Comparison of filtration performance of commercially available automotive cabin air filters against various airborne pollutants

Building and Environment 161 (2019) 106272

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Comparison of filtration performance of commercially available automotive cabin air filters against various airborne pollutants

T

Ki Joon Heoa,b, Jung Woo Noha, Byung Uk Leeb, Yeonsang Kima,**, Jae Hee Jungc,d,e,* a

Technical Textile R&D Group, Korea Institute of Industrial Technology (KITECH), Ansan, 15588, Republic of Korea Aerosol and Bioengineering Laboratory, Department of Engineering, Konkuk University, Seoul, 05209, Republic of Korea c Center for Environment, Health, and Welfare Research, Korea Institute of Science and Technology (KIST), Seoul, 02792, Republic of Korea d Green School, Korea University, Seoul, 02841, Republic of Korea e Division of Energy & Environment Technology, KIST School, Korea University of Science and Technology, Seoul, 02792, Republic of Korea b

ARTICLE INFO

ABSTRACT

Keywords: Air pollution Automotive cabin Air filter Particulate matter Volatile organic compounds

Controlling air pollutants in the automobile cabin environment has become increasingly important owing to the health risks of exposure to high concentrations of harmful air pollutants. To protect daily commuters and drivers against the harmful effects of air pollution, modern automobiles are commonly equipped with automotive cabin air filters (ACAFs). Thus, understanding the filtration performance of ACAFs is essential for assessing cabin indoor air quality. In this study, six original equipment manufacturer (OEM) and nine after-market ACAFs were selected and their particulate matter (PM) filtration performance against standard particle (ISO 12103-1 A2 dust), filter pressure drop under various airflow velocities, and gas removal performance against standard test gases (n-butane and toluene) were estimated. In the PM filtration test, the lowest filtration performance occurred at a particle size range of 0.3–0.5 μm, with a filtration efficiency of 50–90%, and the filtration performance increased with increasing particle size. The PM filtration performance of OEM ACAFs (72.3 ± 13.81%) was higher than that of after-market ACAFs (56.4 ± 23.72%). All test OEM ACAFs satisfied the Korea Air Cleaning Association standard filtration performance guidelines for 0.3–0.5-μm particles (> 50%), but four of the nine after-market ACAFs did not satisfy the standard guidelines. In addition, only one of the six OEM ACAFs simultaneously satisfied the standard removal performance guidelines for n-butane (> 70% after 1 min and > 45% after 5 min) and toluene (> 80% after 1 min and > 70% after 5 min). This study provides valuable baseline information on ACAFs for understanding and improving cabin indoor air quality.

1. Introduction In urban environments, automobiles are one of the most significant sources of airborne pollutants [1–5]. Automobiles generate a complex mixture of gases and particles formed by incomplete combustion, volatilization of unused fuel, and the release of engine lubricating oil. These airborne pollutants contain various chemical compounds such as particulate matter (PM), carbon monoxide (CO), sulfur oxides (SOx), nitrogen oxides (NOx), and hydrocarbons, and they are suspected etiological agents for respiratory diseases because these airborne mixtures can be inhaled or attach to the human body [6–11]. The public health risk of exposure to automotive airborne pollutants has led to regulatory emissions standards for passenger cars, such as Euro-5 and Euro-6. To meet emissions requirements, various emission reduction technologies have been applied to modern automobiles *

[12–14]. However, automobile drivers remain at high risk of exposure to airborne pollutants [15,16]. Previous studies have found that the PM concentration on roads is around 25 times higher than that in the ambient air environment [17–19]. Highly concentrated PM penetrates the automotive cabin environment through automotive heating, ventilation, and air conditioning (HVAC) systems [20]. Zhu et al. reported that the concentration of ultrafine PM can be 10 times higher in vehicle cabins on the road than in background ambient air [21]. Fruin et al. reported that 33–45% of total daily PM exposure occurs while commuting and traveling in automobiles [17]. This high-level PM exposure in automobile cabins can cause various adverse health effects, potentially releasing toxic and carcinogenic substances into the bloodstream [22–24]. PM < 2.5 μm can penetrate deep into the human respiratory tract, such as the bronchi and alveoli [25]. A previous study also found that the concentrations of airborne gaseous pollutants such as NOx and

Corresponding author. Center for Environment, Health, and Welfare Research, Korea Institute of Science and Technology (KIST), Seoul, 02792, Republic of Korea. Corresponding author. Technical Textile R&D Group, Korea Institute of Industrial Technology (KITECH), Ansan, 15588, Republic of Korea. E-mail addresses: [email protected] (Y. Kim), [email protected] (J.H. Jung).

