| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 96 === 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.
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