying engine
conditions (Yamada et al., 2011). Another contributing
factor is the difficulty in exhaust
gas measurement (Yamada et al., 2011; Rashid et al., 2013). On-road
measurements of exhaust gas are difficult, due to the continually evolving
chemical composition, requiring techniques capable of providing detailed
chemical speciation in real time or near-real time. Furthermore, the vast
number of gaseous compounds in exhaust emissions often involves lengthy
quantification processes. The detailed chemical characterisation of exhaust
gas with varying engine conditions, however, can considerably aid emission
inventories and provide a greater understanding of exhaust emissions on local
air quality. In addition, this information could serve to influence the
design of emission control devices, reducing the emission rates of
potentially harmful unregulated exhaust gas components.
On-road measurements of unregulated exhaust gas emissions are often performed
in tunnels, on roadsides, or motorways (e.g. Gentner et al., 2013; Liu et
al., 2015; Ježek et al., 2015; Zavala et al., 2006; Jiang et al., 2005;
Kristensson et al., 2004; Fraser et al., 1998; Miguel et al., 1998; Staehelin
et al., 1998). These measurements provide a compositional overview of the
exhaust emissions from the on-road vehicular fleet, consisting of a vast
range of vehicle types (e.g. light-duty, heavy-duty), emission control
devices (e.g. with or without exhaust gas recirculation), and fuel
composition (e.g. ultra-low sulfur diesel, ULSD; super unleaded petrol;
premium unleaded petrol; biofuel; among others). These measurements, however,
do not allow the effect of different engine conditions or emission control
devices on the exhaust gas composition to be investigated. Dynamometer
engines or chassis dynamometers can afford compositional insight into exhaust
emissions with varying engine conditions, providing a high degree of control
and reproducibility (Tadano et al., 2014; Louis et al., 2016). Several
studies have used dynamometers to investigate compositional changes in
unregulated exhaust gas emissions with the use of different transient driving
cycles, for petrol engines (Pang et al., 2014; Baldauf et al., 2005), diesel
(Yamada et al., 2011; Cross et al., 2015; Schauer et al., 1999; Zhao et al.,
2015; Ballesteros et al., 2014; Nelson et al., 2008; Siegl et al., 1999;
Westerholm et al., 1991), or both (Alves et al., 2015; Chirico et al., 2014;
Alkurdi et al., 2013; Caplain et al., 2006; Schmitz et al., 2000; Louis et
al., 2016). Driving cycles (often performed with chassis dynamometers) are
designed to simulate real-world driving conditions, allowing the exhaust
emissions from individual vehicles to be investigated. However, these driving
cycles offer limited information on the effect of combustion or specific
engine conditions (e.g. engine load, speed) on unregulated exhaust emissions,
due to the averaging of emissions over entire driving cycles and the lack of
steady-state engine conditions (Cross et al., 2015), compositional
information which is easily obtained with the use of a single-engine
dynamometer rig (Chin et al., 2012; Cross et al., 2015; Zhu et al., 2013;
Schulz et al., 1999; Machado Corrêa and Arbilla, 2008). Recent studies
have focused on the measurement of intermediate-VOCs (IVOCs) in diesel
exhaust emissions, primarily due to advances in instrumentation to allow the
detection of these species. IVOCs have an effective saturation concentration
(C∗) of 103 to 106 µg m-3 and reside
almost exclusively in the gas-phase at atmospheric conditions (Donahue et
al., 2006). IVOCs comprise a considerable fraction of diesel exhaust
emissions, with studies attributing ∼20 to 60 % of the non-methane
organic gases to IVOCs (Schauer et al., 1999; Siegl et al., 1999; Zhao et
al., 2015; Gordon et al., 2014). IVOC diesel exhaust emissions are relatively
poorly characterised, yet contribute significantly to SOA formation (Gentner
et al., 2017). Recently, Cross et al. (2015) investigated IVOC diesel exhaust
emissions using a dynamometer rig. It was found that IVOC diesel exhaust
emissions were highly dependent on engine power. At low engine loads, the
exhaust gas composition was dominated by saturated hydrocarbons from unburnt
fuel. At high engine loads, however, the exhaust gas composition changed,
including newly formed unsaturated hydrocarbons and oxidised compounds from
incomplete combustion (Cross et al., 2015). Furthermore, Chin et al. (2012)
found the composition of VOC and IVOC diesel exhaust emissions (generated
under steady-state engine conditions) depended on engine load, fuel type
(ULSD and biodiesel), and emission control devices.
List of experiments performed, including the experimental dates,
descriptions, chamber conditions, and engine operating parameters.
Experiment
Experiment date
Experiment description
Engine conditions
Exhaust emission⋆
rpm
Throttle
Load
Torque
DOC
Engine
Fuel
Exhaust
Chamber
Chamber
NO
NO2
Particle
(%)
(%)
(Nm)
temp.
burnt
dilution
temp.
RH
(g kg-1)
(g kg-1)
mass
(∘C)
(g)
ratio
(∘C)
(%)
(mg kg-1)⋆⋆
1
30 Jul 2014
Warm high loada
2500
57
40
75
Yes
460
6.13
166
21.0
54.8
27.6
3.2
302
2
31 Jul 2014
Warm high loada
2500
57
40
75
Yes
450
2.45
313
21.8
52.8
23.4
1.9
235
3
1 Aug 2014
Warm high load
2500
57
40
75
No
450
2.45
325
31.6
61.6
21.7
2.5
198
4
5 Aug 2014
Warm with loadb
2000
40
30
50
Yes
**
0.41
1158
32.9
52.0
26.1
0.5
220
5
8 Aug 2014
Warm with loadb
2000
40
30
50
Yes
300
8.27
60
32.6
58.7
20.7
3.1
268
6
6 Aug 2014
Cold Startc
1150
0
0
1.5
Yes
85
0.59
389
32.7
58.8
7.2
15.0
1159
7
7 Aug 2014
Cold Startc
1150
0
0
2
Yes
83
0.59
564∗∗∗
27.4
52.8
7.9
14.6
915
8
6 Aug 2014 (2)
Cold loaded∗
1500
30
20
32
Yes
169
1.19
312
27.5
51.8
17.3
9.8
121
9
6 Aug 2014 (3)
Warm idle following load∗
1180
0
0
0.3
Yes
150
1.18
775
25.9
45.7
13.1
0.9
168
10
13 Nov 2014 (1)
Warm with loadb
2000
40
30
50
Yes
300
0.41
840
22.4
53.2
31.5
2.0
351
11
13 Nov 2014 (2)
High rpm, 53 % load
3000
75
53
112
Yes
700
3.28
353
23.3
53.4
21.7
1.9
2146
12
14 Nov 2014 (1)
High rpm, 30 % load
3000
48
30
50
Yes
345
1.74
198
22.8
53.0
39.2
5.3
600
13
14 Nov 2014 (2)
High rpm, 40 % load
3000
57
40
70
Yes
445
2.39
191
23.6
48.2
44.6
8.6
655
14
25 Nov 2014
Cold Startc
1150
0
0
2
Yes
**
0.59
564∗∗∗
22.2
39.7
6.3
10.6
1159
15
1 Oct 2015
Warm with loadb
2000
40
28
50
Yes
292
1.57
331
31.5
56.4
12.8
0.5
298
16
29 Sep 2015
Warm with loadb
2000
40
28
50
Yes
293
1.57
337
28.0
67.1
12.2
0.5
241
Superscript letters a, b, and c highlight replicate experiments using the same
engine conditions. Fuel batch A used in experiments 1 to 9 and fuel batch B
used in experiments 10 to 16, see Sects. 2.3 and 3.1.1 for further
information. ∗ is the sequence of engine conditions performed, see
Sect. 2.1 and Fig. S1. ∗∗ No engine temperature measurement (engine
thermocouple non-responsive). ∗∗∗ Estimated exhaust dilution
ratio based on pneumatic valve introduction time. ⋆ Expressed as
emission rates (i.e. mass of emission per kg of fuel burnt). ⋆⋆ Wall-loss
corrected.
