2.1 Field sampling site

Pollutant emissions from HDTs were measured at a roadside location in Gothenburg, Sweden (Fig. 1). The HDTs passed the sampling location with an average speed of 27 km h⁻¹ and acceleration of 0.7 km h⁻¹ s⁻¹ on a slight uphill slope (∼2^∘). Under such uphill driving conditions, vehicles are expected to emit higher levels of pollutants than during downhill and cruise driving. This will be further examined in Sect. 3.3.

Figure 1(a) Sampling site at the roadside in Gothenburg, Sweden, (b) schematic of the experimental set-up. HDT (heavy-duty truck), RSD (remote sensing device), CPC (condensation particle counter), EEPS (Engine Exhaust Particle Sizer™ spectrometer), TD (thermodenuder), and HR-ToF-CIMS^* (high-resolution time-of-flight chemical ionization mass spectrometer, the data from the HR-ToF-CIMS will be presented elsewhere) as well as examples of signals from three passing HDTs are shown. Concentrations of CO₂, particle number (PN), non-volatile PN (black line), particle mass (PM), non-volatile PM (black line), and NO_x from (c) a typical Euro III HDT, (d) a typical Euro VI HDT, and (e) a Euro VI HDT with low PN emission are shown.

Download

2.2 Air sampling

The sampling of the emissions was conducted in line with Hallquist et al. (2013), i.e. extractive sampling of passing HDT plumes. Air was continuously drawn through a flexible copper tube to the instruments inside a container. A similar experimental set-up was previously applied to on-road bus emission measurements (Liu et al., 2019). Particles were measured by an EEPS (Engine Exhaust Particle Sizer™, model 3090, TSI Inc.) in the size range of 5.6–560 nm with high time resolution (10 Hz), while total particle number was measured by a butanol-based condensation particle counter (CPC, model 3775, TSI Inc., 50 % cut-off diameter of 4 nm). Particle numbers measured by the two instruments showed a good correlation (R²=0.73) (Fig. S1 in the Supplement). A second EEPS measured the outflow of a TD (thermodenuder, Dekati Ltd.), enabling estimations of particle volatility. The data were corrected for size-dependent losses in the TD. The temperature inside the TD heating zone was set to 250 ^∘C with a residence time of ∼0.6 s, which is generally sufficient to evaporate nearly all the organics and sulfates from the particles (Huffman et al., 2008). However, organics with extremely low volatility (organic saturation mass concentration, $C^{*}<{\mathrm{10}}^{-\mathrm{5}}$ µg m⁻³ at 298 K) may still be retained even at this high temperature (Gkatzelis et al., 2016). Thus, in this study, we define the “non-volatile components” as particle components that remain after passing through the TD operating at 250 ^∘C. Differences in counting efficiencies between the two EEPS were accounted for by size-dependent correction factors (typically less than 10 %), which were retrieved by simultaneous sampling of ammonium sulfate particles by both EEPS instruments and direct comparison of their measured size distributions (Fig. S2). Black carbon (BC) and the mixture of BC and brown carbon (BrC) were measured by an Aethalometer at 880 and 370 nm, respectively (model AE33, Magee Scientific Inc.). The determination of particle mass concentrations by the integrated particle size distribution (IPSD) method requires information on particle density. Particle sphericity and unit density were assumed due to a lack of detailed knowledge about the chemical composition of the emitted particles. Figure S3 shows that there is a good linear relationship at EF_PM values larger than 1 mg (kg fuel)⁻¹ between the BC mass measured by the Aethalometer and the non-volatile particle mass measured by the EEPS, but, assuming sphericity and unit density, the EEPS mass is lower, which indicates a potential underestimation of the effective non-volatile particle density. Compared to the EEPS, the detection limit of the Aethalometer is 5 times higher, which may influence the correlation between BC and particle mass (PM) at low mass loading conditions (Fig. S3). There have been several studies on the morphology and density of combustion-generated particles and its detailed dependence on combustion and dilution condition (e.g. Maricq and Ning, 2004; Ristimaki et al., 2007; Liu et al., 2009; Zheng et al., 2011; Quiros et al., 2015). However, to be consistent, and to compare with a majority of previously reported data, unity density was used for further discussion and comparisons. CO₂ was measured by a non-dispersive infrared gas analyser (LI-840, LI-COR Inc.). NO_x and NO were measured simultaneously by two separate chemiluminescent analysers (model 42i, Thermo Scientific Inc.), and the NO₂ concentration was calculated from the difference between the NO_x and NO concentrations. SO₂ was measured by a pulsed fluorescence gas analyser (model 43c, Thermo Scientific Inc.). A remote sensing device (RSD) (OPUS Inspection Inc.) was used to measure the gaseous emission factors of CO, NO_x, and hydrocarbon (HC). Briefly, the instrument was set up with a transmitter and a receiver on the same side of the truck lane and a reflector on the opposite side. Co-linear beams of infrared (IR) and ultraviolet (UV) light are emitted and cross-reflected through the plume, and light attenuation data related to respective pollutant concentrations are measured. Gas pollutant concentrations were determined relative to CO₂ as measured by the RSD. NO_x and NO measured by the gas analysers and the RSD were in agreement (R²=0.53 and 0.66, respectively; Figs. S4, S8a). The high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS) shown in Fig. 1 was used to characterize the chemical composition of organic and inorganic compounds in the gas and particle phases (emitted from the HDTs). However, the extensive chemical characterization is beyond the scope of this work and will be presented elsewhere.

