Polycyclic Aromatic Compounds COMPARISON OF

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COMPARISON OF PAHS, NITRO-PAHS AND OXY-PAHS ASSOCIATED WITH AIRBORNE PARTICULATE MATTER AT ROADSIDE AND URBAN BACKGROUND SITES IN DOWNTOWN TOKYO, JAPAN a

b

c

Yuki Kojima , Koji Inazu , Yoshiharu Hisamatsu , Hiroshi Okochi d

, Toshihide Baba & Toshio Nagoya

a

a

a

Department of Resources and Environmental Engineering, School of Science and Technology , Waseda University , Shinjuku-ku, Japan b

Department of Chemistry and Biochemistry , Numazu National College of Technology , Numazu, Japan c

Field Science Center , Tokyo University of Agriculture and Technology , Fuchu, Japan d

Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering , Tokyo Institute of Technology , Midori-ku, Yokohama, Japan Published online: 19 Nov 2010.

To cite this article: Yuki Kojima , Koji Inazu , Yoshiharu Hisamatsu , Hiroshi Okochi , Toshihide Baba & Toshio Nagoya (2010) COMPARISON OF PAHS, NITRO-PAHS AND OXY-PAHS ASSOCIATED WITH AIRBORNE PARTICULATE MATTER AT ROADSIDE AND URBAN BACKGROUND SITES IN DOWNTOWN TOKYO, JAPAN, Polycyclic Aromatic Compounds, 30:5, 321-333 To link to this article: http://dx.doi.org/10.1080/10406638.2010.525164

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Polycyclic Aromatic Compounds, 30:321–333, 2010 C Taylor & Francis Group, LLC Copyright ISSN: 1040-6638 print / 1563-5333 online DOI: 10.1080/10406638.2010.525164

Comparison of PAHs, Nitro-PAHs and Oxy-PAHs Associated with Airborne Particulate Matter at Roadside and Urban Background Sites in Downtown Tokyo, Japan Yuki Kojima1 , Koji Inazu2 , Yoshiharu Hisamatsu3 , Hiroshi Okochi1 , Toshihide Baba4 , and Toshio Nagoya1 1 Department of Resources and Environmental Engineering, School of Science and Technology, Waseda University, Shinjuku-ku, Japan 2 Department of Chemistry and Biochemistry, Numazu National College of Technology, Numazu, Japan 3 Field Science Center, Tokyo University of Agriculture and Technology, Fuchu, Japan 4 Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Midori-ku, Yokohama, Japan

Atmospheric polycyclic aromatic hydrocarbons (PAHs), nitro-PAHs and oxy-PAHs are emitted from primary sources. Some nitro-PAHs and oxy-PAHs can also arise from secondary formation in the atmosphere. To assess the relative importance of these sources, the polycyclic aromatic compound (PAC) concentrations were determined at a roadside (Roadside site) and on a rooftop (Urban Background site) in downtown Tokyo Japan. The concentrations of PAHs, 1-nitropyrene and oxy-PAHs at the Roadside site were higher than those at the Urban Background site, while 2-nitrofluoranthene levels were the same at both sites. However, the mean ratios of concentrations at the Urban Background site to the Roadside site were in the order 1,8-naphthalic anhydride>9,10anthraquinone>PAHs or 1-nitropyrene or acenaphthenequinone or benzanthrone. This suggests that in addition to vehicle emissions, a considerable fraction of some of the oxy-PAHs studied originates from another source, which might be secondary formation by atmospheric PAH degradation, and this contribution varied among the oxy-PAHs.

Received 28 October 2009; accepted 14 April 2010 Address correspondence to Y. Kojima, Department of Resources and Environmental Engineering, School of Science and Technology, Waseda University, 3-4-1 Okubo, Shinjukuku 169-8555, Japan. E-mail: [email protected]

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Yuki Kojima et al. Key Words: oxy-PAHs, nitro-PAHs, PAHs, airborne particulate matter, roadside


