Spatial Distributions of PM2.5 Concentrations, Chemical Constituents, and Acidity for PM2.5 Pollution Events and Their Potential Source Contribution: Based on Observations from a Nationwide Air Quality Monitoring Network for 2018-2019

In this study, nationwide air quality monitoring stations were classified into three groups by the hierarchical clustering method with PM2.5 time-series for high PM2.5 pollution periods of 2018 similar to 2019. In this way, we grouped the regions with similar PM2.5 variations. Then, we allocated the locations of 6 supersites for PM2.5 chemical composition measurements (operated by the National Institute of Environmental Research) into three clustered groups to investigate the differences in chemical compositions, potential source contribution regions, and aerosol acidity. For three high PM2.5 pollution periods, potential source regions contributing to each supersite (and supersite groups) did not differ significantly. However, chemical compositions and acidity of PM2.5 varied significantly among supersites (even among supersites grouped as the same cluster with similar PM2.5 time-series). Our findings suggest that local emissions of precursors play an important role in regional secondary PM2.5 formation, although PM2.5 mass concentration variations were controlled by synoptic weather conditions. Particularly, aerosol acidity seems to be affected by regional ammonia emissions, which has important implications because acidity affects aerosol hygroscopicity and thus the formation of secondary organic/inorganic aerosols.