Emissions of volatile organic compounds from interior materials of vehicles

Building and Environment 170 (2020) 106599

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Emissions of volatile organic compounds from interior materials of vehicles Shen Yang a, b, *, Xudong Yang b, Dusan Licina a a

� Human-Oriented Built Environment Lab, School of Architecture, Civil and Environmental Engineering, Ecole Polytechnique F�ed�erale de Lausanne, CH-1015, Lausanne, Switzerland b Department of Building Science, Tsinghua University, CN-100084, Beijing, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Indoor air quality (IAQ) Source apportionment Vehicular environment Material emission Air temperature

People spend a substantial amount of daily time in vehicle environments being exposed to variety of airborne chemicals. High concentrations of volatile organic compounds (VOCs) inside vehicle cabins have been of increasing concern owing to various health risks. Yet, there is a limited knowledge and data about VOC emissions from interior vehicular materials. In a controlled small-scale ventilated chamber, we quantified dynamic VOC emission characteristics for four commonly-used vehicular interior materials (leather coat of seats (LCS), dash­ board (DB), pillar (PI) and ceiling (CE)) exposed to the two interior air temperatures (25 and 60 � C). The emission strengths of VOCs increased with temperature elevation by 3–36 times, but not linearly for every group of compounds. The level of individual VOC compounds emitted was a strong function of time, indicating the importance of a continuous sampling approach. Correlation analysis of individual VOCs revealed different emission mechanisms among the major compounds. The source apportionment results indicated that the LCS materials were associated with emissions of aromatics, carbonyls and other VOCs at 25 � C, while special at­ tentions should be paid to the DB and the PI for their predominant contributions to extremely elevated level of aliphatic compounds at 60 � C. The results of this study are potentially useful for database of VOC emissions from vehicular materials, development of improved standard testing methods and for improved modeling of VOC emissions.

1. Introduction Volatile organic compounds (VOCs) are recognized as important gaseous pollutants due to their adverse effects on human comfort and health [1–3]. Wide existence of VOCs in various indoor environments has aroused public concerns: VOCs have been detected in residences [4], offices [5], aircraft cabins [6], vehicle cabins [7], etc. Rapid growth in motor vehicle possession has increased amount of time spent commuting which triggered a considerable interest in investigating VOCs in the micro-environments of vehicle cabins. Several studies have reported that VOC concentrations inside vehicle cabins are comparable to, or even higher than, that in buildings [8–11]. The high VOC concentrations in vehicles may be attributed to combined effects of VOCs intrusion from outdoors [12,13], fuel combustion [14, 15], emissions from interior materials [16], human-related emissions [17], and others [18,19]. The VOC emissions from interior materials in vehicle cabins account for a considerable fraction of the total VOCs (TVOCs), especially in new automobiles [20]. Previous research

examined VOC concentrations inside vehicle cabins at 20–25 � C when automobiles were parked with windows and doors closed. The study results showed that aliphatic compounds, aromatic compounds and al­ dehydes were the major detected VOCs in the test vehicles, and that TVOC concentration could reach as high as 8000 μg/m3 [21]. This concentration exceeds the standard limit for buildings by more than 10 times [22], indicating extremely high VOC emission strengths of interior materials in vehicle environments. While that study brought important new knowledge about VOC emission strength from interior vehicle materials in actual use, the study did not differentiate emissions from individual materials. Segregating emissions from different vehicular materials is important for VOC source apportionment and improved source control in vehicle cabins. Thus, more attentions are required on VOC emission characteristics of each major vehicular material. Research with specific emphasis on VOC emissions from individual interior materials of vehicle cabins are presently limited. Existing investigation methods can be categorized into two classes, including high-temperature headspace measurement and assessment in ventilated

� * Corresponding author. Human-Oriented Built Environment Lab, School of Architecture, Civil and Environmental Engineering, Ecole Polytechnique F� ed�erale de Lausanne, CH-1500, Lausanne, Switzerland. E-mail address: [email protected] (S. Yang). https://doi.org/10.1016/j.buildenv.2019.106599 Received 13 June 2019; Received in revised form 13 December 2019; Accepted 14 December 2019 Available online 17 December 2019 0360-1323/© 2019 Elsevier Ltd. All rights reserved.

