Investigation of Air Humidity Affecting Filtration Efficiency and Pressure Drop of Vehicle Cabin Air Filters

Bin Xu1,2*, Ya Wu1, Zhongping Lin3, Zhiqing Chen3

1 Department of Environmental Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China 2 State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, 1239 Siping Road, Shanghai 200092, China 3 School of Mechanical Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China

ABSTRACT

Vehicle cabin air filters are exposed to humid air more frequently than any other air filters during routine use. The filtration performance of several commercially-available cabin air filters was investigated, along with the humid exposure period, using laboratory-based measurements. The averaged filtration efficiency and pressure drop were measured at ~70% and 75 Pa, respectively. Significant increases in filtration efficiency (up to 15%) and pressure drop (up to 250 Pa) were observed as the humid exposure time increased. Filtration efficiency increased ~15% and pressure drop increased 250 Pa as 140 g water was absorbed, which represents ~60 minutes humid exposure at a relative humidity of 90%. The pressure drop increased significantly at the beginning of the humid exposure due to the greater water absorption capacity of dryer dust in the filter. The dust load had a significant effect on the changes in filtration efficiency and pressure drop. The filtration efficiency and pressure drop of the 12-month used filter increased 2 times faster than that of the new filter at the same exposure conditions. The filtration efficiency and pressure drop were explicitly expressed as functions of the water absorption mass in the filter. Two coefficients were empirically derived and successfully accounted for the effects of humid exposure on filtration efficiency and pressure drop.

Keywords: Cabin filter; Particle filtration; Humid air; Filtration efficiency; Pressure drop.

INTRODUCTION frequently than any other air filters. Vapor and droplets in the humid air may be absorbed by the dust loaded in the In the last decade, many studies have identified vehicle cabin filter, which negatively affects the filters’ performance, cabin as a microenvironment for human exposure to leads to the malfunction of cabin filters or even the failure Particulate Matters (PM) (Zhu et al., 2007; Kaminsky et of vehicle ventilation system. Thus, it is essential to al., 2009; Geiss et al., 2010; Knibbs and de Dear, 2010; investigate the influence of the humid air on the cabin Bigazzi and Figliozzi, 2012; Hudda et al., 2012). Extensive filters’ performance along the filter usage periods. studies have been conducted to investigate the parameters Previously, several studies have been conducted to assess that determine the PM entry to the in-cabins and measures to the performance of cabin air filters. Pui et al. (2008) found reduce the in-cabin PM concentrations (Xu and Zhu, 2009; that the in-cabin PM concentration was reduced significantly Knibbs et al., 2010; Hudda et al., 2011). Both experimental with the recirculated air passing through the cabin filter. Qi and numerical studies have reported a positive effect of et al. (2008) experimentally evaluated the vehicle cabin air cabin air filter on reducing the in-cabin PM concentration filters’ filtration efficiency that varied from 20% to 70%. (Pui et al., 2008; Qi et al., 2008; Xu et al., 2011; Xu and Xu et al. (2011) investigated the cabin filters’ performance Zhu, 2013). An appropriate cabin filter under good condition under different air velocities and dust loadings. Up to 10% leads up to a 40% reduction of in-cabin PM exposure (Xu increase of filtration efficiency and 45 Pa increase of pressure et al., 2011; Xu and Zhu, 2013). During the vehicles’ routine drop were observed as the filter was used for 20 months. use, vehicle cabin filter are exposed to humid air more Xu et al. (2013) conducted extensive measurements on the performance of air filters that were used in the airliner cabins and reported a much greater filtration efficiency (86%–99%) and pressure drop (150–250 Pa) than vehicle * Corresponding author. cabin filters’. However, the performance of cabin air filters Tel.: +86-13916186347, Fax: +86-21-65981831 under humid exposure condition is still unknown. The E-mail address: [email protected] knowledge of humid air affecting the filtration efficiency