**

https://doi.org/10.1016/j.buildenv.2019.106272 Received 25 April 2019; Received in revised form 1 July 2019; Accepted 7 July 2019 Available online 08 July 2019 0360-1323/ © 2019 Elsevier Ltd. All rights reserved.

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necessary to confirm the general performance of commercially available ACAFs. In addition, modern ACAFs are composed of multi-layer composite filters for the simultaneous removal of harmful gases and PM. Most previous studies have only focused on and evaluated the PM filtration efficiency [21,36,38]; few studies have focused on the performance of gaseous pollutants elimination [39]. It is especially difficult to find research results testing the removal of PM and hazardous gases simultaneously by ACAFs. In addition, previous studies did not consider the possibility that commercial ACAF performance varies according to type and manufacturer, as they only tested three or fewer filter samples. In this study, we aimed to confirm the overall performance of ACAFs against target air pollutants under the standard test protocol, and to compare performance by the type of filter manufacturer: original equipment manufacturer (OEM) vs. after-market. We evaluated the performance of six OEM and nine after-market ACAFs against PM (ISO 12103-1 A2 test dust particle) and gaseous pollutants (n-butane and toluene). We believe that these findings provide an initial baseline understanding of the relationship between ACAF performance and automobile cabin IAQ. 2. Materials and methods Fig. 1. Schematic diagram illustrates that the automotive cabin air filter removes harmful airborne pollutants and purifies cabin indoor air.

2.1. Preparation of ACAFs test Most of the commonly available ACAFs on the market are composite filters with dust filtration layers, absorption layers with granules of activated carbon, and support layers. ACAFs were divided into two groups according to the filter manufacturer: OEM or after-market. Six OEM and nine after-market ACAFs were selected for testing. Fig. 2 and Table 1 provide detailed images and information for each of the filters. ACAFs of different sizes, weights, numbers of pleats, and intended automobile type were selected to represent a wide range of cabin air filters. These test ACAFs are commonly used by the public and have high market share in the Republic of Korea, and were purchased at an automobile service center. They are made of nonwoven fibers without detailed manufacturers’ specifications for fiber diameter, basis weight, or packing density.

SOx in vehicle cabins remained noticeably higher than their concentrations in ambient air [26]. The emission of volatile organic compounds (VOCs) from automobile cabin indoor materials is one of the main causes of poor automotive cabin air quality [27–30]. Recently, “sick car syndrome” has been highlighted as a result of the identification of toxic VOCs such as BTEX (benzene (B), toluene (T), ethylbenzene (E), and xylenes (X)), which are emitted from the dashboard, door panels, seat coverings, and flooring materials of automobiles, and ozone is created through photochemical reactions with NOx [31–33]. These gaseous pollutants can cause eye, nose, and throat irritations, allergic skin reactions, headaches, and fatigue through daily commutes, and drivers are often exposed to high concentrations of airborne pollutants emitted from surrounding automobiles or interior materials [34]. Although there are two fundamental control strategies for improving cabin indoor air quality (IAQ), source control and air cleaning, the pollution source of cabin indoor air is generally unreachable. Therefore, a possible solution for passengers is to use an air cleaning system to improve automotive cabin air quality, as shown Fig. 1. Previous studies of on-road exposure to air pollutants reported that PM concentrations in automobile cabins are slightly lower than in outdoor road concentrations when the HVAC system is being operated [21,35]. Lee and Zhu, and Pui et al. reported that PM concentrations in automobile cabins were reduced to the minimum level when the HVAC system was on recirculation mode [36]. An approximately 25% reduction in PM was achieved without the use of automotive cabin air filters (ACAFs) because PM adheres to the duct wall when cabin air passes through the HVAC system. However, the PM reduction performance increased to ~70% when ACAFs were used in the automobile cabin HVAC system [37]. Thus, modern automobiles are commonly equipped with ACAFs to reduce automotive cabin exposure to various airborne pollutants. However, the performance of the commonly available ACAFs against airborne pollutants is not assured, because legally enforced certification parameters for ACAF filtration performance (e.g., N95 and H13 grade) have not yet been established. Only test method specifications for ACAFs, such as DIN 71460, ISO/TS 11155, and SPS-KACA014-0144, have been established. Xu et al. compared the performance of three kinds of ACAFs against ultrafine PM, and revealed that the filtration performance of test ACAFs varied from 20 to 80% depending on the filter type [38]. Therefore, it is

