PROCEEDINGS OF 1995
CANADIAN MERCURY NETWORK WORKSHOP
[INLINE]
AIR/SURFACE EXCHANGE OF MERCURY VAPOR IN FORESTS: THE IMPORTANCE OF DRY DEPOSITION AND RE-EMISSION IN THE OVERALL BIOGEOCHEMICAL CYCLE OF MERCURY
S. E. Lindberg1,2
1Visiting Professor, University of Lund, Lund, Sweden;
2Permanent address: Environmental Sciences Division, Oak Ridge
National Laboratory,
PO Box 2008, Oak Ridge,
Tennessee 37763-6038 USA.
Atmospheric sources are recognized to be significant in the cycling
of Hg in the biosphere, yet there are no reliable data on
air/surface exchange rates of Hg in temperate forests. The EPRI
Mercury Air/Surface Exchange (MASE) Project involves development of
field micrometeorological methods and laboratory chamber techniques
combined with modeling approaches for determining exchange rates for
atmospheric Hg over environmental surfaces such as soils, surface
water, and vegetation. We will summarize here the results of our
studies since 1990 of atmospheric Hg deposition, surface emission,
and re-emission and their roles in the overall biogeochemical cycle
in forests. Our work has been concentrated in a temperate deciduous
forest in eastern North America, but we have also measured Hg
exchange rates in a boreal forest in Sweden (Lindberg et al. in
press), and are planning similar studies in northern conifer
forests, and subtropical wetlands.
We have developed and extensively tested the micrometeorological
modified Bowen ratio (MBR) method for Hg. The MBR method, which has
been used to quantify fluxes of other trace gases in forests (eg.
SO2 and O3), involves measurement of the concentration gradients of
Hg and of reference gases (eg. CO2 and H2O) over the surface of
interest. The gradients of CO2 and H2O are combined with direct
measurements of their fluxes measured by fast-response sensors and
eddy correlation to compute turbulent mixing coefficients. These are
used with the measured gradients of Hg to estimate fluxes. We
developed a sampler which uses 12 replicate mass flow controllers to
measure air concentrations of Hg vapor with the precision and
accuracy necessary for quantifying the small concentration gradients
over environmental surfaces. Tests for bias between adjacent
samplers using 12 replicate gold-trap Hg collectors indicated no
significant bias among samplers (Kim and Lindberg 1994). We
consistently achieve a precision of <1-2% for 1-2 h samples of Hg in
air at background levels (1-3 ng/m3; all analyses are by cold vapor
atomic fluorescence).
We recently tested the MBR method over Hg-contaminated soils with
excellent success (the data behaved as predicted from first
principals; Lindberg et al 1995). To provide comparative data from
an independent method, we also designed a dynamic plant exposure
chamber to measure Hg fluxes to soils and tree seedlings over a wide
range of concentrations in the laboratory (Hanson et al 1995), as
well as a dynamic soil chamber to measure Hg fluxes over a range of
soil types (eg. Kim and Lindberg 1995).
Our field data suggest that measured Hgo gradients over terrestrial
surfaces are quite small, generally a few percent of the mean
concentrations (on the order of 0.02-0.20 ng/m3 over height
intervals of ~1-10 m), and are bidirectional, suggesting that the
true net long-term fluxes will be difficult to quantify because both
dry deposition and emission are important. During our forest canopy
sampling intensive at Walker Branch Watershed (WBW) in Tennessee, we
found significant emissions of Hgo from the plant canopy which
strongly decreased at night and as drought stress increased,
suggesting a stomatally controlled process (Lindberg in press). We
also found that Hg dry deposition was most evident under conditions
of plume impaction and when the canopy surfaces were wet from rain.
The laboratory plant chamber data confirmed the emission of Hgo from
plants at background Hgo concentrations, but found dry deposition at
elevated Hgo (>~10 ng/m3), suggesting the existence of a
compensation point for atmospheric Hgo exchange in plants (Hanson et
al. 1995). Similar results have since been found at a forest
plantation remote from local emissions of Hg, where the net flux of
Hgo over the trees was clearly upward at background Hgo levels
(~1.5-2.0 ng/m3). We also found net emission of Hg from background
forest soils (soil Hg ~0.5 =B5g/g) at rates which were dependent on
soil temperature, turbulent mixing, and soil Hg concentration. The
emission rates of Hgo were generally in the range of 5-10 ng m-2 h-1
over soils, and 30-80 ng m-2 h-1 over the forest canopy (Kim et al.
1995, Lindberg in press).
Measured fluxes of Hg in precipitation, throughfall, and litterfall
indicate that the WBW forest receives a significant loading of
atmospheric Hg. Our initial estimates of the overall fluxes of Hg in
this watershed suggest that atmospheric inputs are of the same
magnitude as total surface emissions if one assumes that the Hg flux
in litterfall represents an atmospheric deposition source. If the Hg
accumulated in litter is derived entirely from soil uptake, the
forest at Walker Branch appears to behave as a net source of Hg to
the troposphere. However, recent data suggest that litterfall is
derived primarily from the atmosphere.
