Collaborative Research: Magnitude and Pathway of Gaseous Atmospheric Mercury Deposition in Forests

汞(Hg)既来自自然来源，也来自人类活动。该项目的目标是在两个不同的森林地点对全年的汞进行测量。这个丰富的数据集将被用来更好地了解汞如何在树木、土壤和大气中移动。了解汞的分布很重要，因为它是一种有毒物质，会对人类的神经造成负面影响，并与其他大气气体发生反应。该项目的知识将有助于评估汞的全球分布，从而实现国际公认的减少汞污染的目标。此外，该项目每年将为一名博士后研究人员、两名研究生和一名本科生提供部分支持。大气汞沉积通过各种干湿(如降雨)沉积过程，包括植物吸收气态汞的过程。当植物死亡或落叶时，植被中的汞被转移到土壤中，贡献了土壤中54-94%的汞。在没有直接测量的情况下，气态元素汞(Hg[0])的沉降量是从凋落物和被动膜中推断出来的，但它们不是净汞(0)沉降量的理想指标，因为它们不能考虑再排放。因此，该项目的第一个目标是量化两种不同季节的森林--落叶温带雨林和常绿亚热带雨林--中汞(0)净气体干沉降量(总沉降量减去排放量之和)的大小和时间动态。将使用大型塔上的微气象测量来测量汞(0)净沉降量，这是直接、非侵入性和延长时间测量生态系统一级包括所有下层汇和源的净汞(0)交换的唯一可用的方法。第二个目标是通过部署两个相应的通量系统将汞(0)通量分配到树冠和土壤贡献中--一个在林冠上方测量生态系统级的汞(0)交换，另一个在树冠下方量化土壤贡献。冠层汞(0)通量将按差值计算。通量分配将提供全球和区域化学输送模式中限制汞(0)沉积所需的年、季节和日汞(0)汇(例如，到冠层)和源强度(例如，来自土壤)。最后，这项工作的第三个目标是通过比较汞(0)的通量与二氧化碳、臭氧、水蒸气和羰基硫化物的通量来阐明沉积的途径。所有这些痕量气体在生态系统中都有不同的汇和来源，并且它们的冠层程度、气孔、叶肉和土壤对通量的贡献也不同。通量之间的比较，包括季节性、昼夜和组成通量，将使我们能够量化汞(0)交换与光合作用活动、气孔导度、叶内酶活性、外部角质层吸收和土壤交换的耦合程度。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Mercury (Hg) is released from both natural sources and human activities. The goal of this project is to make measurements of mercury for an entire year in two contrasting forest locations. This rich dataset will be used to better understand how Hg moves through trees, soil, and the atmosphere. Understanding the distribution of mercury is important because it is a toxic substance, which can cause negative neurological effects in humans and react with other atmospheric gases. The knowledge from this project will be useful for assessing the global distribution of a mercury so that international recognized goals for reducing Hg pollution can be achieved. Additionally, the project will provide partial support for a postdoctoral researcher, two graduate students, and one undergraduate student per year.Atmospheric mercury deposits via various dry and wet (e.g., rainfall) deposition processes, including gaseous mercury uptake by plants. Mercury in vegetation is transferred to soils when plants die off or shed leaves, contributing 54-94% of mercury in soil. In the absence of direct measurements, gaseous elemental mercury (Hg[0]) deposition is inferred from litterfall and passive membranes, yet these are not ideal proxies for net Hg(0) deposition as they don?t account for re-emission. Consequently, first objective of this project is to quantify the magnitude and temporal dynamics of net gaseous dry Hg(0) deposition (sum of gross deposition minus emission) in two forests with different seasonalities, a deciduous temperate forest and an evergreen subtropical rain forest. Net Hg(0) deposition will be measured using micrometeorological measurements on large towers, the only available method for direct, non-intrusive and time-extended measurements of net Hg(0) exchange at the ecosystem level encompassing all underlying sinks and sources. The second objective is to partition Hg(0) fluxes into canopy and soil contributions via deployment of two corresponding flux systems - one above the forest canopy to measure ecosystem-level Hg(0) exchange and a second system below the canopy to quantify soil contributions. Canopy Hg(0) fluxes will be calculated by difference. Flux partitioning will provide annual, seasonal and diurnal Hg(0) sink (e.g., to canopies) and source strengths (e.g., from soils) needed to constraint Hg(0) deposition in global and regional chemical transport models. Finally, the third goal of this work is to elucidate pathways of deposition by comparing Hg(0) fluxes to those of carbon dioxide, ozone, water vapor, and carbonyl sulfide. All these trace gases have different sinks and sources in ecosystems and vary in their degree of canopy, stomatal, mesophyll and soil contributions to fluxes. Comparison among fluxes, including seasonality, diurnality and component fluxes will allow us to quantify the degree to which Hg(0) exchange is coupled to photosynthetic activity, stomatal conductance, enzymatic activity within leaves, external cuticular uptake and soil exchange.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.