Abstract. The exchange of gaseous elemental mercury, Hg(0), between the atmosphere and terrestrial surfaces remains poorly understood mainly due to difficulties in measuring net Hg(0) fluxes on the ecosystem scale. Emerging evidence suggests foliar uptake of atmospheric Hg(0) to be a major deposition pathway to terrestrial surfaces. Here, we present a bottom-up approach to calculate Hg(0) uptake fluxes to aboveground foliage by combining foliar Hg uptake rates normalized to leaf area with species-specific leaf area indices. This bottom-up approach incorporates systematic variations in crown height and needle age. We analyzed Hg content in 583 foliage samples from six tree species at 10 European forested research sites along a latitudinal gradient from Switzerland to Northern Finland over the course of the 2018 growing season. Foliar Hg concentrations increased over time in all six tree species at all sites. We found that foliar Hg uptake rates normalized to leaf area were highest at the top of the tree crown. Foliar Hg uptake rates decreased with needle age of multi-year old conifers (spruce and pine). Average species-specific foliar Hg uptake fluxes during the 2018 growing season were 18 ± 3 µg Hg m⁻² for beech, 26 ± 5 µg Hg m⁻² for oak, 4 ± 1 µg Hg m⁻² for pine and 11 ± 1 µg Hg m⁻² for spruce. For comparison, the average Hg(II) wet deposition flux measured at 5 of the 10 research sites during the same period was 2.3 ± 0.3 µg Hg m⁻², which was four times lower than the site-averaged foliar uptake flux of 10 ± 3 µg Hg m⁻². Scaling up site-specific foliar uptake rates to the forested area of Europe resulted in a total foliar Hg uptake flux of approximately 20 ± 3 Mg during the 2018 growing season. Considering that the same flux applies to the global land area of temperate forests, we estimate a foliar Hg uptake flux of 108 ± 18 Mg. Our data indicate that foliar Hg uptake is a major deposition pathway to terrestrial surfaces in Europe. The bottom up approach provides a promising method to quantify foliar Hg uptake fluxes on an ecosystem scale.

中文翻译：

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自下而上量化整个欧洲的叶面汞吸收通量

摘要。大气和陆地表面之间气态元素汞Hg（0）的交换仍然知之甚少，这主要是由于在生态系统规模上难以测量净Hg（0）通量。越来越多的证据表明，叶面吸收大气中的Hg（0）是到达地面的主要​​沉积途径。在这里，我们提出了一种自下而上的方法，通过将标准化为叶面积的叶面Hg摄取率与特定于物种的叶面积指数相结合，来计算对地上树叶的Hg（0）摄取通量。这种自下而上的方法结合了冠高和针头年龄的系统变化。我们在2018年生长季节期间，从瑞士到芬兰北部的纬度梯度分析了10个欧洲森林研究站点的6种树种的583个叶子样品中的汞含量。在所有地点的所有六个树种中，叶片汞的浓度均随时间增加。我们发现，在树冠顶部，归一化至叶面积的叶汞摄取率最高。随着多年生针叶树（云杉和松树）针叶年龄的增长，叶片汞的吸收率降低。2018年生长期中，特定物种的平均叶片汞吸收通量为18±3 µg Hg m^-2为山毛榉，26±5微克汞柱米^-2对于橡木，4±1微克汞柱米^-2为松树和11±1微克汞柱米^-2为云杉。为了进行比较，在同期的10个研究站点中的5个站点上测得的平均Hg（II）湿沉降通量为2.3±0.3 µg Hg m ^-2，比该位置的平均叶面吸收通量10±低四倍。 3 µg汞m ^-2。将特定地点的叶面吸收率提高到欧洲森林地区，导致2018年生长季节的总叶面汞吸收通量约为20±3 Mg。考虑到相同的通量适用于全球温带森林土地面积，我们估计叶面汞的吸收通量为108±18 Mg。我们的数据表明，叶面汞的吸收是欧洲陆地表面的主要沉积途径。自下而上的方法提供了一种有前途的方法，可以在生态系统规模上量化叶片汞的吸收通量。