Particle and gaseous emissions from individual diesel and CNG buses

Abstract

In this study size-resolved particle and gaseous emissions from 28 individual diesel-fuelled and 7 compressed natural gas (CNG)-fuelled buses, selected from an in-use bus fleet, were characterised for real-world dilution scenarios. The method used was based on using CO2 as a tracer of exhaust gas dilution. The particles were sampled by using an extractive sampling method and analysed with high time resolution instrumentation EEPS (10 Hz) and CO2 with a non-dispersive infrared gas analyser (LI-840, LI-COR Inc. 1 Hz). The gaseous constituents (CO, HC and NO) were measured by using a remote sensing device (AccuScan RSD 3000, Environmental System Products Inc.). Nitrogen oxides, NOx, were estimated from NO by using default NO2/NOx ratios from the road vehicle emission model HBEFA3.1. The buses studied were diesel-fuelled Euro III–V and CNG-fuelled Enhanced Environmentally Friendly Vehicles (EEVs) with different after-treatment, including selective catalytic reduction (SCR), exhaust gas recirculation (EGR) and with and without diesel particulate filter (DPF). The primary driving mode applied in this study was accelerating mode. However, regarding the particle emissions also a constant speed mode was analysed. The investigated CNG buses emitted on average a higher number of particles but less mass compared to the diesel-fuelled buses. Emission factors for number of particles (EFPN) were EFPN, DPF = 4.4 ± 3.5 × 1014, EFPN, no DPF = 2.1 ± 1.0 × 1015 and EFPN, CNG = 7.8 ± 5.7 ×1015 kg fuel−1. In the accelerating mode, size-resolved emission factors (EFs) showed unimodal number size distributions with peak diameters of 70–90 nm and 10 nm for diesel and CNG buses, respectively. For the constant speed mode, bimodal average number size distributions were obtained for the diesel buses with peak modes of ~10 nm and ~60 nm. Emission factors for NOx expressed as NO2 equivalents for the diesel buses were on average 27 ± 7 g (kg fuel)−1 and for the CNG buses 41 ± 26 g (kg fuel)−1. An anti-relationship between EFNOx and EFPM was observed especially for buses with no DPF, and there was a positive relationship between EFPM and EFCO.

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Atmos. Chem. Phys., 13, 5337–5350, 2013
www.atmos-chem-phys.net/13/5337/2013/
doi:10.5194/acp-13-5337-2013
© Author(s) 2013. CC Attribution 3.0 License.
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Particle and gaseous emissions from individual diesel and
CNG buses
˚
A. M. Hallquist1, M. Jerksj¨
o1, H. Fallgren1, J. Westerlund2, and ˚
A. Sj¨
odin1
1IVL Swedish Environmental Research Institute, Gothenburg, Sweden
2University of Gothenburg, Department of Chemistry and Molecular Biology, Atmospheric Science, Gothenburg, Sweden
Correspondence to: ˚
A. M. Hallquist (asa.hallquist@ivl.se)
Received: 14 July 2012 – Published in Atmos. Chem. Phys. Discuss.: 22 October 2012
Revised: 17 March 2013 – Accepted: 20 March 2013 – Published: 27 May 2013
Abstract. In this study size-resolved particle and gaseous
emissions from 28 individual diesel-fuelled and 7 com-
pressed natural gas (CNG)-fuelled buses, selected from an
in-use bus fleet, were characterised for real-world dilution
scenarios. The method used was based on using CO2as
a tracer of exhaust gas dilution. The particles were sam-
pled by using an extractive sampling method and analysed
with high time resolution instrumentation EEPS (10Hz)
and CO2with a non-dispersive infrared gas analyser (LI-
840, LI-COR Inc. 1 Hz). The gaseous constituents (CO, HC
and NO) were measured by using a remote sensing de-
vice (AccuScan RSD 3000, Environmental System Prod-
ucts Inc.). Nitrogen oxides, NOx, were estimated from NO
by using default NO2/ NOxratios from the road vehicle
emission model HBEFA3.1. The buses studied were diesel-
fuelled Euro III–V and CNG-fuelled Enhanced Environmen-
tally Friendly Vehicles (EEVs) with different after-treatment,
including selective catalytic reduction (SCR), exhaust gas
recirculation (EGR) and with and without diesel particu-
late filter (DPF). The primary driving mode applied in this
study was accelerating mode. However, regarding the parti-
cle emissions also a constant speed mode was analysed. The
investigated CNG buses emitted on average a higher num-
ber of particles but less mass compared to the diesel-fuelled
buses. Emission factors for number of particles (EFPN) were
EFPN,DPF =4.4±3.5×1014, EFPN,no DPF =2.1±1.0×1015
and EFPN,CNG =7.8±5.7×1015 kgfuel−1. In the accelerat-
ing mode, size-resolved emission factors (EFs) showed uni-
modal number size distributions with peak diameters of 70–
90 nm and 10nm for diesel and CNG buses, respectively. For
the constant speed mode, bimodal average number size dis-
tributions were obtained for the diesel buses with peak modes
of ∼10 nm and ∼60 nm.
Emission factors for NOxexpressed as NO2equivalents
for the diesel buses were on average 27±7 g (kg fuel)−1
and for the CNG buses 41±26 g (kg fuel)−1. An anti-
relationship between EFNOxand EFPM was observed espe-
cially for buses with no DPF, and there was a positive rela-
tionship between EFPM and EFCO.
1 Introduction
It is acknowledged that combustion processes, especially
traffic-related emissions, contribute significantly to total par-
ticulate air and gaseous pollutants in urban environments.
Many epidemiological studies have shown that particles have
adverse health effects (Pope and Dockery, 2006). Particles
also have an effect on climate either directly via scattering
and absorption of radiation or indirectly via its influence on
the formation of clouds.
When measuring particle emissions, mass basis is often
used. This implies that such data are dominated by large par-
ticles. Numerically vehicle exhaust is dominated by ultra-
fine particles (UFPs), i.e. particles with a diameter <100 nm
(Janhall et al., 2004; Harrison et al., 1999; Kumar et al.,
2010). Therefore an alternative way of presenting particle
emissions is needed – i.e. looking at the number of particles
emitted – to enable accounting for the small particles that on
a mass basis are negligible. Further, health risks are proba-
bly dominated by the UFPs (Donaldson et al., 1998; Delfino
et al., 2005; Valavanidis et al., 2008). Thus, there is an ob-
vious need to ascertain the emission of particles from traffic
regarding number and size in order to establish effective air
quality management strategies.
Published by Copernicus Publications on behalf of the European Geosciences Union.

5338 ˚
A. M. Hallquist et al.: Particle and gaseous emissions from individual diesel and CNG buses
Particles measured in close vicinity of the emission source
are primary, i.e. emitted as particles from the tailpipe, or sec-
ondary, i.e. formed during the expansion and cooling of the
hot exhaust gases. The former are often in the form of ag-
glomerates of solid phase material, whereas the latter are
more volatile (Morawska et al., 2008). Additionally, traffic
contributes to the formation of secondary organic aerosols
(SOAs); however, the magnitude of this contribution is very
uncertain (Robinson et al., 2007). This is a chemically in-
duced particle formation (time scales of hours to days) which
is very important on a regional and global scale (Hallquist et
al., 2009).
The particle emissions from any combustion source can be
derived from the emission ratio of the particle concentration
to a co-emitted trace gas, such as CO2or NOx(Janhall and
Hallquist, 2005). Knowing the emission factor for the cho-
sen trace gas (EFgas), an emission factor for particle number
(EFPN) or mass (EFPM) can be estimated (Hak et al., 2009).
EFPN/PM =1part
1gas ×EFgas,(1)
where 1part and 1gas are measured changes in the concen-
tration of particle number/mass and trace gas, respectively.
Alternative ways of measuring particle emissions from vehi-
cles are at the kerbside, often giving values for the average
fleet, or by chassis dynamometer, measuring vehicles indi-
vidually (e.g. Janhall et al., 2004; Wang et al., 1997; Ban-
Weiss et al., 2010). However, in the latter case it is difficult,
if not impossible, to accurately mimic the real-world dilution.
Additionally, there are chase-car experiments where the test
vehicle is followed by an instrumented vehicle (e.g. Pirjola et
al., 2004; Vogt et al., 2003). A challenge with this method is
to avoid being influenced by other vehicles as well as keeping
the distance between the target vehicle and the chasing vehi-
cle constant. Knowledge about emissions from the on-road
fleet under real-world conditions is crucial. In a recent study,
EFPN was measured at the kerbside for individual vehicles
for real-world dilution (Hak et al., 2009).
