| 摘要: | 每年春天中南半島及亞洲大陸生質燃燒旺盛，高濃度污染物會隨盛行西風傳輸。在傳輸過程中，氣膠前驅物經由各種化學反應影響，產生二次氣膠，伴隨原生氣膠對太陽輻射進行散射和吸收，以及形成雲凝結核對氣候變遷產生重大影響。本文從2011年3月至4月，在位於西風帶下風的臺灣鹿林山大氣背景監測站(海拔高度2,862公尺)觀測大氣氣膠，目的是瞭解不同氣流來源傳輸氣膠特性動態變化，推論雲霧、超細微粒生成事件可能影響氣膠特性變化機制，並以模式模擬推估不同氣流來源氣膠pH值。 觀測期間鹿林山逆推氣流軌跡主要是生質燃燒(BB)、自由大氣、人為污染(Anthropogenic)三種類型。當受到生質燃燒影響時，CO日變化趨勢減弱，但是氣體NOy、氣膠吸光係數、氣膠散光係數，仍呈現規律日變化，代表鹿林山監測站，可能還受到本地谷風傳輸影響。透過更能代表自由大氣的夜晚數據，可以評估生質燃燒PM2.5與PM10氣膠質量濃度相對於背景大氣各增加為3.6、3.5倍；受BB類型與谷風傳輸的本地污染交互影響時，各增加為3.6、3.8倍；顯示受到生質燃燒氣流影響時，即使加上本地污染，也僅造成粗氣膠略微增加。透過即時氣膠水溶性離子層析儀(Particle-Into-Liquid-Sampler coupled to an Ion Chromatograph, PILS-IC)，顯示短時間SO42-、NO3-、NH4+與K+趨勢關係良好，加上氮的氧化比值(Nitrogen Oxidation Ratio, NOR)隨著雲霧事件發生而增大，表示BB類型氣流與谷風傳輸的本地污染整合作用，且受到異相反應或者光化學反應影響。此外，在凌晨時段容易觀測到相對溼度下降、O3濃度上升，推論是受到邊界層高度下降影響，同時也發生氣溫和氣膠Ca2+、Mg2+離子濃度上升現象，這可能是受到夜晚山風影響。發生雲霧事件時，掃描移動度微粒粒徑分布監測儀(Scanning Mobility Particle Sizer, SMPS)量測到氣膠數目濃度粒徑分布以accumulation mode為主，與生質燃燒氣膠主要成分粒徑一致，表示雲霧事件受到生質燃燒氣團傳輸影響。另一方面，超細微粒生成事件氣膠數目濃度粒徑分布呈現出accumulation mode與Aitken mode，顯示氣膠在超細微粒生成發生的核凝現象。值得提出的是PILS-IC觀測到氣膠水溶性NO2-離子，表示有光化學反應發生，因為雲霧液滴NO2可以透過光化學反應生成HNO3或HONO。在超細微粒事件日Aitken mode的尖峰粒徑約為30 nm，粒徑很小表示光化學反應可能發生在觀測站附近。本文觀測期間鹿林山超細微粒生成事件僅發生三次，顯示大多數光化學反應地點，並不在測站周邊。最後，透過AIM2模式模擬觀測期間不同氣流來源氣膠pH值，變動範圍約在0至-2之間。與文獻相較，鹿林山氣膠pH值明顯較低，推論是受到傳輸的氣膠水溶性離子以及大氣相對溼度影響。利用多元廻歸分析評估氣膠pH值影響因子，顯示SO42-、NO3-為負影響，相對濕度、NH4+為正影響。The dense pollutants from active biomass burning (BB) in Indochina and Asian mainland are transported by prevailing westerly every spring. During long-range transport, secondary aerosol produced from aerosol precursors through various chemical reactions accompanying with primary aerosol play an important role in climate change by scattering and absorbing solar radiation and/or forming cloud condensation nuclei. This study observed atmospheric aerosol under prevailing westerly at Lulin Atmospheric Background Station (LABS, 2,862 m a.s.l.) in Taiwan from March to April in 2011. The objectives are to understand dynamic variations of aerosol properties of different air masses, to infer possible mechanisms affecting variations of aerosol properties during cloud-fog and ultrafine particle formation events, and to estimate aerosol pH values of different air masses. The BB, free troposphere, and anthropogenic are the three major types of backward trajectories at LABS during the observation period. The diurnal variation of CO was weakening, while NOy, aerosol absorption and scattering coefficients were keeping regular diurnal patterns when under the influence of BB. This may indicate the influence of local valley breeze on LABS. Through more representative nighttime data, the concentrations of PM2.5 and PM10 from BB relative to background air are estimated to increase to 3.5 and 3.6 folds, respectively, while the increases are 3.6 and 3.8 folds, respectively, when adding local valley breeze transport to BB. This shows that coarse particles are slightly increased with the addition of local pollution to the transported BB plume. The well correlated short term SO42-、NO3-、NH4+, and K+ concentrations measured by Particle-Into-Liquid-Sampler coupled to an Ion Chromatograph (PILS-IC) plus higher values of Nitrogen Oxidation Ratio (NOR) in cloud-fog events implies aerosol properties not only affected by the integration of BB type of air masses with local valley breeze but also by the heterogeneous or photochemical reactions. In addition, the reduced relative humidity (RH) and increased O3 concentration during early morning is considered to be the effect of lowering boundary layer. Meanwhile, the increased concentrations of Ca2+ and Mg2+ accompanies with high ambient temperature are inferred to be caused by mountain breeze during the night. Aerosol number size spectra measured by Scanning Mobility Particle Sizer (SMPS) are dominated by accumulation mode for cloud-fog events, which is consistent with the size ranges of major chemical species of BB aerosol. This demonstrates the effect of BB plume on cloud-fog events. In contrast, aerosol number size spectra are presented with both accumulation and Aitken modes to show the occurrence of nucleation during the events of ultrafine particle formation. It is noted that NO2- is observed by PILS-IC to evidence the occurrence of photochemical reactions because HNO3 or HONO can be formed from photochemical reactions of NO2 in cloud-fog droplets. The peak diameter of Aiken mode is around 30 nm, which indicates photochemical reactions may occur near LABS because the peak size is small. During the observation period, only three events of ultrafine particle formation occurred, which implies most photochemical reaction sites are not near LABS. Finally, aerosol pH values from various air masses were modeled through AIM2 model to reveal in a range of 0 to -2. Compared with literature values, the pH range of LABS aerosol is apparently lower, which is thought due to the influence of transported water-soluble ions and atmospheric RH. Employing multiple linear-regression analysis on factors influencing aerosol pH values, SO42- and NO3- are found having negative effect, while RH and NH4+ are positive. |
