长白山和哀牢山偏远山地森林系统与大气间汞的交换通量研究

Recognition of the global dimensions of the human-caused mercury pollution in the 1970's has set in motion decades of research and policy development that have culminated last year in the UN's legally binding Minimata Convention. The treaty will restrain the rate of increases in Hg use and emissions to the atmosphere in the hopes of reducing Hg accumulation in biota and therefore human exposure to this neurotoxic contaminant. East and Southeast Asia are currently responsible for about 40% of global anthropogenic emissions. About 75% of the mercury from this region comes from China, which is about one-third of the global total. Global climate change may complicate the response of global ecosystems to mercury emission reductions, through its profound effects on many aspects of the movement and chemical transformations of mercury in the environment. Since Hg can exist as a gas (primarily Hg0) though, the balance of Hg deposition and re-emission between land and atmosphere influences the rate at which Hg emission controls will influence Hg in soils, water and ultimately the biota. A key barrier to defining this balance is the precision, temporal resolution and spatial scale with which land-atmosphere Hg0 exchange (flux) is measured. Ecosystem-level flux sampling approaches - i. e. micro-meteorological (MM) techniques already implemented for a broad range of trace gases should also be utilized for Hg0, which has previously received very little attention in Hg research community. SKLEG has taken the lead in China to advance experimental field techniques to measure such Hg0 fluxes. The exchange of Hg between plants and the atmosphere is an important process of the biogeochemical cycle of Hg, particularly as canopy processes enhance atmospheric deposition to terrestrial ecosystems. Here, we propose to use relaxed eddy accumulation-technique in conjunction with continuous in-canopy gradient measurements to for the first time quantify net ecosystem Hg0 vapor exchange for forest ecosystems and address the fine-scale source/sink distribution of Hg0 in the canopy of two contrasting remote upland forests of China. The proposed work will be a long-term study with a high time-resolution (40 min) covering a broad seasonal record in an old-growth subtropical evergreen forest (Mt. Ailao) and the period of spectacular surface layer airborne mercury depletion episodes (June - August) in a deciduous broad-leaf forest (Mt. Changbai).

汞污染可在全球传输，2013年签署的水俣公约旨在控制汞排放以减少汞在生物体积累及暴露风险。东亚和东南亚地区人为源汞排放占全球份额的40%，其中我国排放占全球年排放总量的1/3。汞的化学转化及迁移受气候变化影响，生态系统对汞减排的响应可能更为复杂。大气中汞主要是单质态，汞减排将直接影响地表和大气间汞沉降和释放的平衡进而影响汞进入土壤、水体和生物体。精确、高时空分辨率的观测地表与大气间Hg0的交换通量是定量这一过程的关键。生态系统尺度通量观测的微气象方法已广泛应用于多种微量气体，而在Hg0通量上的研究还较少。植被加强了大气汞的沉降，植物与大气间汞交换的过程在汞地球化学循环中尤为重要。本研究拟在我国长白山落叶阔叶林和哀牢山亚热带常绿林森林系统采用弛豫涡旋积累技术结合林冠内梯度的同步测定开展长期、分季节、高分辨率（40min）的观测，以定量偏远山地森林系统汞交换通量和厘清林冠内Hg0的动力学过程。

本研究工作主要内容分为云南哀牢山亚热带森林生态系统和吉林长白山温带森林生态系统观测测量，根据两个研究点实验条件因地制宜，长白山森林系统利用梯度法测量森林通量，云南哀牢山森林系统利用弛豫涡旋法测量通量，并利用气象仪器测量相关气象参数。改进了微气象法中弛豫涡旋法与梯度法，更加精准测量大气汞浓度及其变化，提高了研究方法的运用前景。首次长时间高分辨率测量不同森林系统多高度处大气汞浓度，建立了高精准的森林大气汞廓线。首次利用弛豫涡旋法大时间尺度高时间分辨率测量亚热带偏远背景区森林生态系统与大气之间交换通量。首次精准量化了森林系统干沉降，完善了全球汞生物地球化学循环，对汞的迁移转换有了更加清晰地认识，对估算汞的自然源排放量具有重要意义。首次对比了不同地域森林生态系统通量变化与气候变化之间的关系。证明我国大气汞浓度近年来有所降低，指示在汞地球化学循环过程中与大气作用的净通量过程减弱，说明我国具有较强得履约能力。