Ⅲ. RESULTS AND DISCUSSION
3.4 Seasonal/Monthly variation of particulate matter
3.4.1 Seasonal variations of particulate matter
Figure 20 demonstrates the annual mean concentrations of PM10, PM2.5, PM10-2.5, and PM2.5/PM10 at six locations in Ulsan. In the case of PM10, the highest concentrations were observed in spring > winter >
summer > autumn in stations of DS, SN, YE, and HS, while spring > winter > autumn > summer in station of NS and SAM stations. The similar tendency was also reported from the previous studies (Bi et al., 2007; Wu et al., 2009). For PM2.5, seasonal variations at DS, SN, YE and HS stations were similar to PM10 in the order of spring > winter > summer > autumn. That in NS station was spring > summer >
winter > autumn and that in SAM station was winter > spring > summer > autumn. Seasonal concentration patterns of NS and SAM stations were different from other areas. However, in general, all six stations showed the lowest values of PM2.5 concentration in autumn. This results were also supported by the previous studies (Fang et al., 2017; Wu et al., 2009).
The concentration of PM10-2.5 was obtained by subtracting PM2.5 concentrations from PM10
concentration. In particular, PM10-2.5 concentration in spring showed higher levels than those of PM2.5, unlike other seasons. This might be due to the influence of large particles contained in the yellow dusts coming from the deserts of China and Mongolia into Korea in spring. The ratio of PM2.5 to PM10
concentrations was used to determine the contribution of PM2.5 contained in PM10. The PM2.5/PM10
ratios for the two years had the highest value of 0.58 at SAM and SN, and 0.54 at NS, 0.57 at DS, 0.48 at YE, and 0.48 at HS stations, respectively. PM2.5/PM10 ratios were higher in SAM and SN stations with high traffic volume than in other areas. The high PM2.5/PM10 ratios might be due to secondary dust generation and the low PM2.5/PM10 ratios might be due to the primary emission sources or yellow dusts caused by long-range transport. Seasonal patterns were shown in the order of summer (0.59) > autumn (0.56) > spring (0.53) > winter (0.47) in NS station. DS and SAM stations showed the same pattern:
summer (0.62, 0.66) > winter (0.57, 0.58) > spring (0.55, 0.55) > autumn (0.55, 0.53). In the case of SN station, the order was winter (0.62) > spring (0.58) > autumn (0.57) > summer (0.56) and for YE station:
winter (0.54) > summer (0.47) > autumn (0.47) > spring (0.44). HS station, located in the industrial area, had the order of spring and winter (0.50) > summer (0.46) > autumn (0.45). In spring, PM2.5/PM10
ratio showed low values except for HS station due to the influence of the coarse particles from yellow dusts and high values in summer due to the contribution of secondary aerosols caused by photochemical reactions at high temperatures.
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Figure 20. Seasonal variation of PM10 and PM2.5 at each station for 2 years.
The quality of the atmosphere in Korea in spring changed greatly under the influences of yellow dusts.
Figure 21 and 22 shows the 120-hour backward trajectories of the observed yellow dust days in 2015 and 2016, respectively. The backward trajectories were calculated and clustered using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLT) 4.8 model provided by National Oceanic and Atmospheric Administration (NOAA)’s air resource laboratory. The yellow dusts blown from the desert of China and Mongolia by the westerly wind increased the concentrations of PM (Clarke et al., 2014;Kim et al., 2012; Wang and Lin, 2015).
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Figure 21. 120-hr backward trajectories ending in Ulsan at altitudes of 1000m using NOAA HYSPLIT Model on yellow dust event in 2015.
Figure 22. 120-hr backward trajectories ending in Ulsan at altitudes of 1000m using NOAA HYSPLIT Model on yellow dust event in 2016
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In order to understand the influence of the yellow dusts, during the spring period, the density at the time of dusting was compared with the usual concentration (Figure 23). The concentrations of PM during normal days were averaged over all days without the yellow dust observation in spring. The concentration of PM10 during the yellow dust period was the highest at DS station (137.9 µg/m3) and lowest at SAM station (102.6 µg/m3). Besides PM2.5 concentration was the highest at HS station (42.1 µg/m3) and the lowest at YE station (31.4 µg/m3). In comparison between yellow dust days and normal days, PM10 concentrations increased about 2.1-2.7 times and PM2.5 concentrations increased 1.3-1.5 times. It indicates that coarse particles might be transported more than fine particles during yellow dusts (Lee et al., 2004; Yeh et al., 2015).
Figure 23. Comparison of the concentrations of (a) PM10 and (b) PM2.5 between non-yellow dust and yellow dust periods.
In summer and autumn, not only the diffusion of air was strong but also the frequency of rainfall was concentrated in late summer, therefore the effect of cleaning polluted air caused the concentration of PM to be low. The concentration of PM10 might be greatly affected by the washout phenomenon from the atmosphere due to frequent precipitation. In the case of PM2.5, due to unstable atmospheric conditions, they might flow into the upper atmosphere and act as condensation nuclei which form raindrops (Spracklen et al., 2011). In summer, the atmospheric flow over the Korean Peninsula was accelerated and the south and east winds did not favor fine particles from the northwest, and pushed them into the western sea (Lee et al., 1997). In autumn, the PM were actively diffused due to high mixing height (Gupta et al., 2007). The reason for the lowest concentration in the autumn was the rapid increase in precipitation in autumn due to typhoons that occurred from September 28th to October 6th in 2016. Figure 24 compares the average concentrations of PM10 and PM2.5 during typhoon and non- typhoon periods. The reduction rates of PM10 and PM2.5 were similar, which shows that rainfall had greatly influenced the PM concentrations (Li et al., 2015).
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Figure 24. Comparison of the concentrations of (a) PM10 and (b) PM2.5 between non-typhoon and typhoon periods.
In winter, the concentrations of PM sharply increased in comparison to other seasons due to the increase in energy consumption and the generation of high-concentration smog in winter. (Li et al., 2017b; Szidat et al., 2007). In addition, there were effects of fugitive emission sources such as illegal incineration and reverse phenomenon in winter (Lee et al., 2015). According to the data provided by Korea Coal Association, the consumption of anthracite in winter was much higher than in other seasons.
Figure 25. Anthracite consumption by season.
3.4.2 Monthly variation of particulate matter
Figure 26 shows monthly changes in PM10 and PM2.5 concentrations at six stations in Ulsan. In the case of PM10, there was a similar trend at all 6 stations. The highest concentration was observed in May at all stations except for SAM station due to the effects of yellow dusts, then decreased until September and increased again from October. The mean concentrations in the total of six stations were the highest
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in May (57.7 µg/m3) and the lowest in September (31.1 µg/m3) (Hieu and Lee, 2010). HS station showed the highest concentration among six stations regardless of the month. The monthly variation of PM2.5
was similar to that of PM10. The mean concentrations at six stations were the highest in May (30.2 µg/m3) and the lowest in September (16.4 µg/m3). In particular, PM10 concentrations continued to increase in the spring due to yellow dusts, whereas PM2.5 concentrations tended to decrease in April.
Therefore, it indicates that PM2.5 might be not greatly affected by the yellow dusts.
Figure 26. Monthly variations of (a) PM10 and (b) PM2.5 concentrations at 6 stations for 2 years.