由環保署測站的觀測以及模式(CMAQ)模擬結果發現白天污染物容易在西半部沿海地區生成，經由海風傳送至內陸山區，埔里盆地內的污染物濃度會在中午過後升高;晚上地表受到輻射冷卻，在近地表形成逆溫層，大氣呈現穩定的狀態，因此讓污染物滯留在近地表，無法擴散，但有部分的污染物會被離岸風傳送到西部平原，埔里地區的污染物濃度在午夜過後下降;白天邊界層發展，垂直混合作用增強，降低盆地內污染物的濃度。 ;Puli basin is located in central mountainous area of Taiwan. The thermally driven circulations such as the upslope/downslope wind and mountain/valley wind generally develop when the synoptic forcing is weak. During high PM2.5 concentration episode, air pollutants mainly produced in the western coastal region can be transported into inland area by sea breeze, and the air quality becomes bad in Puli basin. Air pollutants dispersion is strongly associated with local circulations and atmosphere PBL structure.
In this study, the thermally driven circulations and its subsequent influence on the PBL structure and air pollutant dispersion Puli basin are investigated by using high-resolution mesoscale meteorological model (WRF) and observation data from the Central Weather Bureau (CWB) and air quality monitoring station of Environmental Protection Administration (EPA) in Taiwan.
The upslope winds develop because of the different heating between sloping surface and adjacent plain and leads to the wind divergence and subsidence over the basin in the morning. With the onset of valley wind, the wind speed increase and westerly flow dominate over the basin. Additionally, the atmosphere turns stable to well-mixed condition quickly due to the solar heating and subsiding warming. On the other hand, the radiative cooling and cold air advection produced by downslope and mountain winds form the inversion layer during the night. The surface layer is decoupled from the flow above. Besides, the sea breeze penetrates into mountainous region in the afternoon. The mountain wave is induced by the interaction between the sea breeze and topography. The wind speed increase because of the combination of sea breeze and valley winds.
According to observation data from EPA and CMAQ model results, air pollutants are mostly produced in the western plain and transported by the sea breeze into mountainous region. And the PM2.5 concentration observed at Puli station raise sharply in the afternoon. The inversion layer associated with the radiative cooling and cold air advection forms and causes pollutants accumulate near the surface. With the onset of the mountain wind, the pollutants tend to be transported plainward and the PM2.5 concentration recorded at Puli station decrease. After sunrise, the convective boundary layer develops, the vertical mixing becomes strong and dilutes the concentrations near the surface.
