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

Second, the chemical is transported (often over 10s – 100s – 1000s of km) through
the atmosphere to a more remote region (Beyer et al., 2000; Ikonomou et al., 2002;
Wania, 2003; Wania and Dugani, 2003). The chemical must be sufficiently persistent
in the atmosphere to survive this long journey. Third, dry (gaseous or particulate) or
wet deposition of the chemical to the more remote region can occur. Importantly, on
being deposited to an environmental compartment (soil, vegetation, water bodies,
snow/ice) POPs might be re-emitted (i.e. a secondary emission of ‘old’/previously
deposited POP).
It has been hypothesised that a net transfer of POPs from warm source/usage areas
to colder remote regions via LRAT (i.e. a global re-distribution) is occurring (Wania
and Mackay, 1993; 1995; 1996; Jones and de Voogt, 1999; Wania and Su, 2004).
Repeated temperature-controlled air-surface exchange provides the key mechanism
for this so-called ‘cold condensation’ process. Furthermore, ‘global fractionation’ may
occur, whereby different POPs travel different distances from a common source area.
The extent of fractionation would be dependent on compound physical-chemical
properties - notably vapour pressure and air-surface partitioning. Compounds with
higher volatility will undergo LRAT and condense in colder (polar) regions, while less
volatile compounds will be deposited closer to the source area. This could happen
either in one single event of release followed by deposition, or alternatively in a
series of ‘hops’ through repeated air-surface exchange influenced by changes in
temperature (the ‘grasshopper effect’) (Wania and Mackay, 1996). Fractionation does
not necessarily lead to a net increase of POPs in polar regions (i.e. cold
condensation) – it could arise simply following a single primary emission pulse,
where different compounds undergo LRAT, deposition and loss processes to slightly
different extents.
This general concept of a temperature-driven re-cycling and re-distribution of POPs
has provided the motivation for numerous studies designed to look for evidence of
global cold condensation and fractionation. There is no doubt that fractionation and
cold condensation do occur (Wania and Mackay, 1996; Ockenden et al., 1998; Meijer
et al., 2003a; Jaward et al., 2004a), but it appears that these processes exert a
relatively minor influence on the global mass balance of POPs (Meijer et al., 2003b;
Wania and Su, 2004). This is likely because a number of other key factors influence
the global cycling and atmospheric distribution of POPs, and these can confound or
lessen the importance of the cold condensation/fractionation processes. Key among
them are:
• The potential for ongoing (often diffusive) primary sources of POPs, even
now, long after many POPs have been regulated and banned (e.g. Jaward et
al., 2004b);
• The patterns of atmospheric circulation and air mass origin (e.g. Cortes et al.,
2000);
• The capacity of environmental compartments to store or degrade POPs. This
is intimately linked to the amounts and forms of organic matter in those
compartments (e.g. Meijer et al., 2003b);
• The timescales required for large-scale re-cycling and exchange to occur.
All these issues need to be borne in mind when considering the potential impacts of
underlying climate changes on the global cycling of POPs.
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