Emission Characteristics of Regulated and Unregulated Air Pollutants from Heavy Duty Diesel Trucks and Buses

Received: June 25, 2018
Revised: October 10, 2018
Accepted: October 11, 2018

Download Citation: RIS|BibTeX|https://doi.org/10.4209/aaqr.2018.05.0195  

Cite this article:

Jung, S., Mun, S., Chung, T., Kim, S., Seo, S., Kim, I., Hong, H., Chong, H., Sung, K., Kim, J. and Hong, Y. (2019). Emission Characteristics of Regulated and Unregulated Air Pollutants from Heavy Duty Diesel Trucks and Buses. Aerosol Air Qual. Res. 19: 431-442. https://doi.org/10.4209/aaqr.2018.05.0195

HIGHLIGHTS

Keywords: Volatile organic compounds; Aldehydes; Polycyclic aromatic hydrocarbons; Heavy duty diesel trucks and buses; After-treatment systems.

INTRODUCTION

Since 1990, the major pollutants (carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NO_x), and particulate matter (PM)) emitted from vehicles in Korea have been systematically regulated with stringent emission standards and inventories. However, to date, the characterization and monitoring of unregulated air pollutants (volatile organic compounds (VOCs), aldehydes, polycyclic aromatic hydrocarbons (PAHs)) has been unsatisfactory. Furthermore, no emission standards currently exist for the control of such emissions from automobile sources.

The Seoul Metropolitan Area, which includes Seoul, Incheon, and other regions of the Gyeonggi Province, has low air quality as a result of pollution emitted from road vehicles. In 2016, the total number of road vehicles was approximately 10 million (MLTMA, 2017) due to the dense population of this area. Chen et al. (2009) noted that the number of vehicles has increased sharply in recent years, causing traffic congestion, low vehicle speeds, longer travel length and increased air pollution. Among the various road vehicles in Seoul Metropolitan Area, trucks contribute 49.7 and 60.0% of the NO_x and PM_2.5 emissions, respectively, while buses are responsible for 32.6% of the VOC emissions (NIER, 2016). As high emitters of pollution in the area of interest, the emission characteristics of heavy duty diesel trucks and buses therefore require investigation. Moreover, driving patterns, such as frequent stop-and-go driving, high traffic volumes, and growing congestion in the Seoul Metropolitan Area lead to middle and low vehicle speeds, which maintain low exhaust gas temperature.

Automobile-related NO_x, unburned HC, CO and PM emissions are major pollutants significantly impacting the urban environment (Keogh et al., 2009), and vehicle emissions are main sources of gaseous and particulate matter pollution in urban atmosphere (Jung et al., 2017). It has also been reported that the primary precursors of photochemical smog are formed by gaseous emissions under specific conditions in the presence of sunlight (Finlayson-Pitts and Pitts, 1997). However, Jiang et al. (2018) reported that NO_x emissions vary with driving cycle and the PM, HC, and CO emissions from heavy duty diesel trucks have been found to be low under the majority of test conditions, as these vehicles are equipped with diesel particulate filters (DPFs) and selective catalytic reduction (SCR) systems for PM and NO_x reduction. VOCs are important because they contribute to pollution through harmful photochemical processes (Streit, 2013), and VOCs have been classified as unregulated air pollutants due to their range of harmful effects on human health (Chang et al., 2001; Ho et al., 2002; Yang et al., 2007). According to a report by the U.S. EPA (2002), investigations into VOCs rather than that of total HCs are of particular importance in the context of health effects and environmental impacts.

In addition, George et al. (2014) reported that carbonyls, formaldehyde, and acetaldehyde, are the major VOCs from heavy duty trucks equipped with the latest after-treatment systems, comprising 72% of the total VOC emissions. Furthermore, Nelson et al. (2008) reported that the VOC emissions from heavy duty diesel trucks and buses were 29.8 mg km^–1 of formaldehyde, 13.2 mg km^–1 of acetaldehyde, 6.1 mg km^–1 of benzene and 3.4 mg km^–1 of toluene. Moreover, Hu et al. (2013) indicated that gaseous PAHs comprised a large portion of the total PAHs emitted from heavy duty diesel vehicles without DPF, and revealed that this was not linked to the PM remaining in the exhaust line after DPF system.

