Evaluation of Car Air Filters ’ Efficiency as Active Samplers for Polycyclic Aromatic Hydrocarbons and Heavy Metals

In this study, an innovative and inexpensive approach, based on car engine air filters (CAFs), was used to monitor polycyclic aromatic hydrocarbons (PAHs) and heavy metals (HMs). CAFs were collected from two taxi garages in Guangzhou and analyzed for PAHs and heavy metals. The total concentrations of eight high molecular weight PAHs (ΣPAHs) ranged from 923 to 11378 μg/CAF. The average ΣPAHs concentration in winter was slightly higher than in summer, a seasonal signal that is observed in most studies. The most abundant individual PAH was chrysene, followed by benzo[a]anthracene, benzo[b]fluoranthene and benzo[k]fluoranthene. Concentrations of benzo[a]pyrene (BaP) varied between 8.22 and 632 μg/CAF. Based on estimated air volumes that pass through each CAF, atmospheric PAHs and heavy metals were estimated. ΣPAHs ranged from ~20 to ~200 ng/m for the average air volume estimate, being comparable to those reported previously using a high volume air sampler. BaP-equivalent carcinogenic potency were between 1 and 15.0 ng/m (again, for the average scenario). The average concentrations of heavy metals in CAFs decreased in order of Zn > Cu > Pb > Cd.


INTRODUCTION
Urban air pollution can cause acute or chronic adverse effects on human health via exposure to both gaseous (oxides of nitrogen, oxides of sulfur, oxides of carbon etc.) and particulate pollutants (organic and inorganic).Among various particulate pollutants, heavy metals (HMs) and polycyclic aromatic hydrocarbons (PAHs) are of most concern and have been intensively studied in past decades because some of these compounds are toxic even at low concentration in the air (Vassilakos et al., 2007;Vijayanand et al., 2008;Yang et al., 2010;Delgado-Saborit et al., 2011).HMs associated with particles in urban air can penetrate in the deeper parts of lungs and catalyse the oxidative stress in the body cells, eliciting inflammatory injuries in the airway and lungs (Nel, 2005).Inhalation of airborne trace metals can therefore have long-term effects and serious impact on human health (Massey et al., 2013).
Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous pollutants that are mainly derived from incomplete combustion of organic materials.Some of these compounds are highly carcinogenic and/or mutagenic, several compounds have been considered as probable and possible carcinogens to humans (IARC, 1983(IARC, , 1991)).Because PAHs have been frequently detected in the outdoor and indoor air of different urban area (Li et al., 2006;Masih et al., 2010b;Yang et al., 2010;Delgado-Saborit et al., 2011;DEFRA, 2012;Hung et al., 2012;Katsoyiannis et al., 2012;Masih et al., 2012;Alkurdi et al., 2013), they have been identified as an important class of toxic air pollutants in the atmosphere of urban areas.PAHs are present in both gas and particle phases influencing human health through inhalation and/or ingestion (Li et al., 2006;Ma et al., 2010;Masih et al., 2010a;Ma et al., 2011).Zhang et al. (2009) reported that 1.6% of the lung cancer morbidity in China was due to inhalation exposure to ambient air PAHs.Thus, it is very important to monitor continually the levels of PAHs in ambient air and assess these exposure rates.
As described by Katsoyiannis et al. (2012), atmospheric pollutants in ambient air are monitored mainly by conventional sampling methods including high volume air sampling via glass fiber filters and polyurethane foam disks using active sampling (Lee et al., 2007;Shi et al., 2010;Yang et al., 2010;Delgado-Saborit et al., 2011;Birgul et al., 2012;Martellini et al., 2012;Cheng et al., 2013) or by passive sampling techniques (e.g., semi-permeable membrane devices, Liu et al., 2006;Piccardo et al., 2010).Since the sampling sites selected for these methods are stationary and limited, the air concentrations are somewhat site-specific.However, Zhang et al. (2011) and Katsoyiannis et al. (2012) suggested that car air filters (CAFs) from taxis could act as "moving" high volume air samplers and provide city integrated air concentrations with low variability for particle-bound contaminants.
CAFs are used in vehicles to filter out engine-destroying dirt and maintain engine performance.Among various air filters (i.e., cotton, foam, and paper), paper air filters are widely used because they are disposable and inexpensive.City taxis/cabs normally change CAFs every 15-20 day, circulate almost exclusively within local urban area, have access to city centers and congestion zones, therefore analyses of pollutants associated with taxi CAFs could provide scientists with results that correspond broadly to a defined area and time (Katsoyiannis et al., 2012).
To try the hypothesis of the afore mentioned study and to expand it to analysis of heavy metals, the concentrations of PAHs and heavy metals in taxi CAFs of Guangzhou were investigated, and their atmospheric concentrations were estimated.The seasonal variation of PAHs and heavy metals and carcinogenic potency are also discussed.

