**3. Diurnal variation of particle size distribution of PAHs**

## **3.1. Preliminary measurement**

One of the most influential and distinctive factors for diurnal PAH concentration variations was expected to be diurnal variations of vehicle emissions. Thus, it was considered most appropriate to investigate diurnal variations of particle size distributions of PAHs in accordance with morning and evening traffic peak hours and two off-peak hours in between, namely daytime and night time, in total four different time periods of the day for separate measurements. Preliminary PAS real-time monitoring was conducted to see if the PAS signals show morning and evening peaks corresponding to the traffic. The measurements were conducted from March 27th to April 2nd, 2006, at R6 and from April 21st to 23rd, 2006 at CC, right before the MOUDI air sampling, respectively. Figure 7 presents the results from the PAS signals. Although the timing of the PAS signal peaks were somewhat varied on different days, the morning and evening peak hours and the daytime and overnight off-peak hours were confirmed. Based on these observations, the following four time periods were decided for the MOUDI air sampling durations at each site to investigate diurnal variations of particle size distributions of PAHs: 6:00-10:00 (morning (m)), 12:00-16:00 (daytime (d)), 18:00-21:00 (evening (e)) and 22:00-5:00 (overnight (o)) at R6; and 6:00-9:30 (m), 13:00-17:00 (d), 18:30-21:30 (e) and 22:00-5:00 (o) at CC.

Temporal Variation of Particle Size Distribution of Polycyclic Aromatic Hydrocarbons at Different Roadside Air Environments in Bangkok, Thailand 37

> ) (ng/m<sup>3</sup> )

Air sampling was conducted using the MOUDI during the four selected time periods for three consecutive days. The sampling periods were April 3rd (Mon.)-6th (Thu.), 2006, at R6 and April 24th (Mon.)-27th (Thu.), 2006, at CC. Table 2 shows the 13 particle size-fractioned PAH concentrations (ng/m3) during the four time periods of the day. Particulate matter was collected cumulatively on the same filters in each time period, while the air sampling using

Time period PAH Total Detection limit of day < 0.18 0.18~0.31 0.31~0.56 0.56~1.0 1.0~1.8 1.8~3.2 3.2~5.6 5.6~10 10~18 18 < (ng/m<sup>3</sup>

Particle size fraction (μm)

Morning Phe 0.064 0.072 0.061 0.026 0.020 0.017 0.0091 0.017 0.012 0.010 0.308 0.0015 Ant 0.021 0.024 0.018 0.013 0.015 0.013 0.0060 0.012 0.0092 0.018 0.151 0.0015 Fluo 0.065 0.074 0.054 0.024 0.017 0.014 0.0075 0.014 0.0090 0.0090 0.288 0.0015 Pyr 0.12 0.13 0.097 0.042 0.030 0.022 0.010 0.020 0.013 0.014 0.497 0.0015 BaA 0.074 0.094 0.065 0.023 0.014 0.012 0.007 0.014 0.013 0.012 0.329 0.0029 Chr 0.079 0.11 0.086 0.033 0.021 0.014 0.009 0.019 0.015 0.016 0.400 0.0029 BbF 0.19 0.17 0.15 0.046 0.017 0.014 0.010 0.017 0.013 0.015 0.642 0.0037 BkF 0.10 0.11 0.078 0.039 0.018 0.0090 0.0089 0.012 0.0092 0.015 0.398 0.0037 BeP 0.20 0.19 0.16 0.045 0.017 0.011 0.011 0.012 N.D. N.D. 0.65 0.0018 BaP 0.22 0.21 0.18 0.052 0.029 0.017 0.015 0.022 N.D. N.D. 0.74 0.0037 IP 0.14 0.11 0.086 0.034 N.D. N.D. N.D. N.D. N.D. N.D. 0.37 0.0037 DahA 0.023 0.010 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.033 0.015 BghiP 0.27 0.19 0.18 0.059 N.D. N.D. N.D. N.D. N.D. N.D. 0.69 0.0037