Xu et al., Aerosol and Air Quality Research, 14: 1066–1073, 2014 1067 and pressure drop is limited. The understanding of the effect were used as the particle source since the cabin air filters of humid exposure on cabin filter performance at different were exposed to the ambient air under the practical filter usage periods is even less. In addition, existing condition. The ambient air was injected into the testing theoretical studies do not account for the humid exposure system through a fan and a nozzle airflow meter that was effect on particle filtration. calibrated according to ISO9300-2005 standard before the To fill this important knowledge gap, this study was measurements. DI water vapor and droplets were generated designed to experimentally characterize the PM filtration from a sprayer that was connected with a peristaltic pump efficiency and pressure drop in response to various relative (Model BT100-2, Baoding Longer precision pump Co.). humidity or water absorptions at different cabin filter usage The humidity was controlled by the injected water flow periods. The theoretical equations were then revised to rate from the pump. The relative humidity was monitored extend its application to calculate the filtration efficiency by an indoor air quality monitor-Qtrak (Model TSI 7565, and pressure drop under humid exposure condition. The TSI Inc. USA). The filter upstream duct was designed long relationship between any two of the studied parameters enough to uniformly mix the airflow and the vapor on the (filtration efficiency, pressure drop, relative humidity, and cross section area of the test tunnel. The test filter was set water absorption mass) was explicitly presented. up until the uniformity of filter face velocity was reached. The entire testing system was maintained with a positive METHODS pressure (5–15 Pa) over ambient to prevent outside air and humid leaking into the system. An Optical Particle Sizer Sample Filters and Testing System (OPS, Model TSI 3330, TSI Inc. USA) was used to measure The filtration efficiency and pressure drop were measured particle number concentrations (particles/cm–3) in the 0.3– with four 6-month used vehicle cabin air filters, one new 5 µm range alternately upstream and downstream of the filter, one 3-month used filter, one 9-month used filter, and test filter. The filtration efficiency was calculated as η = 1 one 12-month used filter. These usage periods (3 months, 6 – downstream number concentration/upstream number months, 9 months, 12 months) were chosen to represent the concentration. The pressure drop across the test filter was common range of the vehicle cabin air filters’ usage period. continuously monitored using a manometer (Manometer 475 The 6-month used filters were manufactured for Toyota Mark III, Dwyer Instruments Inc. USA) in the experiment. Prado, Nissan Teana, Volkswagen Passat, respectively. The other filters that were manufactured for the same vehicle Test Protocols and Data Analysis model of Toyota Prado SUV were selected from the The filtration efficiencies and pressure drops across the maintenance workshop. All of the test filters are made of test filters were measured at the airflow rate of 150 m3/h, pleated glass-fiber papers. The cabin air filters for Toyota which was the most commonly used vehicle ventilation Prado were 190 mm in width, 200 mm in length, and 30 mm airflow rate under the actual driving conditions (Xu et al., in thickness. By multiplying the pleat number and the pleat 2011). The measurement was conducted at a relatively surface areas, filter surface areas were calculated as 0.5 m2. consistent temperature (20 ± 2°C). To investigate the humid The Solid Volume Fraction (SVF) of the test filters are 5– exposure affecting filtration performance, the filtration 10%. efficiencies and pressure drops were measured at three An Europe Standard EN-779 classified testing system Relative Humidity (RH: 35%, 62%, 90%). This RH range (European Air Filter Test Standard. EN 779:2002) was covers most of the cabin air filters’ RH exposure levels under used in the experiment. Fig. 1 illustrated the schematic of real driving condition. It was found that small water droplets the testing system setup. The particles in the ambient air exist in the air upstream of the filter at the RH of 90%, and

Fig. 1. Experimental schematic of the experimental test system.

