Steven E. Lindberg

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

Abstract 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.

The role of atmospheric deposition in an eastern U.S. deciduous forest

Atmospheric sources contributed significantly to the annual flux of trace metals and sulfate to the forest floor of Walker Branch Watershed, a forested catchment in the southeastern United States. Atmospheric deposition supplied from 14% (Mn) to≈40% (Zn, Cd, So4=) to 99% (Pb) of the annual flux to the forest floor; the remainder was attributable to internal element cycling. The measured water solubility of these metals in suspended and deposited particles indicates that they may be readily mobilized following deposition. Dry deposition constituted a major fraction of the total annual atmospheric input of Cd and Zn (≈20%), SO=(≈35%), Pb(≈55%), and Mn (≈90%); however, wet deposition rates for single events exceeded dry deposition rates by one to four orders of magnitude. Interception of rain by the canopy resulted in loss of Cd, Mn, Pb, Zn, and SO= from the canopy, but uptake of H+ which increased with increasing free acidity of the incoming rain, and with increasing residence time of the rain on the leaf surface.

Atmospheric Chemistry, Deposition, and Canopy Interactions

One of man’s impacts on geochemical cycles involves the atmospheric chemistry and deposition of several major and trace elements. The critical links between increased atmospheric emissions and effects on ecosystems are the rates at which, and the mechanisms by which, atmospheric constituents are transported to the earth’s surface and made available to receptor organisms. Forest vegetation is a particularly important sink for atmospheric emissions; because of the reactivity and large surface area of forest canopies, they are effective receptors of airborne material delivered by both wet and dry deposition processes. In the eastern United States, forest canopies represent the initial point of interaction between the atmosphere and the biosphere for ~50% of the land surface area. On a global scale, we estimate that the combined surface area of all forest canopy leaves and needles is of the same order as that of the total global land surface area (~1014 m2).

High-precision measurements of mercury vapor in air: Design of a six-port-manifold mass flow controller system and evaluation of mass flow errors at atmospheric pressure

We constructed an atmospheric sampling system for Hg vapor that utilizes a single vacuum pump connected via a manifold to six separate mass flow controllers (MFC). The manifold system reduces the size and power requirements for collection of replicate samples, is ideally suited for use on meteorological towers, and achieves the precise control of air-sampling volumes required for computing air/surface exchange rates from concentration gradients of Hg vapor. In testing our air sampling systems, we found consistent calibration errors between the manufacturers calibrations and a standard bubble flow meter. Errors as high as 30% decreased systematically with increasing flow rate to values of 3–5% at near-maximum flow. The relative error patterns established between adjacent MFC units in each system were found to be relatively stable over time. Using gold-coated sand amalgamation traps for Hg vapor and the flow correction factors computed from our calibrations, we routinely achieve precision for replicate measurements of Hg vapor in background air of 0.5–2% (expressed as relative standard errors of mean concentrations of 1.5–3.5 ng/m3). Application of the flow correction factors measurably decreases the level of bias between mean concentrations of Hg vapor measured with adjacent sampling systems and is necessary to reduce uncertainty associated with quantifying gradients in atmospheric concentrations.

Emissions of mercury vapor from tree bark

Abstract Measurements of the rate of elemental Hg vapor (Hg°) emissions from the bark of red maple (Acer rubrum L.), yellow-poplar (Liriodendron tulipifera L.), chestnut oak (Quercus prinus L.) and white oak (Quercus alba L.) were conducted in a controlled laboratory chamber with a mean air Hg° concentration of 1.6 ng m−3. Measured Hg° emissions for the four bark species studied ranged from a maximum of 10.8 ng m−2 h−1 for white oak to a minimum of 1.2 ng m−2 h−1 for red maple. Chestnut oak, yellow-poplar, and red maple bark all had similar Hg° emission rates with a mean of 1.9 ng m−2 h−1, but the mean emission rates from white oak were up to five times greater. This discrepancy was correlated with higher rates of evaporation from the white oak bark samples. When compared to published values of Hg° emissions from foliage and soils, it was concluded that bark surfaces would contribute less than 10% of all Hg° emissions from a forest landscape.