Σ13 PAHs 1.6 1.5 1.2 0.44 0.20 0.14 0.094 0.16 0.094 0.109 5.5 Ratio(%) 28.4 26.9 22.1 7.9 3.6 2.6 1.7 2.9 1.7 2.0 100.0 Daytime Phe 0.042 0.097 0.11 0.049 0.045 0.039 0.032 0.033 0.039 0.034 0.51 0.0034 Ant 0.023 0.045 0.053 0.030 0.057 0.037 0.028 0.027 0.035 0.033 0.37 0.0034 Fluo 0.045 0.081 0.095 0.055 0.048 0.022 0.026 0.023 0.022 0.021 0.44 0.0034 Pyr 0.075 0.16 0.18 0.091 0.067 0.032 0.034 0.032 0.028 0.027 0.73 0.0034 BaA 0.048 0.086 0.095 0.059 0.055 0.027 0.047 0.024 0.038 0.032 0.51 0.0067 Chr 0.073 0.10 0.12 0.058 0.061 0.039 0.059 0.038 0.048 0.038 0.64 0.0067 BbF 0.11 0.21 0.22 0.079 0.048 0.040 0.045 0.033 0.039 N.D. 0.83 0.0084 BkF 0.064 0.12 0.14 0.067 0.043 0.047 0.035 0.028 0.036 N.D. 0.58 0.0084 BeP 0.077 0.19 0.21 0.053 0.023 N.D. 0.027 N.D. N.D. N.D. 0.58 0.0042 BaP 0.099 0.18 0.17 0.049 0.064 N.D. 0.085 N.D. N.D. N.D. 0.65 0.0084 IP 0.19 0.21 0.24 N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.64 0.0084 DahA N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.034 BghiP 0.23 0.26 0.27 0.035 N.D. N.D. N.D. N.D. N.D. N.D. 0.80 0.0084

Σ13 PAHs 1.1 1.7 1.9 0.63 0.51 0.28 0.42 0.24 0.29 0.19 7.3 Ratio(%) 14.8 23.9 26.2 8.6 7.0 3.9 5.8 3.3 3.9 2.6 100.0

Σ13 PAHs 1.3 2.1 1.9 0.65 0.33 0.38 0.36 0.27 0.27 0.25 7.9 Ratio(%) 17.1 26.9 23.9 8.2 4.2 4.9 4.6 3.5 3.5 3.2 100.0 Overnight Phe 0.017 0.036 0.031 0.020 0.021 0.014 0.013 0.019 0.011 0.013 0.20 0.0010 Ant 0.0088 0.017 0.013 0.011 0.011 0.012 0.010 0.022 0.0071 0.010 0.122 0.0010 Fluo 0.014 0.027 0.026 0.016 0.020 0.012 0.014 0.032 0.0074 0.010 0.177 0.0010 Pyr 0.028 0.052 0.049 0.031 0.038 0.023 0.018 0.037 0.011 0.013 0.30 0.0010 BaA 0.024 0.034 0.031 0.014 0.015 0.0083 0.016 0.031 0.0077 0.011 0.190 0.0021 Chr 0.023 0.043 0.039 0.026 0.024 0.014 0.014 0.031 0.0070 0.010 0.231 0.0021 BbF 0.022 0.055 0.053 0.012 0.027 0.013 0.018 0.028 0.011 0.012 0.25 0.0026 BkF 0.032 0.058 0.070 0.031 0.033 0.0094 0.020 0.030 0.011 0.013 0.307 0.0026 BeP 0.035 0.086 0.082 0.035 0.033 0.012 0.016 0.012 0.0086 0.011 0.329 0.0013 BaP 0.041 0.112 0.099 0.049 0.027 0.019 0.029 0.023 0.011 0.013 0.42 0.0026 IP 0.065 0.067 0.083 0.032 0.019 N.D. N.D. 0.017 N.D. N.D. 0.28 0.0026 DahA 0.017 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.017 0.010 BghiP 0.11 0.11 0.11 0.044 0.030 0.009 0.012 0.027 0.010 0.021 0.48 0.0026

Evening Phe 0.050 0.096 0.096 0.049 0.036 0.036 0.037 0.038 0.035 0.032 0.50 0.0028 Ant 0.029 0.053 0.046 0.031 0.028 0.037 0.030 0.031 0.035 0.025 0.35 0.0028 Fluo 0.058 0.10 0.084 0.054 0.034 0.039 0.035 0.030 0.033 0.027 0.49 0.0028 Pyr 0.075 0.18 0.15 0.075 0.042 0.059 0.052 0.040 0.034 0.030 0.73 0.0028 BaA 0.073 0.11 0.12 0.035 0.039 0.032 0.037 0.033 0.036 0.039 0.55 0.0056 Chr 0.076 0.15 0.11 0.048 0.037 0.034 0.033 0.038 0.033 0.036 0.60 0.0056 BbF 0.12 0.25 0.23 0.051 0.053 0.029 0.033 0.037 0.041 0.033 0.89 0.0070 BkF 0.13 0.17 0.13 0.047 0.063 0.033 0.035 0.027 0.027 0.030 0.69 0.0070 BeP 0.10 0.17 0.22 0.051 N.D. 0.022 0.021 N.D. N.D. N.D. 0.58 0.0035 BaP 0.10 0.25 0.30 0.12 N.D. 0.062 0.050 N.D. N.D. N.D. 0.89 0.0070 IP 0.19 0.20 0.15 0.033 N.D. N.D. N.D. N.D. N.D. N.D. 0.57 0.0070 DahA 0.049 0.080 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.13 0.028 BghiP 0.29 0.30 0.26 0.055 N.D. N.D. N.D. N.D. N.D. N.D. 0.90 0.0070

