高斯煙流模式(Gaussian plume model) 雖可用來模擬煙流之濃度場分佈，但此模式只能運用在均勻流中且無法準確估計大氣穩定度的影響，亦無法由高斯煙流模式模擬粒狀物質的擴散或計算下沖現象(Downwash)所造成之濃度。因此研究中結合歐氏-拉氏法(Eulerian-Lagrangian Approach)發展一套數值模式，可用來計算氣狀污染物和粒狀污染物在開闊地區和建築物後方的濃度場。
本研究中先以拉氏法的隨機移動模式(Random Walk Model)，模擬不同大氣穩定度下氣狀和粒狀污染物在開闊地區的擴散，並計算不同下游距離及地表處的濃度場分佈。模式模擬建築物所造成的煙流下沖現象，歐氏法先採用大渦流模式(Large Eddy Simulation, LES)計算建築物周圍的平均風速場，依據此風速場再以拉氏法的隨機移動模式模擬氣狀污染物和粒狀污染物的擴散，並計算濃度場分佈。
模擬結果並與前人之現場實驗結果比較以證實模式的可靠性。本研究中並利用該模式探討大氣穩定度、排放高度、粒狀物質粒徑、密度等參數對擴散現象的影響。

Prediction of downwash phenomenon behind buildings has been an important topic in air pollution research. It is well known that Gaussian plume model cannot be used to predict concentration distribution when downwash happen. In this study, an Eulerian-Lagrangian approach was adapted to develop a numerical model to simulate gaseous and particulate pollutants. The velocity field around the building was calculated by a Large Eddy Simulation (LES) model. Based on the velocity field, concentration distribution was determined by a Lagrangian random walk model. The simulation results were compared with the experimental results and showed good agreement. Furthermore, the effect of atmospheric stability, discharge height, particle diameter and density were investigated by the present model.

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