Chapter 3: Identification of source areas of polycyclic aromatic hydrocarbons in Ulsan, Korea
3.3. Results and discussion
3.3.1. Seasonal source areas of the gaseous PAHs
Long-range transport of the gaseous PAHs in South Korea, to the best of my knowledge, has not been well investigated in previous studies. This limitation could be because the gaseous PAHs have shorter atmospheric half-lives compared to the particulate ones (Mackay et al., 2006b), and the frequent reaction of the gaseous compounds to free radicals (Keyte et al., 2013b). However, the effect of the monsoon system and the suitable weather conditions (e.g., less precipitation, low solar radiation, and high air pressure) might contribute to the long-range transport of the gaseous PAHs in Northeast Asia. A discussion on this issue is provided in the following sections.
3.3.1.1. Source areas of the gaseous PAHs in spring
In the spring, both PSCFm and CWTm highlighted that the gaseous PAHs in Ulsan could be contributed by those originating from northeastern China (i.e., Heilongjiang, Jilin, and Liaoning) and North Korea (Figures 3-3a and 3-4a). The gaseous PAHs coming from these areas could account for the highest contribution (47.6%) among the transported PAHs.
Moreover, the air parcels from these areas accounted for approximately 45% of the total trajectories in spring (Figure 3-5a) and mostly appeared in March and April when an increase of active fire and thermal anomalies was observed (Figures 3-6a and 3-6b). The gaseous PAHs formed during the combustion process in these areas could be then transported to the receptor site in Ulsan by the surface-wind. In addition, the lower rainfall levels along the routes of trajectories crossing northeastern China (i.e., Heilongjiang, Jilin, and Liaoning) and the East Sea in spring (Figure 3-7a) could limit wet deposition of the gaseous PAHs. Added to this, atmospheric half-lives of some PAHs that are dominant in the gaseous phase (i.e., Phe, Pyr, and
41
Flt) (Nguyen et al., 2018) range from 55 to 170 h (Mackay et al., 2006b). Meanwhile, the air parcels could spend approximately 72 h (Kim et al., 2016a) to traverse northeastern China (i.e., Heilongjiang, Jilin, and Liaoning) and the East Sea before arriving in Ulsan. Thus, the gaseous PAHs originated from those areas might be long-range transported to the study area.
Figure 3-3. PSCF of the gaseous PAHs shown in (a) modified and (b) conventional approaches in four seasons. Names of several Chinese provinces and cities marked by numbers and color circles are shown in Figure 3-2.
Figure 3-4. CWT of the gaseous PAHs shown in (a) modified and (b) conventional approaches in four seasons.
42
Figure 3-5. Trajectory cluster analysis in the (a) spring, (b) summer, (c) fall, and (d) winter.
The percentage numbers denote the contribution of each cluster.
43
Figure 3-6. Locations of active fire and thermal anomalies over Northeast Asia in (a) March, (b) April, (c) May, (d) June, (e) July, (f) September, (g) October, (h) January, and (i) February.
The red lines are the monthly mean air trajectories in the corresponding months. Data were obtained from the MODIS Collection 6 Near real-time Hotspot
(https://earthdata.nasa.gov/firms).
44
Figure 3-7. Rainfall levels (mm/h) along the backward air trajectories in the (a) spring, (b) summer, (c) fall, and (d) winter. Data were obtained from the HYSPLIT 4 model and GDAS 1 meteorological data.
This finding is also supported by the CBPF plot, suggesting that the gaseous PAHs in Ulsan could be derived from both common ground level (e.g., vehicle exhaust) and distant (e.g., long- range transport) sources (Figure 3-8a). These emission sources could be at the north of the sampling site and associated with the low to high wind speeds (2–6 m/s) for the local and distant sources, respectively. Moreover, the high probabilities of these emission sources (0.8–
1.0) could suggest their comparable contributions (Uria-Tellaetxe and Carslaw, 2014) to the gaseous PAHs in Ulsan during the spring.
45
3.3.1.2. Source areas of the gaseous PAHs in summer and fall
In summer and fall, the CBPF plots for the gaseous PAHs showed high probabilities at relatively low wind speed (< 3 m/s) (Figures 3-8b and 3-8c), reflecting that the gaseous species could be primarily derived from local emission. Particularly, the dispersion of PAHs under such wind speed conditions could be within a relatively small area (e.g., a few kilometers). In addition, the prevailing wind in Ulsan during summer and fall is the southeasterly wind, blowing from the southeast of the city and passing over the industrial and urban areas prior to reaching the sampling site (Figure 3-1). Especially, the petrochemical and non-ferrous industrial complexes in Ulsan have been mentioned to be the important emission sources of PAHs due to coal/coke combustion and heavy oil usage for their operation (Choi et al., 2012b;
Nguyen et al., 2018). Hence, the gaseous PAHs derived from such industrial activities and other sources in the urban area (e.g., vehicle exhaust) could obviously affect the sampling site.
Figure 3-8. CBPF plots for the gaseous PAHs in (a) spring, (b) summer, (c) fall, and (d) winter.
Additionally, the PSCFm and CWTm suggested that the gaseous PAHs in Ulsan could also be affected by emission sources in eastern China in the summer (i.e., Jiangsu, Shanghai, and Zhejiang), and northeastern China (i.e., Hebei and Jilin) as well as North Korea in the fall (Figures 3-3 and 3-4). However, the greater rainfall levels (Figure 3-7) and solar radiation in summer and fall could promote the wet deposition and photochemical degradation of the gaseous PAHs during their transport, leading to the insignificant influence of the regional emissions on the gaseous PAHs in Ulsan. This finding is also in line with that confirmed by the CBPF results mentioned above. The effect of air pollutants from the upwind (i.e., China and North Korea) to the downwind areas (i.e., South Korea and Japan) was also reported to weaken in summer and fall due to the Asian monsoon system (Bhardwaj et al., 2019; Inomata et al., 2017; Zhang et al., 2011b). The regional areas suggested by the PSCFm and CWTm could be
46
due to the trailing effect (Hsu et al., 2003a), locating upwind of the emission sources as source areas. The trailing effect could be stemmed from the equal distribution of weighting functions along the routes of trajectories (Hsu et al., 2003b).
3.3.1.3. Source areas of the gaseous PAHs in winter
In winter, the PSCFm and CWTm revealed that northern China (i.e., Hebei, Tianjin, and Shandong) and northeastern China (i.e., Jilin and Liaoning) might contribute to the increase of gaseous concentrations in Ulsan (Figures 3-3d and 3-4d) and accounted for 92.3% of the transported PAHs. The CBPF analysis also suggested a distant emission source with medium wind speed (3˗4 m/s) toward the northwest of the sampling site (Figure 3-8d). The gaseous PAHs coming from this direction would be contributed by those emitted from the regional sources through trans-boundary transport as described above. The combustion process, such as coal combustion for heating (Figures 3-6h and 3-6i), was expected to be one of the main emission sources of the gaseous PAHs in these areas. Additionally, the CBPF plot for the gas phase in winter also validated the common ground source (e.g., vehicle emission) at the northwest (Figure 3-8d) associating with the low wind speed (< 1 m/s). Because the probability of this source (0.25) was higher than that of the distant source (0.1–0.15), the influence of the local sources on the gaseous PAHs might be more important in Ulsan during the winter.
3.3.2. Seasonal source areas of the particulate PAHs