| 摘要: | 中南半島地區是全球生質燃燒活動的活躍地區之一，生質燃燒產生大量人為氣膠中，吸光性氣膠，主要為黑碳(Black Carbon; BC)與褐碳(Brown Carbon; BrC)，影響該地區氣膠光學特性及輻射收支。本研究透過氣膠觀測網(AErosol RObotic NETwork; AERONET)觀測及反演產品進行分析，瞭解該地區氣膠光學特性及氣膠成分，同時使用MODIS火點資料，瞭解不同時空中生質燃燒活躍程度，並分析該狀態下的氣膠特徵。中南半島地區所架設的AERONET站，有部分已經累計有10年左右的觀測，並且AERONET太陽光度計會進行定期且標準化的校正程序，在不同測站間進行氣膠光學特性及成分比較時，各站儀器間的差異會降至最低。因此本研究將透過AERONET氣膠光學產品資料分析，以進一步瞭解中南半島氣膠光學特性及成分特徵。
研究方法參照前人研究所提出的主要氣膠種類群落分析及氣膠成分反演法，前者透過消光埃指數(EAE)對單次散射反照率(SSA)的群落作為主要氣膠種類的判別，後者則是透過折射指數(Refractive index; RI)及Maxwell-Garnett 模式反演最佳成分比率。在氣膠光學特性結果當中，中南半島北方氣膠光學厚度值在2及3月最高(0.8左右)，7月過後數值較低(0.4以下)，細顆粒佔比(FMF)在Chiang Mai站 10月至4月高，其餘月份低；各站的SSA約在0.8-0.9以2、3月最低；而AOD及FMF在中南半島北方相對於南方而言整體數值高，SSA數值較低，氣膠特性在地理空間分布以北方消光程度大，以及偏向吸光性的細顆粒，位置愈向南方，氣膠較以粗顆粒氣膠佔多數，且整體氣膠量較北方來得少；同樣的結果反映在各站的氣膠粒徑分布中，乾季以偏向細顆粒氣膠體積多於粗顆粒氣膠。以上現象透過主要氣膠群落分析，結果1及4月接近存在城市/工業類氣膠及細顆粒的混合類氣膠的群落，3月則分布接近生質燃燒氣膠，AOD大於第三四分位數情形下，有資料密集分布於生質燃燒氣膠類別。進一步分析氣膠成分的反演結果，並與化學採樣結果相互比較，兩者的相關係數為0.6。在中南半島北部方面，BC氣膠佔比在月分布上，2、3月最高(5~6%在CM站)也對應SSA偏低之情形，值得一提的是，3月份並非觀測到最高的BC占比(也就是最低的SSA)，主要可能原因為氣流來源不同，3月可能受到當地生質燃燒與境外印度緬甸空氣塊傳輸雙重影響，而5月雨季開始，生質燃燒活動降低，沙塵類氣膠佔的比例普遍上升(粗顆粒比例變多)，造成FMF的下降。相較之下，中南半島南方(Bac Lieu站)氣膠成分多數月分由沙塵類氣膠為主導，屬於自然源氣膠粗顆粒特性。
分析2014年及2015年3月，DAK站與CM站的AOD高峰值日及非峰值日的氣膠特徵，吾人發現，在非峰值日CM站及DAK站的氣膠光學特性趨勢一致，表示氣膠分布空間均勻；高峰值日，兩站的氣膠光學特性有所不同，AOD趨勢在DAK站為上午低下午高，CM站則是趨勢相反，推論CM上午的AOD高值是由前一天污染物累積，並於峰值當日觀測而得，而DAK則是在下午高生質燃燒活動時，觀測出高值，兩站的觀測上時序變化差異，推論是因為測站所在海拔高度以及距離生質燃燒的不同所造成。在BC與BrC佔比變化方面，非事件日的BC佔比數值變化不規律；相較之下，BC氣膠佔比在事件日存在峰值，BrC在事件當日佔比下降。本研究的成果有助於整體瞭解中南半島生質燃燒月份、氣膠光學特性、氣膠成分及事件日與非事件日的特性差異，並輔助未來觀測實驗規劃、模式模擬驗證、衛星反演驗證及輻射驅動力估算之研究。
;Aerosol chemical components (i.e., black carbon (BC), brown carbon (BrC)) regulate the optical properties of aerosol, which are highly sensitive to atmospheric radiative forcing estimation. Previously, through an intensive international field experiment, i.e. The 7 SEAS project, the complex aerosol environment of Indochina was investigated. However, the role of absorbing aerosols during biomass burning season on the environment is still unclear. In this paper, the long-term cimel sunphotometer AERONET (Aerosol Robotic Network) measurements and its inversion products from 6 sites in Indochina have been studied to understand the seasonal and spatial characteristic differences in aerosol optical properties and the mixture of BC with other aerosol components in smoke haze.
The long-term data analysis revealed that the monthly mean aerosol optical depth (AOD) was higher in the months of March and April than the rest of months observed by all AERONET sites in the region, inferring the influence of regional biomass-burning activities. From the aerosol size distribution, the number of small particles was even larger than large particles during dry period. Due to the spatial differences in geographic and land use patterns, the AERONET sites in northern Indochina show a higher AOD compared to that of in southern Indochina. In comparison to the yearly-mean aerosol optical properties, a lower single-scattering albedo (SSA) and higher fine-mode fraction (FMF) values in February and March, suggested the domination of smaller and stronger absorbing particles during this period.
Two methods (i.e., aerosol type cluster and aerosol component retrieval) had been applied to determine the aerosol type and chemical components during the biomass-burning season. The correlation coefficient between component retrieval and chemical sampling was 0.6 at DAK site. For Chiang Mai site (northern Indochina), the cluster method revealed biomass-burning aerosol type in February and March, when a large fraction of absorbing aerosols (BC 5% and BrC 40%) were also observed according to the result from aerosol component retrieval. It is worth to mention that the peak of biomass burning month (i.e. March) in northern Indochina may not necessary with high BC fraction (i.e., lowest SSA) once dust particles are mixed. For southern Indochina (Bac Lieu site), a mixed aerosol type with low absorbance was determined.
Furthermore, we categorized the days into event days and non-event days based on AOD value during biomass-burning months (March in 2014 and 2015). As a result, the tendency of aerosol optical properties between DAK and CM site showed similar pattern, which suggested that aerosol spatial distribution was more homogeneous during non-event days. As contrast, AOD and AE tendency were different at DAK and CM site during event days. We suggested that the peak AOD value of event day at CM site was due to aerosol accumulation from previous day. For DAK site, the peak AOD value of event day happened in the afternoon which may due to nearby source region. The difference of geographic location and altitude between two sites cause the inhomogeneity of aerosol optical properties over the region. For the BC and BrC fractions during even day, BC fraction showed higher value while BrC fraction showed lower for both CM and DAK sites. This results of this study will help us to understand the seasonal variability of aerosol optical properties, the aerosol composition, and the event day and non-event day of biomass-burning month over Indochina peninsula. The results will also be useful in future field experiment planning, model/satellite retrievals evaluations, and aerosol radiative forcing estimation. |
