Persistence is the ability of a chemical to stay unchanged in the environment for a long time. Persistent chemicals are able to move long distances from the place where they are emitted. Thus they can be found in areas far from the human activities that result in emissions or discharges. Transport happens typically in the atmosphere, but may also happen in rivers and oceans. Persistence contributes to long-range transport in two ways:

• Persistence, e.g. in soil, can create a long-term reservoir of chemical which is continuously emitted to the air or water which can carry it to remote regions.
• Persistence, e.g. in air or water, means that the chemical can survive a long journey unchanged.

The ability of a chemical to survive unchanged during long-range transport is described by its half-life, i.e. the time taken for half of the chemical to disappear from the environment. If we know how quickly a chemical disappears (its half-life) and how quickly the environment (e.g. the atmosphere) carries it, then we can work out how long a journey the chemical can make before it disappears.

Most of the chemical will have disappeared within five half-lives: only about 3% will remain. So the length of journey possible is the distance travelled in five half-lives. Typical natural air speeds are 1 m/sec, i.e. 86 km/day. So for meaningful long-range transport through the atmosphere for, say, 1000 km, a chemical will need to have a half-life of at least 2 days.

The details of long-range transport will be affected by other factors, such as:

• How the chemical is distributed between environmental compartments (air, water, soil, and sediment).
• How environmental conditions (e.g. temperature) change during the journey, and how these changes affect the half-life of the chemical.

However, the simple calculation can be used to get a good idea of whether a chemical is likely to undergo long-range transport.

Air and water move: soil and sediment are essentially stationary. Long-range transport of chemicals in the environment thus takes place almost exclusively in air and water. River transport may be important on a regional basis, but few rivers are more than 1000 km long and most long-range transport in water takes place in the oceans that cover 80% of the earth’s surface. The simple relationship between half-life and the speed at which air or water moves can give a good idea of the relative importance of long-range transport through the atmosphere and in the oceans.

Typical natural air speeds in the atmosphere are 1 m/sec, i.e. 86 km/day. Ocean water moves much more slowly, typically hundreds of times more slowly than air. An air mass can move from the tropics to the polar regions in just five to six days. But it can take 10 to 15 years for a water mass to move from the equator to the Arctic. The slower oceanic movement is due to the many obstacles (land masses) and the indirect routes followed by ocean currents.

Based on the relative speeds at which air and ocean water move, a chemical will need to have a half-life some 400 times greater in water if it is to survive a 1000 km ocean journey than it needs to survive the same journey by air. In other words, the atmosphere can be a significant factor in long-range transport for chemicals with a half-life in air greater than about 2 days, but the ocean will only be a significant contributor for a chemical with a half-life in water greater than 2 years.

A further factor reduces the relative significance of the oceans as a means of long-range transport for environmental chemicals. Only a relatively small proportion of chemicals are sufficiently non-volatile and sufficiently soluble in water to remain in significant amounts in water that is in contact with air. During a long slow ocean journey, most chemicals will move from the ocean into the atmosphere and take the ‘fast track’ to remote regions if their atmospheric half-life is long enough.

In summary, the relative contributions of the atmosphere and the oceans to the long-range transport of an environmental chemical depends on:

• The stability of the chemical in the environment, i.e. its half-life in air and in water.
• The distribution of the chemical between environmental compartments (e.g. between the ocean and the atmosphere), i.e. physico-chemical properties such as volatility (vapour pressure) and water solubility (polarity).
• The relative speeds at which air and water masses move over long distances in the atmosphere and the oceans respectively.

Jenson, J., Adare, K and Shearer, R. (eds) 1997. Canadian Arctic Contaminants Assessment Report. Ministry of Indian Affairs and Northern Development Ottawa, Ont.

Arctic Pollution Issues: A State of the Arctic Environment Report. 1997. AMAP Oslo, Norway.