| 摘要: | 氣膠吸濕特性為大氣氣膠的基本特性之一，因為大氣氣膠吸水導致含水量的改變將影響到氣膠沉降特性、粒徑分布、光學特性及化學反應。本文使用一套能量測收集在濾紙上氣膠的含水量量測系統(Chang and Lee, 2002 ; Lee and Chang, 2002)，量測在鄉村(台北縣石門鄉)、都市(新莊微粒超級測站)、高山(鹿林山)三個地點採集的PM2.5細氣膠含水量，提供在高濕度環境(相對濕度90%)的大氣氣膠含水量。三個地點採樣期間為2006年3月份，鄉村與都市測站採樣期間有受到沙塵影響，高山測站則受到生質燃燒影響。 對於氣膠含水量因子的探討，本文分析氣膠水溶性離子、二元酸(dicarboxylic acid)、有機碳、元素碳，並開發出氣膠水可溶有機碳(water-soluble organic carbon, WSOC)分析方法。本文藉由ISORROPIA模式(Nenes et al., 1998)模擬出無機氣膠含水量，比較實測氣膠含水量，並以分析出的氣膠有機成分，探討氣膠化學成分對大氣氣膠含水量的影響。 實場觀測結果顯示鄉村大氣氣膠中水可溶有機碳平均濃度為2.52 μgC m-3，水可溶有機碳佔PM2.5質量約7.3±1.8%；高山生質燃燒氣膠中水可溶有機碳平均濃度為4.38 μgC m-3，水可溶有機碳佔PM2.5質量約24.0±5.7 %；都會區大氣氣膠中水可溶有機碳平均濃度為2.75 μgC m-3，水可溶有機碳佔PM2.5質量約9.1± 2.8%。石門/鹿林山/新莊三個地點氣膠WSOC/ (Sulfate+Nitrate)含量百分比，在石門平均為14 %；鹿林山約為120 %；新莊約為30 %。三個測站氣膠低分子量二元酸的物種分布，以oxalic acid為最多，其次為malonic acid與succinic acid，而glutaric acid含量最少。 在探討高氣膠含水量釋放行為(effluorescence)以及氣膠含水量，當WSOC/ (Sulfate+Nitrate)混合氣膠中的水可溶有機碳含量越多則氣膠遲滯現象會越不明顯，鹿林山具有高比例的有機碳使得鹿林山生質燃燒氣膠的遲滯現象不明顯。本文控制環境溫度於25℃、相對溼度於30%，發現氣膠仍含有水量，氣膠AMC範圍為1.00~1.41。因此，採樣後濾紙秤重時，須將整個環境絕對濕度控制得更低，否則氣膠含水量會造成質量濃度的高估。石門/鹿林山/新莊大氣有機/無機混合氣膠含水量與ISORROPIA推估無機水溶性氣膠含水量平均值相差皆在4 μg m-3內，表示大氣氣膠主要是以無機水溶性氣膠去吸附水量，無機水溶性氣膠組成以硫酸鹽與銨鹽含量最高，ISORROPIA模擬顯示氣膠硫酸銨含量高則氣膠含水量就越高。大氣氣膠若含有高比例的WSOC會在低相對濕度下幫助氣膠吸水，並造成氣膠在相對濕度20%時，仍無法完全脫附水分成為乾氣膠。 Aerosol hygroscopicity is one of the most fundamental properties of atmospheric aerosols. The variations of aerosol mass change (AMC) due to water uptake will affect aerosol deposition characteristics, size distribution, optical property, and heterogeneous chemical reactions. In this study, PM2.5 at rural (Shi-Men in Taipei County), urban (Hsin-Chuang Aerosol Supersite), and mountain (Lu-Lin Mountain) sites are collected for measuring aerosol water content at high humid (90% RH) by using an aerosol water mass measuring system (Chang and Lee, 2002 ; Lee and Chang, 2002). The rural and urban samples were affected by Yellow dust transported from Asian Continent, while mountain samples was influenced by transported biomass burning from Southeast Asia during the field trip conducted in March 2006. To investigate aerosol properties in affecting AMC, this study analyzes water-soluble inorganic ions, dicarboxylic acids, organic and elemental carbons, and water-soluble organic carbon (WSOC) in the collected aerosols. The ISORROPIA model (Nenes et al., 1998) is then utilized to simulate AMC of water-soluble inorganic ions for the comparison with the measured AMC. This will enable the study on AMC from the influence of aerosol organic components. The averages of WSOC and its fractions in in PM2.5 in the field observation are 2.52 μgC m-3 and 7.3±1.8% for the rural site, 4.38 μgC m-3 and 24.0±5.7% for the mountain site, 2.75 μgC m-3 and 9.1±2.8% for the urban site, respectively. For the investigation of the influence of aerosol organic components on AMC, the ratios of WSOC/(Sulfate+Nitrate) are calculated to result in 14% at rural site, 120% at mountain site, and 30% at urban site, respectively. The analyzed concentrations of low-molecular dicarboxylic acids in abundance are in the descending order of oxalic acid, malonic acid, succinic acid, and glutaric acid. In the study of aerosol behavior in the efflorescence mode and AWC, the highest mixing ratio of WSOC/(Sulfate +Nitrate) at the mountain site shows an inhibition of aerosol hysteresis in the efflorescence mode. For an environment controlled at 25℃ and 30% RH, AMC is found ranging from 1.00-1.41 in this study. It suggests that filter-based aerosol mass determined from weighing may be overestimated unless the absolute humidity of the weighing room is tightly controlled. As the differences of measured and modeled AWC using ISORROPIA for all three sites are within ±4 μg m-3, it indicates inorganic species are mainly responsible for AMC. Among aerosol inorganic water-soluble ions, sulfate and ammonium ions are the highest anion and cation, respectively. The modeled AMC from ISORROPIA shows that concentrations of sulfate and ammonium ions are positively related to AMC. High fraction of WSOC in aerosol tends to help increase aerosol water uptake under low RH envieonment in the deliquescence mode. This high fraction of WSOC in aerosol also inhibits complete water release even in the 20% RH envieonment in the efflorescence mode. |
