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

POPs exist in the atmosphere in the gas phase and associated with particles. Their
partitioning is controlled by temperature (Bidleman, 1988; Falconer and Bidleman,
1994) and can be quite different in different environments – the tropics compared to
the Arctic, for example.
4. Reaction rates:
POPs are degraded in the environment, in the atmosphere by reactions with the
hydroxyl radical, and in soils/sediments by microbially mediated processes (e.g.,
Anderson and Hites, 1996; Totten and Eisenreich 2000, Mandalakis et al., 2003).
Despite all these obvious examples of the influence of temperature on the emissions
and environmental distributions of POPs, it must be remembered that regional or
global increases in ambient temperatures attributed to underlying changes in climate
are ‘small’ – averaging only 1-2 0 C over time frames of decades or longer. These are
unlikely to be sufficient – by themselves – to substantially impact global POPs cycles.
After all, timeframes of decades are sufficient to see the ‘rise and fall’ of POP
compound production/use/restrictions/bans, while reaction rates and environmental
partitioning phenomena generally only increase by a factor of 2-3 for a 10 0 C
increase in temperature.
The role of air-surface exchange
As noted above, there is evidence that gas phase POPs can ‘hop’ between
environmental surfaces and the atmosphere (Gouin et al., 2004). The clearest
evidence for this comes from measurements of the diurnal cycling of atmospheric
concentrations (highest in the day, lowest at night) that are positively correlated with
temperature. However, such cycling is not observed in all studies, and appears to be
a strong function of the underlying surface. The clearest evidence for such T-driven
cycling comes from studies over vegetated surfaces - a peat bog in Minnesota and
grassland in the UK, for example (Hornbuckle and Eisenreich, 1996; Lee et al., 1998)
as well as urban areas (Brunciak et al., 2002; Dachs et al., 2002). In contrast, studies
over the open ocean suggest that other factors control atmospheric concentrations
over a typical 24-hour cycle (Jaward et al., 2004c). When diurnal cycling has been
observed, it is clearest where ambient temperatures show the largest diurnal
amplitude and where the surface compartment undergoes large diurnal shifts in
temperature. These observations are potentially important with respect to possible
influences of climate change, because underlying changes that affect the nature and
partitioning properties of environmental surfaces may impact the rate and magnitude
of the air-surface exchange of POPs. Hence – it is hypothesised - increased
desertification, changes in land use and vegetative cover, changes in the extent of
the ice sheets, and changes in the rates of primary productivity in the oceans may all
exert an influence on global POP cycling.
Vegetation:
Vegetation is a compartment that actively participates in air-surface exchange of
POPs. The waxy surfaces of leaves can provide an important storage compartment
for these compounds (Barber et al., 2004), are in intimate contact with the
atmosphere, and can undergo substantial diurnal and seasonal changes in
temperature. It is not a compartment with a particularly large capacity to store POPs,
compared to soils or sediments for example, but it can:
• scavenge POPs from the atmosphere;
• enhance the rates of deposition to forested areas relative to adjacent
clearings, and on vegetated areas relative to adjacent bare soil;
• store POPs in quantities which vary as a function of the plants lipid content;
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