ICCA Discussion Paper
Evaluating Long-Range Transport and
Persistence is the ability of a substance to stay unchanged in the environment for a long time.
Long-range transport is the potential for a substance to travel from its point of release to a remote
region, typically hundreds of kilometers distant.
TH (half-life) is the time it takes for half of the amount of substance to be removed from the
H (Henry’s Law constant) or air-water partition coefficient (Kaw) is a parameter that describes
the concentration ratio of a substance in equilibrium between air and water.
Kow is the octanol to water partition coefficient and is a surrogate measure for the affinity of many
organic substances to lipid and organic matter.
Biosphere is the zone of the earth from the crust into the surrounding atmosphere that contains
There is growing international concern with identifying and managing environmentally persistent
substances that are both transported to and deposited into the biosphere of regions far from the place
where they were used and released. As discussed in Technical Background Sheet 2 ("The Link
Between Persistence and Long-Range Transport"), atmospheric transport is believed to be the
primary mode for conveying persistent substances to these remote regions. For this reason, it has
been proposed that chemicals with half-lives (TH) in air >2 days have the potential for long-range
transport through the atmosphere. Justification for the 2 day criterion is illustrated in Figure 1, which
shows the results of a simple model that estimates the amount of a substance that will be transported
to a remote region as a function of the atmospheric TH of the substance and the distance (or time)
traveled. Results from the model illustrate that < 3% of a chemical will be transported a distance of
1000 km or more if the atmospheric TH of the chemical is < 2 days. In contrast, > 20% of a chemical
having a atmospheric TH of 10 days will be transported an estimated 2000 km.
Evaluating Long-Range Transport and Deposition Potential
As illustrated above, simple advective transport models and the air half-life criterion only consider
the potential for a substance to undergo long-range transport. When used as a stand-alone criterion,
atmospheric TH can not identify the potential for a substance transported to remote regions to
partition out of the atmosphere and deposit in the biosphere Substances with TH > 2 days that do not
partition out of the atmosphere may be transported long distances but will remain in the air and not
deposit. If there is no deposition, then there is no potential for exposure to organisms, including
humans, that live in these remote regions.
Using Partition Coefficients to Screen for Deposition Potential
Deposition of a substance depends on physical and chemical properties, for example vapor pressure
and water solubility, that control the ability of the substance to partition out of the atmosphere and
into the biosphere. Water is a substantial component of the biosphere. Therefore, the solubility of a
substance in air and water may be used to estimate the potential for the substance to partition from
the atmosphere to the biosphere. The relationship between solubility in air and water is described by
the Henry’s Law constant (H), which is the ratio of the substance’s solubility in air (which is
represented by the vapor pressure) to the substance’s solubility in water. The potential for a
substance to partition from air to water decreases with increasing H. Therefore, the larger the
Henry’s Law constant the lower the potential for a substance to deposit into the biosphere.
Using Henry’s Law Constant
When evaluating the combined potential for long range transport to and deposition in remote
regions, both the air half-life and Henry’s Law constant can be used. The strong influence of the
combined effects of TH and H on the potential for long-range transport and deposition of a substance
into a remote region can be illustrated by expanding the model used to generate Figure 1 to include
partitioning. Results from the illustrative model (Figure 2) demonstrate that for substances with H ³
1, 1% or less is deposited into the biosphere of the remote region (i.e., greater than 99% remains in
the atmosphere), regardless of TH. In contrast, the model predicts that deposition of known POPs, for
example DDT, PCBs and hexachlorobenzene, all of which
have H < 0.1, would be greater than 50%. These model
results also demonstrate the importance of log Kow (i.e.,
partitioning between octanol and water) on the potential for
deposition. Log Kow is important because it represents the
potential for the substance to partition from water to
organic (i.e., biological) materials, such as plants, animals
and organic coatings on soils (see "Technical Background
Sheet 3: What is Bioaccumulation?") Results from this
simple model that incorporates the combined effects of TH
and H, suggest that substances with a non-dimensional
Henry’s Law constant greater than 1 (i.e., 2.5 x 103 Pa-
m3/mol) and log Kow greater than 5 will not deposit into
the biosphere of remote regions, regardless of the half-life
of the substance in the atmosphere.
Using Partition Coefficients (Kaw and Kow) and Multi-Media Models to Screen for Deposition
Although the Henry’s Law Constant can be used for evaluating the potential for a substance to
partition between the atmosphere and biosphere, there are many other processes that can contribute
to the deposition of a chemical in the biosphere of remote regions in addition to partitioning from air
to water. These processes include dry deposition and wet deposition. Multi-Media models (Technical
Background Sheet Number 8) are useful tools for evaluating the potential interactions of and
consequences on the fate of a chemical from all the processes that effect the partitioning. Figure 3*
shows the results of such a multi-media modeling analysis for several chemicals. For this discussion
it is important to note that chemicals in Group A (upper left corner of the graph) will reside
predominantly in the air (i.e., > 99%). Therefore, chemicals that have log Kaw (air-water partition
coefficient or non-dimensional Henry’s Law constant) and log Kow in the range that places them in
group A will not have the potential for long range transport and deposition in the biosphere of
remote regions. Therefore, chemicals that have a Kaw > 0.01 and log Kow in the range of 1 to 5 will
not have the potential for long-range transport and deposition in the biosphere of remote regions.
Both of these approaches illustrate ways to use partition coefficients and half-life in air to identify
chemicals with the potential for long-range transport and deposition and provide scientifically-sound
screening approaches for identifying substances that have the potential for both long range transport
to and deposition into the biosphere of remote regions.
For More Information
-range transport and deposition: the role of
Henry’s Law constant. Final report
processes and models. The Science of the Total Environment 160/161:25-38.
chemicals. The Science of the Total Environment 160/161:211-232.
* Figure 3 is from van de Meent, D., McKone, T., Parkerton, T., Matthies, M., Scheringer, M.,
Wania, F., Purdy, R., and Bennett, D. 1999. Persistence and Transport Potential of Chemicals in a
Multimedia Environment. In Proceedings of the SETAC Pellston Workshop on Criteria for
Persistence and Long-Range Transport of Chemicals in the Environment, 14-19 July 1998, Fairmont
Hot Springs, British Columbia, Canada. Society of Environmental Toxicology and Chemistry,