Emission Characteristics of Hazardous Air Pollutants from Medium-Duty Diesel Trucks Based on Driving Cycles

: Studies on the characteristics of hazardous air pollutants (HAPs) in the emissions of medium-duty diesel trucks are signiﬁcantly insufﬁcient compared to those on heavy-duty trucks. This study investigated the characteristics of regulated pollutants and HAPs, such as volatile organic compounds (VOCs), aldehydes, and polycyclic aromatic hydrocarbons (PAHs), and estimated non-methane hydrocarbon (NMHC) speciation in the emissions of medium-duty diesel trucks. Ten medium-duty diesel trucks conforming to Euros 5 and 6 were tested for four various driving cycles (WLTC, NEDC, CVS-75, and NIER-9) using a chassis dynamometer. In an urban area such as Seoul, CO and NMHC emissions were increased because of its longer low-speed driving time. NOx emissions were the highest in the high-speed phase owing to the inﬂuence of thermal NOx. PM emissions were almost not emitted because of the DPF installation. Alkanes dominated non-methane volatile organic compound (NMVOC) emissions, 36–63% of which resulted from the low reaction of the diesel oxidation catalyst. Formaldehyde emissions were the highest for 35–53% among aldehydes irrespective of driving cycles. By sampling the particle-phase of PAHs, we detected benzo(k)ﬂuoranthene and benzo(a)pyrene and estimated the concentrations of the gas-phase PAHs with models to obtain the total PAH concentrations. In the particle portion, benzo(k)ﬂuoranthene and benzo(a)pyrene were over 69% and over 91%, respectively. The toxic equivalency quantities of benzo(k)ﬂuoranthene and benzo(a)pyrene from NIER-9 (cold) for both Euro 5 and Euro 6 vehicles were more than ﬁve times higher than those of NIER (hot) and NEDC. In the case of NMHC speciation, formaldehyde emissions were the highest for 10–45% in all the driving cycles. Formaldehyde and benzene must be controlled in the emissions of medium-duty diesel trucks to reduce their health threats. The results of this study will aid in establishing a national emission inventory system for HAPs of mobile sources in Korea.


Introduction
Air pollution caused by various transportation sources poses a serious threat to human health. The Seoul Metropolitan Area (SMA) is significantly affected by mobile sources of pollution as the number of registered vehicles in the region is more than ten million as of 2019 [1]. Therefore, the management of automobile emissions is required in areas of high population density. The SMA is a metropolitan area composed of Seoul in its center, the Gyeonggi Province, and Incheon, and the contribution of automobiles to air pollution is high in this region. According to the national air pollutant emissions [2] data for 2017, medium-duty trucks are a major source of emissions as they contribute high proportions of CO (37.4%), NOx (25.1%), and PM 2.5 (19.9%) in the SMA. Even though many studies on emissions from heavy-duty trucks have been conducted, domestic and overseas studies on emissions from medium-duty trucks are insufficient.
This study investigated the characteristics of regulated pollutants and HAPs in the emissions of medium-duty diesel trucks using various test modes. This article presents measurement-based characteristics of regulated pollutants and speciation of HAPs that reflect the domestic situation in Seoul, and pollutants for priority management are identified.

Test Vehicle and Fuel
In this study, a total of ten medium-duty diesel trucks, five each conforming to Euro 6 and Euro 5, were selected based on data of their market share derived from the statistics of registered vehicles obtained from the Korea Automobile Manufacturers Association [25] to investigate the characteristics of their emissions in terms of regulated pollutants and HAPs. Table 1 lists the specifications of the test vehicles. The fuel used for testing satisfied the diesel emission standards of the Clean Air Conservation Act of Korea, and its properties are listed in Table 2.

Test Cycles
To investigate the characteristics of air pollutants from the emissions of medium-duty diesel trucks, test cycles were classified based on vehicle speed and by simulating the driving characteristics on major arterial roads, including those in Seoul. The National Institute of Environment Research (NIER)-9 driving cycle, with an average speed of 34-35 km/h, is one of the 15 driving cycles developed by the NIER to simulate real driving conditions on various urban roads in Korea. Apart from NIER-9, various certification cycles including the worldwide harmonized light duty test cycle (WLTC), the new European driving cycle (NEDC), and the constant volume sampler (CVS)-75 were used. The WLTC are chassis dynamometer tests for the determination of emissions and fuel consumption from light-duty vehicles. It consists of four parts for low, medium, high and extra-high speed sections. The NEDC is a driving cycle, designed to assess the emission levels of car engines and the fuel economy in passenger cars. It consists of four repeated ECE-15 urban driving cycles (UDC) and one extra-urban driving cycle (EUDC). The CVS-75 is federal test procedure (FTP)-75, which has been used for emission certification and fuel economy testing of light-duty vehicles in the United States. It consists of a cold-start transient, a stabilized, and a hot-start transient phase. In addition, 50% of the maximum load was reflected in the test vehicles, and the test was conducted three times or more. Table 3 lists the specifications of the test cycles. The driving patterns of the considered driving cycles are shown in Figure 1. The various test cycles of this study include certification cycles for export to Europe, America, and Asia, making it possible to compare and analyze the results of this study in other countries.