This study investigates the compositional changes of unregulated exhaust
emissions with varying engine conditions (i.e. engine load, speed, and
“driving scenarios”) and emission control devices (with or without a diesel
oxidative catalyst, DOC) using
a dynamometer rig. In contrast to previous studies, this work combines both
detailed chemical speciation and groupings of VOC-IVOCs based on their
structure and functionality, providing a more detailed compositional overview
of the effect of different engine conditions on exhaust gas emissions. This
work also investigates the effect of different exhaust dilution ratios on the
exhaust gas composition, simulating the chemical and physical transformations
of the exhaust gas emissions at varying ambient dilutions (i.e. near to
downwind of an emission source). The emissions from a light-duty 1.9 L
Volkswagen diesel engine were investigated. Exhaust emissions from different
engine conditions were introduced into an atmospheric chamber which was used
as a “holding-cell” for sampling, allowing lower time resolution techniques
to be used. In total, 16 individual and 8 groups of compounds were measured
in the exhaust gas using thermal desorption comprehensive two-dimensional gas
chromatography coupled to a flame ionisation detector
(TD-GC × GC-FID). The effect of different engine conditions and
emission control devices on the composition and abundance of the speciated
VOC-IVOCs is discussed, along with the reproducibility of the engine exhaust
emissions/chamber sampling system and the potential impacts of our findings.
Experiment design
Experiments
A series of experiments were performed in July to August 2014, November 2014,
and September to October 2015, as a part of the project COMbustion PARTicles
in the atmosphere (Com-Part). Experiments were designed to systematically
characterise the chemical and physical transformations of primary and
secondary particles emitted from a light-duty diesel engine under a range of
atmospheric dilution and oxidation conditions. The results shown here focus
on the effect of different engine conditions on the composition and abundance
of VOC-IVOCs in the raw exhaust emissions, which formed a subset of the total
number of experiments performed. The experimental descriptions and engine
operating parameters discussed here can be found in Table 1. A range of
engine conditions were studied, including (i) engine speed, ranging from
1150 rpm (idle) to 3000 rpm (maximum engine output); (ii) engine load,
ranging from 0 % (no load) to 53 % (the maximum load which could be
safely applied to by the dynamometer in the experimental setup);
(iii) emission control devices (with and without the DOC) and; (iv) “driving
scenarios” (see below for further details). Exhaust dilution ratios were
varied to represent a range of ambient conditions from near to downwind of an
emission source, capturing the chemical and physical transformations of
semi-volatiles in the exhaust emissions with varying ambient dilutions.
The majority of experiments focused on steady-state engine conditions, where
selected engine running parameters were applied to the engine and the engine
temperature allowed to stabilise, prior to the introduction of the exhaust
emissions into the Manchester Aerosol Chamber (MAC). The MAC was filled with
clean air prior to the introduction of the exhaust emissions. Steady-state
engine conditions are defined here as a constant engine temperature within
±10 % of the steady-state average. Steady-state engine conditions
were not performed for the cold-start (exps. 6, 7, and 14; Table 1) and
“driving-scenario” (exps. 8 and 9; see Table 1) experiments, with the
cold-start experiments requiring exhaust injection into the MAC after ∼1 to 2 min of engine start-up (i.e. cold-engine). The driving scenario
experiments involved a sequence of engine conditions, with the injection of
the exhaust emissions into the MAC after the completion of the sequence, but
prior to achieving steady-state engine conditions. The sequence of engine
conditions used in the driving scenario experiments are discussed in
Sect. 3.3 and shown in Fig. S1 in the Supplement. All experiments except
experiment 3 (see Table 1) were performed with the DOC. This allowed the
combined effect of the DOC and different engine conditions on the exhaust
emissions to be observed, i.e. engine conditions most representative of
on-road diesel vehicles.
Schematic of dynamometer and sampling system. Arrows display air
flow direction. CPC is the condensation particle counter. DMPS is the
differential mobility particle sizer. DOC is the diesel oxidative catalyst.
TD is the thermal desorption unit. GC × GC-FID is the comprehensive
two-dimensional gas chromatography flame ionisation detector. See Sect. 2.3
for further information.
Chamber setup
Experiments were performed in the MAC located within the University of
Manchester, UK. The MAC consists of an 18 m3 fluorinated ethylene
propylene Teflon bag with the following dimensions; 3 m
(L) × 3 m (W) × 2 m (H). The chamber is supported
by three rectangular aluminium frames, two of which are free moving, allowing
the chamber to expand and collapse as sample air flow is introduced or
extracted. Purified air is used within the chamber and is humidified prior to
introduction. A suite of instruments was used to measure chamber temperature
(series of cross-calibrated thermocouples), relative humidity (Dewmaster
chilled mirror hygrometer, Edgetech Instruments, USA), CO2 (model
6262, Li-Cor Biosciences, USA), NOx (model 42i, Thermo
Scientific, MA, USA), O3, (model 49C, Thermo Scientific, MA, USA),
and VOC-IVOCs (GC × GC-FID, see below for further details). Particle
number, mass, and diameter were measured using a differential mobility
particle sizer (DMPS; Williams et al., 2007) consisting of a differential
mobility analyser (DMA; Winklmayr et al., 1991) and a condensation particle
counter (CPC; model 3010, TSI Inc., USA). Further technical information
regarding the chamber design can be found in Alfarra et al. (2012).
Engine and exhaust sampling system
The emissions from a light-duty Volkswagen (VW) 1.9 L diesel engine were
investigated. A schematic of the dynamometer and exhaust sampling system is
shown in Fig. 1. The engine had 4 cylinders with a capacity of 1896 cm3
and a compression ratio of 19.5 : 1. The engine was mounted on an eddy
current dynamometer rig (CM12, Armfield Ltd., Hampshire, UK) and the exhaust
connected to a new (0 mileage hours) retrofitted DOC. The DOC was purchased
from a local garage (Oldham Tyre and Exhaust, Oldham, UK) and consisted of a
mix of platinum and rhodium. No diesel particulate filter was used,
conforming to Euro 4 emission control regulations. The auto-equivalent
version of this engine has been used in several VW Polo and Jetta models in
the early 2000's and was chosen as an example of a light-duty diesel engine.
The after-treatment was selected to meet Euro 4 emission control regulations
required for such models. Engine running parameters (i.e. speed, load, and
throttle) were controlled using a dedicated software package (Armfield Ltd.,
Hampshire, UK) on a separate PC. Engine load is the amount of breaking force
(mechanical resistance) applied to the engine during operation, simulating
weight (i.e. load) and/or resistance. Engine throttle controls the amount of
fuel injected into the combustion chamber required for speed (rate of
movement, measured in rpm) and load. Engine torque (force generated in the
crankshaft of the engine) was not controlled but is included in Table 1 for
reference. The engine temperature was measured via an in-built thermocouple
located inside the engine exhaust pipe, next to the engine. The DOC
temperature has been inferred from the measured exhaust temperature. The DOC
temperature will be lower than the measured exhaust temperature due to the
DOC being located further down the exhaust pipe. A 2 m long, 2-inch bore,
stainless steel tube with a computer-controlled pneumatic valve was used to
allow the engine emissions to be introduced into the MAC or diverted to
waste. The timed control of the pneumatic valve allowed
for a proportion of the exhaust
emissions to be introduced into the chamber, controlling dilution. The final
exhaust dilution ratios were calculated from the measured CO2
concentration prior to and after the introduction of the exhaust emissions.
The exhaust dilution calculations will be discussed in further detail in a
separate publication. The engine was fuelled with standard European (EN590
specifications, Euro 5 compliant) ULSD obtained from a local fuelling
station. Two batches of fuel were obtained, the first in June 2014 (batch A)
and the second in November 2014 (batch B). A second batch of fuel was
required due to a considerable increase in the number of planned experiments.
