Relevance of dew formation and stomatal conductance on mercury fluxes
A.D. Converse and T.M. Scanlon
Department of Environmental Science, University of Virginia, Charlottesville, Virginia 22904
Introduction
Results:
Results:
Conclusions
Atmospheric mercury (Hg) deposition is a growing global concern,
even in pristine regions far from point sources. Once deposited, it can
be transformed into the potent neurotoxin methylmercury, which is
hazardous to humans and wildlife. It is necessary to quantify the total
Hg deposition to ecosystems in order to better understand Hg cycling
and to develop appropriate legislation to reduce Hg pollution.
Previous studies suggest that wet surfaces enhance surficial Hg
deposition. Regular dew formation could therefore result in increased
Hg deposition, but only one dew study has been previously reported.
Plants accumulate Hg over the course of the growing season and
stomatal conductance has been suggested to play a role. If stomata are
open (high conductance), more gaseous Hg could enter or leave the
plant.
Dew collection and modeling
Stomatal conductance and mercury fluxes
Dew modeling
Dew depth (dew volume/surface area) is determined through
micrometeorological techniques by summing the negative
evapotranspiration (E) indicative of dew formation. The calculated
depth is representative of dew forming on natural surfaces, rather than
an artificial collection table.
Using eddy covariance to determine E tends to underestimate dew
formation, particularly in evenings when there are low wind speeds,
wet conditions, and/or a stable atmosphere. Jacobs et al. (2006)
reported a dew deposition model that does not incorporate eddy
covariance equipment and is based on the surface energy budget:
Mercury fluxes were measured using the aerodynamic method in the
summer and fall of 2008. A diurnal pattern is visible in the summer
The surface energy budget model to predict dew depth better matched
the collected dew samples and onsite observations in comparison to
the eddy covariance technique. This suggests that the model is more
appropriate for our site. In similar sites with low-lying vegetation it
could provide a good estimate of dew depth without the difficulties
associated with early morning dew collection.
The mass of Hg deposited in dew can be compared to
precipitation data. A ‘back-of-the envelope’ calculation indicates that
dew only contributes a small portion of total deposition in comparison
to rainfall in the summer (Table 2). In regions or seasons with less
rainfall, dew might provide a larger portion of the wet Hg deposition
budget.
Combining this equation with free water evaporation, then followed by
Penman’s substitution results in:
This study has two major goals:
1) To quantify the contribution of dew to Hg deposition
using an energy balance technique combined with dew
sampling.
The surface energy budget model, eddy covariance, and physical
sampling were used to determine dew depth in the summer and fall.
Figure 1 displays a single evening/morning of dew formation and
Figure 2 displays the entire summer period (fall data not shown).
Surface energy budget
0.1
Materials and methods
Eddy covariance
0
12 am
2 am
4 am
6 am
8 am
Figure 1: One night of dew formation (August 8th). Eddy covariance underestimates
dew formation in comparison to other methods.
Magnitude of
Hg flux (ng m -2 h-1)
Dew (mm)
0.05
Surface energy budget
Eddy covariance
UV radiation
0.2
-50
-50
Relative humidity
Surface wetness
Nov 14
y = 85.2x + 17.8
R2 = 7.3E-3
p = 0.72
75
50
25
25
0.01
0.02
0.03
0
0.01
Dew (mm)
0.02
0.03
Stomatal conductance
-1
Figure 4: The magnitude of daytime mercury flux as a function of stomatal
conductance. This is no significant correlation was between Hg fluxes and stomatal
conductance in the summer or fall (p> 0.05).
Fall
Aug 6
Aug 7
Aug 8
Aug 9
Aug 10
Aug 11
Aug 12
Hg flux (ng m -2 h-1)
100
50
50
0
0
-50
-100
0
Figure 2: Summer dew collection and modeling. The upper panel (a) indicates the
dew depths determined using three separate techniques. The lower panel (b) displays
Hg concentrations in the collected dew samples. The [Hg] was non-detect on August 11th
and a dew sample was not collected August 12th due to technical difficulties.
Stomatal conductance
Stomatal conductance is not correlated with absolute Hg fluxes.
This does not disprove the idea of gaseous Hg entering and leaving
stomatal cavities, but it certainly is a curious observation. Significant
correlations between temperature, UV radiation, PAR and short wave
radiation with Hg fluxes are present in the summer.
Jacobs, A. F. G., Heusinkveld, B. G., Kruit, R. J. W., & Berkowicz, S. M. (2006).
Contribution of dew to the water budget of a grassland area in the Netherlands.
Water Resources Research, 42(3).
