大台北地區大氣汞濕沉降之時序變化

Abstract

本研究主要探討在2004/10~2005/12期間大台北地區大氣汞濕沉降通量的時序變化。以超純淨鍍鐵弗龍的自動集雨器採集雨水，共採集65個降雨事件，包括颱風、沙塵暴和梅雨。使用冷蒸汽原子螢光法測定雨水中汞金屬總量，部分樣品則測定溶解態、顆粒態以及活性態的汞。總汞濃度範圍1~65 ng/L，權重平均濃度為7±11 ng/L (n = 65)，溶解態、顆粒態以及活性態分別為4±8 ng/L (n = 23)、 3±2 ng/L (n = 30)、0.4±1.4 ng/L (n = 9)，雨水中以溶解態所佔比例最高，達63%。不論一般或是特殊氣象事件之降雨，濃度跟降雨量皆呈現負相關，表示有明顯的清除現象。從總汞濃度與沉降通量的時序變化中，發現有明顯的季節性變化，總汞濃度有冬高 (12±18 ng/L，n =18 )、夏低 (4±2 ng/L，n =22)的趨勢，冬季高值主要原因為東北季風所帶之中國陸源性及污染性物質，夏季濃度較低則因降雨量增加而稀釋減少。沉降通量高值則集中在夏、秋之際，因來自海源的颱風事件在短時內挾帶大量降雨所造成。總汞之濕沉降通量約21.7μg/m2/yr，高於東亞地區其他國家如日本，因本地降雨量較高所致。沙塵暴、梅雨以及颱風等特殊事件所帶來的雨水總汞分別為65 ng/L (n = 1)、2±3 ng/L (n = 20)、8±3 ng/L (n = 6)，以沙塵暴為最高；但其沉降通量所佔總通量比例，颱風便高達40%，梅雨次之為10%，沙塵暴僅佔0.1%。

Abstract Temporal variation of wet deposition flux of mercury (Hg) in metropolitan Taipei, Taiwan was investigated from October 2004 to December 2005. A total of 65 individual events, including typhoon, Asian dust and plum rain events, was collected by automatic Teflon-coated rain collector. Total Hg concentration was measured by Cold Vapor Atomic Fluorescence Spectrophotometry (CVAFS) for all rain samples and dissolve-, particle- and operation-defined reactive-phases Hg were measured in some selected events as well. Total Hg concentration ranged from 1~65 ng/L with a volume-weighted mean of 7±11 ng/L (n = 65). Dissolve-phase Hg averaged 4±8 ng/L (n = 23) about 63% of total Hg while particle-phase and reactive Hg were 3±2 ng/L (n = 30) and 0.4±1.4 ng/L (n = 9), respectively. A negative relationship between total Hg concentration and precipitation amount was observed in all precipitation events, signifying the scavenging of atmospheric particulates and colloids bearing Hg. A significant seasonal variation in total Hg showed the highest concentration in the winter (12±18 ng/L，n =18 ) as result of terrestrial and anthropogenic matters from mainland China during northeast monsoon and lowest in summer (4±2 ng/L，n =22) due to a dilution effect caused by a huge of precipitation. There was additionally a maximum of wet Hg deposition flux the in summer and fall mainly associated with typhoon-induced rainfalls of marine materials. The annual total Hg deposition flux was about 21.7μg/m2 positively correlation with the precipitation amount which was much higher in Japan. The highest concentration was observed in Asian dust event about 65 ng/L (n = 1) relative to those in plum rain (2±3 ng/L, n = 20) and typhoon events (8±3 ng/L , n = 6). Further, typhoon events accounted for 40% of total Hg deposition flux, plum rain for 10% and Asian dust only for 0.1%.