Photocatalytic Degradation of Polycyclic Aromatic Hydrocarbons in Fine Particulate Matter (PM2.5) Collected on TiO2-Supporting Quartz Fibre Filters

Airborne fine particulate matter (PM2.5) pollution is known to have adverse effects on human health, and owing to their carcinogenic and mutagenic nature, polycyclic aromatic hydrocarbons (PAHs) are of particular concern. This study investigated the effect of ultraviolet (UV)-induced photocatalysis on the degradation of PAHs in PM2.5, employing titanium dioxide (TiO2)-supporting quartz fibre filters. A TiO2 layer was formed on the quartz fibre filters, and airborne PM2.5 was collected using an air sample at a flow rate of 500 L/min for 24 h. The PM2.5 samples were subsequently irradiated with ultraviolet rays at 1.1 mW/cm2. The amounts of nine targeted PAHs (phenanthrene, PHE; anthracene, ANT; pyrene, PYR; benzo[a]anthracene, BaA; chrysene, CHR; benzo[b]fluoranthene, BbF; benzo[k]fluoranthene, BkF; benzo[a]pyrene, BaP; and benzo[g,h,i]perylene, BgP) gradually decreased during the treatment, with half-lives ranging from 18 h (PHE) to 3 h (BaP), and a significantly greater reduction was found in comparison with the PAHs collected in the control (non-TiO2 coated) quartz fibre filters. However, the degradation rates were much faster when the PAHs were in direct contact with the TiO2 layer. As PM2.5 is a mixture of various kinds of solids, co-existing components can be a rate-determining factor in the UV-induced degradation of PAHs. This was demonstrated by a remarkable increase in degradation rates following the removal of co-existing salts from the PM2.5 using water treatment.


Introduction
The air pollution caused by particulate matter with a size of 2.5 µm or less (PM 2.5 ) is one of the main health risks globally, causing significant excess mortality and reductions in life expectancy [1]. Increased mortality associated with exposure to PM 2.5 has been documented in numerous epidemiological studies, linked to pulmonary and cardiovascular diseases [2][3][4][5][6][7]. PM 2.5 is a mixture of various solids and droplets emitted and/or generated from natural and anthropogenic sources and, therefore, the possibility of adverse health effects may depend on the chemical contaminants in the PM 2.5 at a receptor site [8].
Polycyclic aromatic hydrocarbons (PAHs) are a class of organic compounds containing two or more fused rings of carbon and hydrogen atoms, and represent ubiquitous contaminants in the environment [9]. These compounds are generated mainly from anthropogenic sources, such as fossil-fuel burning, coal production, oil manufacturing, oil spills, wood preservation, tobacco smoke, various forms of cooking, and occasionally from natural sources such as volcanoes and forest fires [10,11]. When emitted into the air, PAHs readily condense onto the PM 2.5 owing to the lower vapour pressure. PAHs have been identified as important mediators of adverse health effects due to their carcinogenic and mutagenic nature [12][13][14][15]. For example, the International Agency for Research on Cancer (IARC) and mutagenic nature [12][13][14][15]. For example, the International Agency for Research on Cancer (IARC) has classified several PAHs into the groups A (carcinogenic to humans), 2A (probably carcinogenic to humans), and 2B (possibly carcinogenic to humans) [16] depending on their level of carcinogenicity. Of the hundreds of known PAHs, 16 have been designated as high-priority pollutants by the United States (U.S.) Environmental Protection Agency (EPA) [17] because of their potential toxicity to humans and other organisms as well as their prevalence and persistence in the environment [18].
The photocatalysis of titanium dioxide (TiO2) has been extensively studied for its application in environmental remediation [19], which relies on the generation of active oxidants (e.g., OH radicals) to trigger the oxidative degradation of a broad range of organic compounds. Although photocatalytic oxidation has been applied to the degradation of PAHs in soil and water samples [20,21], no previous studies have reported its application for the treatment of PAHs in atmospheric PM2.5.
Air-filtration technology (e.g., filter masks and air-cleaning devices) is widely used to prevent personal exposure to PM2.5. However, the PM2.5 trapped on the surface of the filters still has hazardous properties if not subsequently treated owing to its potentially toxic constituents, including PAHs. Therefore, there is a potential risk of human contact with the condensed PM2.5 when replacing or cleaning filter-units. We previously developed a TiO2-supporting quartz fibre filter (TiO2 filter) for the collection of PM2.5 and subsequently applied this to the degradation of the carbonaceous components in PM2.5 [22]. Our results showed that the total carbon content in PM2.5, measured by a carbon analyser, gradually decreased with time during ultraviolet (UV) irradiation, and carbon dioxide (CO2) was the major product [22]. However, the application of this kind of treatment remains untested for individual organic compounds such as PAHs. Therefore, here, we aimed to investigate the effect of UV-induced photocatalysis on the degradation of PAHs in PM2.5 following its collection on a TiO2 filter ( Figure 1).