The fuel batches were of the same specification and obtained from the same
local fuelling station. Batch A was used in experiments 1 to 9 and batch B in
experiments 10 to 16 (see Table 1 and Sect. 3.1.1). The sulfur content was <10 ppm. Further information regarding the standard European ULSD fuel
specifications can be found in the EU directive 2009/30/EC (EU, 2009).
TD-GC × GC-FID
VOC-IVOC exhaust emissions were measured using thermal desorption
comprehensive two-dimensional gas chromatography with a flame ionisation
detector (TD-GC × GC-FID) operating at 200 Hz. A TT24-7 thermal
desorption unit (Markes International, Llantrisant, UK) with an air server
attachment was used for sample collection. The inlet of the TD unit was
connected to MAC using ∼2.5 m of heated 1/4′′ stainless steel
tubing. The stainless steel tubing was heated to ∼70 ∘C to
reduce condensational losses of VOCs. An in-line unheated particulate filter
prevented sampled particles from entering the TD unit. The in-line filter was
replaced prior to each experiment to minimise particulate loadings. A clean
air diaphragm pump (model PM25602-86, KNF Neuberger, Oxfordshire, UK) was
used to extract an overflow of sample air from the MAC, a proportion of which
was sampled into the TD unit. Two sequential glass traps cooled to
-20 ∘C in an ethylene glycol bath were used to remove water vapour
from the sampled air. No significant VOC losses have been found using this
method of water vapour removal (Lidster, 2012). Air samples were trapped onto
Tenax sorbent tubes (Markes International, Llantrisant, UK) held at
-10 ∘C during sampling (26 min sampling duration) and heated to
230 ∘C upon desorption.
An Agilent 7890 GC (Agilent Technologies, Wilmington, USA) with a modified
modulation valve, consisting of a 6-port, 2-way diaphragm valve (Valco
Instruments, Texas, USA) and 50 µL sample loop (Thames Resteck, UK)
was used (see Lidster et al., 2011, for further information). Cryogenic
cooling (liquid CO2, BOC, UK) was used to refocus the sample on the
head of the primary column upon desorption. Compound separation was achieved
using a primary 25 m 5 % phenyl polysilphenylene-siloxane column (BPX5,
SGE, Ringwood, Australia) with a 0.15 mm internal diameter and
0.4 µm film thickness, and a secondary 7 m polyethylene glycol
column (BP20 SGE, Ringwood, Australia) with a 0.25 mm internal diameter and
0.25 µm film thickness. Helium (CP grade, BOC, UK) was used as the
carrier gas. Primary and secondary column pressures were controlled using an
electronic pneumatic control (Agilent 7890 EPC) and were set at 50 and
23 psi, respectively. The modulator was heated to 120 ∘C with a
5 s cycle time, comprising of 0.3 s injection and a 4.7 s sample
introduction. The oven temperature programme consisted of a two-stage ramp;
holding at 70 ∘C for 1 min, increasing to 160 ∘C at
16 min (6 ∘C min-1), then 200 ∘C at 20 min
(10 ∘C min-1) with an additional 2 min hold, giving a total
runtime of 22 min. The FID heater was set to 300 ∘C with a hydrogen
flow of 30 mL min-1 (CP grade, BOC, UK) and an air flow of
300 mL min-1 (BTCA 178 grade, BOC UK).
A National Physical Laboratory (NPL30, Teddington, UK) gas standard was used
to monitor instrument variability over the course of the experiments.
VOC-IVOC concentrations were determined using either the NPL gas standard or
the relative response factors (RRF) of liquid standards (see Supplement for
further information). Calibrations were performed weekly using the NPL gas
standard, or more frequently during instrument maintenance periods. The
instrument detection limits for the investigated compounds can be found in
the Supplement of Dunmore et al. (2015). Compounds with carbon numbers
greater than C15 were not measured due to instrument temperature
constraints; the boiling points of these compounds are too high to be removed
from the column at the maximum operating temperature of the modulator. The
exhaust injection times into the MAC, GC × GC-FID exhaust sampling
start and end times, and the number of replicate measurements performed in
each experiment, is shown in Table S2 in the Supplement. Only samples where
no changes had been made to the chamber conditions were analysed. The exhaust
emissions were blank subtracted using the chamber background measurement/s
prior to the introduction of the exhaust emissions. The average sampling
start time was 13 min after the injection of the exhaust emissions into the
MAC. No apparent losses of the VOC-IVOCs were observed during sampling; the
average relative standard deviation from replicate measurements of the
investigated VOC-IVOCs over the longest sampling duration (∼2 h) was
6.4 % (exp. 6, see Table 1 and Table S2 in the Supplement).
Liquid fuel analysis
The two batches of ULSD fuel (see Sects. 2.3 and 3.1.1) were analysed using
comprehensive two-dimensional gas chromatography (model 6890N, Agilent
Technologies, UK) coupled to a time-of-flight mass spectrometer (Pegasus 4D,
Leco, MI, USA) (GC × GC-TOFMS). Compound separation was achieved
using a primary 15 m 5 % phenyl polysilphenylene-siloxane column (BPX5,
SGE, Ringwood, Australia) with a 0.25 mm film thickness and 0.25 mm
internal diameter, and a secondary 2 m 50 % phenyl
polysilphenylene-siloxane column (BPX50, SGE, Ringwood, Australia) with a
0.25 mm film thickness and 0.25 mm internal diameter. Neat ULSD fuel was
introduced into the GC × GC-TOFMS using a Gerstel multipurpose
sampler (MPS 2, Gerstel, USA) with dedicated controller (model C506, Gerstel,
USA). A 1 µL injection volume was used with a split ratio of
100 : 1. The transfer line was set to 270 ∘C. Cryo-jet modulation
cooling was used to achieve comprehensive two-dimensional separation. Helium
(CP grade, BOC, UK) was used as the carrier gas with a constant flow rate of
1.5 mL min-1. The oven starting temperature was set to 65 ∘C
with a 0.2 min hold, followed by a temperature ramp of
4 ∘C min-1 to 240 ∘C, with a further 10 min hold.
The modulator and secondary oven temperature was set to 15 and 20 ∘C
above the oven temperature, respectively. The TOFMS acquisition rate was set
to 50 spectra per second, with a scan range of mass-to-charge (m/z) 35 to
500. The data were analysed using Leco ChromaTOF software version 4.51.6
(Leco, MI, USA). Compounds were identified using the National Institute of
Standard and Technology (NIST) standard reference database (version 11).
An annotated chromatogram displaying the speciated VOC-IVOCs.
Chromatogram axis, x is the first dimension separation (boiling point,
increasing from left-to-right), y is the second dimension separation
(polarity, increasing from bottom-to-top). Colour scale represents peak
intensity, increasing from blue to red. Letters refer to compound groupings;
A is the single-ring aromatics with two carbon substitutions, B is the
single-ring aromatics with three carbon substitutions, C to I is the C7
to C13 aliphatics grouped by carbon number (i.e. C is the C7
aliphatics, D is the C8 aliphatics etc.). Numbers refer to individual
compounds; 1 is benzene, 2 is toluene, 3 is ethyl benzene, 4 is
meta/para-xylene (co-elution), 5 is ortho-xylene, 6 is styrene, 7 is
1,3,5-trimethyl benzene, 8 is 1,2,4-trimethyl benzene, 9 is 1,2,3-trimethyl
benzene, 10 is heptane, 11 is octane, 12 is nonane, 13 is decane, 14 is
undecane, 15 is dodecane, 16 is tridecane, and 17 is tetradecane (not
quantified). Aromatic and aliphatic banding often observed with this
technique are shown (cf. Hamilton and Lewis, 2003, and Dunmore et al., 2015).
The start and end of each aliphatic grouping is marked by the lower and
higher carbon number n-alkane (i.e. nonane marks the start of the C9
aliphatic grouping, decane marks the end of this group).