INTRODUCTION Anthropogenic airborne particulate matter is released from various sources through combustion of organic compounds, such as in power plants, open burning, and automobiles (1−3). In an urban area like downtown Tokyo, automobiles are the dominant source of airborne particulate matter. The soluble organic fraction of airborne particulate matter includes a number of organic compounds that are detrimental to human health. Inhalation of airborne particulate matter can cause several diseases, such as asthma, allergies, or lung cancer (4−7). Polycyclic aromatic compounds (PACs), including polycyclic aromatic hydrocarbons (PAHs) and their derivatives such as nitro polycyclic aromatic hydrocarbons (nitro-PAHs), are well-known as mutagenic pollutants associated with airborne particulate matter. They are released to the atmosphere by incomplete combustion such as in diesel engines (1−3, 8−12). Some nitro-PAHs can also form by OH or NO3 radical-initiated reactions of parent PAHs with NO2 (13−15). Recently, we found that the annual mean concentrations of the secondary nitro-PAHs 2nitrofluoranthene and 2-nitropyrene had remained the same or increased over the past 10 years in downtown Tokyo Japan, despite significant concentration decreases for the parent PAHs and NO2 (Kojima et al., 2010). This suggests that secondary PACs have an increased influence on the toxicity of airborne particulate matter. Oxy-PAHs are major products of atmospheric oxidation of PAHs (2, 16−18). For example, it has been reported that phenanthrene can be degraded to several oxy-PAHs such as 9,10-phenanthrenequinone, 9-fluorenone, dibenzopyranone (17, 18). Recently, oxy-PAHs have been reported to be mutagenic and carcinogenic (4−6, 19) and to enhance allergic inflammation (7, 20−22). Allergic inflammation can occur when oxyPAHs generate reactive oxygen species (ROS), which cause oxidative stress. Additionally, some oxy-PAHs are parent compounds of secondary nitro-PACs that are also toxic (23−26). For example, benzanthrone is reported to be a parent compound of the secondary nitrobenzanthrones, which are strong direct-acting mutagenic compounds (25, 26). Despite these concerns about oxy-PAHs, there are limited reports on the atmospheric occurrence of them (27−30), and particularly their sources. Oxy-PAHs could be expected to form not only primarily during incomplete combustion (9, 31, 32) but also secondarily by PAH oxidation in the atmosphere. However, to our knowledge, the contribution of secondary formation to the ambient burden has only been assessed for 9,10-phenanthrenequinone (30). Thus, the most important purpose of this study was to investigate the relative contributions of secondary formation to oxy-PAHs. In most previous reports, the simultaneous observation of atmospheric concentrations of nitro-PAHs at several sampling sites has been conducted at urban–suburban sites or roadside–residential sites that were at least a few kilometers apart (26, 33−35). These were used to investigate PAH nitration during long-term transport. In comparison, the two sampling sites in this study were a roadside and the rooftop of an eighteenstory building that were separated by a linear distance of only approximately 170 m. These sites allowed us to investigate the atmospheric occurrences of PACs during their dilution into the background atmosphere after emission from vehicles at the roadside. Furthermore, by comparing the samples collected at the two sites, we could clearly assess the relative contribution of primary sources versus secondary formation of PACs. This was possible due to the following assumptions: a) with no traffic on the road then the concentrations of both primary and secondary pollutants at the two sampling sites


Roadside and Urban Background PAC Concentrations should be almost the same because of the short liner distance between the two sites, and these could be regarded as the background concentrations; b) with heavy traffic on the road, the concentrations of primary pollutants at the roadside should be equal to the background levels plus those from direct vehicle emissions, whereas concentrations of secondary pollutants at the two sites should be almost the same. This is described in detail in results and discussion. In this study, PACs associated with airborne particulate matter were collected at a roadside and on the rooftop of an eighteen-story building in downtown Tokyo, Japan. The PAC concentrations were compared to investigate the relative contributions of primary sources and secondary formation, with a focus on oxy-PAHs. Additionally, the concentrations of gas-phase PAHs at the two sites were determined because most atmospheric formation of nitro- and oxy-PAHs has been reported to be through the reaction of gas-phase PAHs (14, 18).

