or change in solar radiation being received), then the DMS-producing plankton
might respond in such a way as to reduce the change. For example, if the air tem-
perature increased then the resulting warming of surface seawater would lead to
increased production of DMS by the plankton. This in turn would increase the
flux of DMS across the sea surface and so raise the number of CCN in the atmos-
phere. The resulting enhanced cloudiness would tend to cool the atmosphere, so
opposing the warming which initiated the cycle. The process would work in
reverse for an initial cooling. If correct, this feedback loop would imply that
marine phytoplankton are able to regulate to some degree, at least, the temper-
ature of the atmosphere and thus the Earth’s climate.
This idea was tested by examining ice cores from Antarctica for their content
of DMS atmospheric oxidation products (MSA and nss-SO 42 - ) over the last glacial
cycle (as discussed earlier for CO 2 ; Fig. 7.10). The results, shown in Fig. 7.24,
clearly indicate that both MSA and nss-SO 42 - were at higher concentrations during
the last glaciation than since its termination about 13 000 years ago. This is the
opposite of what would be predicted if planktonic DMS production were reduc-
ing any temperature change. Although the results do not support the notion of
plankton regulating climate, it is now widely accepted that without CCN formed
from DMS the amount of cloudiness, and hence the climate, over large parts of
the globe would be significantly different both now and in the past.7.4 Persistent organic pollutants
Finally, we turn to organic pollutants as examples of exotic chemicals (i.e. those
introduced by human manufacture) impacting on the global environment.
Organic pollutants are considered persistent when they have a half-life (i.e. the
time taken for their concentration to decrease by 50%) of years to decades in a
soil or sediment and of several days in the atmosphere. Organic pollutants persist
in the environment if they are of low solubility, low volatility or resistant to degra-
dation (see Section 4.10.1 & Box 4.16). Stable aromatic compounds, and highly
chlorinated compounds, for example polychlorinated biphenyls (PCBs), poly-
chlorinated dibenzo-p-dioxins and furans (PCDD/Fs), 2,2-bis-(p-chlorophenyl)-
1,1,1-trichloroethane (DDT) and hexachlorocyclohexane (HCH) (Fig. 7.25) are
good examples. The deleterious health effects of these molecules on humans and
other animals are widely documented being potentially carcinogenic (PCBs,
PCDD/Fs, DDT, HCHs), mutagenic (PCDD/Fs) and able to disrupt immune,
nervous and reproductive systems.7.4.1 Persistent organic pollutant mobility in the atmosphere
Many persistent organic pollutants (POPs) are semivolatile organic compounds
(SVOCs) having vapour pressures (see Box 4.14) between 10 and 10-^7 Pa. At these
vapour pressures SVOCs can evaporate (volatilize) from soil, water or vegetation
into the atmosphere. However, as vapour pressure is temperature dependent
(see Box 4.14), it follows that at lower temperatures (lower vapour pressures)274 Chapter Seven