McKay, Donald. "Front matter" Multimedia Environmental Models ...

©2001 CRC Press LLC
f = 15/0.5 = 30
and the rates of output, Df, are 12 and 3 mol/h, totaling 15 mol/h as before.
It is apparent that the air D value is larger and most significant. D values can be
added when they are multiplied by a common fugacity. Therefore, it becomes
obvious which D value, and hence which process, is most important. We can arrive
at the same conclusion using partition coefficients, but the algebra is less elegant.
Note that how the chemical enters the environment is unimportant, all sources
being combined or lumped in I, the overall input. This is because, once in the
environment, the chemical immediately achieves an equilibrium distribution, and it
“forgets” its origin.
6.2.4 Advective Processes
In an evaluative environment, there are several advective flows that convey
chemical to and from the environment, namely,
1. inflow and outflow of air
2. inflow and outflow of water
3. inflow and outflow of aerosol particles present in air
4. inflow and outflow of particles and biota present in water
5. transport of air from the troposphere to the stratosphere, i.e., vertical movement
of air out of the environment
6. sediment burial, i.e., sediment being conveyed out of the well mixed layer to depths
sufficient that it is essentially inaccessible
7. flow of water from surface soils to groundwater (recharge)
It also transpires that there are several advective processes which can apply to
chemical movement within the evaluative environment. Notable are rainfall, water
runoff from soil, sedimentation, and food consumption, but we delay their treatment
until later.
In situations 1 through 4, there is no difficulty in deducing the rate as GC or Df,
where G is the flowrate of the phase in question, C is the concentration of chemical
in that phase, and the Z value applies to the chemical in the phase in which it is
dissolved or sorbed.
For example, aerosol may be transported to an evaluative world in association
with the inflow of 10 12 m 3 /h of air. If the aerosol concentration is 10 –11 volume
fraction, then the flowrate of aerosol G Q is 10 m 3 /h. The relevant concentration of
chemical is that in the aerosol, not in the air, and is normally quite high, for example,
100 mol/m 3 . Therefore, the rate of chemical input in the aerosol is 1000 mol/h. This
can be calculated using the D and f route as follows, giving the same result.
If Z Q = 10 8 , then
f = C Q/Z Q = 100/10 8 = 10 –6 Pa
D AQ = G QZ Q = 10 ¥ 10 8 = 10 9