Chassis Dynamometer Emission Measurement System
As shown in Figure 2, the concentrations of pollutants were measured using a chassis dynamometer emission measurement system. The measurement system consists of a chassis dynamometer, an auxiliary operation device, a dilution tunnel, a constant volume sampler, and an exhaust gas analyzer. In the chassis dynamometer, set mechanical loads were applied to reflect the driving conditions of the vehicles, such as acceleration, deceleration, and constant speed. According to the set driving cycle, the gas emitted from the exhaust pipe during vehicle driving was mixed with air in the dilution tunnel and allowed to flow into the constant-volume sampler. The mixed gas was collected in a sample collection bag in the constant volume sampler for analysis using an exhaust gas analyzer. The quantity of emissions was calculated by converting into weight unit per traveled distance (g/km). Among the emissions, CO and CO 2 were analyzed using nondispersive infrared (NDIR) spectroscopy, and total hydrocarbons (THC) and CH 4 were analyzed using a heated flame ionization detector (HFID). The NOx was analyzed using a chemiluminescence detector (CLD). Table 4 lists the specifications of the exhaust gas analyzer.  PM was collected by diluting the exhaust gas with air at a certain ratio while driving using the chassis dynamometer. After collecting part of the exhaust gas and diluting it with external air in the dilution tunnel, the PM was collected using a quartz filter. A microbalance (CP2P-F, Sartorius) was used to measure the weight of the PM collected in the filter, and the measurement was performed in a weighing chamber at a temperature of 20 ± 5 • C and a relative humidity of 47 ± 5% for accurate measurement. PM measurements were performed in the following sequence. First, a filter, the weight of which was measured at a constant temperature in a humidity chamber, was installed in the filter holder of the PM sampler, and a PM sample was collected by suctioning the exhaust gas from the exhaust pipe at a constant flow rate and a constant speed. During this process, the temperature of the exhaust gas that penetrates through the filter was maintained at 52 • C or less. The filter in which the PM was collected was stored in a Petri dish to ensure that it remained intact. The filter containing the PM sample remained in the weighing chamber at a temperature of 20 ± 5 • C and a relative humidity of 47 ± 5% for 24 ± 4 h, and the weight difference before and after the measurement was obtained. The number of particles was measured using a condensation particle counter (CPC, MEXA-2000 SPCS).

Analysis Method for HAPs
For VOCs, a sample was collected in a 5-L Tedlar bag through a purge line after collecting diluted exhaust gas in a sample collection bag during the vehicle driving and analysis processes. VOCs were analyzed using the US EPA TO-14A [26] method. In the sample concentration and pretreatment process through a thermal desorption system, the moisture of the sample was removed using a Nafion dryer using the purge and trap technique, and VOCs were concentrated and adsorbed at a low temperature of −15 • C in the cold trap part. The concentrated sample was heated to 300 • C for desorption and injected into the gas chromatograph/mass spectrometer (GC/MS) for analysis. A 100-ppb standard gas containing a mixture of 56 materials, including ozone precursors, was used as the standard material. The calibration curve was confirmed at 2, 5, 10, 20, and 50 ppb, and the coefficient of determination (R 2 ) for the concentration and area was 0.998. Table 5 lists the analytical conditions used. Aldehydes were sampled by collecting the exhaust gas at a flow rate of 2 L/min through the dilution tunnel of the engine dynamometer and the pitot tube of the portable emission measurement system after connecting the Waters ozone scrubber and the 2,4-dinitrophenylhydrazine cartridge to the front and rear ends. The sample was stored in a freezer at −4 • C or less before pretreatment. For the extraction solvent, acetonitrile was filtered with a GHP filter (47 mm and 0.2 µm) before use in the extraction of the cartridge. The extraction volume was set to 5 mL. The sample was placed in a dedicated 2-mL vial and analyzed using ultra-performance liquid chromatography (UPLC) based on the US EPA TO-11A [27] method. A 40 µg/mL standard solution from Supelco, a mixture of 13 materials, was mixed and diluted and used as the standard material, and the calibration curve was confirmed at concentrations of 100, 200, 500, 800, and 1000 ng/mL. The R 2 for the concentration and area was 0.997. Table 6 lists the analytical conditions used. The PM was collected in quartz filters for the analysis of PAHs. The front and rear filters that collected PM were placed in the 11 mL extraction cell of the accelerated solvent extractor. The empty space of the cell was filled with diatomite to prevent errors during extraction. The accelerated solvent extractor performed sample extraction at a high temperature and pressure, and it extracted approximately 20 mL with dichloromethane for 15 min at a temperature of 100 • C and a pressure of 2000 psi. For the analysis of PAHs, the standard material was analyzed using the US EPA TO-13A [28] method. A 100 µg/mL standard solution from Supelco, a mixture of 16 materials, was diluted and used as the standard material. The calibration curve was confirmed at 0.1, 0.25, 0.5, 1, and 2 µg/mL, and the R 2 for the concentration and area was 0.995. Table 7 lists the analytical conditions used.