(a)

Σ13 PAHs 0.436 0.69 0.68 0.32 0.30 0.145 0.18 0.31 0.102 0.14 3.3 Ratio(%) 13.2 21.0 20.7 9.8 9.0 4.4 5.4 9.3 3.1 4.1 100.0

**3.2. Results and discussion** 

**Figure 8.** Preliminary real-time monitoring for the selection of four time periods corresponding to peak and off-peak hours of PAS signals for MOUDI air sampling [35]

### **3.2. Results and discussion**

36 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

**3.1. Preliminary measurement** 

**3. Diurnal variation of particle size distribution of PAHs** 

One of the most influential and distinctive factors for diurnal PAH concentration variations was expected to be diurnal variations of vehicle emissions. Thus, it was considered most appropriate to investigate diurnal variations of particle size distributions of PAHs in accordance with morning and evening traffic peak hours and two off-peak hours in between, namely daytime and night time, in total four different time periods of the day for separate measurements. Preliminary PAS real-time monitoring was conducted to see if the PAS signals show morning and evening peaks corresponding to the traffic. The measurements were conducted from March 27th to April 2nd, 2006, at R6 and from April 21st to 23rd, 2006 at CC, right before the MOUDI air sampling, respectively. Figure 7 presents the results from the PAS signals. Although the timing of the PAS signal peaks were somewhat varied on different days, the morning and evening peak hours and the daytime and overnight off-peak hours were confirmed. Based on these observations, the following four time periods were decided for the MOUDI air sampling durations at each site to investigate diurnal variations of particle size distributions of PAHs: 6:00-10:00 (morning (m)), 12:00-16:00 (daytime (d)), 18:00-21:00 (evening (e)) and 22:00-5:00 (overnight (o)) at R6; and 6:00-9:30 (m), 13:00-17:00 (d), 18:30-21:30 (e) and 22:00-5:00 (o) at CC.

**Figure 8.** Preliminary real-time monitoring for the selection of four time periods corresponding to peak

and off-peak hours of PAS signals for MOUDI air sampling [35]

Air sampling was conducted using the MOUDI during the four selected time periods for three consecutive days. The sampling periods were April 3rd (Mon.)-6th (Thu.), 2006, at R6 and April 24th (Mon.)-27th (Thu.), 2006, at CC. Table 2 shows the 13 particle size-fractioned PAH concentrations (ng/m3) during the four time periods of the day. Particulate matter was collected cumulatively on the same filters in each time period, while the air sampling using




Temporal Variation of Particle Size Distribution of Polycyclic Aromatic Hydrocarbons at Different Roadside Air Environments in Bangkok, Thailand 39

Concurrent with the three-day air sampling using the MOUDI, PAS real-time monitoring was also conducted using the PASs, which showed similar trends of diurnal variations to those observed in the preliminary measurements. At R6, the PAH signal values sharply increased from approximately 5 am and reached morning peaks between 9 and 10 am during the three days. Daytime concentrations were lower than in the morning. In the daytime and evening, several small peaks appeared. From around midnight to 5 am, concentrations were remarkably low. At CC, sharp morning peaks were observed at approximately 7 am on April 24th and around 8 am on April 27th. In the evening, broader peaks appeared between 4 and 9 pm, and then the concentration decreased. The sharp increase in the morning was observed at both sites. This observation is consistent with that in previous reports [12,15-16]. The sharp morning peaks can be explained by both the strong atmospheric stability caused by the inversion layer and an increase in emissions from the morning traffic. Although the total traffic volume was smaller at R6 (74,000 vehicle/day on April 5th (Mon.)) than that at CC (92,000 vehicle/day on April 24th (Wed.)), higher concentrations were observed at R6 throughout the day, possibly due to the covered configuration that restricted the atmospheric dilution effect. According to the traffic monitoring, congestion occurred during the daytime at both sites, but daytime PAS signal levels at CC were constantly low compared with the large fluctuations of the daytime levels

Table 3 shows the average local meteorological data during the three-day monitoring periods. At both sites, the mean wind directions were almost stable at southwest, which situates the both sampling locations at downwind of the road emissions. At R6, the wind speed, which ranged from 0.2-0.6 m/s, was low compared with that at CC. The observation implies the limited dispersion and long residence time of airborne PAHs within the site. On the other hand, the wind speed at CC was much higher, ranging 1.3-2.8 m/s, implying faster dispersion. Solar radiation, which promotes photochemical decomposition of PAHs [26], was more than 40% lower at R6 throughout the day because of the elevated highway and the large buildings along the R6 road. This road configuration may also explain the higher

Temperature (°C) Solar radiation (W/m2) Relative humidity (%) m d e o m d e o m d e o R6 29.5 35.9 31.2 29.7 26.1 284.8 0.2 0 85.3 57.9 76.3 83.8 CC 27.9 29.1 29.6 28.0 129.3 762.5 12.7 0.4 69.5 61.9 55.9 68.2

**Table 3.** Table 3. Local meteorological data; average during the four time periods of the day (April 3-

Wind speed (m/s) Wind direction m d e o m d e o R6 0.2 0.6 0.3 0.5 SW WSW SW W CC 1.9 1.3 1.8 2.8 SSW SSW SSW SW

at R6.

daytime concentrations at R6.