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chambers. The headspace test method has been commonly applied by vehicle manufacturers, as recommended in Standard VDA277 [23] in Germany. In the method, a targeted material is sealed in a sampling bag (e.g., a Tedlar bag) at a high temperature to accelerate VOC emissions for a short period. Afterwards, VOCs inside the bag are sampled and analyzed to generate emission strength and composition profiles of the tested material. However, the method neglects dynamic VOC emission characteristics of interior materials, which is essential to VOC assess­ ment in vehicle cabins. In contrary, in the ventilated chamber method, a targeted material is placed in a chamber with controlled air exchange rate and temperature, so that the test conditions can mimic realistic vehicle environments. ISO [24] developed an assessment standard to measure VOCs emitted from vehicle interior materials in a small-scale ventilated chamber with air exchange rate of 0.4 h 1 and air tempera­ ture at 65 � C. Existing data of VOC emissions from vehicular materials are very rare at present. Yang et al. [25] conducted multi-time VOC sampling in ventilated chamber test to investigate dynamic emissions of aromatics (benzene, toluene, p-xylene, ethylbenzene and styrene) released from floor mats of vehicles. The results indicated that aromatics emission strength of the floor mat decayed with time. Questions remained open about VOC emission strengths and compositions from other vehicular interior materials, which is essential for VOC source apportionment in vehicles. A unique difference between vehicle cabin environment and build­ ings is the much broader air temperature range in vehicles. Owing to combined effects of large surface ratio of windows, solar radiation and low heat insulation, air temperature of vehicle interiors could be as high as 70 � C [26]. Previous field tests reported strong positive correlations between interior air temperature and VOC concentrations in vehicle cabins [27,28]. Given the presence of wide range of air temperatures inside vehicles, it is necessary to characterize the VOC emissions from interior materials under a broad air temperature range. The objective of this study is to investigate dynamic VOC emission strengths and composition of four major vehicular materials, and their dependence on the interior air temperature. We examined the emissions from four commonly-used major vehicular interior materials — leather coat of seat (LCS), dashboard (DB), pillar (PI) and ceiling (CE). The experiments were performed in a well-controlled ventilated chamber with the two different air temperature set points (25 and 60 � C). The results are of potential use for enriching database of VOC emissions from vehicular materials, VOC source apportionment and control, for devel­ opment of enhanced standard testing methods, and for improved modeling of VOC emissions in vehicles.

piece except the PI due to its irregular shape. The original size PI samples (805 cm2 area each) were used for emission testing. Fig. S1 illustrates the four materials used for material emission testing. 2.2. Experimental protocol We utilized a small-scale, well-controlled environmental chamber (Fig. 1) in accordance to the ASTM D5116 [39] for material emission tests, which is more flexible in setting of environmental parameters [41]. The chamber (53 cm � 40 cm � 25 cm) was equipped with the ceiling-mounted mixing fan to provide uniform air mixing and with the water bath to control the air temperature inside. The synthetic air (mix of 20.9% of oxygen in nitrogen) was supplied from pressurized cylinder (Beijing Zhaoge Gas Technology, qualified by Tsinghua University, China). Throughout the experiments, the air flow was controlled by a valve and a flow controller at 0.34 L/min, corresponding to a calculated air exchange rate of 0.4 per hour, in accordance with that of ISO stan­ dard [24]. Relative humidity inside the chamber was controlled by humidity controller via adjusting the valves at the wet and dry clean air flow paths. An automatic sensor and data logger (WSZY-1, Beijing, China) was placed in the exhaust to monitor the temperature and hu­ midity continuously. The logger was calibrated before the whole experiment. The precision of the logger for temperature and relative humidity was 0.1 � C and 0.1% respectively. One night before each test, the chamber was cleaned with methanol and deionized water, and ventilated at 0.4 h 1 air exchange rate with synthetic air throughout the night to eliminate any residual pollutants from previous experimental runs. Our previous experiments have demonstrated the control precision of the environmental chamber system [17,45–47]. The VOC emissions from one sample of each material was tested at the air temperature of 25 � C and relative humidity at 50%. The other material samples were tested at the air temperature of 60 � C and relative humidity of 8%. Both scenarios corresponded to the same value of ab­ solute humidity of around 9.9 g/kg of dry air. The two experimental conditions represented typical comfortable and extreme environmental temperature conditions that can be encountered in vehicle cabins. At the beginning of each test, the targeted material sample was put on the bottom of the chamber. The outer surface of the material was exposed to the chamber air to ensure one-sided emissions from material, as commonly encountered in vehicle cabins, while the other side and the edges were sealed with non-emissive aluminum tapes. For the LCS and PI materials, the exposed surface was artificial leather. The smoother side of the DB sample and the textile surface of the CE sample were exposed. The VOCs were measured at the exhaust branch by drawing the air through a Tenax-TA tube by a sampling pump (QC-II, Beijing Municipal Institute of Labor Protection, China) at 300 mL/min for 10 min. The flow rate of the sampling pump was calibrated with the soap film flowmeter (BUCK M-1, A.P. BUCK, Inc., US) before each experi­ ment. Based on preliminary tests, we conducted three VOC measure­ ments during the first 2 h to capture the sharp concentration fluctuations. Afterwards, the sampling interval steadily dropped until twelfth hour. Then, after one-night pause, we obtained at least two VOC samples in the second-day experiment before the 26th hour. At least 11 VOC samples were obtained for each test.

