2.1 Test Vehicles and Routes

Taking into account that more than 99.4% of the diesel vehicles currently in China are compliant with Euro III–V, three typical LDDTs compliant with Euro III, Euro IV and Euro V, respectively, were selected from the market and their specifications are provided in Table I. These trucks have similar dimensions and powers, and their biggest difference is their aftertreatment technology. To eliminate the impact of fuel quality, all the diesel fuel used in the study was from a specified filling station, conforming to the China VI standard.

The test route was designed to simulate the real driving conditions of most diesel trucks in Zhengzhou, Henan province. The total length of the test route was approximately 68 km, including 14 km of urban roads, 18 km of connection roads and 36 km of highway. VOCs were sampled only when the trucks travelled on the urban and highway roads and cold start emissions of VOCs were not included during the whole test. Table II shows the driving condition parameters during each road type. The average speeds on urban and highway roads were 18.1–20.8 km h⁻¹ and 72.8–76.5 km h⁻¹, respectively. Driving conditions on urban roads are more aggressive than those on highway roads. The average accelerations on urban and highway roads were 0.22–0.26 m s⁻² and 0.10–0.13 m s⁻², respectively. During the measurement, the trucks were not in service and the load of each truck was approximately 500 kg during the experiment, containing the PEMS equipment, four batteries, two testers and one driver.

2.2 Volatile Organic Compounds Sampling and Analysis

Under real driving conditions, some gaseous emissions may transform to secondary fine particles when the exhaust is cooled or diluted with the ambient atmosphere. Thus, VOC emissions might be overestimated if sampled directly from the vehicle exhaust because the temperature is very high. Therefore, a combined PEMS (Sensors Inc, USA) was employed to sample the exhaust VOC emissions. The schematic diagram of the emission testing and sampling system is shown in Figure 1.

The microproportional sample system (MPS), a partial flow dilution system, was used to dilute and cool the exhaust from tailpipe. After the MPS, the gas temperature decreased from about 120ºC to about 40ºC. Two 3.2 L SUMMA^® canisters (Entech Instruments Inc, USA) were used to sample the VOCs during each test trip, one for the urban roads and the other for the highway roads. VOC emissions during the connection roads section were not sampled because the actual running speed could not meet the requirement due to unexpected road repairing. The sampling flow rate was controlled by a passive restrict valve at 0.1 L min⁻¹. Teflon^TM tubes were used to connect the canister and PEMS system to minimise the adsorption of VOCs. A laptop was used to control the system and collect data from the test module. It should be noted that these trucks were driven by their owners throughout the test to ensure these trucks were running under ordinary working conditions and each vehicle was tested twice to enhance the reliability of the results.

Analysis of the VOCs was carried out following the United States Environmental Protection Agency (US EPA) TO-15 method by a gas chromatography-mass selective detector (GC-MSD) (22). Samples collected in the SUMMA^® canister were preconcentrated using an 8900DS preconcentrator (Nutech Instruments Inc, USA) with three cold traps and a canister autosampler (Nutech Instruments Inc, USA, mode 3600DS). The moisture, CO₂ and methane would be removed through the traps. Then the concentration of the individual VOCs in the samples was determined by a GC-MSD system (7890A GC with a 5975 MSD, Agilent Technologies Inc, USA). Separation of the VOCs was achieved through a capillary column (60 mm × 0.25 mm internal diameter, 1.4 μm film thickness, DB-624 column, Agilent Technologies Inc). During sampling and analysis, strict quality assurance and quality control procedures were conducted to assure the data quality (22). The detection limits of the target non-methane hydrocarbons ranged from 7 parts per trillion by volume (pptv) to 141 pptv and the accuracy of the measurements was about 1–10%. Detailed description of the analysis procedures can be found in our previous study (23).

A total of 102 VOC species were identified and quantified, including 29 alkanes, 35 halocarbons, 17 aromatics, nine alkenes, five carbonyls and seven other compounds, which are presented in Table III. Due to the detection limitation of GC-MSD used in this study, some species (ethane, ethylene, propylene, acetylene, formaldehyde) were not detected and included.