A schematic of the experimental set-up is given in Fig. 1 along with some examples of typical temporal profiles of pollutant concentrations in the plumes from Euro III and Euro VI HDTs. A camera at the roadside recorded the HDT plate numbers, which were used for identification and to obtain engine Euro type information. All the instruments were at least operated at 1 Hz sampling frequency to capture rapidly changing concentrations during the passage of a HDT, which is sufficiently fast to measure pollutant concentration peaks (typically 5 to 20 s in duration) as shown in Fig. 1c–e. In general, the duration of a peak was around 5 s. It was slightly longer for NO_x (around 20 s), which limits measurements of high-frequency passages. Euro III HDTs typically emitted a significant PN, PM, NO_x, and non-volatile components (Fig. 1c). More than 95 % of Euro III, Euro IV, Euro V, and EEV HDTs had measurable particle emission signals. Significant differences in low particle and gaseous emissions were evident for Euro VI HDTs (Fig. 1d and e).

2.3 Data analysis

The exact time of individual HDTs passing the sampling inlet was determined from the camera recordings, and the associated plume pollutant concentrations were integrated to calculate corresponding pollutant emission factors of individual HDTs as described by Hallquist et al. (2013). Emissions of gases and particles from individual HDTs were normalized by the CO₂ concentration to compensate for different degrees of dilution during sampling (Janhäll and Hallquist, 2005). CO₂ peak concentrations exceeding 4 times the standard deviation of the background signal were used as the base criterion for successful plume capture. Peaks in NO_x, PN, PM, and BC concentrations concurrent with that of CO₂ signify the presence of co-emitted pollutants in a HDT plume. Emission factors (EFs) of gaseous and particle emissions for individual HDTs can then be expressed in units of amount of pollutant emitted per kilogram fuel burned based on the carbon balance method (Ban-Weiss et al., 2009; Hak et al., 2009):

where EF_pollutant is the emission factor of the respective pollutant. The time interval of t₁ to t₂ represents the period when the instruments measured the concentration of an entire pollutant peak from an individual HDT (see Fig. 1c–e). Dallmann et al. (2012) and Preble et al. (2015) used the concepts of inflection points to identify t₁ and t₂. In our study, t₁ and t₂ were determined similarly: t₁ is the point before the pollutant concentration intensity increases abruptly and t₂ is when the intensity becomes relatively flat and undistinguishable compared to background levels. It is noted that the integrated peak intensity is insensitive to the exact location of t₂ since the added integrated signals at or beyond this point are fluctuating around zero. t₁ and t₂ were determined independently of each pollutant peak to account for differences in the time response of individual instruments to the exhaust plume. ${\int }_{t_{\mathrm{1}}}^{t_{\mathrm{2}}}\left(\right[\mathrm{pollutant}{]}_t-{\left[\mathrm{pollutant}\right]}_{t_{\mathrm{1}}})\mathrm{d}t$ and ${\int }_{t_{\mathrm{1}}}^{t_{\mathrm{2}}}\left(\right[{\mathrm{CO}}_{\mathrm{2}}{]}_t-{\left[{\mathrm{CO}}_{\mathrm{2}}\right]}_{t_{\mathrm{1}}})\mathrm{d}t$ are the changes in concentration of a pollutant and CO₂, respectively, during this time interval. EF${}_{{\mathrm{CO}}_{\mathrm{2}}}$ of 3158 g (kg diesel fuel)⁻¹ was used as the emission factor of CO₂, assuming complete combustion and a carbon content of 86.1 % as given in Edwards et al. (2014). Emission factors for plumes with pollutant concentrations lower than our set detection limit (4 times the standard deviation of the pollutant background signal) were replaced by the minimum value among all recorded emission factors (EF_min) rather than being omitted to avoid inflating emissions from low-emitting HDTs.

3.1 Fleet compositions

A total of 675 resolved plumes from 556 individual HDTs for the carriage of goods with weights exceeding 12 t were identified. There were 330 Swedish HDTs with Euro type information, 46 Swedish HDTs from which Euro type information was not available, and 180 foreign-licensed non-Swedish HDTs. Among the 330 Swedish trucks, Euro III, Euro IV, Euro V, EEV, and Euro VI HDTs accounted for 3 %, 5 %, 30 %, 5 %, and 57 %, respectively (Fig. S5).

3.2 Emissions variability

Differences in operating and ambient conditions may lead to differences in pollutant emission factors for the same HDT. In this study, we utilized measurement data from 55 HDTs which passed the sampling location repeatedly, yielding a total of 137 plumes. The average pollutant emission factors of each HDT plotted against the individual plume measurements of the corresponding HDT are shown in Fig. S6. In general, the emission factors of PM, non-volatile PN, and NO_x showed little variation (R²≥0.77) among multiple passages of the same HDT; however, higher variability was observed in the PN emissions. This is likely related to variations in the formation of nucleation mode particles from volatile compounds, which are more sensitive to driving (Zheng et al., 2014) and dilution conditions. In the following discussion, for HDTs with multiple passages, the average pollutant emission factors of all the detected plumes were used for that individual HDT.

