Insights from mercury stable isotopes on terrestrial-atmosphere exchange of Hg(0) in the Arctic tundra
Insights from mercury stable isotopes on terrestrial-atmosphere exchange of Hg(0) in the Arctic tundra
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DOI：
10.5194/bg-16-4051-2019
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发表时间：
2019-10-18
期刊：
影响因子：
4.9
通讯作者：
Obrist, Daniel
中科院分区：
文献类型：
--
作者：
Jiskra, Martin;Sonke, Jeroen E.;Obrist, Daniel
冻土带通过储存大气中的汞沉积并将其运送到北冰洋，在北极汞（Hg）循环中起着关键作用。最近的一项研究表明，70%的大气汞沉积到冻土带是通过植被和土壤对气态元素汞（GEM或Hg(0)）的吸收发生的。控制北极冻土带陆地-大气Hg(0)交换的过程是核心，但仍未得到充分研究。本文将大气、间隙雪气和土壤孔隙气中Hg(0)的稳定同位素分析与美国阿拉斯加州北部Toolik野外站冻土带生态系统中Hg(0)通量的测量相结合。在冬季黑暗月份，行星边界层（PBL）条件和Hg(0)浓度全天基本稳定，出现小Hg(0)净沉降。在春季，卤素引起的大气汞耗竭事件（AMDEs）发生，AMDEs之后Hg(0)的快速再排放导致Hg(0)的净排放通量。在夏季短暂的无雪生长期，植被对大气Hg(0)的吸收增强了向北极苔原的大气Hg(0)净沉降。在夜间，当PBL条件稳定时，生态系统对大气Hg(0)的吸收导致大气Hg(0)的耗竭。夜间大气Hg(0)的下降伴随着大气Hg库中较轻的Hg(0)同位素的耗竭。富集因子epsilon Hg-202（植被）（吸收率）= -4.2(+/- 1.0)，与植被优先吸收轻Hg(0)同位素一致。Hg(0)通量测量表明，Hg(0)在太阳辐射最强的白天部分再发射。土壤孔隙空气中Hg(0)浓度相对于大气Hg(0)浓度下降，同时土壤孔隙空气中较轻的Hg(0)同位素富集，epsilon Hg-202（土壤空气-大气）= -1.00（+/- 0.25）‰，（EHgsoil空气-大气）-Hg-199 = 0.07（+/- 0.04）‰。这些土壤孔隙空气中Hg(0)的首次Hg稳定同位素测量结果与之前在天然腐植酸氧化Hg(0)过程中观察到的分异相一致，表明非生物氧化是观测到的土壤Hg(0)吸收的原因。Hg稳定同位素指纹图谱与Hg(0)通量测量和PBL稳定性评估相结合，证实了植被对Hg(0)的吸收在北极冻土带Hg(0)的陆-气交换中起主导作用。
The tundra plays a pivotal role in the Arctic mercury (Hg) cycle by storing atmospheric Hg deposition and shuttling it to the Arctic Ocean. A recent study revealed that 70% of the atmospheric Hg deposition to the tundra occurs through gaseous elemental mercury (GEM or Hg(0)) uptake by vegetation and soils. Processes controlling land-atmosphere exchange of Hg(0) in the Arctic tundra are central, but remain understudied. Here, we combine Hg stable isotope analysis of Hg(0) in the atmosphere, interstitial snow air, and soil pore air, with Hg(0) flux measurements in a tundra ecosystem at Toolik Field Station in northern Alaska (USA). In the dark winter months, planetary boundary layer (PBL) conditions and Hg(0) concentrations were generally stable throughout the day and small Hg(0) net deposition occurred. In spring, halogen-induced atmospheric mercury depletion events (AMDEs) occurred, with the fast re-emission of Hg(0) after AMDEs resulting in net emission fluxes of Hg(0). During the short snow-free growing season in summer, vegetation uptake of atmospheric Hg(0) enhanced atmospheric Hg(0) net deposition to the Arctic tundra. At night, when PBL conditions were stable, ecosystem uptake of atmospheric Hg(0) led to a depletion of atmospheric Hg(0). The night-time decline of atmospheric Hg(0) was concomitant with a depletion of lighter Hg(0) isotopes in the atmospheric Hg pool. The enrichment factor, epsilon Hg-202(vegetation) (uptake) = -4.2 parts per thousand (+/- 1.0 parts per thousand) was consistent with the preferential uptake of light Hg(0) isotopes by vegetation. Hg(0) flux measurements indicated a partial re-emission of Hg(0) during daytime, when solar radiation was strongest. Hg(0) concentrations in soil pore air were depleted relative to atmospheric Hg(0) concentrations, concomitant with an enrichment of lighter Hg(0) isotopes in the soil pore air, epsilon Hg-202(soil air-atmosphere) = -1.00 parts per thousand (+/- 0.25 parts per thousand) and (EHgsoil air-atmosphere)-Hg-199 = 0.07 parts per thousand (+/- 0.04 parts per thousand). These first Hg stable isotope measurements of Hg(0) in soil pore air are consistent with the fractionation previously observed during Hg(0) oxidation by natural humic acids, suggesting abiotic oxidation as a cause for observed soil Hg(0) uptake. The combination of Hg stable isotope fingerprints with Hg(0) flux measurements and PBL stability assessment confirmed a dominant role of Hg(0) uptake by vegetation in the terrestrial-atmosphere exchange of Hg(0) in the Arctic tundra.