| 摘要: | 本研究針對台南都會及沿海地區2002年1月及4月代表冬春兩季之PM2.5氣膠微粒水溶性離子組成、含水率及酸鹼性，進行兩地冬春季氣膠不同特性、組成及其時空變異探討。
氣膠樣品在相對濕度40±5 %的調理下，台南都會與沿海地區冬春兩季PM2.5氣膠微粒水溶性離子主要成份均為SO42-、NO3-、NH4+，在冬春兩季三物種合佔都會地區PM2.5質量濃度的44.6 %及36.1 %，沿海地區則分別為44.6 %、38.0 %，顯示兩地之冬季二次氣膠組成對PM2.5的貢獻均較春季為高。以PM2.5NR(Neutralization ratio)值計算，在都會地區冬季之PM2.5氣膠呈中性；而春季之PM2.5氣膠呈鹼性。沿海地區酸鹼特性與都會地區氣膠相似，冬季PM2.5 氣膠呈中性，而春季呈鹼性。其主因乃由於台南沿海地區以養殖漁業為主，魚池內因高密度的養殖及大量曝氣，氨氮濃度及總氮量會隨魚體排泄物增加而增高。由都會及沿海兩地之SOR(硫氧化比率)及NOR(氮氧化比率)發現，SOR高於0.4及NOR高於0.15時，所代表光化反應相對強烈，其天氣狀態為相對溼度低且當時之O3濃度較SOR及NOR低時為高。此外，SOR高時風速強烈，但NOR高時其風速較低，顯示硫物種藉較遠處傳輸氧化轉化成氣膠硫酸鹽，但氮物種由氣態NO2轉化成氣膠NO3-，多為當地之交通污染滯留轉化。
此外，本研究成功建立DRIFT光譜之氣膠微粒官能基負荷濃度校正，開發一套氣膠SO42-、NO3-、NH4+含量快速且不具有破壞性之分析方法。由DRIFT光譜顯示SO42-吸收波數隨著SO32-增減而改變，間接證實SO42-由SO2變成SO32-後再轉化為SO42-，此外，由DRIFT的快速量測顯示南台灣二次氣膠其結合形態以(NH4)2SO4、NH4NO3、NH4HSO4為主。
台南都會及沿海之PM2.5氣膠含水率，在相對濕度60±2 % 控制下，其沿海地區日夜PM2.5氣膠平均含水率分別為31.3±8.4 %、38.0±7.2 %，都會地區日夜PM2.5氣膠平均含水率分別為25.7±16.9 %、24.3±21.5 %，顯示冬春兩季沿海地區PM2.5氣膠含水率均明顯高於都會地區。而以日夜PM2.5氣膠含水率比較，除冬季日間都會氣膠含水率略高於夜間外，其沿海冬春兩季及都會春季都會之氣膠含水率均以夜晚較高。以控制於相對濕度60±2 %之調理環境測得氣膠含水量，其結果與Lee et al. (1998)的降濕模式所計算之含水量值尚有明顯差異，應與未量測之水溶性有機碳的吸濕特性未能加以計算有關。
The chemical composition, water content, and acidity of atmospheric PM2.5 aerosols in a Tainan urban and coastal area were evaluated in January (winter) and April (spring) of 2002.
Controlling relative humidity (RH) at 40±5 %, it was found that NH4+, SO4-2 and NO3- were the dominant water-soluble ionic species in both winter and spring. These accounted for an average 44.6% and 36.1% of PM2.5 mass at the urban site in winter and spring, respectively, and 44.6% and 38% at the coastal site, demonstrating that secondary aerosols were a larger part of the PM2.5 mass in winter at both sites. The average neutralization ratio (NR) value of PM2.5 in the Tainan urban area was neutral (NR is 1.0±0.2) in winter, and alkaline (NR is 2.5±0.4) in spring. The average value in the Tainan coastal area was likewise neutral in winter and alkaline in spring. The reason for this is that Tainan coastal areas are primarily agricultural and aquafarm areas with high-density cultivation and high aeration, and hence high levels of ammonium nitrate and total nitrogen, and aquatic animal wastes. It was found that a SOR (sulfur oxidation ratio) value of >0.4 and a NOR (nitrogen oxidation ratio) value of >0.15 represented a relatively strong photochemical reaction, low relative humidity, and high O3 concentration. Additionally, strong wind speeds led to a high SOR value, but weak wind speeds led to a high NOR value. This demonstrates that the gaseous S species (SO2) that form sulfate aerosols are transported from far away, whereas the N species that form gaseous NO2 and subsequently NO3- are of local origin (due to local traffic pollution emissions).
This study also succeeded in establishing DRIFT spectra for calibration of aerosol functional group loading and a quick, nondestructive method to determine NH4+, SO4-2 and NO3- content in PM2.5 aerosols. The DRIFT spectrum showed SO4-2 absorptive wavelength changes followed by SO3-2 changes, and indirectly proves that SO2 converts to SO3-2, which then converts to SO4-2. It also shows that the main associated types of secondary aerosols in southern Taiwan are (NH4)2SO4, NH4NO3 and NH4Cl.
The water content (mass concentration) of atmospheric PM2.5 aerosols controlled at RH 60±2% was 31.3±8.4% and 38.0±7.2% at the coastal site in the daytime and nighttime, respectively, and 25.76±16.9% and 24.3±21.5% at the urban site. The water content was clearly higher in the coastal area in both winter and spring. Apart from urban winter, when daytime water content was slightly higher than nighttime, nighttime values were higher than daytime. Controlling RH at 60±2%, the measured water content was clearly higher than and different from values calculated using the water ascending model (Lee et al., 1998). It may be that the water absorptive properties of soluble organic carbons in aerosols are not taken into consideration in this model. |