3.3 Emissions factors (EFs) of particles and gaseous pollutants

Figure 2a and b show the box-and-whisker plots of PM and PN emission factors (EFs) for different Euro classes. Generally, both PM and PN emissions decreased with more stringent Euro emission standards, and especially for Euro VI where larger changes in emission characteristics were evident. These decreasing trends are statistically significant at the 95 % confidence interval (CI) using the Jonckheere–Terpstra test, a non-parametric test for trends in ordered groups. In addition to PM and PN, the emission trends of BC, NO_x, CO, and HC with respect to the level of stringency of Euro standards were statistically examined. Using Euro III HDTs (median EF_PM=586 mg (kg fuel)⁻¹) as a baseline, the median EF_PM for Euro IV, Euro V, EEV, and Euro VI HDTs have been reduced by 78.1 %, 86.1 %, 88.9 %, and 99.8 %, respectively. In particular, Euro VI HDTs have a median EF_PM of only 1.4 mg (kg fuel)⁻¹ (Fig. 2a). While it is noted that Euro III to Euro VI standard certifications are based on chassis dynamometer cycle measurements, the Euro VI regulations have started to include additional off-cycle and in-service conformity testing. The Euro emission standard of transient testing for heavy-duty engines gives emission limits as brake-specific emission factors, as mass (g) of a specific pollutant or number (particles) per kilowatt-hour (kWh). In order to enable a comparison with the Euro emission standard, the EFs in grams (or particles) per kilograms fuel were converted using a brake-specific fuel consumption (BSFC) of 231.5 g kWh⁻¹. This is the average value for the long-haul and regional delivery cycles of chassis dynamometer tests of a typical Euro VI truck (Rexeis et al., 2018). The uncertainty of the BSFC for different Euro class HDTs operating over a wide range of engine conditions is generally within 25 % (Mahmoudzadeh Andwari et al., 2017; He et al., 2017; Dreher and Harley, 1998; Heywood, 1988). Note that our measurements represent points of time similar to those in a cycle where the particle emissions can be most prominent. Looking at a whole cycle, this value will be averaged; hence, the results from our instantaneous plume measurements may represent an upper limit of the emissions. In Fig. 2a, the right y axis gives the EFs converted into units of g kWh⁻¹ and the Euro standards are shown as blue crosses.

Figure 2(a) EF_PM, (b) EF_PN, (c) EF_BC, (d) EF${}_{{\mathrm{NO}}_{\mathrm{2}}}$ and EF${}_{{\mathrm{NO}}_x}$ for Euro III to Euro VI and non-Swedish HDTs. Non-detectable pollutant emission signals for captured plumes have been replaced by EF_min. For box-and-whisker plots, the top and the bottom line of the box are 75th and 25th percentiles of the data, the red line inside the box is the median, and the top and bottom whiskers are 90th and 10th percentiles. Note that the median EF_BC of Euro VI HDTs overlaps with the bottom of the box. EF${}_{{\mathrm{NO}}_x}$ values in (d) are in NO₂ equivalents. HDTs with either EF${}_{{\mathrm{NO}}_{\mathrm{2}}}$ or EF${}_{{\mathrm{NO}}_x}$ lower than the detection limits of the instruments were removed in (d) for illustration purposes. Note that the comparison with the emission standard is only indicative as they are based on test cycle performance.

Download

In general, the measured median EF_PM values are lower than the Euro standards for all HDT types. In particular, the median EF_PM for Euro VI HDTs is more than 1 order of magnitude lower than the Euro standard regulation value since diesel particulate filters (DPFs) are required for these Euro VI HDTs to comply with PM and PN standards (Williams and Minjares, 2016). No information about potential retrofits of tested HDTs was available for the vehicles measured in this study. The effectiveness of DPFs in reducing particle emissions has been confirmed by various studies (Martinet et al., 2017; May et al., 2014; Mendoza-Villafuerte et al., 2017; Moody and Tate, 2017; Preble et al., 2015). For example, Bergmann et al. (2009) illustrated that post-DPF PM concentrations decreased by 99.5 % compared with pre-DPF for the New European Driving Cycle (NEDC). In real-world measurements, at least ∼90 % reductions in PM emissions compared to typical pre-DPF levels have been reported (Bishop et al., 2015). Euro emission standards for PM of Euro IV and Euro V heavy-duty diesel engines are the same, while the measured median EF_PM of the Euro V fleet was around 1.5 times lower than that of the Euro IV fleet. The control of diesel engine emissions typically requires a compromise between NO_x and particle emission reduction (Clark et al., 1999). The NO_x emission standard is more stringent for Euro V compared to Euro IV (a factor of 43 % lower), and hence Euro V HDTs are generally equipped with SCR or EGR to reduce NO_x. In contrast, Euro IV engines are rarely equipped with NO_x after-treatment systems, and thus they must achieve the NO_x emission limits by tuning the engine performance parameters at the expense of higher PM emissions (Preble et al., 2018; Van Setten et al., 2001). In each of the Euro III, Euro IV, Euro V, and EEV classes, 25 %–50 % of all the measured HDTs had an EF_PM higher than their corresponding Euro standards. As described previously, this comparison with the Euro standard is relative and indicative. The higher emissions from individual HDTs may indicate deterioration of engine performance due to wear caused by ageing, mileage accumulation, or inadequate maintenance. Our study shows that Euro VI HDTs generally have low PM emissions, but HDTs from older Euro classes frequently exceeded their PM emission limits, suggesting that improved maintenance and suitable retrofitting of older engines are needed.

