| Summary: | 碩士 === 國立中興大學 === 環境工程學系 === 92 === This study mainly investigated the chemical compositions and the deposition fluxes of wet precipitation in Taichung area. We also evaluated the characteristics of acid precipitation of different climate using backward trajectory of airflow and statistics analysis. The samples of the rainwater were collected at the Shalu and the Tungshin station which located in the interior areas of Taichung. We gathered totally 82 samples of the rainwater from January to December in 2003, and analyzed pH and ionic concentrations of Cl-, NO3-, SO4=, Na+, NH4+, K+, Mg2+ and Ca2+.
The annual rainfall of Taichung was around 722 mm. It was only 30%~50% of those obtained in the previous years. The average volume-weighted pH value was 4.7. The major anions contributing to the acidity of rainwater were SO4= and NO3-. Their average volume-weighted concentrations were 35.2 and 24.1 μeqL-1, respectively. Neutralizing cations were primarily NH4+ and Na+. Their average volume-weighted concentrations were 40.9 and 15.4 μeqL-1, respectively. The analytical result of the rainwater showing the main factors of the polluted acid rain in Taichung were inorganic secondary aerosols (NO3-, nss-SO4= and NH4+), seasalt aerosols (Cl-, Na+ and Mg2+) and activities of agriculture (K+).
As the northeast monsoon prevailed during winter season, the concentrations of nss-SO4= and ratio of nss-SO4=/NO3- in rainwater were significantly high. It indicated that Taichung area might receive foreign pollutants via a long-range transport from China. When the Pacific high dominated the region, NO3- concentrations in rainwater were significantly elevated and the ratio of nss-SO4=/NO3- was low. It indicated the prominence of acidic substances contributed by local sources. We calculated that around 21% nss-SO4= in rainwater via a long-range transport from northeast and southeast of China. Furthermore, the wet deposition fluxes of nss-SO4=, NO3-, NH4+ and Na+ were 14.0, 13.1, 6.5 and 2.2 Kgha-1yr-1, respectively.
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