Along with particles, nitrogen oxides, NOx, are depicted
as being the most problematic pollutant from internal com-
bustion engines (Lopez et al., 2009). In order to meet the
lower NOxand particle emission levels introduced for heavy
duty vehicles (HDVs), exhaust gas after-treatment has be-
come necessary. To reduce particle emissions from HDVs,
diesel particulate filters (DPFs) are widely used. An example
of after-treatment technology to reduce NOxis selective cat-
alytic reduction (SCR), which can be found in power plants,
ships and lately also in HDVs. The most common method is
SCR with urea injection due to urea’s low toxicity and ease in
handling, but direct injection of NH3can also be used. In the
SCR system the urea/water mixture (e.g. AdBlue®) is first
added to the exhaust gas which becomes hydrolysed to NH3
and CO2. In the SCR catalyst section NH3reacts with NOx
to form N2and H2O. Another common approach to reduce
NOxemissions is exhaust gas recirculation (EGR). By keep-
ing a low combustion temperature and low oxygen content
the formation of NOxis unfavourable; this can be achieved
by recirculating a small fraction of the exhaust gas back to
the cylinders.
Emissions from new HDVs in Europe are regulated by
Euro standards. Currently in force since 2008 is the Euro V
standard, and the Euro VI standard will be implemented
in 2013. Enhanced Environmentally Friendly Vehicle, EEV,
is a voluntary environmental standard which requires lower
emission levels than Euro V. It was introduced together with
the Euro IV and Euro V emission standards as an incentive
to develop vehicles with even lower emission levels than re-
quired by regulations, and is mostly applicable to CNG heavy
duty vehicles.
In order to meet the challenges with increased transporta-
tion, decreased oil resources and enhanced greenhouse gas
emissions, the European Union has decided on a 10 % substi-
tution of traditional fuels in the road transport sector (petrol
and conventional diesel) by alternative fuels by the year
2020. However, the emissions from vehicles using alterna-
tive fuels have to be thoroughly studied to avoid introduction
of air pollutants that can have severe health/environmental
effects or other so far unknown effects or, alternatively, to
establish the advantages from using these fuels.
In the literature there are some studies that have compared
the particle emissions from diesel-fuelled and CNG-fuelled
buses (Jayaratne et al., 2008, 2009; Wang et al., 1997; Ull-
man et al., 2003; Lanni et al., 2003; Norman et al., 2002;
Clark et al., 1999). This study takes these investigations fur-
ther by determining both gaseous (NOx, CO and HC) and
size-resolved particle emission factors for CNG and diesel
buses belonging to different Euro classes with various after-
treatment equipment, i.e. EGR and SCR, for real-world dilu-
tion scenarios.
2 Experimental method
In this study particle and gaseous emissions from individ-
ual vehicles were determined by measuring the concentration
change in the diluted exhaust plume compared to the concen-
trations before the passage and relative to the change in CO2
concentration. By this method it is not necessary to measure
absolute concentrations as the relation to CO2is assumed
to be constant during dilution (Jayaratne et al., 2005, 2010;
Canagaratna et al., 2004; Shi et al., 2002; Hak et al., 2009).
In addition, this method enables deriving size-resolved EFs
(Janhall and Hallquist, 2005).
In total 35 different buses were studied, 28 diesel buses
and 7 CNG buses. A summary of their technical characteris-
tics including fuel used, Euro class, after-treatment system,
year taken into service and kilometres travelled is shown in
Table 1.
The measurements were performed at five different loca-
tions in connection to the bus depots with limited influence
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˚
A. M. Hallquist et al.: Particle and gaseous emissions from individual diesel and CNG buses 5339
Table 1. Technical data of the buses studied.
Bus no Euro FuelaAfter-treatmentbYear taken Distance travelled
class into service (103km)
1 IIIcDiesel SCR, DPF 2004 525
2 IIIcDiesel SCR, DPF 2004 516
3 III Diesel DPF 2003 454
4 III Diesel DPF 2002 995
5 III Diesel DPF 2002 584
6 III Diesel DPF 2002 523
7 III Diesel – 2004 232
8 III Diesel – 2004 285
9 IV Diesel EGR, DPF 2006 393
10 IV Diesel EGR, DPF 2006 3.74
11 IV Diesel EGR 2008 116
12 IV Diesel EGR 2006 597
13 IV Diesel EGR 2010 182
14 EEVdCNG – 1999 598
15 EEV CNG – 2004 397
16 EEV CNG – 2004 365
17 EEV CNG – 2008 157
18 EEV CNG – 2008 153
19 EEV CNG EGR 2004 450
20 EEV CNG EGR 2004 482
21 V Diesel SCR, DPF 2009 55.8
22 V Diesel SCR 2009 n.ae
23 V Diesel SCR 2007 347
24 V Diesel SCR 2007 307
25 V Diesel SCR 2009 171
26 V Diesel SCR 2007 336
27 V Diesel SCR 2007 351
28 V Diesel SCR 2007 143
29 V Diesel EGR, DPF 2009 123
30 V Diesel SCR 2007 28.6
31 V Diesel SCR 2007 3924
32 V Diesel SCR 2007 209
33 V Diesel SCR 2007 371
34 V Diesel SCR 2009 104
35 V Diesel SCR 2010 71.2
aDiesel = MK1 <10 ppm S
bSCR=selective catalytic reduction, EGR=exhaust gas recirculation, DPF=diesel particulate filter
cModified Euro III, now classified as Euro V
dEEV=Enhanced Environmentally Friendly Vehicle
en.a = not available.
from other traffic. Each bus passed the remote sensing and
EEPS instrumentation in two driving modes: (1) acceleration
from standstill to about 20 kmh−1, and (2) constant speed of
about 20 km h−1. Before the buses were measured they were
driven a distance, assuring the engines to be fully warmed
up. Each bus was tested at least three times, but often more
repetitions were performed.
2.1 Particle sampling
The sampling of the particle emissions was conducted ac-
cording to Hak et al. (2009), i.e. an extractive sampling of
the passing bus plumes where the sample was continuously
drawn through a cord-reinforced flexible conductive tubing.
The particles were measured with an EEPS (Engine Exhaust
Particle Sizer Spectrometer, Model 3090, TSI Inc.). With this
instrument, particle size distributions both regarding mass
and number can be obtained in the size range of 5.6–560nm
and with a time resolution of 10 Hz. When determining the
mass of particles emitted, spherical particles with unit den-
sity were assumed. The CO2concentration was measured
with a non-dispersive infrared gas analyser (LI-840, LI-COR
Inc.) with a time resolution of 1Hz (Fig. 1).
In order to prevent the influence of the ambient temper-
ature on the measurements for the different measurement
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5340 ˚
A. M. Hallquist et al.: Particle and gaseous emissions from individual diesel and CNG buses
CO2
TD
EEPS
RSD
RSD
Fig. 1. Schematic of the experimental set-up used. EEPS (En-
gine Exhaust Particle Sizer Spectrometer, Model 3090, TSI Inc.),
RSD (Remote Sensing Device, AccuScan RSD 3000, Environmen-
tal System Products Inc.) and TD (thermodenuder; Dekati).
days, the extracted sample flow was heated to 298K be-
fore the analysis using a thermodenuder (TD; Dekati). Size-
dependent aerosol losses within the TD were accounted for
(user manual).
2.2 Gas sampling
The gaseous constituents NO, HC and CO were measured by
using a remote sensing device (AccuScan RSD 3000, Envi-
ronmental System Products Inc.). This equipment was set up
with a transmitter and a receiver on one side of the passing
lane and a reflector on the other (Fig. 1). The principle of this
instrument has been described in detail elsewhere (Burgard
et al., 2006) and will only be briefly presented here. This in-
strumental set-up generates and monitors a co-linear beam
of IR and UV light emitted and reflected. Concentrations
are determined relative to the concentration of CO2with a
time resolution of 100 Hz. For detecting CO, HC and CO2
the absorptions in the IR region at 2150cm−1, 2970 cm−1
and 2350 cm−1, respectively, are used. For NO the absorption
in the UV region at 227nm is used. The instrumental noise
of the used RSD 3000 unit was estimated with the method
described in Burgard et al. (2006) using a dataset from an
earlier remote sensing study, comprising more than 20000
on-road emission measurements on passenger cars. The de-
tection limits were then estimated as three times the stan-
dard deviation of the noise and were determined to be 18g
(kg fuel)−1, 14g (kgfuel)−1and 5 g (kg fuel)−1for CO, HC
and NO, respectively.