Results and discussion
The effect of different engine loads, speeds, “driving
scenarios”, and emission control
devices (i.e. with or without DOC) on the VOC-IVOC emission rates from a
light-duty diesel engine fuelled with USLD is discussed. In total, 16
individual and 8 groups of VOC-IVOCs were speciated in the exhaust gas using
TD-GC × GC-FID. The individual compounds included 9 single-ring
aromatics: benzene, toluene, ethyl benzene, meta- and para-xylene (grouped),
ortho-xylene, styrene, 1,3,5-TMB, 1,2,4-TMB, and 1,2,3-TMB, and 7 n-alkanes
from n-heptane to n-tridecane. Grouped compounds consisted of C7 to
C13 branched aliphatics grouped by carbon number and single-ring
aromatics with three carbon substitutions (i.e. those in addition to the
trimethylbenzene isomers above). The emission rates of n-tridecane and the
C13 branched aliphatic grouping were not measured in some experiments
due to a shift in the instrument retention time, resulting in these species
not being observed. The saturation concentration (C∗,
µg m-3, Donahue et al., 2006) of the speciated compounds
ranged between 105 and 108 µg m-3, classifying
these species as intermediate to volatile organic compounds (VOC-IVOCs). The
most abundant volatility fraction of IVOCs from diesel exhaust emissions were
measured, see Zhao et al. (2015) for further information. An annotated
chromatogram displaying the speciated compounds is shown in Fig. 2. The use
of two different stationary phases in GC × GC allows compounds to be
separated by two physical properties, such as boiling point and polarity, as
shown here (see Fig. 2). This two-dimensional separation creates a
characteristic space where compounds are grouped by similar physical
properties (e.g. aromatic and aliphatic banding) (see Fig. 2, cf. Hamilton
and Lewis, 2003; Dunmore et al., 2015), aiding in the identification of
unknowns. This characteristic space, in combination with the use of
commercially available standards and the elution patterns observed in
previous work using this instrument (Dunmore et al., 2015) allowed 8 compound
groupings to be identified. The identification of all the individual
compounds (except styrene, see Supplement) were confirmed using commercially
available standards. The emission rates of the individual and grouped
compounds and their percentage contribution to the total speciated VOC-IVOC
emission rate (hereafter referred to as ∑SpVOC-IVOC) are shown in
Tables S3 to S6. No corrections have been made for gas-phase absorption to PM
in this work. Gas-phase absorption to PM is negligible due to the relatively
high vapour pressures of the compounds speciated, low VOC-IVOC mixing ratios,
and the small amount of aerosol mass present after exhaust dilution.
Comparison of measured VOC-IVOC emission rates in replicate
cold-start experiments (exps. 6 and 7) (a). Comparison of the
percentage contribution of the individual and grouped compounds to the ∑SpVOC-IVOC emission rates in exps. 6 and 7 (b). The emission rates
of tridecane and the C13 branched aliphatic grouping has not been
included in (b) to allow direct comparison between other experiments
where these species were not measured. Error bars represent the calculated
uncertainty in the measured emission rates, see Sect. S1.1 in the Supplement
for further information.
Experimental reproducibility
The reproducibility of the measured VOC-IVOC emission rates with different
engine conditions and exhaust dilution ratios were investigated and are
discussed below. The emission rates from two replicate cold-start experiments
(1150 rpm, 0 % load, exps. 6 and 7, see Table 1) and two replicate warm
with load (WWL) experiments (2000 rpm, 30 % load, exps. 15 and 16, see
Table 1) are shown in Figs. 3 and 4, respectively. Both replicate experiments
were performed with similar exhaust dilution ratios. The emission rates from
the replicate cold-start and WWL experiments displayed excellent
reproducibility, considering the vast number of variables in these
experiments (e.g. combustion and DOC hydrocarbon, HC, removal
efficiency). All emission rates,
except styrene in one experiment (< limit of detection), were observed to
be within the calculated uncertainty (see Supplement). The emission rates
from two replicate warm high-load experiments (2500 rpm, 40 % load,
exps. 1 and 2, see Table 1) with different exhaust dilution ratios is shown
in Fig. 5. The exhaust dilution ratios in these experiments were 166 and 313
in experiments 1 and 2 (see Table 1), respectively. The emission rates in
these experiments are relatively comparable. Only one measurement of the
exhaust emissions was performed in each experiment. The majority of
experiments had a minimum of two replicate measurements of the exhaust
emissions (see Table S2), possibly accounting for slight differences observed
in the measured VOC-IVOC emission rates.
Comparison of measured VOC-IVOC emission rates in replicate WWL
experiments (exps. 15 and 16, 2000 rpm, 30 % load) (a).
Comparison of the percentage contribution of the individual and grouped
compounds to the ∑SpVOC-IVOC emission rates in exps. 15 and
16 (b). The emission rates of tridecane and the C13 branched
aliphatic grouping has not been included in (b) to allow direct
comparison between other experiments where these species were not measured.
Error bars represent the calculated uncertainty in the measured emission
rates, see Sect. S1.1 for further information.
Comparison of measured VOC-IVOC emission rates in replicate warm
high-load experiments 1 and 2 (exps. 1 and 2, 2500 rpm, 40 %
load) (a). Comparison of the percentage contribution of the
individual and grouped compounds to the ∑SpVOC-IVOC emission rates in
exps. 1 and 2 (b). The emission rates of tridecane and the C13
branched aliphatic grouping has not been included in (b) to allow
direct comparison between other experiments where these species were not
measured. Error bars represent the calculated uncertainty in the measured
emission rates, see Sect. S1.1 for further information.
Comparison of measured VOC-IVOC emission rates in replicate warm
with load experiments 4 and 5 (2000 rpm, 30 % load) (a).
Comparison of the percentage contribution of the individual and grouped
compounds to the ∑SpVOC-IVOC emission rates in exps. 4 and
5 (b). The emission rates of tridecane and the C13 branched
aliphatic grouping has not been included in (b) to allow direct
comparison between other experiments where these species were not measured.
Error bars represent the calculated uncertainty in the measured emission
rates, see Sect. 1.1 for further information.
The emission rates from two replicate WWL experiments (2000 rpm, 30 %
load, exps. 4 and 5, see Table 1) at the extremes of the investigated exhaust
dilution ratios are shown in Fig. 6. The exhaust dilution ratios were 1158
and 60 in experiments 4 and 5, respectively. The emission rates in these
experiments displayed some disagreement. The engine thermocouple was
unresponsive during one of these experiments (exp. 4, see Table 1).
Consequently, it is not known if steady-state engine conditions were achieved
prior to the introduction of the exhaust emissions into the MAC and whether
the engine temperature upon injection was comparable to the replicate
experiment, possibly accounting for the observed differences in the VOC-IVOC
emission rates. Nevertheless, no experiments with such large differences in
the exhaust dilution ratios have been directly compared in the following work
and where engine conditions are compared, experiments with similar exhaust
dilution ratios and engine temperatures have been used. These experiments
highlight the importance of replicate measurements and the comparison of
VOC-IVOC emission rates from experiments with similar engine temperatures. A
propagation of errors was calculated to determine the experimental
uncertainty in the measured emission rates and is discussed in detail in the
Supplement. Briefly, the experimental uncertainty includes, (i) the standard
deviation in the replicate measurements of the calibration standard and the
reported uncertainty in the standard mixing ratios (where applicable), (ii) a
5 % standard deviation in the chamber volume, and (iii) an additional
20 % error in the emission rates of compounds integrated using the GC Image
software package, see the Supplement for further information. The
experimental uncertainty in the measured emission rates for the investigated
VOC-IVOCs ranged from 6 to 50 % with an average of 22 %.
Comparison of measured VOC-IVOC emissions rates in replicate
cold-start experiments 6 and 7 (fuel batch A) with cold-start experiment 14
(fuel batch B) (a). Comparison of the percentage contribution of the
individual and grouped compounds to the ∑SpVOC-IVOC emission rates in
experiments 6, 7, and 14 (b). The emission rates of tridecane and the
C13 branched aliphatic grouping has not been included in (b) to
allow direct comparison between other experiments where these species were
not measured. Error bars represent the calculated uncertainty in the measured
emission rates, see Sect. S1.1 for further information.