PM2.5 Collection Using the TiO2 Filter
As a quartz fibre filter is commonly used for sampling PM2.5 and withstands temperatures up to 1000 °C, it was used as the supporting material for the TiO2. Following the method of Misawa et al. [22], a TiO2 layer was formed on the surface of the individual quartz fibres by calcinating titanium tetraisopropoxide (TTIO). By considering the suction pressure of the air sampler and adjusting the TTIO concentration, the amount of TiO2 supported on the filter was estimated as approximately 0.05 g/filter [22]. The actual amount was subsequently determined as the difference in mass before and after calcination, which was calculated to be 0.0478 ± 0.0051 g/filter (n = 3).

PM 2.5 Collection Using the TiO 2 Filter
As a quartz fibre filter is commonly used for sampling PM 2.5 and withstands temperatures up to 1000 • C, it was used as the supporting material for the TiO 2 . Following the method of Misawa et al. [22], a TiO 2 layer was formed on the surface of the individual quartz fibres by calcinating titanium tetraisopropoxide (TTIO). By considering the suction pressure of the air sampler and adjusting the TTIO concentration, the amount of TiO 2 supported on the filter was estimated as approximately 0.05 g/filter [22]. The actual amount was subsequently determined as the difference in mass before and after calcination, which was calculated to be 0.0478 ± 0.0051 g/filter (n = 3).
The PM 2.5 samples were collected using a high-volume air sampler to pass outdoor air through the TiO 2 and quartz filters at a flow rate of 500 L/min for 24 h on the roof of the Tokai University building, Kanagawa, Japan. Figure 2 shows the high-volume air samplers used and the typical appearance of PM 2.5 collected on the TiO 2 filter. The photocatalytic degradation rate is dependent on the UV intensity. Based on Misawa et al.'s [22] observations of significant decreases in the carbonaceous content of the PM 2.5 at a peak wavelength of 365 nm at 1.1 mW/cm 2 , we employed the same irradiation conditions in this study. Figure 3 shows the scanning electron microscope (SEM) images of individual PM 2.5 particles collected on the quartz and TiO 2 filters before and after UV-irradiation for 72 h. While no remarkable changes were observed for the PM 2.5 collected on the quartz filter (as a control), numerous particles disappeared from the TiO 2 filter, which we attributed to the photocatalytic degradation of organic compounds. To quantify this effect, the PAH contents in the PM 2.5 were measured before and after the UV-irradiation treatment.
Catalysts 2021, 11, x FOR PEER REVIEW 3 of 12 The PM2.5 samples were collected using a high-volume air sampler to pass outdoor air through the TiO2 and quartz filters at a flow rate of 500 L/min for 24 h on the roof of the Tokai University building, Kanagawa, Japan. Figure 2 shows the high-volume air samplers used and the typical appearance of PM2.5 collected on the TiO2 filter. The photocatalytic degradation rate is dependent on the UV intensity. Based on Misawa et al.'s [22] observations of significant decreases in the carbonaceous content of the PM2.5 at a peak wavelength of 365 nm at 1.1 mW/cm 2 , we employed the same irradiation conditions in this study. Figure 3 shows the scanning electron microscope (SEM) images of individual PM2.5 particles collected on the quartz and TiO2 filters before and after UV-irradiation for 72 h. While no remarkable changes were observed for the PM2.5 collected on the quartz filter (as a control), numerous particles disappeared from the TiO2 filter, which we attributed to the photocatalytic degradation of organic compounds. To quantify this effect, the PAH contents in the PM2.5 were measured before and after the UV-irradiation treatment.     The PM2.5 samples were collected using a high-volume air sampler to pass outdoor air through the TiO2 and quartz filters at a flow rate of 500 L/min for 24 h on the roof of the Tokai University building, Kanagawa, Japan. Figure 2 shows the high-volume air samplers used and the typical appearance of PM2.5 collected on the TiO2 filter. The photocatalytic degradation rate is dependent on the UV intensity. Based on Misawa et al.'s [22] observations of significant decreases in the carbonaceous content of the PM2.5 at a peak wavelength of 365 nm at 1.1 mW/cm 2 , we employed the same irradiation conditions in this study. Figure 3 shows the scanning electron microscope (SEM) images of individual PM2.5 particles collected on the quartz and TiO2 filters before and after UV-irradiation for 72 h. While no remarkable changes were observed for the PM2.5 collected on the quartz filter (as a control), numerous particles disappeared from the TiO2 filter, which we attributed to the photocatalytic degradation of organic compounds. To quantify this effect, the PAH contents in the PM2.5 were measured before and after the UV-irradiation treatment.

