## 1 Introduction  
The novel coronavirus (COVID-19) was reported in Wuhan, China at the end of 2019 and spread rapidly around the world, leading the World Health Organization to declare a pandemic in March 2020 [1]. By August, the number of confirmed cases worldwide exceeded 29.5 million, with 9.33 million deaths [2]. South Korea has experienced two coronavirus incidents in the past decade, middle east respiratory syndrome coronavirus (MERS) and COVID-19. Although COVID-19 has a lower fatality rate than MERS or severe acute respiratory syndrome (SARS) [3], the highest reproduction number was estimated to be 3.54 in the early stages of the epidemic in Korea [4], making it more contagious than any other coronavirus. Many countries where COVID-19 has been considered an international concern due to the spread of the virus have taken steps ranging from urging residents to stay at home to social distancing efforts and even lockdowns [5; 6; 7; 8; 9; 10; 11; 12]. Starting with the first case on January 20, Korea's spread rate rapidly increased around the metropolitan areas, reporting the second-largest number of confirmed cases in the world until February. Social distancing to prevent the spread of coronavirus began on February 29, and at the end of April, the daily new confirmed cases remained below 10, entering a period of stabilization. However, in August, large-scale cluster infections were confirmed across the country and the second pandemic began, compelling the government to issue stronger social distancing guidelines. The increase in the cumulative number of confirmed cases and social distancing period in Korea are shown in Figure 1.  
Many recent studies and media have reported that COVID-19 control actions have not only slowed the spread of the virus [13; 14; 15], but also brought local and short-term changes in air quality [16; 17; 18; 19; 20; 21]. Reduced human and industrial activities have decreased air pollutant emissions, and significant improvements in global air quality are being observed in 2020 [22; 23; 24]. In particular, in central China where the first COVID-19 confirmed case was reported, a significant decrease in particulate matter (PM) was observed immediately after lockdown. Compared to the same period in the previous three years, from January to March 2020, fine particles (PM\({}_{2.5}\)) and respiratory suspended particles (PM\({}_{10}\)) decreased by 30.1 to 40.5%, respectively, and the concentration of nitrogen dioxide (NO\({}_{2}\)) was significantly reduced by 27.9% [24]. A decrease in air pollutant levels was also reported in Korea, which adopted social distancing, a regulation somewhat weaker than a COVID-19 lockdown [12; 25; 26]. The reduction in traffic-related emissions due to social distancing and reductions in transboundary pollutants from the neighboring countries lowered the PM\({}_{2.5}\), PM\({}_{10}\), and NO\({}_{2}\) concentrations by 45%, 36%, and 20%, respectively, in March compared to the previous three years [25]. The improved air quality in March was very unusual because, traditionally in Korea, the air quality in spring deteriorates appreciably due to the influence of Asian dust [27]. Meanwhile, in Sao Paulo, a representative megacity in Brazil where the confirmed cases increased rapidly, the concentration of PM\({}_{2.5}\), PM\({}_{10}\), and NO\({}_{2}\) also decreased by 45%, 46%, and 58%, respectively. This improvement in air quality prevented 802 premature deaths and saved about USD 720 million in health costs [11]. Similarly, in Korea, a study focused on Seoul to evaluate the health costs associated with PM\({}_{2.5}\) decreases, but only the overall change over four months was observed. Thus, detailed evaluations according to short-term changes in air quality were not performed [26]. Also, no research has evaluated only PM\({}_{2.5}\) but also air pollutants representative of Korea, such as PM\({}_{10}\) and NO\({}_{2}\)  
Figure 1: The cumulative number of COVID-19 cases worldwide. The highlighted shadow represents the level of social distancing (SD) in Korea.