Ⅲ. Result and discussion
3.1. Concentration
3.1.1. AAS VOCs concentration
The seasonal variation of VOC concentration varies by meteorological conditions and the strength of emission sources in each season (Hsu et al., 2018; Kumar et al., 2017). Considering wind direction and wind speed in each season from Figure 11, there was dominant northwesterly wind in autumn and winter.
It indicates that emitted VOCs from industrial area blows to the East Sea. However, there were also easterly and southeasterly winds in the summer from industrial areas, which pollute the urban areas.
Due to coastal location, Ulsan has regular sea-land breezes that can actively circulate atmosphere. In Figure 12, Summertime had precipitation, while the cold weather had decreases in temperature, solar radiation, and relative humidity.
Figure 13 and Table 5 show the seasonal concentration trends of AVOCs and BVOCs. There was a seasonal difference in the concentrations of the Σ24 VOCs in this study. The concentration in summer (25.52 µg/m3) was statistically higher than those in autumn (16.37 µg/m3) and winter (11.87 µg/m3) (rank-sum test, p < 0.05). This seasonal variation is similar to a previous study in China (An et al., 2014). During the entire sampling period, BTEX means concentration was high and similar to the previous study in Ulsan (Kim et al., 2019). The mean concentration of m,p-xylenes (7.08 µg/m3) was the highest, followed by o-xylene (6.75 µg/m3), toluene (4.50 µg/m3), and ethylbenzene (3.11 µg/ m3) in summer. As there were frequent easterly and southeasterly winds, VOCs could have been transported to the sampling site in an urban area and contributed to the high concentration in summer due to the large emissions of xylenes, toluene, and ethylbenzene from the automobile and shipbuilding industries (MOE, 2020b). In autumn, the concentration of toluene (8.16 µg/m3) was higher than those of m,p- xylenes (2.42 µg/m3), and ethylbenzene (1.95 µg/m3). In winter, toluene (5.86 µg/m3) also showed the highest mean concentration, followed by benzene (1.95 µg/m3) and m,p-xylenes (1.65 µg/m3). With less easterly and southeasterly wind blowing through the industrial areas in the autumn and winter than in the summer, there was statistically higher toluene (rank-sum test, p 0.05) and lower xylenes and ethylbenzene. Toluene and benzene concentrations increased with the weak southeasterly wind. It indicates that there was less transportation from industrial areas. Additional source identification will be discussed in the post section.
Among BVOCs, there was a high concentration of isoprene (0.69 µg/m3) in summer compared with autumn (0.01 µg/m3) and winter (0.29 µg/m3). Seasonal isoprene concentration and emission are known to be correlated with temperature and solar radiation (Filella & Penuelas, 2006; Fuentes et al., 2000).
Additionally, isoprene showed a stronger correlation with temperature and solar radiation in the summer (Spearman's correlation, r = 0.697, r = 0.650, p < 0.01) than in the cold season, autumn (Spearman's
21
correlation, r = 0.366, r = 0.380, p < 0.05). Regarding monoterpenes, summer concentrations of α- pinene (0.74 µg/m3) and β-pinene (0.37 µg/m3) were predominant. Autumn and winter, on the other hand, had a lower concentration of α-pinene (0.06 µg/m3 and 0.00 µg/m3) and β-pinene (0.02 µg/m3 and 0.00 µg/m3). Monoterpenes are known to be actively emitted with a high correlation with temperature (Kaser et al., 2013; Tingey et al., 1980).
For the concentration fraction in Figure 13, there was a higher concentration fraction of BVOCs in summer (8%) than in autumn (2%) and winter (1%). However, the fraction of AVOCs concentration was dominant in all seasons and it increased in autumn (98%) and winter (99%) than summer (92%). It indicates that AVOCs are dominant in all seasons but BVOCs are not much low concentration to ignore in summer.
