Abstract
Sulfur dioxide (SO<sub>2</sub>) is a colorless and highly toxic gas that is emitted during the combustion of compounds containing sulfur (S). It can also occur naturally during volcanic eruptions. SO<sub>2</sub> is widely recognized as a toxic air pollutant and is one of the causative substances affecting the generation of secondary ultrafine dust such as particulate matter 2.5 (PM2.5). Our previous study (Chang & Lee, 2022) presents the observations on the recent changes in nitrogen dioxide (NO<sub>2</sub>) pollution in Seoul before and after the COVID-19 pandemic. This paper compares the average monthly SO<sub>2</sub> densities in Seoul before and after the COVID-19 pandemic. The analysis produced the following results: (i) The average monthly SO<sub>2</sub> density in Seoul decreased significantly in 2020 compared to that before the COVID-19 pandemic; (ii) The average difference in the SO<sub>2</sub> monthly density in Seoul in the period from January 2021 to December 2021 and that from January 2020 to December 2020 is not statistically significant. In contrast to the studies that limit their discussions to average values, as that presented in a report by the Ministry of Environment in the Republic of Korea, the analyses of this study produced statistically valid results. These results could be partly interpreted as the effects of the partial resumption of the suspended non-industrial combustion plants in Seoul after the first half of 2020. Furthermore, the paper observes that the order-of-magnitude of the sample variances of monthly SO<sub>2</sub> density in Seoul during recent three years (2021, 2020, and 2019) is -7. Finally, implications of the findings are delineated.
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