Geology Reference
In-Depth Information
7.1.2.1 Influence of Lifetime on Environmental Behaviour. Some
knowledge of environmental lifetimes of chemicals is very valuable in
predicting their environmental behaviour. In relation to the atmosphere,
there is an interesting relationship between the spatial variability in the
concentrations of an atmospheric trace species and its atmospheric
lifetime.
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Compounds such as methane and carbon dioxide with a long
lifetime with respect to removal from the atmosphere by chemical
reactions or dry and wet deposition (see Section 7.2 of this chapter)
show little spatial variability around the globe, as their atmospheric
lifetime (several years) exceeds the timescale of mixing of the entire
troposphere (of the order of a year). On the other hand, for a short-lived
species such as nitrogen dioxide, removal by chemical means or dry or
wet deposition occurs much more quickly than atmospheric mixing and
hence there is very large spatial variability, with concentrations some-
times exceeding 100 ppb in urban areas, while remote atmosphere
concentrations can be a the level of a few parts per trillion. By analogy,
short-lived species also show a much greater hour-to-hour and day-to-
day variation at a given measuring point than long-lived species for
which local sources impact only to a modest degree on the existing
background concentration.
This illustration using the atmosphere can be taken somewhat further
in relation to other environmental media. Lifetimes of highly soluble
species such as sodium and chloride in the oceans are long compared to
the mixing times and therefore variations in salinity across the world's
oceans are relatively small (see Chapter 4). In contrast, where soils are
concerned, mixing times will generally far exceed lifetimes and extreme
local hot spot concentrations can be found where soils have become
polluted.
Lifetime also influences the way in which we study the environmental
cycles of pollutants. In the case of reactive atmospheric pollutants, it is
the reaction rate, or rate of dry or wet deposition, which determines the
lifetime. We are therefore concerned mainly with the rates of these
processes in determining the atmospheric cycle. In the case of longer-
lived species, such as persistent organic compounds like PCBs and
dioxins, chemical reaction rates are rather slow and these compounds
can approach equilibrium between different environmental media such
as the atmosphere and surface ocean or the atmosphere and surface soil,
with evaporation exceeding deposition during warmer periods and wet
and dry depositions replacing the contaminant in the soils or oceans in
cooler weather conditions. Both the kinetic approach dealing with
reaction rates and the thermodynamically based approach considering
partition between environmental media will be introduced in this