This study therefore aimed to investigate the emission characteristics of regulated (NO_x, PM, CO, and NMHC) and unregulated (VOCs, aldehydes, and PAHs) air pollutants from heavy duty diesel trucks and buses under urban driving conditions in the Seoul Metropolitan Area. In addition, a comparison of emission characteristics and the NMHC speciation for unregulated air pollutants was carried out with respect to the different after-treatment systems (DPF + exhaust gas recirculation (EGR) and SCR).

EXPERIMENTAL METHODS

 
Test Vehicles and Fuel

Four heavy duty diesel trucks and buses, which represent the majority of in-use trucks and buses in Korea, were selected for investigation of their emission characteristics. The tested vehicles were model year 2011–2013, all of which met the Euro 5 emission standards. These vehicles were equipped with two different after-treatment systems (DPF + EGR and SCR). The specifications of the test vehicles are shown in Table 1. The test fuel satisfied the diesel quality standards based on the Korean air quality conservation laws given in Table 2.

Test Cycles

The heavy duty diesel trucks and buses were tested on a chassis dynamometer (AVL Zöllner) following the National Institute of Environment Research (NIER)-9 mode according to vehicle type and considering the operating speed for heavy duty trucks and buses in the study area. The NIER-9 mode (average speed: 34–35 km h^–1) is one of the 15 driving cycles developed by the NIER to simulate actual vehicle operating conditions on various urban roads in Korea. The effects of cold and hot start conditions were also investigated in the test cycles considering a 50% load. The specifications of the driving cycles are given in Table 3.

Test Equipment and Emission Analysis

A chassis dynamometer system, a constant volume sampler (CVS-7400T, Horiba), a dilution tunnel, and an exhaust gas analyzer were used for the purpose of this study. The heavy duty diesel vehicle chassis dynamometer had a maximum power, a total inertia weight, a maximum velocity, and a roller diameter of 10,096 N, 454–5443 kg, 200 km h^–1, and 1219.2 mm, respectively. The exhaust gas was analyzed with diluted ambient air in the CVS. Table 4 shows the specifications of the exhaust gas analyzer (MEXA-7200D, Horiba), and Fig. 1 shows a schematic representation of the vehicle emission test system. PM was collected in quartz filters using a PM sampling holder connected to the CVS. The quartz filters were weighed at the proper temperature (20 ± 5°C) and humidity (47 ± 5%) to obtain the PM mass data.

Fig. 1. Schematic diagram of vehicle emission test system.

Following analysis of the regulated pollutants, the exhaust gas was collected in a 5 L Tedlar bag. The sampled gas was transferred to the automatic sampler at low temperature using a thermal desorber (Unity 2, Markes Int.). After passing the sampled gas through a purge and trap at –15°C, the VOCs were condensed, and moisture was removed using a Nafion dryer. Gas chromatography/mass spectrometry (GC/MS) was used to analyze sampled gas (see Table 5 for details regarding the pretreatment and analytical conditions). VOCs standard gas (56 Component VOC Mix, Supelco Inc.) was analyzed to calibrate the sampled gas.

The 2,4-dinitrophenylhydrazine (DNPH) method was used to analyze all aldehyde components. After passing the diluted exhaust gas through the DNPH cartridge (Waters), ultra-performance liquid chromatography (UPLC) was used to analyze the concentrated acetonitrile-containing solution (for analytical conditions, see Table 6). Aldehyde standard solution consisting of 13 components (Carbonyl-DNPH Mix 1, Supelco Inc.) was analyzed to calibrate the concentrated sample.

The PAHs were collected in quartz filters that had been placed in an 11 mL extraction cell of the accelerated solvent extractor. The accelerated solvent extractor was then extracted using 20 mL dichloromethane for 15 min at a pressure of 13,789.5 kPa and temperature of 100°C. GC/MS was used to analyze the PAHs (for analytical conditions, see Table 7) according to the 16 EPA PAH compounds present in the 16 component PAH mixture (PAHs Mix, Supelco Inc.).

The values of repeatability, linearity, zero drift and span drift for the exhaust gas analyzer were 0.1, 1.8, 0, and 2% for CO; 0.1, 1.5, 0, and 0.3% for CO₂; 0.2, –1.4, 0%, and 0.1% for THC; and 0.3, –1.7, 0, and 0.3% for NO_x, respectively. VOC, aldehyde (Carbonyl-DNPH), and PAHs standards were used for calibration, and the calibration curves were calculated for a minimum of five standard concentrations. The blanks values were subtracted from the sample values with respect to the VOCs, aldehydes and PAHs. Linearity was confirmed by R² values of 0.993–0.998 for the calibration curves. In addition, the method detection limit (MDL), precision and accuracy were within the ranges of 0.06–0.65 ppb, 1.0–13.4%, and 77.4–101.8% for the VOCs; 25.8–60.1 ppb, 4.1–17.7%, and 75.4–113.9% for the aldehydes; and 7.2–61.7 ppb, 2.4–16.5%, and 94.0–119.0% for the PAHs, respectively. These results were within the accepted ranges of the Korean quality control standards. 

