Uptake of Gaseous Elemental Mercury by a Rainforest: Insights from a Tropical Glasshouse Used as a Dynamic Flux Chamber

Vegetation uptake of gaseous elemental mercury (Hg0) is the main deposition pathway to terrestrial environments. However, the fluxes and processes of forest-atmosphere Hg0 exchange remain ill-constrained, especially in rainforests. To help address this, we used the 1 ha Masoala Rainforest hall of the Zoo Zurich as a dynamic flux chamber to measure net rainforest Hg0 fluxes and even calibrate Hg0 deposition velocities with turbulence measurements. The net Hg0 flux correlated well with CO2 assimilation, showing peak Hg0 uptake at noon. The interquartile range of Hg0 uptake spanned from 1.69 to 3.45 ng m-2 h-1 during the day and from 0.01 to 0.44 ng m-2 h-1 at night. The study results revealed a Hg0-specific canopy resistance (Rc = 1000 s m-1), which underlined the importance of stomatal uptake as a dominant Hg0 deposition pathway in the rainforest. Even though a dynamic flux chamber, however large, is only an approximation of a real rainforest, our findings underline the value of whole-rainforest flux studies both for constraining Hg exchange with the atmosphere and resolving the role of specific mechanisms.