EXPERIMENTAL Sampling of Airborne Particulate Matter and Gas-phase PAHs Airborne particulate matter was sampled on a quartz fiber filter (20 × 25 cm2 , Pallflex Product, 2500QAT-UP) by a high-volume air sampler (HV-1000F, Shibata Co., Tokyo, Japan) equipped with an impactor stage to eliminate any particles larger than 10 µm in aerodynamic diameter. The sample flow rate was 0.5 m3 min−1 . After collection, all the filters were stored at −30◦ C until they were analyzed. The sampling train for the gas-phase PAHs consisted of a filter holder with a quartz fiber filter (47 mm in diameter, Pallflex Product, 2500QAT-UP), a home-made glass cartridge (35 mm i.d.) packed with 5 g of XAD-2 resin (Supelpak 2, Supelco Inc., Bellefonte, USA), and a pump with a flow rate of 20 L min−1 . After collection, the XAD-2 resin cartridges were also stored at –30 ◦ C until they were analyzed. Sampling was conducted at a roadside on Meiji-Street facing the Okubo Campus (School and Graduate School of Science and Engineering, Waseda University) in Shinjuku-ward in downtown Tokyo, and on the rooftop of a 70 m high eighteen-story building on the campus, (represented by the Roadside site and Urban Background site in Figure 1(b)). In the surrounding area there are many roads with traffic densities of >40,000 cars a day (represented by heavy gray lines in Figure 1 (a)), such as MeijiStreet. However, apart from vehicles there are no other strong primary PACs sources (e.g., incinerators or power plants) located within the surrounding few kilometers. The high-volume air sampler on the sidewalk at the Roadside site was placed as close to the roadway as possible. There were significant differences in the concentrations of the primary pollutants such as PAHs and 1-NP between the two sites (This is described in detail in results and discussion). This indicates that, compared with the particles collected on the rooftop, the particles collected at the Roadside site contain a large amount of new particles that have just been emitted from vehicles on the road. Consequently, relative to the roadside site we considered the rooftop to be an “Urban Background site.” Twenty-four-hour sampling campaigns were performed on February 12, 16 and 18, 2009. Sampling was conducted in winter to minimize artifacts from PAC degradation by ozone (O3 ) during air sampling. The O3 concentrations during the sampling days (Table 1) were low enough that artifacts would not affect the PAC concentrations according to a previous report (36). For the four oxy-PAHs in our study we measured only the particle-phase concentrations, as reports indicate they exist almost exclusively in the particle-phase in winter (37).

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Figure 1: The sampling locations in downtown Tokyo, Japan. The heavy gray lines in (b) indicate busy traffic roads with a traffic density of >40,000 cars per day.

Materials Benzo[a]pyrene (BaP), benzo[k]fluoranthene (BkF), and indeno[1,2,3-cd]pyrene (IP) were obtained from Wako Pure Chemical Industries. Naphthalene (NA), acenaphthalene (ACthy), acenaphthene (AC), anthracene (ANT), phenanthrene (PH), pyrene (PY), benzo[ghi]perylene (BghiP), perylene (PER), acenaphthenequinone (ACQ), 9,10anthraquinone (9,10-AQ), 1,8-naphthalic anhydride (1,8-NA) and benzanthrone (BA) were obtained from Sigma-Aldrich Chemical Co. Fluoranthene (FL) and 1-nitropyrene (1-NP) were obtained from Tokyo Kasei Kogyo. 2-Nitrofluoranthene (2-NF) was obtained from Chiron AS (Trondheim, Norway). All the chemicals were used without further purification. The chemical structural formulas of selected oxy-PAHs in this study are shown in Scheme 1.

Table 1: Meteorological data and the concentrations of ozone (Ox ) and CO during the three sampling campaignsa. Sampling date Wind direction Wind speed [ m s−1 ] Temperature [ ◦ C ] Sunlight intensity [ MJ m−3 ] R. H. [ % ] Obx [ ppb ] CO [ ppm ] aAverage

2/12/09

2/16/09

NNW 2.4 10.4 9.2 50 11 0.79

NNW 3.7 10.4 5.2 47 17 0.60

2/18/09 NNW 2.4 6.2 9.1 37 13 0.64

value from 24 hourly data provided by a government pollution monitoring station (Shinjuku-ku Honcho Station) located close to the sampling sites. bO concentrations were based on the ultraviolet absorption method (APOA-370, Horiba). x

Roadside and Urban Background PAC Concentrations O O

O 9,10-Anthraquinone

(9,10(9,10-AQ)

O

O

1,8-NaphthalicAnhydride (1,8(1,8-NA)

O

O

Acenaphthene -quinone (ACQ)

O Benzanthrone

(BA)


Scheme 1.