RESULTS AND DISCUSSION

 
Emission Characteristics of Regulated Air Pollutants

Emission characterization was initially performed to measure the quantities of regulated air pollutants emitted from the heavy duty diesel trucks and buses equipped with different after-treatment systems. Thus, the NO_x, PM CO and NMHC emissions from the test trucks and buses subjected to the NIER-9 mode test driving cycle with different start conditions of cold or hot engine temperatures are represented in the units of “mass emitted per kilometer” i.e., g km^–1) as shown in Fig. 2. More specifically, Fig. 2(a) indicates that the NO_x emissions from trucks equipped with two different after-treatment systems were relatively similar in the driving cycle with the cold temperature engine start, 5.63 ± 0.40 g km^–1 for the DPF + EGR system and 5.50 ± 0.26 g km^–1 for the SCR system. For the hot start condition, the SCR-equipped truck had noticeably lower NO_x emissions (4.27 ± 0.45 g km^–1) than the DPF + EGR equipped truck did (4.86 ± 0.06 g km^–1). In general, SCR catalysts are expected to chemically react at temperatures of ≥ 200°C, where NO_xconversion efficiencies reduce the quantities of NO_x emissions in the exhaust tailpipe (Misra et al., 2013). In the case of NO_x measurement, similar values were obtained for the two different after-treatment systems, likely due to the low exhaust gas temperature, under which the required chemical reaction could not efficiently occur to reduce the NO_x emissions in the SCR system. The findings of Jiang et al. (2018) support this, as they observed that NO_x emissions for SCR-equipped vehicles were strongly correlated with the SCR temperature.

Fig. 2. Emission characteristics of (a) NO_x, (b) PM, (c) CO, and (d) NMHC according to vehicle type for cold and hot start conditions.

In contrast, the buses examined herein were found to have higher NO_x emissions relative to the trucks due to their higher engine power and larger engine displacement. In addition, NO_x emissions under the cold start condition (14.24 ± 0.38 g km^–1) for the DPF + EGR system were similar to that of the SCR system (14.76 ± 0.32 g km^–1), while the SCR-equipped bus showed slightly higher NO_xemissions relative to the DPF + EGR-equipped buses under hot start conditions (13.85 ± 0.46 and 11.40 ± 0.57 g km^–1, respectively). Although the NO_x emissions of heavy duty diesel trucks with similar engine displacement and power have generally received little attention, Jiang et al. (2018) showed a wide variation in the reported NO_x emissions from diesel buses complying with the Euro 5 standard, 0.41–13.98 g km^–1 under the idle, creep, transient, and cruise phases. Upon comparison of the data for the two different after-treatment systems, it was apparent that relatively low combustion temperatures during operation of the EGR system resulted in lower NO_x emissions than from the SCR-system. Indeed, previous studies (Zeldovich et al., 1985; Gallus et al., 2017; Ramos et al., 2018) have indicated that NO_x tends to form in the high temperature combustion region, and that it is highly dependent on the EGR mass flow rate with respect to EGR valve operation. Accordingly, the EGR operation may lowerthe NO_x emissions from DPF + EGR-equipped bus compared to SCR-equipped bus. Furthermore, for these cases, there were noticeable differences in the NO_x emissions between the two start conditions. These differences can be attributed to the EGR and the chemical reaction in the SCR system. For the hot start condition, it is important to note the difference in NO_x emissions between the DPF + EGR and SCR systems.