Photocatalytic Degradation of PAHs in PM 2.5
Simultaneous sampling of the PM 2.5 was conducted using two co-located high-volume air samplers with the TiO 2 and quartz (non-TiO 2 coated) filters, respectively. The concentrations of PM 2.5 collected on the TiO 2 and quartz filters were very similar at 20 ± 7.0 µg/m 3 (n = 3) and 19 ± 7.5 µg/m 3 (n = 3), respectively. As these 24 h mean concentrations were slightly lower than the World Health Organisation (WHO) air quality guidelines (25 µg/m 3 ), the status of the PM 2.5 air pollution at the study site was not severe.
Among the 16 PAHs designated as high-priority pollutants by the U.S. EPA, 9 were targeted in this study (phenanthrene, PHE; anthracene, ANT; pyrene, PYR; benzo[a]anthracene, BaA; chrysene, CHR; benzo[b]fluoranthene, BbF; benzo[k]fluoranthene, BkF; benzo[a]pyrene, BaP; and benzo[g,h,i]perylene, BgP), considering the higher volatility (i.e., a lower fraction in the particle phase) of PAHs containing 2-3-member rings [23] and the analytical sensitivity of each compound. Table 1 shows the average amount of these different PAHs detected in the three pairs of PM 2.5 samples before UV-irradiation (the initial amount of the PAHs, m 0 ). No significant differences were found between the amounts on the two types of filters. Generally, the PAHs detected as trace components: in total, the nine PAHs accounted for only 0.006% of the collected PM 2.5 material. The PM 2.5 samples were subsequently irradiated with UV light (1.1 mW/cm 2 ) and the amounts of the nine target PAHs (m) were measured at 6, 12, and 24 h after irradiation. Figure 4 shows the trends in the PAHs after being normalised by the initial amounts (m/m 0 , %). In the case of the quartz filter, a slight decrease was observed for each PAH and the m/m 0 percentages after 24 h were as follows: PHE, 60 ± 4.2%; ANT, 75 ± 5.0%; PYR, 74 ± 9.9%; BaA, 69 ± 6.0%; CHR, 82 ± 9.2%; BbF, 84 ± 9.7%; BkF, 83 ± 11%; BaP, 85 ± 9.3%; and BgP, 83 ± 97%. These reductions likely resulted from a combination of heat-induced evaporation by UV-irradiation and UV-induced decomposition [24,25].
By assuming first-order kinetics, the half-lives (t 1/2 ) of the PAHs in the PM 2.5 can be obtained based on the obtained degradation rates (k (/h)), using the m/m 0 trends shown in Figure 4, where the following applies:    21 h. However, these times appear to be too long for this method to be effectively employed as a practical remediation strategy. As a comparison, the molecules of the nine PAHs were placed in direct contact with the TiO2 and quartz filters by adding an aliquot of standard PAH solution (acetonitrile/methanol) and subjected to UV light at 1.1 mW/cm 2 . The initial amounts of the PAHs were as follows: PHE, 85 ng/filter; ANT, 38 ng/filter; PYR, 176 ng/filter; BaA, 96 ng/filter; CHR, 98 ng/filter; BbF, 38 ng/filter; BkF, 40 ng/filter; BaP, 93 ng/filter; and BghiP, 150 ng/filter. These amounts were comparable to those present in the PM2.5 samples collected from the field ( Table 1). As shown in Figure 5, after UV-irradiation for just 0.5 h, the amounts of the target PAHs immediately decreased to below or close to detection limits in the case of the TiO2 filter. As several PAHs are known to be labile to UV [24,25], the amounts of the PAHs-especially ANT and BaP-immediately decreased, even in the case of the quartz filters. This indicates the action of a rate-regulating factor in the degradation of PAHs in PM2.5.  However, these times appear to be too long for this method to be effectively employed as a practical remediation strategy. As a comparison, the molecules of the nine PAHs were placed in direct contact with the TiO 2 and quartz filters by adding an aliquot of standard PAH solution (acetonitrile/methanol) and subjected to UV light at 1.1 mW/cm 2 . The initial amounts of the PAHs were as follows: PHE, 85 ng/filter; ANT, 38 ng/filter; PYR, 176 ng/filter; BaA, 96 ng/filter; CHR, 98 ng/filter; BbF, 38 ng/filter; BkF, 40 ng/filter; BaP, 93 ng/filter; and BghiP, 150 ng/filter. These amounts were comparable to those present in the PM 2.5 samples collected from the field ( Table 1). As shown in Figure 5, after UV-irradiation for just 0.5 h, the amounts of the target PAHs immediately decreased to below or close to detection limits in the case of the TiO 2 filter. As several PAHs are known to be labile to UV [24,25], the amounts of the PAHs-especially ANT and BaP-immediately decreased, even in the case of the quartz filters. This indicates the action of a rate-regulating factor in the degradation of PAHs in PM 2.5 .