1068 Xu et al., Aerosol and Air Quality Research, 14: 1066–1073, 2014 this represents the high humid and raining conditions. The exhibits a similar filtration efficiency curve as a function of filtration efficiencies and pressure drop across the filter were particle size. Since the vehicle cabin air filters were designed measured continuously as the water was absorbed. On the as a medium-efficiency filter, the filtration efficiency was other hand, in order to investigate the filtration performance measured in the fine particle size range from 0.3 µm to 5 µm. change as the dust desorbed the water, the measurement The filtration efficiencies varied significantly from 35% to continued by stopping water injection into the system. The 100% with the greatest increase observed for larger particles. ambient RH is 35 ± 3% in the measurement. For particles larger than 5 µm, the filtration efficiency is It should be noted that the continuous water absorption in close to 100%, which implied the fact that the dust collected this study may be potentially different from the intermittent in the cabin air filter contains substantial large suspended water absorption characteristics occurred in real-world. To dust that are enriched in metal elements (e.g., Zn, Cu and 2– generalize the results of the humid affecting the cabin filters’ Cr) and ions (e.g., SO4 ) (Zhao et al., 2006). The pressure performance at different humid exposure frequencies, the drops were found consistent at 60–75 Pa for all the test filters. filtration efficiency and pressure drop were expressed as a function of the water absorption mass at various filter usage Effect of RH and Water Absorption on the Filtration periods. The filter was weighted every five minutes to Performance calculate the water absorption mass in the filter. The data The vapor or droplets were consistently absorbed by the were collected after the filter face velocities and relative dust in the filter. It leads to the change of filtration humidity were observed to be stabilized within 5% and performance, e.g., filtration efficiency and pressure drop. 10% difference, respectively. The averaged filtration efficiency that was calculated as the mean of the filtration efficiencies at 6 particle size bars Theoretical Calculation from 0.3 µm to 5 µm was used to express the filtration To extend the theoretical calculation of filtration efficiency efficiency as a function of water absorption. Due to the and pressure drop under humid exposure condition, the limited number of size bars, the relationship between filtration frequently used equations (Eqs. (1) and (2), Rao and Faghri, efficiency and water absorption was not derived on a size- 1988; Brown, 1993) were revised in this study. segregated basis. This is a limitation of this study. Also, it should be noted that humid exposure affects filtration η = 1 – exp(–4αEh/πdf) (1) efficiency of smaller particles (diameter < 0.3 µm) due to the varying diffusion effect, which is not considered in this study. 2 Δp/h = f(α)(4μU/df ) (2)

100 where η is the filtration efficiency of the filter, α is the (a) Solid Volume Fraction (SVF), E is the total Single Fiber Efficiency (SFE), can be calculated as the Eq. (3) (Hinds, 80 1999), h is filter thickness, df is fiber diameter, Δp is the pressure drop, f(α) is dimensionless pressure drop and can 60 be then obtained as Eq. (4) (Davis, 1973). µ is the air viscosity, U is filter face air velocity. 40 Filter A Filter B 20 E = 1 – (1 – ED)(1 – ER)(1 – EI) (3) Filtration efficiency, % Filter C Filter D 3/2 3 f(α) = 64α (1 + 56α ) (4) 0 .1 1.0 10.0 Particle diameter, m where ED, ER, and EI are the single fiber filtration 200 efficiencies due to diffusion, interception and inertia (b) impaction, respectively (Hinds, 1999). To extend the theoretical calculation for the filtration 150 efficiency and pressure drop under the humid exposure condition, two coefficients “B” and “C”, defined as the ratio of the filtration efficiency and pressure drop with 100 water absorption to the filtration efficiency and pressure drop without water absorption, were added in Eqs. (1) and 50 (2). The detailed derivations of the coefficient “B” and “C” Pressure drop, Pa are discussed in below section. 0 RESULTS AND DISCUSSION ABCD Test filters Filtration Efficiency and Pressure Drop for Test Filters Fig. 2. Measured particle (a) filtration efficiencies and (b) Fig. 2 shows the filtration efficiencies and pressure drops pressure drops of four 6-month used cabin air filters. The of the 6-month used filters. All test filters used in this study airflow rate is 150 m3/h.

