Restructuring of PM2.5 Composition and Source Contributions Across COVID-19 Social Distancing Periods: A Case Study at an Urban Monitoring Site in Seoul, South Korea

This study examined whether changes in social activity associated with the Republic of Korea’s COVID-19 social distancing measures were accompanied by a simple reduction and recovery of PM2.5 or by source-specific temporal restructuring. Observations collected at an urban monitor-ing site in Gwangjin-gu, Seoul, from February 2018 to March 2024 were classified into pre-distancing (pre-D), social distancing (during-D), and post-distancing (post-D) periods. PM2.5 mass, chemical composition, secondary aerosol indicators, and PMF-derived source contributions were compared across periods, and generalized additive models were additionally applied to account for meteorological and seasonal variability. PM2.5 concentrations remained significantly lower during both the during-D and post-D periods than during the pre-D period after meteorological adjustment. Seven PMF sources were resolved. Secondary inorganic aerosols remained the dominant source and showed no significant period effect after adjustment. Metal-related industry decreased during the during-D period and partially rebounded during the post-D period, while Cr-rich industry and mixed combustion remained lower than during the pre-D period. Construction dust remained elevated during both the during-D and post-D periods, whereas paved road dust showed a significant increase during the post-D period after meteorological adjustment. These findings indicate source-specific temporal restructuring of PM2.5 at the study site rather than a uniform reduction-and-recovery pattern. The results highlight the importance of source-oriented management targeting secondary aerosol precursors, non-exhaust traffic emissions, construction-related fugitive dust, and industrial sources.