2. Materials and methods 2.1. Test materials We tested four brand new and commonly-used surfaces of vehicle interiors — leather coat of seat (LCS), dashboard (DB), pillar (PI) and ceiling (CE). The materials were obtained from National Automobile Quality Supervision and Inspection Center, China in their original packaging. The four materials are unique because of their large exposed surface area that they occupy in vehicles. As per manufacturer’s speci­ fication, the typical exposed material surface areas are the following: LCS ¼ 5.25 m2; DB ¼ 2.04 m2; PI ¼ 1.35 m2; and CE ¼ 2.14 m2. The 9 mm thick LCS, used for covering vehicle seats, was composed of a polyurethane foamed material covered by artificial leather. The DB, the panel facing the front passengers of a vehicle, was a piece of 4 mm thick polypropylene board of which one surface was smoother. The 3 mm thick PI was designed as a vertical support of window area, made by artificial leather covering the polypropylene board. The 2 mm thick CE made of textile and polyurethane foamed material was used to cover the interior ceiling of vehicles. We obtained two samples from each material type for testing at the two air temperatures. To normalize the emission area and initial test conditions, each sample was cut into 50 cm � 30 cm

2.3. VOC sample analysis The Tenax-TA tubes were analyzed with gas chromatography – mass spectrometry (GC-MS, 6875/5975B, Agilent, USA) coupled with Thermo-desorber (TD, Markes, UK). The sorbent tubes experienced two desorption stages: primary (tube) desorption at 250 � C for 10 min and secondary (trap) desorption at 300 � C for 3 min. The flow rate of carrier gas (nitrogen) for thermal desorption was set to 55 mL/min. Then analytes were chromatographically resolved on an HP-VOC capillary column of 30 m in length and 0.2 mm in diameter. Temperature program of the column was set at a total time of 26 min, including the two sub 2

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Fig. 1. Schematic of the laboratory-scale ventilated environmental chamber configuration for VOC emission test.

processes: temperature rise from 40 � C to 250 � C at 10 � C/min, and then held steady for 6 min. The helium carrier gas was set at a constant pressure of 138 kPa for a flowrate of 3 mL/min. The MS was operated in the total ion scan mode, in which the entire mass range (m/z from 30 to 300) was scanned at a frequency of 2.5 Hz. The peak spectrum was checked with the method recommended by the US National Institute of Standards and Technology (NIST) and confirmed by retention time comparison with that obtained for standard compounds from Institute for Environmental Reference Materials of Ministry of Environmental Protection, China.

2.5. Estimation of VOC emission strengths The VOC emission strength of the tested materials can be estimated on basis of VOC mass-balance equation in the chamber, as shown in Equation (1): dCðtÞ A ¼ EðtÞaCðtÞ dt V

(1)

where. C(t), VOC concentration inside the chamber, μg/m3; t, time, h; A, surface area of the tested material, m2; V, volume of the environmental chamber, m3; E(t), VOC emission strength of the tested material, μg/m2h; a, air exchange rate of the chamber, 1/h. Average VOC emission strength between sampling intervals can be calculated with measured VOC concentration using central differential method with transformation as:

2.4. Quality control and quality assurance Quantification of individual VOCs was completed with external standard eight-point calibration method as described in detail in our previous study [29]. Summary of sample analysis quality control including calibration data can be seen in Table S1. Commonly detected VOCs, including benzene, toluene, ethylbenzene, m/p-xylene, o-xylene, acetic acid butyl ester, styrene, and undecane, were quantified with standard compounds (purity > 99.9%, Aladdin). Previous studies have proven the effectiveness of Tenax tubes sampling the major carbonyls found in this study, such as hexanal and decanal [6,17,30–32,42]. Concentrations of other VOCs were semi-quantified to their toluene equivalents as stated in our previous research [30–32]. The TVOCs were calculated as toluene-equivalent by summing up all the compounds detected in the Tenax-TA tube [32]. The sampled volume was corrected for temperature in calculation of VOC concentrations. To ensure safe sampling volume, we checked if breakthrough of major compounds existed at both temperatures by connecting two Tenax tubes head-to-tail during sampling in our preliminary tests. We did not detect target compounds from the second tube with the sample volume in the study, indicating acceptable capture performance of the adsorbents at different temperatures. In addition, it is noteworthy that the Tenax-TA tubes sampling may be not suitable for detection of VOCs with boiling point <70 � C (called VVOCs), particularly for aldehydes compared with the 2, 4-dinitrophenylhydrazine (DNPH) sampling method. [43, 44] There­ fore, VVOCs, such as acetone, were excluded from data interpretation, even though they were detected in GC-MS. Similarly, we only considered aldehydes � C6 in analyzing the results of aldehydes [30]. Our previous studies indicated the relative standard deviations (RSD) less than 8% for individual VOCs (seen in Table S1) and less than 15% for TVOC [6,31]. The laboratory blanks of each sorbent tube were checked prior to sampling procedure and were subtracted from corre­ sponding sampled results. Instrument errors from TD-GC/MS and sam­ pling procedure was minimized by routine operational maintenance and standard sample calibrations. Before the targeted material sample was placed into the chamber, the VOC concentration in the chamber was measured to ensure that back­ ground concentration of individual VOC was below 2 μg/m3 and TVOC concentration <15 μg/m3.

