| 摘要: | 過去數十年間，中南半島生質燃燒活動對區域空氣品質、大氣能見度和氣候衝擊效應受到廣泛討論。然而，文獻對中南半島生質燃燒傳送機制與區域氣候反饋作用仍未清晰明瞭，為釐清大氣動力與中南半島生質燃燒活動間的交互作用，本研究結合MERRA-2再分析場、衛星和地面觀測，選取MERRA-2 2013-2015年3/1-4/15日瞬時資料與2000-2019年3、4月月平均資料，針對7-SEAS/BASELInE實驗研究區域，分析中南半島生質燃燒活動，本研究提出三項主要成果：(1)生質燃燒傳送機制理論模型；(2)四種生質燃燒傳送的氣候因子；(3)中南半島生質燃燒氣膠效應與區域氣候回饋。
針對成果(1)生質燃燒傳送機制理論模型，透過個案分析大氣氣膠水平與垂直分布，並與連結綜觀尺度大氣(如：鋒面系統、低層噴流(Low Level Jet, LLJ))，結果顯示，3-4月期間大陸性高壓外延使氣旋式環流(相對低壓)生成於中南半島，抬升並累積生質燃燒氣膠於高度3km處，當鋒面生成時，鋒前LLJ將生質燃燒氣膠傳送至下風處，於3-4月期間此過程以3-5天為循環周而復始。而在垂直方向上，鋒面系統南側700 hPa LLJ攜帶生質燃燒氣膠，同時鋒面南側所伴隨的大氣上升運動，將使生質燃燒氣膠保留於自由大氣層內，直到氣膠擴散至鋒面北側時，下沉氣流將氣膠傳送至地表
針對成果(2)生質燃燒傳送的氣候因子，透過合成分析與主成分分析法，分析長期氣候生質燃燒污染物與區域氣候之關係性，將氣候對生質燃燒傳送的影響歸類為四項因子：(1)孟加拉灣反氣旋環流、(2)南海反氣旋環流、(3)太平洋副熱帶高壓與(4)中南半島至台灣強西風帶。中南半島至台灣強西風帶與鋒前LLJ發生頻率正相關，當天氣場中鋒前LLJ與生質燃燒傳送事件頻率增加時，在氣候場中強西風帶則會更加增強。孟加拉灣反氣旋環流增強時，造成穩定大氣並減少降水，增強源區生質燃燒活動。南海反氣旋環流增強時，將利於中南半島至台灣強西風帶增強，同時促使南海區域大氣穩定，減少氣膠移除作用，使生質燃燒氣膠傳送至下風處。太平洋副熱帶高壓強弱與中南半島至台灣西風帶強弱相關，當太平洋副熱帶高壓增強時，連帶強化中南半島至台灣西風帶，反之亦然。而當太平洋副熱帶高壓向北偏移時，則會造成中南半島至台灣西風帶產生南風分量，改變生質燃燒傳送路徑，使生質燃燒累積於源區。生質燃燒傳送氣候特徵因子可進一步與大尺度氣候特徵指數連結，結果顯示生質燃燒活動受聖嬰現象(ENSO)影響，而印度洋偶極振盪(Indian Ocean Dipole)與生質燃燒活動無明顯相關。在聖嬰年(El Niño)孟加拉灣反氣旋環流、南海反氣旋環流及中南半島至台灣強西風帶增強，造成生質燃燒活動增強，且更容易傳送至南海及台灣區域。
針對成果(3)中南半島生質燃燒氣膠效應與區域氣候回饋，生質燃燒氣膠長期滯留於中南半島至台灣700 hPa處，對中南半島北部大氣短波氣膠輻射效應(ARE)造成20%的大氣層內淨輻射通量改變，此結果間接說明，中南半島生質燃燒期間ARE在大氣能量平衡上扮演重要角色，並造成地球環境系統某種程度上熱力與動力調整。由3-4月中南半島生質燃燒與5月份夏季季風相關性顯示，中南半島生質燃燒排放與傳送增強時，夏季季風則會同時增強，而生質燃燒氣膠累積於中南半島時，夏季季風則會減弱，說明中南半島生質燃燒氣膠效應，可能進一步影響5月夏季季風肇始。然而，氣膠效應對於區域氣候變遷的回饋機制相當複雜，加入數值模式的模擬驗證，可以做為未來生質燃燒氣膠反饋作用的主要研究方向。
;Impacts of long-range transported biomass-burning aerosols from Indochina on regional air quality, atmospheric visibility and climate effects have been discussed extensively in the literature over the past few decades. However, the transport mechanism and regional climate feedback of Indochina biomass-burning aerosols is still not fully understood. To clarify the interaction between atmospheric dynamics and biomass-burning aerosols in the Indochina, we present results of the MERRA-2 dataset, satellite and in-situ observation in spring 2000-2019 over the 7-SEAS/BASELInE experiment region. This study finally proposes three main analysis results of the biomass-burning activities in the Indochina: (1) A 3D conceptual model of biomass-burning transport mechanism; (2) Four meteorological factors attributed to biomass-burning activities; (3) The biomass-burning aerosol effect and its regional climate feedback.
