Intrinsic Mechanisms for High-Concentrated PMs in Southern Taiwan: Combined Effects by PBLH, LCFs and Large-Scale Subsidence

Abstract Subsidence and weak winds caused by the wintertime leeward effect have long been considered as key contributors to air quality deterioration in southwestern Taiwan, particularly in Kaohsiung City (KHC). However, similar meteorological conditions also occur in summer yet air quality remains much better. This indicates that subsidence and weak winds alone may not fully explain the seasonal differences in PM accumulation. In this study, we investigate whether the intensity of large-scale subsidence (LSS), particularly its ability to suppress daytime PBL growth, plays a more critical role on PM. ERA5 data were used to investigate PBLH and LSS. Leeside-effect cases on May 26 and January 23, 2017 (summer and winter), were selected for analysis. The Weather Research and Forecasting (WRF) model was used to simulate these cases for diagnosing the combined impacts of LSS, PBLH, and LCFs on PM accumulation. In KHC, seasonal variations in low PMs during summer are negatively correlated with high PBLH. However, with respect to diurnal PMs, high PM levels are observed despite high PBLH during the daytime. Under the combined effects of PBLH, LCFs, and LSS, the back-and-forth movement of weak nighttime offshore winds and daytime sea breezes plays a crucial role in PM accumulation over KHC. At night, offshore winds over KHC, driven by the divergent airflow of a leeside vortex, are typically accompanied by a low PBLH that suppresses the upward transport of PM. During the day, LSS inhibits the effect of high PBLH on PM transport to higher altitudes. Subsequently, the sea breeze carries the PMs from the northwest back to KHC, causing PMs to settle over land. In contrast, during summer, high surface temperatures lead to a high PBLH and upward motion, allowing PM to be transported to altitudes exceeding 1000 m, moving it far away from KHC. Graphical abstract Schematics of the combined leeside effect due to PBLH (dotted red arrow), LCFs (lee vortex – green cylinder; surface divergence wind – green circle; sea breeze – purple arrow), and large-scale flow (LSS – red arrow; NE: northeasterly – blue arrow). The triangular black area represents the terrain, and the dotted circular arrow indicates the back-and-forth movement of LCFs from nighttime to daytime (top figure represents nighttime, bottom figure represents daytime).