Extracted ion chromatogram of m/z 57 (dominate aliphatic fragment
ion) for the liquid diesel fuel samples analysed using
GC × GC-TOFMS. (a) Fuel batch A (see Sect. 3.1.1 for
further information). (b) Fuel batch B. Chromatogram axes:
x is the primary column, first dimension separation (boiling point,
increasing from left-to-right) and y is the secondary column, second dimension
separation (polarity, increasing from bottom-to-top). Colour scale represents
peak intensity, increasing from blue to red. Chromatograms have been
normalised to allow direct comparison of peak intensity between
chromatograms. Dashed box highlights an approximate carbon number range of
C7 to C12, determined from the NIST library identification of
individual compounds.
Calculated diesel oxidative catalyst (DOC) hydrocarbon removal
efficiency for the speciated VOC-IVOCs. Determined from measured emission
rates of the speciated VOC-IVOCs in two replicate experiments with (exp. 2,
see Table 1) and without (exp. 3) a DOC.
Emission without catalytic
Emission with catalytic
Removal efficiency
converter (mg kg-1)
converter (mg kg-1)
(%)
Individual compounds
Benzene
19.50±1.75
1.88±0.17
90.4±9.0
Toluene
3.89±0.37
1.58±0.15
59.3±9.4
Ethyl benzene
1.56±0.36
0.05±0.01
97.1±22.8
m/p-xylene
2.98±0.62
0.13±0.03
95.7±20.9
o-xylene
2.17±0.55
0.22±0.06
89.7±25.3
Styrene
2.74±0.69
0.01±0.004
99.5±25.3
1,3,5-TMB
2.26±0.36
0
100a
1,2,4-TMB
2.45±0.21
0
100a
1,2,3-TMB
1.92±0.20
0
100a
Heptane
2.37±0.14
0.53±0.03
77.4±5.7
Octane
4.96±0.57
0
100a
Nonane
11.72±1.08
2.44±0.22
79.2±9.2
Decane
33.83±3.11
7.30±0.67
78.4±9.2
Undecane
49.76±4.57
32.34±2.97
35.0±9.2
Dodecane
137.65±12.64
156.60±14.38
0b
Groupings
Branched aliphatics
C7
4.41±1.00
1.25±0.28
71.6±22.6
C8
18.77±4.76
3.68±0.93
80.4±25.4
C9
46.78±10.70
6.65±1.52
85.8±22.9
C10
76.81±18.78
17.33±4.24
77.4±24.4
C11
71.95±16.43
31.97±7.30
55.6±22.8
C12
86.36±20.80
74.41±17.92
13.8±24.1
Aromatic substitutions
C3
14.18±3.13
5.20±1.15
63.3±22.1
Total groupings
Aliphatics
545.37±37.22
334.50±24.74
38.66±11.7
Aromatics
53.65±3.81
9.07±1.17
83.09±2.6
Total speciated
599.02±37.41
343.58±24.77
42.64±9.7
a Compound not observed (< instrument limit of
detection). b No
observed decrease in concentration. TMB is trimethyl benzene. Errors
represent the calculated experimental uncertainty, see the Supplement for
further information.
ULSD fuel: batch A and B
Two batches of ULSD fuel were used in the experiments (see Sect. 2.3). The
emission rates from three replicate cold-start experiments, two using fuel
batch A (exps. 6 and 7, see Table 1) and one using fuel batch B (exp. 14),
are shown in Fig. 7. From Fig. 7, it can be observed that there is a
considerable difference in the emission rates of the C7 to C12
branched aliphatics between replicate experiments 6 and 7, and experiment 14.
The emission rates of the C7 to C12 branched aliphatics decreased
by a factor of ∼4 with the use of fuel batch B (exp. 14). The excellent
agreement of the emission rates between replicate cold-start experiments 6
and 7 suggests the compositional differences observed in experiment 14 is the
result of a slight difference in the fuel composition between batches A and
B. GC × GC-TOFMS was used to further investigate any compositional
differences between the fuel batches. An extensive analysis of the diesel
fuel was not performed. The aim of this analysis was to investigate whether
there were any apparent differences in the fuel composition that would
prevent a direct comparison of the emission rates from fuel batches A and B.
Extracted ion chromatograms for m/z 57 (dominant aliphatic fragment ion)
from fuel batches A and B are shown in Fig. 8a and b, respectively. The
chromatograms were normalised to the total peak area to allow direct
comparison of peak intensity between the chromatograms. The highlighted
regions in Fig. 8 display straight- and branched-chain aliphatics with a
carbon number range of approximately C7 to C12 (determined from the
NIST library). The peak intensities in the chromatograms from fuel batches A
and B are largely comparable, except for the highlighted regions, where a
slightly lower peak intensity is observed in fuel batch B. As a result, the
emission rates from fuel batches A and B have not been directly compared in
the following work. The reason for the observed compositional differences
between fuel batches A and B is unclear, although it suggests a possible
change in the refining process between the purchase of both fuel batches.
DOC removal efficiency
The HC removal efficiency of the DOC was investigated by performing two
replicate experiments (exps. 2 and 3, Table 1) with and without the DOC. The
additional back pressure created due to the in-line DOC appeared to have no
effect on engine operation, allowing a direct comparison between both
experiments. The engine speed and load were set to 2500 rpm and 40 %
load, respectively. The DOC HC removal efficiency is strongly dependant on
working temperature. Below 200 ∘C the DOC HC removal efficiency is
close to 0 %, rising sharply to near 100 % HC removal efficiency at
∼430 ∘C (Korin et al., 1999; Roberts et al., 2014; Majewski
and Khair, 2006; Russell and Epling, 2011). The steady-state engine
temperature in both experiments was 450 ∘C. Thus, the DOC was near
maximum HC removal efficiency. The HC removal efficiency was calculated using
the equation shown in Roberts et al. (2014). The removal efficiency of the
investigated DOC for the speciated compounds is shown in Table 2. The DOC
removed 43±10 % (arithmetic mean ± experimental uncertainty;
see Sect. 3.1 and the Supplement for further information) of the ∑SpVOC-IVOC emissions. The compound class-dependant removal efficiencies for
the investigated DOC were 39±12 % and 83±3 % for the
aliphatics (branched and straight-chain) and single-ring aromatics,
respectively. A typical DOC is expected to remove 50 to 70 % of the total
HC emissions (Johnson, 2001; Alam et al., 2016). For the investigated
compounds, the total DOC HC removal efficiency is at the lower limit of this
expectation. The DOC removal efficiency for styrene; meta-,
para-, and ortho-xylene; ethylbenzene (C2 aromatic
substitution grouping); and benzene
was greater than 90 %. In addition, the trimethylbenzenes (TMB) were not
observed with the use of the DOC (∼100 % removal efficiency). This
high HC removal efficiency, however, was not observed for all the single-ring
aromatics. Toluene had a relatively poor removal efficiency in comparison, at
59±9 %. Furthermore, the removal efficiency of the unspeciated
C3 aromatic substitution grouping (i.e. less branched aromatic isomers
of TMB) was determined to be 63±22 %, suggesting the isomeric
structure influences removal efficiency, possibly the result of reactivity
and/or adsorption to the metal binding sites in the DOC (cf. Salge et al.,
2005; Russell and Epling, 2011).
Generally, the HC removal efficiency decreased with increasing carbon chain
length. This was particularly evident with the branched aliphatics, with the
removal efficiency decreasing from 72 % to 14 % from C7 to
C12, with a sharp decrease in the removal efficiency from C10 to
C12. Analogous to the branched aliphatics, the n-alkanes displayed the
same rapid decrease in the HC removal efficiency between n-decane and
n-dodecane, with the DOC observed to have no effect on the emission of
n-dodecane. The removal of n-alkanes in the DOC have been found to
decrease with increasing carbon chain length, a result of the greater number
of adjacent sites in the DOC required to achieve absorption (Yao, 1980;
Russell and Epling, 2011), supporting the results shown here. However,
recently Alam et al. (2016) investigated the HC removal efficiency of a
similar specification DOC (i.e. mixed platinum and rhodium) for C12 to
C33 n-alkanes, among other species. It was found that the DOC HC
removal efficiency did not continue to decrease with increasing carbon chain
length, rather decreasing from C12 to C16, followed by an increase
from C17 to C23 and further decrease from C24 to C32. Few
studies have investigated the HC removal efficiency of individual species and
grouped counterparts, expressing DOC HC removal efficiency as total HC, with
no reference to possible compositional and structural effects, which based on
the results shown in this work and Alam et al. (2016), require further study.
