Passive air sampling of VOCs, O 3 , NO 2 , and SO 2 in the large industrial city of Ulsan, South Korea: spatial-temporal variations, source identification, and ozone formation potential.

Concerns about volatile organic compounds (VOCs) have increased due to their toxicity and secondary reaction with nitrogen oxides (NO X ) to form ozone (O 3 ). In this study, passive air sampling of VOCs, O 3 , NO 2 , and SO 2 was conducted in summer, fall, winter, and spring from 2019 to 2020 at six industrial and ten urban sites in Ulsan, the largest industrial city in South Korea. Over the entire sampling period, the concentration of toluene (mean: 8.75 μg/m 3 ) was the highest of the 50 target VOCs, followed by m,p-xylenes (4.52 μg/m 3 ), ethylbenzene (4.48 μg/m 3 ), 3-methylpentane (4.40 μg/m 3 ), and n-octane (4.26 μg/m 3 ). Total (Σ 50 ) VOC levels did not statistically differ between seasons, indicating that large amounts of VOCs are emitted into the atmosphere throughout the year. On the other hand, O 3 , NO 2 , and SO 2 exhibited strong seasonal variation depending on the meteorological conditions and emission sources. The spatial distribution of Σ 50 VOCs, NO 2 , and SO 2 indicated that industrial complexes were major sources in Ulsan, while O 3 had the opposite spatial distribution. Using a positive matrix factorization model, five major sources were identified, with industrial effects dominant. Aromatic compounds, such as m,p,o-xylenes, toluene, and 1,2,4-trimethylbenzene, significantly contributed to O 3 formation. The VOC/NO 2 ratio and O 3 concentrations suggested that reducing VOC emissions is more effective than reducing NO 2 emissions in terms of preventing the secondary formation of O 3 . The findings of this study allow for a better understanding of the relationship between VOCs, O 3 , NO 2 , and SO 2 in industrial cities.