The influence of COVID-19 pandemic on PM(2.5) air quality in Northern Taiwan from Q1 2020 to Q2 2021

The study aimed to investigate the PM(2.5) variations in different periods of COVID-19 control measures in Northern Taiwan from Quarter 1 (Q1) 2020 to Quarter 2 (Q2) 2021. PM(2.5) sources were classified based on long-range transport (LRT) or local pollution (LP) in three study periods: one China lockdown (P1), and two restrictions in Taiwan (P2 and P3). During P1 the average PM(2.5) concentrations from LRT (LRT-PM(2.5)(-)(P1)) were higher at Fuguei background station by 27.9% and in the range of 4.9–24.3% at other inland stations compared to before P1. The PM(2.5) from LRT/LP mix or pure LP (Mix/LP-PM(2.5)(-)(P1)) was also higher by 14.2–39.9%. This increase was due to higher secondary particle formation represented by the increase in secondary ions (SI) and organic matter in PM(2.5-P1) with the largest proportion of 42.17% in PM(2.5) from positive matrix factorization (PMF) analysis. A similar increasing trend of Mix/LP-PM(2.5) was found in P2 when China was still locked down and Taiwan was under an early control period but the rapidly increasing infected cases were confirmed. The shift of transportation patterns from public to private to avoid virus infection explicated the high correlation of the increasing infected cases with the increasing PM(2.5). In contrast, the decreasing trend of LP-PM(2.5-P3) was observed in P3 with the PM(2.5) biases of ∼45% at all the stations when China was not locked down but Taiwan implemented a semi-lockdown. The contribution of gasoline vehicle sources in PM(2.5) was reduced from 20.3% before P3 to 10% in P3 by chemical signatures and source identification using PMF implying the strong impact of strict control measures on vehicle emissions. In summary, PM(2.5) concentrations in Northern Taiwan were either increased (P1 and P2) or decreased (P3) during the COVID-19 pandemic depending on control measures, source patterns and meteorological conditions.