Effect of Water-Soluble Salts on the Photocatalytic Degradation of PAHs
When the catalytic degradation of environmental contaminants is conducted on a homogeneous interface (solid-solid), the efficiency potentially depends on the contact between the targets and the catalyst. Di Sarli et al. [26] showed the importance of strategies that avoid or minimise the segregation between soot particles and the catalyst to regenerate diesel particulate filters. Lee and Choi [27] conducted the solid-phase photocatalytic reaction on the soot/TiO 2 interface and described that the migrating nature of the active oxidants should be taken into account in the quantitative understanding of photocatalytic reaction mechanisms. As shown in Figure 2, a few particles remained on the TiO 2 filter surface even after UV-irradiation. These particles mainly consisted of inorganic constituents including sulphates, nitrates, metal oxides, elemental carbon (which is inert to TiO 2 photolysis), and porous carbonaceous particles [22]. Mayama et al. [28] observed the structure and composition of individual aerosol particles using a time-of-flight secondary mass spectrometer (TOF-SIMS) and found black carbon, which is an incomplete-combustion effluent that usually contains PAHs, present on the surface of sulphate surrounded by organic matter, probably non-volatile fatty acids [29]. Based on these observations, the PAHs in PM 2.5 that are not in direct contact with the TiO 2 layer, and co-existing solid components may prevent active oxidants from reacting with the PAHs. Therefore, we attempted to change the structure of the PM 2.5 collected on the TiO 2 filter by removing water-soluble salts.

