Geology Reference
In-Depth Information
where t is the time, u, v and w are the components of the wind vector in
x, y and z accounting for the horizontal and vertical large-scale trans-
port. Small-scale turbulence can be accounted for using K
z
, the turbulent
diffusion coefficient, P
i
and L
i
are the chemical production and loss
terms and S
i
are the sources owing to emissions. Cloud processes
(vertical transport, washout and aqueous phase chemistry) are repre-
sented in a cloud processing term. The application of this type of
equation is the basis of chemical modelling. In this chapter we will
concentrate mainly on the chemical terms in this equation and the
processes that control them, but inherently, as the study of tropospheric
photochemistry is driven by observations, these must be placed within
the framework of Equation (2.6).
2.5 TROPOSPHERIC OXIDATION CHEMISTRY
Though atmospheric composition is dominated by both oxygen and
nitrogen, it is not the amount of oxygen that defines the capacity of the
troposphere to oxidise a trace gas. The ''oxidising capacity'' of the
troposphere is a somewhat nebulous term probably best described by
Thompson.
13
The total atmospheric burden of O
3
, OH and H
2
O
2
determines the
''oxidising capacity'' of the atmosphere. As a result of the multiple
interactions among the three oxidants and the multiphase activity of
H
2
O
2
, there is no single expression that defines the earth's oxidising
capacity. Some researchers take the term to mean the total global OH,
although even this parameter is not defined unambiguously.
Figure 8 gives a schematic representation of tropospheric chemistry,
representing the links between emissions, chemical transformation and
sinks for a range of trace gases.
Atmospheric photochemistry produces a variety of radicals that exert
a substantial influence on the ultimate composition of the atmosphere.
14
Probably the most important of these in terms of its reactivity is the
hydroxyl radical, OH. The formation of OH is the initiator of radical-
chain oxidation. Photolysis of ozone by UV light in the presence of water
vapour is the main source of hydroxyl radicals in the troposphere, viz
O(
1
D)
þ
O
2
(
1
D
g
)
O
3
þ
hn(l
o
340 nm)
-
(2.7)
O(
1
D)
þ
H
2
O
-
OH
þ
OH
(2.8)