The overall PM emission results are presented in Fig. 2(b). For the trucks, the test values ranged 0.0022–0.0023 g km^–1 for the cold start condition and 0–0.0005 g km^–1 for the hot start condition. For the buses, PM emissions of close to zero were recorded for the DPF + EGR system, while these emissions ranged 0.0001–0.0006 g km^–1 for the SCR system. These results suggest that identification of the differences in PM emissions between the after-treatment systems is difficult due to the low PM emission levels in both the trucks and the buses. It is also important to compare our results with those reported in the literature. More specifically, Jiang et al. (2018) reported PM emissions of 0.0006–0.002 g km^–1 from experiments on a diesel bus with a similar displacement and power, while Quiros et al. (2018) reported PM emissions of 0.003 and 0.011 g km^–1 for heavy duty diesel vehicles equipped with DPF and DPF + SCR systems, respectively. In addition, PM emissions from the DPF + EGR-equipped diesel truck decreased with higher start temperatures, whereas similar values were recorded for the cold and hot start conditions in the SCR-equipped case. In theory, PM emissions depend on the flame temperature and the local flame equivalence ratio in the combustion chamber, and so the engine produces higher PM emissions at low flame temperatures and at high flame equivalence ratios. Furthermore, Ramos et al. (2018) reported that the quantity of PM can increase upon increasing the EGR mass flow rate. Accordingly, higher PM emissions in the DPF + EGR case can be attributed to low combustion temperature and air deficiency due to EGR valve opening. Moreover, PM filtration in the DPF also reduced PM emissions.

The CO and NMHC emissions for the diesel trucks and buses equipped with the two different after-treatment systems are depicted in Figs. 2(c) and 2(d), respectively. In the case of the trucks, CO emissions were 0.94–2.51 g km^–1 under the cold start condition and 0.71–2.01 g km^–1 under the hot start condition, while NMHC emissions were 0.06–0.23 g km^–1 under the cold start condition and 0.04–0.25 g km^–1 under the hot start condition. For the buses, CO emissions were 0.94–1.03 and 0.71–0.84 g km^–1 under the cold and hot start conditions, respectively, while NMHC emissions were0.07–0.11 and 0.06–0.16 g km^–1, respectively. This result reveals that both the start conditions and the after-treatment systems can influence the CO and NMHC emissions. In addition, Jiang et al. (2018) reported that CO and NMHC emissions were lower in cruise mode than in creep mode, reporting CO emissions of 0.04 and 3.13 g km^–1, respectively, in addition to NMHC emissions of 0.006 and 0.28 g km^–1, respectively. Furthermore, Alves et al. (2015) noted that CO and HC emissions under cold start conditions were higher than those under hot start condition for in-use vehicles on chassis dynamometer urban cycles. The literature further reveals that CO forms due to incomplete combustion, and that NMHC is formed due to the evaporation of fuel and the chemical reaction of unburned HCs in the exhaust (Heywood, 1988; Kennedy et al., 2015). In particular, Mollenhauer et al. (2007) reported that NMHC forms in the zone of lean air-fuel mixture and excess fuel under over-fueling by late injection, while the high CO concentration can be attributed to the rich air-fuel mixture where incomplete conversion of the HCs to CO₂ takes place due to air deficiency. In addition, compared to the SCR-equipped vehicle, the DPF + EGR-equipped vehicles exhibited higher CO emissions, and the increased NMHC formation can be attributed to the EGR operation and excess fuel due to over-fueling by late injection for DPF regeneration. These results indicate that DPF had a significant effect on the reduction of CO, while the hot combustion temperature influenced the NMHC reduction. Furthermore, the cold start condition resulted in higher CO emissions than the hot start condition. In contrast, the NMHC emissions from the DPF + EGR-equipped truck and bus under the hot start condition were higher than those under the cold start condition, whereas the NMHC emissions from the SCR-equipped truck and bus under the cold start condition were higher than those recorded under the hot start condition. These findings can be attributed to lack of heat from the cylinder walls under the cold condition, as this heat is required to evaporate the fuel and the fuel-rich mixtures. 

Emission Characteristics of Unregulated Air Pollutants

The NMVOC emissions from the DPF + EGR-equipped vehicles were composed of 11.2–14.2 mg km^–1 of alkanes, 3.2–11.8 mg km^–1 of alkenes, 5.3–9.1 mg km^–1 of aromatics, and 0.7–1.0 mg km^–1 of cycloalkanes, whereas those from the SCR-equipped vehicles were 4.5–12.7 mg km^–1 of alkanes, 3.0–8.5 mg km^–1 of alkenes, 2.0–9.6 mg km^–1 of aromatics, and 0.6–2.2 mg km^–1 of cycloalkanes. Fig. 3 shows the compositions of the NMVOCs emitted from the trucks and buses subjected to different driving cycles under the cold or hot start temperatures. Among the various NMVOCs, alkanes were the most abundant, accounting for 44.5–52% of the total for the trucks and 42.6–59.4% for the buses. In addition, for both trucks and buses, higher quantities of alkenes were detected from the SCR-equipped vehicles, while those equipped with the DPF + EGR system emitted higher quantities of aromatics. The emission trends of the cycloalkanes were similar to those of the alkenes. The above results therefore indicate that alkane emissions were higher than those of alkenes, cycloalkanes, and aromatics, likely due to the efficiency of oxidation catalysts toward alkanes being lower than for the other components. Similar results have also been reported in the literature, with alkane, aromatic, and alkene emissions of 46.1–48.5, 24.2–25.8, and 22.3–25.1%, respectively, being reported from heavy duty diesel trucks and buses (NIER, 2017). In addition, Knafl et al. (2006) reported that the VOCs gave different conversion efficiencies through diesel oxidation catalysts, corresponding to 63–80, 74–83, and 92–99%, for alkanes, aromatics, and alkenes, respectively.

