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 10-7Pa. 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)

Age (thousands of years)

Present

Age (thousands of years)

Present

Fig. 7.24 150000-year record of methane sulphonic acid (MSA) concentration, non-sea-salt-sulphate (nss-SO:|-) aerosol concentration and temperature reconstruction (from oxygen isotope data) from an Antarctic ice core. MSA and nss-SOf aerosol concentrations are both high during very cold conditions, e.g. the last glacial period between 18 and 30 thousand years ago (see text for discussion). After Legrand et al. (1991).

Fig. 7.24 150000-year record of methane sulphonic acid (MSA) concentration, non-sea-salt-sulphate (nss-SO:|-) aerosol concentration and temperature reconstruction (from oxygen isotope data) from an Antarctic ice core. MSA and nss-SOf aerosol concentrations are both high during very cold conditions, e.g. the last glacial period between 18 and 30 thousand years ago (see text for discussion). After Legrand et al. (1991).

these SVOCs will condense from the atmosphere back into, or on to, soils, water and vegetation. Given the global poleward movement of air masses (see Section 1.3.:), SVOCs can be transported large distances from their place of manufacture and used in temperate industrial areas to remote polar regions. The low temperatures in polar environments promote condensation of the SVOCs trapping them on the cold land surface and in its vegetation. The overall process has been likened to a global 'distillation system' (Fig. 7.26). Most SVOCs probably require a number of transportation 'hops' to arrive in polar regions, although more volatile organic compounds in rapidly moving air masses may make the journey in a single 'hop' (Fig. 7.26).

Fig. 7.25 Chemical structures of the contrasting persistent organic pollutants (POPs), polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and furans (PCDDs/PCDFs), p,p'-dichlorodiphenyl trichloroethane (DDT) and hexachlorocyclohexane (HCH). Symbols x and y indicate the possible number of chlorines attached to the ring structures.

Fig. 7.25 Chemical structures of the contrasting persistent organic pollutants (POPs), polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and furans (PCDDs/PCDFs), p,p'-dichlorodiphenyl trichloroethane (DDT) and hexachlorocyclohexane (HCH). Symbols x and y indicate the possible number of chlorines attached to the ring structures.

Condense 1

Volatilize

Volatilize

1 Multiple volatilization-transport-condensation events: 'leapfrogging'

2 Single volatilization-transport-condensation events

Fig. 7.26 Global distillation of persistent organic pollutants.

1000-

"D

■ Zooplankton 10-12-

Biomagnification

■ Arctic cod ■ Zooplankton

Tropic level

1000)

^ ^ ^ ^ Ê2 1 ■ ■ ■ ■ 1

Tropic level

Zooplankton ■

Tropic level Tropic level

Fig. 7.27 Bioaccumulation (a,c) and biomagnification (b,d) of persistent organic pollutants (POPs) with increasing trophic level (i.e. position occupied by a species in the food chain). Trophic level 2 animals are primary consumers (herbivores) while higher-level animals are carnivores.

Fig. 7.27 Bioaccumulation (a,c) and biomagnification (b,d) of persistent organic pollutants (POPs) with increasing trophic level (i.e. position occupied by a species in the food chain). Trophic level 2 animals are primary consumers (herbivores) while higher-level animals are carnivores.

Once in the polar environment SVOCs interact with the local ecosystem. Many POPs are hydrophobic (dislike water) and are therefore lipophilic (affinity for fats; see Box 4.14). Consequently, if animals ingest POPs they are partitioned into the organism's fat reserves. This partitioning stores the compound in fats, excluding it from metabolism and excretion. This scenario potentially allows POP concentrations to increase up the trophic level within food chains (Fig. 7.27). We should note here the distinction between bioaccumulation, where higher organisms have increased body burdens of POPs on account of the organisms being bigger, and biomagnification, where higher organisms not only have increased body burdens of POPs but also exhibit a higher concentration of POPs per unit mass of lipid (Fig. 7.27). It was the bioaccumulation and biomagnification of the insecticide DDT (Fig. 7.25) that was highlighted in Silent Spring, Rachel Carson's seminal book of the 1960s. In it, Carson showed how the decline of common North American bird species was due to consumption of food laden with this poison.

The indigenous humans of the arctic rely on local animals, particularly those high up the food chain such as seals and caribou, for their food source. Animals like seals are rich in fat and thus people living in the arctic can have very high exposure to POPs through their diet. For example, a Beluga whale washed up on the St Lawrence seaboard contained PCBs in excess of 50mgkg-pid. This PCB concentration classified the whale as toxic waste. It is sadly ironic that POPs contaminate arctic environments, ecosystems and peoples who were never involved in their manufacture or use.

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