Driving scenarios
The VOC-IVOC emission rates from several “driving scenarios” were
investigated. The driving scenarios included either (i) a single applied
engine load and speed, and injection before a steady-state engine temperature
had been achieved, or (ii) a sequence of different engine loads and speeds,
during which steady-state engine temperature was achieved. These experiments
were performed to gain a greater insight into the factors controlling
VOC-IVOC emission rates. Three experiments were performed, cold-start
(exp. 6), cold loaded (exp. 8), and warm idle following load (WIFL, exp. 9).
The engine conditions used in each of these experiments can be found in
Fig. S1. Cold-start included a cold-engine start (idling speed and 0 %
load) with the injection of the exhaust emissions into the MAC after ∼1–2 min. Cold loaded included a cold-engine start followed by the
immediate application of 1500 rpm and 20 % load, with a 1 min hold
before injection. Steady-steady engine temperatures were not achieved during
the cold-start or cold-loaded experiments. WIFL included a cold-engine start,
followed by the immediate application of 2000 rpm and 28–30 % load with
a 7 min hold (during which a steady-state engine temperature was achieved),
followed by 1 min of idling speed (1150 rpm) and 0 % load before
injection. The ∑SpVOC-IVOC emission rates in each experiment were 9268±699, 2902±199, and 1438±96 mg kg-1 in the cold-start, cold-loaded, and WIFL experiments,
respectively. The application of 1500 rpm and 20 % load for 1 min (cold
loaded) resulted in a decrease in the ∑SpVOC-IVOC emissions by a factor
of ∼3, in comparison to the cold-start engine conditions; highlighting
the importance of engine combustion efficiency on VOC-IVOC emission rates.
The VOC-IVOC compositional profiles and emission rates in the driving
scenario experiments can be observed in Fig. 9.
Effect of different driving scenarios on measured VOC-IVOC emission
rates (a) and the contribution of the individual and grouped
compounds to the ∑SpVOC-IVOC emission rate (b).
CS is the cold start (exp. 6), CL is the cold loaded (exp. 8), and WIFL is the warm
idle following load (exp. 9, see text for further information). The emission
rates of tridecane and the C13 branched aliphatic grouping have not been
included in (b) to allow direct comparison between other experiments
where these species were not measured. Error bars represent the calculated
uncertainty in the measured emission rates, see Sect. S1.1 for further
information.
The engine temperature in the cold-start and cold-loaded experiments was 85
and 169 ∘C, respectively. In the WIFL experiment, the engine
temperature reached 290 ∘C during steady-state, decreasing to
150 ∘C upon injection. The ∑SpVOC-IVOC emission rate was lower
in the WIFL experiment than observed in the cold-loaded experiment, where a
higher engine temperature was measured upon injection. The HC removal
efficiency of the DOC below 200 ∘C is close to zero (Korin et al.,
1999; Roberts et al., 2014; Majewski and Khair, 2006; Russell and Epling,
2011), suggesting the lower ∑SpVOC-IVOC emission rate observed in the
WIFL experiment is the result of increased combustion efficiency from the
higher engine speed and load applied before idling conditions. Engine
“warm-up” increases the temperature of the lubricant, coolant, and engine
components, reducing friction and increasing combustion efficiency, thus
resulting in less unburnt fuel emissions in the exhaust gas (cf. Roberts et
al., 2014). This increased combustion efficiency in the WIFL experiment is
also supported by the exhaust gas composition (see Fig. 9b). The engine
temperatures in all three experiments were below 200 ∘C and
consequently the DOC had a minimal effect on HC removal in these experiments.
Therefore, the observed compositional changes in exhaust gas is the result of
increasing combustion efficiency from the cold-start to WIFL experiments.
Straight-chain alkanes are more easily fragmented during combustion than
branched aliphatics (Fox and Whitesell, 2004). The sequential increase in the
abundance of the C7 to C12 n-alkanes in the exhaust gas from the
cold-start to WIFL experiment suggests higher molecular weight n-alkanes
which have not been measured (>C13, abundant in diesel fuel and
lubricate oil) undergo increasing fragmentation with increasing combustion
efficiency, resulting in a higher percentage contribution of smaller
n-alkanes (i.e. C7 to C12) to the ∑SpVOC-IVOC emission
rates. The relationship between internal combustion efficiency and engine
temperature is relatively linear (e.g. Mikalsen and Roskilly, 2009), with the
exception of high engine loads and relatively low speeds (not performed
here), where the engine combustion efficiency and temperature eventually
plateau due to a too lean air to fuel ratio, resulting in incomplete
combustion (see Heywood, 1988, for further information). The percentage
contribution of the C3 aromatic substitution grouping to the ∑SpVOC-IVOC emission rates displayed no obvious change with increasing
combustion efficiency (within the calculated uncertainty). However, the
abundance of the C2 aromatic substitution grouping and toluene generally
decreased with increasing combustion efficiency, with the percentage
contribution observed to plateau in the cold-start and cold-loaded
experiments, followed by a decrease in the WIFL experiment.
Effect of different engine loads on measured VOC-IVOC emission
rates (a) and the percentage contribution of the individual and
grouped compounds to the ∑SpVOC-IVOC emission rate at 0 (exp. 14), 30
(exp. 12), 40 (exp. 13, and 53 % (exp. 11) engine load (b). The
emission rates of tridecane and the C13 branched aliphatic grouping have
not been included in (b) to allow direct comparison between other
experiments where these species were not measured. For comparison, the
percentage contribution of the individual and grouped compounds to the ∑SpVOC emission rate in a cold idle experiment (exp. 14) has been included on
the left of (b), see text for further details. Error bars represent
the calculated uncertainty in the measured emission rates, see Sect. S1.1
for further information.
Engine load
The effect of different engine loads, at a constant speed, on the VOC-IVOC
emission rates is discussed below. Three experiments were performed at 30,
40, and 53 % engine load (exps. 12, 13, and 11, respectively; see
Table 1). The GC × GC-FID was not operational during lower engine
load experiments (not presented here). The ∑SpVOC-IVOC emissions were
observed to decrease with increasing engine load, with ∑SpVOC-IVOC
emission rates of 1019±65, 365±24, and 70±4 mg kg-1 at
30, 40, and 53 % load, respectively (see Table S7). This trend of
decreasing VOC emission rates with increasing engine load has been observed
in a number of previous studies for light-duty and medium-duty diesel
vehicles (Cross et al., 2015; Shirneshan, 2013; Chin et al., 2012; Yamada et
al., 2011) and can be explained by considering the engine operation. At low
engine temperatures (i.e. low engine loads and idling conditions), the fuel
flow is increased to provide easily combustible conditions within the engine
cylinder. This additional fuel flow creates a rich fuel to air ratio, where
there is insufficient oxygen to burn the fuel, resulting in incomplete
combustion and higher VOC-IVOC emission rates from the unburnt fuel. As the
engine temperature increases (e.g. with increasing engine load), the
in-cylinder oxidation rate increases as the fuel components become more
easily combustible at higher temperatures, increasing combustion efficiency
and decreasing VOC-IVOC emission rates (Heywood, 1988). The effect of
different engine loads, at a constant speed, on the VOC-IVOC emission rates
is shown in Fig. 10a. The carbon number distribution of the n-alkanes and
branched aliphatics at 30 % and 40 % engine load are comparable.
