| Summary: | The COVID-19 pandemic has led countries to take action, which has included practicing social distancing or lockdown. Many cities are experiencing air quality improvements due to human activity restrictions. The purpose of this study was to compare the air quality between 2020 and the previous three years, focusing on the two cities (Seoul and Daegu) where coronavirus is spreading the fastest in Korea. Significant decreases in PM<sub>2.5</sub>, PM<sub>10</sub>, CO, and NO<sub>2</sub> were observed in both cities. In particular, compared to the same period of 2017-2019, in March 2020, PM<sub>2.5</sub> showed remarkable reductions of 36% and 30% in Seoul and Daegu, respectively. The effects of social distancing have maximized improvements in air quality due to reduced transboundary pollutants. The PM<sub>2.5</sub>/PM<sub>10</sub> ratio was significantly reduced after social distancing, indicating that the contribution of traffic-related PM<sub>2.5</sub> declined. Air quality improved overall from January to July, and the most noticeable drop in the air quality index (AQI) was observed in April. These findings indicate that relatively weak social distancing measures compared to a COVID-19 lockdown can help reduce air pollutant levels. At the same time, however, changes in air quality in the neighboring countries caused by COVID-19 control action are affecting Korea.
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