We are now developing the first complete mass balance of mercury in
a temperate forest in N. America (Lindberg in press). Some surprises
with interesting implications have surfaced in our results: 1) Trees
are both sources and sinks for airborne Hg; 2) The measured flux of
Hgo over forest soils is also bidirectional; 3) The flux of Hg in
throughfall consistently exceeds that in rain, suggesting leaching
of internal Hg or washoff of dry deposited aerosol and water soluble
gaseous Hg. Since plants appear to efficiently exclude soil Hg from
root uptake based on published laboratory studies, the Hg in
throughfall appears to be derived from dry deposition; 4) Tree ring
studies have confirmed stomatal uptake and internal binding of Hg in
trees near point sources where air concentrations exceed the
compensation point for atmospheric Hg uptake (Turner et al in
prep.); 5) The single largest Hg flux in this and other forests
occurs in litterfall, the source of which appears to be atmospheric
deposition, but which must yet be confirmed; and 6) A preliminary
budget suggests that a significant fraction of deposited Hg may be
re-emitted to the atmosphere, and that the sink capacity of forests
for Hg may need reevaluation. Clearly, it will be important to
derive better estimates of the fluxes of Hg in terrestrial systems
to understand their role in the global Hg cycle, and their potential
to either behave as so-called "natural" sources of Hg, or as
significant sources of re-emitted Hg. Recent data from this forest
and other North American forests will be discussed with the
objective of understanding the sources and exchange mechanisms of
various pathways of Hg movement.
Acknowledgments. Research sponsored by the Electric Power Research
Institute under contract with ORNL (managed by Lockheed-Martin ORNL
Inc., for the U.S. Department of Energy under contract
DE-AC05-84OR21400).
References and Recent Publications
Meyers, T.P., M.E. Hall, and S.E. Lindberg. Use of the modified Bowen ratio technique to measure fluxes of trace gases. Atmos. Envir. (submitted).
Lindberg, S. E. Forests and the Global Biogeochemical Cycle of Mercury: The Importance of Understanding Air/vegetation Exchange Processes. IN: Baeyens, W. Ed. Regional and Global Mercury Cycles, NATO Advanced Science Institute Series, Novosibersk, Siberia, July, 1995. Kluwer Academic Publishers, Dordrecht, Holland (in press).
Lindberg, S. E., Meyers, T. P., and J. Munthe. 1996. Evasion of mercury vapor from the surface of a recently limed acid forest lake in Sweden. Water, Air, Soil, Pollut. (in press).
Kim, K.-H., Lindberg, S. E., and Meyers, T. P. 1995. Micrometeorological measurements of mercury fluxes over background forest soils in eastern Tennessee. Atmos. Envir. 27:267-282.
Stratton, W. J. and S. E. Lindberg. 1995. Use of a refluxing mist chamber for measurement of gas-phase water-soluble mercury (II) species in the atmosphere. Water, Air, Soil, Pollut. 80: 1269-1278.
Lindberg, S.E., K-H. Kim, T.P. Meyers, and J.G. Owens. 1995. A micrometeorological gradient approach for quantifying air/surface exchange of mercury vapor: Tests over contaminated soils. Envir. Sci. Technol. 29:126-135.
Lindberg, S. E., K.-H. Kim, and J. Munthe. 1995. The precise measurement of concentration gradients of mercury in air over soils: a review of past and recent measurements. Water, Air, Soil, Pollut. 80: 383-392.
Vermette, S.J., M.E. Peden, T.C. Willoughby, S.E. Lindberg, and A.D. Weiss. 1995. Methodology for the sampling of metals in precipitation: Results of the National Atmospheric Deposition Program (NADP) pilot network. Atmos. Envir. 29: 1221-1230.
Kim, K.-H and S. E. Lindberg. 1995. Design and initial tests of a dynamic enclosure chamber for measurements of vapor-phase mercury fluxes over soils. Water, Air, Soil, Pollut. 80: 1059-1068.
Vermette, S.J., S.E. Lindberg, and N. Bloom. 1995. Field tests for a regional mercury deposition network: Sampling design and test results. Atmos. Envir. 29: 1247-1252.
Johnson, D. W. and S. E. Lindberg. 1995. Sources, sinks, and cycling of Hg in forested ecosystems. Water, Air, Soil, Pollut. 80: 1069-1077.
Lindberg, S.E. and S.J. Vermette. 1995. Workshop on sampling mercury in precipitation for the National Atmospheric Deposition Program. Atmos. Envir. 29: 1219-1220.
Hanson, P. J., S. E. Lindberg, T. A. Tabberer, J. G. Owens, and K.-H. Kim. 1995. Foliar exchange of mercury vapor: evidence for a compensation point. Water, Air, Soil, Pollut. 80: 373-382.
Kim, K.-H. and Lindberg, S. E. 1994. 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. J. Geophys. Res. 99: 5379-5384.
Lindberg, S.E., J.G. Owens, and W. Stratton. 1994. Application of throughfall methods to estimate dry deposition of mercury. IN J. Huckabee and C. Watras, (Eds.), Mercury as A Global Pollutant, pp. 261-272. Lewis Publ.
Lindberg, S.E., T.P. Meyers, G.E. Taylor, R.R. Turner, and W.H. Schroeder. 1992. Atmosphere/surface exchange of mercury in a forest: Results of modeling and gradient approaches. J. Geophys. Res. 97: 2519-2528.
Lindberg, S. E., R.R. Turner, T.P Meyers, G.E Taylor, and W.H.
Schroeder. 1991. Atmospheric concentrations and deposition of airborne
Hg to Walker Branch Watershed. WASP 56:577-594. [INLINE]
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