Sample Collection
A CAF is located in the front part of the vehicle and filters the air that goes into the engine protecting the latter from the air particulate matter, and maintaining the engine performance at high levels.
Guangzhou, with an area of 7434 km 2 , is located in southern China and belongs to the Pearl River Delta region.Guangzhou is not only one of the most densely populated areas (the population being up to14.8 million in 2010) (Guangzhou population an family planning Bureau, 2011), but is also the economic and cultural center of South China.In Guangzhou, around 2.4 million vehicles circulate (in 2012) (http://auto.163.com/12/0711/10/864GPKNP00084TV6.html), and there are about 18000 taxis operating daily.Among various companies of taxis, three taxi companies own around 50% of the fleet.In this study, all CAFs sampled were from taxis carrying a 1.8 or 2.0 L engine volumes and were CAFs deployed/used for 15 to 20 days.
CAFs were collected from the official service garages of two main Taxi groups in Guangzhou.Seventeen CAFs deployed in taxis during the period 10-31 of August 2012 (presented hereafter as summer samples), and eleven CAFs deployed in taxis during the period 12 th of November to 2 th of December 2012 (presented hereafter as winter samples) were collected and analyzed for PAHs and HMs.Three new CAFs purchased from the official service garages were taken as the field blacks and were opened to open air for 30-min time in each garage, and then transported to the lab together with the collected-used CAFs.

Sample Treatment and Analysis of PAHs
The extraction procedure for PAHs was a modification of the method used by Katsoyiannis et al. (2012).Briefly, three grams of each filter (consisting of many small pieces (< 0.5 cm) from various parts of the CAF surface) were spiked with phenanthrene-d 10 and extracted in triplicate with a 30 mL solution of hexane/acetone (1:1) in an ultrasonic bath (KH-250E, Kunshan, China) for 30 min.The extracts were centrifuged at 3000 rpm for 5 min.The organic extracts were combined and concentrated to 1.0 mL by a rotary evaporator (52-A, Yarong, China).The extracts were cleanedup by a combined column of silica gel and alumin (Cai et al., 2007).PAHs were eluted with 50 mL of dichloromethane.This fraction was collected and concentrated in a rotary evaporator again.Finally, the extracts were transferred to a 2 mL amber glass vial and evaporated at room temperature under a gentle stream of nitrogen and rediluted to 1 mL of hexane.
PAHs in extracts were determined using Agilent 7890 gas chromatography with 5975 mass selective detector (GC-MSD) equipped with a Rtx-5MS capillary column (30 m × 0.25 mm i.d., 0.25 µm film thickness) in the electron impact mode (70 eV).The column temperature was initiated at 60°C (hold 1 min), and increased to 110°C at 35 °C/min, followed by an increase to 275°C at 8.0 °C/min, then increased to 280°C at 25.0 °C/min (hold for 10 min).Injector temperature was at 250°C, and the temperatures for both the transfer line and ion source were 250°C.The carrier gas was helium at a constant flow rate of 1 mL/min.An aliquot of 1 µL was injected in splitless mode with a solvent delay of 4 min.Mass range m/z 50-500 was used for quantitative determinations.Target compounds were identified based on their mass spectra and retention times.

CAF Treatment for Heavy Metals Analysis
The samples were treated by dry ashing method.The dry ashing procedure is a modification of Laing et al. (2003) and Momen et al. (2006).Briefly, two gram sample of CAF as well as field blacks was weighed into a porcelain crucible and heated on an electric hot plate at low temperature until carbonized (no smoke).Then, the crucible was transferred into the muffle furnace and pre-ashed at a temperature of 250°C and subsequently ashed at 500°C for 2 h.After ashing was completed (white or a semi-gray ash), the crucible was left to cool.The ash was dissolved with 2 mL 1:1 (V:V) HCl and then diluted to 25 mL with double distilled water.The concentrations of heavy metals (Cu, Zn, Pb, and Cd) in the solutions were measured with atomic absorption spectroscopy (Z-2300, Hitachi, Japan).The average recoveries of Cu, Zn, Pb and Cd ranged from 85% to 96%.In the field blacks, the concentrations of Cu, Zn, Pb and Cd in solutions were below 0.04, 0.2, 0.01, 0.003 µg/mL.respectively, being far lower than those in the used-CAF solutions.