Branched aliphatics display an increase in abundance from C7, reaching
peak concentration at C10, followed by a decrease to C13, similar
to that observed in Bohac et al. (2006). Straight-chain alkanes do not
display the same increase and decrease in abundance, with the emission rates
of n-nonane and n-dodecane greater than n-undecane, displaying no
obvious trend. At 53 % engine load, the emission profile changes. The
most abundant n-alkane and branched aliphatic grouping shifts to higher
carbon numbers at higher loads, changing from n-nonane to n-undecane and
from C10 to C12 branched aliphatics. The n-alkanes now display a
sequential increase and decrease in their emission factors, as observed with
the branched aliphatics. This compositional shift to higher carbon number
species under higher engine loads has also been observed in Chin et
al. (2012) for n-alkanes from an Isuzu 1.7 L diesel engine fuelled with
ULSD. Whilst no explanation was provided for this observation, Chin et
al. (2012) found the most abundant n-alkane shifted from n-nonane at
idling conditions (800 rpm) with no load to n-tridecane at 2500 rpm with
maximum applied engine load (900 brake mean effective pressure, kPa).
The percentage contribution of the individual and grouped VOC-IVOCs to the
∑SpVOC-IVOC emission rate in each experiment is shown in Fig. 10b. The
percentage composition from a cold-start experiment (exp. 14) has also been
included on the left of Fig. 10b to provide a comparison between cold idle
engine conditions (which has a compositional profile most similar to unburnt
fuel) and different engine loads. The percentage contribution of the
individual and grouped VOC-IVOCs to the ∑SpVOC-IVOC emission rate
changed considerably with different engine loads. All aromatics, except
benzene, displayed a non-monotonic behaviour with increasing engine load;
their percentage contribution is high at cold idle and 40 % load, with a
smaller contribution at 30 % and 53 % load. This non-monotonic
behaviour has also been observed in Cross et al. (2015). Cross et al. (2015)
investigated the load-dependant emissions from a 5.9 L medium-duty diesel
engine fuelled with ULSD. It was found that the fractional contribution of
oxidised species and aromatics (not explicitly mentioned but shown in the
data) varied inconsistently with increasing engine load. The reason for this
non-monotonic behaviour is currently unclear. The percentage contribution of
benzene generally decreased with increasing engine load. Interestingly, the
percentage contribution of the n-alkanes continued to decrease from cold
idle to 40 % load, followed by a considerable increase at 53 % load.
At 53 % load, the n-alkanes represented 55 % of the ∑SpVOC-IVOC emission rate, 1.6 times greater than observed in the cold-start
experiment. Conversely, branched aliphatics displayed the opposite trend. The
percentage contribution of the branched aliphatics continued to increase from
cold idle to 40 % load, followed by a considerable decrease at 53 %
load, to approximately the same percentage contribution observed in
the cold-start experiment.
This change in the percentage contribution of the n-alkanes and branched
aliphatics at 53 % engine load, can be explained by considering the DOC
HC removal efficiency and the internal combustion temperature. From cold idle
to 40 % engine load, the engine temperature increased from <100 ∘C at cold idle to 445 ∘C at 40 % load. The DOC HC
removal efficiency is thus increasing from near zero at cold idle to near
maximum at 40 % load. At 53 % load, the steady-state engine
temperature reached 700 ∘C. The DOC HC removal efficiency was near
maximum at 40 % load and it is therefore unlikely that the DOC would
account for such a considerable shift in the percentage contribution of the
n-alkanes and branched aliphatics at 53 % load. This shift in the
composition is most likely the result of the considerably higher engine
temperature, resulting in the fragmentation of higher molecular weight
n-alkanes from increased internal combustion efficiency, as observed with
the driving scenario experiments, resulting in a higher percentage
contribution of C7 to C12 n-alkanes to the ∑SpVOC-IVOC
emission rate at 53 % load in comparison to cold idle. The compositional
profiles from 0 to 40 % engine load display the combined effect of
increasing engine combustion efficiency and DOC HC removal efficiency,
possibly explaining why the percentage contribution of C7 to C12 n-alkanes do not increase with increasing engine combustion efficiency (i.e.
the DOC is likely masking the effect of increasing combustion efficiency on ∑SpVOC-IVOC emissions). These experiments also provide additional information
on the effect of the DOC on the exhaust gas composition. The observation of
increasing n-alkane abundance with increasing engine combustion efficiency
suggests that the increase in the abundance of the branched aliphatics at
cold idle (exp. 14), 30 % (exp. 12), and 40 % engine load (exp. 13),
respectively, is the result of the DOC fragmenting higher molecular weight
branched aliphatics with increasing HC removal efficiency; indicating that
the branched aliphatics are more easily fragmented in the DOC than
n-alkanes, possibly the result of the fewer binding sites required in the
DOC for adsorption.
Total speciated VOC-IVOC emission rate measured in each experiment
(refer to Table 1) divided into aliphatic and aromatic emissions
rates (a). Experiments ordered from left-to-right by decreasing
VOC-IVOC emission rates. Engine temperature, speed, and load in each
corresponding experiment are shown in (b). (a) Fuel batch A
used (see Sects. 2.3 and 3.1.1). (b) Fuel batch B used. No DOC in
exp. 3. Sequence of engine conditions performed in exps. 9 and 8 (see
Sect. 2.1 and Fig. S1). No engine temperature measurement for exps. 4 and 14
(engine thermocouple unresponsive). Error bars represent the calculated
uncertainty in the measured emission rates, see Sect. S1.1 for further
information.
Combustion and DOC removal efficiency
The measured ∑SpVOC-IVOC emission rates in each experiment (ordered
from highest to lowest) are shown in Fig. 11, along with the corresponding
engine load, speed, and temperature. The ∑SpVOC-IVOC emission rates
varied significantly with different engine conditions, ranging from 70 to
9268 mg kg-1. The aliphatics represented 56 to 97 % of the ∑SpVOC-IVOC emission rates, with the single-ring aromatics accounting for the
remainder. The highest ∑SpVOC-IVOC emissions were observed in a
cold-start experiment (exp. 6) with no applied load and idling speed
(1150 rpm). Conversely, the lowest ∑SpVOC-IVOC emissions were observed
in the experiment with highest applied engine load and speed (exp. 11,
3000 rpm, 53 % load). The ∑SpVOC-IVOC emission rates were observed
to decrease with increasing engine load and temperature, and to a lesser
degree engine speed. This result is consistent with increased combustion
efficiency and DOC HC removal efficiency with increasing engine temperature,
similar to that observed in previous studies (e.g. Cross et al., 2015; Chin
et al., 2012).
Urban driving conditions are characterised by low engine speed, load, and
exhaust gas temperatures (cf. Franco et al., 2014; EEA, 2016). Conversely,
motorway or highway driving typically result in higher engine temperatures,
due to increased engine speed and load. The results from this study show that at
low engine loads and speeds, the emission rates of unregulated VOC-IVOCs per
kilogram of fuel burnt are considerably greater than emitted at higher
engine speeds and loads (emission rates were 65 times greater from a
cold-start than at maximum applied engine speed and load). Furthermore, it
was found that the exhaust gas composition varied with combustion efficiency
and DOC HC removal efficiency, both which are strongly dependent on working
temperature (Korin et al., 1999; Roberts et al., 2014; Majewski and Khair,
2006; Russell and Epling, 2011). The effect of combustion efficiency on the
exhaust gas composition was observed at engine temperatures below ∼150 ∘C (below the working temperature of the DOC) and at the
maximum applied engine speed and load (700 ∘C), where combustion
efficiency dominated over the effect of the DOC on the exhaust gas
composition. At engine temperatures ranging from 150 to 450 ∘C, the
combined effect of engine combustion and DOC HC removal efficiency on the
exhaust gas composition was observed.