Analysis After collection of airborne particulate matter, each filter was thoroughly cut into small pieces and put in a flask, and an internal standard added. Then particle associated PACs were extracted ultrasonically for 20 min twice with dichloromethane (DCM). For gas-phase PAH analysis, each entire XAD-2 cartridge was directly extracted ultrasonically for 20 min twice with DCM. The DCM extracts were filtered to remove any solid material. The filtrates were concentrated to 500 µL under a N2 flow. Then the residues were purified by SPE with a silica gel solid phase for PAH and oxy-PAH analysis (Discovery SPE DSC-Si Silica) and alumina-acidic and alumina-basic solid phases for nitro-PAH analysis (Bond Elut JR-AL-A and JR-AL-B, Varian). The purified solutions were concentrated to 500 µL under a N2 flow, and submitted to HPLC or GC/MS analysis. Particle-phase PAHs were determined by HPLC with fluorescence detection. The system consisted of a detector (RF-10AXL, Shimadzu), a pump (LC-10AD, Shimadzu), a system controller (SCL-10A, Shimadzu), a degasser (DGU-20A5, Shimadzu), a column oven (CTO-10Avp, Shimadzu), and a column (3.0 mm i.d. × 250 mm, Pegasil ODS, Senshu Pak). The mobile phase was acetonitrile/water (8/2, v/v). The flow rate was 0.5 mL min−1 . The fluorescence detector was set to the optimum excitation and emission wavelengths for each target PAH. Gas-phase PAHs and particle-phase oxy-PAHs were determined by GC/MS. GC/MS analyses were performed on a 30 m DB-5MS column (0.25 mm i.d. and 0.25 µm film thickness) using an HP6890 GC interfaced to an HP5973 MS detector. The initial column temperature was 100◦ C and this was increased at 6◦ C min−1 to 300◦ C and held at this temperature for 10 min. MS detection was performed in selected ion monitoring (SIM) mode. For nitro-PAH analysis, the in-line reduction and chemiluminescence detection HPLC method was employed, following that previously reported (38). The system consisted of a chemiluminescence detector (CLD-10A, Shimadzu), four pumps (LC-10ADvp and LC-10ATvp, Shimadzu), a six-port switching valve (FCV-12AH, Shimadzu), a column oven (CTO-10ACvp, Shimadzu), and a system controller (CBM-20A, Shimadzu). Two separation columns (3.0 mm i.d. × 250 mm, 201TP54, Vydac and 5C18-MS-II, Nacalai Tesque), a concentration column (2.0 mm i.d. × 5 mm, Cadenza CD-C18, Imtakt), and a reducing column (4.0 mm i.d. × 10 mm, NPpak-R, Jasco) were employed. The mobile phase for the first separation and reduction of nitro-PAHs was 75% ethanolacetate buffer (pH 5.5), and that for the second separation was acetonitrile/imidazoleperchloric acid buffer (pH 7.6) (1:1, v/v). Each flow rate was 0.5 mL min−1 . The mobile phase for concentration was 10 mM ascorbic acid in water, and the flow rate was 2 mL min−1 when necessary. The chemiluminescence reagent solution was 8 mM H2 O2 and 0.64 mM bis(2,4,6-trichlorophenyl)oxalate in acetonitrile.

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Roadside Urban Background

120 100 80 60 40 20 0

(× 25)

NA

ACthy

AC

PH

ANT

PY

FL

PY

FL

BkF

BaP

PER

BghiP

IP

(b)

Particle-Phase PAHs / pmol m−3


Gas-Phase PAHs / pmol m−3

(a)

5.0 4.0 3.0 2.0 1.0 0.0

Figure 2. Atmospheric concentrations of (a) gas, (b) particle, and (c) gas + particle phase PAHs at the two sampling sites. Each column and error bar represents the mean and maximum/minimum concentrations, respectively. NA concentrations are in 25 × pmol m−3 units.