Occurrence of PAHs in CAFs
Table 1 presents the descriptive statistics of PAH concentrations in CAFs, expressed as µg/CAF.All eight individual PAHs were detected and their total concentrations (Σ PAHs ) ranged from 923 to 11378 µg/CAF.The most abundant individual PAH was Chr, followed by BaA, BbF and BkF.It can be seen that on average, the concentrations of PAHs are lower as the number of rings increases.It is very interesting to note the range of concentrations of individual PAHs, which spans few orders of magnitude for all PAHs, an interesting result taking into account all the uncertainty associated with this sampling method.The study of Katsoyiannis et al. (2012) observed ranges of one order of magnitude for 10 CAFs analyzed for a city (Manchester, UK) that is much smaller than Guangzhou.The comparison between the two studies is given in Fig. 1 (where Chr is excluded because it was not reported in the latter study).It is interesting to note that while the average Σ PAHs and BaP are similar, the profile for the other PAHs is totally different.Manchester is an urban area with reduced industrial activities (compared to some decades ago), while Guangzhou still have a lot of industrial activities and is also affected by regional pollution of the Pearl River Delta region.Yet, we see that the Σ PAHs concentrations in CAFs are identical.This is likely evidence that the emissions from vehicles are those that mostly affect the PAHs fingerprint in CAFs.
Concerning the seasonal variation, the average Σ PAHs concentrations were just slightly higher in winter (4379 µg/CAF against 4049µg/CAF during summer) (Table 1).This difference is not considered significant and probably confirms that the PAHs occurring at street level are mainly emitted by vehicles, a process that is happening at the same extent at all periods throughout the year.The temperature difference between summer (ranging from 24 to 37°C) and winter (ranging from 7 to 26°C) (Fig. 2) is probably not affecting neither the concentration of PAHs at street level, nor the fate of PAHs on the CAF, once sampled.Additionally, the PM 10 concentrations (varying between 11-89 µg/m 3 ) in both summer and winter were quite similar (Fig. 3).Seasonality of particle-bound PAHs concentrations in Guangzhou has been reported by other researchers (Li et al., 2006;Tan et al., 2006;Duan et al., 2007;Yang et al., 2010).The seasonality of PAHs is a fact seen also in most studies worldwide (e.g., Katsoyiannis et al., 2011) but the aforementioned groups also pointed out that the seasonal variation can be affected by meteorological conditions such as atmospheric temperature, wind, storm, humidity.
Regarding the individual PAHs, some are significantly higher in summer (Fig. 4).The PAH that exhibits much higher concentrations during winter is Chr with average of 2633 µg/CAF, against 2218µg/CAF in summer.

Atmospheric PAH Levels Estimated Based on CAF Concentrations
Following the method of Katsoyiannis et al. (2012), PAH atmospheric concentrations can be derived through the CAF-PAH concentrations (Table 1) and the air flow.As mentioned above, CAF filters the air that goes into the engine.Thus, the air flow during the engine's operation can be calculated by Eq. (1) (Pope, 2009): (1)   where, CFM (cubic feet per minute) is the engine's air flow rate (ft 3 /min), RPM (revolutions per minute) is the engine speed (rpm/min), VE is the volumetric efficiency (for unmodified engines is about 85%), and CID (cubic inch displacement) refers to the engine volume (in 3 ).In China, the vehicle exhaust is generally expressed using liter (L) (1 liter = 61.02cubic inches).In this study, the engine volumes of taxis were 1.8 L and 2.0 L (Table 2), respectively.
If VE and CID of Eq. ( 1) are constant (for a specific taxitype), then the only parameter that results in uncertainty is the engine speed (RPM).As described by Katsoyiannis et al. (2012), the engine speed of taxis would generally range from 1500 to 3000 rpm when it cruises normally.In Guangzhou, according to the information from the taxis driver/garage experts, taxis circulate daily for 19 to 22 h (two shifts), and the CAF is changed normally every 15-20 days (Table 2).
In this study, we assumed four different driving scenarios (Table 2) for the estimation of the total air volume passed through a CAF.Following these scenarios and Eq. ( 1), the air volume that passes through a CAF varies from 20763 to 71233 m 3 , with the average/conservative scenario being 44794 m 3 (Table 2).
Using the two extreme and the average scenarios, the atmospheric PAH concentrations were estimated and are presented in Table 3.For the most conservative scenario (71233 m 3 of air) Σ PAHs varied between 18.52 and 159.72 ng/m 3 (in summer) with an average of 56.84 ng/m 3 .During winter, Σ PAHs varied between 12.96 and 130.02 ng/m 3 (in summer) with an average of 61.47 ng/m 3 .
The Σ PAHs concentrations estimated based on CAF analysis are comparable to those reported for Guangzhou in air particles (Li et al., 2006;Tan et al., 2006;Duan et al., 2007;Yang et al., 2010) or PM 2.5 -bound PAHs (Gao et al., 2011(Gao et al., , 2012)).In the Manchester study the PAH levels   estimated through CAFs were many times higher than the concentrations measured by means of Hi-Volume air sampling (filter and polyurethane foam).The use of CAFs gives us an estimate of the PAH concentrations at road level, inside the traffic, therefore it offers insight about the exposure that people who spend big parts of their daily lives in the traffic undergo.
Regarding individual PAHs (Table 3), their estimated average concentrations for the conservative scenario were 23.63 ng/m 3 for Chr, 6.87 for BaA, 6.18 ng/m 3 for BbF, 2.57 ng/m 3 for BkF and 1.45 ng/m 3 for BaP.Especially for BaP, the range of estimated air concentrations was from 0.12 to 8.87 ng/m 3 and in most cases the level of BaP was well below 1.0 ng/m 3 .The latter value is mentioned here,  , 2004).BaP is used as a biomarker for PAHs and is also among the most carcinogenic PAHs.

