| 摘要: | 生物放射源為一主要大氣氣膠來源之一，特別是在森林區域。當這些自然形成的氣膠顆粒在大氣當中進行反應，不但會改變其化學組成，也會改變氣膠對能見度以及太陽輻射總量的影響。其中一個造成改變的因子為人為汙染源，當人為汙染物質隨著氣流被運送至森林中，就可能與當中的生物氣膠進行反應，而促進生物二次有機氣膠生成。因此，了解人為活動對生物二次有機氣膠生成之影響，有助於預測此途徑生成之氣膠顆粒對大氣氣膠總量的影響。本研究建立二次有機氣膠追蹤物的分析方法，利用熱脫附質子轉移反應飛行時間質譜儀(TD-PTR-TOF-MS)測量有機氣膠追蹤物，藉以說明在副熱帶森林地區，二次有機氣膠生成受城市汙染源的影響。相較於一般質譜儀，質子轉移反應質譜儀有(1)靈敏度高(偵測極限達pptv)；(2)可進行多種有機化學物質的分析，以及(3)檢測速度快等優點，因此非常適合用於大氣中有機氣膠顆粒之追蹤。
起初，本研究利用常見的有機氣膠追蹤物(如生質燃燒追蹤物：左旋葡萄糖)進行評估，發現質子轉移反應飛行時間質譜儀訊號與各物種在濾紙上沉積量呈線性關係。然而，我們也同時發現有許多碎裂片段存在於質子轉移反應質譜儀的漂浮管柱中。接著，於2013年8月在台北氣膠與輻射觀測站(Taipei Aerosol and Radiation Observatory, TARO)進行氣膠樣本(PM1)採集，利用質子轉移反應飛行時間質譜儀進行分析後，將結果與使用DRI 2001 碳分析儀分析結果做比對。發現質子轉移反應飛行時間質譜儀所測得之總有機物氣膠量(TOM)小於DRI 2001 碳分析儀分析結果27%。由第一階段實驗結果判斷此低估可能來自於少部分碎裂物質於漂浮管中未被偵測，以及揮發性有機物的損失所造成。儘管如此，兩種儀器的分析結果呈良好的相關性(R2=0.8578)。質譜圖顯示鄰苯二甲酸(phthalic acid)及戊二酸(glutaric acid)為氣膠樣本中主要物質，分別占7.0及9.4%的總有機物氣膠量。此階段實驗結果所測得之兩種人為汙染追蹤物，將用來協助評估人為汙染源運輸至生物放射源區對當地大氣組成分的影響。
熱脫附質子轉移反應飛行時間質譜儀架設於台灣副熱帶溪頭森林地區，探討此區二次有機氣膠來源，以及其微物理過程之影響。得到圖譜結果後，利用正定矩陣分解方法(PMF)對結果進行分析，顯示都市污染源(30%)、生物源(7%)及生質燃燒源(7%)為此區主要二次有機氣膠貢獻源。都市及生質燃燒追蹤物，如：鄰苯二甲酸、左旋葡萄糖、琥珀酸主導有機物質量濃度，因為人為放射源會產生相對較大的顆粒，促使生物源放射顆粒易附著於其上，進而抑制生物氣膠的生長，改變其物理特性。另外，採樣地區高溼度特性也對氣膠顆粒的形成，以及化學組成分的分配扮演著重要角色，直接影響有機物質存在於氣膠顆粒中的量。
除了典型人為污染追蹤物分析，氣膠質量濃度、以及其他化學物質分析結果顯示：森林中，有機氮化物(organonitrates,ONs)也是重要的化學組成分之一。ONs主要由環境中揮發性有機物質與氮氧化物進行化學反應而生成的產物。在熱脫附質子轉移反應飛行時間質譜儀分析方法中，利用硝酸異丙酯(isopropyl nitrate, IPN)作為ONs標準品，可在m/z於45.992得NO2+訊號值來表示ONs的濃度，實驗顯示20%的IPN可轉換成NO2+，且NO2+的訊號值與ONs濃度有良好的線性關係。 分析溪頭PM1氣膠樣本結果發現ONs濃度約占總有機物氣膠量約4%，此結果可能低於實際值五倍，因為受到高濕度干擾，ONs易分解成其他物質，而造成低估的結果。ONs與N100 nm的相關性暗示ONs濃度可能影響氣膠的生長，此外inferred branching值為5.41%指出ONs可能為氮氧化物在溪頭森林環境中反應的最終產物，終止一系列氮氧化物和OHx在大氣中繼續生成臭氧或其他有機物質的反應。
總結，研究結果顯示人為污染追蹤物(都市、生質燃燒及有機氮化物)約佔有機氣膠總量的59%，表示人為活動對台灣地區副熱帶森林中有機氣膠生成有重要影響。利用熱脫附質子轉移反應飛行時間質譜儀測定氣膠中有機追蹤物，不但有助於了解二次有機氣膠的來源和生成機制，也提供關於在生物環境中重要大氣反應的資訊。
;Biogenic environment, particularly forest, is among the main source regions of atmospheric aerosols. Changes in the natural formation of particulate matter in these regions may inherently lead to alteration in visibility conditions and in radiative forcing of climate. One of the principle causes of the deviation from the pristine condition is the transport of anthropogenic pollutants to the forestry areas. Probing the influence of the human-related activities on biogenic secondary organic aerosols may provide a projection of global aerosol budget in the coming years. In this study, a Thermo-Desorption Proton-Transfer-Reaction Time-of-Flight Mass Spectrometer (TD-PTR-TOF-MS) was developed to comprehensively characterize the particle phase organic markers to account the major sources of anthropogenic perturbation (e.g. urban) in a subtropical forest. PTR-TOF-MS as a detector of organic compounds in particle phase is highly beneficial due to its high sensitivity, a wide range of organic compounds detected and rapid mixing ratio quantification scheme.
