| 摘要: | 空氣污染除了受到人為排放影響，天氣型態也扮演著重要角色。臺灣在冬季受大陸冷高壓天氣系統影響，盛行風為東北風，中南部地區在東北季風影響時，因位於背風面，呈現靜風穩定大氣條件。
臺灣大氣邊界層觀測實驗計畫(Taiwan atmospheric PBL Observation, Modeling, and Data Assimilation experiments, T-POMDA)，於2021年1月1日至6日，搭乘海研船三號進行海上觀測實驗，包含探空氣球釋放，並架設10-m氣象塔，收集海面以及大氣邊界層觀測數據。配合海研三號所安排的船期以及行經路線，從高雄港出發往北至中部外海，並返回高雄港。臺中以北因受東北季風影響，海況較差，無法進行觀測。臺中至雲林外海為本次觀測重點區域，探空氣球施放次數較為頻繁。本文結合綜觀氣象、當地天氣特性，來說明大氣邊界層垂直結構特性。
海上觀測期間，受大陸冷高壓天氣系統配置影響，綜觀氣象條件主要受東北季風以及高壓迴流環流結構影響。東北季風盛行時，高屏沿岸呈現弱風尾流特性，而高壓迴流天氣型態下，盛行東風因中央山脈地形阻擋，西半部整體風場微弱，大氣擴散條件不佳，皆有利於污染物濃度的累積。而在此情況下，大氣邊界層垂直結構發展對污染物的擴散行為，也更為重要。
在陸地，白天熱力作用加熱地表，使混合層發展，熱力作用在中午最旺盛，混合邊界層高度為當日最高，晚上隨著輻射冷卻增強，底層形成穩定邊界層，使邊界層高度下降。然而在海洋，因海水的日夜溫差較小，使中午熱力作用較不易加熱海面，夜晚及清晨的輻射冷卻也較不明顯，因此海洋邊界層高度的日變化相比陸地是較小的。
分析結果顯示，中部外海因受管道效應(Channel Effect)影響，風速較高，熱力作用也較明顯，在大氣較不穩定的條件下，邊界層的高度較高，較好的擴散條件使PM2.5濃度較低；然而，南部外海因位於中央山脈較背風面，環境綜觀風不易影響該地，使風速較低，熱力作用也因此不明顯，大氣穩定的情況下使邊界層高度較低，不利的污染物擴散條件使高污染事件更容易發生。
;Apart from anthropogenic emissions, air pollution also plays an important role in weather patterns. In winter, Taiwan is affected by Asian continental anticyclone system, and the prevailing wind is northeasterly. Under the influence of the northeast monsoon, central and southern regions become stable atmospheric condition, because they are located at the leeside area of Central Mountain Range.
Taiwan atmospheric PBL Observation, Modeling, and Data Assimilation experiments (T-POMDA) took R/V Ocean Researcher 3 to do the experiment from January 1 to 6, 2021, including the casting of sounding balloons, and erected a 10-meter meteorological tower, to collect observed data from sea surface and atmospheric boundary layer. In line with the schedule and track arranged by R/V Ocean Researcher 3, departing from Kaohsiung Port, to the north to outer-sea of central area, and returned to Kaohsiung Port. Due to the influence of the northeast monsoon to the north of Taichung, the sea conditions were poorer, and the observation couldn’t be carried out. The outer-sea of Taichung to Yunlin is the key area of the observation, so the sounding balloons were released more frequently. This paper combines synoptic weather and local weather characteristics to illustrate the vertical structure characteristics of the planetary boundary layer.
During marine observation, affected by the configuration of Asian continental anticyclone system, synoptic meteorological conditions were mainly affected by the northeast monsoon and the structure of continental high-pressure peripheral circulation. When the northeast monsoon prevailed, the coast of KP area presented the characteristic of the weak wind outflow. While in the weather pattern of high-pressure peripheral circulation, due to the barrier of the Central Mountain Range, the wind prevailed easterly, and the wind field of west side was totally weak, the atmospheric diffusion condition was bad, so that could easily accumulate the pollutants. The development of vertical structure of the planet boundary layer is also more important to the diffusion of pollutants in the situation.
At land, the surface is heated by the thermal action in daytime, and the mixing layer develops, the thermal action is strongest at noon, mixing boundary layer height is highest in the day. Through the enhancement of radiative cooling at nighttime, the bottom layer forms stable boundary layer, and the boundary layer height decreases. However, at the ocean, due to the smaller difference of sea temperature, the thermal action won’t be easier to heat the sea surface at noon, and radiative cooling at night and dawn is less obvious, so the diurnal variation of marine boundary layer height is smaller than land.
The analysis results show that central offshore is influenced by Channel Effect, the wind speed is higher, and the thermal action is more obvious. Under the condition of relatively unstable atmosphere, the boundary layer height is higher, and the better pollutants diffusion condition makes PM2.5 concentration lower. However, the southern offshore is located at more leeside area of Central Mountain Range, the synoptic wind won’t be easier to influence the area, the wind speed is lower, and the thermal action is less obvious. The condition of stable atmosphere makes the boundary layer height lower, the unfavorable pollutant diffusion condition makes severe pollution events occurred easily. |
