The Long Range Transport of Atmospheric Pollutants

At first glance, it seems amazing to discover that relatively nonvolatile organochlorines and PAHs can eventually travel thousands of kilometers by air from their point of release and end up contaminating relatively pristine areas of the world such as the Arctic. Some quantitative understanding of this long-range transport of atmospheric pollutants (LRTAP) has been made using principles of physical chemistry.

By a global fractionation (or distillation) process, pollutants travel at different rates and are deposited in different geographical regions depending upon their physical properties. Most persistent organic pollutants have sufficient volatility to evaporate—often rather slowly—at normal environmental temperatures from their temporary locations at the surface of soil or water bodies. However, because the vapor pressure of any chemical increases exponentially with temperature, evaporation is favored in tropical and semitropical areas, so these geographic regions are rarely the final resting places for pollutants. In contrast, cold air temperatures favor the condensation and adsorption of gaseous compounds onto suspended atmospheric particles, most of which are subsequently deposited onto the Earth's surface. Thus the Arctic and Antarctic regions are the final resting places for relatively mobile pollutants that are not deposited at lower latitudes because of their high volatility. Unfortunately, these compounds degrade even more slowly in these regions because temperatures there are so cold.

Example of pollutants that migrate to polar regions are the highly cWo-rinated benzenes; PAHs having three rings; and PCBs, dioxins, and furans that have only a few chlorines (see Table 12-2). Substances with even greater volatility, such as naphthalene and the less chlorinated benzenes, are not deposited even at the cold temperatures of polar regions; consequently, they continue their worldwide travels more or less indefinitely until they are chemically destroyed, usually by reaction initiated by the hydroxyl radical.

As implied in Table 12-2, the mobility of a chemical increases as the vapor pressure of its condensed form (as measured by that of the supercooled liquid at 25°C) increases. In addition, mobility increases as the temperature of condensation of the vapor form of the pollutant gas decreases. Thus, substances that do not condense until the temperature drops to —30°C or lower eventually accumulate in polar regions, where such air temperatures are common. Substances having condensation temperatures below --50oC remain airborne indefinitely, since not even polar regions sustain such temperatures for long.

DDT is an intermediate case on these transport scales. It does evaporate sufficiently rapidly (supercooled liquid vapor pressure is 0.005 pascals), but its relatively high condensation temperature of 13°C (55°F) means that much of it becomes permanently deposited at mid-latitudes (especially in the winter) and only a small fraction of it migrates to the Arctic.

Although PCBs are predicted by the model to deposit mainly in temperate areas rather than migrating en masse to the Arctic, the migration that does occur is sufficient that animals there are quite contaminated by these

Predicted Mobilities of Persistent Airborne Pollutants

Global Transport Behavior

Low Mobility

Relatively Low Mobility

Relatively High Mobility

Vapor pressure of liquid at 25°C in pascals*

Condensation temperature

10^4 30°C

10^2 1 -10°C -50°C

Examples PAHs

Chlorobenzenes PCBs

4-8 CI 2-4 CI ■

3 rings 5-6 CI 1-4 CI 0-1 CI

1-2 rings 0-4 CI 0-1 CI

Pesticide examples


Toxaphene Chlordane

HCB Dieldrin Hexachloro-cyclohexane

Moth balls

Source: Adapted mainly from F. Wania and D. Mackay, "Tracking the Distribution of Persistent Organic Pollutants," Environmental Science and Technology 30 (1996): 390A-396A. *For the supercooled liquid.

Source: Adapted mainly from F. Wania and D. Mackay, "Tracking the Distribution of Persistent Organic Pollutants," Environmental Science and Technology 30 (1996): 390A-396A. *For the supercooled liquid.

chemicals. The world record for PCB contamination, 90 ppm, is held by polar bears in Spitsbergen, Norway. Even breast milk is higher in PCBs for women who live in far northern areas than in more temperate ones, a result partially of their high-fat diet since organochlorines are known to accumulate in such a medium.


DDE has a 25°C vapor pressure (for its supercooled liquid) of 0.0032 Pa and a condensation temperature of ^2°C. Is DDE more or less volatile than DDT? Predict whether a larger or smaller fraction of the fraction that does vaporize will be deposited at polar latitudes compared to DDT itself.

Owing to the variations in air temperature during their transport, most molecules of mobile pollutants experience several successive cycles of evaporation and condensation as they migrate gradually toward colder climates.

FIGURE 12-4 Calculated variation with time in the geographic distribution of an airborne pollutant released at the Equator (EQ). [Source: F. Wania and D. Mackay, "Tracking the Distribution of an Airborne Pollutant/' Environmental Science and Technology 30 (1996): 390A.)

Tropic Subtropic Temperate Subpolar Polar

EQ Latitude

Tropic Subtropic Temperate Subpolar Polar

EQ Latitude

This 'grasshopper effect" is illustrated in Figure 12-4 for a pulse of a relatively mobile pollutant that was emitted near the Equator at time t0. At a later time tj, the majority of the pollutant mass is still present in tropical regions, but at a subsequent time t2 it has moved mainly to the subtropics. Whether it eventually ever moves ("hops") from temperate and subpolar regions to polar ones (at a later time t6) depends upon whether or not its mobility is sufficiently high.

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