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Fig. 3 illustrates the averaged filtration efficiency and the beginning of the humid exposure. This is because the pressure drop as a function of filter’s humid exposure time as the vapor and droplets were absorbed continuously. In at various RHs. It was observed that, after 60 min humid absorption capability of the loaded dust in the filter decreased exposure at RH of 90%, filtration efficiency and pressure Fig. 3(b), the pressure drop curves at 20 min and 40 min drop increased significantly by 20–25% and 170–200 Pa, with respect to 90% RH and 62% RH were bended slightly. respectively. This is because the dust in the filter absorbed This might due to the different absorption type before and the vapor or droplets and increased in sizes that led to a after the bending point. After a certain amount of water narrower and more curving air pathway inside filters. absorption, the vapor might deposit on the surface of the Narrower air pathway that caused larger fiber projected area dust instead of dust inside. After that point, the size of the vertical to the airflow direction results in a greater filtration loaded dust did not increase significantly that led to a slighter efficiency. Also, narrower air pathway led to a significant pressure drop increase. air velocity increase inside filter media that caused the It was also found that the water can be desorbed as the RH increase of pressure drop across the filter. More curving air reduced, which led to the decreases of filtration efficiency pathway increased the possibility of particles leaving air and pressure drop. In the experiment, the vapor injection streamlines and attaching on the fiber due to the inertial was stopped to investigate the filtration performance change effect. In addition, air pathway with more curves led more as the water was desorbed. As shown in Fig. 3, the dust airflow turns that results in an increased pressure drop. desorbed the water faster than it absorbed the vapor. For From Fig. 3, it was also seen that the filtration efficiency example, at the RH of 90%, the pressure drop increased and pressure drop increased more significantly with greater 200 Pa using 60 min; while it only took 40 min for the RH. Larger RH led more vapor absorption and faster dust pressure drop to decrease 200 Pa. size increase. Quantitatively for example, it took ~17 min In this study, the water absorption by the dust was faster for the pressure drop to increase 100 Pa at the RH of 90%. than the intermitted absorption under the practical driving On the other hand, it took ~40 min for the same pressure condition. This experimental approach can be reasonably drop increase at the RH of 62%. It was noted that the applied to estimate the filtration performance change caused filtration efficiency and pressure drop increased faster at by humid exposure since the absorption mechanisms are the same. It should be noted that the dust loaded in the test filters was from the particles in the on-road atmosphere in 100 RH-90% (a) China. Different loaded dust might lead to different filtration Stop water injection RH-62% performance changes due to different water absorption RH-35% characteristics. Furthermore, to apply these results at different 90 humid exposure frequencies, the changes of filtration Stop water injection efficiency and pressure drop were expressed as a function of the absorbed water mass. 80 Fig. 4 showed the water absorption mass as a function of filter’s humid exposure time. As expected, the water absorption mass is linearly proportional to the filter’s humid

Filtration efficiency, % Filtration efficiency, exposure time. Larger RH led to more vapor exposure that 70 resulted in a greater water absorption rate. Substituting the filter’s humid exposure time by the mass 0 20 40 60 80 100 120 140 160 180 of water absorbed, the filtration efficiency and pressure drop 400 were expressed as a function of the water absorption mass. RH-90% (b) 350 RH-62% RH-35% 160 300 RH-90% 140 RH-62% 250 120 RH-35%

200 100

150 80 Presure drop, Pa Presure drop, 60 100 40 50 0 20 40 60 80 100 120 140 160 180 20

Time, min Water absorbed in the filter, g 0 Fig. 3. The relationship between (a) filtration efficiency, (b) 0 20406080100 pressure drop and filter’s humid exposure time at different Time, min RH conditions. 6-month used filter for Toyota Prado were Fig. 4. The relationship between the water absorption mass tested. and the filter’s humid exposure time at different RH levels.

