Micrometeorological measurements of mercury vapor fluxes over background forest soils in eastern Tennessee

We used the modified Bowen ratio method to estimate the fluxes of vapor-phase elemental Hg (Hg0) over background forest soils during the summer and fall of 1993. Fluxes were derived from the concentration gradients of total gaseous Hg between sampling heights of 25 and 165 cm and the concurrently determined turbulent diffusion coefficients of reference trace gases (i.e. H2O or CO2). The concentration and gradient data of Hg0 measured during the campaigns generally fell in relatively narrow ranges of 1.52–3.68 and −0.16 to 0.32 ng m−3 (over 140 cm), respectively: means ( ± 1 S.D.) for the corresponding; emission and deposition fluxes were found to be 7.5 ± 7.0 (n= 30) and −2.2 ± 2.4 ng m−2 h−1 (n=9), respectively. From the data collected during a series of sequential measurements, reproducible patterns of diurnal exchange emerged: (1) small bidirectional fluxes of Hg0 in the morning, (2) peak emissions near midafternoon, and (3) generally insignificant exchange during the nighttime. The fluxes of Hg over soil surfaces appear to be driven by a combined effect of several meteorological factors, including wind speed, vertical mixing, and soil temperature. Comparison of environmental conditions for both emission and deposition events showed that the direction of fluxes may be strongly influenced by the stability conditions of the boundary layer. The overall results of our emission and dry-deposition measurements in concern-with recent studies of wet-deposition rates in the forest ecosystem suggest that source strengths of this forest soil system may be of the same order of magnitude as sink strengths.