臺灣博碩士論文加值系統

本研究使用觀測資料和高解析模式，探討細懸浮微粒濃度(fine particulate matter, PM2.5)的空間分布在台灣冬春季連續弱綜觀天氣下的變化。
根據台灣天氣型態資料庫、地面降雨和位於台灣東南風場下的上游測站石垣島探空資料，在2008至2019年台灣冬春季中，有302天被定義為弱綜觀天氣型態。另外，將台灣各個地面空氣品質測站的日均PM2.5濃度與各個測站該年冬春季均PM2.5濃度的比值定義為各測站的PM2.5比率，再計算弱綜觀天氣的日子下各個地面測站PM2.5比率大於1的頻率，此無因次頻率指標(無單位)代表各個測站在弱綜觀天氣下PM2.5相較於季節背景的加強程度。在弱綜觀天氣下PM2.5被增強的高頻率(大於0.65)測站位置，和背景東南風場下的背風側弱風區域相符。在連續弱綜觀天氣的日子下，台灣區域綜觀風場會由東南東風向轉為南南東風向，而位於背風側測站的頻率指標也會提高至大於0.8；此外，在連續弱綜觀天氣下，台北盆地PM2.5加強的頻率會在第二天大幅增加。觀測分析結果顯示連續弱綜觀日在台灣北部的日均PM2.5相較於各個測站冬春季背景有最顯著的增加；此外，在台灣複雜地形下，上游風場的風向改變對於背風側局地環流有重要的影響，並且會進一步影響粒狀汙染物在背風側的分布和傳送方向。
為了進一步探討背景風場的方向轉移對細粒狀汙染物傳送的影響，我們利用具有高解析度台灣地形的渦度向量模式，在兩組不同風向110度(東南東風)和140度(南南東風) 的背景風場下，以被動示蹤物(tracer) 模擬台灣重要人為排放源的的局地傳送。兩組實驗皆顯示白天台灣中北部地區的氣流由背風渦漩所引導的西南風場主導，因此從中部排放的被動示蹤物容易傳向台灣北部陸地，並且多數汙染物會局限在背風渦漩和山區之間。此外，南南東風的模擬中，台灣西北端背風渦漩的向北移動能在背風側引發更大範圍的西南回流，將被動示蹤物傳至更北的地區。整體而言，數值模擬結果顯示台灣複雜地形背風側渦漩的位置和移動，對上游風向十分敏感，並且對汙染物的向北傳送扮演著重要角色。

In this study, we investigate the spatial-temporal distribution of fine particulate matter (PM2.5) during the consecutive days of weak synoptic weather in winter and spring using both observational data and large-eddy simulations.The 302 weak synoptic weather days in spring and winter during 2008-2019 are selected based on the Taiwan Atmospheric Events Database, the rainfall stations data, and the sounding profiles at Ishigaki Island, an upstream site under the background southeasterly wind condition. The enhanced PM2.5 ratio index (no unit) is defined as the frequency of PM2.5 ratio > 1 in a specific station under the 302 weak synoptic days. In this index, the PM2.5 ratio is the daily PM2.5 concentration divided by the annual mean PM2.5 at each Taiwan Environment Protection Administration (EPA) station. The enhanced PM2.5 ratio index corresponds to the possibility that PM2.5 is enhanced relative to the seasonal background concentration. Under the weak southeasterly synoptic wind condition, the stations with high-frequency (>0.65) of enhanced PM2.5 are located at the lee-side of Central Mountain Range, consistent with the trapping effects caused by mountain blocking. Moreover, pollution enhancement evolves into a more serious condition in the consecutive weak synoptic days, and the high-frequency areas extend northward to Taipei basin on the 2nd day with the synoptic boundary layer wind in the Ishigaki soundings shifting more southerly. Overall we identify from the observational analysis that more serious pollution events occurred when the weak synoptic condition persists in continuous days, and the transition of the synoptic-scale wind field plays an important role in controlling the local circulation and hence pollution dispersion on the lee side.
To understand the pollutants transport under different background wind conditions, the idealized simulations are performed using the vector vorticity equation model with Taiwan topography (TaiwanVVM). Two experiments are conducted with southeasterly background wind direction of 110 (ESE) and 140 (SSE) degrees. Passive tracers are emitted in six locations representing anthropogenic sources of the power plants, industrial park, city centers, and airport in Central Taiwan. Both simulation results show that, in daytime, the tracers are mainly transported to Northern Taiwan and trapped over the land by the southwesterly wind induced by the lee vortices. In addition, relative to the ESE experiment, the tracers in the SSE (140 degrees) experiment are prone to a more northward transport, as the lee vortex quickly shedding northward induces persistent southwesterly winds over North Taiwan. The results from TaiwanVVM simulations show the change of position and movement of lee vortices are sensitive to the transition of background wind and can play a key role in the northward expansion of air pollution areas in Northern Taiwan under weak synoptic conditions.

誌謝 i
中文摘要 ii
Abstract iv
Contents vii
Figure Captions viii
Table Captions xi
1. Introduction 1
2. Method 7
2.1 Meteorological and PM2.5 Data 7
2.2 Definition of Weak Synoptic Weather Days 7
2.3 The Enhanced PM2.5 ratio index 9
2.4 Model Description and Experiment Setup 9
3 Results 13
3.1 The Enhanced PM2.5 ratio index and Wind Flow under Weak Synoptic Weather Days 13
3.2 The Pollutant Enhancement in Consecutive Weak Synoptic Weather Days 14
3.3 Numerical Simulations 17
4 Discussion 20
4.1 The Quantification of Mountain Blocking Effect and Lee Vortices 20
4.2 The Vertical Distribution of the Particulate Pollution 21
5 Conclusion and Future Work 24
References 26
Figures 32
Tables 43

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