For PN emissions, EF_PN shows an overall trend similar to EF_PM. However, a large data scatter was evident for Euro VI HDTs, likely due to the sensitivity of nucleation mode particle formation to changes in driving conditions (Fig. 2b). Zheng et al. (2014) reported high concentrations of nucleation mode particles under uphill driving conditions but low concentrations under cruise and downhill driving conditions. It is important to note that the median EF_PN of Euro VI HDTs was significantly lower than those from the other Euro type HDTs, which indicates efficient PM removal by the DPF without compromising on total PN emission. Nonetheless, the decrease in particle number in the accumulation mode removes particle surface area available for condensation, and therefore it favours nucleation of organics from fuel and lubrication oil. Le Breton et al. (2019) confirmed the contribution of lubrication oil in bus emissions. Besides, DPFs can act as a sulfur reservoir, and, when excess sulfur is released, SO₂-to-SO₃ conversion can take place once the after-treatment temperature reaches a critical level (Herner et al., 2011). In this case, total particle number emissions can increase due to nucleation from gas-phase sulfuric acid. Since the fuel sulfur content is low, more than 90 % of Euro VI HDTs had an EF${}_{{\mathrm{SO}}_{\mathrm{2}}}$ value lower than the threshold; in this study, organics would play a more important role in the formation of nucleation mode particles.

Figure 2c shows that the median EF_BC was reduced by more than 99 % for Euro VI HDTs compared to Euro III HDTs, and the median EF_BC of Euro VI HDTs was even at the threshold (0.2 mg (kg fuel)⁻¹). The BC emissions generally showed a decrease from Euro III to Euro VI HDTs (Jonckheere–Terpstra test, p<0.01), which is similar to the EF_PM trend with the exception of Euro IV HDTs. Compared with Euro V HDTs, the median EF_BC of Euro IV HDTs is 35 % lower; however, the emission of the mixture of BC and BrC from Euro IV HDTs is higher (Fig. S7a). Euro IV HDTs had the highest BrC contribution to the total light-absorbing substances among all the Euro classes (Fig. S7b).

Figure 2d compares the emissions of NO₂ and NO_x from different Euro class HDTs. The vertical lines represent the different Euro standards. HDTs with either EF${}_{{\mathrm{NO}}_{\mathrm{2}}}$ or EF${}_{{\mathrm{NO}}_x}$ values lower than the detection limits of the instruments were removed in Fig. 2d for illustration purposes, while all the presented statistical analyses include all the data as outlined above. In general, Euro VI HDTs exhibit a more than 90 % reduction in both median and average EF${}_{{\mathrm{NO}}_x}$ compared to Euro III HDTs. This is consistent with Carslaw et al. (2011), who estimated a 93 % NO_x reduction from Euro III to Euro VI for heavy goods vehicles (HGVs) in the United Kingdom. Relatively, the Euro V HDTs had a larger fraction exceeding their Euro standard, which may be due to the combined effects of poor engine tuning and the inactivity (low temperature) or deterioration of SCR systems. Newer engines tend to exhibit a higher NO₂ emission fraction at a similar NO_x level, and the Euro VI HDTs show a relatively low median EF${}_{{\mathrm{NO}}_{\mathrm{2}}}$ with a large range of data scatter and several high emitters. A continuous increase in EF${}_{{\mathrm{NO}}_{\mathrm{2}}}$/EF${}_{{\mathrm{NO}}_x}$ was evident from Euro IV to Euro VI HDTs (Fig. S8b). This trend is consistent with Kozawa et al. (2014), who reported an increase in the share of NO₂ to total NO_x from Euro III to Euro V vehicles. Euro VI HDTs have a higher NO₂ fraction, because the DOC upstream of the filter is used to convert NO to NO₂ to control the soot loading in the DPF and facilitate the passive regeneration (Van Setten et al., 2001). A failure of the NO₂ reduction due to the inactivity of the SCR, resulting from low exhaust gas temperature, may yield a higher NO₂ emission (Bishop et al., 2010; Heeb et al., 2010; Herner et al., 2009; May et al., 2014; Thiruvengadam et al., 2015). A more significant decrease in NO_x than NO₂ emissions of Euro VI HDTs may cause an increase in EF${}_{{\mathrm{NO}}_{\mathrm{2}}}$/EF${}_{{\mathrm{NO}}_x}$.

