| 摘要: | 本文於2013年春季分別於中南半島生質燃燒源區泰國清邁山區(海拔1,536公尺)及台灣鹿林山大氣背景監測站(海拔2,862公尺)進行氣膠採樣。研究目的是分別探討近生質燃燒(BB)源區及背景點傳輸老化氣膠特性，此外，本文也彙整2010-2013年中南半島及2003-2013年鹿林山氣膠特性，探討兩地中、長期氣膠化學特性及來源貢獻。
研究結果顯示2013年清邁PM10氣膠受PM2.5氣膠主導，BB事件日PM2.5氣膠水溶性離子以硫酸根離子及銨根離子為優勢物種，氣膠碳成分以OC3及EC1-OP為主，水可溶有機碳(WSOC)佔有機碳比例為61%，二元酸以Oxalate濃度最高，至於氣膠單醣無水化合物則明顯以左旋葡萄糖為主。生質燃燒指標物種nss-K+、OC3、EC1-OP及Levoglucosan彼此間相關性判定係數R2都達0.6以上(N=38)，印證本地區氣膠受BB影響。利用特定氣膠成分比值性質，可推測PM2.5氣膠來自開放式森林燃燒，燃燒樹種可能混合軟木及硬木，燃燒狀態為燜燒狀態。2010-2013年中南半島近BB源區氣膠成分各年代比例接近，陽離子、陰離子、有機碳、元素碳佔PM2.5質量濃度平均比例分別為 5.7±0.6%、12.2±1.3%、40.9±4.1%和 7.4±1.0%，應可代表中南半島BB PM2.5氣膠主要成分比例。
彙整2003-2013年鹿林山PM2.5氣膠觀測資料顯示：生質燃燒(BB)類型質量濃度、水溶性離及碳成分濃度都高於其他氣流類型，特別是生質燃燒指標物種如：nss-K+、NO3-、OC3、EC1-OP、levoglucosan等特別凸顯。非生質燃燒(NBB)類型的水溶性離子比例則高於BB類型，表示偏向受到人為污染。利用碳成分優勢物種、char-EC/soot-EC及OC/EC比值，也可確認BB類型氣膠的BB特徵和NBB類型的偏向來自機動車輛排放影響。
利用2010年及2013年上風處(泰國清邁)及下風處(鹿林山)觀測數據，顯示nss-K+、OC3及EC1-OP的比例變化較其他成分穩定，適合當作BB長程傳輸氣膠指標物種。本文利用穩定生質燃燒物種nss-K+¬探討氣膠傳輸老化特性，顯示NH4+、NO3 、SO42-、OC1、OP及二元酸部分物種在傳輸過程有增益現象，單醣無水化合物則在傳輸過程會有降解現象。
Positive Matrix Factorization (PMF)解析出2003-2013年鹿林山逆推BB軌跡類型PM2.5氣膠污染源共有6個主要類型，依高低序分別為BB mixing secondary aerosol (28.9%)、BB (26.3%)、BB mixing sea salt (15.0%)、BB mixing Dicarboxylates (14.7%)、BB mixing soil dust (9.1%)及Vehicle emissions (5.9%)，PMF來源推估顯示約94% PM2.5質量濃度來自BB煙團在長程傳輸過程混合其他污染源。在NBB期間，PMF解析出4個主要污染源，其中二次氣膠貢獻PM2.5質量濃度最多達70.2 %，氣流多半源自於海洋(包含大平洋與南中國海)，也有少許氣流混合海洋及人為污染物來自於中國大陸沿岸並透過低層大氣傳輸至鹿林山。
;This work collected atmospheric aerosols at Chiang Mai (1,536 m a.s.l., Thailand) in the northern part of Indochina Peninsula and Mt. Lulin Atmospheric Background Station (LABS, 2,862 m a.s.l., Taiwan), respectively, during the springtime of 2013. The objectives of this study were to investigate aerosol characteristics in the near-source biomass burning (BB) area and transported and aged aerosols at a background site. In addition, aerosol characteristics of Indochina Peninsula observed during the period of 2010-2013 and that of LABS during 2003-2013 were also summarized to explore middle- and long-term aerosol chemical characteristics and source contributions.
The results showed that PM2.5 dominated PM10 at Chiang Mai in 2013. For BB events, sulfate and ammonium ions were the major species in PM2.5 water-soluble ions (WSIs); while OC3 and EC1-OP were significant in aerosol carbonaceous components. The fraction of water-soluble organic carbon in organic carbon (OC) was 61% and oxalate was the dominant component of dicarboxylates. As for anhydrous sugars, levoglucosan was undoubtedly the most significant component. The BB tracers such as nss-K+, OC3, EC1-OP, and levoglucosan were correlated well with each other (with the coefficient of determination R2≧0.6, N=38), which confirmed that aerosols were affected by BB in the area. Inferred from the characteristics of specific aerosol component ratios, PM2.5 aerosol was contributed from open forest burning from a mix of softwood and hardwood and the combustion state was smoldering. The yearly component fractions of PM2.5 from near-source BB during 2010-2013 were close to each other. The PM2.5 fraction of cations, anions, OC, elemental carbon were 5.7±0.6%, 12.2±1.3%, 40.9±4.1%, 7.4±1.0%, respectively. The resulted fractions of PM2.5 should be typical for Indochina BB aerosol.
The PM2.5 aerosol observations at LABS showed that PM2.5 mass, WSIs, and carbonaceous content from BB backward trajectory group were all higher than that of other trajectory groups especially for BB tracers such as nss-K+, NO3-, OC3, EC1-OP and levoglucosan as summarized from 2003 to 2013. In contrast, the mass fractions of PM2.5 WSIs of non-BB (NBB) were higher than that of BB, an indication of more anthropogenic influence. By employing prevailing carbonaceous component, char-EC/soot-EC, and OC/EC ratios, aerosols from BB backward trajectory group were confirmed with BB characteristics and that from NBB trajectory group were more toward to vehicle emissions.
This work compared PM2.5 component ratios at the upwind Chiang Mai Thailand (Mt. Suthep in 2010 and Mt. Ang Khang in 2013) with the corresponding values at the downwind LABS to find that the ratios of nss-K+, OC3, and EC1-OP were quite stable. It implies that these three PM2.5 components are appropriate for acting as BB aerosol tracers through long-range transport. As a result, aerosol aging characteristics were investigated by using stable nss-K+ to show the enhancements of NH4+, NO3 , SO42-, OC1, OP, and dicarboxylates and the degradation of anhydrosugars during transport.
Six source types were resolved from Positive Matrix Factorization (PMF) for PM2.5 aerosols classified into BB backward trajectory group at LABS from 2003 to 2013. The resolved source types in source contributions from high to low are BB mixing secondary aerosol (28.9%), BB (26.3%), BB mixing sea salt (15.0%), BB mixing Dicarboxylates (14.7%), BB mixing soil dust (9.1%), and Vehicle emissions (5.9%). PMF source apportionment indicated that 94% of PM2.5 mass concentration was contributed from BB plume mixed with other source contributions in the path of long-range transport. In contrast, four source types were resolved from PMF and secondary aerosol contributed predominantly to 70.2% of PM2.5 mass concentration during the NBB period. The NBB air masses were mostly originated from ocean (including Pacific Ocean and South China Sea) but few with a mix of marine contributions and anthropogenic pollutants from China coastline and transported through lower atmosphere to LABS. |