6th, 2006 at R6 and April 24-27th, 2006 at CC).

(b)

**Table 2.** Particle size-fractioned 13 PAHs concentrations (ng/m3) in the four time periods of the day a) Rama6; b) Chockchai4

the MOUDI was repeated for three consecutive days. After the sampling on the first and second days, the filters were kept in plastic cases and carried until the sampling on the second and third days. (N.B. Unfortunately, filter samples of CC overnight were mishandled and size-segregated PAH concentration data and particle weight data are not available. However, total atmospheric PAH concentrations are available using the amount of air pumped in by the MOUDI equipment as shown in Table 2.)

Concurrent with the three-day air sampling using the MOUDI, PAS real-time monitoring was also conducted using the PASs, which showed similar trends of diurnal variations to those observed in the preliminary measurements. At R6, the PAH signal values sharply increased from approximately 5 am and reached morning peaks between 9 and 10 am during the three days. Daytime concentrations were lower than in the morning. In the daytime and evening, several small peaks appeared. From around midnight to 5 am, concentrations were remarkably low. At CC, sharp morning peaks were observed at approximately 7 am on April 24th and around 8 am on April 27th. In the evening, broader peaks appeared between 4 and 9 pm, and then the concentration decreased. The sharp increase in the morning was observed at both sites. This observation is consistent with that in previous reports [12,15-16]. The sharp morning peaks can be explained by both the strong atmospheric stability caused by the inversion layer and an increase in emissions from the morning traffic. Although the total traffic volume was smaller at R6 (74,000 vehicle/day on April 5th (Mon.)) than that at CC (92,000 vehicle/day on April 24th (Wed.)), higher concentrations were observed at R6 throughout the day, possibly due to the covered configuration that restricted the atmospheric dilution effect. According to the traffic monitoring, congestion occurred during the daytime at both sites, but daytime PAS signal levels at CC were constantly low compared with the large fluctuations of the daytime levels at R6.

38 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

Time period PAH Total Detection limit

Particle size fraction (μm)

Morning Phe 0.051 0.077 0.070 0.044 0.039 0.037 0.039 0.034 0.036 0.035 0.46 0.0035 Ant 0.020 0.021 0.019 0.018 0.017 0.016 0.021 0.016 0.019 0.020 0.19 0.0035 Fluo 0.031 0.040 0.046 0.026 0.023 0.020 0.022 0.016 0.022 0.019 0.27 0.0035 Pyr 0.058 0.069 0.068 0.040 0.029 0.027 0.025 0.022 0.022 0.023 0.38 0.0035 BaA 0.031 0.030 0.023 0.019 0.018 0.016 0.024 0.010 0.014 0.025 0.21 0.0070 Chr 0.057 0.061 0.048 0.025 0.021 0.020 0.023 0.022 0.032 0.034 0.34 0.0070 BbF 0.094 0.13 0.12 0.11 N.D. N.D. N.D. N.D. N.D. N.D. 0.46 0.0087 BkF 0.093 0.079 0.069 0.038 N.D. N.D. N.D. N.D. N.D. N.D. 0.28 0.0087 BeP 0.10 0.17 0.13 0.039 N.D. N.D. N.D. N.D. N.D. N.D. 0.45 0.0044 BaP 0.10 0.14 0.10 0.036 N.D. N.D. N.D. N.D. N.D. N.D. 0.38 0.0087 IP 0.059 0.11 0.058 N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.23 0.0087 DahA N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.035 BghiP 0.13 0.20 0.15 N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.48 0.0087

