Chapter 5: Spatial distribution and temporal variation of air-water and air-soil exchange of
5.3. Results and discussion
5.3.4. Distribution of PAHs among environmental media
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The air-soil exchange behaviors of PAHs also showed their spatial distributions. In particular, the exchange fluxes and ff values of almost PAHs at the industrial site were higher than those at the other sites (Figure 5-6). Additionally, they were mostly higher than 0.7 (i.e., Ant) and between 0.3 to 0.7 (i.e., Flu, Phe, Flt, Pyr, BaA, and Chr), suggesting the net equilibrium and volatilization of these species, respectively. The highest PAH concentrations in both the air and soils of the industrial site might lead to the equilibrium conditions of the PAHs, reflecting the delay of mass transfer between the air and soils.
For the seasonal variations, the exchange fluxes and ff values in summer were higher than those in the other sampling periods (Figure 5-6). Moreover, statistically significant differences were also found between the exchange fluxes or ff of summer and those of the other seasons (Mann- Whitney rank-sum test, p < 0.05). The higher air temperature and the lower atmospheric concentrations of PAHs in the summer could enhance the escape of PAHs from soils to the air.
In addition, the lowest flux and ff values were shown in the middle spring, however, no statistical differences were observed between the flux or ff of middle spring and those of the late spring or fall. As the air-soil partitioning could be strongly affected by meteorological conditions, such as air temperature (Wang et al., 2011a), this observation could be because of the relatively similar air temperature in the middle spring to those in late spring and fall.
Regarding the 3-ring PAHs (i.e., Flu, Phe, and Ant), they mostly experienced net volatilization or equilibrium during the sampling campaigns. However, for the 4-ring PAHs (i.e., Flt, Pyr, BaA, and Chr), they reached the equilibrium condition or volatilization in the summer and shifted to net deposition in the other seasons, especially in the middle spring when the air temperature was lowest during the study period. This result also reflected a sensitivity of the medium PAHs (i.e., 4-ring PAHs) to the air temperature as reported in previous studies (Bozlaker et al., 2008; Degrendele et al., 2016; Wang et al., 2011a).
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Table 5-2. Percentage distributions of PAHs among the air, surface water, and soils.
PAHs Air Water Soil
3 to 6 rings 1.01 0.03 98.96
3 rings 6.46 0.16 93.39
4 rings 0.64 0.03 99.33
3 and 4 rings 1.84 0.05 98.11
As shown in Table 5-2, the PAHs mostly distributed in the soils, followed by the air and surface water. However, it should be noted that the air and water PAHs considered in this study were primarily in the gaseous and dissolved phases, respectively. Therefore, a consideration of PAHs in both gaseous and particulate phases for the air, as well as dissolved and particulate phases for the surface water, could increase the distributions of PAHs in these environmental media.
The highest fractions of PAHs in the soils (over 90%) could be because the PAHs have the affinity to bind to the soil organic matter, leading to their accumulation in the soils. Meanwhile, PAHs in the air and surface water could be washed out and degraded easier by several processes, such as photochemical reaction, deposition, and advection.
In the soil, the heavy PAHs having higher toxicity are more dominant due to their less mobility and stronger sorption to organic matters of soil particles. Additionally, these heavy species tend to more accumulate in the soils and mostly undergo soil-water exchange through overland runoff. In the water, they would deposit onto bottom sediments rather than dissolve in the water due to their low water solubility (Mackay et al., 2006a). Moreover, the heavy PAHs have less ability to undergo air-soil exchange to become the atmospheric chemicals because of their low vapor pressure (Mackay et al., 2006a). Since the heavy species are more toxic, their low distributions in the air and their strong accumulation in the soil might contribute to low cancer risk for human health through inhalation intake and dermal absorption. However, the re- suspension of soil particles containing the heavy PAHs (e.g., road dust) from soils to the air under strong wind speed could affect human health and this issue should be considered regarding human health risk assessment for the soil PAHs.
The distribution of PAHs in the air was the second highest, however, the atmospheric PAHs should be paid attention because the original sources of these compounds (i.e., biomass burning and fossil fuel combustion) would emit them directly into the air. Especially, for Ulsan city, the petrochemical and non-ferrous industrial activities are believed to be the noticeable source of PAHs because their production processes can have coke/coal/heavy oil combustion, one of the
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emission sources of PAHs. In addition, the monsoon system in Ulsan, especially during summer, could bring PAHs emitted in the industrial area to the others (i.e., urban and residential areas) (Nguyen et al., 2018). Moreover, as South Korea locates downwind of other countries in the Northeast Asia (i.e., China and North Korea), the monsoon system, especially in winter and spring (Inomata et al., 2017), could also bring PAHs originated from upwind areas (i.e., China and western areas of South Korea) to Ulsan and contribute to the local pollution. Therefore, although the PAHs were demonstrated to secondly distribute in the air, the atmospheric PAHs in Ulsan should be preferentially studied.
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