Table 1 compares the average emission data of PM and PN of the current work with previous studies according to the HDT type and gives information on measurement methods used and driving conditions. Generally, the EF_PM and EF_PN in this study are within the reported ranges of HDV emissions in the literature. Our estimated EF_PM values of Euro III HDTs are comparable to those of Euro III buses in Hallquist et al. (2013) and Pirjola et al. (2016). HDTs and buses within the same Euro class emit similar amounts of PM. Watne et al. (2018) show that DPF-retrofitted Euro III buses have much lower particle EFs. While EF_PM is highly dependent on driving conditions such as speed and acceleration, the average EF_PM of Euro IV, Euro V, and EEV HDTs of this study (172, 146, and 78 mg (kg fuel)⁻¹, respectively) are comparable to previous studies (Hallquist et al., 2013; Pirjola et al., 2016; Watne et al., 2018). The average EF_PM of Euro VI HDTs (5 mg (kg fuel)⁻¹/1.1 mg km⁻¹) is within the range of emissions from HDVs with DPFs, e.g. 0.6–20.5 mg km⁻¹, for a recent chassis dynamometer test (Jiang et al., 2018) and 2.5–8.7 mg km⁻¹ for road measurements in California (Quiros et al., 2016). Note that size ranges and measurement methodologies may differ among the studies as listed in Table 1. Since most of the particle emissions related to road traffic combustion are below 560 nm (Fig. 4), the size range in our study is comparable to most other wider-range PM measurements. Larger particles from road measurements of total PM may include non-combustion-related particles, e.g. resuspended road dust, tire particles, and brake-wear particles, and they should be interpreted with caution. In contrast to EF_PM, a much less obvious decrease in average EF_PN was observed across different Euro classes. The reason for the high average particle emission for EEV and Euro VI is likely due to high emissions of nucleation mode particles from a number of HDTs.

Table 1Comparison of the average emission data^a for PM and PN from the present study with literature data. Bold font was used for better illustration of different Euro classes.

^a Given errors are at 95 % CI. ^b Standard deviation. ^c ELPI, electrical low-pressure impactor. ^d Average fuel consumption of 0.26 L km⁻¹ for HDV during long-haul and regional delivery tests (Rexeis et al., 2018); the density of 0.815 kg dm⁻³ (Hallquist et al., 2013) for diesel particles was assumed for unit conversion. ^e MSS, micro soot sensor. ^f SP-AMS, soot particle aerosol mass spectrometer. ^g OHMS, on-road heavy-duty vehicle emissions monitoring system.

Download XLSX

Figure 3(a) EF_PM and EF_PN of individual HDTs in this study and previous studies and (b) the relationship between EF_PM and EF_SPN of Euro VI HDTs. Dashed red lines represent Euro emission standards (horizontal: PM emission standard; vertical: SPN emission standard). HDTs with either EF_PM or EF_PN values lower than the detection limits of the instruments were removed in (a) for illustration purposes. Note that the comparison with the emission standard is only indicative as they are based on test cycle performance.

Download

In Fig. 3a, EF_PM and EF_PN of individual HDTs in this study and selected previous studies are plotted. HDTs with either EF_PM or EF_PN lower than 0.07 mg (kg fuel)⁻¹ or 2.8×10¹¹ particles (kg fuel)⁻¹, respectively, were removed from the figure (24 % of the data) for illustration purposes. Their corresponding measurements were below the detection limits of the instruments, while the presented statistical analyses include all the data as outlined above. Generally, both EF_PM and EF_PN exhibited a decreasing trend from Euro III to Euro IV and from Euro V to EEV HDTs (Jonckheere–Terpstra test, p<0.01). Overall, Euro VI HDTs had drastically lower PM emissions but highly scattered PN emissions. Older Euro type buses retrofitted with DPF were shown to have reduced particle emissions, and some retrofitted Euro III buses (black open triangles in Fig. 3a) may perform as well as Euro VI HDTs, indicating the effectiveness of retrofitting older HDTs.

Figure 4(a–e) Mean and median size-resolved EF_PN and EF_{non-volatile PN} of different Euro class HDTs and (f) comparisons of mean size-resolved EF_PN of HDVs in this study and previous studies. Shaded regions in (a–e) represent the statistical 95 % confidence interval. One HDT with extremely different EF in (d) was excluded from the analysis and shown in blue.

Download

In more recent European standards, PN regulation has been introduced. The SPN as defined by the European Particle Measurement Programme (PMP) is the number of particles which remains after passing through an evaporation tube with a wall temperature of 300–400 ^∘C (Zheng et al., 2011). The PMP only measures and regulates solid particles with a diameter larger than 23 nm, because measurements of smaller particles in the nucleation mode have poor repeatability (Martini et al., 2009). SPN larger than 23 nm was integrated into the European emission regulation in 2013 for Euro VI heavy-duty engines (Giechaskiel et al., 2012). A potential issue of evaporation measurements is that a fraction of the less than 23 nm particles can also be formed downstream of the European PMP methodology through renucleation of semi-volatile precursors (Zheng et al., 2012, 2011). In our study, the TD temperature of 250 ^∘C is lower than the maximum temperature used by the PMP (300–400 ^∘C) and does not follow the exact operation specifications of the PMP. However, Amanatidis et al. (2018) summarized that TD is a suitable alternative approach for the removal of volatile particles. Particles larger than 23 nm downstream of the TD were measured by the EEPS, and we integrated the size bins from 23.5 to 560 nm to represent the SPN. Figure 3b compares the EF_PM and EF_SPN of Euro VI HDTs. Generally, after-treatment control systems could not reduce SPN emissions as effectively as PM emissions, indicating that more control of SPN emission of Euro VI HDTs may be necessary.