2.3 Calculation of the Emission Factors and Ozone Formation Potential

EF per kilometre of a certain pollutant was calculated with the corresponding concentration, total exhaust volume and running distance during the test process. Prior to calculation, the results of the VOC measurements were time-aggregated. The total exhaust volumes in various driving conditions were the integration of the instantaneous exhaust flow rates, and the same for the total running distance. The EF of compound i was calculated as Equations (i)–(iii):

(i)

(ii)

(iii)

where V (m³) is the total exhaust volume of the sampling process; V_ins (m³ s⁻¹) is the instantaneous exhaust flow rate; DR_ins is the instantaneous dilution ratio of MPS; S (km) is the distance that the test vehicle travelled during the sample period; S_j is the travel distance at j second, which is equal to the value of instantaneous speed at time j recorded by the global positioning system (m s⁻¹); EF_i (mg km⁻¹) is the EF of compound i; C_i (parts per billion by volume) is the concentration of compound i; M_i (g mol⁻¹) is the molar mass of compound i; and V_m (l mol⁻¹) is the molar volume of compound i. The volumes and concentration data were all normalised to the standard ambient temperature and pressure condition (273.15 K, 101.33 kPa). The total EFs of the VOCs in a certain driving mode were summed by the individual VOC EFs in the driving mode.

The OFP refers to the amount of ozone generated by VOCs per unit mass (mg O₃ mg⁻¹ VOCs), which can reflect the ozone formation capacity of VOC species. In most cases, ratios of VOCs to NOx from the diluted exhaust were much higher than 20 in this study, which illustrated VOCs had the greater effect on the ozone formation (24). Therefore, the MIR scenarios developed by Cater (25) was applicable to evaluate the OFP of VOC species here. The OFP of a certain VOC is calculated according to Equation (iv) (26, 27):

(iv)

where OFP_i (mg O₃ km⁻¹) is the ozone formation of compound i; and MIR_i (mg O₃ mg⁻¹ VOCs) is the maximum incremental reactive of compound i obtained from Cater (25, 28). The total OFPs of a certain driving mode were summed by the individual VOC OFPs of the driving mode.

3.1 Regulated Gaseous Emissions

Figure 2 presents the EFs of regulated gaseous pollutants of three LDDTs compliant with different standards. Obviously, NOx, CO and HC emissions from LDDT-3 (Euro V) were the lowest and those from LDDT-1 (Euro III) were the highest, except for CO. In general, updated emission standards had a great effect on the reduction of regulated gaseous emissions. This is mainly because the three trucks adopted different aftertreatment technologies to meet different emission standards (29). For example, both selective catalytic reduction (SCR) and diesel oxidation catalyst (DOC) were utilised by LDDT-3 to be compliant with Euro V standards. SCR was often used to purify the NOx emissions and the DOC device could oxidise the CO and HC emissions efficiently (30–32). It is not difficult to understand why LDDT-1 produced the worst emissions because there is no aftertreatment requirement for Euro III trucks in most of China.

As shown in Figure 2, NOx, CO and HC emissions under urban conditions were significantly higher than those under highway conditions. To be specific, NOx, CO and HC emissions under urban conditions were 1.3–1.8 times, 1.4–2.2 times and 2.5–4.1 times those under highway conditions. This phenomenon could be explained by the fact that the combustion quality in the engine was associated with the operation speed and frequent acceleration and deceleration (18, 33, 34). During this experiment, no traffic signals were encountered on the highway and the average speed was up to 73.8 km h⁻¹. However, there were 26 traffic signals on the urban roads and the average speed was only 19.4 km h⁻¹. In this operating condition, the combustion was insufficient and the temperature of aftertreatments might not be high enough for proper function, which caused the emissions to deteriorate.

3.2 Volatile Organic Compound Speciation Profiles

Average weight percentage of individual VOC species of the entire trip was calculated based on the test trucks. On the whole, alkanes were the dominant group, accounting for 65.5 ± 10.3% of the total VOCs, followed by aromatics, carbonyls and alkenes, taking up 19.6 ± 5.0%, 5.4 ± 1.9% and 4.4 ± 1.8%, respectively. Additionally, though 35 halocarbons were quantified, they only took up 3.6 ± 1.5% of the VOCs. Thus, the following discussions on the VOCs are mainly focused on alkanes, aromatics, alkenes and carbonyls.