2.2. Preparation of test airborne pollutants To evaluate the PM filtration of commercially available ACAFs, ISO 12103-1 A2 test dust particles were used as the standard PM. These dust particles are the representative test particles used in automotive cabin filter testing standards such as DIN 71460 part 1 and SPS-KACA0140144, and are also widely used in aerosol research due to their similar properties to PMs in the indoor air environment [40–42]. To evaluate the gaseous pollutant removal performance of ACAFs, two species of gaseous pollutants with different molecular weights, n-butane and toluene, were selected. Toluene is a toxic gas that deteriorates indoor air quality and poses a threat to human health. N-butane and toluene are among the five most abundant species, contributing over 38% of the total measured VOCs emitted in automobile exhaust [43–45]. Toluene was measured at high concentrations of 53.5–266.0 μg/m3 [28]; n-butane has also been observed at high concentrations (11.7 ppb) in automotive cabin air environments [43]. Therefore, these gases have been included among the representative test materials (n-butane, toluene, and SO2) for automotive cabin filter testing standards, such as DIN 71460 Part 2 and SPS-KACA014-0144, and are therefore widely used in adsorption performance testing of ACAFs. 2.3. PM filtration performance test Fig. 3(a) shows the filtration test setup used to measure the PM filtration efficiency of test ACAFs according to the SPS-KACA014-0144 standard. Following SPS-KACA014-0144, standard ISO 12103-1 A2 dust 2

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Fig. 2. Detailed images of commercial automotive cabin air filters.

particles were generated at a concentration of 20 mg/m3air by a solid aerosol generator (SAG410, Topas GmbH, Dresden, Germany), and carried by filtered air at a flow rate of 300 m3/h. The air used for testing had 50% relative humidity and a temperature of 23 °C, following the standard conditions defined in SPS-KACA014-0144. The dust filtration efficiencies (η) of the test ACAFs were calculated using the following equation:

=1

Coutlet , Cinlet

each experimental condition. 2.4. Gas removal performance test Fig. 3(b) shows the experimental setup for the gas removal evaluation of the ACAFs according to standard SPS-KACA014-0144 [44]. The test conditions given in the standards are 23 °C and 50% relative humidity. The input volumetric content of n-butane and toluene as test gases was 80 ppm. Exposure for 1 min to 80 ppm corresponds to 2 months of exposure to the actual atmospheric concentration [28]; this high concentration reduces the testing time. Thus, many researchers have performed lab-scale filtration tests using high concentrations of pollutants, called accelerated life testing [39,45]. This test concentration used here is also the standard concentration for test methods DIN 71460 Part 2 and SPS-KACA014-0144. N-butane and toluene were carried by filtered air at flow rate of 150 m3/h. Gas concentrations were measured using a heated flame ionization detector (FID 3006, SICK AG,

(1)

where Coutlet and Cinlet represent the particle concentrations (particles/ cm3air) of the ISO 12103-1 A2 dust aerosol at the inlet and outlet of the filter, respectively. The size and number concentration of dust aerosols were measured with a particle size spectrometer (LAP321, Topas GmbH). The pressure drops of all test filters were measured using pressure transmitters (FCX-AII, Fuji Electric S.A.S., Clermont-Ferrand, France). All measurements were repeated more than three times for

Table 1 Detail of commercial automotive cabin air filters used in filtration performance test with information of type, size, number of pleated, weight, type of material of each layer. Type

OEM

After-market

O1 O2 O3 O4 O5 O6 A1 A2 A3 A4 A5 A6 A7 A8 A9

Size (W x L x H) (mm)

Number of pleats

242*197*30 252*225*30 240*203*34 206*251*25 224*204*30 250*177*35 252*225*30 252*225*30 240*203*34 240*203*34 204*252*24 208*235*25 224*200*28 224*200*28 246*176*35

