台北陽明山區與嘉義鄉間酸沉降之化學特性探討

Abstract

本研究在台北陽明山區與嘉義民雄鄉同步收集雨水標本，採樣時間自2006年1月至2007年12月為期兩年，共計307個雨水標本。分析項目包括pH值（酸鹼計）、導電度（導電度計）及主要離子成分（離子層析儀），並探討酸沉降之化學特性。 水化學之時空特性顯示，陽明山降雨之酸鹼值加權平均為4.38，水質偏酸，酸雨發生率為94%，呈全面性酸化現象。酸化原因為當地既有物質及火山背景所提供的致酸離子之外，尚有來自中國大陸之長程傳輸貢獻，尤其東北季風盛行時，pH值更低。季節性差異以夏、冬兩季最為明顯，夏季雨水pH值較高，各項離子濃度較低，冬季反之。離子濃度貢獻比依次為Na+＞SO42-＞Cl-＞Ca2+＞Mg2+＞NH4+＞NO3-＞K+。硫酸根與硝酸根為主要致酸物質。嘉義地區降雨酸鹼值之加權平均為5.11，水質較北部山區為鹼，酸沉降發生率為41%。高酸鹼度由農產活動及天然鹼性塵土所提供，當地降水品質受自然來源影響多過於人為活動。夏、秋兩季雨水pH值較低，冬季較高。離子濃度貢獻比依次為NH4+＞Cl-＞K+＞SO42-＞Ca2+＞Na+＞Mg2+＞NO3-。海水氯鈉比值分析結果顯示，陽明山樣區降水有氯虧損現象，約佔47%。 降水離子來源，陽明山降水普遍受海鹽影響，其次為當地火山背景、人為活動及境外傳輸；嘉義降水主要受農耕活動影響，其次是塵土或海鹽等。此外，SO42-在各因子間呈中度相關，顯示長程輸送雖在其他強勢因子中並不突顯，但仍相當重要。在沈降量變化方面，陽明山之酸沉降高於嘉義2－6倍，且硫酸根比硝酸根沉降量多，主要是受離子濃度影響，其次為雨量。天氣類型方面，發生酸沉降多屬東北季風、沙塵暴或鋒面系統，降水受境外傳輸影響遠大於當地。 比較其他地區降水，SO42-、NO3-與人為污染相關；Na+、Cl-、Mg2+與海鹽相關；NH4+、K+與農業活動有關；Ca2+、K+、Mg2+則與當地塵土相關；Cl-、SO42-與火山作用相關。

Wet-only rainwater samples (N = 307) were collected from January 2006 to December 2007 in the last two years at the Yangmingshan site in Taipei and the rural site in Chiayi. The chemical composition of the rainwater was analyzed for pH, conductivity, and major ions by using the pH meter, conductivity meter and Ion Chromatography, respectively. The analysis results will be used to discuss the chemical characteristics of the acid precipitation at these two experiment sites. The pH value from Yangmingshan experiment site revealed the rainwater is acidic with a volume-weighted mean pH of 4.38. The incidence of acidic deposition is 94% and reveals comprehensively acidification phenomenon. The acidification was caused by the input from local materials and volcanic activities at Tatun volcanoes or the long-range transmissions from the Mainland China. During the northeast monsoon prevailed period, the pH values were lower than other seasons. The seasonal variability of chemical characteristics shows a distinct difference between summer and winter. Despite the higher pH values in the summer, most of the ions were higher in the winter. The concentration of ions follows a general pattern as Na+> SO42-> Cl-> Ca2+> Mg2+> NH4+> NO3-> K+. The SO42- and NO3- are the major acidification factors. Corresponsively, the rainwater samples collected from Chiayi were more alkaline with a volume-weighted mean pH of 5.11. The incidence of acidic deposition is 41%. The higher values of pH were attributed to the neutralization by agricultural activities and natural alkaline local dusts. The quality of rainwater was largely affected by the natural sources than the anthropogenic activities. The seasonal variability of pH values was lower in the summer and autumn, but higher in the winter. The equivalent concentration of components followed the order: NH4+> Cl-> K+> SO42-> Ca2+> Na+> Mg2+> NO3-. Furthermore, the Cl-/Na+ ratios point to the chlorine loss was a universal phenomenon at Yangmingshan experiment sites, about 47%. As for the sources of ions, Yangmingshan is largely influenced by the oceanic source; the local volcanic activities, anthropogenic sources and the long-range transmissions are also playing important roles as ion providers. By contrast, the agricultural activities, natural local dust and sea salt may be more important for Chiayi experiment site. Additionally, the long-range transmissions are the most important source for SO42- at both experiment sites. The deposition fluxes of SO42- and NO3- at Yangmingshan site were 6 and 2-fold higher than Chiayi site, respectively. The concentration of the ions is the major factor affects the deposition fluxes of SO42- and NO3-, and the rainfall plays less important role. According to the weather pattern, most of the acidic deposition occurred during the northeast monsoon and Asian dust storm prevailed season or when the frontal systems paced around Taiwan. It implies that the quality of rainwater is deeply affected by the long-range transmissions than the local inputs. Several source-types of ions have been identified through comparing the results with the observations from other regions. It appears the SO42- and NO3- are related to the anthropogenic pollution; Na+, Cl- and Mg2+ have good correlation with sea salt spray source; NH4+ and K+ are originated from the agricultural activities; Ca2+, K+ and Mg2+ are associated with the local natural dust; and the volcanic degassing process may influence the flux of Cl- and SO42- at Yangmingshan experiment site.