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

Increases aerosol loading: gaseous Hg 0 is not generally readily scavenged by
aerosol particles, with the exception of black carbon, in fact activated charcoal is a
useful adsorbent for Hg 0 (g). Oxidised Hg compounds however are extremely prone to
scavenging both by dry and deliquesced aerosol particles due to their involatility and
usually high solubility. Should atmospheric oxidation rates of Hg increase, see above,
and at the same time aerosol loading increases the atmospheric concentration of Hg
associated with atmospheric particles would be expected to increase significantly.
The transport of oxidised Hg compounds on particles depositing to water bodies
could become the major pathway for water-soluble Hg compounds to reach aquatic
environments. Washout of particles by precipitation would also introduce these
oxidised compounds directly to water bodies, and also indirectly via run-off.
One aspect of increased aerosol loading which may have particular consequences
for the marine environment is the increased production of sea salt particles resulting
from changes in global wind fields (Sander et al., 2003). An increase in sea salt
aerosol, would increase the concentration of reactive halogen (particularly Br) in the
MBL, accelerating Hg 0 (g) oxidation. If combined with an increase in acid gases, or
their precursors, from anthropogenic emissions which are necessary to acidify sea
salt particles and initiate halogen activation, the effect on the oxidation rate of Hg 0 (g)
in the MBL would be
noticeable. Using the box
model AMCOTS Hedgecock
and Pirrone (2004) suggest
that a decrease of liquid water
content (LWC) (due to
deliquesced sea salt particles)
would reduce the rate of Hg 0
depletion. Increases in sea salt
loading increase oxidation and
decrease Hg 0 (g) residence time.
The fall off seen in Figure
VI.D.1 as LWC increases is
because under the model
% Hg0(g) depletion
100
90
80
70
60
50
40
30
20
10
0
conditions for the remote MBL the concentrations of acid gases were not high
enough to titrate the sea salt aerosol alkalinity. If the acid gas concentration is
increased in the model the fall-off in Hg 0 (g) depletion is not seen. Increases in LWC
beyond 300% showed a more rapid initial effect on Hg 0 depletion, which then
diminished. This was due to more rapid halogen activation, followed by a more rapid
decline in the concentrations of active halogen compounds.
Changes in plant growth regimes: evapotranspiration rates would change
increasing where biological growth occurs faster. Most Hg in leaves comes from the
atmosphere, changes in uptake from the atmosphere, but the dynamics of Hg
release from leaf litter are not really understood.
The occurrence of Hg in European soils especially from anthropogenic emissions
and deposition over the centuries raises the distinct possibility that Hg emissions
from soils will increase. One scenario under consideration is, in a warmer and drier
environment under climate change, Hg emissions will increase due to soil OM
oxidation releasing it into surface and coastal waters. This increased Hg load has
the capacity to contaminate aquatic food webs beyond any new emission sources.
This is just one of the possible feedbacks in a climate-changed world.
196
33% 66% 133% 200% 300% 1000%
Atmospheric Sea Salt LWC
Figure VI.D.1 – Modeled Hg 0 depletion rate
(%) with LWC of sea salt aerosol.