PM2.5 episodes in central Taiwan under weak southwesterly winds in spring

The Pacific high-pressure system extended westward, causing weak southwesterly winds around Taiwan from April 7 to 9, 2019. 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. 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 sunrise, heterogeneous reactions between N ₂ O ₅ and water vapor dominated, followed by the conversion of organic nitrate (NTR2) into HNO ₃ . 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 . OC reduction depends mainly on controlling OC produced via combustion. Another way is to reduce low-volatility/semivolatile primary organic aerosols (POAs). Finally, we explored the acidity/alkalinity of aerosols. When the PM _2.5 concentration exceeded 50 µg m ^− 3 , the neutralization ratio (NR) was close to 1.