Calibrations were conducted every 1.5–2h of measure-
ments by using a certified gas mixture containing 1510ppm
propane, 1580 ppm NO, 1600 ppm NOx, 3.00 % CO and
12.8 % CO2in N2(AGA Gas). The gaseous data was re-
trieved from the RSD system as ppm or %.
2.3 Calculation of emission factors (EFs)
Particle emission factors were derived by assuming the CO2
concentration to be directly proportional to the fuel consump-
tion, hence assuming complete combustion. For the gaseous
constituents also the measured HC and CO were accounted
for. In the calculations a carbon fraction of 0.865 and 0.749
for diesel and CNG fuel, respectively, was used. In this study
the emission factors are presented as mass or number per
kg fuel used. The gaseous pollutant emission factor for each
compound (CO, HC or NO) per kilogram of fuel burnt was
for diesel-fuelled vehicles calculated by applying Eq. (2)
(Burgard et al., 2006) and for CNG-fuelled vehicles by ap-
plying Eq. (3):
EFgas =CFFuel ×SF×Mgas
MC×
gas
CO2
1+CO
CO2+6HC
CO2,(2)
EFgas =CFFuel ×SF×Mgas
MC×
gas
CO2
1+CO
CO2+4.3HC
CO2,(3)
where CFFuel is the carbon mass fraction of the fuels, Mgas
and MCare the molar mass of CO, HC, NO and C, respec-
tively, and SF is a scaling factor. The RSD unit is calibrated
with propane, and the hydrocarbons in the exhaust gas from
diesel vehicles are assumed to be similar to the calibration
gas, hence the molar mass of propane was used as MHC in
Eq. (2). In Eq. (3) the molar mass of methane was used as
this is the major constituent of CNG. The scaling factor is
only applicable for determining HC; for all the other gaseous
compounds SF is equal to 1. An SF is used to compensate
for the known difference between non-dispersive infrared
(NDIR)-based measurements and flame ionization detector
(FID)-based measurements, a factor of 2 for diesel-fuelled
vehicles (Singer et al., 1998) and a factor of 4.3 for CNG-
fuelled vehicles (Stephens et al., 1996; Singer et al., 1998).
The factor of 6 in Eq. (2) arises from the carbon atoms per
molecule of propane multiplied with the scaling factor of 2.
Since the remote sensing device measures NO and not
NO2, the reported NOxemission factors have been estimated
from measured NO and the default NO2/NOxratios from the
HBEFA 3.1 road vehicle emission model (HBEFA3.1, 2010);
see Table 2. The NOxemission factors were calculated by us-
ing Eq. (4):
EFNOx=EFNO
1−NO2
NOx,(4)
where EFNO is expressed as grams of equivalent NO2per kg
fuel. Reporting NOxemissions as equivalent NO2complies
with HDV emission standards (Shorter et al., 2005).
In order to be able to compare with studies expressing
EFs in mass/number per km, the EFs in this study were
re-calculated by using the average fuel consumption re-
ported for the tested diesel and CNG buses, 0.38L km−1and
0.735 Nm3km−1, respectively. For the calculations a density
of 0.815 kg dm−3and 0.70 kg m−3was assumed (Swedish
Environmental Protection Agency, 2013). These EFs (in
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˚
A. M. Hallquist et al.: Particle and gaseous emissions from individual diesel and CNG buses 5341
Table 2. HBEFA 3.1 Emission factors, fuel consumption (FC) and NO2to NOxratios for Ubus Std >15–18t Urban Access
Road/30/Stop + Go.
EFPN EFPM EFNOxFC NO2/NOx
1014 (kg fuel)−1g (kg fuel)−1g (kg fuel)−1g km−1%
Euro III 8.3 0.70 37 444 7
Euro III 1.6 0.18 37 448 30
DPF
Euro IV 4.1 0.18 23 357 21
EGR
Euro IV 0.69 0.012 23 365 25
EGR, DPF
Euro V 0.68 0.012 14 372 25
EGR, DPF
Euro V 2.0 0.20 38 353 7
SCR
Euro V 0.20 0.0078 37 360 25∗
SCR, DPF
CNG EEV 0.072 0.17 44 510 25
∗This NO2/NOxratio has also been used in this study for Euro III buses with SCR and DPF.
20
1
Figure 2.
Example of emission signals from three successive individual passages of the same 2
bus in accelerating mode. Particle number (red line) and CO
2
concentration (black line). 3
4
5.0×10
5
1.0×10
6
1.5×10
6
2.0×10
6
2.5×10
6
0
200
400
600
800
1000
1200
1400
1600
10:33 10:35 10:36 10:37 10:39
Particles (# cm
-3
)
CO
2
(ppm)
Time (hh:mm)
Fig. 2. Example of emission signals from three successive individ-
ual passages of the same bus in accelerating mode. Particle number
(red line) and CO2concentration (black line).
number/masskm−1) will be a lower limit as the fuel con-
sumption during acceleration is expected to be higher.
2.4 Modelling
The measured EFs (both particles and gaseous) were also
compared to modelled EFs by using the HBEFA 3.1 (2010).
This model provides EFs in gkm−1for six main categories
of road vehicles: passenger cars, light duty vehicles, heavy
goods vehicles, urban buses, coaches and motorcycles (in-
cluding mopeds). These main categories are further divided
into size classes, type of fuel and emission standards. For all
Euro IV and Euro V HDVs the model provides EFs sepa-
rately for vehicles with SCR and for vehicles with EGR. For
the class urban buses EFs are also provided for vehicles both
with and without DPF. Furthermore, the emission factors are
given for a large number of traffic situations based on emis-
sion measurements according to different sets of real-world
driving cycles (HBEFA3.1, 2010).
The measured EFs in this work were compared to mod-
elled data for a standard urban bus (15–18tons). The driv-
ing pattern was classified according to the HBEFA 3.1 traffic
situation scheme as urban access road with a posted speed
of 30 km h−1and with stop-and-go traffic. The stop-and-go
traffic flow is defined as a driving cycle including many ac-
celerations from standstill which was considered to be the
driving pattern that best described the driving pattern used
in the present measurements for the accelerating mode. All
EFs were recalculated from gkm−1to g kg−1by using the
specific fuel consumption given in HBEFA 3.1. Used emis-
sion factors, fuel consumption and NO2to NOxratios are
presented in Table 2.
3 Results and discussion
3.1 Emission signal
An example of typical signals in number of particles and
CO2concentration during a bus passage is shown in Fig. 2.
In this figure three successive bus passages for the same ve-
hicle are displayed for the accelerating mode. The shape of
the CO2peak is broader than the particle peak, which is due
to the use of a small volume before the CO2analyser, ex-
tending the time available for the instrument to process the
gas sample in order to prevent concentration peaks out of
the instrument’s measurement range. In Table 3 the measure-
ment results for all the tested buses are presented. Generally
there is higher variation in the data for the constant speed
mode tests compared to the accelerating mode tests, which is
primarily due to difficulties for the drivers to keep the same
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5342 ˚
A. M. Hallquist et al.: Particle and gaseous emissions from individual diesel and CNG buses
Table 3. EF for particle number (EFPN), mass (EFPM) and gaseous compounds for all the buses studied in accelerating mode (acc) and
constant speed mode (const). Stated errors are at the statistical 95 % confidence interval.