Ci Ci Δti 1;i Ei

1;i

¼

1

A ¼ Ei V

1;i a

V Ci Ci ð A Δti 1;i

1

Ci þ Ci 2 þa

1

(2)

Ci þ Ci 1 Þ 2

(3)

where. Ci and Ci-1, measured VOC concentration at the i and i-1 sample respectively, μg/m3; Δti-1,i, time interval between the i-1 and i sample, h; Ei-1,i, average VOC emission strength within the time interval, μg/ (m2h). 3. Results and discussion 3.1. TVOC emissions and VOC profile Fig. 2 shows the variation of TVOC concentration and VOC compo­ sition as the result of exposure of the LCS sample to the two air tem­ peratures. At 25 � C, TVOC concentration increased at the beginning of the emission to 200 μg/m3 by the second hour. After the fifth hour, the concentration gradually decreased to around 170 μg/m3 and remained relatively stable by the end of the experiment. In the initial 11 h, car­ bonyls dominated the emitted VOCs, accounting for more than 45% of TVOC, followed by aromatic compounds and other compounds. The composition of VOCs was time dependent. After the LCS was exposed for 20 h, aromatic compounds dominated the emissions, accounting for 40% of TVOCs while fraction of carbonyls dropped to 25%. These results corroborate the findings from previous studies which showed that con­ centration of carbonyls and alcohols in vehicles with leather seats is significantly higher than those with fabric seats at typical comfortable air temperatures [33]. At 60 � C, the peak TVOC concentration was around 670 μg/m3 — 3 � higher than the concentration at 25 � C. After the second hour, the rate 3

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Fig. 2. Longitudinal variation of TVOC concentration and VOC composition as the result of exposure of the LCS sample material to (a) 25 � C and (b) 60 � C.

Fig. 3. Longitudinal variation of TVOC concentration and VOC composition as the result of exposure of the DB sample material to (a) 25 � C and (b) 60 � C. 4

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of TVOC decline was substantially sharper compared to the lower air temperature. After 20 h, the TVOC concentration declined to around 100 μg/m3, which was below the time-matched concentration recorded at 25 � C. Carbonyls and aromatics remained significant fraction of the emitted TVOCs, however, high air temperature also triggered high emissions of aliphatic compounds. After 20 h of experiments, aliphatic compounds became the most dominant VOC group, exceeding 30% mass fraction of TVOCs. In contrary, the proportion of aliphatic compounds in TVOCs at 25 � C was below 5% in all VOC samples. Fig. 3 presents the variation of TVOC concentration and VOC composition as the result of exposure of the DB material to the two air temperatures. At 25 � C, the TVOC concentration increased to 50 μg/m3 during the initial 4 h, and then gradually decreased to 20 μg/m3. Compared to the emission test from the LCS, the TVOC concentration emitted from the DB was on average 5 � lower. As for the VOC com­ positions, aliphatic compounds accounted for over 60% of TVOCs for majority of samples. At 60 � C, the presence of aliphatic compounds in the air further increased to above 85% (Fig. 3b). Relative to 25 � C, the emissions of TVOC increased by as much as 35 � when the DB material was exposed to 60 � C. Fig. 4 presents the longitudinal variation of TVOC concentration and VOC composition as the result of exposure of the PI material to the two air temperatures. At 25 � C, the TVOC concentration increased to 110 μg/ m3 after initial 2 h, and then gradually decreased to 20 μg/m3. Aliphatic compounds, aromatics and carbonyls were the major VOCs emitted from the PI during the first-day experiment, while after 20 h, aromatics became the predominant ones, accounting for more than 60% of TVOCs. Similar to the DB, at 60 � C aliphatic compounds dominated the profile of VOC emissions from the PI, accounting for above 80%. The emitted TVOC concentrations from the PI were over 20 � higher at 60 � C than at 25 � C. Fig. 5 presents the time-dependent variation of TVOC concentration and VOC composition as the result of exposure of the CE material to the