Emission Characteristics of Regulated Pollutants
The results of the tests conducted in various test cycles (WLTC, NEDC, CVS-75, and NIER-9) using a chassis dynamometer for medium-duty diesel trucks are presented in Figure 3. As shown in Figure 3, as the regulatory standard for diesel vehicles is strengthened from Euro 5 to Euro 6, CO (0.021-0.55 g/km (15-61%)), NMHC (0.001-0.015 g/km (8-64%), and NOx (0.02-0.17 g/km (5-54%)) showed a tendency to decrease. CO and NMHC emissions were the highest in the NEDC. This appears to be because the acceleration and deceleration rates of the engine speed were higher, and the low-speed driving time was longer than that of the other modes. In urban areas such as Seoul, due to heavy traffic, the low-speed driving period is long because of idling and frequent stops, resulting in high CO and NMHC emissions. In the NIER-9, CO emissions were 2.4-3.2 times higher and NMHC emissions were 1.4-1.5 times higher because the cold-start condition had a higher combustion instability and a lower catalytic activity than that of the hot-start condition [6,16,29,30]. In the WLTC, NOx emissions were 1.5-5.8 times higher than those in the other cycles. This appears to be due to the influence of thermal NOx caused by the highspeed section [15,16]. Euro 6 showed a 43% reduction in NOx emissions compared to Euro 5 owing to the lean NOx trap (LNT). As shown in Figure 4, the PM emissions of mediumduty diesel trucks were less than 0.001 g/km. It appears that the PM emissions were significantly low owing to the installation of the DPF in the test vehicles. Particle numbers (PN) in the emissions from the Euro 6 vehicles ranged from 8.2 × 10 10 to 1.2 × 10 11 #/km, which were lower than those from the Euro 5 vehicles (1.0 × 10 11 to 1.3 × 10 11 #/km).  As a limiting point, the engine and reduction device technology are advanced in Euro 5 and higher vehicles, so the emissions are low and the standard deviation is relatively large. Therefore, the difference is not large from a statistical point of view, but Euro 6 is considered to have a slight decrease compared to Euro 5.