Effect of Water-Soluble Salts on the Photocatalytic Degradation of PAHs
When the catalytic degradation of environmental contaminants is conducted on a homogeneous interface (solid-solid), the efficiency potentially depends on the contact between the targets and the catalyst. Di Sarli et al. [26] showed the importance of strategies that avoid or minimise the segregation between soot particles and the catalyst to regenerate diesel particulate filters. Lee and Choi [27] conducted the solid-phase photocatalytic reaction on the soot/TiO2 interface and described that the migrating nature of the active oxidants should be taken into account in the quantitative understanding of photocatalytic reaction mechanisms. As shown in Figure 2, a few particles remained on the TiO2 filter surface even after UV-irradiation. These particles mainly consisted of inorganic constituents including sulphates, nitrates, metal oxides, elemental carbon (which is inert to TiO2 photolysis), and porous carbonaceous particles [22]. Mayama et al. [28] observed the structure and composition of individual aerosol particles using a time-of-flight secondary mass spectrometer (TOF-SIMS) and found black carbon, which is an incomplete-combustion effluent that usually contains PAHs, present on the surface of sulphate surrounded by organic matter, probably non-volatile fatty acids [29]. Based on these observations, the PAHs in PM2.5 that are not in direct contact with the TiO2 layer, and co-existing solid components may prevent active oxidants from reacting with the PAHs. Therefore, we attempted to change the structure of the PM2.5 collected on the TiO2 filter by removing water-soluble salts.  Figure 6 shows the changes in the amounts of water-soluble ions (Na + , NH 4 + , K + , Mg 2+ , Ca 2+ , Cl − , NO 3 − , and SO 4 2− ) before and after treatment with Milli-Q water (0.5 L). Overall, 93% of the ions were easily removed by the water treatment. Even though PAHs are insoluble in water, 30% of the nine targeted PAHs were removed by the water treatment, probably due to volatilisation by the subsequent drying process (40 • C for 1 h) and/or coelution with finer particles. The PM 2.5 samples with and without the water treatment were then irradiated with UV at 1.1 mW/cm 2 and the target PAHs were measured at intervals of 2.5 h for 7.5 h. Compared to the TiO 2 filter without the salts removed, remarkable decreases in m/m 0 were observed for all PAHs in the treated (salt-removed) PM 2.5 samples (Figure 7). This demonstrates that co-existing solid components in PM 2.5 can act as a rate-regulating factor in the UV-induced degradation of PAHs, likely by reducing the light intensity reaching photo-labile PAH molecules and preventing the diffusion of active oxidants generated by the TiO 2 layer. As such, the overall degradation rate of the PAHs was affected by the structure of the solid mixture of PM 2.5 collected on the TiO 2 filter.
Catalysts 2021, 11, 400 7 of 12 creases in m/m0 were observed for all PAHs in the treated (salt-removed) PM2.5 samples (Figure 7). This demonstrates that co-existing solid components in PM2.5 can act as a rateregulating factor in the UV-induced degradation of PAHs, likely by reducing the light intensity reaching photo-labile PAH molecules and preventing the diffusion of active oxidants generated by the TiO2 layer. As such, the overall degradation rate of the PAHs was affected by the structure of the solid mixture of PM2.5 collected on the TiO2 filter.  of 2.5 h for 7.5 h. Compared to the TiO2 filter without the salts removed, remarkable decreases in m/m0 were observed for all PAHs in the treated (salt-removed) PM2.5 samples (Figure 7). This demonstrates that co-existing solid components in PM2.5 can act as a rateregulating factor in the UV-induced degradation of PAHs, likely by reducing the light intensity reaching photo-labile PAH molecules and preventing the diffusion of active oxidants generated by the TiO2 layer. As such, the overall degradation rate of the PAHs was affected by the structure of the solid mixture of PM2.5 collected on the TiO2 filter.

