季風與大河主導之東海表水溶解態元素汞時空變化探討：控制因子、收支與環境蘊含

Elucidating Spatiotemporal Variations in Dissolved Elemental Mercury in the River-Dominated and Monsoon-Influenced East China Sea: Drivers, Budgets and Environmental Implications

陳奕勝(Yi-Sheng Chen)

指導教授 ： 曾鈞懋

國立臺灣大學/理學院/海洋研究所/碩士(2019年)

https://doi.org/10.6342/NTU201903380

摘要

溶解態元素汞(dissolved elemental mercury, DEM)及其海氣交換通量在河流及季風主導的東海表層海水中呈現明顯的時空分布差異。空間分布上，DEM濃度在近岸之長江沖淡水中濃度較高(90±20 – 260±40 fM)，在遠岸之黑潮水中濃度較低(60±10 – 160±40 fM)，可能反映由長江輸入汞的影響。此外，在夏季季風盛行期間沿岸水區域內可能因湧升流而使DEM濃度呈現明顯的高值(290±20 – 320±70 fM)。整體平均DEM濃度不僅受各水團之濃度影響，也與受到長江流量及季風方向、強度所控制之各水團組成比例有關。季節分布上，在各個水團中的平均DEM濃度皆在夏季航次呈現明顯的高值(120±30 –320±70 fM)且大致與各水團平均表水溫呈正相關。此平均DEM濃度季節分布趨勢可能與能夠同步影響各水團的因子有關，如表水溫、日照量以及風速。在夏季受到較高表水溫及日照而增強的DEM現場生成速率加上低風速導致的低氣體交換速率將使得DEM濃度提升。反之在冷季低DEM現場生成速率以及高氣體交換速率將導致較低的DEM濃度。此外，DEM生成速率亦與表水溫呈良好的正相關，然而其與表水溫的斜率在遠岸水團是近岸水團之兩倍。東海元素汞之海氣交換通量主要受DEM濃度及表水溫所控制，因此同樣呈現夏季高逸散通量(670±380 pmol m-2 d-1)、冬季DEM與大氣氣態元素汞(Gaseous Elemental Mercury, GEM)之濃度接近平衡(海氣交換通量20±60 pmol m-2 d-1)之季節分布差異。東海全年元素汞之逸散量約為45 kmol yr-1，相較東亞區域大氣汞沉降通量，東海全年而言為大氣元素汞的淨匯(net sink)，唯獨在夏季受到高逸散通量所致而為大氣元素汞之淨源(net source)。在全球海洋汞循環中，東海之汞年逸散量呈現不等比例之貢獻，以僅約0.1%之海洋表面積貢獻全球海洋元素汞逸散量之0.3%。而東海在汞海氣循環中的角色及貢獻可能會因表水溫升高或長江流量增加等環境變遷而改變。

關鍵字

汞循環 ； 元素汞 ； 海氣交換 ； 現場生成 ； 東海 ； 長江 ； 邊緣海

並列摘要

Distinct spatiotemporal distributions of sea surface dissolved elemental mercury (DEM) and its air-sea exchange flux were observed in the river-dominated and monsoon-influenced East China Sea (ECS). Spatially, DEM concentrations were higher in the nearshore Changjiang Diluted Water (CDW) (90±20 – 260±40 fM) and Shelf Mixing Water (SMW) (70±20 – 170±10 fM) than in the offshore Kuroshio Water (KW) (60±10 – 160±40 fM), potentially reflecting riverine Hg inputs from the Changjiang. During the monsoon-driven coastal upwelling in summer, Coastal Water (CW) also featured elevated DEM (290±20 – 320±70 fM). Seasonally, cruise-mean DEM concentrations in all water masses were elevated in summer (120±30 –320±70 fM) and were positively correlated with sea surface temperature (SST). Such a pattern could be attributed to factors that synchronously affect each water mass such as SST, insolation and wind speed. Higher SST or insolation that enhances DEM production and lower gas exchange velocities due to lower wind speed during the summer monsoon would increase DEM concentrations, while low production rates along with higher gas exchange velocities would lead to lower DEM concentrations during the winter monsoon. Moreover, the first-order DEM production rates were positively related with SST but the SST relationships varied between the nearshore and the offshore water masses, which likely resulted from differences in dissolved organic matter concentration or light penetration. A similar seasonal pattern in air-sea exchange flux of DEM was found in which strong evasion fluxes occurred in summer (670±380 pmol m-2 d-1), while in winter DEM was close to being in equilibrium with gaseous elemental mercury in the atmosphere (20±60 pmol m-2 d-1). Annually, ca. 45 kmol yr-1 of Hg was emitted to the atmosphere, making the ECS a net sink of Hg annually, but a source in summer compared to the regional Hg deposition. Overall, the ECS poses a disproportionate Hg0 evasion to the atmosphere, accounting for ~0.3% of ocean Hg evasion in ~0.1% sea surface area.

並列關鍵字

Mercury cycling ； Elemental mercury ； Air-sea exchange ； In-situ production ； East China Sea ； Changjiang ； Marginal Sea

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