two air temperatures. At 25 � C, the TVOC concentration increased to 35 μg/m3 during the beginning 2 h. Afterwards, it decreased gradually to lower than 5 μg/m3. As for the VOC compositions, aromatics dominated the VOC emissions, accounting for more than 60% of TVOCs for majority of samples. When the CE was exposed to 60 � C, the predominant com­ pounds became carbonyls and aromatics, which consisted over 90% of the mass fraction of TVOCs. The emitted TVOC concentration from the CE sample tripled at 60 � C compared to 25 � C. Compared to the emis­ sions from the above three materials, the concentration of TVOCs emitted from the CE was substantially lower at both two temperatures. It is expected to find that the concentration decay from the second day of the experiments became slower than that in the first day, seen in all the four materials at both air temperatures. The phenomena were in consistent with the results from previous studies about VOC emissions from vehicular and building materials [25,40], which can be explained by mass transfer mechanisms — at the beginning of material emissions, the VOC concentrations in the chamber are mainly driven by convective mass transfer from the material surface to the chamber air. Afterwards, the inside concentration of VOCs of the surface becomes lower due to emission, and then the VOC concentrations in the chamber are domi­ nated by diffusion process, which is much slower than the convective one, leading to slower decay rate of VOC concentrations. Another possible reason is that most materials tested in this study were complex ones with several layers. With time evolution, though the upper layer emitted less and less VOCs, VOCs which emanated from down layers transferred the material surface and then spread to the chamber air, leading to near steady or even increased concentrations. When comparing the emitted VOC compositions from the LCS, the DB and the PI, we found that they shared common emission character­ istics that can be explained by their material compositions. The composition of the PI material included artificial leather as that of the LCS, which led to common emissions of carbonyls and aromatics at 25 � C. Similarly, the PI and the DB both consisted of polypropylene board,

Fig. 4. Longitudinal variation of TVOC concentration and VOC composition as the result of exposure of the PI sample material to (a) 25 � C and (b) 60 � C. 5

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Fig. 5. Longitudinal variation of TVOC concentration and VOC composition as the result of exposure of the CE sample material to (a) 25 � C and (b) 60 � C.

which caused the high aliphatic compound release from the two mate­ rials, particularly at 60 � C. Emission strengths of VOCs from the LCS at 25 � C and 60 � C are shown in Fig. 6 (a) and (b), respectively. The emission strengths of VOCs from the LCS decayed with time at both air temperature conditions. The emission strengths of most VOCs were on average 2-4 � higher at 60 � C, compared to 25 � C. The emission strengths of carbonyls, aromatics and other compounds were the strongest among emitted compounds. Fig. 6 (c) and (d) presents emission strengths of VOCs from the DB at the two temperatures. Analogous to the LCS material, the emission strengths of VOCs from the DB exhibited decline trend over time. The VOC emission rate was one to two orders of magnitude higher at elevated temperature. The aliphatic compounds emitted from DB had higher emission strengths relative to other compounds. Similar trends can also be observed in emission strengths of VOCs from the PI, as shown in Fig. 6 (e) and (f). The VOC emission strengths of the CE were much lower than the above three materials, as seen in Fig. 6 (g) and (h). The emission strengths of VOCs from the CE at 60 � C were even lower compared to that from the LCS at 25 � C. Several previous field tests have reported that VOC concentrations inside vehicle cabins decrease with time; that VOC concentrations in used vehicles are significantly lower than those in new ones; and posi­ tive correlation between air temperature and VOC concentrations [21, 27,33]. Results from this study can explain the above findings in field tests from the perspective of vehicular material emissions. The main cause for the decay-with-time characteristic of VOC emissions is continuous decrease of the residual VOCs embedded in the materials. Furthermore, the elevated air temperature stimulates the VOC off-gassing from the materials owing to increased molecular desorption and molecular motion speed which increases VOC diffusion coefficient [34]. A joint influence of the two mechanisms contributed to enhanced VOC emission strengths from the interior materials in vehicle cabins. The next section summarizes influences of the material type and air

