Atmosphere‐surface exchange of mercury in a forest: Results of modeling and gradient approaches

Abstract

We have modified recently published dry deposition models to estimate deposition velocities (Vd) for Hg in both fine aerosol and vapor form to forest canopy surfaces. Aerosol and total vapor phase Hg concentrations in air previously measured at Walker Branch Watershed in Tennessee have been used with model results to estimate dry deposition to a deciduous forest. The concentration data confirm that airborne Hg is dominated by vapor forms at this site and exhibits concentrations moderately above continental background levels. The modeled Vd values reflect published data which suggest that dry deposition of Hg vapor is strongly controlled by surface transport processes, notably stomatal and mesophyll resistances, the latter dominating. Weekly mean Vd values ranged from 0.006 (winter) to 0.12 (summer) cm s−1. We have also measured concentration gradients of Hg vapor in air above this forest to estimate air‐surface exchange during short‐term experiments. While the model results indicate that the canopy is a sink for Hg vapor, the concentration profiles suggest that the forest soils are a source during some periods, the combined effect of which is net Hg fluxes in the upward direction. Application of a detailed canopy turbulence model yielded soil emission rates of the order of 50 ng Hg m−2 h−1, ∼10% of which is deposited in the canopy. Our modeled dry deposition estimates plus limited measurements of wet deposition in this area suggest that dry and wet deposition may be comparable in magnitude.

Estimation and Mapping of Wet and Dry Mercury Deposition Across Northeastern North America

“…van Hove et al, 2002) and it lacks representation of canopy emission of GEM Lindberg and Meyers 2001;. Xu et al (1999) side-stepped modeling a compensation point by separately calculating GEM deposition according to Lindberg et al (1992) and formulating a separate model for canopy emission of GEM consistent with the very limited (pre-1999) understanding of controls on this process. Net GEM deposition was taken to be the net of the two model results.…”

“…Recent measurements of GEM emission from vascular plant canopies suggest that some biological process (bioreduction) at the root-soil interface may be responsible for the production of and apparent supersaturation of Hg 0 in the xylem of plants exhibiting net GEM emission . Rea et al (2002) measured mercury in the foliage of deciduous trees over the course of the growing season and found that total foliar mercury accumulation was substantially less than GEM deposition estimated following Lindberg et al (1992). Rea et al (2001) determined that Hg p and RGM dry deposition are rapidly washed off foliar surfaces, and, therefore, foliar accumulation of mercury most likely represents GEM assimilation.…”

“…The Lindberg et al (1992) model clearly overestimates GEM deposition (S.E. Lindberg, personal communication, 2002) because it does not include a compensation point for mercury (Hanson et al, 1995) as is necessary for modeling atmospheric species with bi-directional fluxes (e.g.…”

“…Dry deposition of Hg p and RGM Dry deposition of Hg p and RGM were estimated using the approach of Lindberg et al (1992) with some differences and additions described below. Dry-deposition velocities for aerosol particles and gasses were estimated using a big-leaf model designed for complex terrain and multi-species canopies (Miller et al, 1993a, b).…”

Strong and weak, unsteady reconfiguration and its impact on turbulence structure within plant canopies

“…These fluxes between the terrestrial vegetation and the atmosphere impact micro and mesoscale meteorology, regional and global climate change, carbon balance and cycling, as well as hydrology. 1, 2 Fluxes of trace gases and aerosols at the canopyatmosphere interface also determine the important role of vegetated surfaces on the removal of air pollutants (e.g., tropospheric ozone, 3, 4 heavy metals, 5 and aerosols 6 ). Transport of biogenic particles emitted from forests and crops such as pollens, 7 seeds, 8 and spores 9 also play an important role in ecological and agricultural processes.…”