Bus no Euro EFPN,acc EFPN,const EFPM,acc EFPM,const EFCO,acc EFa
NOx,acc
class # (kgfuel)−1# (kg fuel)−1mg (kg fuel)−1mg (kg fuel)−1g (kg fuel)−1g (kg fuel)−1
1014 1014
1 IIIb1.9 ±0.2 1.1 ±0.2 62 ±11 41 ±12 <18 22 ±3
2 IIIb23 ±1c9.7 ±0.5 2465 ±1352c142 ±23 52 ±10 28 ±3
3 III 0.46 ±0.34 4.2±2.6 31 ±19 273 ±161 <18 24 ±16
4 III n.ad3.4 ±1.0 171±126 151 ±41 <18 30 ±5
5 III 0.11 ±0.01 0.12±0.04 6.7 ±3.1 n.a <18 <5e
6 III 11 ±2 n.a 681 ±236 n.a <18 19±2
7 III 33 ±6 n.a 1566 ±419 n.a 25±14 22 ±7
8 III 45 ±13 n.a 2074 ±619 n.a 36±17 <5
9 IV 13 ±0.1 3.1±0.5 650 ±45 61 ±12 <18 <5
10 IV 5.1 ±0.6 2.6 ±0.7 177 ±23 58 ±8<18 20 ±2
11 IV 39 ±23 47 ±42 1883 ±908 489 <18 9 ±3
12 IV 44 ±7 n.a 3089 ±818 n.a 52±35 <5
13 IV 13 ±8 5.8±1.8 562 ±469 91 ±34 <18 19 ±5
14 EEV 173 ±25 n.a 36 ±25 n.a <18 9±3
15 EEV 45 ±41 n.a 15 ±9 n.a <18 43 ±21
16 EEV 1.4 ±1.0 n.a 3.5 ±1.6 n.a <18 59±9
17 EEV 155 ±33 n.a 60 ±15 n.a <18 77±4
18 EEV 144 ±12 n.a 49 ±24 n.a <18 89±27
19 EEV 11 ±7 5.6±9.4 3.0 ±1.4 1.9 ±0.5 <18 <5
20 EEV 13 ±4 20±7 0.38 ±0.22 n.a <18 <5
21 V 2.9 ±0.5 2.4±0.5 76 ±14 46 ±12 <18 63 ±5
22 V 4.4 ±1.5 2.7±0.5 125 ±52 47 ±13 <18 45±5
23 V 8.4 ±0.9 5.2±1.7 175 ±36 63 ±23 <18 50±2
24 V 11 ±1 20 ±4 184 ±14 204 ±109 <18 38 ±6
25 V 12 ±1 7.4 ±3.3 242 ±26 56 ±26 <18 27±12
26 V 11 ±1 12 ±5 181 ±11 205 ±147 <18 49 ±0
27 V 8.3 ±1.4 4.1±0.7 178 ±42 61 ±15 <18 42±27
28 V 15 ±6 3.2 ±0.6 318 ±167 41 ±8<18 29±11
29 V 0.36 ±0.45 0.095 ±0.028 3.8 ±2.8 4.9 ±5.2 <18 <5
30 V 5.8 ±0.5 5.0±0.3 298 ±25 77 ±19 19 ±21 58 ±4
31 V 7.6 ±2.9 33±16 240 ±87 509 ±264 28 ±9 43 ±4
32 V 15 ±6 3.9 ±2.7 766 ±429 398 ±260 <18 20±2
33 V 7.2 ±0.9 n.a 232 ±77 n.a <18 51 ±6
34 V 92 ±42 n.a 165 ±66 n.a <18 17 ±20
35 V 5.0 ±2.0 15±5 246 ±128 385 ±275 <18 15±11
aIn NO2equivalents
bModified Euro III, now classified as Euro V
cOmitted when calculating average size distributions and total numbers
dn.a = not available
eLess than 8 g (kgfuel)−1NO as NO2equivalents.
constant speed/rpm while passing the measurement equip-
ment on repeated occasions. However, vehicles identified as
high-emitters in the accelerating mode were also generally
identified as high-emitters in the constant speed mode (Ta-
ble 3).
3.2 EFpart for different Euro classes
In Fig. 3 the derived EFPN and EFPM for each Euro class are
shown for the accelerating mode. Generally, higher EFs were
obtained for buses without DPFs regarding both number and
mass of particles emitted. The CNG buses emitted on average
a higher number of particles compared to the diesel-fuelled
buses, which is in line with previous studies (Jayaratne et al.,
2008, 2010). When comparing the average EFPN of the in-
vestigated diesel-fuelled buses with the CNG-fuelled buses
for the accelerating mode, the EFPN for CNG buses were
about five times higher (1.6±0.7 ×1015 vs. 7.8 ±5.7 ×1015
(kg fuel)−1), which is similar to results obtained by Jayaratne
et al. (2008) (4.0×1015 vs. 2.1 ×1016 (kg fuel)−1), when
Atmos. Chem. Phys., 13, 5337–5350, 2013 www.atmos-chem-phys.net/13/5337/2013/

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A. M. Hallquist et al.: Particle and gaseous emissions from individual diesel and CNG buses 5343
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A. M. Hallquist et al.: Particle and gaseous emissions from individual diesel and CNG buses 7
a)
˚
A. M. Hallquist et al.: Particle and gaseous emissions from individual diesel and CNG buses 7
4.0×10
15
8.0×10
15
1.2×10
16
1.6×10
16
2.0×10
16
01234567
EF
PN
# (kg fuel)
-1
w DPF
wo DPF
HBEFA w DPF
HBEFA wo DPF
Euro class average
Average
E_III E_IV
w. EGR
E_V
w. SCR
E_V
w. EGR
EEV
w. EGR
E_III
w. SCR
EEV
0
500
1 000
1 500
2 000
2 500
3 000
3 500
01234567
EF
PM
mg (kg fuel)
-1
E_III E_IV
w. EGR
E_V
w. SCR
E_V
w. EGR
EEV
w. EGR
E_III
w. SCR
EEV
Fig. 3. EFPN (a) and EFPM (b) for all the buses studied divided into Euro class for the driving mode acceleration. Without DPF (white
circles), with DPF (red circles), average of all represented Euro classes (dashed line), average of an individual represented Euro class (solid
line). Crosses are EFs obtained by the HBEFA 3.1 model with DPF (red) and without (black).
using the same fuel C-content assumption as in this study.
However, in the case of mass of particles, the emissions from
the CNG buses were on average lower compared to diesel
buses.
Figure 3 also shows that a diesel bus with DPF for
the accelerating mode emits on average 5 times less
than a diesel bus without DPF regarding number of
particles and 3 times less regarding mass of particles
(4.4±3.5 ×1014 vs. 2.1 ±1.0 ×1015 kg−1and 206 ±175
vs. 696±398 mg kg−1).
Regarding number of particles, only buses without DPF
were having EFs above the average EF of all tested vehi-
cles (see Fig. 3). The largest scatter in EFPN was, however,
obtained for the CNG-fuelled buses. Out of the 15 highest
PN-emitting buses, there were five gas buses (in total 7 CNG
buses were tested) and 13 had no DPF installed. Regard-
ing mass of particles, vehicles emitting above the average
EFPM of all tested buses belonged to all Euro classes, ex-
cept for buses representing Euro V with EGR and the CNG-
fuelled buses. The 15 highest PM-emitting buses were only
diesel-fuelled buses; 12 had no DPF and four of the total five
tested Euro IV with EGR buses were among these vehicles.
The higher masses obtained for EGR-equipped buses with-
out DPF may be due to the decrease in oxygen content when
some of the exhaust gas is re-circulated, which favours soot
formation (Seinfeld and Pandis, 1998; Maricq, 2007).
For comparison, modelled values of EFPN and EFPM us-
ing the HBFA 3.1 model are shown in Table 2. The mod-
elled values are generally significantly lower than the mea-
sured values. A possible explanation for this can be different
driving modes, acceleration versus route, including start and
stops but also constant speed mode. As indicated by Table 3,
www.atmos-chem-phys.net/13/1/2013/ Atmos. Chem. Phys., 13, 1–15, 2013
b)
˚
A. M. Hallquist et al.: Particle and gaseous emissions from individual diesel and CNG buses 7
4.0×10
15
8.0×10
15
1.2×10
16
1.6×10
16
2.0×10
16
01234567
EF
PN
# (kg fuel)
-1
w DPF
wo DPF
HBEFA w DPF
HBEFA wo DPF
Euro class average
Average
E_III E_IV
w. EGR
E_V
w. SCR
E_V
w. EGR
EEV
w. EGR
E_III
w. SCR
EEV
0
500
1 000
1 500
2 000
2 500
3 000
3 500
01234567
EF
PM
mg (kg fuel)
-1
E_III E_IV
w. EGR
E_V
w. SCR
E_V
w. EGR
EEV
w. EGR
E_III
w. SCR
EEV
Fig. 3. EFPN (a) and EFPM (b) for all the buses studied divided into Euro class for the driving mode acceleration. Without DPF (white
circles), with DPF (red circles), average of all represented Euro classes (dashed line), average of an individual represented Euro class (solid
line). Crosses are EFs obtained by the HBEFA 3.1 model with DPF (red) and without (black).
using the same fuel C-content assumption as in this study.
However, in the case of mass of particles, the emissions from
the CNG buses were on average lower compared to diesel
buses.