-1
100
5
3800 ng m-2
(m s )
Summer
(b)
Rain
determined from average dew depth and
concentration of 6 summer dew samples.
Rainfall data are from the Mercury
Deposition Network (MDN) site at Big
Meadows (average of 2003-2007).
Literature Cited
A positive correlation to Hg fluxes is present with temperature, UV
radiation (Figure 5), photosynthetically active radiation (PAR), and
incident shortwave radiation in the summer. No significant correlation
between Hg fluxes and other parameters are present in the fall.
10
64 ng m-2
y = -112.3x + 15.9871
R2 = 4.1E-3
p = 0.48
75
50
Dew
Table 2: Summer wet Hg
deposition from dew and
rainfall. Deposition from dew
Two more collection campaigns for Hg fluxes, dew and stomatal
conductance will take place in the winter and spring of 2009.
100
y =-0.056x + 1.9
R2 = 6.0E-4
p = 0.80
Malcolm, E. G., & Keeler, G. J. (2002). Measurements of mercury in dew:
Atmospheric removal of mercury species to a wetted surface. Environmental Science
& Technology, 36(13).
Acknowledgments
Ami Riscassi diligently helped in all field work, including the early morning dew
collections. Others offered assistance in set-up and tear down: Clara Funk, Leslie
Piper, Kelly Hokanson , Thushara Gunda, and Karen Vandecar. Liz Garcia (NPS)
fielded all of our questions concerning the NDAP and MDN. Finally, we’d like to
thank the National Park Service and the staff at the Big Meadows Ranger station for
graciously allowing us to set-up camp in the park. This project is funded by grants
from the NSF and GAANN (Graduate Assistance in Areas of National Need).
-50
y = 4.58x - 2.16
R2 = 0.10
p < 0.05
20
40
UV radiation
-2
Air temperature
-100
Nov 7
Fall
100
Soil moisture
Atmospheric [Hg]
Aug 12
Summer Hg
deposition
Stomatal conductance
0.1
0
PAR radiation
0
(m s )
[Hg] (ppt)
Soil temperature
0
Physical dew collection
Precipitation
Net radiation
50
0
0
Surface
temperature
50
Stomatal conductance was estimated using the Penman-Monteith
equation. It is compared to absolute value of Hg fluxes since gaseous
elemental Hg is believed to enter and leave through stomatal openings
(deposition and emission, respectively).
0.05
Atmospheric [H2O]
and [CO2]
100
Summer
0.15
Wind speed and
direction
100
Figure 3: Mercury fluxes measured in the August and November. Vertical dashed
lines indicate 12pm. Mercury fluxes ranged from -75.6 to 106.6 ng m-2 h-1 over the two
seasons. The average Hg flux was 2.9 ng m-2 h-1 in the summer, and -0.12 ng m-2 h-1 in
the fall.
Physical dew collection
(a)
Table 1. Equipment used in field
campaigns. The photo on the left
displays the flux tower during set-up
and the Big Meadows site.
Fall
-100
Aug 7
s
 v q
Rn  G  
v E 
s
s   rav
2) Compare Hg fluxes to stomatal conductance using
the Penman-Monteith equation.
Our field site is in Big Meadows, a high elevation wetland and the
largest open area within Shenandoah National Park, VA. Vegetation
consists of non-woody grasses with a summer canopy height of
approximately 60 cm. Samples were collected during 1-weeklong
campaigns in August (summer) and November (fall), 2008.
Eddy covariance equipment was mounted on a portable flux
tower at a reference height of 2.4 m and measurements were averaged
over 20 minute intervals. Gaseous atmospheric Hg was alternated
sampled at two inlet heights (Tekran 2537) and fluxes were determined
using the aerodynamic method. Table 1 lists all the equipment used.
Dew samples were collected using trace-metal techniques
following a modified protocol of Malcolm et al. (2002). Mercury
analysis was conducted at the University of Virginia using cold vapor
atomic fluorescence (Tekran 2600).
Summer
Hg flux (ng m -2 h-1)
v E  Rn  G  H
Hg fluxes but no pattern was seen in the fall (Figure 3).
(W m )
60
100
0
For further information
20
40
60
UV radiation
-2
Please contact adc9x@svirginia.edu. More information on this and related projects
can be obtained at www.hydrology.evsc.virginia.edu/Scanlon_Lab.
(W m )
Figure 5: Daytime mercury fluxes as a function of ultraviolet (UV) light. There is a
significant correlation between UV radiation in the summer (p<0.05) but not in the fall.
A link to the pdf of this poster is also available.