## 3 Results and Discussion  
### Comparision of Air Pollutant Levels  
The time-series of PM\({}_{2.5}\), PM\({}_{10}\) and NO\({}_{2}\) concentrations in Seoul and Daegu for the control period (2017-2019) and COVID-19 period in 2020 are shown in Figures 3-5, respectively. The concentration of PM\({}_{2.5}\) during the observation was significantly lower than that of the control period, with an average reduction of 30.7 ug/m\({}^{3}\). For PM\({}_{2.5}\), it was reduced by 44.2 ug/m\({}^{3}\) and 42.6 ug/m\({}^{3}\) in Seoul and Daegu, immediately after implementing SD (week 8) respectively (Figure 3). Significant reductions in PM\({}_{2.5}\) were also observed even before SD (weeks 1-7) due to changes in transboundary PM\({}_{2.5}\) concentrations [25]. Especially in China, the plant operations temporarily stopped in early January due to the New Year holiday, reducing industrial emissions. Also at the end of January, strict COVID-19 lockdown measures centered on Hubei Province were implemented, which curbed PM\({}_{2.5}\) man-made emissions due to traffic volume and industrial development [24]. The impact lasted until March, leading to a decrease in PM\({}_{2.5}\) concentrations in South Korea. Korea also showed a sharp drop in traffic after SD was implemented and a huge reduction in PM\({}_{2.5}\) concentrations in the early SD period as a result of decreased anthropogenic emissions as human activity decreased [12]. Since March 22 nd, SD efforts were further strengthened (week 14) and another significant PM\({}_{2.5}\) decrease was observed. In particular, Seoul showed its greatest reduction during the observation period, with a decrease of nearly 80% from 57.1 ug/m\({}^{3}\) to 11.6 ug/m\({}^{3}\). PM\({}_{2.5}\) concentration changes were immediately apparent in the first SD efforts and two weeks after the implementation of strict SD (week 16), probably related to reductions in regional atmospheric congestion. Figure 4: Comparison of average PM\({}_{10}\) concentrations by week between 2020 and the previous three years in (**a**) Seoul and (**b**) Daegu. Highlighted shadow represents the level of social distancing. The error bar means standard deviation. Figure 3: Comparison of average PM\({}_{2.5}\) concentrations by week between 2020 and the previous three years in (**a**) Seoul and (**b**) Daegu. Highlighted shadow represents the level of social distancing. The error bar means standard deviation. The changes in PM\({}_{10}\) concentrations were similar to those of PM\({}_{2.5}\), as shown in Figure 4. Immediately after the implementation of SD measures, PM\({}_{10}\) concentrations were reduced by 59.5 ug/m\({}^{3}\) and 63.3 ug/m\({}^{3}\) in Seoul and Daegu, respectively. As described earlier, this was interpreted as the result of reductions in transboundary PM from the neighboring countries and decreases in PM due to SD. In particular, after the implementation of SD, PM\({}_{10}\) concentrations in both cities were reduced by 76% compared to the control period. The effect of the enhanced SD is weak compared to that observed in PM\({}_{2.5}\) but still led to a continued decrease in PM\({}_{10}\) concentrations. We estimated the contribution of particles to the atmospheric PM by their size using the PM\({}_{2.5}\)/PM\({}_{10}\) ratio (Figure 5). The ratio is widely used to identify the source of PM. A high ratio indicates a high contribution of PM\({}_{2.5}\), which represents artificial emission generation, and a low ratio represents the contribution of coarse particulates from natural sources such as Asian dust or wildfires [55; 56]. Before and after SD, the PM\({}_{2.5}\)/PM\({}_{10}\) ratio decreased significantly, which was related to reductions in anthropogenic PM\({}_{2.5}\) due to the restriction of human activities. Interestingly, notable changes in the PM\({}_{2.5}\)/PM\({}_{10}\) ratio were also observed in the first two weeks following COVID-19 control actions.