Ⅲ. Result and discussion
3.3. Formation potential of SOA and ozone from VOCs
3.3.4. SOAFP estimation from PAS
In the same way as AAS SOAFP calculation, 18 VOCs measured in passive sampling sites were calculated. In Figure 30, there is SOAFP contribution of different VOC species in summer, autumn, and winter. In summer, Although SOAFP contribution of BTEX was dominant as 66%, toluene (20%) showed the highest SOAFP contribution among calculated VOC species followed by α-pinene (14%), benzene (14%), ethylbenzene (12%), o-xylene (12%) and m,p-xylenes (9%). α-Pinene is known as the main contributor to SOAFP and SOA pollution was reported by monoterpenes in previous studies (Cheng et al., 2018; Zhang et al., 2015). As rural sites are included in passive sampling sites, the contribution fraction of SOAFP could be different from active sampling results. As there is no yield of isoprene to SOA formation, there was only monoterpenes contribution to SOAFP. The fraction of BVOCs SOAFP contribution in summer was 25% and AVOCs SOAFP contribution was 75%. In autumn and winter, BTEX contribution to SOAFP was dominant: toluene (25%, 25%), benzene (20%, 42%), ethylbenzene (19%, 10%), o-xylene (13%, 9%), and m,p-xylenes (12%, 7%). Compared with summer, the contribution of BVOCs was lowered to 4% in autumn and 0% in winter. Lowered SOAFP contribution of BVOCs was estimated with the relation of lower BVOCs concentration and emission from lowered plant activities in the cold season.
In Figure 31, there are SOAFP contribution fractions of VOC species in industrial sites, urban sites, and rural sites during sampling periods. In industrial sites, benzene (27%) showed the highest OFP contribution followed by toluene (27%), ethylbenzene (12%), and o-xylene (10%). The OFP
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contribution of BVOCs was only 7%: α-pinene (3%) and β-pinene (1%). Urban sites showed a similar OFP contribution fraction with industrial sites. As urban could be affected by both anthropogenic and biogenic sources with the distance of traffic and residential area and forest, there were some BVOC species contributions: α-pinene (7%), β-pinene (3%), and camphene (2%). Despite those SOAFP contributions, urban sites were dominantly affected by AVOCs. On the other hand, BVOCs showed much higher contribution to SOAFP in rural sites. SOAFP contribution of BVOCs was 21%: α-pinene (13%), β-pinene (4%), and camphene (3%). The fraction of BVOCs contribution in urban sites was three times higher than in industrial sites.
Figure 32 shows the spatial distribution of SOAFP in each season. With the southeasterly seasonal wind coming from East Sea, there was a low PM2.5 concentration in summer. On the other hand, industrial and rural sites showed both high SOAFP and PM2.5 concentrations. Those sites were affected by the high SOAFP contribution of benzene, toluene, and α-pinene. In Figure S9, it was suspected that urban and rural sites had high PM2.5/ PM10 ratios which indicate anthropogenic sources, vehicle exhausts, and secondary formation (Munir, 2017; Xu et al., 2017). However, there was a limitation that PM2.5/ PM10 ratio can be changed by significant PM concentrations at time and space. There is also high uncertainty that ratios were assumed with consistent primary emissions and loss rates of PM10 and PM2.5
from dry and wet depositions are equal.
In autumn and winter, PM2.5 concentration was higher. In cold seasons, PM2.5 pollution is easily affected by long-range transport by northwesterly seasonal wind. There were high SOAFP In R2, I2, and I3. They were affected by high toluene SOAFP contribution with near emission sources. With the toluene emission sources near the R2 site, rural sites could be also dominated by AVOCs to contribute SOAFP (Figure S4). I2 and I3 showed low PM2.5/ PM10 ratios and they indicate that PM10 could be more directly emitted by industrial emissions from mechanical sources (Munir, 2017). With those ratios, around I2 and I3, there were also high S O2 and NO2 concentrations which are precursor gases for SOA formation. However, except for those sites, other sites also showed high PM2.5. As there is dominant PM2.5 long-range transportation with northwesterly wind during the cold season in Korea, SOA from transported air mass should be considered (Jeong et al., 2011).
Figure 33 shows the top 12 VOC species contributing to concentration and SOAFP during the passive sampling period. Among 12 VOC species, toluene had the highest concentration contribution (26%) with the highest SOAFP contribution (24%). The top 5 VOC species that contributed to concentration were BTEX and their concentration contribution was 79%. They also contribute 79% to SOAFP similar to previous studies and it indicates that BTEX is the main VOC species that contribute SOAFP dominantly in Korea (Kim et al., 2022). Among BVOCs, α-pinene showed the highest concentration contribution (7%) with the highest SOAFP contribution (7%) followed by β-pinene (2%, 2%),
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camphene (2%, 2%), and limonene (1%, 1%). BVOCs contribution was lowered in SOAFP than OFP, and their SOAFP contribution was not much higher than the concentration contribution. SOAFP contribution of BVOCs was less dominant than OFP contribution. it is caused by high concentration and yield of AVOCs and no effect of isoprene. For the SOAFP, BTEX is needed to be regulated to reduce SOA formation in industry, urban and rural sites and it needs distinguished control of SOA formation with O3 formation control.
Figure 30. Contribution of VOC species to total SOAFP in (a) summer, (b) autumn and (c) winter
Figure 31. Contribution of VOC species to total SOAFP in (a) industrial, (b) urban and (c) rural sites
(a) (b) (c)
(a) (b) (c)
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Figure 32. Spatial distribution of SOAFP and PM2.5 in (a, d) summer, (b, e) autumn and in (c, f) winter
Figure 33. Contribution of VOC species to concentration and total SOAFP during PAS sampling period
(a) (b) (c)
(d) (e) (f)
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