Discussion
Diesel exhaust emissions contain thousands of compounds ranging from ∼C5 to C22, with contributions of up to C33 from
lubricant oil (Alam et al., 2016; Gentner et al., 2017). Only a proportion of
these emissions were speciated in this study. Of the measured compounds,
branched aliphatics generally dominated the exhaust gas composition. An
increasing contribution of branched aliphatics in the exhaust gas was
observed with increasing engine temperature from ∼150 to 450 ∘C
and is likely due to increasing DOC HC removal efficiency. However, below the
working temperature of the DOC (<150 ∘C), the proportion of
n-alkanes in the exhaust gas were observed to increase with increasing
combustion efficiency and could be important in urban environments;
straight-chain alkanes are more efficient at producing SOA than their
branched counterparts (Presto et al., 2010; Tkacik et al., 2012; Lim and
Ziemann, 2009). Previous studies have suggested that liquid-fuel-based emission
factors are consistent with unburnt fuel in diesel exhaust emissions. For
example, Gentner et al. (2013) showed the majority of VOC and IVOC diesel
exhaust emissions were within 70 % uncertainty of liquid-fuel-based
emission factors. This work shows that as combustion efficiency increases, the
contribution of smaller, more volatile (C7 to C13) n-alkanes in
the exhaust gas also increases, the result of increased fragmentation of
higher molecular weight n-alkanes (>C13, not measured) likely
from the fuel and lubricating oil. This may suggest diesel liquid-fuel-based
estimates of SOA yields may be inconsistent with diesel exhaust SOA yields,
particularly at high engine temperatures (i.e. high engine loads and speeds).
Literature emission rates (expressed as mass emitted per kg of fuel
burnt) from diesel exhaust emissions using different vehicle types and engine
conditions.
Reference
Compounds
Vehicle
Engine
Emission
Fuel
Driving
Load
Speed
Emission
studied
type
size (L)
control
type
cycle
(%)
(rpm)
rate
devices
(mg kg-1)
Cross et al. (2015)
IVOCs
MDDV
5.9
None
ULSD
–
0
idle
220
Gordon et al. (2014)
NMOG
HDDV
10.8
None
USLD
UC
–
–
6100
MDDV
6.6
DOC
ULSD
UDDS
–
–
1000
MDDV
5.9
None
USLD
UC
–
–
700
Zhao et al. (2015)
IVOC
MDDV*
12.8, 14.9
DPF
USLD
Creep + idle
–
–
600
MDDV*
12.8, 14.9
DPF
USLD
UDDS, UC, Hi-Cruse
–
–
20
MDDV*
6.6, 5.9, 10.8
None
USLD
Creep + idle
–
–
4000
MDDV*
6.6, 5.9, 10.8
None
USLD
UDDS, UC, Hi-Cruse
–
–
700
Off-road LDDV
2.2
None
ULSD
4-mode EPA TRU
–
–
700
Gordon et al. (2013)
NMOG
Off-road LDDV
2.2
None
ULSD
4-mode EPA TRU
–
–
2000
* is the average emission rate of multiple experiments and
vehicles with similar engine and driving cycle conditions, see Zhao et
al. (2015) for further information. UC is the unified driving
cycle. UDDS is the urban dynamometer driving schedule. EPA TRU is the
Environment Protection Agency transportation refrigeration unit, see Gordon
et al. (2013) for further information. NMOG – nonmethane
organic gases. LDDV – light duty diesel vehicle. MDDV – medium duty diesel
vehicle. HDDV – heavy duty diesel vehicle. DOC – diesel
oxidative catalyst. DPF – diesel particulate filter. ULSD – ultra-low
sulfur diesel.
The comparison of emission rates is difficult between studies due to the vast
number of differences (types of speciated compounds and volatility range,
vehicular types, emission control devices, etc.). Furthermore, few studies
have investigated the effect of different engine conditions on the exhaust
gas composition. The majority of studies have investigated diesel exhaust
emissions using chassis dynamometers, averaging emissions over entire driving
cycles and often reporting emission rates as mass emitted per distance
travelled; emissions and units which are not directly comparable with the
emission rates shown in this work. The different types of instruments used to
measure diesel exhaust emissions and the difficulties in the measurement of
low volatility species has, in-part, resulted in considerable variation in
the types of “speciated” compounds between studies, further compounding the
difficultly in the direct comparison of emission rates. For example, Zhao et
al. (2015) reported speciated and unspeciated IVOC emission rates from both
medium-duty and heavy-duty diesel vehicles. In their study, speciated IVOCs
included straight- and branched-chain alkanes, alkylcyclohexanes,
unsubstituted and substituted polycyclic aromatic hydrocarbons, and
alkylbenzenes with a volatility range of C∗ 102 to
106 µg m-3. Similarly, Cross et al. (2015) measured IVOC
emission rates from a medium-duty diesel engine. The emission rates reported
in their study were based on compounds with a volatility range of C∗ ∼ 103 to 107 µg m-3 and included
cycloalkanes, bicycloalkanes, tricycloalkanes, straight and branched-chain
aliphatics, and groupings of “aromatics”, “oxidised” and “remainder”.
Gordon et al. (2014) measured the emission rates of non-methane organic gases
from medium-duty and heavy-duty diesel engines. In their study, speciated
compounds included single-ring aromatics, straight- and branched-chain
aliphatics, cycloalkanes and non-aromatic carbonyls with a volatility range
of C∗ ∼106 to 109 µg m-3. In this
study, the emission rates from a light-duty diesel engine were reported,
based on the emissions of straight and branched-chain aliphatics and
single-ring aromatics with two and three carbon substituents, with a carbon
range of C6 to C13 and a volatility range of C∗ ∼105 to 108 µg m-3. The investigated chemical
composition and volatility range can have a considerable impact on the
reported emission rates (e.g. Zhao et al., 2015). The emission rates from
comparable experiments in the studies discussed above can be observed in
Table 3. However, a direct comparison of these emission rates with the
results shown here has not been performed due to the differences in vehicular
type (medium-duty and heavy-duty vs. light-duty vehicles) and the volatility
and chemical composition of the speciated compounds. Further studies are
required, providing emission rates of individually speciated compounds (where
possible) to facilitate direct comparison.
Tailpipe sampling requires instruments capable of providing instantaneous
measurements of the chemical composition. This rapid analysis time comes at
the expense of detailed chemical speciation. The use of an atmospheric
simulation chamber in this study allowed instruments requiring longer
sampling times to be used, such as the GC × GC-FID. The chamber
allowed detailed chemical speciation of the exhaust gas composition to be
instantaneously measured under specific ambient temperatures and engine
conditions, which would not have been possible with direct tailpipe sampling.
Thus, the chamber sampling method is complimentary to tailpipe measurements,
allowing a more thorough characterisation of the exhaust gas composition
through the identification of individual hydrocarbon components. The emission
control devices used in this study were Euro 4 compliant. Euro 4 emission
control regulations were first implemented for all new vehicles from
approximately January 2006, with Euro 5 emission control regulations starting
after January 2011. Currently ∼20 % of the European Union diesel
fleet are Euro 4 compliant (ACEA, 2017). The emission rates from only one
diesel engine were investigated in this study. However, several compositional
changes in the exhaust gas were comparable with previous studies, suggesting
the effect of engine combustion and DOC HC removal efficiency on the exhaust
gas composition are relatively consistent between studies. In recent years,
emission regulations have focused on reducing NOx emissions
from diesel vehicles with the introduction of emission control technologies,
such as exhaust gas recirculation, lean-burnt
NOx traps, and selective catalytic reduction (Yang et al.,
2015). However, to our knowledge there are no further emission control
technologies planned for the reduction of total hydrocarbon mass or
unregulated VOCs. To reduce the effect of diesel exhaust emissions on local
air quality, further technologies must be developed to reduce emission rates,
specifically from cold engine conditions (i.e. poor combustion efficiencies)
and below the working temperature of DOCs. The experimental approach and
results presented in this work will support further studies investigating the
effect of different combustion engines, emission control devices, and
atmospheric conditions on the composition and evolution of exhaust gas
emissions. To our knowledge, this is the first study using an atmospheric
simulation chamber to separate the effects of the DOC and combustion
efficiency on the exhaust gas composition.