25 30 33 34 28 34 27 32 33 28 35 35 26 29 30

Total basis weight (g/ m2)

532 555 531 422 412 525 427 428 364 438 416 444 418 441 431

Filtration layer basis weight (g/m2)

Description of layer 1st layer (inlet)

2nd layer

3rd layer

4th layer

5th layer

272 263 136 198 161 181 277 154 154 275 225 239 238 321 227

Scrim Scrim PET PET PET PET PET PET PET PET PET PET PET PET PET

PP* PP A.C A.C A.C A.C A.C A.C A.C A.C A.C A.C A.C A.C A.C

PET** PET PP PP PP PP PP PP PP PP PP PP PP PP PP

A.C.*** A.C.

PET PET

* PP melt blown filter, ** PET spun bond filter, *** Activated carbon. 3

Scrim Scrim Scrim Scrim Scrim Scrim Scrim

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Fig. 3. Schematic diagram of the experimental setup used in the filtration test. (a) PM filtration performance test set-up and (b) gas removal performance test set-up.

Waldkirch, Germany) at both the inlet and outlet of the test ACAFs.

of each ACAF was calculated by comparing the dust particle number concentrations at the inlet and outlet (Eq. (1)). Fig. 4(b) shows the PM filtration efficiency of ACAFs as a function of particle size. Although the minimum filtration efficiency occurred for particles in the size range 0.3–0.5 μm for all test filters, the filtration performance of the test ACAFs increased from 65.9 ± 20.73% to 99.0 ± 1.57% when the particle size increased from 0.3–0.5 μm to 5.0–10.0 μm. These results are consistent with previous findings that particles of around 300 nm size are the most difficult to capture, and the filtration efficiency increases rapidly for particles larger than 300 nm due to particle filtration mechanisms [47]. Qi et al. and Zhu et al. also reported that the particle size with the minimum filtration performance from ACAFs is approximately 300 nm because this intermediate size is too small to be removed by interception and impaction, but too large for effective diffusion [21,35]. However, interception and impaction effects increase rapidly for particles larger than 300 nm, and these effects are thus important as particle size increases. In our tests, all OEM filters satisfied the standard filtration efficiency at 0.3–0.5 μm defined by SPS-KACA014-0144 (> 50%), whereas only five of the nine after-market filters met the standard efficiency as shown Fig. 4(b). The filtration efficiency of OEM ACAFs (72.3 ± 13.81%) was

2.5. Surface area and pore size of activate carbon The BET surface area and pore size of activated carbons were measured using nitrogen sorption at 77 K defined by ISO 15901–2. Prior to the experiments, the samples were outgassed at 408 K for 15 h. The isotherms were determined by N2 adsorption using BJH method in a Micrometric accelerated surface area and porosimetry system (ASAP2010, Micromeritics, Georgia, USA). The total pore volume was calculated from the amount of vapor adsorbed at a relative pressure closed to unity assuming that the pores are filled with the condensate in liquid state [46]. 3. Results and discussion 3.1. PM filtration performance To quantify the PM filtration efficiency, the ACAFs were exposed to standard dust particles and Fig. 4(a) shows the particle number concentration of generated standard dust particles. The filtration efficiency

Fig. 4. Filtration performance of automotive cabin air filters. (a) Number particle concentration of generated ISO A2 dust particle. (b) Filtration efficiency of filters by particle size. Black dash line indicates standard for 0.3–0.5 μm filtration performance (> 50%) defined by Korea Air Cleaning Association (SPS-KACA014-0144).

4

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Fig. 5. Filtration performance according to manufacturer type. (a) 0.3–0.5 µ m filtration efficiency of ACAFs according to manufacture type. Boxes indicate the 25th–70th percentiles; lines and black circles inside the boxes indicate the median and mean, respectively; whiskers represent the maximum and minimum values. Blue dash circle and green circle represents high-performance group and low-performance group of after-market, respectively. * indicates t-test p-value < 0.05. (b) 0.3–0.5 µ m filtration efficiency of ACAFs according to price. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

