Geology Reference
In-Depth Information
(ii) r
b
, the boundary layer resistance describes the transfer through a
laminar boundary layer (approximately 1mm thickness) at the
surface;
(iii) r
s
, the surface (or canopy) resistance is the resistance to uptake by
the surface itself. This can vary enormously, from essentially zero
for very sticky gases such as HNO
3
vapour, which attach irre-
versibly to surfaces, to very high values for gases of low water
solubility, which are not utilised by plants (e.g. CFCs).
Since these resistances operate essentially in series, the total resistance,
R, which is the inverse of the deposition velocity, is equal to the sum of
the individual resistances.
R
¼
1
n
d
¼
r
a
þ
r
b
þ
r
s
ð
7
:
8
Þ
Some trace gases have a net source at the ground surface and diffuse
upwards; an example is nitrous oxide.
Whether the flux is downward or upward, it is driven by a concen-
tration gradient in the vertical, dc/dz. The relationship between flux, F,
and concentration gradient is
F
¼
K
z
d
c
d
z
where K
z
is the eddy diffusivity in the vertical (a measure of the
atmospheric conductance). Fluxes, and thus deposition velocities, can
be estimated by measurement of a concentration gradient simultane-
ously with the eddy diffusivity.
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It is usually assumed that trace gases
transfer in the same manner as sensible heat (i.e. convective heat
transfer, not radiative or latent heat) or momentum. Thus, the eddy
diffusivity for either of these parameters is measured usually from
simple meteorological variables (gradients in temperature and wind
speed).
A few substances are capable of showing both upward and downward
fluxes. An example is ammonia. Ammonium in the soil, NH
4
1
,isin
equilibrium with ammonia gas, NH
3(g)
NH
4
1
þ
H
2
O
NH
3(g)
þ
H
3
O
1
"
(7.9)
when atmospheric concentrations of ammonia exceed equilibrium
concentrations at the soil surface (known as the compensation point),
the net flux of ammonia is downwards. When atmospheric concen-
trations are below the equilibrium value, ammonia is released into the
air.
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