Weight percentages of the top 15 VOC species from the exhaust are presented in Table IV. These species accounted for approximately 83.4% of the total VOCs. Dodecane, n-undecane, naphthalene, n-decane and acetone were the major species, and their total weight percentages were over 80.1%. These results are partially consistent with the results obtained by Wang et al. (14), who indicated that n-decane, n-undecane and n-dodecane were the most abundant species. However, a study by Yao et al. (20) showed that carbonyls were the top group, which could account for 42.7–69.2% of the total VOCs. The difference was mainly attributed to the different VOC species quantified between the two studies. For example, Yao et al. (20) reported formaldehyde and acetaldehyde took up 47.9% and 21.0% of carbonyls, while these two species were not detected in this study.

3.3 Effect of Standards on Volatile Organic Compound Emissions

The mean EFs and weight percentages for each VOC group for the entire trip of the three test trucks are plotted in Figure 3. The total VOC EFs of LDDT-1 (Euro III), LDDT-2 (Euro IV) and LDDT-3 (Euro V) were 186.9 ± 34.9 mg km⁻¹, 106.5 ± 26.2 mg km⁻¹ and 61.1 ± 16.9 mg km⁻¹, respectively. In other words, the VOC emissions decreased significantly as the standards tightened gradually from Euro III to Euro V. Most of the other species also showed a decreasing trend. Especially, dodecane and n-undecane presented the most significant decline, from 85.2 ± 3.7 mg km⁻¹ and 38.6 ± 11.7 mg km⁻¹ for Euro III to 16.7 ± 2.8 mg km⁻¹ and 9.7 ± 3.0 mg km⁻¹ for Euro V, respectively. The trend was partially consistent with that found by Zhang et al. (10), though the VOC EFs were a little higher than those in this work. This might be mainly attributed to the fact that Zhang et al. (10) employed tunnel measurement, which included evaporative emissions.

Most VOC groups presented similar variation trends as the emission standards changed, especially for the dominant groups. For example, both alkane and aromatic emissions decreased noticeably as the standards varied from Euro III to Euro V. The progress in engine technology and application of aftertreatment devices played a major role in the subtraction of VOCs emissions. Additionally, there were no significant differences between emissions of carbonyls, alkenes and halocarbons from LDDT-1 (Euro III) and LDDT-2 (Euro IV), but they were much higher than those of LDDT-3 (Euro V). No coherent order was observed for other emissions among these diesel trucks, possibly because the absolute values of these species were too small to quantify accurately. On the whole, implementing stringent emissions standards could reduce most of the VOC species effectively in the freight transportation sector.

Figure 3 indicates that alkanes were the dominant group in tailpipe VOCs emissions from the test LDDTs, accounting for 57.2–80.0%, followed by aromatics (12.5–22.9%), carbonyls (3.1–7.7%) and alkenes (2.2–6.5%). This result was consistent with that observed by Wang et al. (14) (carbonyls < aromatics < alkanes) but inconsistent with that by Yao et al. (20) (alkenes < aromatics < alkanes < carbonyls). Discrepancy of the quantified VOC species was the main cause of the inconsistency. It can also be found that the proportion of alkanes decreased significantly, from 80.01% for LDDT-1 to 57.15% and 60.41% for LDDT-2 and LDDT-3, respectively. Additionally, LDDT-2 and LDDT-3 had similar VOC group distributions, while the aromatics weight percentage of LDDT-2 was significantly related to that of LDDT-3. This is probably due to the different aftertreatment used in LDDT-2 and LDDT-3 (as shown in Table III). Jung et al. (35) also observed that heavy-duty trucks equipped with DPF emitted higher quantities of aromatics compared with those with SCR.

Figure 4 shows the EFs of the top 15 VOC species from the exhaust of LDDTs. The EF of dodecane for LDDT-3 (Euro V) was 51.0% and for LDDT‐2 (Euro IV) it was only 19.6% relative to LDDT‐1 (Euro III). For several other species, LDDT-3 had the lowest EFs, while the EFs of LDDT-2 and LDDT-1 were comparable or even higher, such as naphthalene, acetone, 2-propenal. A hypothesis is that much higher temperatures and more oxidising conditions during the DPF regeneration process favour carbonyl formation (36). However, there is no direct evidence that DPF regeneration occurred. Additionally, there were several individual species whose emissions were not affected by the emission standards. Overall, most of the top 15 VOC species presented a decreasing trend as the emission standards tightened.