Σ13 PAHs 0.84 1.1 0.91 0.39 0.15 0.14 0.15 0.12 0.14 0.16 4.1 Ratio(%) 20.2 27.5 22.0 9.5 3.6 3.3 3.7 2.9 3.5 3.8 100.0 Daytime Phe 0.050 0.072 0.082 0.042 0.039 0.039 0.038 0.033 0.047 0.044 0.48 0.0032 Ant 0.024 0.026 0.034 0.024 0.018 0.022 0.022 0.014 0.024 0.022 0.23 0.0032 Fluo 0.037 0.050 0.051 0.033 0.023 0.024 0.019 0.014 0.026 0.030 0.31 0.0032 Pyr 0.050 0.081 0.073 0.049 0.032 0.035 0.029 0.016 0.028 0.021 0.42 0.0032 BaA 0.042 0.045 0.044 0.026 0.020 0.028 0.030 0.015 0.027 0.028 0.31 0.0064 Chr 0.059 0.054 0.044 0.025 0.014 0.023 0.018 0.014 0.025 0.020 0.30 0.0064 BbF 0.11 0.12 0.086 0.055 0.041 0.028 N.D. 0.032 0.040 0.046 0.56 0.0081 BkF 0.057 0.045 0.059 0.028 0.018 0.017 N.D. 0.021 0.023 0.018 0.29 0.0081 BeP 0.11 0.10 0.097 0.032 N.D. N.D. N.D. N.D. 0.024 0.025 0.40 0.0040 BaP 0.056 0.055 0.061 0.012 N.D. N.D. N.D. N.D. 0.030 N.D. 0.21 0.0081 IP 0.052 0.128 0.099 N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.28 0.0081 DahA N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.032 BghiP 0.070 0.14 0.11 0.030 N.D. N.D. N.D. N.D. N.D. N.D. 0.35 0.0081

Σ13 PAHs 0.72 0.92 0.84 0.36 0.21 0.22 0.16 0.16 0.29 0.25 4.1 Ratio(%) 17.6 22.2 20.5 8.6 5.0 5.3 3.8 3.8 7.1 6.2 100.0 Evening Phe 0.048 0.070 0.086 0.064 0.053 0.042 0.042 0.033 0.035 0.033 0.51 0.0038 Ant 0.018 0.025 0.034 0.031 0.022 0.026 0.021 0.022 0.023 0.020 0.24 0.0038 Fluo 0.030 0.048 0.059 0.040 0.028 0.027 0.024 0.023 0.020 0.018 0.32 0.0038 Pyr 0.050 0.082 0.080 0.051 0.044 0.041 0.032 0.026 0.030 0.021 0.46 0.0038 BaA 0.006 0.036 0.033 0.030 0.029 0.020 0.031 0.020 0.023 0.033 0.26 0.0076 Chr 0.008 0.049 0.048 0.029 0.027 0.020 0.024 0.025 0.020 0.019 0.27 0.0076 BbF 0.10 0.12 0.11 0.051 0.039 0.031 N.D. N.D. N.D. N.D. 0.45 0.0094 BkF 0.038 0.050 0.060 0.035 0.023 0.015 N.D. N.D. N.D. N.D. 0.22 0.0094 BeP 0.096 0.12 0.12 0.044 0.041 N.D. N.D. N.D. N.D. N.D. 0.41 0.0047 BaP 0.077 0.075 0.087 0.054 0.057 N.D. N.D. N.D. N.D. N.D. 0.35 0.0094 IP 0.044 0.082 0.10 N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.23 0.0094 DahA N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.038 BghiP 0.065 0.11 0.14 N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.32 0.0094

Σ13 PAHs 0.58 0.87 0.95 0.43 0.36 0.22 0.17 0.15 0.15 0.14 4.0 Ratio(%) 14.4 21.5 23.6 10.6 9.0 5.5 4.3 3.7 3.7 3.6 100.0 Overnight Phe 0.45 0.0026 Ant 0.28 0.0026 Fluo 0.33 0.0026 Pyr 0.41 0.0026 BaA 0.24 0.0051 Chr *data not available* 0.35 0.0051 BbF 0.42 0.0064 BkF 0.25 0.0064 BeP 0.25 0.0032 BaP 0.24 0.0064 IP 0.07 0.0064 DahA N.D. 0.026 BghiP 0.22 0.0064

) (ng/m3 )

of day < 0.18 0.18~0.31 0.31~0.56 0.56~1.0 1.0~1.8 1.8~3.2 3.2~5.6 5.6~10 10~18 18 < (ng/m3

(b) **Table 2.** Particle size-fractioned 13 PAHs concentrations (ng/m3) in the four time periods of the day

Σ13 PAHs 3.5

the MOUDI was repeated for three consecutive days. After the sampling on the first and second days, the filters were kept in plastic cases and carried until the sampling on the second and third days. (N.B. Unfortunately, filter samples of CC overnight were mishandled and size-segregated PAH concentration data and particle weight data are not available. However, total atmospheric PAH concentrations are available using the amount of air

pumped in by the MOUDI equipment as shown in Table 2.)

a) Rama6; b) Chockchai4

Ratio(%)

Table 3 shows the average local meteorological data during the three-day monitoring periods. At both sites, the mean wind directions were almost stable at southwest, which situates the both sampling locations at downwind of the road emissions. At R6, the wind speed, which ranged from 0.2-0.6 m/s, was low compared with that at CC. The observation implies the limited dispersion and long residence time of airborne PAHs within the site. On the other hand, the wind speed at CC was much higher, ranging 1.3-2.8 m/s, implying faster dispersion. Solar radiation, which promotes photochemical decomposition of PAHs [26], was more than 40% lower at R6 throughout the day because of the elevated highway and the large buildings along the R6 road. This road configuration may also explain the higher daytime concentrations at R6.