Figure 3 also shows that a diesel bus with DPF for
the accelerating mode emits on average 5 times less
than a diesel bus without DPF regarding number of
particles and 3 times less regarding mass of particles
(4.4±3.5 ×1014 vs. 2.1 ±1.0 ×1015 kg−1and 206 ±175
vs. 696±398 mg kg−1).
Regarding number of particles, only buses without DPF
were having EFs above the average EF of all tested vehi-
cles (see Fig. 3). The largest scatter in EFPN was, however,
obtained for the CNG-fuelled buses. Out of the 15 highest
PN-emitting buses, there were five gas buses (in total 7 CNG
buses were tested) and 13 had no DPF installed. Regard-
ing mass of particles, vehicles emitting above the average
EFPM of all tested buses belonged to all Euro classes, ex-
cept for buses representing Euro V with EGR and the CNG-
fuelled buses. The 15 highest PM-emitting buses were only
diesel-fuelled buses; 12 had no DPF and four of the total five
tested Euro IV with EGR buses were among these vehicles.
The higher masses obtained for EGR-equipped buses with-
out DPF may be due to the decrease in oxygen content when
some of the exhaust gas is re-circulated, which favours soot
formation (Seinfeld and Pandis, 1998; Maricq, 2007).
For comparison, modelled values of EFPN and EFPM us-
ing the HBFA 3.1 model are shown in Table 2. The mod-
elled values are generally significantly lower than the mea-
sured values. A possible explanation for this can be different
driving modes, acceleration versus route, including start and
stops but also constant speed mode. As indicated by Table 3,
www.atmos-chem-phys.net/13/1/2013/ Atmos. Chem. Phys., 13, 1–15, 2013
Fig. 3. EFPN (a) and EFPM (b) for all the buses studied divided into Euro class for the driving mode acceleration. Without DPF (white
circles), with DPF (red circles), average of all represented Euro classes (dashed line), average of an individual represented Euro class (solid
line). Crosses are EFs obtained by the HBEFA 3.1 model with DPF (red) and without (black).
using the same fuel C-content assumption as in this study.
However, in the case of mass of particles, the emissions from
the CNG buses were on average lower compared to diesel
buses.
Figure 3 also shows that a diesel bus with DPF for
the accelerating mode emits on average 5 times less
than a diesel bus without DPF regarding number of
particles and 3 times less regarding mass of particles
(4.4±3.5 ×1014 vs. 2.1 ±1.0 ×1015 kg−1and 206 ±175
vs. 696±398 mg kg−1).
Regarding number of particles, only buses without DPF
were having EFs above the average EF of all tested vehi-
cles (see Fig. 3). The largest scatter in EFPN was, however,
obtained for the CNG-fuelled buses. Out of the 15 highest
PN-emitting buses, there were five gas buses (in total 7 CNG
buses were tested) and 13 had no DPF installed. Regard-
ing mass of particles, vehicles emitting above the average
EFPM of all tested buses belonged to all Euro classes, ex-
cept for buses representing Euro V with EGR and the CNG-
fuelled buses. The 15 highest PM-emitting buses were only
diesel-fuelled buses; 12 had no DPF and four of the total five
tested Euro IV with EGR buses were among these vehicles.
The higher masses obtained for EGR-equipped buses with-
out DPF may be due to the decrease in oxygen content when
some of the exhaust gas is re-circulated, which favours soot
formation (Seinfeld and Pandis, 1998; Maricq, 2007).
For comparison, modelled values of EFPN and EFPM us-
ing the HBFA 3.1 model are shown in Table 2. The mod-
elled values are generally significantly lower than the mea-
sured values. A possible explanation for this can be different
driving modes, acceleration versus route, including start and
stops but also constant speed mode. As indicated by Table 3,
EFPN/PM was generally lower for constant speed mode com-
pared to acceleration. Modelled EFPN was the lowest for
www.atmos-chem-phys.net/13/1/2013/ Atmos. Chem. Phys., 13, 1–14, 2013
Fig. 3. EFPN (a) and EFPM (b) for all the buses studied divided into Euro class for the driving mode acceleration. Without DPF (white
circles), with DPF (red circles), average of all represented Euro classes (dashed line), average of an individual represented Euro class (solid
line). Crosses are EFs obtained by the HBEFA 3.1 model with DPF (red) and without (black).
using the same fuel C-content assumption as in this study.
However, in the case of mass of particles, the emissions from
the CNG buses were on average lower compared to diesel
buses.
Figure 3 also shows that a diesel bus with DPF for
the accelerating mode emits on average 5 times less
than a diesel bus without DPF regarding number of
particles and 3 times less regarding mass of particles
(4.4±3.5 ×1014 vs. 2.1 ±1.0 ×1015 kg−1and 206 ±175
vs. 696±398 mg kg−1).
Regarding number of particles, only buses without DPF
were having EFs above the average EF of all tested vehi-
cles (see Fig. 3). The largest scatter in EFPN was, however,
obtained for the CNG-fuelled buses. Out of the 15 highest
PN-emitting buses, there were five gas buses (in total 7 CNG
buses were tested) and 13 had no DPF installed. Regard-
ing mass of particles, vehicles emitting above the average
EFPM of all tested buses belonged to all Euro classes, ex-
cept for buses representing Euro V with EGR and the CNG-
fuelled buses. The 15 highest PM-emitting buses were only
diesel-fuelled buses; 12 had no DPF and four of the total five
tested Euro IV with EGR buses were among these vehicles.
The higher masses obtained for EGR-equipped buses with-
out DPF may be due to the decrease in oxygen content when
some of the exhaust gas is re-circulated, which favours soot
formation (Seinfeld and Pandis, 1998; Maricq, 2007).
For comparison, modelled values of EFPN and EFPM us-
ing the HBFA 3.1 model are shown in Table 2. The mod-
elled values are generally significantly lower than the mea-
sured values. A possible explanation for this can be different
driving modes, acceleration versus route, including start and
stops but also constant speed mode. As indicated by Table 3,
EFPN/PM was generally lower for constant speed mode com-
pared to acceleration. Modelled EFPN was the lowest for
CNG buses and highest for diesel buses, whereas the oppo-
site was found in this study. A reason for this can be that the
particle number emissions that the HBEFA model is based on
often follow the PMP protocol, involving heating the particle
sample to 300 ◦C, and the CNG particles are suggested to be
volatile (Jayaratne et al., 2012).
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5344 ˚
A. M. Hallquist et al.: Particle and gaseous emissions from individual diesel and CNG buses
For the constant speed mode higher EFs were also gener-
ally obtained for buses without DPF. However, too few CNG
buses were analysed in this driving mode to make a compar-
ison between EFs for CNG buses and diesel buses.
Table 4 is a summary of the average EFPN and EFPM for
diesel buses with and without DPF and for CNG buses ob-
tained in this study (recalculated tokm−1) and a compari-
son to other studies. Generally, the average EFs obtained for
number of particles are within the reported ranges for diesel
buses but somewhat higher for the CNG-fuelled buses. The
average EFPM measured for diesel buses in this study are also
within the ranges reported in other studies. In Table 4 most
EFPM data is for larger particle size ranges. However, as most
particles related to road traffic combustion are below 560nm,
as is shown in Figs. 4 and 5, the particle size range used in
this study is comparable to PM10 and PM2.5. However, im-
portant to note is that road measurements of PM10 and PM2.5
can include non-combustion-related particle emissions, e.g.
re-suspension, and can hence be higher. It is a large varia-
tion in the reported data regarding the mass emitted for CNG
buses and the data reported in this study are similar to results
by Jayaratne et al. (2009) and Nylund et al. (2004).
In Lopez et al. (2009) a Euro IV diesel-fuelled bus
equipped with EGR and DPF and a Euro IV diesel-fuelled
bus equipped with SCR were analysed for a full driv-
ing cycle for which EFPM were determined to be 49±1
and 73±4 mg vehicle−1km−1, respectively. In this study
no Euro IV with SCR were studied, but Euro V were
studied, and the average EFPM for these buses (when ex-
cluding one extreme) was 68±11 mg vehicle−1km−1. Two
Euro IV diesel-fuelled buses equipped with EGR and DPF
were tested: one gave similar EFPM to Lopez et al. (2009),
55 mg vehicle−1km−1, and the other significantly higher
EFPM, 201 mg vehicle−1km−1.
The data presented in this study (Table 3) is a reflection of
the true variation in an in-use regional bus fleet, where the
variation found between similar buses (e.g. regarding fuel
type and after-treatment technology) within the same Euro
class can be due to engine specifics, maintenance and mal-
function.
