Climate Change and the European Water Dimension - Agri ...

Many studies during the 90’s have indicated an atmospheric lifetime of Hg 0 of around
one year, based on mass balance considerations. Field measurements in the Arctic
and Antarctic during polar spring have, however, shown that under these specific
conditions Hg 0 can behave as a reactive gas with a lifetime of minutes to hours
during MDEs (e.g. Schroeder et al., 1998, Ebinghaus et al., 2002). These MDEs
occur only during a limited time of a few weeks and are not representative of the
overall behaviour of atmospheric Hg 0 . Recent modeling and several Arctic field
studies suggest that atmospheric Hg has the shortest lifetime when air temperatures
are low and sunlight and deliquescent aerosol particles are plentiful (Hedgecock and
Pirrone, 2004). Thus the modeled lifetime for a clear sky condition is actually shorter
at mid-latitudes and high latitudes than near the Equator, and for given latitude and
time of year, cooler temperatures enhance the rate of Hg 0 oxidation. Under typical
summer conditions (for a given latitude) and cloudiness, the lifetime (τ) of Hg 0 in the
MBL is calculated to be around 10 days at all latitudes between the Equator and 60°
N. This value is much shorter than the generally accepted atmospheric residence
time for Hg 0 of a year or more. Given the relatively stable background concentrations
of Hg 0 which have been measured, continual replenishment of Hg 0 must take place,
suggesting a 'multi-hop' mechanism for the distribution of Hg, as originally suggested
by Mackay et al. (1995), rather than solely aeolian transport with little or no chemical
transformation between source and receptor.
Primary effects
The effects of increased air temperature are not obvious, but higher temperatures
favour ozone production, increase the oxidation rate of Hg 0 to more soluble forms,
and may alter the partitioning between the gas and particulate phases which would
effect deposition patterns over time and spatial scales.
An increase of seas temperature may cause significant increase of elemental Hg
emission rates from the seas that would lead to increase of Hg re-cycling between
the atmosphere and, possible impacts on coastal zones. Increase in seas
temperature may cause significant changes in biological activity of the oceans that
would lead to changes in the rate and spatial distribution of Hg methylation and
therefore Hg redistribution between the biotic and abiotic marine systems.
Changes in precipitation patterns as an increase in precipitation frequency and
intensity would cause an increase in Hg deposition (input) to marine waters. Higher
frequencies of extreme events that tend to result in increased run-off rather than the
water being absorbed by the soil would tend to increase Hg loadings transported to
rivers and thus seas.
Secondary effects
Higher O3 concentration: it would determine higher rates of Hg oxidation in the
atmosphere, thus more local dry and wet deposition rates to surface waters.
Increased average wind speeds: it would cause higher sea salt aerosol
production, possibly higher levels of reactive inorganic Br compounds, and increase
in Hg oxidation that would increase deposition fluxes.
Less sea ice and snow cover in the Arctic: with greater areas of ocean exposed
MDEs could become more frequent, if more bare land is uncovered the almost direct
re-emission of Hg from the snow could be reduced increasing the Hg loading to the
Arctic ecosystem.
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