**Table 3.** Table 3. Local meteorological data; average during the four time periods of the day (April 3- 6th, 2006 at R6 and April 24-27th, 2006 at CC).

Diurnal variations of particle size distribution of the 13 PAHs concentrations are shown in Figure 8 with classification of semi-volatile 3-4 ring PAHs and non-volatile 5-6 ring PAHs, respectively. Each graph of the size distribution is shown as Lungren type plots [27]. An overall trend of the size distribution was consistent with that in previous studies that concentrations of PAHs were found to be highly dependent upon the size of particulate matter, with the greatest concentrations being the submicron size range (e.g., [7, 28-30]). The higher concentrations at the submicron range can be explained by the condensation mechanism because larger specific surface areas are associated with such particles (e.g., [31]). On the other hand, there were clear differences of the size distribution between the groups of 3-4 ring and 5-6 ring PAHs. Contribution of coarser, or above 1μm range, was larger for 3-4 ring PAHs, while 5-6 ring PAHs were dominantly distributed in the finer size ranges at both of the sites, regardless of the time periods. The results regarding the relationship of PAH size distribution between ring number of PAHs and associated particle size were consistent with that in many other studies conducted previously, not only at Bangkok but also at various places (e.g., [7,13,32]). The Kelvin effect explains this trend. It determines the relationship between the particle diameter and vapor pressure, where more volatile species, namely lower weight PAHs, are associated with larger diameter particles.

Temporal Variation of Particle Size Distribution of Polycyclic Aromatic Hydrocarbons at Different Roadside Air Environments in Bangkok, Thailand 41

> 3-4 rings (R6, evening) C = 3.03 ng/m3

> 5-6 rings (R6, evening) C = 4.58 ng/m3

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2

dC/C(dlogDp)

dC/C(dlogDp)

5-6 rings (R6, overnight) C = 2.03 ng/m3

3-4 rings (R6, overnight) C = 1.15 ng/m3

0.01 0.1 1 10 100 Particle Diamter (μm)

> 3-4 rings (CC, evening) C = 1.91 ng/m3

5-6 rings (CC, evening) C = 1.99 ng/m3

0.01 0.1 1 10 100 Particle Diamter (μm)

0.01 0.1 1 10 100 Particle Diamter (μm)

0.01 0.1 1 10 100 Particle Diamter (μm)

0.01 0.1 1 10 100 Particle Diamter (μm)

0.01 0.1 1 10 100 Particle Diamter (μm)

**Figure 9.** Size distributions of 3-4 ring and 5-6 ring PAH concentrations in the four time periods of day.

(b)

0.01 0.1 1 10 100 Particle Diamter (μm)

0.01 0.1 1 10 100 Particle Diamter (μm)

(a)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

5-6 rings (CC, daytime) C = 2.00 ng/m3

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0.0 0.2 0.4 0.6 0.8 1.0 1.2

dC/C(dlogDp)

dC/C(dlogDp)

3-4 rings (CC, daytime) C = 1.87 ng/m3

0.0 0.2 0.4 0.6 0.8 1.0 1.2

dC/C(dlogDp)

dC/C(dlogDp)

5-6 rings (R6, daytime) C = 4.08 ng/m3

3-4 rings (R6, daytime) C = 3.01 ng/m3

0.01 0.1 1 10 100 Particle Diamter (μm)

> 0.0 0.2 0.4 0.6 0.8 1.0 1.2

dC/C(dlogDp)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

dC/C(dlogDp)

0.01 0.1 1 10 100 Particle Diamter (μm)

PAH contents. An exceptional case was that low molecular PAHs up to Pyrene in the accumulation mode showed higher contents than those in the ultrafine mode in the daytime at both sites. It might again imply accelerated accumulation processes in the daytime. For example, accumulation mode particles went through photochemical reactions with VOCs accompanying condensation of low molecular weight gaseous phase PAHs in the daytime. According to previous reports [33-34] Zielinska (2004), elevated concentrations of high molecular weight PAHs, especially BghiP, indicate significant contribution of gasoline exhaust. As shown in the figure, BghiP content in the ultrafine mode were remarkably high in all the four time periods at R6 and in the morning at CC, again implying significant

a) Rama6; b) Chockchai4

0.01 0.1 1 10 100 Particle Diamter (μm)