Fig. 3. NMVOCs composition according to vehicle type for cold and hot start conditions.

Table 8 summarizes the five major chemical components in the NMVOC emissions from the heavy duty diesel trucks and buses. For the diesel truck equipped with the DPF + EGR systems, the major components were 4.79–5.44 mg km^–1 of propylene and 4.26–4.45 of mg km^–1 1-butene, although n-nonane, n-decane, dodecane, and benzene were also detected. In contrast, the NMVOCs from the SCR-equipped diesel truck comprised mainly 1.59–1.84 mg km^–1 of benzene, 1.39–1.63 mg km^–1 of propylene, and 1.49–1.62 mg km^–1 of 1-butene, although n-heptane, n-decane, dodecane, and n-octane were also present. In the case of the buses, 3.91–4.73 mg km^–1 of n-nonane, 3.1 mg km^–1 of dodecane, and 1.7–2.27 mg km^–1 of n-decane were the major components obtained from the DPF + EGR system, while 5.78–6.66 mg km^–1 of propylene and 2.93–3.4 mg km^–1 of benzene were the main constituents in the SCR system. These results indicate that the chemical components of the NMVOC emissions were strongly dependent on the after-treatment system employed, the engine start conditions, and the vehicle type, as these types of emissions are produced upon the incomplete combustion of diesel fuels or lubricating oils. We also note that Hong et al. (2018) reported that the main components of NMVOCs from heavy duty diesel trucks equipped with after-treatment systems were 11.3–16.1% of propylene, 15.5% of benzene, and 10.3–11.3% of 1-butene.

The aldehyde emissions from the trucks and buses subjected to different driving cycles with cold and hot start conditions are outlined in Fig. 4. For the trucks, the formaldehyde, acetaldehyde, and acrolein emissions for the SCR-equipped and DPF + EGR systems were 5.12 and 9.08–10.62, 2.17 and 3.39, and 0 and 2.75–4.81 mg km^–1, respectively. For the buses, the corresponding values were 8.67–14.61, 3.66–5.09, and 1.50–2.46 mg km^–1, respectively. Similar results were also previously reported, in that the most dominant carbonyl compounds from meidum-duty diesel trucks were formaldehyde (9.2 mg km^–1) and acetaldehyde (3.5 mg km^–1) (Siegl et al., 1999). In addition, Nelson et al. (2008) noted formaldehyde and acetaldehyde emissions of 29.8 and 13.2 mg km^–1, respectively, from diesel vehicles, while Yao et al. (2015) reported that formaldehyde andacetaldehyde comprised 47.9 and 21.0% of the carbonyl emissions from heavy duty diesel trucks.

Fig. 4. Emission characteristics of aldehydes according to vehicle type for (a) cold and (b) hot start conditions.

The overall PAH emission results are presented in Fig. 5. As shown in this figure, the naphthalene and phenanthrene emissions were 1.13–1.72 and 0.91–1.31 µg km^–1 under the cold and hot start conditions in the trucks, and 0–1.57 and 0–1.19 µg km^–1 under identical conditions in the buses, respectively. It was also reported that the main PAH components emitted from heavy duty diesel vehicles were naphthalene (25.2 ± 20.8%), pyrene (24.5 ± 11.7%), and phenanthrene (16.3 ± 7.4%) (Shah et al., 2005). In addition, the amounts of naphthalene and phenanthrene from the DPF + EGR-equipped truck decreased with higher start temperatures, whereas the SCR-equipped vehicles showed similar values under both the cold and hot start conditions. In the case of the buses, the emission of naphthalene and phenanthrene also decreased with higher start temperatures, with virtually no naphthalene being detected under hot start conditions. Furthermore, phenanthrene was barely detected under either start condition. According to Nelson et al. (2008), naphthalene and phenanthrene emissions of 2605 and 215 µg km^–1 were recorded from diesel vehicles without after-treatment systems, whereas Hu et al. (2013) reported that the after-treatment systems in heavy duty diesel vehicles efficiently abated more than 90% of the total PAHs. Furthermore, Nelson et al. (2008) showed that PAH emissions were lowered where high-quality fuels were employed. It was therefore apparent that the fuels employed in this study satisfied the high-quality diesel fuel standards in terms of the ultra-low sulfur, PAHs, lubrication, and aromatic compounds, as listed in Table 2.