RESULTS AND DISCUSSION Meteorological data and O3 and CO concentrations during the sampling campaign (Table 1) were provided by a government pollution monitoring station (Shinjuku-ku Honcho Station) located close to the sampling sites. O3 concentrations were based on the ultraviolet absorption method (APOA-370, Horiba). PAC concentrations at the sampling sites could be affected by meteorological conditions, particularly the wind direction and speed. However, no relationship was found between meteorological conditions and PAC concentrations in this study. Figures 2(a) and (b) show the atmospheric concentrations of gas- and particle-phase PAHs, respectively, at the two sampling sites. Gas-phase PAHs with more than five rings were not detected in the XAD-2 resin samples. Particle-phase PAHs with either two or three rings are highly volatile, and mostly exist in the gas-phase in the atmosphere (8, 37). These PAHs are not discussed in this study because their volatility would lead to large artifacts in the data due to their loss during filtration sampling, and this could undermine the credibility of any conclusions. Remarkable decreases in concentration from the Roadside site to the Urban Background site were observed for all gasand particle-phase PAHs (Figures 2(a) and (b)). This indicates that primary emission from vehicles was the main source of PAHs in the surrounding area, and that atmospheric dilution of PAHs occurred dramatically from the road to the urban background atmosphere in Tokyo. To assess the magnitude of the decreases in concentration, the ratios of PAH concentrations at the Urban Background site to those at the Roadside site (CUB /CRS ratio) were calculated (Figure 3). A low ratio indicates that a significant decrease occurred in the concentration of PAHs from the Roadside site to the Urban

Roadside and Urban Background PAC Concentrations

(b)

(c)

0.8 0.6 0.4

Gas-Phase PAHs

Particle-Phase PAHs

FL

PY

IP

BghiP

BaP

PER

BkF

FL

PY

FL

PY

ANT

PH

AC

ACthy

0.2 0.0


(a)

NA

CUB / CRT Ratios

1.0

total PAHs (Gas + Particle)

Figure 3. The ratios of concentrations of (a) gas- and (b) particle-phase PAHs at the Urban Background site to those at the Roadside site (CUB /CRS ). Each column and error bar represents the mean and maximum/minimum ratios, respectively.

Background site. Overall, the ratios for gas-phase PAHs (average 0.36) tended to be lower than those of particle-phase PAHs (average 0.49). The ratios for PY in the gasphase (average 0.32) on all three sampling days were also lower than those in the particle-phase (average 0.51). The main reason for the lower ratios might be a higher gas-phase PAH degradation rate than the heterogeneous gas-particle reaction during transport in the atmosphere. Differences in the dilution rate from the road into the urban background atmosphere between gas- and particle-phase PAHs could also affect the ratio. However, the mean ratio for the gas-phase varied depending on the compounds, and that for ACthy (0.14) was the lowest of all the PAHs studied. In previous laboratory experiments (39), the rate constant for the gas-phase reaction of ACthy with hydroxyl radicals, nitrate radical and O3 was reported to be highest of the PAHs selected for our study. Thus, this might have contributed to ACthy having the lowest ratio in our results. 1-NP is a representative primary nitro-PAH (3, 11, 33–35). As might be expected, we found its concentration associated with particulate matter was also significantly decreased from the Roadside site to the Urban Background site (Figure 4 (a)). It had an average CUB /CRS ratio of 0.43, which was also on the same level as that of the particlephase PAHs at 0.49 (Table 2). In contrast, no remarkable differences were found between the 2-NF concentrations at the two sites, and its average CUB /CRS ratio was close to 1 at 0.97. This result indicates that 2-NF was not directly emitted from vehicles. This is consistent with previous reports that 2-NF is formed in the atmosphere via an OH or NO3 radical-initiated reaction of FL with NO2 (13, 14, 33−35), and it is not detected in particulate matter emitted directly from sources such as auto and diesel exhausts (3). The particles collected at the Roadside site consisted of two particle types. The first of these were background particles that had a relatively long residence time in the atmosphere. These were equivalent to those collected at the Urban Background site due to the short distance between the two sampling sites. The background particles could be expected to contain a significant amount of secondary pollutants such as 2-NF. The other particles were new ones that had just been emitted from vehicles on the road facing the sampling site in this study, and these had short residence times of less than a

