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Factors controlling the photochemical degradation of methylmercury in coastal and oceanic waters

机译：控制沿海和海洋水体中甲基汞光化学降解的因素

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Many studies have recognized abiotic photochemical degradation as an important sink of methylmercury (CH3Hg) in sunlit surface waters, but the rate-controlling factors remain poorly understood. The overall objective of this study was to improve our understanding of the relative importance of photochemical reactions in the degradation of CH3Hg in surface waters across a variety of marine ecosystems by extending the range of water types studied. Experiments were conducted using surface water collected from coastal sites in Delaware, New Jersey, Connecticut, and Maine, as well as offshore sites on the New England continental shelf break, the equatorial Pacific, and the Arctic Ocean. Filtered water amended with additional CH3Hg at environmentally relevant concentrations was allowed to equilibrate with natural ligands before being exposed to natural sunlight. Water quality parameters - salinity, dissolved organic carbon, and nitrate - were measured, and specific UV absorbance was calculated as a proxy for dissolved aromatic carbon content. Degradation rate constants (0.87-1.67 day(-1)) varied by a factor of two across all water types tested despite varying characteristics, and did not correlate with initial CH3Hg concentrations or other environmental parameters. The rate constants in terms of cumulative photon flux values were comparable to, but at the high end of, the range of values reported in other studies. Further experiments investigating the controlling parameters of the reaction observed little effect of nitrate and chloride, and potential for bromide involvement. The HydroLight radiative transfer model was used to compute solar irradiance with depth in three representative water bodies - coastal wetland, estuary, and open ocean - allowing for the determination of water column integrated rates. Methylmercury loss per year due to photodegradation was also modeled across a range of latitudes from the Arctic to the Equator in the three model water types, resulting in an estimated global demethylation rate of 25.3 Mmol yr(-1). The loss of CH3Hg was greatest in the open ocean due to increased penetration of all wavelengths, especially the UV portion of the spectrum which has a greater ability to degrade CH3Hg. Overall, this study provides additional insights and information to better constrain the importance of photochemical degradation in the cycling of CH3Hg in marine surface waters and its transport from coastal waters to the open ocean.
机译：许多研究已将非生物光化学降解视为阳光照射的地表水中甲基汞（CH3Hg）的重要汇，但速率控制因素仍然知之甚少。这项研究的总体目标是通过扩展所研究的水类型范围，来增进我们对光化学反应在各种海洋生态系统地表水中CH3Hg降解中相对重要性的理解。使用从特拉华州，新泽西州，康涅狄格州和缅因州的沿海站点以及新英格兰大陆架断裂，赤道太平洋和北冰洋的近海站点收集的地表水进行了实验。在暴露于自然阳光下之前，将过滤后的水与环境相关浓度的CH3Hg进行补充，使其与天然配体平衡。测量了水质参数-盐度，溶解的有机碳和硝酸盐-，并计算了特定的UV吸光度作为溶解的芳族碳含量的代表。尽管特性有所变化，但在所有测试的水类型中，降解速率常数（0.87-1.67天（-1））变化了两倍，并且与初始CH3Hg浓度或其他环境参数无关。就累积光子通量值而言，速率常数与其他研究报告的值范围相当，但处于较高水平。进一步研究反应控制参数的实验观察到硝酸盐和氯化物的影响很小，并且可能涉及溴化物。 HydroLight辐射传递模型用于计算三个代表性水体（沿海湿地，河口和开阔海洋）中具有深度的太阳辐照度，从而确定水柱综合速率。在三种模式水类型中，从北极到赤道的所有纬度范围内，还模拟了光降解每年造成的甲基汞损失，估计全球脱甲基率为25.3 Mmol yr（-1）。由于所有波长（尤其是光谱中具有更强降解CH3Hg能力的紫外线）的穿透性增加，在公海中CH3Hg的损失最大。总体而言，这项研究提供了更多的见识和信息，以更好地限制光化学降解在海洋表层水中CH3Hg循环以及其从沿海水域到远洋的运输中的重要性。

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