Shown in Table 2 are the average EFs of gaseous pollutants (NO_x, CO, HC) in this study compared with other studies. EF${}_{{\mathrm{NO}}_x}$ generally decreased from the Euro III (43.3 g (kg fuel)⁻¹) to Euro VI (3.1 g (kg fuel)⁻¹) class, and they are in good agreement with reported values for HDTs in the literature. The EF${}_{{\mathrm{NO}}_x}$ of Euro III HDTs is moderately higher in this study. Note that the EF${}_{{\mathrm{NO}}_x}$ and EF_PM values of EEV were much higher in Pirjola et al. (2016), in which only a limited number (3–4) of vehicles were tested and hard braking was common in approaching a 90^∘ turn before accelerating again. The ratio of EF${}_{{\mathrm{NO}}_{\mathrm{2}}}$ to EF${}_{{\mathrm{NO}}_x}$ generally agrees with the projection in Kousoulidou et al. (2008), on-road plume chasing measurements in Lau et al. (2015), and remote sensing studies in the UK (Carslaw and Rhys-Tyler, 2013). Carslaw et al. (2019) reported a decreasing trend of EF${}_{{\mathrm{NO}}_{\mathrm{2}}}$/EF${}_{{\mathrm{NO}}_x}$ with vehicle mileage for Euro 6 light-duty diesel vehicles, while no significant trend was identified for Euro VI HDTs in this study. There may be other parameters influencing the NO_x emission. For example, Ko et al. (2019) reported that the NO_x emissions from Euro VI diesel vehicles were 29 % higher in a traffic jam than in smooth traffic conditions. The temperature of the exhaust and DPF regeneration may also influence the EF${}_{{\mathrm{NO}}_x}$.

Table 2Comparison of the average emission data^a for NO_x, NO₂/NO_x, CO, and HC from the present study with literature data. Bold font was used for better illustration of different Euro classes.

^a Given errors are at 95 % CI. ^b In NO₂ equivalents. ^c RSD data. For the RSD data sets of multiple individuals, negative values were replaced by zero when calculating the averages. ^d Standard deviation. ^e Median. ^f Average fuel consumption of 0.26 L km⁻¹ for HDV during long-haul and regional delivery tests (Rexeis et al., 2018); the density of 0.815 kg dm⁻³ (Hallquist et al., 2013) for diesel particles was assumed.

Download XLSX

Compared with EF${}_{{\mathrm{NO}}_x}$, EF_CO decreased less pronounced from Euro III to Euro VI HDTs (57 %). Compared with newer Euro class HDTs, a larger fraction of HDTs in older Euro classes have an EF_CO exceeding the Euro standards, which indicates that engine deterioration may have a serious effect on the CO emissions (Fig. S8c). Hallquist et al. (2013) reported a positive relationship between EF_CO and EF_PM, i.e. high CO indicates incomplete combustion which favours soot formation. DPFs may also reduce CO in addition to PM (Hallquist et al., 2013), which is in agreement with the lowest CO emission of 15.5 g (kg fuel)⁻¹ observed for DPF equipped Euro VI HDTs in this study. HC emission was relatively low for all HDT types, and no obvious decreasing trend was evident for EF_HC from Euro III to Euro VI HDTs (Jonckheere–Terpstra test, p=0.895) (Fig. S8d and Table 2), for which the Euro VI limit is more than a factor of 3 lower than the preceding Euro V/IV standards.

The 46 Swedish HDTs without available Euro information emitted similar levels of particle and gaseous pollutants to Euro VI HDTs, and they were thus likely equipped with newer Euro class engines.

Compared with the fleet of non-Swedish HDTs, the Swedish HDT fleet generally have a lower median and average EF${}_{{\mathrm{NO}}_x}$, but there are no significant differences in the EF of other pollutants (Fig. 2 and Tables 1 and 2). The differences in EF${}_{{\mathrm{NO}}_x}$ are significant at the statistical 95 % CI using the Kolmogorov–Smirnov test, used in favour of the typical Student's t test to account for non-normality of the EF distributions. As information of Euro class, engine types, and treatment technologies of non-Swedish HDTs is not available, we cannot further explore why there is a difference between the two fleets.

In addition to engine Euro type, pollutant emission trends were also investigated with respect to five different vehicle manufacturers (M1, M2, M3, M4, and M5). EF_PM, EF_PN, EF_BC, and EF${}_{{\mathrm{NO}}_x}$ of HDTs under the same Euro class but from different manufacturers are compared in Fig. S9. Since EF data were not normally distributed, statistical significance is assessed with a Kruskal–Wallis test. It is a non-parametric analogue of the one-way ANOVA test. The p values are calculated at the statistical 95 % confidence level. No significant group difference (p>0.05) was observed in EF_PM, EF_PN, EF_BC, or EF${}_{{\mathrm{NO}}_x}$ for Euro V HDTs, i.e. HDTs (from five different manufacturers) show comparable emission characteristics. EF_PM, EF_PN, and EF${}_{{\mathrm{NO}}_x}$ of Euro VI HDTs show no dependency on manufacturers, but a significant difference was observed between M2 and M5 in EF_BC of Euro VI HDTs (p=0.016). (No analysis on Euro III, Euro IV, and EEV HDTs was conducted due to the limited number of vehicles from each manufacturer).

