Natural and Anthropogenic Mercury Sources
35
both as sources and as sinks of atmospheric Hg, although emission events were
observed more frequently and had a larger magnitude (Kim et al. 1995).
Considering the wet deposition inputs, it appears that background soils may not
significantly change the atmospheric Hg pool, but small variations in the mass
balance could have large impacts on the global scale. Hg emission rates measured
averaged 7.5 ± 7.0 ng m- 2 h- I over soils containing 0.35-0.82 pg g-I total Hg.
These concentrations, however, are slightly elevated compared to the background
concentrations generally assumed today, indicating that true background soils
may indeed be neutral towards surface exchange of atmospheric Hg. Studies over
background forest soils in Sweden (Xiao et al. 1991) found fluxes of lower
magnitudes, but these flux chamber measurements were somewhat hampered by
blank problems (Kim and Lindberg 1995). However, it was noted that net Hg
emissions occur during summer at a rate of 0.3 ± 0-4 ng m- 2 h- ', while net
deposition was observed in winter at a rate of 0.9 ± 0.4 ng m- 2 h- I . Unfortunately, no Hg soil contents are given to correlate with the flux magnitudes. In a
related study using flux chamber methods, background forest soils in Tennessee,
USA showed Hg emission fluxes between 2 and 7 ng m -2 h -I, while open field
soils in the same region exhibited greater volatilizion rates of between 12 and
45 ng m -2 h -I, although the open field sites had a smaller Hg soil content than
the forest sites. This difference was attributed to increased solar irradiation over
unshaded soils. These results give an estimate of 1000 T Hg year-I which are
emitted globally from all background soils to the atmosphere, with two thirds
coming from sunlight-exposed open field soils (Carpi and Lindberg 1998).
It has been noted that vast areas of open-field agricultural soils in the world are
treated with municipal sewage sludge as fertilizer. In studies of sewage sludgeamended soils, it was found that sludges containing 7.3 ± 2.5 pg g-I Hg when
applied to background soils result in a release on the order of 25 ± 10 ng m -2 h- I
when kept in the shade. However, this emission increased dramatically when
soils were exposed to sunlight and emissions rose to an average of
460 ± 120 ng m- 2 h- I . In these experiments, solar radiation was shown to
induce Hg emissions more directly than indirectly via soil temperature changes
(Carpi and Lindberg 1997), and that the HgH contained in the sludge must be
reduced in situ to Hg(o) when exposed to sunlight, perhaps by photoreduction
reactions. It was calculated that in the EU and the USA alone, sludge-amended
soils anually release 5 t of Hg to the atmosphere, making them a small source on
the global scale, but important on a regional scale in otherwise uncontaminated
areas where this practice is common. In addition, during the same studies, it was
shown that the sewage sludge-amended soils also release methylated Hg
compounds to the atmosphere at a rate of 12 to 24 pg m -2 h -I, identifying the
first-ever emission of gaseous MeHg to the atmosphere (Carpi et al. 1997). If
upscaled, sewage soil treatment in the EU and USA would liberate 1 kg of MeHg
annually. Considering that vast areas outside the Western world may be
impacted by sludge applications, these soils could have an important regional or
global contribution.
All studies show that the Hg emission flux is more complex than simple phase
transfer of Hg( 0), because the phase transfer enthalpy (derived from temperature
dependencies) is always twice as high as for the pure compound, indicating that
35
both as sources and as sinks of atmospheric Hg, although emission events were
observed more frequently and had a larger magnitude (Kim et al. 1995).
Considering the wet deposition inputs, it appears that background soils may not
significantly change the atmospheric Hg pool, but small variations in the mass
balance could have large impacts on the global scale. Hg emission rates measured
averaged 7.5 ± 7.0 ng m- 2 h- I over soils containing 0.35-0.82 pg g-I total Hg.
These concentrations, however, are slightly elevated compared to the background
concentrations generally assumed today, indicating that true background soils
may indeed be neutral towards surface exchange of atmospheric Hg. Studies over
background forest soils in Sweden (Xiao et al. 1991) found fluxes of lower
magnitudes, but these flux chamber measurements were somewhat hampered by
blank problems (Kim and Lindberg 1995). However, it was noted that net Hg
emissions occur during summer at a rate of 0.3 ± 0-4 ng m- 2 h- ', while net
deposition was observed in winter at a rate of 0.9 ± 0.4 ng m- 2 h- I . Unfortunately, no Hg soil contents are given to correlate with the flux magnitudes. In a
related study using flux chamber methods, background forest soils in Tennessee,
USA showed Hg emission fluxes between 2 and 7 ng m -2 h -I, while open field
soils in the same region exhibited greater volatilizion rates of between 12 and
45 ng m -2 h -I, although the open field sites had a smaller Hg soil content than
the forest sites. This difference was attributed to increased solar irradiation over
unshaded soils. These results give an estimate of 1000 T Hg year-I which are
emitted globally from all background soils to the atmosphere, with two thirds
coming from sunlight-exposed open field soils (Carpi and Lindberg 1998).
It has been noted that vast areas of open-field agricultural soils in the world are
treated with municipal sewage sludge as fertilizer. In studies of sewage sludgeamended soils, it was found that sludges containing 7.3 ± 2.5 pg g-I Hg when
applied to background soils result in a release on the order of 25 ± 10 ng m -2 h- I
when kept in the shade. However, this emission increased dramatically when
soils were exposed to sunlight and emissions rose to an average of
460 ± 120 ng m- 2 h- I . In these experiments, solar radiation was shown to
induce Hg emissions more directly than indirectly via soil temperature changes
(Carpi and Lindberg 1997), and that the HgH contained in the sludge must be
reduced in situ to Hg(o) when exposed to sunlight, perhaps by photoreduction
reactions. It was calculated that in the EU and the USA alone, sludge-amended
soils anually release 5 t of Hg to the atmosphere, making them a small source on
the global scale, but important on a regional scale in otherwise uncontaminated
areas where this practice is common. In addition, during the same studies, it was
shown that the sewage sludge-amended soils also release methylated Hg
compounds to the atmosphere at a rate of 12 to 24 pg m -2 h -I, identifying the
first-ever emission of gaseous MeHg to the atmosphere (Carpi et al. 1997). If
upscaled, sewage soil treatment in the EU and USA would liberate 1 kg of MeHg
annually. Considering that vast areas outside the Western world may be
impacted by sludge applications, these soils could have an important regional or
global contribution.
All studies show that the Hg emission flux is more complex than simple phase
transfer of Hg( 0), because the phase transfer enthalpy (derived from temperature
dependencies) is always twice as high as for the pure compound, indicating that