Fig. 5. Emission characteristics of PAHs according to vehicle type for (a) cold and (b) hot start conditions.

Table 9 summarizes the 10 major chemical components present in the NMHC emissions from the heavy duty diesel trucks and buses. The portions of the chemical components present in the unregulated pollutants were calculated from the NMVOCs and the aldehydes based on the NMHC emissions. Regardless of the driving conditions employed, formaldehyde was the most abundant component in both trucks and buses (14.0–29.9%), while for the DPF + EGR-equipped truck, the major component was acetaldehyde (7.7–7.9%), and for the SCR-equipped truck, acetaldehyde (12.4%) and benzene (11.0%) were dominant. Similarly, n-nonane (12.3–12.5%) was the main component in the case of the DPF + EGR-equipped bus, whereas propylene (15.5–15.7%) was dominant for the SCR-equipped bus. In a previous study by the EEA (2016), the aldehyde components of the NMVOCs were found to consist of 8.4% formaldehyde and 4.6% acetaldehyde for heavy duty vehicles. In addition, the NIER (2017) noted that for heavy duty diesel trucks and buses, the NMHCs were composed of 22.3% formaldehyde, 8.9% acetaldehyde, and 8.7% propylene. Overall, we found that the composition ratios of the various unregulated pollutants exhibited differences with respect to the vehicle type, the engine start conditions, and the after-treatment system. This was supported by the HEI (1995), who reported that the composition of diesel emissions differed significantly based on the vehicle type, driving conditions, fuel type, lubricant, and after-treatment system employed.

CONCLUSIONS

Objective data and detailed information on the emissions released during real urban driving cycles is necessary for developing effective policies on the reduction of air pollutants from heavy duty diesel vehicles in the Seoul Metropolitan Area. In contrast to previous studies, we reported the emission characteristics and speciation of unregulated air pollutants from heavy duty diesel vehicles while also considering different vehicle types (trucks and buses), engine start conditions (cold and hot temperatures), and after-treatment systems (DPF + EGR and SCR). Additionally, we characterized both the regulated and unregulated emissions of Euro 5-compliant diesel trucks and buses that were equipped with different after-treatment systems (DPF + EGR or SCR). Using identical heavy duty diesel vehicles, differences between the emissions from two driving cycles under cold and hot start conditions were assessed. We found that NO_x emissions were similar for vehicles equipped with either after-treatment system due to the low temperature of the exhaust gas. Moreover, PM emissions from trucks and buses were close to zero under both cold and hot start conditions. For NMVOCs, alkane emissions were higher than alkene, cycloalkane, and aromatic emissions for both vehicle types because of the low efficiency of the oxidation catalysts with regard to alkane. Furthermore, the individual components of the NMVOC emissions were affected by both the after-treatment system and the engine start conditions due to incomplete combustion and the evaporation of liquid fuel. Formaldehyde wasthe most abundant aldehyde as well as the most abundant NMHC, and no PAHs, with the exception of naphthalene and phenanthrene, were detected. These results can be attributed to the differences in vehicle type, engine start condition, and after-treatment system. In the Seoul Metropolitan Area, the low temperature of vehicular exhaust gas during the urban driving cycle leads to heavy air pollution due to the common stop-and-go driving pattern. We therefore expect that our speciation of NMHCs produced by heavy duty diesel trucks and buses will assist in the establishment of a Korean emissions system for hazardous air pollutants from vehicular sources.

ACKNOWLEDGMENTS

This work was supported by a grant from the National Institute of Environmental Research (NIER), funded by the Ministry of Environment (MOE) of the Republic of Korea (ex: NIER-2017-01-01-078).

Aerosol Air Qual. Res. 19 :431 -442 . https://doi.org/10.4209/aaqr.2018.05.0195