南海海域汞的大氣循環及海氣交換研究

Abstract

本研究主要研究南海海域汞的大氣循環及海氣交換，於2003至2007年間，共有十六次航次，搭乘海研一號至南海時序 (Southeast Asian Time-Series Station, SEATS) 測站進行採樣及分析。航次中進行現場測定，觀測大氣中元素汞(GEM, Gaseous elemental mercury, Hg0)及表水中元素汞(DEM, Dissolved elemental mercury) 濃度。在大氣汞方面，各季節平均分別3.2 ± 0.6 ng m-3、2.8 ± 0.4 ng m-3、3.6 ± 0.6 ng m-3、4.5 ± 1.5 ng m-3，其中夏季最低，冬季為最高。在水氣汞方面，各季節平均分別為70 ± 40 fM、160 ± 40 fM、120 ± 30 fM、60 ± 30 fM，呈現一個夏季高、冬季低的趨勢，與大氣汞的趨勢相反。大氣汞與水氣汞濃度的趨勢並不相同，顯示主要影響二者濃度的原因並不相同。本研究使用HYSPLYT模式追蹤當時氣團軌跡發現，冬季盛行東北季風，帶來歐亞大陸的陸緣物質及人為活動汙染物，使得大氣元素汞濃度升高。夏天盛行西南季風和季風交替期間，帶來中南半島和印度洋和西太平洋上方汙染較少的氣團，故大氣元素汞濃度較低。夏季水氣汞濃度較高，冬季則較低，此一現象同溫度變化趨勢一致。利用水氣汞濃度及表水溫度計算溫度係數Q10，結果推測此一季節性變化主要由非生物因素主導。根據海氣交換通量公式估算，汞的海氣交換通量在春天 (60 ± 160 pmol m-2 d-1)、夏天 (580 ± 120 pmol m-2 d-1) 及秋天 (730 ± 210 pmol m-2 d-1) ，由海洋逸散至大氣（源），而冬天則有一個相反的方向(-180 ± 110 pmolm-2d-1)，由大氣傳輸至海洋（匯）。冬季南海元素汞由大氣傳輸至海洋，係因較低的海表面溫度及較高的風速，造成水團的垂直混合較強，並且大氣汞濃度較高的關係，使得元素汞的海氣交換在冬季有此一特殊匯的現象，與其他低緯度海域觀測結果不同。

Atmospheric cycling and air-sea exchange of mercury in the northern South China Sea (SCS) were investigated during 16 cruises of the SEATS (Southeast Asian Time-series Study) station between 2003 and 2007. The GEM (Gaseous elemental mercury, Hg0) and DEM (Dissolved elemental mercury) were measured all together on board ship during the cruise period. The GEM concentrations in spring, summer, autumn and winter averaged ca. 3.2 ± 0.6 ng m-3, 2.8 ± 0.4, 3.6 ± 0.6, and 4.5 ± 1.5, respectively. Additionally, seasonal DEM concentrations were 70 ± 40 fM, 160 ± 40, 120 ± 30, and 60 ± 30, respectively. Unlike the annual GEM pattern, the DEM concentrations were high in summer and low in winter. The opposite temporal trends between the GEM and DEM implied different controlling factors on those concentration variations. Source tracking through backward air trajectory of The HYSPLYT model demonstrated that the GEM concentrations were strongly influenced by seasonal monsoons. In winter, the prevailing northeast monsoon with terrigenous and anthropogenic pollutants resulted in high GEM concentrations. Instead, the summer southwest monsoon and inter-monsoon with less polluted air masses from the Indochina Peninsula and Indian Ocean and west Pacific Ocean resulted in low GEM levels. The DEM concentrations were highly correlated with sea water temperature (SST), showing high in summer and low in winter. The Q10 temperature coefficient calculation of DEM suggested that a seasonal change in DEM was mostly abiologically driven. Annually, the air-sea Hg0 exchange fluxes were estimated ca. 60 ± 160 pmol m-2 d-1, 580 ± 120, and 730 ± 210 in spring, summer and autumn, respectively, indicating that the SCS could be the source of Hg0 to the atmosphere in warm seasons. In winter, the SCS acted as a sink of atmospheric Hg0 (-180 ± 110 pmol m-2 d-1) due to low SST and high wind of the year, enhanced vertical mixing and elevated atmospheric Hg0. This is the first time we observed that ocean behaves as a sink of atmospheric Hg0 in low latitude marginal seas.

Key words: South China Sea (SCS), Atmospheric cycling, Air-sea exchange, Gaseous elemental mercury (GEM) and Dissolved elemental mercury (DEM)