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Roadside Urban Background 8.0

150

(b)

Particle-Phase Oxy-PAHs / pmol m–3

Particle-Phase Nitro-PAHs / fmol m–3


(a)

100

50

0

1-NP

2-NF

6.0

4.0

2.0

0.0

9,10-AQ

ACQ

1,8-NA

BA

Figure 4. The atmospheric concentrations of (a) and nitro-PAHs (b) oxy-PAHs in the particle-phase at the two sampling sites. Each column and error bar represents the mean and maximum/minimum concentrations, respectively.

few minutes. If significant secondary formation did not occur during the dilution of the precursors into the roadside atmosphere after vehicular emission or the dilution from the roadside atmosphere to the urban background atmosphere, the concentrations of the secondary pollutants at the Roadside and Urban Background sites should be the same. This is what we observed for 2-NF, where similar 2-NF concentrations between the two sites suggested that secondary formation during the dilution of FL did not have a significant influence on 2-NF concentration levels. The atmospheric concentrations of all oxy-PAHs decreased from the Roadside site to the Urban Background site (Figure 4 (b)). This indicates that their atmospheric concentrations are strongly affected by vehicle emissions, as was the case for PAHs and 1-NP. However, the CUB /CRS ratios illustrated that the decreases for some oxy-PAHs were not as significant as those for PAHs and 1-NP (Table 2). The average ratios on the three sampling days were in the order 2-NF at 0.97>1,8-NA at 0.69>9,10-AQ at 0.63>gas- and particle-phase PAHs or 1-NP or ACQ or BA at 0.36−0.51. This indicates that some of the oxy-PAHs have an additional source besides primary emissions, and this source might be secondary formation by atmospheric PAH degradation. Particularly, the ratios for 1,8-NA and 9,10-AQ were higher than those for the PAHs and 1-NP on all three sampling days. If 1,8-NA and 9,10-AQ originate from both primary and secondary sources then the average percentage contributions from secondary formation at the Urban Background site can be calculated as 54% (1,8-NA) and 41% (9,10-AQ). The percentage of the secondary oxy-PAH to the total oxy-PAH associated with airborne particulate matter was calculated as Ro − Rp × 100 . Ro (1 − Rp )

(1)

Roadside and Urban Background PAC Concentrations Table 2: The ratios of PAH, oxy-PAH, and nitro-PAH concentrations at the Urban Background site to those at the Roadside site (CUB /CRS ). CUB /CRS Ratio Compounds


a

Averaged gas-phase PAHs Averaged particle-phase PAHs b Nitro-PAHs 1-NP 2-NF Oxy-PAHs 9,10-AQ ACQ 1,8-NA BA aAverage bAverage

2/12/09

2/16/09

2/18/09

Average

0.43 0.54

0.38 0.50

0.28 0.43

0.36 0.49

0.53 0.96

0.43 1.06

0.33 0.91

0.43 0.97

0.73 0.66 0.82 0.46

0.65 0.45 0.67 0.37

0.51 0.43 0.57 0.27

0.63 0.51 0.69 0.37

value of the ratios of seven gas-phase PAHs (NA, AC, ACthy, PH, ANT, PY, and FL). value of the ratios of seven particle-phase PAHs (PY, FL, BkF, BaP, PER, BghiP, and IP).