temperature on the composition of emitted VOCs. It is worth mentioning that though the DB, PI and CE exhibited relatively low emitted TVOC concentrations after 24 h in 25 � C, it does not mean that they would not contribute to vehicle interior VOC pollution. Low emission strength but large exposed surface of materials in vehicles can lead to high VOC levels (discussed in Section 3.3). Moreover, the significant influence of air temperature on VOC emissions from materials, especially for DB and PI, will also contribute to high VOC levels in cars in elevated temperature conditions. 3.2. Concentration of individual VOCs dominating the emissions Fig. 7 presents the time-dependent variation of individual VOCs dominating the emissions from the four materials at the two air tem­ peratures. In all scenarios, the emitted VOCs followed the ‘increasepeak-decrease’ concentration pattern. For the LCS material exposed to 25 � C, the major released VOC was 2-butanone, of which the peak concentration accounted for over 30% of the TVOC, followed by tri­ methylsilanol and two aromatics. With the temperature increase to 60 � C, the predominant individual VOC changed to 1-butanol of which the peak concentration was ~20% of the peak TVOC level. Hexanal and 2,4dimethyl-heptane also accounted for a considerable proportion, as well as benzene and toluene. As for VOC emissions from the DB at the two air temperatures, the 2,4-dimethyl-heptane was the dominant chemical, followed by 4-methyl-octane. The 2,4-dimethyl-heptane was also the major compound during VOC emissions from the PI at both tempera­ tures. Emission of 4-methyl-octane were initiated only when the PI was exposed to 60 � C. Increase in the air temperature changed the pre­ dominant VOC released from the CE from toluene to decanal. Other major individual VOCs emitted from the four materials at the two air temperatures are illustrated in Tables S2–S9. Overall, alterations in air temperature was associated with different release mechanisms and concentrations of individual VOCs that dominate the emissions. 6

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Strong correlations between compounds suggest that they are emitted from the same source and that they have the same release mechanism [35]. As shown in Table 1, the four dominating compounds released from the LCS generally exhibited high correlation at 25 � C, indicating that they share similar emission characteristics. At 60 � C, strong correlations were detected among 1-butanol, hexanal, benzene and toluene, while 2,4-dimethyl-heptane had high correlation with hexanal only. This finding can be attributed to variable emission mechanisms for VOCs embedded in the composite materials of the LCS. At 25 � C, alcohols, carbonyls and aromatics may predominantly origin from the exposed leather [36], whereas at increased air temperature VOC diffusion accelerates, leading to more 2,4-dimethyl-heptane emitted from the deeper layer of materials including adhesive and polyurethane where it origins. For the DB exposed to 25 � C, the two major VOCs exhibited strong correlation, suggesting their similar emission characteristics and homology. When the DB was exposed to 60 � C, there was a strong correlation between the two aliphatic compounds (heptane, 2,4-dimethyl and octane, 4-methyl-), as well as between the two aromatic compounds (benzene and toluene). However, correlation between the two groups of compounds was weak. We suspect that 2, 4-dimethyl-heptane and 4-methyl-octane may originate from the exposed smoother surface, whereas benzene and toluene likely origin from the under-layer rough surface. For the PI material exposed to 25 � C, the two major aliphatic compounds (heptane, 2,4-dimethyl and cyclo­ hexane) and the two aromatics (toluene and p-xylene) both exhibited strong correlations. In contrary, correlations between aliphatic com­ pounds and aromatics were weak. Similarly, when the CE emitted VOCs at 60 � C, the major compounds, decanal and toluene, showed weak correlations. The VOCs from the same category had strong mutual cor­ relations, such as the three major alkanes from the PI at 60 � C, and the two aromatics from the CE at 25 � C. 3.3. Overall emission rates in vehicle cabins With the in-vehicle exposed surface area of each tested material (described in Section 2.1), we can estimate the total emission rates of VOCs from the four materials, and to quantitatively compare contribu­ tions of each material. Through multiplying the time-average emission strengths of VOCs by typical in-vehicle exposed surface areas of each material, we calculated the total emission rates of VOCs from the four materials at the two given temperatures, as shown in Fig. 8. At 25 � C, the LCS contributed the most to in-vehicle VOC emissions among the four materials tested. This is a consequence of the specific air temperature, material type and the large exposed surface area (5.25 m2). The DB mainly contributed to emission of aliphatic compounds (50%), while aliphatic emissions from the PI accounted for over 30%. The contribution of the CE to most VOCs was the lowest compared the other three materials, though its exposed surface area was larger than that of DB and PI. At 60 � C, aliphatic compounds were predominant, 2 � higher than the sum of all other VOCs, owing to strong emissions from DB and PI materials (accounting for 50% and 40%, respectively). The results corroborate findings from other field tests that aliphatic compounds were predominantly detected VOCs in vehicle cabins, particularly at high temperature [37,38]. The source apportionment results can be used to offer specific rec­ ommendations. In order to control VOC levels in new vehicle cabins at comfortable conditions, the LCS should be the priority for source con­ trol. However, considering the wide temperature range in vehicle cabins, we should pay special attention to source control of the DB and the PI for their extremely strong emissions of aliphatic compounds at high temperatures.

Fig. 6. Time-dependent variation of VOC emission strengths as the result of exposure of (a) the LCS to 25 � C, (b) the LCS to 60 � C, (c) the DB to 25 � C, (d) the DB to 60 � C, (e) the PI to 25 � C, (f) the PI to 60 � C, (g) the CE to 25 � C and (h) the CE to 60 � C.