0.01 0.1 1 10 100 Particle Diamter (μm)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.0 0.2 0.4 0.6 0.8 1.0 1.2

dC/C(dlogDp)

dC/C(dlogDp)

5-6 rings (R6, morning) C = 3.49 ng/m3

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0.0 0.2 0.4 0.6 0.8 1.0 1.2

dC/C(dlogDp)

dC/C(dlogDp)

3-4 rings (CC, morning) C = 1.69 ng/m3

5-6 rings (CC, morning) C = 2.28 ng/m3

0.01 0.1 1 10 100 Particle Diamter (μm)

3-4 rings (R6, morning) C = 1.89 ng/m3

0.01 0.1 1 10 100 Particle Diamter (μm)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0.0 0.2 0.4 0.6 0.8 1.0 1.2

dC/C(dlogDp)

dC/C(dlogDp)

In terms of the temporal variations of PAH size distribution, at R6 in the morning, the distribution of 3-4 ring PAHs had a peak in the 0.18-0.31μm range, while the distribution of 5-6 ring PAHs had a peak in the finest range of below 0.18μm. The ratios of the ultrafine mode were clearly higher than those of other time periods, indicating an elevated burden of primary vehicle exhaust emission in the morning rush. From the morning to daytime, the distribution of both 3-4 ring and 5-6 ring PAHs slightly shifted to a coarser range. It implies atmospheric processes of particles, such as coagulation, condensation and photochemical reactions, which lead to growth of particles under stronger solar radiation and less vehicle emissions in the daytime. In the evening, the distribution peaks shifted back to finer ranges possibly due to traffic increase again in the evening rush. In the nighttime, distribution in coarser range increased apparently, possibly due to increased re-suspension of road dust caused by a faster driving speed of motor vehicles.

At CC, as a whole the coarser range contributed more than that at R6. This may be due to faster wind speed at CC enhancing re-suspension of coarse particles and/or faster dispersion of fine particles because of the open-space configuration of the road. Unlike at R6, any explicable trend of diurnal variations of the size distributions could not be identified at CC, rather some fluctuation of the size distribution was observed. This could be due to shorter residence time of particles in the site by the faster dispersion because of the open-structure configuration and faster wind, so atmospheric processes of particle growth might be limited compared to the R6's case.

To further support the discussion of diurnal variations of size distribution of PAHs, PAH contents in particulate matter (μg/g) were analyzed according to the three particle size modes in the four time periods of day (Figure 9). (N.B. Figure 8 showed PAH concentrations per unit air mass.) The PAH contents varied considerably according to the particle size modes and the different time periods. In general, smaller particle size modes had higher

Temporal Variation of Particle Size Distribution of Polycyclic Aromatic Hydrocarbons at Different Roadside Air Environments in Bangkok, Thailand 41

40 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

caused by a faster driving speed of motor vehicles.

compared to the R6's case.

Diurnal variations of particle size distribution of the 13 PAHs concentrations are shown in Figure 8 with classification of semi-volatile 3-4 ring PAHs and non-volatile 5-6 ring PAHs, respectively. Each graph of the size distribution is shown as Lungren type plots [27]. An overall trend of the size distribution was consistent with that in previous studies that concentrations of PAHs were found to be highly dependent upon the size of particulate matter, with the greatest concentrations being the submicron size range (e.g., [7, 28-30]). The higher concentrations at the submicron range can be explained by the condensation mechanism because larger specific surface areas are associated with such particles (e.g., [31]). On the other hand, there were clear differences of the size distribution between the groups of 3-4 ring and 5-6 ring PAHs. Contribution of coarser, or above 1μm range, was larger for 3-4 ring PAHs, while 5-6 ring PAHs were dominantly distributed in the finer size ranges at both of the sites, regardless of the time periods. The results regarding the relationship of PAH size distribution between ring number of PAHs and associated particle size were consistent with that in many other studies conducted previously, not only at Bangkok but also at various places (e.g., [7,13,32]). The Kelvin effect explains this trend. It determines the relationship between the particle diameter and vapor pressure, where more volatile species, namely lower weight PAHs, are associated with larger diameter particles.

In terms of the temporal variations of PAH size distribution, at R6 in the morning, the distribution of 3-4 ring PAHs had a peak in the 0.18-0.31μm range, while the distribution of 5-6 ring PAHs had a peak in the finest range of below 0.18μm. The ratios of the ultrafine mode were clearly higher than those of other time periods, indicating an elevated burden of primary vehicle exhaust emission in the morning rush. From the morning to daytime, the distribution of both 3-4 ring and 5-6 ring PAHs slightly shifted to a coarser range. It implies atmospheric processes of particles, such as coagulation, condensation and photochemical reactions, which lead to growth of particles under stronger solar radiation and less vehicle emissions in the daytime. In the evening, the distribution peaks shifted back to finer ranges possibly due to traffic increase again in the evening rush. In the nighttime, distribution in coarser range increased apparently, possibly due to increased re-suspension of road dust