Preparation of TiO 2 Filter
The TiO 2 filter was prepared following the method described by Misawa et al. [22]. Briefly, a piece of quartz fibre filter (Advantec, Tokyo, Japan, diameter = 110 mm ϕ, thickness = 0.38 mm, SiO 2 > 99%) was added to 2% titanium tetraisopropoxide (Kanto Chemical, Tokyo, Japan, reagent grade) in 2-propanol (Kanto Chemical, reagent grade) solution for 1 h. After drying at ambient air temperature, the filter was calcined at 500 • C Catalysts 2021, 11, 400 8 of 12 for 2 h in an electronic furnace (Thermo Fisher Scientific, Waltham, MA, USA, FB1400) to obtain a TiO 2 layer supported on the quartz fibre filter. The morphology of the TiO 2 layer was observed using a field-emission SEM (Hitachi, Tokyo, Japan, S-4800) after coating with a thin Au/Pt film to obtain higher-quality secondary electron images. The crystalline structure of the TiO 2 was also observed using X-ray diffraction instrument (Rigaku, Tokyo, Japan, Gaiger Flex RAD-C).

Sampling PM 2.5 on the TiO 2 Filter
The airborne PM 2.5 was collected on the TiO 2 filter using a high-volume air sampler (Shibata, Tokyo, Japan, HV-500R) attached to a particle-size selector for the PM 2.5 , with a flow rate of 500 L/min. The sampler was positioned on the roof of the Tokai University building, Kanagawa, Japan (35 • 21 42.4" N, 139 • 16 29.0" E) approximately 20 m above the ground for 24 h. All sampling was conducted between May 2018 and September 2019. Before and after sampling, the filters were conditioned for more than 24 h at a constant temperature (21 • C) and relative humidity (40 ± 4%). Thereafter, weight measurements were obtained using a microbalance. The concentrations of PM 2.5 were subsequently determined from the weight difference and the total sampling volume of air. As a control, the PM 2.5 was collected and measured in the same manner using uncoated quartz filters whose collection efficiency was 99.99% for 0.3 µm of dioctyl phthalate at a rate of 5 cm/s [30].

PAHs in PM 2.5
As both the concentration and chemical composition of PM 2.5 vary daily, simultaneous sampling on the TiO 2 -coated and non-coated fibre filters was conducted three times using two adjacent samplers. The photocatalytic degradation of the PM 2.5 was carried out in ambient air at room temperature (~20 • C). Three UV black lights (Toshiba Lighting & Technology, Kanagawa, Japan, FL15BLB, peak wavelength = 365 nm) were placed in parallel, 10 cm above the surface of the PM 2.5 samples collected on the TiO 2 or quartz filters to ensure uniform irradiation. To quantify the nine target PAHs on the filter samples, two 25 mm ϕ pieces were cut from the filters and used for the high-performance liquid chromatography (HPLC) analysis described in Section 3.4. The sampling was performed after 6, 12, and 24 h of irradiation. During this process, the UV intensity at the surface of the filters was 1.1 mW/cm 2 (Custom, Tokyo, Japan, UVA-365) and the surface temperature during the experiment was 34.2-37.3 • C.

PAHs in Direct Contact with the Filters
As a reference comparison, the molecules of the nine target PAHs were placed in direct contact with the TiO 2 and quartz filters by adding a standard PAH solution (Sigma-Aldrich, Supelco, St. Louis, MO, USA, TCL PAH Mix certified reference material; acetonitrile: methanol (9:1), varied) and irradiated with UV light (1.1 mW/cm 2 ). Two 25 mm ϕ pieces were then cut from the filters and used for HPLC analysis. As the degradation of PAHs was rapid, the PAHs were analysed within 0.5 h of irradiation.

