36
R. Ebinghaus et al.
formation and/or transport processes slow down the volatilization rate. The
whole process has been suggested to be a combination of volatilization of existent
elemental Hg( 0) and reduction of Hg2+ in soil solutions or in water layers on soil
minerals (Lindberg et al. 1998). Observations of increased Hg volatilization from
contaminated floodplains during rain events led to the suggestion of a two-step
mechanism comprising the same reactions: a small initial displacement of soil air
containing Hg(o) and DMM (Wallschlager et al. 1995) followed by formation of
volatile Hg compounds in the liquid phase as a result of reduction of Hg(II) and
dismutation of MeHg (Wallschlager et al. 1997b). These studies also calculated
that direct reduction of wet-deposited Hg(II) is not likely to be a major source of
Hg volatilization. Soil moisture may playa role in mercury emissions, however.
Using a roofto eliminate rainfall over soils at Walker Branch Watershed (WBW),
a strong decrease in soil Hg emissions was measured after 6 weeks of no rainfall
(Advokaat and Lindberg 1996). In fact, these soils became a net sink, exhibiting
consistent mercury uptake compared to continued Hg emission from adjacent
control soils. Rainfall was not the source of the mercury, however, as the authors
once again induced mercury emissions by simply irrigating the treatment plot
with distilled water.
There has been increasing interest in understanding the driving forces behind
air-surface exchange of Hg, especially over soils. In these numerous investigations, several key factors besides Hg content and speciation were identified as
influencing the Hg emission rates from soils. At EFPC, it was demonstrated that
Hg fluxes increase exponentially with both soil and air temperature (Lindberg
et al. 1995). Annual and diel cycles were observed with highest fluxes in summer
and in the afternoon and lowest fluxes in winter and at night (Lindberg et al.
1995; Kim et al. 1995). These observations are probably coupled to temperature
cycles, but also indirectly to biological cycles associated with them. Positive
correlations were observed between emission rates and wind speed as well as
relative humidity and turbulence (Kim et al. 1995). Solar radiation, temperature,
and soil moisture were identified as key parameters affecting Hg emissions from
soils, and it was shown that solar radiation induced the formation of elemental
Hg(o) in surface soils (Carpi and Lindberg 1997). Rain events also increase Hg
flux to the atmosphere, supporting that soil moisture is one important parameter
in the formation of volatile species, probably in liquid-phase reactions
(Wallschlager et al. 1997b).
6
Global Fluxes of Mercury
The combustion of fossil fuels to produce heat, steam, and electricity is the major
anthropogenic source of atmospheric emissions of mercury on a global scale
(Nriagu and Pacyna 1988). Other important anthropogenic sources include waste
incineration, mining, and smelting. Globally, the emission of mercury from
natural surfaces (re-emissions plus natural source emissions) is comparable to
R. Ebinghaus et al.
formation and/or transport processes slow down the volatilization rate. The
whole process has been suggested to be a combination of volatilization of existent
elemental Hg( 0) and reduction of Hg2+ in soil solutions or in water layers on soil
minerals (Lindberg et al. 1998). Observations of increased Hg volatilization from
contaminated floodplains during rain events led to the suggestion of a two-step
mechanism comprising the same reactions: a small initial displacement of soil air
containing Hg(o) and DMM (Wallschlager et al. 1995) followed by formation of
volatile Hg compounds in the liquid phase as a result of reduction of Hg(II) and
dismutation of MeHg (Wallschlager et al. 1997b). These studies also calculated
that direct reduction of wet-deposited Hg(II) is not likely to be a major source of
Hg volatilization. Soil moisture may playa role in mercury emissions, however.
Using a roofto eliminate rainfall over soils at Walker Branch Watershed (WBW),
a strong decrease in soil Hg emissions was measured after 6 weeks of no rainfall
(Advokaat and Lindberg 1996). In fact, these soils became a net sink, exhibiting
consistent mercury uptake compared to continued Hg emission from adjacent
control soils. Rainfall was not the source of the mercury, however, as the authors
once again induced mercury emissions by simply irrigating the treatment plot
with distilled water.
There has been increasing interest in understanding the driving forces behind
air-surface exchange of Hg, especially over soils. In these numerous investigations, several key factors besides Hg content and speciation were identified as
influencing the Hg emission rates from soils. At EFPC, it was demonstrated that
Hg fluxes increase exponentially with both soil and air temperature (Lindberg
et al. 1995). Annual and diel cycles were observed with highest fluxes in summer
and in the afternoon and lowest fluxes in winter and at night (Lindberg et al.
1995; Kim et al. 1995). These observations are probably coupled to temperature
cycles, but also indirectly to biological cycles associated with them. Positive
correlations were observed between emission rates and wind speed as well as
relative humidity and turbulence (Kim et al. 1995). Solar radiation, temperature,
and soil moisture were identified as key parameters affecting Hg emissions from
soils, and it was shown that solar radiation induced the formation of elemental
Hg(o) in surface soils (Carpi and Lindberg 1997). Rain events also increase Hg
flux to the atmosphere, supporting that soil moisture is one important parameter
in the formation of volatile species, probably in liquid-phase reactions
(Wallschlager et al. 1997b).
6
Global Fluxes of Mercury
The combustion of fossil fuels to produce heat, steam, and electricity is the major
anthropogenic source of atmospheric emissions of mercury on a global scale
(Nriagu and Pacyna 1988). Other important anthropogenic sources include waste
incineration, mining, and smelting. Globally, the emission of mercury from
natural surfaces (re-emissions plus natural source emissions) is comparable to