Division of Atmospheric Sciences, Desert Research Institute , 2215 Raggio Parkway, Reno, Nevada, 89512, United States.
Environ Sci Technol. 2014 Feb 18;48(4):2242-52. doi: 10.1021/es4048297. Epub 2014 Jan 30.
Evasion of gaseous elemental Hg (Hg(0)g) from soil surfaces is an important source of atmospheric Hg, but the volatility and solid-gas phase partitioning of Hg(0) within soils is poorly understood. We developed a novel system to continuously measure Hg(0)g concentrations in soil pores at multiple depths and locations, and present a total of 297 days of measurements spanning 14 months in two forests in the Sierra Nevada mountains, California, U.S. Temporal patterns showed consistent pore Hg(0)g concentrations below levels measured in the atmosphere (termed Hg(0)g immobilization), ranging from 66 to 94% below atmospheric concentrations throughout multiple seasons. The lowest pore Hg(0)g concentrations were observed in the deepest soil layers (40 cm), but significant immobilization was already present in the top 7 cm. In the absence of sinks or sources, pore Hg(0)g levels would be in equilibrium with atmospheric concentrations due to the porous nature of the soil matrix and gas diffusion. Therefore, we explain decreases in pore Hg(0)g in mineral soils below atmospheric concentrations--or below levels found in upper soils as observed in previous studies--with the presence of an Hg(0)g sink in mineral soils possibly related to Hg(0)g oxidation or other processes such as sorption or dissolution in soil water. Surface chamber measurements showing daytime Hg(0)g emissions and nighttime Hg(0)g deposition indicate that near-surface layers likely dominate net atmospheric Hg(0)g exchange resulting in typical diurnal cycles due to photochemcial reduction at the surface and possibly Hg(0)g evasion from litter layers. In contrast, mineral soils seem to be decoupled from this surface exchange, showing consistent Hg(0)g uptake and downward redistribution--although our calculations indicate these fluxes to be minor compared to other mass fluxes. A major implication is that once Hg is incorporated into mineral soils, it may be unlikely subjected to renewed Hg(0)g re-emission from undisturbed, background soils emphasizing the important role of soils in sequestering past and current Hg pollution loads.
土壤表面气态元素汞（Hg(0)g）的逸出是大气汞的一个重要来源，但土壤中 Hg(0)的挥发性和固-气分配尚不清楚。我们开发了一种新的系统，可以连续测量土壤孔隙中 Hg(0)g 浓度在多个深度和位置，总共提供了 297 天的测量数据，跨越了美国加利福尼亚内华达山脉的两个森林 14 个月。时间模式显示，在多个季节中，土壤孔隙中的 Hg(0)g 浓度始终低于大气中测量到的水平（称为 Hg(0)g 固定化），浓度范围为大气浓度的 66%至 94%以下。在最深的土壤层（40 厘米）中观察到最低的土壤孔隙 Hg(0)g 浓度，但在最上面的 7 厘米中已经存在明显的固定化。在没有汇或源的情况下，由于土壤基质的多孔性质和气体扩散，土壤孔隙中的 Hg(0)g 水平将与大气浓度达到平衡。因此，我们用矿物土壤中 Hg(0)g 汇的存在来解释矿物土壤中低于大气浓度的土壤孔隙 Hg(0)g 浓度下降，或者用以前的研究中观察到的上土壤中低于大气浓度的 Hg(0)g 浓度下降，矿物土壤中 Hg(0)g 汇可能与 Hg(0)g 氧化或其他过程有关，如土壤水中的吸附或溶解。表面室测量表明白天 Hg(0)g 排放和夜间 Hg(0)g 沉积，表明近地表层可能主导大气 Hg(0)g 交换的净效应，导致典型的昼夜循环，这是由于表面的光化学还原作用和可能来自凋落物层的 Hg(0)g 逸出。相比之下，矿物土壤似乎与这种表面交换解耦，显示出一致的 Hg(0)g 吸收和向下再分配，尽管我们的计算表明这些通量与其他质量通量相比较小。一个主要的影响是，一旦 Hg 被纳入矿物土壤，它可能不太可能从未受干扰的背景土壤中重新释放 Hg(0)g，这强调了土壤在封存过去和当前 Hg 污染负荷方面的重要作用。