The TD-PTR-TOF-MS was initially evaluated against standard compounds that are typically utilized as organic markers for source attribution for atmospheric aerosols (e.g. levoglucosan for biomass burning). The results demonstrated the linearity of the TD-PTR-TOF-MS signals against a wide range of mass loading of specific species on filters. However, it was found that significant fragmentation occurred in the drift tube of the PTR-MS. The instrument was further deployed to analyze a subset of submicron aerosol samples collected at the TARO (Taipei Aerosol and Radiation Observatory) in Taipei, Taiwan during August 2013. The inter-comparison with DRI thermo-optical carbon analyzer indicated that the TD-PTR-TOF-MS underestimated the mass of total organic matter (TOM) in aerosol samples by 27%. The underestimation was most likely due to the fragmentation in PTR drift tube. Besides, condensation loss of low vapor pressure species in the transfer components was also responsible for the underestimation to a certain degree. Nevertheless, it was showed that the sum of the mass concentrations of the major detected ion peaks correlated strongly with the TOM determined by DRI analyzer (R2 = 0.8578). Among the detected ions, phthalic acid and glutaric acid were identified based on the fragmentation pattern obtained from the mass spectra of the authentic substances. Accordingly, it was estimated that phthalic acid and glutaric acid contributed 7.0 and 9.4 %, respectively, to the TOM. The molecular fingerprints of anthropogenic tracers obtained in this urban site were deemed valuable as they can be used to account for the transport of pollutants to a biogenic site.
The TD-PTR-TOF-MS was then deployed at a subtropical forestry station, located in the Xitou Experimental Forest of National Taiwan University in central Taiwan, to probe the composition of organic aerosols (OA). The comprehensive identification of the major extracted peaks and the subsequent source apportionment using Positive Matrix Factorization (PMF) denoted three prominent source clusters of OA mass in Xitou forest. These include urban (30%), biogenic (7%) and biomass burning (7%). Urban and biomass burning tracers such as phthalic acid, levoglucosan, and succinic acid dominated the overall organic mass concentration which clearly indicated the impact of anthropogenic activities in the forest. Moreover, the presence of unnatural pollutants in the forest possibly inhibited the growth of particles through the scavenging of the nuclei particles. The elevated mixing ratio of the tracers observed in this study also hinted the role of the relative humidity (over 90% during the campaign) in the formation and partitioning of these compounds into the particle phase, thereby directly altering the fraction of the organic matter in the submicron particles.
In addition to the typical anthropogenic organic markers, the total mass concentration and molecular identity of specific organonitrates were analyzed using the TD-PTR-TOF-MS. Organonitrates (ONs) are the oxidation products of the volatile organic compounds in the presence of NOx. The peak of NO2+ (m/z 45.992) served as the surrogate of the sum of mixing ratio of organonitrates where isopropyl nitrate (IPN) was utilized to account the degree of fragmentation of ONs to NO2+. The results showed a linear response of TD-PTR-TOF-MS against a wide range of mass loading of ONs and 20% of the IPN dissociates to form NO2+. Analysis of the submicron particles from Xitou forest indicated that the organonitrates only accounted for 4% of the total organic matter, probably underestimated by 5 folds or more. Such inference was justified by the elevated relative humidity (> 90%) that caused the dissociation of ONs in the particle phase. The relationship of organonitrates to some environmental components revealed the impact of ONs in some of the crucial atmospheric processes in the forest. The association of organonitrates to the number concentration of nuclei mode particles (N100 nm) hinted the contribution of ONs to aerosol growth. Furthermore, an inferred branching ratio of 5.41% using the true mass concentration of organonitrates indicated that the ONs were a significant sink of NOx and suppressed the formation of ozone and organic oxidation products in Xitou forest through chain termination of the HOx cycle.
Overall, this study revealed that anthropogenic tracers (urban + biomass burning + organonitrate) contributed a significant fraction (≈59%) of the OA burden in a subtropical forestry area, clearly indicating the strong influence of human-related activities. The characterization of the organic markers in aerosols using the TD-PTR-TOF-MS not only uncover the sources and formation of the secondary organic aerosols but also provided insights on perturbation of anthropogenic pollutants on critical atmospheric phenomena in a biogenic environment. |