For the achievement of (1) 3D conceptual model of biomass-burning transport mechanism, we analyzed the horizontal and vertical distribution of biomass-burning aerosols and linked the synoptic-dynamic meteorology (e.g. frontal system, Low Level Jet (LLJ)) to aerosol transport. From March to April, the cold frontal system extends from Japan to the South China Sea, and the postfront accompanied a strong westerly wind belt at approximately 700 hPa (3 km), transporting biomass-burning aerosols from the source region (i.e., northern Indochina) to the sink region (i.e., Taiwan and West Pacific) via the LLJ stream. The 700 hPa LLJ may have carried the biomass-burning plumes located south of the frontal system and accompanied the upward/downward motion south/north of the frontal system. This downward motion at the north side of the frontal system brought may bring pollution down to the surface and increased surface PM concentration. The life cycle of the synoptic weather pattern is approximately 3-5 days and consistently repeatable throughout March to April.
For the achievement of (2) the four meteorological factors attributed to biomass-burning activities, we identify those factors by using long-term data with composite analysis, principal components analysis and correlation analysis. The four factors include: (1) anticyclone (Monsoon trough) in the Bay of Bengal, (2) relative anticyclone in the South China Sea, (3) Pacific subtropical high, and (4) westerlies from Indochina to Taiwan. The strong westerlies from Indochina to Taiwan positively correlates with the frequency of the postfront LLJ. When the postfront LLJ and biomass-burning transport events occur frequently in the synoptic-scale meteorology, the westerlies from Indochina to Taiwan will also increase in the climate field. The increasing anticyclone over the Bay of Bengal tends to enhance atmospheric stability, reduce precipitation, and turns to strengthen biomass-burning activities in the source region. The increasing relative anticyclone in the South China Sea can enhance westerly from Indochina to Taiwan. At the same time, the relative anticyclone can promote the stability of atmosphere, reduce the deposition efficiency of aerosol, and support the biomass-burning aerosol transport to the downwind region. The strength of the Pacific subtropical high positively correlates with the strength of the westerlies from Indochina to Taiwan. When the Pacific subtropical high northward shift, the circulation generates the southerly wind from Indochina to Taiwan, which changes the biomass-burning transport path and location. We further assess the correlation of attributed factors and different large-scale climate indexes (e.g. ENSO, Indian Ocean Dipole (IOD)). As a result, the biomass-burning activity has a stronger correlation with ENSO, but weak correlation with IOD. In the El Niño year, the increasing anticyclonic in the Bay of Bengal and the South China Sea accompanied the stronger westerlies from Indochina to Taiwan, which enhanced the biomass-burning activities and aerosol transport to the South China Sea and Taiwan.
For the achievement of (3) the biomass-burning aerosol effect and its regional climate feedback, as the aerosol frequently located from Indochina to Taiwan at approximately 700 hPa, which can increase the net radiation flux by 20% in the atmosphere as so-called the shortwave aerosol radiation effect (ARE). Those ARE play an important role in the atmospheric energy budget during biomass-burning period, and adjust the thermal and dynamic in the earth′s environmental system. Furthermore, it can be found that the summer monsoon pattern in May has linkage with biomass-burning activities from March to April. The summer monsoon will be increased when the biomass-burning emissions and transport enhance, and it will be weakened when biomass-burning accumulates in the Indochina. The correlation illustrates that the biomass-burning aerosol feedback may change the precipitation and affect the summer monsoon onset. However, the understanding of the aerosol effect is limited by only using MERRA-2 reanalysis data. To gain a better understanding of the biomass-burning aerosol effect and regional climate feedback mechanism, utilizing a numerical study would greatly benefit in the future. |