3.3 Size-resolved EF, number and mass
In Fig. 4, size-resolved EFPN for each bus class in the ac-
celerating mode are shown, i.e. diesel buses with (Fig. 4a)
and without (Fig. 4b) DPF and CNG buses (Fig. 4c). All
classes show more or less a unimodal number size dis-
tribution. Diesel buses emit larger particles compared to
CNG buses, peak diameter 70–90nm and 10 nm, respec-
tively, which is similar to results reported in Jayaratne
et al. (2009) (80–90 nm and 10–12 nm, respectively). The
lack of larger particles in the emissions from CNG-fuelled
buses decreases the available surface area, hence favour-
ing nucleation over adsorption/condensation of supersatu-
rated vapours. This enhanced nucleation is one reason for
4.0×10
14
8.0×10
14
1.2×10
15
1.6×10
15
5 50 500
dEF
PN
/dlogDp (# kg
-1
)
Dp (nm)
a)
1.0×10
15
2.0×10
15
3.0×10
15
4.0×10
15
5.0×10
15
5 50 500
dEF
PN
/dlogDp (# kg
-1
)
Dp (nm)
b)
5.0×10
15
1.0×10
16
1.5×10
16
2.0×10
16
2.5×10
16
3.0×10
16
3.5×10
16
5 50 500
dEF
PN
/dlogDp (# kg
-1
)
Dp (nm)
c)
Fig. 4. Size-resolved average EFPN for diesel buses (Euro III–V)
with DPF (a) and without DPF (b) and for CNG buses (c) for
the driving mode acceleration. Solid lines represent averages and
dashed lines the statistical 95 % confidence interval. For the data
presented in graph (b) one bus (no. 34) was excluded showing much
higher size-resolved EFPN and with a peak size of ∼17 nm.
the larger average particle number emissions for the tested
CNG buses (Kumar et al., 2010). The mass size distribution
shows that the diesel engines in the accelerating mode pri-
marily emit particles with a diameter of ∼150 nm and that
CNG buses exhibit on average a bimodal mass size distribu-
tion with one mode peaking at about 25 nm and another at
∼125 nm (Fig. 5).
Atmos. Chem. Phys., 13, 5337–5350, 2013 www.atmos-chem-phys.net/13/5337/2013/

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A. M. Hallquist et al.: Particle and gaseous emissions from individual diesel and CNG buses 5345
Table 4. Comparison of emission data for particle number and mass from present study with selected literature data.
PN
Ref Dp range
nm
Speed
km h−1Vehicle type Method Instrument EFPN
# vechicle−1km−1
1014
This study 5.6–560 acc. bus diesel road EEPS 1.4±1.1a
5.6–560 acc. bus diesel road EEPS 6.5 ±3.2b
5.6–560 acc. bus CNG road EEPS 40±29
Beddows and Harrison (2008) >7 HDV aggregated CPC 7.06
Birmili et al. (2009) 10–500 75–90 HDV CFD TDMPS 29.6 ±3.5
Corsmeier et al. (2005) 30–300 85 HDV box model 7.8
Jayaratne et al. (2010) >5 80 bus diesel dynamoneter CPC 1.71
Jayaratne et al. (2010) >5 80 bus CNG dynamoneter CPC 5.4
Jayaratne et al. (2009) 5–160 25–100 %cbus diesel dynamometer SMPS 1.2–18
Jayaratne et al. (2009) 5–160 25–100 %cbus CNG dynamometer SMPS 1.0–14
Jones and Harrison (2006) 11–450 <50 HDV street canyon SMPS 6.36
Keogh et al. (2010) nsdHDV statisticaleCPC 65 (60.19–69.81)
Keogh et al. (2010) ns HDV statisticaleSMPS 3.08
Morawska et al. (2008) 10–30 HDV review 2.14–37.8
Morawska et al. (2008) 18–50 HDV review 1.55–8.2
Morawska et al. (2008) 18–100 HDV review 1.7–10.5
Morawska et al. (2008) 30–100 HDV review 3.19
Wang et al. (2010) 10–700 90–110 HDV road DMPS 17.5
Wang et al. (2010) 10–700 0–50 HDV road DMPS 22.1
Keogh et al. (2010) ns LDV statisticaleCPC 3.63
PM
Ref PM(x) Speed
km h−1Vehicle type Method Instruments EFPM
mg vehicle−1km−1
This study 5.6–560 acc. bus diesel road EEPS 64±54a
5.6–560 acc. bus diesel road EEPS 215 ±123b
5.6–560 acc. bus CNG road EEPS 12±9
Clark et al. (1999) PM d.c bus diesel dynamometer ns 190–1450
Clark et al. (1999) PM d.c bus CNG dynamometer ns 4–100
Jayaratne et al. (2009) PM10 25-100 %cbus diesel dynamometer DustTrak 46.5–668.6
Jayaratne et al. (2009) PM10 25–100 %cbus CNG dynamometer DustTrak 0.01–1.3
Jones and Harrison (2006) PM10 <50 HDV street canyon TEOM 370 ±32
Jones and Harrison (2006) PM2.5<50 HDV street canyon TEOM 179 ±22
Keogh et al. (2010) PM10 Ns HDV statisticaleseveral 538
Keogh et al. (2010) PM2.5Ns HDV statisticaleseveral 302 (236–367)
Lanni et al. (2003) PM d.cfbus diesel dynamometer gravimetric 72
Lanni et al. (2003) PM d.c bus CNG dynamometer gravimetric 86
Lopez et al. (2009) PM d.c bus EIV EGR +DPF on-board MAHA 49 ±1g
Lopez et al. (2009) PM d.c bus EIV SCR on-board MAHA 73 ±4g
Nylund et al. (2004) PM d.c bus diesel dynamometer ns 20–170
Nylund et al. (2004) PM d.c bus CNG dynamometer ns 5–10
Ullman et al. (2003) PM d.c bus diesel dynamometer gravimetric 296
Ullman et al. (2003) PM d.c bus CNG dynamometer gravimetric 84
Wang et al. (2010) PM2.590–110 HDV roadhTEOM 233 ±18
Wang et al. (2010) PM2.50–50 HDV roadiTEOM 628 ±50
Wang et al. (2010) PM10 90–110 HDV roadhTEOM 1087 ±68
Wang et al. (1997) PM d.c bus diesel dynamoneter gravimetric 1960
Wang et al. (1997) PM d.c bus CNG/LNG dynamometer gravimetric 48
Keogh et al. (2010) PM10 Ns LDV statisticaleseveral 153
Keogh et al. (2010) PM2.5Ns LDV statisticaleseveral 33
aDPF
bno DPF
c% of max engine power
dns = not stated
ebased on 667 EFs
fd.c = driving cycle
gsd
hhighway
iurban
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5346 ˚
A. M. Hallquist et al.: Particle and gaseous emissions from individual diesel and CNG buses
Table 5. Comparison of emission data for NOx, VOC and CO from present study with selected literature data.
Ref Speed
km h−1Vehicle type Method EFNOx
g km−1EFVOC
g km−1EFCO
g km−1
This study acc Euro III road 5 ±3<4a5±5
acc Euro IV road 4 ±2<4a5±5
acc Euro V road 11 ±3<4a3±1
acc CNG bus road 21 ±14 <4a<3
Chen et al. (2007) <85 HDV on-board 6.54 1.88 4.96
Clark et al. (1999) d.cbbus diesel dynamometer 28.5–37.5 0.1–0.6c2.5–18.0
Clark et al. (1999) d.c bus CNG dynamometer 10.9–23.8 16.9–32.2c0.2–13.3
Corsmeier et al. (2005) 85 HDV on-road 6.86 ±1.57 – –
Jayaratne et al. (2009) 25–100% bus diesel dynamometer 6.7–18 – –
Jayaratne et al. (2009) 25–100% bus CNG dynamometer 5.5–32 – –
Jones and Harrison (2006) <50 HDV street canyon 5.19 – –
Kristensson et al. (2004) 75 HDV tunnel 8.0±0.8 – –
Lanni et al. (2003) d.c bus diesel DPF dynamometer 38.4 0.1 0.2
Lanni et al. (2003) d.c bus CNG dynamometer 68.9 93.9 76.4
Lopez et al. (2009) d.c bus EIV EGR+ DPF on-board 6.925 0.068c0.250
Lopez et al. (2009) d.c bus EIV SCR on-board 6.121 0.053c1.716
Nylund et al. (2004) d.c bus diesel dynamometer 8–9 0.05–0.4c–
Nylund et al. (2004) d.c bus CNG dynamometer 2–7 0.25–2c–
Ullman et al. (2003) d.c bus diesel dynamometer 22.7 0.6 2.8
Ullman et al. (2003) d.c bus CNG dynamometer 26.1 15.0 7.7
Wang et al. (2010) 90 HDV on-road 9.8±0.29 – –
Wang et al. (2010) 0–50 HDV on-road 11.9 ±0.59 – –
Wang et al. (2008) bus calculatedd18.19 3.71 37.15
Wang et al. (2008) truck calculatedd9.3 2.99 34.79
aIn this study HC
bd.c = driving cycle
cTHC
dcalculated from emission inventory
For the analysis of the average size-resolved EFPN/PM for
buses without DPF (Figs. 4b and 5b), one bus (no. 34) was
excluded showing much higher size-resolved EFPN and with
a peak size of ∼17 nm. For this bus the average size-resolved
EFPM was bimodal with peak sizes of ∼30 nm and ∼190 nm.