where Ro and Rp are the CUB /CRS ratios of oxy-PAH and PAH, respectively. This calculation is based on the following assumptions: a) primary emission from vehicles and secondary formation in the atmosphere are the only sources of oxy-PAHs; b) the atmospheric occurrences of primary oxy-PAHs and PAHs associated with airborne particulate matter are the same, which could lead to the further assumption that the CUB /CRS ratio of a primary oxy-PAH is equal to the average CUB /CRS ratio for the seven particlephase PAHs selected in this study; and c) concentrations of secondary pollutants at the two sites should be almost the same because of no contribution from vehicle emissions. Assumption (c) was supported by experimental data we obtained for the secondaryformed 2-NF (Figure 4 a). As mentioned above, PAH (fluoranthene) nitration in the atmosphere is a major contributor to secondary formation of atmospheric 2-NF. This could lead to the further assumption that the CUB /CRS ratios for secondary oxy-PAHs are equal to 1, as is the case with 2-NF of our results. The calculated contributions on the three days were in the order 2/12/09>2/16/09>2/18/09 for both compounds. For example, those for 1,8-NA on the three days were 74%, 50% and 39%, respectively. Although these daily variations were not related to O3 concentrations (Table 1), which is one of the indicators of atmospheric photo reactivity, the percent contributions might also have been affected by OH or NO3 radical concentrations and the amount of primary oxy-PAHs emitted in association with airborne particulate matter. In addition, this calculation is sometimes inadequate because these assumptions would not be completely valid in the real atmosphere. For example, although particulate phase PAHs were used as an indicator of atmospheric occurrences of primary oxy-PAHs in particle-phase in assumption b) above, the CUB /CRS ratios might not be exactly the same between PAHs and oxy-PAHs, and this is evidenced by the average ratios on the three sampling days for an individual particle associated PAH ranging from 0.44–0.58 (Figure 3 (b)). Thus, the calculated % contribution can only be used in a relative sense. However, these results do give an indication that secondary formation might significantly affect the concentration of some oxy-PAHs like 1,8-NA and 9,10-AQ even in winter, while there might be no or low contributions of secondary formation for ACQ and BA. Furthermore, the contribution in summer could be expected to be higher than that in winter. The parent PAHs of individual oxy-PAHs need to be taken into consideration when investigating the different source contributions among the oxy-PAHs. For example,

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Yuki Kojima et al. there is no known stable parent-PAH for BA in the atmosphere, at least there is none with the same number of rings as BA. This suggests that the contribution of secondary formation for BA should be quite low, and may explain its lower CUB /CRS ratio compared with the other oxy-PAHs in this study. In contrast to BA, the other oxy-PAHs have some known parent PAHs in the atmosphere, such as ANT for 9,10-AQ. 1,8-NA can form by the oxidation of several PAHs such as AC and ACthy, and even from PAHs with more than three rings like benz[a]anthracene (40−42). This could lead to a higher contribution of secondary formation for 1,8-NA, and is consistent with the CUB /CRS ratio for 1,8-NA being the highest of the selected oxy-PAHs on each sampling day in this study. One anomaly in the data was with ACQ, which has stable parent PAHs (AC and ACthy) and the potential for significant secondary formation. Additional factors that would have been expected to lead to a high secondary formation contribution for ACQ were: the mean atmospheric concentration of ACthy (55 pmol m−3 ) was about 300 times higher than that of ACQ (0.18 pmol m−3 ) at the Roadside site, and both previous reports and data from our study suggest that the atmospheric degradation rate of ACthy would be high as mentioned above. However, the CUB /CRS ratio for ACQ was only at the same level as those for particle-phase PAHs, indicating minimal secondary formation. One possible reason for this discrepancy is that the most abundant product of ACthy oxidation in the atmosphere was not ACQ, as has been shown in previous laboratory experiments (40−42). Even if ACQ could be formed by atmospheric oxidation of AC or ACthy, it might readily degrade to further oxidative products like 1,8-NA that is expected to be more stable than ACQ, before it can aggregate to airborne particulate matter. However, further research is required to confirm this. Further studies are needed to develop a better assessment of the contributions of secondary formation for oxy-PAHs. For example, the atmospheric degradation rate differences among particle-phase PAHs and oxy-PAHs were not considered in this study, and this might influence the contribution calculations. Similarly, differences in atmospheric formation rates between oxy-PAHs and nitro-PAHs were not discussed in this study. However, the important findings of this study were: 1) secondary formation might affect the concentrations of oxy-PAHs in downtown Tokyo Japan, even in winter, and 2) the contributions of secondary formation varied among the oxy-PAHs, even for those that have suitable parent PAHs with the same number of rings as seen with ACQ in this study. In the future, it is important to examine the contribution of secondary formation and its mechanism for individual toxic oxy-PAHs by both laboratory experiments and real atmospheric observations.

ACKNOWLEDGMENTS This paper is a part of the outcome of research performed under a Waseda University Grant for Special Research Projects (Project number: 2009A-861).

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