Figure 12. Meteorological parameters during sampling period
Figure 11. Wind rose in (a) summer, (b) autumn and (c) winter in sampling site
(a) (b) (c)
22
Figure 13. Temporal variation of VOCs concentration of AAS
(a) Trend of Σ24 VOCs concentration, (b) Trend of AVOCs and BVOCs concentration, (c) Trend of Isoprene and Monoterpene, and (d) the concentration fraction of AVOCs and BVOCs in summer, autumn and winter
(a)
(b)
(c)
(d)
23
Table 5. Min, max and mean concentration of Σ24 VOCs from AAS with MDL and Detection ratio (μg/m3, %)
No. VOC species Summer autumn winter
MDL Detection ratio
Min Max Mean Min Max Mean Min Max Mean
1 Isoprene ND 4.98 0.69 ND 0.20 0.01 ND 0.52 0.03 0.29 37 BVOC
2 MTBE ND 3.44 0.51 ND 1.40 0.37 ND 2.11 0.39 0.46 87 AVOC
3 Benzene 0.20 3.44 0.83 0.30 7.20 1.61 0.49 6.17 1.95 0.24 100 AVOC
4 Toluene ND 45.62 4.50 ND 30.20 8.16 ND 36.42 5.86 0.33 96 AVOC
5 Ethylbenzene ND 33.33 3.11 ND 14.02 1.95 ND 17.85 0.93 0.20 83 AVOC
6 m,p-Xylenes ND 74.55 7.08 ND 27.32 2.42 ND 39.52 1.65 0.44 79 AVOC
7 o-Xylene ND 44.13 6.75 ND 16.18 1.14 ND 20.93 0.84 0.32 76 AVOC
8 α-Pinene 0.02 2.71 0.74 ND 2.50 0.06 ND 0.05 0.00 0.45 65 BVOC
9 Camphene ND 0.78 0.21 ND 0.46 0.03 ND 0.09 0.01 0.46 73 BVOC
10 1,3,5-Trimethylbenzene ND 4.01 0.07 ND 0.76 0.05 ND 0.66 0.03 0.36 28 AVOC
11 β-Pinene ND 1.87 0.37 ND 0.56 0.02 ND 0.02 0.00 0.63 65 BVOC
12 1,2,4-Trimethylbenzene ND 17.27 0.29 ND 2.63 0.15 ND 2.08 0.12 0.42 23 AVOC
13 Myrcene ND 0.32 0.02 ND 0.11 0.01 ND 0.03 0.00 0.56 61 BVOC
14 d-3-Carene ND 0.09 0.00 ND 0.03 0.00 ND 0.04 0.00 0.45 36 BVOC
15 α-Terpinene ND 0.39 0.01 ND 0.00 0.00 ND 0.00 0.00 0.51 10 BVOC
16 p-Isopropyltoluene ND 0.32 0.06 ND 1.86 0.07 ND 0.22 0.01 0.43 44 BVOC
17 Limonene ND 0.27 0.01 ND 0.18 0.01 ND 0.00 0.00 0.45 9 BVOC
18 Eucalyptol ND 0.77 0.03 ND 0.12 0.01 ND 0.04 0.00 0.45 31 BVOC
19 γ-Terpinene ND 0.04 0.00 ND 0.02 0.00 ND 0.03 0.00 0.46 51 BVOC
20 Terpinolene ND 0.11 0.01 0.00 0.04 0.01 ND 0.04 0.01 0.42 96 BVOC
21 Linalool 0.00 0.17 0.03 0.00 0.23 0.02 ND 0.06 0.01 0.19 100 BVOC
22 Camphor ND 0.49 0.11 0.01 0.41 0.09 ND 0.19 0.03 0.36 94 BVOC
23 Naphthalene ND 1.17 0.09 ND 0.10 0.01 ND 0.17 0.01 0.50 55 AVOC
24 α-Humulene ND 0.16 0.01 ND 0.99 0.04 ND 0.08 0.01 0.49 87 BVOC
24
There is a diurnal mean concentration in each season in Figure 14. The concentration of AVOCs was higher in the daytime (05:00~17:00) (21.16 µg/m3) than nighttime (17:00~05:00) (17.96 µg/m3) with statistical difference (rank-sum test, p < 0.05). A sea-land breeze blows both during the daytime and at nighttime because Ulsan is located along the coast. The emitted VOCs from industrial areas near the coast might be transported to the inland urban during the daytime due to the air circulation in Ulsan. As there was a stable atmosphere with statistically lower wind speed in the nighttime (1.55 m/s) than daytime (2.25 m/s) (rank-sum test, p < 0.01), the concentration of AVOCs peaked occasionally during night.
However, there was no statistical difference in BVOCs between daytime (05:00~17:00) (1.48 µg/m3) and nighttime (17:00~05:00) (1.66 µg/m3) (rank-sum test, p > 0.05). In Figure 15, Even though it is well known that isoprene and monoterpenes are highly emitted in the daytime with increased temperature and light intensity, only the mean concentration of isoprene emitted from broad leaves was higher in the daytime than the nighttime and was moderately correlated with temperature and solar radiation in summer (Spearman's correlation, r = 0.697, r = 0.650, p < 0.01) (Filella & Penuelas, 2006;
Ghirardo et al., 2010; Guenther et al., 1993). Due to its high volatility and absence of storage in plants, isoprene is mainly emitted into the atmosphere as soon as it is synthesized, resulting in high concentrations during the daytime (Ghirardo et al., 2010). On the other hand, the mean concentrations of monoterpenes emitted from conifers and some broad leaves were higher in nighttime than daytime in summer similar to previous studies in urban (Hellén et al., 2012; Panopoulou et al., 2020). Similar to isoprene, monoterpenes are known to emit greater during the daytime, but they are effectively stored in plant tissues throughout the daytime and emitted gradually at night. Emission is regulated by both the concentration of monoterpenes in tissues and temperature-dependent vapor pressures (Laffineur et al., 2011; Lerdau et al., 1994; Lerdau et al., 1997). They had a negative correlation with solar radiation and temperature in summer (Spearman's correlation, r = -0.482, r = -0.571, p < 0.01). As nighttime has a stable atmosphere with low wind speed, monoterpene concentration has a negative correlation with wind speed (Spearman's correlation, r = -0.532, p < 0.01). It causes an accumulation of monoterpenes in the nighttime atmosphere and a rapid decrease of concentration in the morning due to mixing start and a rapid oxidation reaction by sunlight (Järvi et al., 2009; Li et al., 2020). There was no significant diurnal variation of BVOCs in autumn and winter with low concentrations.
25
Figure 14. (a) Diurnal variation of AVOCs and (b) BVOCs in sampling period
Figure 15. (a) Diurnal variation of isoprene and (b) monoterpenes in summer
(a) (b)
(a) (b)
26