efficiencies were observed with O2 (~91.0%) and A4 (~25.3%), respectively (Fig. 4(a)). The A4 filter consists of only two filtration layers with 28 pleats, and its basis weight is only 136 g/m2; in contrast, O2 consists of four filtration layers with 30 pleats, and its basis weight is 263 g/m2, as shown in Table 1. Thus, we analyzed differences in configuration and performance according to the number of filtration layers. First, to confirm the effect of the number of filtration layers, the tested ACAFs were classified into three groups: those with two layers, those with three layers and those with four layers. Fig. 7(a) shows the 0.3–0.5 μm dust filtration efficiency according to the number of filtration layers. The average filtration efficiency increased with increasing number of filtration layers (51.7 ± 20.33%, 2.0 ± 15.20%, and 87.5 ± 3.92% with two, three, and four layers, respectively, p < 0.01). Fig. 7(b) shows 0.3–0.5 μm dust filtration efficiency curves for OEM and after-market ACAFs as a function of their total basis weight. Interestingly, dust filtration efficiency per total basis weight of OEM ACAFs was constant (r2 = 0.3943), whereas that of after-market ACAFs showed large variations (0.33 ± 0.195%, coefficient of variation: 0.5937) compared to OEM ACAFs (0.40 ± 0.062%, coefficient of variation: 0.1545), and decreased with increasing weight (r2 = 0.8185). This result appears to contradict the results of previous studies of filtration efficiency and the basis weight of filters. Greater basis weight facilitates filtration by presenting a larger surface area and thus enhancing the diffusion and interception mechanisms for small particles [48]. This surprising result might be due to differences in the performance of filtration layers produced by different manufacturers and manufacturing methods. Our results indicate that the performance and number of filtration layers have a greater impact on the PM filtration performance of ACAFs than total basis weight. Therefore, selection of OEM ACAFs with a large number of filtration layers is the best option for PM filtration of automotive cabin air, as certification parameters are not currently legally enforced. Recently, ACAFs have been constructed to be thicker or composed of multiple layers to enhance filtration performance against various types of harmful airborne pollutants. These thicker composite filters result in a high pressure drop along with long airflow pathways. Similar to the indoor environment of a building, increased pressure drop in ACAFs leads to a lower flow rate under the same automobile HVAC system operating conditions, and users of ACAFs with high pressure drop are forced to use a higher operating setting than users of ACAFs with low pressure drop to obtain the same amount of clean air. Previous research has shown that increased pressure at the air filter can reduce

Fig. 6. PM filtration efficiency of automotive cabin air filters according to the manufacturer type. Boxes indicate the 25th–70th percentiles; lines and black circles inside the boxes indicate the median and mean, respectively; whiskers represent the maximum and minimum values. * indicates t-test pvalue < 0.05.

higher than that of after-market ACAFs (56.4 ± 23.72%), and this difference was statistically significant (t-test; p < 0.05) as shown in Fig. 5(a). This may be due to the large performance deviation of aftermarket ACAFs. The after-market ACAFs were divided into two groups, a high-performance group (81.8 ± 4.23%) on one side (blue dotted circle) and a low-performance group (36.6 ± 7.36%) on the other (green dotted circle), whereas the OEM ACAFs were evenly distributed over the entire higher performances than standard level. This is due to the lack of legally enforced certification parameters for direct management of ACAF filtration performance. This situation has resulted in large range in after-market ACAF filtration performance, according to price and manufacturer. Fig. 5(b) shows PM filtration performance according to the price of ACAFs. After-market ACAFs showed better PM filtration performance with increasing price (y = 4.00x – 64.913, r2 = 0.9027). However, after-market ACAFs had higher prices despite poorer PM filtration performance than OEM ACAFs. These results indicate the need for legally enforced certification parameters to directly manage filtration performance. However, there was little difference in the filtration efficiency between OEM and aftermarket ACAFs for PM > 0.5 μm (p > 0.5), as shown in Fig. 6. Although the trend in filtration efficiency variation according to particle size was similar among all test ACAFs, there was a large difference in PM filtration efficiency against the same particle size [38]. For 0.3–0.5 μm particle size, the highest and lowest filtration 5

