| 摘要: | 生質燃燒氣膠對於大氣成分的改變與氣候變遷都有不可忽略的貢獻，中南半島為生質燃燒活動最為盛行區域之一，雖然已知氣候可能是此區域生質燃燒旺盛與否的主因，但主因與污染物傳輸變化的影響上所知仍舊有限。故本研究著重於利用NASA/MERRAero氣膠再分析資料，結合MODIS火點、氣膠光學厚度與雲等衛星資料，分析長時期(2003到2014年)中南半島春季生質燃燒氣膠分布與傳輸特徵，並探討其與氣候及區域環流間之關聯性，以及初探生質燃燒氣膠對區域輻射收支、雲微物理與降雨的潛在影響。
本研究主要選擇範圍85°-143°E和8°-44°N，由衛星火點資料分析顯示，中南半島北部為生質燃燒主要發生區域，也是本篇研究主要範圍。結合Niño3.4區域海溫與中南半島火點距平，可將過去十二年之中南半島生質燃燒極端事件分為三種情境：(A)Niño3.4海溫高相關之生質燃燒事件；(B)Niño3.4海溫低相關之強生質燃燒事件；及(C)Niño3.4海溫低相關之弱生質燃燒事件。在情境A中，2008和2011年Niño3.4具低海溫特徵，大陸冷高壓及其伴隨的東北季風較為強勁，勢力延伸至南海及中南半島，因此太平洋高壓勢力在東亞大幅減弱，此情境下東北季風配合850hPa東風帶北移西伸，有利水氣輸送至中南半島，造就一個不利生質燃燒的潮濕環境。相反的，2010年Niño3.4海溫呈現劇烈的正距平，此年東北季風較弱，整體氣候場呈現有利生質燃燒生成環境。在情境B中，2007年為典型的例子，當年Niño3.4海溫無明顯波動，但因太平洋高壓位置西伸至西太平洋邊緣，阻擋大陸冷高壓南下，使中南半島的環流場改由西南風支配，造就旺盛的生質燃燒。在情境C中，當上述大尺度環流接近氣候平均，此時印度高壓減弱可能使更多的水氣從印度洋傳送至中南半島西部，也是2003與2005年生質燃燒活動轉為零星的主要原因。
在污染物長程傳輸方面，2008與2011年等弱燃燒事件年中，由於北方高壓及其伴隨的東北季風影響較強，破壞原本存在於中南半島的相對低壓減弱，使輻合上升運動明顯減弱，污染物較難上升至自由大氣進行長程傳輸。相反的，如在2007年的強燃燒年中，相對低壓特徵明顯，加上大尺度環流配合地形抬升效應，有助污染物向高層傳送，並藉由西風帶向東傳輸，除此之外在強燃燒年中，高層西風帶向下風處的傳輸效率較佳，甚至能將污染物傳送至日本南方海域。
進一步針對極端事件年進行區域大氣輻射及環境衝擊之初探。估算在大氣層頂之氣膠造成的直接輻射通量改變，2007和2010高燃燒年分別為-13.36與-16.24 Wm-2，2008和2011年低燃燒年則為-11.78與-9.64 Wm-2，皆遠大於全球平均輻射驅動力(-0.27 Wm-2)。生質燃燒傳輸路徑上，正行經一片均質的層積雲區域，大量生質燃燒氣膠排放到大氣中並進入到雲系統中，可改變雲微物理特徵。結果顯示，在無大尺度系統影響下，高濃度污染物將成為雲凝結核改變雲微物理特性，雲滴粒徑相較於平均值偏小，造成區域降水量減少或推延降雨時間的可能性。;The aerosols from biomass burning have a non-negligible contribution to atmospheric composition, as well as to climate change. Indochina is one of the prevailing source regions of biomass burning in the world. Despite the large–scale climate has been known a possible factor to biomass-burning actives in this region, the long-thern variability and transport mechanisms in different climate patterns still remain unstudied. Therefore, this study focuses on analyzing long-term (2003-2014) distribution and transport characteristics of biomass-burning aerosols and meteorology in March using NASA/MERRAero data set. Combining with satellite data (e.g., MODIS fire counts, aerosol optical depth, and cloud effective radius) to investigate the correlation between biomass-burning aerosols and regional climate. In addition, the potential impacts of biomass-burning aerosols on regional radiation budget, cloud microphysics, and rainfall distribution are also discussed in this study.
The study domain covers 85°-143°E and 8°-44°N. The northern Indochina is the main biomass-burning region in the dowmain according to satellite data. Analysis of the Niño3.4 sea surface temperature (SST) anomaly and the fire counts in north Indochina, we proposed three climate scenarios associated with fire activites: (A) years with extreme biomass-burning activity associated to El/La Niño climate; (B) years with strong biomass-burning activity under normal climate, and (C) years with weak biomass-burning activity under normal climate. For the scenario A, the years of 2008 and 2011 are typical examples showing weak fire activities corresponding to low Niño3.4 SST. An overwhelming northeast monsoon extends its influence to the South China Sea and Indochina. Meanwhile, the tropical easterlies at 850hPa moves toward northwest. As a result, moist air converges over Indochina, setting an unfavorable condition for biomass burning. In contrast, in 2010, the northeast monsoon shows weaker than climate average due to El Niño climate. The overall climate pattern of 2010 represents a favorable environment for biomass burning. For the scenario B, the year of 2007 shows no correlation to Niño3.4 SST but strong biomass-burning activity were observed. The location of the Pacific High plays an important role in this scenario. When the Pacific High extends westerly to Asian continent, it obstructs continental cold High from moving southward. The wind pattern over Indochina is dominated by southwester. As a result, stronger biomass-burning activity was observed. For the scenario C, the years of 2003 and 2005 showed weak biomass-burning activities under normal climate. A common climate pattern for these two years is that more moist air transported from the Indian Ocean. .
Regarding to the transport of biomass-burning aerosols, we found that the years (i.e. 2008 and 2011 as examples) with weak fire activity are also showing an adverse long-range transport mechanism. In contrast, for strong biomass-burning activity year (i.e. 2007), the large-scale circulation accompanied with orographic lefting can help polluted airmass aloft to higher level and long-range tranport within westerlies.
The radiative and water-cycle effects of biomass-burning aerosols are also investigated in this study. The estimates of aerosol direct radiative forcing at the top of atmosphere for strong and weak biomass years are -13.36 ~ -16.24 Wm-2 and -11.78 Wm-2 ~ -9.64 Wm-2, respectively, showing two order greater than global average of -0.27 Wm-2. In addition to aerosol direct effect, we also assessed the indirect effect. We found that the increase of biomass-burning aerosols can alter the cloud microphysics, in terms of cloud droplet size and cloud fraction. In a normal climate condition, the biomass-burning aerosols tend to decrese cloud droplet size and clould fraction, which in turns reducing precipitation amount or delaing the precipitation time. |