Several previous studies regarding VOC emissions from building materials concluded some experimental and theoretical correlations between temperature and key emission parameters [25,40]. We assume that the temperature-correlated coefficients of VOC emissions vary much with different VOCs and materials, which explains the various increase extents of VOCs with temperature elevation. In addition, some chemical reactions may also exist for some VOCs, which are influenced by temperature as well. Moreover, since most materials tested in this study were complex materials with various layers, influence of tem­ perature on VOC emissions from difference layers can also vary. To further understand their emission characteristics, the emission strength of each dominant VOC was calculated, followed by determining correlation factors among them, as shown in Table 1.

3.4. Implications The emission strengths and compositions of VOCs emitted from vehicular interior materials was strongly dependent on time. Therefore, 7

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Fig. 7. Time-dependent variation of individual VOCs dominating the emission from (a) the LCS at 25 � C, (b) the LCS at 60 � C, (c) the DB at 25 � C, (d) the DB at 60 � C, (e) the PI at 25 � C, (f) the PI at 60 � C, (g) the CE at 25 � C and (h) the CE at 60 � C.

a one-time VOC sampling method that currently exists in regulations may mispredict the VOC emissions from materials. Future regulations should therefore consider continuous VOC sampling method for the emission tests from vehicular interior materials. That approach can offer the basis for improved modelling of VOC emissions from car materials. Furthermore, our study results indicate that VOC emissions from interior materials differ at distinct air temperatures encountered in vehicles. Therefore, the VOC emission test should not be conducted at a singlepoint air temperature. We recommend that future regulatory de­ velopments adopt the VOC emission test that is conducted at distinct air temperatures that typically occurs in vehicle interiors. Components of vehicular interior materials may be more complex than many of common building materials. Many interior materials in vehicle cabins, such as the LCS, are composite ones and have various surface treatments, which may lead to irregular emission characteristics of VOCs, as shown in Section 3.2. This implies that the assumption of

initial uniform VOC distribution, which has been widely applied in modeling of VOC emissions from building materials, must be carefully interpreted for vehicular interior materials. For the purpose of modeling of VOC emissions from interior materials in vehicle cabins, we recom­ mend to that complex materials are treated as multi-layer ones accord­ ing to their compositions and surface treatments. 3.5. Limitations The present study has several limitations. The investigation is partially restricted to specific vehicle materials that are not always representative of all interior materials in vehicles. For the purpose of VOC source apportionment in vehicle cabins, other materials with large exposed surface areas, such as door material and shelf of trunk, need further measurements of their emission characteristics. In addition, VOC emissions can vary even for the same type of materials owing to different 8

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Table 1 Correlation coefficient R2 among compounds dominating the emissions from the two materials at different air temperatures. (*p < 0.05). Material

Temperature (� C)

LCS

25

60

DB

25 60

PI

25

60

CE

25 60

Correlations 2-Butanone Silanol, trimethylBenzene Toluene 1-Butanol Hexanal Heptane, 2,4-dimethylBenzene Toluene Heptane, 2,4-dimethylOctane, 4-methylHeptane, 2,4-dimethylOctane, 4-methylBenzene Toluene Heptane, 2,4-dimethylCyclohexane Toluene p-Xylene Heptane, 2,4-dimethylCyclohexane Octane, 4-methylToluene p-Xylene Decanal Toluene

2-Butanone 1 0.95* 0.93* 0.84* 1-Butanol 1 0.76 0.76 0.82* 0.82* Heptane, 2,4-dimethyl1 0.94* Heptane, 2,4-dimethyl1 0.86* 0.23 0.27 Heptane, 2,4-dimethyl1 0.84* 0.50 0.44 Heptane, 2,4-dimethyl1 0.77* 0.93* Toluene 1 0.98* Decanal 1 0.37

Silanol, trimethyl-

Benzene

Toluene

1 0.91* 0.81* Hexanal

1 0.95* Heptane, 2,4-dimethyl-

1 Benzene

Toluene

1 0.67 0.76

1 0.92*

1

1 Octane, 4-methyl-

Benzene

Toluene

1 0.15 0.17 Cyclohexane

1 0.98* Toluene

1 p-Xylene

1 0.28 0.15 Cyclohexane

1 0.83* Octane, 4-methyl-

1 0.84* 0.84* 0.92* Octane, 4-methyl-

1 0.79* p-Xylene

1

1

1 Toluene 1

Fig. 8. Average emission rates of the four materials in vehicle cabins at the two interior temperatures.