Emission Characteristics of HAPs
As for the components of NMVOCs, alkanes exhibited the highest proportion (36.2-62.5%) in all the test modes of the Euro 6 and Euro 5 vehicles, followed by aromatics (14.4-40.1%), alkenes (8.0-34.9), and cycloalkanes (1.7-12.7%), as shown in Figure 5. Alkanes showed the highest proportion because they have lower reactivity in the DOC than in the other components [31,32].  Table 8 shows the five major chemical components in the NMVOC emissions for each regulatory standard and test mode. For Euro 6, benzene emissions (0.19-1.01 mg/km) were the highest in the WLTC, NEDC, CVS-75, and NIER-9 (hot) cycles, and dodecane emissions (1.72 mg/km) were the highest in the NIER-9 (cold). For Euro 5, the highest emissions were benzene (0.38 mg/km) in the WLTC, propylene (0.91 mg/km) in the NEDC, and dodecane (0.41-1.91 mg/km) in the CVS-75 and NIER-9 (cold and hot) cycles. The rankings of chemical components of the emissions varied based on the regulatory standard and test mode, and they are known to be influenced by emission reduction devices, driving conditions, and engine start conditions [15,16].
As for aldehyde emissions, formaldehyde emissions (0.44-3.71 mg/km) were the highest for all regulatory standards and test modes, followed by acetaldehyde (0.22-1.97 mg/km) and acrolein (0.29-1.69 mg/km) emissions as shown in Figure 6. Formaldehyde (34.6-52.8%) and acetaldehyde (20.5-34.1%) were present in high proportions. Formaldehyde emissions are known to be the highest from diesel trucks, followed by acetaldehyde emissions among the aldehydes [7,22]. A similar tendency (formaldehyde 48% and acetaldehyde 21%) was observed in heavy-duty diesel trucks [33]. In the NEDC, the aldehyde emissions were the highest. In the case of the NIER-9, they were 1.0-1.9 times higher under the cold-start condition compared to the hot-start condition. The aldehydes from biofuels showed that a higher temperature during pyrolysis processing increased the content of aldehydes [23]. In this study, 16 particle-phase PAHs in PM were analyzed, whereas gas-phase PAHs were not sampled. In the atmosphere, PAHs are present as semi-volatile compounds in both the gas and particle phases [34]. For total PAH concentrations, gas-phase PAHs were estimated using the gas-particle partition coefficient (K P ) calculated for five low molecular weight PAH compounds [35] based on previously established models [35][36][37][38] using the following equation: where, K P is the gas-particle partition coefficient (µg/m 3 ), TSP is the concentration of total suspended particulate material (µg/m 3 ), F is the particle phase concentration of PAHs (µg/m 3 ), and A is the gaseous phase concentration of PAHs (µg/m 3 ). Figure 7 shows the total PAH emissions (gas + particle phase) estimated in this study. For the test cycles on the x-axis, only the cycles in which the substances were detected were indicated. For the benzo(k)fluoranthene, 0.14-1.33 µg/km for Euro 6 and 0.12-0.15 µg/km for Euro 5 were estimated. For the benzo(a)pyrene, 0.11-0.15 µg/km for Euro 6 and 0.21-0.28 µg/km for Euro 5 were estimated. The results showed that 68.7-99.8% of benzo(k)fluoranthene and 90.7-98.1% benzo(a)pyrene were in the particle phase. According to Akyüz and Çabuk (2010), high molecular weight PAHs (4-5 rings) are mainly associated with the particle phase. When the K P ·TSP is larger than 1, the compound partitions dominantly to the particle phase; a K P ·TSP smaller than 1 indicates partitioning dominantly to the gaseous phase [39]. It appears that some PAHs were detected in trace amounts because of the high PM reduction rate as DPF was installed in all the test vehicles. Hu et al. (2013) [22] reported that vehicles with DPF can efficiently reduce emission of PAHs by more than 90%. In addition, standards for PAH emissions from high-quality diesel fuel have been made more stringent [21]. According to Li et al. (2020), four species of PAHs from biofuels were released at a temperature of 800 • C. The relative content of PAHs was clearly detected at a temperature of 750 • C rather than 650 • C [23]. In this study, the tested diesel fuel satisfied the high-quality diesel fuel standard specified limits for sulfur, PAHs, lubrication, and aromatic compounds. The toxicity equivalent factor (TEF) method is commonly used and is reliable for the evaluation of carcinogenicity of the identified PAHs. [14,15,18,40,41]. Since Nisbet and LaGoy (1992) put forward the concept of TEF based on the toxicity of PAHs (Table  9), benzo(a)pyrene is usually used as a reference compound to derive the TEQ of PAHs [14,15,18,40,41]. Emission characteristics of (a) benzo(a)fluoranthene and (b) benzo(a)pyrene from mediumduty diesel trucks according to driving cycles. Particle data points represent an average of triplicate measurements with a standard deviation of (a) 0-0.32 µg/km for NEDC, 0.32-2.10 µg/km for NIER-9 (cold), 0-0.37 µg/km for WLTC, and 0-0.46 µg/km for NEDC; (b) 0-0.33 µg/km for WLTC, 0-0.24 µg/km for CVS-75, 0-2.99 µg/km for NIER-9 (cold); and 0-0.84 µg/km for NIER-9 (hot). TEQ is a classic method to evaluate the toxicities of particles, and it is calculated as follows: where, Ci denotes the concentration of an individual i PAH. Figure 8 depicts the TEQ of the two identified PAHs based on driving conditions. For the test cycles on the x-axis, only the cycles in which the substances were detected were indicated. The two identified PAHs had a 5-ring structure, but the TEQ of benzo(a)pyrene was 100 times higher than that of benzo(a)fluoranthene because of a more bay-regionlike benzene ring structure, which could cause more mutagenicity and carcinogenicity compared to linear-structured PAHs [42][43][44]. Notably, the TEQs of benzo(a)pyrene and benzo(a)fluoranthene emitted from NIER-9 (cold) were more than 5 times and 12 times higher than that of NIER (hot) and NEDC for each Euro 5 and Euro 6 vehicles, respectively, implying that sufficient warm-up of vehicles could prevent the emission of carcinogenic aromatic compounds.
Moreover, only two 5-ring PAHs were identified because of the equipped aftertreatment systems such as DPF and LNT, which reduce the particle phase emission of PAHs from the tested vehicles [45].