Carcinogenic Potency of PAHs
Among 16 PAH compounds listed as priority pollutants by USEPA, seven compounds, namely BaA, BbF, BkF, BjF, BaP, InP, and DahA, are considered to have carcinogenic effects.In order to estimate the carcinogenic potencies of PAHs, scientists have used BaP-equivalent carcinogenic power (BaP eq ) as an index for better assessing aerosol carcinogenicity of PAHs (Gao et al., 2011;Martellini et al. 2012).The BaP eq can be calculated by multiplying its concentration with the corresponding toxic equivalent factor (TEF) (Yassaa et al., 2001) as the following Eq.( 2): BaP eq = (BaA × 0.06) + (BbF × 0.07) + (BkF × 0.07) + BaP + (DahA × 0.6) + (InP × 0.08) (2) In this study, the BaP eq was calculated and found to range from 1.06 to 15 ng/m 3 (average of 4.9 and 5.5 ng/m 3 in summer and winter, respectively, Fig. 5).The average levels are comparable to those from two urban areas of South Italy (Bari and Taranto) (Amodio et al., 2009) and in three major cities of India (Kolkata, Mumbai and Chennai) (Cheng et al., 2013), but higher than the mean concentrations of BaP eq (1.93 ng/m 3 ) in PM 2.5 of Guangzhou (Gao et al., 2011).In this study, BaA, BaP and DahA made the largest contribution to BaP eq .

Concentrations of Heavy Metals
Four metal elements were analyzed in this study and their concentrations in CAFs and CAF-estimated air concentrations (ng/m 3 ) are presented in Table 4.The order of average concentrations of heavy metals in both CAF and air was Zn > Cu > Pb > Cd, being consistent with that in the ambient air of the Coimbatore city, Tamilnadu, India ( Vijayanand et al., 2008).Zn concentrations in CAFs ranged from 4557 to 35849 µg/CAF and the average concentration is up to 4621 µg/CAF, while those of Cd varied between 27.8 and 117 µg/CAF with a mean of 63.2 µg/CAF.
The estimated atmospheric concentrations of HMs (Table 3) (again for the conservative scenario, that is volume of 71233 m 3 ) ranged from 24.3 to 219 ng/m 3 for Cu, from 64 to 503.3 ng/m 3 for Zn, and from 12.4 to 85.8 ng/m 3 for Pb, thus being comparable to those in TSP of Guangzhou and Hong Kong (Cu: 30.8-82.3 ng/m 3 ; Zn: 241-1190 ng/m 3 ; Pb: 53.5-269 ng/m 3 ; Cd: 1.61-7.85ng/m 3 ) (Lee et al., 2007), Beijing (Okuda et al., 2004), and in airborne particulate matter of Ulsan, Korea (Lee and Park, 2010).The concentrations of Cd ranged from 0.4 to 1.6 ng/m 3 being again in line with concentrations found in Hong Kong (Lee et al., 2007) and airborne particulate matter in Ulsan, Korea (Lee and Park, 2010), but lower than the values in Guangzhou (Lee et al., 2007) and ambient air in Washington, DC (Melaku et al., 2008).

Fig. 5 .
Fig. 5. Seasonal variation of BaP eq calculated based on CAF-derived PAH air concentrations

Table 4 .
HM concentrations in CAFs (μg/CAF) and air estimates (ng/m 3 ) for the conservative scenario.would reveal the chemicals, or classes of chemicals that can be sampled and efficiently trapped to CAFs.