PM2.5 Episodes in Central Taiwan Under Weak Southwesterly Winds in Spring

Abstract From April 6 to 9, 2019, during the period between two cold fronts from Asian continent affecting Taiwan, the Pacific high-pressure system extended westward, causing weak southwesterly winds around Taiwan. During that time, PM 2.5 event occurred in central Taiwan. We applied the WRF/CMAQ modeling system to explore the PM 2.5 event, with a focus on the maximization of local pollution. The Integrated Process Rate (IPR) is employed to explore the contributions of various mechanisms to the PM 2.5 concentration. At the north of central Taiwan, the increase in the PM 2.5 concentration was caused mainly by horizontal advection (HADV) and to a limited degree by aerosol chemistry (AERO) and vertical diffusion (VDIF). In Taichung City, AERO, VDIF and vertical advection (ZADV) caused the increase in PM 2.5 concentration. In the south of central Taiwan, HADV caused the decrease in the PM 2.5 concentration. The calculated integrated reaction rate (IRR) indicated that gaseous HNO 3 was produced by OH + NO 2 during the day, accounting for almost all the HNO 3 formed at noon. After sunset, heterogeneous reactions between N 2 O 5 and water vapor dominated. Local pollution accounted for approximately half of the total amount of SO 4 2− . The most important way to control local SO 4 2− is to reduce the emission of SO 2 or H 2 O 2 . Organic carbon (OC) reduction depends mainly on controlling OC produced via combustion. Another way is to reduce low-volatility/semivolatile primary organic aerosols (POAs). Additionally, the simulation indicated that the PM 2.5 in central Taiwan was acidic, with pH values was below 2 at noon and between 3 and 5 at midnight. Graphical Abstract