At CC, as a whole the coarser range contributed more than that at R6. This may be due to faster wind speed at CC enhancing re-suspension of coarse particles and/or faster dispersion of fine particles because of the open-space configuration of the road. Unlike at R6, any explicable trend of diurnal variations of the size distributions could not be identified at CC, rather some fluctuation of the size distribution was observed. This could be due to shorter residence time of particles in the site by the faster dispersion because of the open-structure configuration and faster wind, so atmospheric processes of particle growth might be limited

To further support the discussion of diurnal variations of size distribution of PAHs, PAH contents in particulate matter (μg/g) were analyzed according to the three particle size modes in the four time periods of day (Figure 9). (N.B. Figure 8 showed PAH concentrations per unit air mass.) The PAH contents varied considerably according to the particle size modes and the different time periods. In general, smaller particle size modes had higher

**Figure 9.** Size distributions of 3-4 ring and 5-6 ring PAH concentrations in the four time periods of day. a) Rama6; b) Chockchai4

PAH contents. An exceptional case was that low molecular PAHs up to Pyrene in the accumulation mode showed higher contents than those in the ultrafine mode in the daytime at both sites. It might again imply accelerated accumulation processes in the daytime. For example, accumulation mode particles went through photochemical reactions with VOCs accompanying condensation of low molecular weight gaseous phase PAHs in the daytime.

According to previous reports [33-34] Zielinska (2004), elevated concentrations of high molecular weight PAHs, especially BghiP, indicate significant contribution of gasoline exhaust. As shown in the figure, BghiP content in the ultrafine mode were remarkably high in all the four time periods at R6 and in the morning at CC, again implying significant

Temporal Variation of Particle Size Distribution of Polycyclic Aromatic Hydrocarbons at Different Roadside Air Environments in Bangkok, Thailand 43

contribution of gasoline vehicles to the roadside atmospheric PAHs. Especially the BghiP content in the ultrafine mode in the evening at R6 had the highest value. In fact at this time period, the ratio of the gasoline vehicles to the total traffic volume was the highest (63%)

This study clearly showed diurnal variations of particle size distribution of PAHs in four different time periods of day by field measurements on the two different types of roads in Bangkok, Thailand. The data indicated possible influences on the temporal variations of size distribution of PAHs by diurnal changes in traffic emissions, road configurations in relation to dispersion efficiency, meteorological conditions, atmospheric processes of particles such as coagulation, condensation and photochemical reactions under daytime sunlight, also atmospheric processes of PAHs in the accumulation particle mode. In view of people's exposure to vehicular emissions in an urban area with heavy road traffic, it is desired that urban air quality monitoring should be conducted in more comprehensive ways with higher resolutions of time and space, and actual behavior of pollutants be elucidated by various

*Department of Urban Engineering, Graduate School of Engineering, The University of Tokyo, Tokyo,* 

*Department of Environmental Science, Faculty of Science, Chulalongkorn University, Bangkok, Thailand* 

[1] IARC (2011) IARC monographs on the evaluation of carcinogenic risks to humans. http://monographs.iarc.fr/ENG/Classification/index.php. Accessed 13 Apr 2012. [2] IPCS (1998) Selected non-heterocyclic polycyclic aromatic hydrocarbons. Environmetal

[3] Beak SO, Field RA, Goldstone ME, Kirk PW, Lester JN, Perry R (1991) A review of atmospheric polycyclic aromatic hydrocarbons: Sources, fate and behavior. Water Air

[4] European Commission (2001) Ambient air pollution by Polycyclic Aromatic

among the four time periods at the two sites.

Kazuo Yamamoto and Fumiyuki Nakajima

Health Criteria 202. WHO, Geneva.

Hydrocarbons (PAH). Position Paper.

and Soil Pollution 60: 279-300.

*Environmental Science Center, The University of Tokyo, Tokyo, Japan* 

**4. Conclusion** 

advanced approaches.

**Author details** 

Tomomi Hoshiko\*

Tassanee Prueksasit

**5. References** 

Corresponding Author

*Japan* 

 \*

N.B.) The particulate weight data of the ultrafine mode for CC evening, and size distribution data of particle weight for CC overnight were not obtained, thus their mode distribution data are not available in the figure.

**Figure 10.** 13 PAH contents in the three particle modes in the four time periods of day (ultrafine: < 0.18μm, accumulation: 0.18-1.8μm, coarse: 1.8μm <)

contribution of gasoline vehicles to the roadside atmospheric PAHs. Especially the BghiP content in the ultrafine mode in the evening at R6 had the highest value. In fact at this time period, the ratio of the gasoline vehicles to the total traffic volume was the highest (63%) among the four time periods at the two sites.
