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Nitrogen dioxide (NO2) is a reddish-brown, highly reactive gas generated by oxidation of nitrogen monoxide in the atmosphere. It is widely recognized that NO2 is a toxic air pollutant as well as being one of the causative substances of secondary ultrafine dust such as particulate matter 2.5 (PM2.5). Our previous work (Chang and Lee, 2021) presents the recent change of PM2.5 and PM10 in Seoul before and after the COVID-19 pandemic. In this paper, we compare the average NO2 densities in Seoul before and after the COVID-19 pandemic. Our analysis has produced the following results: (i) the average NO2 density in Seoul has decreased significantly in the first half of 2020 compared to that before the COVID-19 pandemic; (ii) the average difference in the NO2 density in Seoul between the period from October 2020 to June 2021 and that from October 2019 to June 2020 is not statistically significant. In contrast to discussing average values only, as was done in a report by the Ministry of Environment in the Republic of Korea, our analyses have produced statistically valid results. Our results could be partly interpreted as the effect related to the partial resumption of economic activity in Seoul after the first half of 2020. We also make comments on the implications of our findings, which are not discussed in our previous work (Chang and Lee, 2021).

STATISTICALLY IDENTIFYING THE PATTERN CHANGE IN THE NITROGEN DIOXIDE (NO2) POLLUTION IN SEOUL AFTER THE COVID-19 PANDEMIC