3.4 Influence of Driving Conditions on Volatile Organic Compound Emissions

Figure 5 shows several VOC group emissions from the exhaust of LDDTs under urban and highway driving conditions, respectively. It can be seen that VOC emissions on highway roads were much lower than those on urban roads. EFs of each VOC group decreased significantly, especially for alkanes and aromatics. Specifically, EFs of alkanes under highway conditions were only 20.4–46.2% of those under urban conditions, which was mainly attributed to the sharp decline of the most abundant alkane species, such as dodecane, n-undecane and n- ecane. For aromatics, the significant reduction of the EFs during highway driving could be attributed to the sharp reductions of naphthalene, 1,2,3-trimethylbenzene and 1,2,4-trimethylbenzene. Lower average speed and more acceleration and declaration were found during the urban road episodes, causing more incomplete combustion on non-highway road driving, resulting in higher VOCs emissions than on highways (37). Caplain et al. (38) also reported that tailpipe emissions in urban driving cycles were approximately four times those in motorway driving cycles. In addition, the reduction degrees of VOC EFs (urban vs. highway) for LDDT-3 were highest while those for LDDT-2 were lowest. This discrepancy is mainly due to the different aftertreatment devices used. For instance, optimum conditions of a DOC + SCR system used for LDDT-3 could be maintained under highway conditions because of the high exhaust temperature, resulting in more efficient reduction.

A breakdown of the C1–C12 VOCs for different driving conditions of the tested trucks is presented in Figure 6. There was no obviously consistent trend in the carbon number distribution of the VOC species between highway and urban road conditions except for C3 and C11, which showed a decreasing trend when driven on the highway compared to urban roads. This phenomenon illustrated that driving conditions had a weak correlation with carbon number distribution. On the whole, the carbon number of the VOCs was concentrated in C3–C4 and C10–C12, showing a distinct ‘double peak’ phenomenon. Lu et al. (39) summarised several previous studies and reached a similar conclusion. VOCs are expected to be a mixture of unburned and partially burned fuel species (40). Propane and acetone are the dominant species in C3–C4 group, and this portion of the VOCs is likely generated as a result of the high efficiency of the diesel engine. For the C10–C12 group, these species are considered to be components of diesel fuel. Durbin et al. (41) reported the C1–C3 species contributed most to non-methane organic gases (NMOG) and ethene, ethyne, acetaldehyde and formaldehyde made the largest contribution. The differences may be attributed to the VOCs species detected between these two studies.

3.5 Ozone Formation Potential

According to the EFs of each VOC species, OFPs based on the travelled distance were calculated and the results are plotted in Figure 7. As expected, the magnitude of OFP based on emission rate presented a decreasing trend. To be specific, LDDT‐1 and LDDT-2 had the higher OFPs, approximately 239.6 ± 57.3 mg O₃ km⁻¹ and 227.7 ± 69.2 mg O₃ km⁻¹, respectively, and that for LDDT-3 was 124.8 ± 47.6 mg O₃ km⁻¹. The OFP values in this work were lower but comparable to those for diesel trucks in some studies (20, 37, 42). The lower OFP values in this study were mainly because DOC and SCR dramatically reduced VOCs emission. Additionally, engine technologies, driving cycles and fuel quality were also important factors.

The chemical structure of OFPs was different from the trend of VOCs emissions based on distance travelled, shown in Figure 3. Aromatics were the primary contributor to OFP, accounting for 49.3–57.6% of the OFPs. It was noteworthy that although alkenes accounted for only approximately 5.0% of the VOC emissions, the OFP contribution of alkenes (13.4–22.3%) was comparable with that of alkanes (13.7–27.9%), which was attributed to the higher MIR scales of alkenes related to alkanes. Similar conclusions have been reached in previous studies. Therefore, priority measures should be taken to reduce the VOCs with high MIR values, such as aromatics and alkenes, to control the formation of ozone originated from diesel exhaust.

The top 20 VOC species ranked by their OFP are given in Figure 8. The contribution of these substances accounted for approximately 90.0% of the total measured OFPs. Naphthalene, 1-butene, dodecane, 1,2,3-trimethylbenzene, 2-propenal, 1,2,4-trimethylbenzene and 3-ethyltoluene were the dominant species in the photochemical ozone formation process, and their OFP values were over 10 mg O₃ km⁻¹. Among the top 20 species, 11 belonged to the aromatic group and four were alkenes, which accounted for a lower mass percentage but higher MIR values. This indicates that substances present in small amounts but with high MIR values should not be ignored.