manufacture process and surface treatment. Thus, future studies regarding emissions of VOCs from vehicular materials need to take the influence of production process into consideration. The settings of air change rate (0.4 1/h) and temperatures (25 � C and 60 � C) were applied according to the ISO standard [24], which focuses on evaluation of VOC emissions from vehicular materials. The experimental conditions were in consistent with air change rates and temperatures reported in parked vehicles [10,26]. However, air change rate of vehicles could vary extensively, particularly in driving conditions [42]. Therefore, to represent material emission behaviors in realistic vehicular interior environment, we suggest more experimental investigations in VOC emissions from vehicular materials under various environmental conditions. The dynamic emission chamber test in this study only lasted for 26 h. Long-term measurement will of course provide more accurate emission

characteristics to mimic the actual vehicle use. However, long-term experiments would be a great challenge for both research experiments and standard development regarding evaluation of vehicular material emissions. One potential solution is to predict the long-term emission in vehicles using VOC emission mechanism model, of which the key pa­ rameters can be obtained by short-term emission test. This study actu­ ally provides some fundamental data for the emission model development and regression, which is beneficial for actual-use predictions. In the study, to investigate the influence of temperature on VOC emissions from vehicular materials, the humidity was controlled consistently. The absolute humidity was controlled in this study, which is inspired by another study reported that instead of relative humidity, absolute humidity should be used as a token humidity influence factor to emission from materials [40]. However, the conclusion has not been 9

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Building and Environment 170 (2020) 106599

confirmed yet because temperature, relative humidity and absolute humidity are combined parameters. Relative and absolute humidity may have different influence mechanisms on VOC emissions from materials. Therefore, further studies are necessary to probe the effect of humidity on vehicular material emissions. It is worthwhile to note that the previous source apportionment is based on chamber test of materials separately. Individual material test results can reveal individual contributions to the indoor air quality, with which we can more effectively propose source control measures. How­ ever, in actual vehicular environment, the materials coexist, meaning that their emissions can influence each other’s. Thus, the emission characteristics of the materials can be distinguished from the indepen­ dent cases. Nevertheless, the coexisting effect would be challenging to consider in chamber experiments and need further investigations.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors would like to acknowledge the support from National Natural Science Foundation of China through Grant No. 51678329. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.buildenv.2019.106599.

4. Conclusion

References

Emissions from interior materials of vehicle cabins are potentially important contributors to accumulated interior air pollution in vehicles. In a well-controlled ventilated chamber, we investigated VOC emission characteristics of four typical vehicular interior materials under the two air temperatures. We found that concentration of emitted VOCs was a function of the material type, air temperature and time. The air temperature to which materials were exposed to was asso­ ciated with distinct emission strengths of individual VOC compounds. At 25 � C, the leather coat of seat (LCS) material mainly emitted carbonyls and aromatics, while at 60 � C, strong emissions of aliphatic compounds were observed. Expectedly, the VOC emission strengths always rose with the increased air temperature. For the dashboard (DB) material, aliphatic compounds were dominating the emissions at both 25 � C and 60 � C, accounting for up to 80% of total emissions. For the pillar (PI) material, the main VOCs emitted at 25 � C were aliphatic compounds, aromatics and carbonyls. However, at 60 � C the emissions were domi­ nated by aliphatic compounds. The predominant VOCs released from the ceiling (CE) material changed from aromatics to carbonyls when the temperature increased from 25 � C to 60 � C. The concentration of individual VOC compounds released from the materials fluctuated with time, suggesting the limitation of the existing one-time sampling approach and importance of a continuous sampling methodology. Furthermore, correlation analysis of individual VOC levels showed that alcohols, carbonyls and aromatics had similar emis­ sion mechanisms for the LCS, which was not the case for aliphatic compounds. For the DB, the predominantly emitted aliphatic com­ pounds, 2,4-dimethyl-heptane and 4-methyl-octane exhibited similar emission characteristics that differed from the aromatics. Similar trends of correlations were observed in major VOC emissions from the PI and the CE materials. In all, the source apportionment results indicated that emissions from the LCS were responsible for majority of VOCs in vehicle cabins. In addition, special attention is required for the DB and the PI materials because of their high rates off-gassing properties of aliphatic compounds at high temperatures. In conclusion, this study characterized a time- and temperaturedependent nature of individual VOCs emitted from common interior materials in vehicles. Future research on dynamic and temperaturedependent VOC emissions from vehicle materials would be important as a basis for enhancing air quality in car environments. Such efforts would also aid our understanding of the emission mechanisms of VOCs and consequent exposures. In addition, semi-volatile organic com­ pounds (SVOCs) are of increased concern in cars, especially at high temperatures. Future efforts in investigation of SVOCs in vehicles and emission of SVOCs from vehicular materials are essential to understand a whole picture of air pollution inside vehicles and control air pollutant levels in vehicular environments.

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