The reason for this discrepancy is not known but could be due
to maintenance or malfunction of this particular bus.
For the constant speed mode the characteristic bimodal
number size distributions were obtained for the diesel
buses with and without DPF, with one mode peaking at
∼10 nm (nucleation mode) and the other at ∼60 nm (soot
mode/accumulation mode) (Fig. 6) (Maricq, 2007). The rea-
son for the different average number size distributions be-
tween accelerating and constant speed mode may be more
available surface area in the accelerating mode, hence favour-
ing adsorption/condensation over nucleation. In acceleration
from standstill the engine load is close to its maximum, and
Jayaratne et al. (2009) also obtained a unimodal number size
distribution for a diesel bus at 100% load.
3.4 Comparison of EFpart and EFgas (NOx, HC and CO)
The highest NOxvalues were obtained for the CNG buses
compared to all the other Euro classes of diesel buses; how-
ever, the scatter was largest for the CNG buses as well
(41±26 g kg−1) (Fig. 7), which is in accordance with Ek-
str¨
om et al. (2005). Possible reasons for this variability may
be vehicle maintenance and variations in the CNG compo-
sition (Shorter et al., 2005; Ayala et al., 2002). The EF for
NOxranged from 4 to 21 g km−1depending on Euro class,
which is in good agreement with reported values for HDVs
and buses in the literature (Table 5). In comparison with the
HBEFA 3.1 model, the measured values for EFNOxare on
average lower for all the tested Euro classes but within the
95 % confidence interval for the Euro V with SCR and EEV
buses. However, for some SCR-equipped buses and CNG
buses higher EFNOxvalues were measured. One reason for
some of the high values regarding SCR may be that it is
critical that the exhaust temperature is high enough for the
SCR to work properly.
In Fig. 8a there is a comparison of EFpart and EFNOx; both
mass and number of particles show an anti-relationship with
NOx, which is especially true when no DPF is installed. In a
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A. M. Hallquist et al.: Particle and gaseous emissions from individual diesel and CNG buses 5347
0
100
200
300
400
500
600
700
800
900
1 000
5 50 500
dEF
PM
/dlogDp (mg kg
-1
)
Dp (nm)
a)
0
500
1 000
1 500
2 000
2 500
3 000
3 500
5 50 500
dEF
PM
/dlogDp (mg kg
-1
)
Dp (nm)
b)
0
10
20
30
40
50
60
70
80
90
5 50 500
dEF
PM
/dlogDp (mg kg
-1
)
Dp (nm)
c)
Fig. 5. Size-resolved average EFPM for diesel buses (Euro III–V)
with DPF (a) and without DPF (b) and for CNG buses (c) for
the driving mode acceleration. Solid lines represent averages and
dashed lines the statistical 95 % confidence interval. For the data
presented in graph (b) one bus (no. 34) was excluded showing a
bimodal EFPM and with peak sizes of ∼30 nm and ∼190 nm.
diesel engine there is a compromise between emissions of
NOxand emissions of particles (Clark et al., 1999), as is
demonstrated by the data in Fig. 8a. For the CNG-fuelled
buses no such trend was observed.
Generally the emission of CO from a diesel engine is low
as the combustion is carried out in an air-rich environment.
This can be seen in the data for the tested buses, where the
CO concentrations for many of the buses are below the de-
2.0×10
14
4.0×10
14
6.0×10
14
8.0×10
14
5 50 500
dEF
PN
/dlogDp (# kg
-1
)
Dp (nm)
a)
0.5×10
15
1.5×10
15
2.5×10
15
3.5×10
15
5 50 500
dEF
PN
/dlogDp (# kg
-1
)
Dp (nm)
b)
Fig. 6. Size-resolved average EFPN for diesel buses (Euro III–V)
with DPF (a) and without DPF (b) for the driving mode constant
speed mode. Solid lines represent averages and dashed lines the sta-
tistical 95 % confidence interval. Dotted lines represent averages for
the accelerating mode.
0
10
20
30
40
50
60
70
80
90
100
01234567
EF
NOx
g (kg fuel)
-1
w DPF
wo DPF
HBEFA
Euro class average
Average
E_III E_III
w. SCR
E_IV
w. EGR
E_V
w. SCR
E_V
w. EGR
EEV EEV
w. EGR
Fig. 7. EFNOxfor all the buses studied divided into Euro class.
Without DPF (white circles), with DPF (red circles), average of
all represented Euro classes (dashed line), average of an individ-
ual represented Euro class (solid line). Crosses are EFs obtained by
the HBEFA 3.1 model.
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5348 ˚
A. M. Hallquist et al.: Particle and gaseous emissions from individual diesel and CNG buses
12 ˚
A. M. Hallquist et al.: Particle and gaseous emissions from individual diesel and CNG buses
2.0×10
15
4.0×10
15
6.0×10
15
8.0×10
15
1.0×10
16
0
500
1 000
1 500
2 000
2 500
3 000
3 500
0 20 40 60 80
EFPM mg (kg fuel)-1
EFPN # (kg fuel)-1
EF
NOx
g (kg fuel)
-1
EF
PN
Euro III w/wo DPF
EF
PN
Euro IV w/wo DPF
EF
PN
Euro V w/wo DPF
EF
PM
Euro III w/wo DPF
EF
PM
Euro IV w/wo DPF
EF
PM
Euro V w/wo DPF
2.0×1015
4.0×1015
6.0×1015
8.0×1015
1.0×1016
0
500
1 000
1 500
2 000
2 500
3 000
3 500
0 10 20 30 40 50 60
EFPM mg (kg fuel)-1
EFPN # (kg fuel)-1
EFCO g (kg fuel)-1
Fig . 8. EF PN (circles) and EF PM (triangles) v ersus the EF for NO x
(a) and v ersus the EF for CO (b) . Euro III (blue symbols), Euro IV
(red symbols) and Euro V (green symbols). Filled symbols repre-
sent b uses with DPF installed and unfilled symbols no DPF .
Generally the emission of CO from a diesel engine is lo w
as the comb ustion is carried out in an ai r -rich en vironment.
This can be seen in the data for the tested b uses, where the
CO concentrations for man y o f the b uses are belo w the de-
tection lim it of the instrument (i.e. belo w 18 g (kg fuel) −1).
Ho we v er , for six of the b uses CO concentrations were mea-
sured (3 times the std of the noise). In Fig. 8b th e EF PM
and E F CO are c ompared, and as is sho wn a positi v e relation-
ship betwe en EF PM and EF CO w as obse rv ed. High CO con-
centration is an indication of incomplete comb ustion, hence
f a v ouring soo t formation, i.e. high EF PM . Re g arding number
of particles there is also a positi v e relationship, ho we v er less
profound (Fig. 8b) than the relationship between EF PM and
EF CO .
The CO e missions are also influenced by DPF . The a v-
erage EF CO for the diesel b uses with DPF tested in this
study , when assi gning v alues belo w 6 (1 times the std of
the noise) to 6 g (kg fuel) −1, were 11 g (kg fuel) −1(10 b uses
in total). F or the b uses without DPF the a v erage EF CO w as
14 g (kg fuel) −1(18 b uses in total); hence DPF is not only
reducing particles b ut CO as well, as reported in A yala et
al. (2002) and Lanni et al. (2001). F or the tested b uses, DPF
had no statistical significant ef fect on the amount of NO x
emitted, which also is in agreement with results reported by
A yala et al. (2002).