Figure 5Cumulative emission factor distribution for (a) PM, (b) PN, (c) NO_x, and (d) NO₂ measured in HDT exhaust plumes with HDTs ranked from the highest to lowest in terms of emission factors.

Download

3.4 Size-resolved EF_PN of volatile and non-volatile particles

Figure 4a–e show the average size-resolved number emission factors (solid lines) simultaneously measured via the bypass and TD lines for different Euro class HDTs. The EF_PN of the volatile components is calculated as the difference of EF_PN measured after the bypass line and the non-volatile component EF_PN measured after the TD line. To differentiate between nucleation and accumulation mode particles, a cut-off particle diameter of 30 nm was used as defined by Kittelson et al. (2002). In general, all Euro III, Euro IV, Euro V, and EEV HDTs showed a bimodal particle number size distribution, with one mode peaking at ∼ 6–10 nm (nucleation mode) and another at ∼ 50–80 nm (accumulation/soot mode) (Maricq, 2007). For Euro VI HDTs, the particle number size distributions were dominated by the nucleation mode. The EF_PN of the accumulation mode particles shows a decreasing trend from Euro III to EEV HDTs. The accumulation mode of the Euro VI HDTs was insignificant. For heavy-duty diesel engines without a particulate filter, nucleation mode particles are mainly formed from organics. For vehicles with DPF, both organics and the fuel sulfur content might influence the formation of nucleation mode particles (Vaaraslahti et al., 2004). Thiruvengadam et al. (2012) found a direct relationship between exhaust nanoparticles in the nucleation mode and the exhaust temperature of the DPF-SCR-equipped diesel engine. These factors lead to high variability in the nucleation mode fraction of EF_PN. Figure 4f shows that HDVs with DPF (dashed lines) exhibited lower emissions of accumulation mode particles, with no significant reduction in nucleation mode particles when compared to HDVs without DPF (solid lines). In general, the absence of significant accumulation mode particles from Euro VI HDTs was consistent with observations made from DPF-equipped HDVs. High emissions of accumulation mode particles from Euro III HDTs were consistent with measurements from HDVs without DPF in previous studies (Liu et al., 2019; Hallquist et al., 2013; Preble et al., 2015).

Most particles in the nucleation mode evaporate after passing through the TD. Sakurai et al. (2003b) reported that volatile compounds in diesel particles are mainly comprised of unburned lubrication oil. The non-volatile components in the nucleation mode may consist of metallic ash, from lubrication oil or fuel additives (Sakurai et al., 2003a), or some organic compounds of extremely low volatility (Gkatzelis et al., 2016). In the accumulation mode, the particle mode diameter shifted towards lower sizes after passing the TD. In Fig. 4a–e, we also present the median size distribution (dashed lines). There is a small difference between mean and median size distributions in the accumulation mode, while a bigger difference occurs in the nucleation mode. The latter mode is more dynamic, and there are larger possibilities for extreme values skewing the averages.

To be consistent with previous studies, which overwhelmingly report average size distributions, we choose to utilize the average size distributions for the discussions below. The volatilities of particle emissions in the accumulation and nucleation modes have been evaluated by calculating the average EF_PN and EF_PM fraction remaining (after heating) of particles emitted from Euro III-VI HDTs (Fig. S10). In general, the EF_PN fraction remaining in the nucleation mode was lower than that in the accumulation mode across all HDTs in all Euro classes. In terms of particle mass, the nucleation mode and accumulation mode showed similar EF_PM fractions remaining from Euro III to EEV HDTs, while Euro VI HDTs had a much lower EF_PM fraction remaining in the nucleation mode than in the accumulation mode. Compared with other Euro class HDTs, Euro VI HDTs had the lowest EF_PN and EF_PM fraction remaining in both nucleation and accumulation mode. Around 94 % of the particles by number and 55 % of the particles by mass (or volume) in total were evaporated. Alfoldy et al. (2009) reported that if the same amount of volatile mass in the nucleation mode and accumulation mode were inhaled, 48 % and 29 % of the mass would deposit in the lung, respectively, implying that volatile mass in the nucleation mode would exert a 1.5 times stronger effect.