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Fig. 7. Filtration performance of automotive cabin air filters according to configuration. (a) 0.3–0.5 µ m filtration efficiency of ACAFs according to the number filtration layers. Boxes indicate the 25th–70th percentiles; lines and black circles inside the boxes indicate the median and mean, respectively; whiskers represent the maximum and minimum values. Blue dot circle and green dot circle represents high-performance group and low-performance group of after-market, respectively. ** indicates t-test p-value < 0.01. (b) Normalized PM filtration by total basis weight. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

where η is the filtration efficiency and ΔP is the pressure drop. According to Equation (2), the higher the QF indicates the more efficient the air filter. To determine the optimal performance of the air filters, the QF was calculated based on the experimental data from the 0.3-μm dust filtration efficiency and the pressure drop at 1.5 m/s. Fig. 9 shows the QFs of test ACAFs, with the QF of a commercial HEPA filter shown for comparison (blue dotted line). Overall, there was significant variation in the QFs of the test ACAFs (0.0049–0.0361 Pa−1). The relatively low filtration performance group (A1, A4, A5, and A8) inevitably showed low QF values. A6 and A7 constituted the high-performance filtration group among after-market ACAFs, but their QFs were lower than the QFs of A1, A2, and A9, with similar filtration performance due to their high pressure drops. With the exception of O2, which had the highest QF value (0.0361 Pa−1), the rest of the test ACAFs showed lower QFs than a commercial HEPA filter (0.0292 Pa−1). This was due to the relatively low filtration efficiency of the test ACAFs compared to that of the HEPA filter (> 99.995% filtration efficiency for 0.3-μm PM). Additionally, the activated carbon layer in ACAFs can decrease the QF value due to the increased pressure drop. The average QFs of OEM and after-market ACAFs were 0.0190 ± 0.0090 and 0.0137 ± 0.0087 Pa−1, respectively. However, the difference between

Fig. 8. Variation in the pressure drop of the automotive cabin air filters according to the face velocity.

vehicle fuel economy by 1–15% [49]. Due to the particular characteristics of automobiles, ACAFs affect not only the cabin environment but also the engine parts [50]. Therefore, ACAFs with a large pressure drop decrease automobile acceleration time by reducing the amount of air entering the engine system. In other words, a high-pressure-drop ACAF requires greater fuel consumption, resulting in more exhaust emissions from an automobile [51]. Therefore, due to the trade-off between filter pressure drop and filtration efficiency, optimization is critical for the economic feasibility and effectiveness of ACAFs. Fig. 8 shows the pressure drop of the test ACAFs as a function of the air face velocity. The face velocity was set in the range of 0.5–3.0 m/s to include the actual cabin air filter face velocity of 0.6–1.8 m/s [52]. The trend toward increasing pressure drop with air face velocity was similar for all test ACAFs. Interestingly, A6 showed the highest pressure drop despite ranking fifth of 15 in PM filtration performance, whereas A1 showed the lowest pressure drop, as was expected, despite ranking fourteenth of 15 in filtration performance. In general, to determine if an air filter is efficient, the filter quality factor (QF) is widely used, based on the following formula [48,53,54]:

QF =

ln(1

) P

,

Fig. 9. Quality factor of automotive cabin air filters. Blue dot line indicates QF value of commercial HEPA filter. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

(2) 6

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Fig. 10. Gas removal performance of 15 commercial automotive cabin air filters according to the elapsed time. (a) N-butane and (b) toluene removal performance. Black and red bar represent gas removal performance after 1 min and 5 min elapsed. Black and red dot line indicate standard guide line of gas removal performance after 1 min and 5min, respectively, as defined by Korea Air Cleaning Association (SPS-KACA014-0144). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

these was not statistically significant (t-test; p > 0.05). These results indicate that the filtration performance and energy consumption of automobile HVAC systems can vary dramatically depending on the type of filter. In addition, ACAFs are exposed to humid air more frequently than any other air filters during routine use, which may affect their filtration performance. Significant increases in filtration performance and pressure drop were observed as the exposure time to humid air increased [55]. In addition, the lifetime of the filter was reduced by half with elevated humidity [56]. While high humidity is likely to have a significant impact on filter performance and energy consumption by automobile HVAC systems, the standard test method does not account for this possibility.