Re g arding total h ydrocarbon (H C), emissions abo v e the
detection l imit (14 g (kg fuel) −1) were not found for an y of
the b uses in this study . Compared to the literature data sho wn
in T able 5, v alues abo v e the detectio n limit of our instrumen-
tation were only reported for som e CNG-fuelled b uses.
4 Atmospheric implications and conclusions
The me thod of using a high time resolution particle instru-
ment and CO 2concentration as a tracer of the comb ustion
source for determining EF PN and EF PM from indi vidual v e-
hicles for real-w o rld dilution sho wed to be v ery successful
re g arding reprodu cibility , costs and number of v ehicl es stud-
ied. This method enabled measuremen ts of not only particle
number b ut also size, as well a s mass.
Compressed natural g as b uses are more adv antageous re-
g arding emissions of particle mas s compared to diesel b uses.
Ho we v er , in accelerating mode, generally CNG b uses emit
more particles by number compared to diese l-fuelled b uses,
and these particles are smaller ( Dp∼10 nm compared to
∼80 nm) and presumably more v olatile. The f act that CNG
b uses emit high number of particles in accelera ting mode,
e.g. at b us stops where man y people may be standing w aiting
for b us es, is an important aspect. Ho we v er , the health impact
of these pa rticles v ersus diesel par ticles is still a matter of
discussion.
This study sho ws that DPF mark edly reduces emissions
of particles both by mass and number as well as CO emis-
sions also for real-w orld dilution. Reducing the number of
soot mode particles does not cause a se v ere increase in nu-
cleation mode particles as is the case for some of the tested
CNG-fuelled v ehicles without particle filter .
There w as a lar ge v ariation in NO xemissions from the
tested SCR-equip ped b uses. This is most lik ely due to dif fer -
ences in engine and e xhaust temperature, which in fluence the
ef ficienc y of the SCR to reduce N O xemissions. In particular
this has implications for NO 2population e xposur e in urban
areas and is thus a health issue that needs to be in v estig ated
further .
Compared to other types of v ehicles, the a v erage EF PN for
a diese l-fuelled b us without DPF is v ery similar to results
obtained for a diesel passenger car without DPF (Hak et al.,
2009) when looking at the number of parti cles emitted per kg
fuel used (2.1 ±1.0 ×10 15 kg −1vs. 2.1 ±0.3 ×10 15 kg −1).
The mean EF PN for DPF-equipped diesel-fuelled b uses were
in the same order as an old pe trol car (4.4 ±3.5 ×10 14 kg −1
vs. 4.2 ±3.0 ×10 14 kg −1) (Hak et al., 2009). Ho we v er , w hen
taking fuel consumption into con sideration, there w as a lar ge
dif ference. Di esel-fuelled b uses without DPF are then emit-
ting more part icles per km −1than a diesel passenger car
without DPF , whereas DPF-equipped diesel b uses are similar
to a d iesel passenge r car without DPF (6.5 ±3.2 ×10 14 and
1.4 ±1.1 ×10 14 km −1vs. 1.2 ±0.2 ×10 14 km −1). On a v e r -
age the CNG-fuelled b us i n v estig ated in this study emitted a
Atmos. Chem. Ph ys., 13, 1– 14 , 2013 www .atmos-chem -ph ys.net/13/1/2013/
˚
A. M. Hallquist et al.: P artic le and gaseous emissions fr om indi vidual diesel and CNG b uses 7
a)
b)
Fig. 8. EFPN (circles) and EFPM (triangles) versus the EF for NOx
(a) and versus the EF for CO (b). Euro III (blue symbols), Euro IV
(red symbols) and Euro V (green symbols). Filled symbols repre-
sent buses with DPF installed and unfilled symbols no DPF.
tection limit of the instrument (i.e. below 18g (kg fuel)−1).
However, for six of the buses CO concentrations were mea-
sured (3 times the std of the noise). In Fig. 8b the EFPM
and EFCO are compared, and as is shown a positive relation-
ship between EFPM and EFCO was observed. High CO con-
centration is an indication of incomplete combustion, hence
favouring soot formation, i.e. high EFPM. Regarding number
of particles there is also a positive relationship, however less
profound (Fig. 8b) than the relationship between EFPM and
EFCO.
The CO emissions are also influenced by DPF. The av-
erage EFCO for the diesel buses with DPF tested in this
study, when assigning values below 6 (1 times the std of
the noise) to 6 g (kg fuel)−1, were 11 g (kg fuel)−1(10 buses
in total). For the buses without DPF the average EFCO was
14 g (kg fuel)−1(18 buses in total); hence DPF is not only
reducing particles but CO as well, as reported in Ayala et
al. (2002) and Lanni et al. (2001). For the tested buses, DPF
had no statistical significant effect on the amount of NOx
emitted, which also is in agreement with results reported by
Ayala et al. (2002).
Regarding total hydrocarbon (HC), emissions above the
detection limit (14 g (kg fuel)−1) were not found for any of
the buses in this study. Compared to the literature data shown
in Table 5, values above the detection limit of our instrumen-
tation were only reported for some CNG-fuelled buses.
4 Atmospheric implications and conclusions
The method of using a high time resolution particle instru-
ment and CO2concentration as a tracer of the combustion
source for determining EFPN and EFPM from individual ve-
hicles for real-world dilution showed to be very successful
regarding reproducibility, costs and number of vehicles stud-
ied. This method enabled measurements of not only particle
number but also size, as well as mass.
Compressed natural gas buses are more advantageous re-
garding emissions of particle mass compared to diesel buses.
However, in accelerating mode, generally CNG buses emit
more particles by number compared to diesel-fuelled buses,
and these particles are smaller (Dp∼10nm compared to
∼80 nm) and presumably more volatile. The fact that CNG
buses emit high number of particles in accelerating mode,
e.g. at bus stops where many people may be standing waiting
for buses, is an important aspect. However, the health impact
of these particles versus diesel particles is still a matter of
discussion.
This study shows that DPF markedly reduces emissions
of particles both by mass and number as well as CO emis-
sions also for real-world dilution. Reducing the number of
soot mode particles does not cause a severe increase in nu-
cleation mode particles as is the case for some of the tested
CNG-fuelled vehicles without particle filter.
There was a large variation in NOxemissions from the
tested SCR-equipped buses. This is most likely due to differ-
ences in engine and exhaust temperature, which influence the
efficiency of the SCR to reduce NOxemissions. In particular
this has implications for NO2population exposure in urban
areas and is thus a health issue that needs to be investigated
further.
Compared to other types of vehicles, the average EFPN for
a diesel-fuelled bus without DPF is very similar to results
obtained for a diesel passenger car without DPF (Hak et al.,
2009) when looking at the number of particles emitted per kg
fuel used (2.1 ±1.0 ×1015 kg−1vs. 2.1 ±0.3 ×1015 kg−1).
The mean EFPN for DPF-equipped diesel-fuelled buses were
in the same order as an old petrol car (4.4±3.5×1014 kg−1
vs. 4.2 ±3.0×1014 kg−1) (Hak et al., 2009). However, when
taking fuel consumption into consideration, there was a large
difference. Diesel-fuelled buses without DPF are then emit-
ting more particles per km−1than a diesel passenger car
without DPF, whereas DPF-equipped diesel buses are similar
to a diesel passenger car without DPF (6.5 ±3.2 ×1014 and
1.4±1.1 ×1014 km−1vs. 1.2 ±0.2 ×1014 km−1). On aver-
age the CNG-fuelled bus investigated in this study emitted a
higher number of particles than a diesel passenger car both
with respect to kg fuel burnt and per km driven.
In the data the typical trade-off trend between emission
of NOxand particles (PN and PM) was observed, especially
for vehicles without DPF, as well as a positive relationship
between emissions of CO and PM/PN.
Atmos. Chem. Phys., 13, 5337–5350, 2013 www.atmos-chem-phys.net/13/5337/2013/

˚
A. M. Hallquist et al.: Particle and gaseous emissions from individual diesel and CNG buses 5349
The data presented in this study demonstrate the variation
in gas and particle emissions of the in-use fleet of a regional
public bus service, where variations found between similar
buses can be due to engine specifics, maintenance or mal-
function.
Acknowledgements. This work was financed by V¨
asttrafik, the
Foundation for the Swedish Environmental Research Institute, and
the Graduate School Environment and Health, the University of
Gothenburg. The drivers and the personnel at the measurement sites
are gratefully acknowledged for their assistance and hospitality.
Donald H. Stedman and Gary Bishop of Denver University are
acknowledged for valuable input regarding the RSD evaluation.
Edited by: T. Pet¨
aj¨
a
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