The COVID-19 pandemic has prompted governments around the world to impose mitigation strategies of unprecedented scales, typically involving some form of restrictions on social activities and transportation. The South Korean government has been recommending a collection of guidelines now known as social distancing, leading to reduced human activities. This study analyzes changes in the concentrations of fine particulate matter (PM_2.5) during the 30-day periods before and since the start of social distancing on 29 February 2020 using measurement data from air quality monitoring stations at various locations of the seven major cities of South Korea, namely, Seoul, Busan, Incheon, Daegu, Daejeon, Gwangju, and Ulsan. All seven cities experienced decreased levels of PM_2.5 concentration by up to 25% and smaller fluctuations during the period of social distancing. Inter-city comparisons show that the PM_2.5 concentration changes are positively correlated with the city-wide PM_2.5 emission fractions for mobile sources and negatively correlated with the city-wide PM_2.5 emission fractions for combustion and industrial process sources. In addition, the meteorological influences favorable for transboundary pollutant transport have weakened during the period under COVID-19 social distancing. Intra-city comparisons show that decreases in the intra-city variability of PM_2.5 concentration were larger in coastal cities than in inland cities. Comparisons between the inter- and intra-city variabilities in the PM_2.5 concentration changes under social distancing highlight the importance of taking into account intra-city variabilities in addition to inter-city variabilities.

COVID-19 大流行促使世界各国政府实施规模空前的缓解战略，通常涉及对社会活动和交通的某种形式的限制。韩国政府一直在推荐一系列现在被称为社交距离的指导方针，从而减少人类活动。本研究利用美国七个主要城市不同地点的空气质量监测站的测量数据，分析了 2020 年 2 月 29 日开始保持社交距离之前和之后的 30 天期间细颗粒物 (PM _2.5 ) 浓度的变化。韩国，即首尔、釜山、仁川、大邱、大田、光州、蔚山。_{所有七个城市的 PM 2.5}水平均有所下降在保持社交距离期间，集中度最高可达 25%，波动更小。城市间比较表明，PM _2.5浓度变化与移动源的全市PM _2.5排放比例呈正相关，与燃烧和工业过程源的全市 PM _{2.5排放比例呈负相关。}此外，在 COVID-19 社会疏远期间，有利于污染物跨界运输的气象影响已经减弱。城市内比较表明，沿海城市 PM _2.5浓度的城市内变异性下降幅度大于内陆城市。PM中城市间和城市内变化的比较_2.5社交距离下的集中度变化突出了除城市间差异外考虑城市内差异的重要性。