Changes in the Concentration and Source Analysis of Water-Soluble Inorganic Ions in PM_2.5 in Tianjin During the COVID-19 Epidemic

This study evaluates the trends in PM2.5 and related atmospheric pollutants, the seasonal distribution of their components, and the contribution differences of various PM2.5 sources during the COVID-19 pandemic (2020—2022). Continuous offline PM2.5 sampling was conducted in Tianjin’s urban area from 2020 to 2022. The water-soluble inorganic ion composition and potential sources were analyzed. The results show a significant decrease of 29.4% in PM2.5 concentrations during the pandemic, compared to the period before. The concentrations of gaseous precursors such as SO2, NO2, CO, and O3, as well as major water-soluble inorganic ions, also declined. The primary factor for the reduction in PM2.5 emissions was the pandemic restrictions. During the three years of the pandemic, the seasonal distribution of PM2.5 and its components was highest in winter, followed by spring and autumn, and lowest in summer. This pattern was consistent with the pre-pandemic period. Back trajectory clustering and potential source analysis indicated that short-range transport from nearby regions was the main source of PM2.5 in Tianjin. PMF source apportionment results revealed that secondary transformation was the primary source of PM2.5, with the contribution from traffic sources increasing annually. In winter, coal-fired sources in power plants exhibit the highest contribution proportion to energy generation.