PAHs in PM 2.5 after Salt Removal
To investigate the effect of water-soluble salts on the degradation of PAHs in PM 2.5 , three pairs of PM 2.5 samples were collected on the TiO 2 filters using two adjacent samplers. The PM 2.5 samples collected by the first sampler were used for the degradation tests described in Section 3.3.1 following treatment with water. These samples were vacuum filtrated with deionised water (0.5 L), dried in an oven for 1 h at 40 • C, and subsequently used for the degradation test. For the second set of samples, the PM 2.5 samples were dried in an oven for 1 h at 40 • C without water treatment. In both cases, PAH contents were measured 2.5 and 7.5 h after UV-irradiation.

Determination of PAHs
Two 25 mm ϕ pieces of the filters were cut into small fragments and transferred into a centrifuge tube. The PAHs were then extracted in 10 mL of dichloromethane (Kanto Chemical, reagent grade) for 20 min (two cycles of 10 min each) using an ultrasonic bath to ensure complete extraction. Each extract was then centrifuged for 10 min at 3000 revolutions per minute (rpm). The resulting supernatant (5 mL) was transferred into another centrifuge tube to which 30 µL of dimethyl sulfoxide (FUJIFILM Wako Pure Chemical, Osaka, Japan, reagent grade) was added. After evaporating the dichloromethane under a gentle nitrogen flow, 0.97 mL of acetonitrile was mixed with the residue. The final samples were dissolved in acetonitrile (Kanto Chemical, HPLC grade), filtered using a disposable cellulose acetate membrane filter (Advantec, DISMIC-25CS, pore size = 0.20 µm), transferred to brown vials, and stored at −20 • C until further analysis.
The amounts of target PAHs in the extracts of two 25 mm ϕ filter pieces were determined using high-performance liquid chromatography with fluorescence detection (HPLC-FL), and subsequently converted to those in a whole filter (100 mm ϕ, an effective sampling area) by multiplying by a factor of 8, (100/2) 2 /[2 (25/2) 2 ], to have a unit of ng/filter. The HPLC-FL system consisted of a Shimazdu (Kyoto, Japan) LC-6A pump, an RF-20A fluorescence detector, a CTO-10A column oven, and a C-R6A recorder. The following analytical conditions were used: column ϕ = 4.6 × 250 nm; particle size = 5 µm (GL Science, Tokyo, Japan, Inertsil ODS-P); the eluent consisted of the mixed solution containing 80% acetonitrile and 20% ultrapure water (Milli-Q), at a rate of 1.2 mL/min (isocratic); oven temperature = 40 • C; injection volume = 20 µL. The excitation (Ex) and emission (Em) wavelengths were programmed as follows: 0-16 min = Ex 250 nm and Em 400 nm; and 16-40 min = Ex 286 nm and Em 433 nm. The nine targeted PAHs consisted of 3-member ring (PHE and ANT), 4-member ring (PYR, BaA, and CHR), 5-member ring (BbF, BkF, and BaP), and 6-member ring (BgP) types. A five-point calibration was performed using a dilution series of certified reference material (Sigma-Aldrich, Supelco, TCL PAH Mix certified reference material; acetonitrile: methanol (9:1), varied). The typical HPLC chromatograms are shown in Figure 8. Correlation coefficients (r) for linear regressions of the calibration curves were >0.99 for all PAHs. The limit of detection (LOD) for each PAH was defined as three times the standard deviation of the blank samples. The LODs were 4.5 ng/filter for PHE, 0.26 ng/filter for ANT, 0.13 ng/filter for PYR, 0.13 ng/filter for BaA, 0.26 ng/filter for CHR, 0.15 ng/filter for BbF, 0.04 ng/filter for BkF, 0.04 ng/filter for BaP, and 0.18 ng/filter for BgP.

Determination of Water-Soluble Ion
Following the method described by Ota et al. [31], inorganic ions (Na + , NH4 + , K + , Mg 2+ , Ca 2+ , Cl -, NO3 -, and SO4 2-) were determined using ion chromatography (IC). These ions are major components of the water-soluble salts in the PM2.5 samples in Japan. For the samples with and without water treatment, the filters were cut into 14 mm φ pieces using a pierce punch. These were transferred into a 50 mL test tube and the water-soluble inorganic ions Peak response Time(min) (1) (2) (8) Peak response Time(min) Figure 8. Typical high-performance liquid chromatography (HPLC) chromatograms of (a) extracts from PM 2.5 before UV-irradiation and (b) standard solution at a dilution ratio of 10,000 times.

