Geology Reference
In-Depth Information
For some trace gases, for example, nitric acid vapour, dry deposition
represents a major sink mechanism. In this case the process may have a
major impact upon atmospheric lifetimes.
Worked example
Dry deposition is frequently the main sink for ozone in the rural
atmospheric boundary layer. What is the lifetime of ozone with
respect to this process?
Assuming a typical dry deposition velocity of 1 cm s
1
and a bound-
ary layer height of 1000 m, (H),
½
O
3
ð
mgm
3
s
1
Þ¼
Flux
ð
mgm
2
s
1
Þ
Mixing depth
ð
m
Þ
¼
n
d
½
O
3
H
¼
k
½
O
3
d
dt
where k
¼
n
d
/H
By analogy with Equation (7.4),
t
¼
H
n
d
¼
1000
0
:
01 s
¼
28 h
Thus taking the boundary layer as a discrete compartment, the
lifetime of ozone with respect to dry deposition is around 1 day. The
lifetime in the free troposphere (the section of the atmosphere above the
boundary layer) is longer, being controlled by transfer processes in and
out, and chemical reactions. The stratosphere lifetime of ozone is
controlled by photochemical and chemical reaction processes.
Dry deposition processes are best understood by considering a resist-
ance analogue. In direct analogy with electrical resistance theory, the
major resistances to deposition are represented by three resistors in
series. Considering the resistances in sequence, starting well above the
ground, these are as follows:
(i) r
a,
the aerodynamic resistance describes the resistance to transfer
downwards towards the surface through normally turbulent air;
