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

Therefore,
©2001 CRC Press LLC
N = Df = 10 9 ¥ 10 –6 = 1000 mol/h
Treatment of transport to the stratosphere is somewhat more difficult. We can
conceive of parcels of air that migrate from the troposphere to the stratosphere at
an average, continuous rate, G m 3 /h, being replaced by clean stratospheric air that
migrates downward at the same rate. We can thus calculate the D value. As discussed
by Neely and Mackay (1982), this rate should correspond to a residence time of the
troposphere of about 60 years, i.e., G is V/t. Thus, if V is 6 ¥ 10 9 and t is 5.25 ¥
10 5 h, G is 11400 m 3 /h. This rate is very slow and is usually insignificant, but there
are situations in which it is important.
We may be interested in calculating the amount of chemical that actually reaches
the stratosphere, for example, freons that catalyze the decomposition of ozone. This
slow rate is thus important from the viewpoint of the receiving stratospheric phase,
but is not an important loss from the delivering, or tropospheric, phase. Second, if
a chemical is very stable and is only slowly removed from the atmosphere by reaction
or deposition processes, then transfer to the troposphere may be a significant mechanism
of removal. Certain volatile halogenated hydrocarbons tend to be in this class.
If we emit a chemical into the evaluative world at a steady rate by emissions and
allow for no removal mechanisms whatsoever, its concentrations will continue to
build up indefinitely. Such situations are likely to arise if we view the evaluative
world as merely a scaled-down version of the entire global environment. There is
certainly advective flow of chemical from, for example, the United States to Canada,
but there is no advective flow of chemical out of the entire global atmospheric
environment, except for the small amounts that transfer to the stratosphere. Whether
advection is included depends upon the system being simulated. In general, the
smaller the system, the shorter the advection residence time, and the more important
advection becomes.
Sediment burial is the process by which chemical is conveyed from the active
mixed layer of accessible sediment into inaccessible buried layers. As was discussed
earlier, this is a rather naive picture of a complex process, but at least it is a starting
point for calculations. The reality is that the mixed surface sediment layer is rising,
eventually filling the lake. Typical burial rates are 1 mm/year, the material being
buried being typically 25% solids, 75% water. But as it “moves” to greater depths,
water becomes squeezed out. Mathematically, the D value consists of two terms,
the burial rate of solids and that of water.
For example, if a lake has an area of 10 7 m 2 and has a burial rate of 1 mm/year,
the total rate of burial is 10,000 m 3 /year or 1.14 m 3 /h, consisting of perhaps 25%
solids, i.e., 0.29 m 3 /h of solids (G S) and 0.85 m 3 /h of water (G W). The rate of loss
of chemical is then
G SC S + G WC W = G SZ Sf + G WZ Wf = f(D AS + D AW)
Usually, there is a large solid to pore water partition coefficient; therefore, C S
greatly exceeds C W or, alternatively, Z S is very much greater than Z W, and the term