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

Table VI.D.2.– Mercury depletion (%) as a function of relative humidity
(rh) and O3 concentration (O3 concentrations were derived from IPCC
(2001) WG-1 scenarios).
standards are often exceeded) and OH proves to be a major Hg 0 (g) oxidant even in
the MBL. The impact of increasing O3 concentrations in the atmosphere, including
the boundary layer seen in some IPCC modelling scenarios, would therefore have an
influence on the atmospheric oxidation rate of Hg 0 (g), which in turn would influence
Hg deposition both in terms of dry and wet deposition. The nature of this influence is
not however entirely clear. Over the land Hg oxidation would certainly occur more
rapidly, although both relative humidity (which influences OH concentration) and O3
concentration play a part as seen in Table VI.D.II. The figures reported in Table
VI.D.2 were obtained using a version of the Chemical Balance Model-IV (CMB-IV) to
which atmospheric Hg chemistry has been added.
Due to its relative involatility, Hg(II)(g) produced in the atmosphere is readily
scavenged by rain and also by particulate matter. If O3 and particulate matter
concentrations increase, the probability is that continental air would contain more
oxidised Hg(II)(g) and Hg associated with particulates than previously, especially if the
air were humid. Deposition of Hg would therefore occur closer to sources, or would
occur where dryer continental air meets more humid air. Thus there is the possibility
that Hg deposition would increase substantially in coastal areas. Hg sources located
near coasts could provide much more local contamination than has so far been the
case.
The cycling of Hg in the MBL is not yet entirely understood, certainly Hg 0 (g) is
oxidised quite rapidly under conditions favouring the release of reactive halogen
compounds from sea salt aerosols, and it has been suggested that changes in global
wind fields as a result of climate change would alter the distribution of the acid gases
which promote halogen activation and also tend to result in higher production rates of
sea salt aerosol. This would increase the rate of Hg 0 (g) oxidation in the MBL.
Interestingly though a concurrent increase of the boundary layer O3 concentration
would actually contrast this increase because higher O3 would produce higher HO2
which is the primary sink for active Br compounds in the MBL. To test this hypothesis
the AMCOTS box model (Hedgecock and Pirrone 2001; 2003) which has already
been used to study the lifetime of Hg 0 (g) in the MBL has been run with higher than
usual O3 concentrations (as suggested in IPCC (2001) WG-1 scenarios). Using
typical seasonal cloud cover and air temperature values for the three latitudes used,
increasing the initial O3 concentration in the model, actually results in less Hg 0 (g)
depletion in most instances.
Why the MBL is important in the Hg Cycle?
The fact that in spite of rapid oxidation in the MBL the background Hg 0 (g)
concentration, at least hemispherically, points to an extremely dynamic exchange of
Hg between the MBL and marine waters. Unfortunately not all the processes that
regulate this exchange are at all well understood, but the perturbation of the
processes that contribute could result in an imbalance, leading to far greater Hg input
into coastal and marine waters.
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O3 concentration
% Hg 0 (g) depletion after 7 days 30 ppb 60 ppb 80 ppb
10% r.h. 12.5 18 21
40% r.h. 19.5 28 33
70% r.h. 24 34 40