3.5 Emissions from high emitters

Figure 5 shows the cumulative emission distributions for PM, PN, NO_x, and NO₂ emissions, with HDTs ranked in order from dirtiest to cleanest. The plots show a significant skewness towards a small fraction of HDTs with a high fraction of total emissions (deviation from 1 : 1 line) for each pollutant, indicating the importance of “high emitters”. The disproportionate skewed distribution of pollutants is a common feature of on-road emission measurements (Preble et al., 2018, 2015; Dallmann et al., 2012). The highest-emitting 10 % of HDTs in each pollutant were responsible for 65 % of total PM, 70 % of total PN, 44 % of total NO_x emissions, and 69 % of total NO₂. The distribution of NO_x has the least skewness compared with the other pollutants. If the 10 % highest emitters for each pollutant were removed, the corresponding average EF for PM, PN, NO_x, and NO₂ would decrease by 62 %, 67 %, 38 %, and 66 %, respectively. However, the high emitters for each pollutant are different. For example, Euro III HDTs account for 70 % and 67 % of the top 3 % emitters for PM and BC emissions, respectively, while Euro VI HDTs account for 80 % and 56 % of the top 3 % emitters for PN and NO₂ emissions, respectively. Here, top 3 % emitters were chosen as the reference because Euro III HDTs only accounted for 3 % of the total number of HDTs. Lau et al. (2015) similarly reported that not all high emitters were members of the lower Euro classes and that high emitters for a particular pollutant may not simultaneously be high emitters for other pollutants.

Figure 6The average EFs of (a) PM, (b) PN, (c) BC, and (d) NO_x with respect to the registration year of HDTs. Error bars represent the statistical 95 % confidence interval. Black arrows mark the years that the particular type of HDT examined in this study was first registered.

Download

3.6 Fleet characteristics

Figure 6a–d show the average EFs of PM, PN, BC, and NO_x with respect to the registration year of the HDTs. Triennial average EFs were calculated, with truck registration years divided into 5 bins (2002–2005, 2006–2008, 2009–2011, 2012–2014, and 2015–2017). The arrows in Fig. 6d show the years that the particular type of HDTs examined in this study was first registered. Coupled with the possible phaseout of older fleets, the HDTs with more advanced engines gradually accounted for a higher proportion of the total fleet. There is a significant improvement during the last years and the transition to widespread adoption of Euro VI will take real-world on-road emissions into a new era of much lower contributions to air pollution.

Figure 7Relative contributions of kilometres driven by (a) Swedish and non-Swedish HDTs and (b) Swedish Euro 0, Euro I, Euro II, Euro III, Euro IV, Euro V, and Euro VI HDTs on Swedish roads during 2018. Approximation of contributions of pollutants emitted from Swedish HDTs in each Euro class to the total (c) PM, (d) PN, (e) BC, and (f) NO_x emissions.

Download

To estimate, for each Euro class, the typical contribution of air pollution emissions, we utilized the number of kilometres driven by HDTs on Swedish roads. During 2018, 4.1×10⁹ and 9.2×10⁸ km were driven by Swedish and non-Swedish diesel HDTs on Swedish roads, contributing to 82 % and 18 % to the total distances travelled by diesel HDTs, respectively (Fig. 7a). The numbers of kilometres driven by Swedish Euro 0, Euro I, Euro II, Euro III, Euro IV, Euro V, and Euro VI diesel HDTs were 2.8×10⁷, 5.0×10⁶, 5.4×10⁷, 2.0×10⁸, 3.1×10⁸, 1.3×10⁹, and 2.2×10⁹, respectively (HBEFA 3.3, 2019). In Fig. 7b, the relative contributions of kilometres driven by Swedish Euro 0 to Euro VI HDTs are shown. Zhang et al. (2014) reported no statistically significant difference in fuel consumption among Euro II to Euro IV buses under a real-world typical bus driving cycle in Beijing. In this study, we assume the fuel consumption per kilometre and fuel density are the same across the different Euro class HDTs, and, adopting the average fuel-based EFs calculated in this study (Tables 1 and 2), the approximations of contributions of pollutants emitted from Swedish HDTs in each Euro class to the total PM, PN, BC, and NO_x emissions are depicted in Fig. 7c–f. Due to a lack of corresponding emission information, pollutant average EFs of Euro 0, Euro I, and Euro II HDTs were assumed to be at the same level as those of Euro III HDTs representing lower bound estimates. Euro 0–II HDTs accounted for less than 2.2 % of the grand total distance driven but totally contributed to 16 %, 13 %, 6 %, and 4 % of BC, PM, NO_x, and PN emissions. Euro III HDTs only accounted for 5 % (Fig. 7b) of the total fleet but disproportionally contributed to 37 %, 30 %, 16 %, and 10 % of BC, PM, NO_x, and PN emissions. Euro IV HDTs also exhibited disproportionally high PM and NO_x emissions. A fraction of 32 % of HDTs belonged to the Euro V category, they contributed to 53 %, 42 %, 34 %, and 32 % of NO_x, PM, BC, and PN emissions, respectively. Upgrading, replacing, or making Euro 0 to Euro V HDTs obsolete would be necessary for mitigating a large part of the PM, PN, BC, and NO_x emissions. Euro VI HDTs accounted for the highest fraction of the total fleet (53 %), but only contributed to 2 %, 6 %, 12 % and 47 % of PM, BC, NO_x, and PN emissions, indicating successful overall pollution reduction with the introduction of more Euro VI HDTs. Using the median EFs as references, the emission contributions from Euro VI HDTs to the total pollutant emissions would be even lower (Fig. S11). These data provide useful information to predict future pollutant emission trends and to guide policy analysis and implementation. Since the predicted transition in emissions from road transport would be significant, chemical transport model or cost-assessment models need to gain fast access to emission factors for new generation HDTs to be able to provide a better estimation of near-future air pollution levels.