directly related to the gas removal performance [57]. Gas molecules attached to pores reduce the available pore volume to which other gaseous pollutants can be adsorbed. Therefore, the general gas removal performance of filters with an adsorbent layer decreases as the amount of adsorbed gas and concentration of inlet gas increases. The highest nbutane removal efficiencies after 1- and 5-min elapsed times were ~81.4% and ~53.6%, respectively, for O6. In our tests, only two of the 15 filters, O3 and O6, satisfied the standard n-butane removal efficiency guidelines (SPS-KACA014-0144; > 70% after 1-min elapsed time, > 45% after 5-min elapsed time). None of the after-market ACAFs satisfied these standard guidelines. Fig. 10(b) shows the toluene removal efficiencies after 1- and 5-min elapsed time. O6 had the highest removal performance, and O4, O6, and A7 satisfied the toluene removal efficiency standards (SPS-KACA014-0144; > 80% after 1-min elapsed time, > 70% after 5-min elapsed time). Only O6 simultaneously satisfied the n-butane and toluene removal efficiency standards. However, we used only high inlet concentrations based on standard test methods. Testing the lower concentration of gas typical of real automotive cabin indoor air would likely lead to worse ACAF performance and thus less ACAFs satisfying the removal efficiency standards [39,45]. In our tests, the toluene removal performance of cabin filters was generally higher than the n-butane removal performance. This may be due to the difference in molecular weight between toluene and n-butane. Activated carbon functions through the process of adsorption, whereby gaseous pollutant molecules are trapped inside the pore structure of the carbon substrate. The gas adsorption performance of

3.2. Gas removal performance In recent years, composite ACAFs have been used in cars to remove harmful gases from incoming external air and generating inside the cabin by gas adsorption onto activated carbon layers. Thus, ACAFs must have high removal performance and durability against gaseous pollutants. Fig. 10 shows the gas removal performance of test ACAFs on nbutane and toluene. The guidelines for gas removal efficiency after the 1- and 5-min elapsed times defined by the Korea Air Cleaning Association (SPS-KACA014-0144) are indicated by black and pink dotted lines, respectively. The tested filters exhibited a wide range in gas removal performance, and their gas removal efficiencies decreased gradually with elapsed time. The status of pores on the adsorbent is 7

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Table 2 Summary of surface pore characteristics of five OME and two after-market activated carbons. Sample

BET surface area (m3/g)

Langmuir surface area (m3/g)

Total pore volume (cm3/ g)

Average pore size (Å)

O2 O3 O4 O5 O6 A1 A2

1,114.7 690.4 602.8 858.4 691.8 316.2 599.3

1,484.0 912.0 808.1 1,149.4 914.8 423.0 794.6

0.536 0.336 0.303 0.429 0.348 0.170 0.429

19.232 19.454 20.123 19.996 20.106 21.561 19.996

inlet concentration [59]. We observed that the breakthrough curve shape was similar among the tested ACAFs; however, the breakthrough time differed with the filter type. The gas adsorption capacity of the air filters was obtained by integration of the complete breakthrough curves. The gas adsorption capacity is defined as follows [60]:

q= co V

(t ) dt ×

1 1 × × M× 10 6, 0.082 273 + k

(3)

where q is the adsorption capacity (g/m2); co is the inlet gas concentration (ppm); V is the airflow rate (L/min); t is the elapsed time (min); and M is the molecular weight (g/mol) of the test gas. Fig. 12(a) shows the n-butane outlet concentration of O1 as a function of elapsed time and adsorption capacity for n-butane. The breakthrough time of nbutane was very short. This is due to the high inlet concentration of nbutane (80 ppm). If the inlet concentration of the gas is reduced, resulting in reduced diffusion velocity into the pores of the activated carbon, the breakthrough time may take longer to reach equilibrium [57]. In a real cabin environment, gaseous pollutants collect slowly and steadily at low concentrations, and the breakthrough time may be longer. The highest n-butane adsorption capacity was 4.6 g/m2 (O3), and lowest was 0.2 g/m2 (A1). The adsorption capacity increased linearly with the activated carbon content (y = 0.413x – 0.018, r2 = 0.8863), as shown in Fig. 12(b). This trend was also observed in analysis of toluene adsorption capacity according to activated carbon content (y = 0.572x – 0.115, r2 = 0.6288), as shown in Fig. 12(c). However, the correlation coefficient was only 0.63, and the toluene adsorption capacity of filter O2, containing 290 g activated carbon, was significantly higher than that of O3, with 395 g activated carbon, indicating that other factors affect the capacity for toluene adsorption. This finding may be associated with the surface area or pore size of activated carbon [61]. Table 2 shows the surface pore characteristics of five OEMs and two after-market activated carbon filters. The adsorption capacity increased linearly with the BET surface area (y = 0.041x –