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Fig. 5 illustrates the relationship between the filtration mass of water absorption in the measurement range (0–140 performance and the water absorption mass in the filter. The g)Coefficient B and C were regressed as a function of the result suggests that the filtration efficiency is significantly water absorption mass. This is because “B” and “C” are related to the water absorption mass in the filter instead of directly associated with the increased SVF due to the water RH. The filtration efficiency is directly determined by the absorption. From Fig. 6, it was found that the coefficient characteristic of the air pathway in the filter media that is “B” as a function of water absorption was derived as y = related to the water absorption. On the other hand, the 0.003x + 1.032, R2 = 0.98; the coefficient “C” as a function increase of pressure drop is related to both of the RH and of water absorption was derived as z = 0016x + 1.456, R2 water absorption mass. Quantitatively for example, with = 0.89, where z is “C” and x the water absorption mass. As the same absorbed water mass, the increase of pressure drop the water was continuously absorbed in the filter, the air at the RH of 90% was 50 Pa larger than the pressure drop volume fraction inside the filter media was reduced. Greater at the RH of 62%. This is because higher RH caused faster B indicates narrower air pathways inside filter media. droplet absorption in the loaded dust that led to greater To further verify if the filtration efficiency and pressure dust size increase and narrower air pathways. drop with experimentally determined coefficient “B” and “C” agree with experimental results under different Expansion of Theoretical Calculations with Water conditions, Eqs. (5) and (6) are used to calculate the filtration Absorption efficiency and pressure drop for the filters at the RH of In order to extend the application of the theoretical 90%. Fig. 7 illustrates the comparison between the theoretical calculation on the filtration efficiency and pressure drop filtration efficiency and pressure drop corrected with under water absorption in the filter, two coefficients (B coefficients and experimental data at the RH of 90%. No and C) were introduced and added in Eqs. (1) and (2). Eqs. significant difference was observed. (1) and (2) are then revised to Eq. (5) and (6). Other than filtration efficiency and pressure drop, a parameter “Filter quality factor = –lnP/Δp”, in which “P = η = 1 – exp(–4αBEh/πdf) (5) exp(–4αEh/πdf)” is the penetration factor, was commonly used to determine the filter quality. By substituting “P” 2 Δp/h = Cf(α)(4μU/df ) (6) and “Δp” into the filter quality factor calculation, filter quality factor can be calculated as Eq. (7). Therefore, a Fig. 6 shows the relationship between “B”, “C” and the

(a) 100 1.6 RH-90% (a) RH-62% 1.4 90

1.2

80 Coefficient B y=0.003x+1.023, R2=0.98 1.0

Filtration efficiency, % efficiency, Filtration 70 .8 0 20406080100120140160

0 20 40 60 80 100 120 140 160 5 400 (b) RH-90% (b) 350 RH-62% 4

300 3 250

200 z=0.016x+1.456, R2=0.89 Coefficient C 2 150 Presure drop, Pa

100 1

50 0 20 40 60 80 100 120 140 160 0 20 40 60 80 100 120 140 160 Water absorbed in the filter, g Water absorbed in the filter, g Fig. 6. Coefficients “B” and “C” as a function of the water Fig. 5. (a) Filtration efficiency and (b) Pressure drop as a absorption mass. The experimental data at the RH of 62% function of the water absorption mass in the filter. was used.

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Measurement data-filtration efficiency Stop water injection Measurement data-pressure drop 100 Theoretical prediction-filtration efficiency (a) Theoretical prediction-pressure drop New 6 months 100 400 12 months 90

80 300 80 60

40 200 Filtration efficiency, % 70

20 Pressure drop, Pa Filtration efficiency, % 0 20 40 60 80 100 120 140 100 400 0 0 20 40 60 80 100 120 140 New (b) 350 6 months Water absorbed in the filter, g 12 months 300 Fig. 7. Comparison of filtration efficiency and pressure drop between modeled results and experimental results at the RH 250 of 90%. 200

150 coefficient “D = B/C” can be used to express the filter quality Pa drop, Presure factor under water absorption condition. 100

