| 摘要: | 臺灣的空氣污染在不同的季節具有相當迥異的時空間特性，是當前臺灣國人 需要共同面對的嚴肅科學議題。空氣污染分為境內與境外，除了臺灣境內污染源如工業源、移動污染源及其他污染源外，若適當的環境風場盛行時，來自東亞大陸上游的空氣污染物會透過長程傳輸經過臺灣，導致環境品質下降，對人類健康與生態系統造成負面影響。為可以提前掌握污染物未來濃度趨勢，CMAQ (Community Multiscale Air Quality)模式在模擬過程中使用具可信度的資料及標準的程序，以利於增強臺灣對空氣污染問題的管理及控制能力。然而模式的準確率除了和氣象場及解析度有關，污染源實際的排放趨勢也是主要的因素，當前模式使用的東亞大陸排放量為MIX 2010，已是相對過時的排放量數據，對於模擬近年的境外污染傳輸事件誤差也較大。 本研究透過TROPOMI 衛星觀測以及使用CMAQ 模式，去耦合直接法 (Decoupled Direct Method, DDM)，可提供污染物及排放量之間的非線性關係，用於推估地面氮氧化物(NOX)人為排放量。針對2023年1月1日至3月5日的東亞排放量進行平均推估，並挑選實驗期間臺灣三個顯著的境外污染事件，評估其排放量改善之效益。
個案一為2023年1月18日至1月22日。個案期間的大氣條件屬於東北季風盛行之環境，1月18日冷高壓逐漸自蒙古地區東移至黃河中上游流域，華北一帶的污染物傳輸至華中地區，使華中一帶PM2.5平均濃度高達100µg/m3。1月19日，來自華北及華中上風處的人為污染物傳輸至臺灣內陸，此時境外污染已開始影響臺灣各空品區，富貴角測站可量測到30µg/m3的PM2.5濃度，同時，修正組較Base組再多2至3µg/m3。個案二為三個個案中最顯著的境外污染事件，2月18日冷高壓逐漸自西伯利亞東移，強勁的風場使大量中國華北及華中工業區的污染物帶至海面上，並在2月19日移入臺灣北方，惡化北部的空氣品質，而挾帶進來的污染 物也對中南部東北季風的尾流弱風區形成一系列的光化反應，加劇該區域的空氣品質狀況，Base組的中南部PM2.5濃度大約平均為40µg/m3，而在修正組中提升2至3µg/m3。個案三在2月28日至3月2日，有兩波冷高壓中心自東亞地區東移至韓國海面，在2月28日時整體的大氣環境為弱綜觀，因此臺灣主要以境內污染為主，而後的3月1日冷高壓中心往東南方移動，使環境風場從東風轉為東北風，污染趨勢轉為境外污染，各地測站PM2.5濃度大約平均為45µg/m3。而不同的冷高壓風向也對中南部的沿海及內陸區域有不同的污染物分布情況。整體使用修正過後的東亞排放量來模擬境外傳輸對臺灣之 PM2.5濃度影響有改善之效益，三個個案PM2.5濃度皆呈現上升之趨勢，尤其以境外傳輸進來的硝酸鹽上升最為顯著，以及境外所帶入的污染物在光化反應下生成的污染物也能夠在修正的排放量中得到顯著的模擬成效。;Air pollution in Taiwan exhibits significantly different spatiotemporal characteristics across various seasons, making it a critical scientific issue that the people of Taiwan must face together. Air pollution can be classified into domestic and external sources. In addition to domestic sources, such as industrial, mobile, and other pollution sources, pollutants from upstream regions of East Asia can be transported over long distances to Taiwan during favorable meteorological conditions, leading to a decline in environmental quality and causing negative impacts on human health and ecosystems. To predict future pollutant concentration trends in advance, the CMAQ (Community Multiscale Air Quality) model employs reliable data and standardized procedures during simulations, which aids in enhancing Taiwan′s ability to manage and control air pollution. However, the model′s accuracy is not only related to meteorological fields and resolution, but also to the actual emission trends of pollution sources. The East Asian emission inventory currently used in the model is the MIX 2010, which is relatively outdated, leading to larger errors in simulating recent cross-border pollution transport events.
This study uses TROPOMI satellite observations and the Decoupled Direct Method (DDM) provided by the CMAQ model to capture the nonlinear relationship between pollutants and emissions, enabling the estimation of anthropogenic nitrogen oxide (NOx) emissions at the surface level. An average estimation of East Asian emissions was conducted for the period from January 1 to March 5, 2023. Additionally, three significant cross-border pollution events affecting Taiwan during the experimental period were selected to evaluate the benefits of emission reductions.
Case one occurred from January 18 to January 22, 2023. The atmospheric conditions during this period were characterized by the prevailing northeast monsoon. On January 18, a cold high-pressure system gradually moved eastward from Mongolia to North China, causing pollutants from North China to be transported to Central China, worsening air pollution in that region. On January 19, anthropogenic pollutants from the upwind areas of North and Central China were transported to the interior of Taiwan, and at this point, foreign pollution had begun to affect air quality across various regions in Taiwan.
Case two represented the most significant incident of foreign pollution among the three cases. On February 18, a cold high-pressure system gradually moved eastward from Siberia, and strong winds carried a large amount of pollutants from the industrial areas of North and Central China over the sea. On February 19, these pollutants reached northern Taiwan, deteriorating air quality in the northern region. The pollutants brought in also triggered a series of photochemical reactions in the weak wind area of the northeast monsoon′s tailwind in central and southern Taiwan, exacerbating air quality conditions in that area. In Case Three, from February 28 to March 2, two cold high-pressure centers moved eastward from East Asia to the sea near Korea. On February 28, the overall atmospheric environment was characterized by weak synoptic conditions, so Taiwan mainly experienced domestic pollution. On March 1, the high-pressure center moved southeastward, causing the environmental wind field to shift from easterly to northeasterly, changing the pollution trend to transboundary pollution. The different wind directions of the high-pressure systems also led to varying pollutant distributions between coastal and inland areas in central and southern Taiwan.
Overall, using the revised East Asia emission data to simulate the impact of foreign transport on Taiwan showed improvements, particularly in the case of nitrates brought in from abroad. The pollutants generated from photochemical reactions of the imported pollutants also demonstrated significant simulation effectiveness within the revised emission quantities. |