Fig. 11. N-butane and toluene breakthrough curves. (a) N-butane and (b) toluene outlet concentrations of all tested automotive cabin air filters according to the elapsed time. Black and blue dashed lines indicate the inlet and saturation concentrations of test gas, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

activated carbon increased with increasing gas molecular weight (M) [58]. Thus, toluene (M = 92.4 g/mol), with a higher molecular weight than n-butane (M = 58.12 g/mol), is more highly adsorbed onto activated carbon. In addition, n-butane removal performance noticeably decreased after 5-min elapsed time, whereas toluene removal performance was maintained. The difference in n-butane removal performance after 1min and 5-min elapsed times (23.4 ± 6.59%) was higher than that of toluene (7.2 ± 9.48%). This is related to the lifespan of activated carbon filters, which is estimated from the breakthrough time or the gas adsorption capacity. Fig. 11 shows the n-butane and toluene outlet concentration of tested ACAFs as a function of elapsed time. Breakthrough curves have a characteristic “S” shape. The breakthrough time was determined when the outlet gas concentration reached 5% of the

Fig. 12. N-butane and toluene adsorption performance of 15 commercial composite cabin air filters. (a) N-butane outlet concentration of O1 according to the elapsed time. Black and blue dot line indicate inlet concentration and saturation concentration of n-butane, respectively. Gray zone represents n-butane adsorption capacity of O1. (b) N-butane adsorption capacity of tested automobile cabin air filters. (c) Toluene adsorption capacity of tested automotive cabin air filters. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 8

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3.788, r2 = 0.8771), Langmuir surface area (y = 0.029x – 1.756, r2 = 0.7598), total pore volume (y = 80.768x – 4.474, r2 = 0.7425) and average pore size (y = –13.643x + 298.73, r2 = 0.8745) of activated carbon. These results indicated that the surface area and pore size of activated carbon affect the toluene adsorption capacity of ACAFs. However, the breakthrough time and adsorption capacity of toluene were approximately 3 and 10 times higher than those of n-butane, respectively (Figs. 11 and 12). Thus, the lifespan expectancy of the toluene removal performance of ACAFs was longer than that of the nbutane removal performance. These results indicate that the gas removal performance of ACAFs can vary significantly depending on the type of exposed gas and filter.

[10] [11] [12] [13] [14] [15]

4. Conclusions

[16]

In this study, the performance of commercially available ACAFs (15 types) against various harmful airborne pollutants was evaluated under the standard test protocol (SPS-KACA014-0144). OEM filters not only had good PM filtration performance, but also satisfied standard guidelines. However, some of the after-market filters showed very poor performance against PM. The filter QF values also varied depending on the ACAF type. Only one of the tested ACAFs showed a higher QF value than that of a commercial HEPA filter. Overall, the OEM ACAFs with a large number of filtration layers showed better performance than other filters. Regarding the gas removal performance of ACAFs, the tested OEM ACAFs generally performed better than after-market ACAFs, even though only one of the 15 tested ACAFs simultaneously satisfied the nbutane and toluene removal efficiency standards. Given the wide range of ACAF performance in this study, it is need to establish the legally enforced certification parameters and clear that more attention should be given to the selection of commercial ACAFs. Careful selection is expected to play an important role in protecting automotive cabin IAQ against various harmful airborne pollutants. This study will help inform the selection of suitable ACAFs and provides baseline information for understanding and improving automotive cabin IAQ.

[17] [18] [19] [20] [21] [22] [23]

[24] [25] [26] [27]

Notes

[28]

The authors declare no competing financial interests.

[29]

Acknowledgments

[30]

This research was supported by the KITECH Institutional Program (JA-19-0038), and, in part, by the Ministry of Environment, Republic of Korea, via the Public Technology Program Based on Environmental Policy (2016000160008). It was also partly supported by a National Research Foundation of Korea grant funded by the Korean government (MSIT) (No. 2019R1A2C2002398) and the KIST Institutional Program.

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