2 50 Filter quality factor 4αBEh πd[Cf(4ffμU/d )] 0 20 40 60 80 100 120 140 B 2 Time, min 4αEh πd/[f(4ffμUd)](7) C Fig. 8. (a) Filtration efficiency and (b) pressure drop as a BlnP function of humid exposure time for filters at different usage  periods. 90% RH was used in the measurement. C Δp

Effect of Humid Exposure on the Filtration Performance regressed at different filter usage periods, as shown in Fig. 8. at Various Dust loads Humid exposure led to consistent increases of filtration Filter usage period determined the mass of dust loaded efficiency and pressure drop. The increasing rate for long- in the filter, which potentially affect the water absorption time used filter is slightly larger than short-time used filter. capacity. According to the literature, 3-month, 6-month, 9- For example, 50 g water absorption caused 120 Pa and 150 Pa month and 12-month usages represent 0.8 g, 1.5 g, 2.4 g pressure drop increase for 6-month and 12-month used filters, and 3 g dust loaded in the filter (Xu et al., 2011). Fig. 8 respectively. Dramatic pressure drop increase potentially illustrated the changes of filtration efficiency and pressure led to significant decrease of airflow rate in the vehicle drop as the filter was exposed in the humid air at different ventilation system. This finding can be applied to facilitate usage periods. For clarity, only the measurement results of the maximum humid exposure period to prevent from the filters with three filter usage periods (new, 6-month, ventilation malfunction in the practical driving condition. 12-month) were shown in Fig. 8. It can be seen that, for the new filter, the effect of humid exposure on the filtration CONCLUSIONS performance is negligible. On the other hand, the filtration efficiency and pressure drop increased significantly in In summary, the effect of humid exposure on the vehicle response to the increased humid exposure time for used cabin air filter’s performance was evaluated and investigated filters. With the same humid exposure time, 12-month filter at different RHs at different filter usage periods. For test usage led to 8–12% increase of filtration efficiency and 50– cabin air filters, the averaged filtration efficiency and pressure 100 Pa increase of pressure drop, respectively. This can be drop were measured at ~70% and 75 Pa, respectively. explained by the fact that the absorption capacity of the Significant increase of filtration efficiency (up to 15%) and dust loaded in the filter is much larger than the filter media. pressure drop (up to 250 Pa) were observed as the humid To further investigate the effect of humid exposure on exposure time increased. Filtration efficiency was influenced the filtration performance of vehicle cabin air filters and most by the mass of water absorbed in the filter. On the other generalize the findings of this study to estimate other cabin hand, pressure drop was affected by the water absorption air filters’ filtration performance, the coefficients “B” and mass and the RH simultaneously. The pressure drop increased “C” as a function of water absorption mass were linearly more significantly at the beginning of the humid exposure

