PM2.5 episodes in northern Taiwan under southerly winds in late winter

When northern Taiwan is influenced locally, PM_2.5 events occur in specific conditions. If it is also affected by long-range transport from mainland China in the northwest, the probability of these PM_2.5 events increases. The current study applies WRF/CMAQ to study such case but focus on the local. From April 7 to 9, 2019, the Pacific high pressure extended westward, causing the prevailing weak southerly wind near Taiwan. The intensity of the Pacific high pressure weakened, and the wind speed near Taiwan also weakened. Therefore, the PM_2.5 concentration in Taiwan increased. In northern Taiwan, the hourly PM_2.5 concentration will inevitably increase to approximately 70 μg m⁻³. The ISAM of the CMAQ model demonstrates that local contributions to daily PM_2.5 concentrations at the four representative monitoring stations in northern Taiwan are 3.5 to 16.2 μg m⁻³, greater than those from central, southern, and eastern Taiwan, from 1.8 to 6.7 μg m⁻³. We use the integrated process rate (IPR) and integrated reaction rate (IRR) of CMAQ to explore the formation mechanisms of PM_2.5. The main causes of the increase in PM_2.5 concentration are horizontal advection (HADV), aerosol processes (AERO), emissions (EMIS), and a small amount of cloud processes and aqueous chemistry (CLDS). The main source of NO₃⁻ is the reaction between OH and NO₂ during the day and the reaction between N₂O₅ and water vapor at night. The gaseous HNO₃ concentration reaches a peak near noon or soon after. The aerosol ANO₃ concentration is high in the early morning. We also use the Sulfur Tracking Method (STM) of CMAQ to explore the sources of SO₄²⁻. Mainly it is from the local reaction of SO₂ and H₂O₂. Finally, we apply AERO7 in CMAQ to explore the main sources of carbon components. The organic carbon (OC) concentration is much greater than the element carbon (EC) concentration, which indicates a strong local photochemical effect in northern Taiwan. OC mainly comes from low-volatility/semivolatile oxidized combustion OC, followed by low-volatility/semivolatile POA. Similar weather conditions occur in autumn, winter, and spring. When the weak southerly wind was around Taiwan, PM_2.5 in northern Taiwan was noticed.