Vertical Profile Measurements of Soil Air Suggest Immobilization of Gaseous Elemental Mercury in Mineral Soil

Evasion of gaseous elemental Hg (Hg-g(0)) 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-g(0) 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-g(0) concentrations below levels measured in the atmosphere (termed Hg-g(0) immobilization), ranging from 66 to 94% below atmospheric concentrations throughout multiple seasons. The lowest pore Hg-g(0) 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-g(0) 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-g(0) in mineral soils below atmospheric concentrations-or below levels found in upper soils as observed in previous studies- with the presence of an Hg-g(0) sink in mineral soils possibly related to Hg-g(0) oxidation or other processes such as sorption or dissolution in soil water. Surface chamber measurements showing daytime Hg-g(0) emissions and nighttime Hg-g(0) deposition indicate that near-surface layers likely dominate net atmospheric Hg-g(0) exchange resulting in typical diurnal cycles due to photochemcial reduction at the surface and possibly Hg-g(0) evasion from litter layers. In contrast, mineral soils seem to be decoupled from this surface exchange, showing consistent Hg-g(0) 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-g(0) re-emission from undisturbed, background soils emphasizing the important role of soils in sequestering past and current Hg pollution loads.