Determination of Water-Soluble Ion
Following the method described by Ota et al. [31], inorganic ions (Na + , NH 4 + , K + , Mg 2+ , Ca 2+ , Cl − , NO 3 − , and SO 4 2− ) were determined using ion chromatography (IC). These ions are major components of the water-soluble salts in the PM 2.5 samples in Japan. For the samples with and without water treatment, the filters were cut into 14 mm ϕ pieces using a pierce punch. These were transferred into a 50 mL test tube and the water-soluble inorganic ions were extracted in 10 mL of ultrapure water (Milli-Q) using a shaker (As-One, Osaka, Japan, SR-1) at 170 rpm for 60 min. After filtration using a disposable cellulose acetate membrane filter, the filtrate was used for the IC analysis.
For the Cl − , NO 3 − , and SO 4 2− measurements, the analytical system consisted of a Thermo Fisher Scientific Dionex™ Aquion™ machine with a chemical suppressor (AMMS 3000) and an AS-AP auto-sampler. The following conditions were used during the measurements: separation column ϕ = 4.0 × 250 mm (Thermo Fisher Scientific, IonPac AS9-HC) with a guard column (Thermo Fisher Scientific, IonPac AG4); the eluent consisted of a 5 mM sodium carbonate solution, with a flow rate of 1.0 mL/min (isocratic); 15 mM of sulfuric acid as the scavenger for the chemical suppressor; oven temperature = 40 • C; and the injection volume = 25 µL. The dilution series of reagent-grade NaCl, NH 4 Cl, KCl, MgCl 2 , CaCl 2 , NaCl, NaNO 3 , and Na 2 SO 4 in ultrapure water were used for calibration and determination. All of the reagents were obtained from Kanto Chemical. The LOD was defined as three times the standard deviation of the blanks for anions and 1.0 µg/mL of the standard solutions for cations. The LODs were 5.4 µg/filter for Na + , 3.0 µg/filter for NH 4 + , 2.3 µg/filter for K + , 3.2 µg/filter for Mg 2+ , 13 µg/filter for Ca 2+ , 1.0 µg/filter, 2.9 µg/filter for NO 3 − , and 3.6 µg/filter for SO 4 2− .

Conclusions
The photocatalytic degradation of hazardous PAHs in PM 2.5 was carried out by coupling air filtration and UV-induced TiO 2 technologies. A TiO 2 layer was formed on the surfaces of the quartz filters, and the airborne PM 2.5 was collected on the filter using a high-volume air sampler. The amounts of nine targeted PAHs on the TiO 2 filters gradually decreased with time during the photo-induction (1.1 mW/cm 2 ), and a significantly greater reduction was observed in comparison to non-coated quartz filter controls. However, degradation rates were much lower compared to the photolysis of the PAHs in direct contact with the TiO 2 layer. As PM 2.5 is a mixture of various kinds of solids, co-existing components including soluble salts can act as a rate-determining factor in the UV-induced degradation of PAHs. Thus, the overall degradation rates of the PM 2.5 collected on the TiO 2 filters are affected by the nano-and micro-structures of the samples. To highlight this, we found that degradation rates were remarkably improved after removing the co-existing salts in the PM 2.5 samples using simple water-treatment. As our goal is to investigate the practical application of this principle in air-cleaning devices to remove the most toxic components from PM 2.5 , the efficacy for the decomposition of other healthrelated chemicals should be further investigated.
Author Contributions: Y.S., K.M. and K.S. designed and conceived this study. K.S. conducted the experiments. K.Y. and X.S. conducted the HPLC-FL and IC analysis, respectively. All authors have read and agreed to the published version of the manuscript.
Funding: A part of this work was supported by JSPS KAKENHI Grant number 26410198.