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100 ISCLAIMER New (a) 6 months 2 12 months y=0.001x+0.77, R =0.92 Reference to any companies or specific commercial 90 products does not constitute its endorsement or recommendation by the National Science Foundation of China. 80 y=0.0009x+0.72, R2=0.95 REFERENCE Coefficient B Coefficient 2 Bigazzi, A., and Figliozzi, M., (2012). Impacts of Freeway 70 y=0.0004x+0.70, R =0.99 Traffic Conditions on In-vehicle Exposure to Ultrafine Particulate Matter. Atmos. Environ. 60: 495–503. 0 20 40 60 80 100 120 140 160 Davies, C.N. (1973). Air Filtration, Academic Press, London. 400 EN 779:2002: Particulate Air Filters for General Ventilation New (b) (HEPA and ULPA). CEN Central Secretariat: Rue de 350 6 months y=1.46x+130, R2=0.89 Strassart, 36, B-1050 Brussels. 12 months 300 Geiss, O., Barrero-Moreno, J., Tirendi, S. and Kotzias, D. (2010). Exposure to Particulate Matter in Vehicle Cabins 250 of Private Cars. Aerosol Air Qual. Res. 10: 581–588. 2 200 y=1.27x+114, R =0.89 Hinds, W.C. (1999). Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles. Second Coefficient C Coefficient 150 ed., Wiley, New York. y=0.77x+78, R2=0.99 Hudda, N., Kostenidou, E., Sioutas, C., Delfino, R.J. and 100 Fruin, S.A. (2011). Vehicle and Driving Characteristics 50 that Influence In-cabin Particle Number Concentrations. 0 20 40 60 80 100 120 140 160 Environ. Sci. Technol. 45: 8691–8697. Water absorbed in the filter, g Hudda, N., Eckel, S., Knibbs, L., Sioutas, C., Delfino, R., Fig. 9. The relationship between (a) coefficient “B”, (b) and Fruin, S. (2012). Linking In-vehicle Ultrafine Particle coefficient “C” and the water absorption mass in the filter Exposures to On-road Concentrations. Atmos. Environ. at different filter usage periods. 62% RH was used in the 59: 578–586. measurement. Kaminsky, J., Gaskin, E., Matsuda, M. and Miguel, A. (2009). In-Cabin Commuter Exposure to Ultrafine Particles on Commuter Roads in and around Hong Kong’s Tseung due to the greater water absorption capacity of dryer dust. Kwan O Tunnel. Aerosol Air Qual. Res. 9: 353–357. It was also found that dust loading posed a significant Knibbs, L., de Dear, R. and Morawska, L. (2010). Effect effect on the change of filtration performance. The filtration of Cabin Ventilation Rate on Ultrafine Particle Exposure efficiency and pressure drop of the 12-month used filter inside Automobiles. Environ. Sci. Technol. 44: 3546–3551. increased 2 times faster than the new filter at the same humid Knibbs, L.D. and de Dear, R.J. (2010). Exposure to exposure condition. The filtration efficiency and pressure Ultrafine Particles and PM2.5 in Four Sydney Transport drop was explicitly expressed as a function of humid exposure Modes. Atmos. Environ. 44: 3224–3227. time and water absorption mass in the filter. Coefficients Kuwabara, S. (1959). The Forces Experienced by Randomly “B” and “C” that were derived as a function of water Distributed Parallel Circular Cylinders of Spheres in a absorption mass were introduced to extend the application Viscous Flow at Small Reynolds Number. J. Phys. Soc. of the theoretical calculations for filtration efficiency and Jpn. 14: 527–532. pressure drop to particles under humid exposure. Theoretical Pui, D., Qi, C., Stanley, N., Oberdörster, G. and Maynard, calculations corrected with “B” and “C” agreed well with A. (2008). Recirculating Air Filtration Significantly experimental measurements under other studied conditions. Reduces Exposure to Airborne Nanoparticles. Environ. The findings of this study can be used to facilitate the Health Perspect. 116: 863–866. maximum vehicle cabin air filter’s exposure period to avoid Qi, C., Stanley, N., Pui, D. and Kuehn, T. (2008). Laboratory malfunction of vehicle ventilation system, or as a reference and On-Road Evaluations of Cabin Air Filters Using to further upgrade the vehicle’s ventilation airflow setting Number and Surface Area Concentration Monitors. to prevent from dramatic pressure drop increase. Environ. Sci. Technol. 42: 4128–4132. Xu, B. and Zhu, Y. (2009). Quantitative Analysis of the ACKNOWLEDGEMENTS Parameters Affecting In-cabin to On-roadway (I/O) Ultrafine Particle Concentration Ratios. Aerosol Sci. This material is based on the work partially supported Technol. 43: 400–410. by the National Science Foundation of China (Grant NO. Xu, B., Liu, S., Liu, J. and Zhu, Y. (2011). Effects of 51208372) and 12th Five-Year National Key Technology Vehicle Cabin Filter Efficiency on Ultrafine Particle R & D Program (2012BAJ02B03). Concentration Ratios Measured In-cabin and On-roadway.

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