The Fine Particulate Pollutants Distribution over the Lee Side of Mountains in Taiwan under Consecutive Cold-season Weak Synoptic Weather Days --- Abstract: This study aims to quantify the enhancement of fine particulate matter (〖"PM" 〗_"2.5" ) by lee side local circulation under the complex topography in Taiwan using observational data. We select the cold-season weak synoptic weather as the flow regime that favors the development of lee side circulation in western Taiwan under the southeasterly synoptic wind. We then define the enhancement index that discerns the areas with the high frequency of 〖"PM" 〗_"2.5" enhancement relative to the background concentration. Under the weak synoptic days, the lee side areas can exhibit the enhancement index values > 0.65. In addition, during the consecutive two-day weak synoptic events, the indices capture the shift of 〖"PM" 〗_"2.5" enhanced areas under the subtle transition of the background wind direction. The areas with 〖"PM" 〗_"2.5" > 0.65 expand on the 2nd day in northern Taiwan, when the number of stations exhibiting the index > 0.8 increases by 3 times than the first day. The idealized large-eddy simulations using TaiwanVVM confirm the formation and shedding of lee vortices under the subtle southerly shifting of background wind can significantly modulate the tracer transport in northern Taiwan. The results highlight the change of position and movement of lee side circulation are sensitive to the transition of background wind and can play a key role in modulating the 〖"PM" 〗_"2.5" pollution in northern Taiwan.