Speciation of HAPs and Selection of Priority Management Pollutants
In this study, HAPs speciation was derived based on the NMHC emissions of mediumduty diesel trucks measured in various test cycles. As shown in Table 10, formaldehyde tended to exhibit the highest proportion (9.8-44.6%) in terms of all the regulatory standards and test modes, and a tendency similar to that of the European Environment Agency (EEA) [46] was observed. The proportion of formaldehyde was relatively the highest in both Euro 6 and Euro 5. The proportion of benzene was higher for Euro 6 than for Euro 5. When the speciation was derived during various test cycles based on each relevant regulatory standard, there were differences in speciation between the overseas driving cycles and the domestic driving cycle, confirming that the consideration of the domestic situation is important. In addition, it is deemed necessary to consider formaldehyde and benzene, which are carcinogens, as priority management pollutants through continuous monitoring.

Conclusions
In this study, measurement-based speciation that reflects the domestic situation was derived, and priority management pollutants were identified by investigating the emission characteristics of regulated pollutants and VOCs, aldehydes, and PAHs using various test modes for medium-duty diesel trucks. In contrast to previous studies, we researched the emission characteristics and speciation of unregulated air pollutants from mediumduty diesel trucks while also considering various driving cycles (WLTC, NEDC, CVS-75, and NIER-9), emission standards (Euro 5 and Euro 6), engine start conditions (cold-and hot-start temperatures), and after-treatment systems (DPF + LNT and DPF). When the emission characteristics of the regulated pollutants from medium-duty diesel trucks were investigated, the CO and NMHC emissions were highest in the NEDC mode because its low-speed driving time was longer compared to the other modes. In the SMA, CO and NMHC were increased during the urban driving cycle. NOx emissions were higher in the WLTC than in the other cycles because of the influence of thermal NOx caused by the high-speed section. PM emissions were 0.001 g/km or less because of the influence of the DPF installation in the test vehicles.
When the emission characteristics of HAPs were investigated, alkanes exhibited the highest proportion among the components of NMVOCs because they have a lower reactivity in DOC than those of the other components. Among the components of NMVOCs, benzene, propylene, and dodecane exhibited the highest emissions. The rankings of the chemical components in terms of emissions differed based on the test mode because of the influence of emission reduction devices as well as the engine and driving conditions. As for aldehydes, formaldehyde emissions were the highest in all the regulatory standards and test modes followed by acetaldehyde and acrolein emissions. For PAHs, only benzo(k)fluoranthene and benzo(a)pyrene were detected in trace amounts because PM emissions were low owing to the influence of the DPF installation. The benzo(k)fluoranthene and benzo(a)pyrene were over 69% and over 91% in the particle phase, respectively. We detected benzo(k)fluoranthene and benzo(a)pyrene in the particlephase PAHs and estimated the concentrations of the gas-phase PAHs with models to obtain the total PAH concentrations. The TEQ of two identified PAHs (benzo(k)fluoranthene and benzo(a)pyrene) from NIER-9 (cold) for the Euro 5 and Euro 6 vehicles each were more than 5 times and 12 times higher than NIER (hot) and NEDC, respectively.
When HAPs speciation was derived from the NMHC emission results of the mediumduty diesel trucks measured in various test cycles, formaldehyde exhibited the highest proportion in all regulatory standards and test cycles, and a tendency similar to that of the EEA was observed. Differences in speciation between the domestic and overseas test cycles indicated the importance of considering the domestic situation. The carcinogens, formaldehyde and benzene, were identified as priority management pollutants that re-quired continuous monitoring. The HAP speciation from medium-duty diesel trucks and the priority management pollutants identified in this study are expected to form the basis for policy implementation that will ensure the reduction of HAPs from domestic mobile sources in the SMA, where automobiles have a significant influence on air pollution. It is expected that the results will be utilized by inventory establishment researchers and air pollution reduction policy makers.

Conflicts of Interest:
The authors declare no conflict of interest.