Geology Reference
In-Depth Information
2.5.1 Nitrogen Oxides and the Photostationary State
From the preceding discussion, it can be seen that the chemistry of
nitrogen oxides are an integral part of tropospheric oxidation and
photochemical processes. Nitrogen oxides are released into the tropo-
sphere from a variety of biogenic and anthropogenic sources including
fossil fuel combustion, biomass burning, microbial activity in soils and
lightning discharges (see Figure 2). About 30% of the global budget of
NO
x
, i.e. (NO
þ
NO
2
) comes from fossil fuel combustion with almost
86% of the NO
x
emitted in one form or the other into the planetary
boundary layer from surface processes.
5
Typical NO/NO
2
ratios in
surface air are 0.2-0.5 during the day tending to zero at night. Over
the timescales of hours to days NO
x
is converted to nitric acid (reaction
(2.23)) and nitrates, which are subsequently removed by rain and dry
deposition.
The photolysis of NO
2
to NO and the subsequent regeneration of NO
2
via reaction of NO with ozone is sufficiently fast, in the moderately
polluted environment, for these species to be in dynamic equilibrium, viz
NO
2
þ
hn
NO
þ
O(
3
P)
-
(2.2)
O(
3
P)
þ
O
2
þ
M
-
O
3
þ
M
(2.20)
O
3
þ
NO
-
NO
2
þ
O
2
(2.24)
Therefore, at suitable concentrations, ambient NO, NO
2
and O
3
can be
said to be in a photochemical steady-state or photostationary state
(PSS),
15
provided that they are isolated from local sources of NO
x
and
that sunlight intensity is relatively constant, therefore
½
O
3
¼
j
2
:
2
½
NO
2
k
2
:
24
½
NO
ð
2
:
25
Þ
The reactions (2.2, 2.20 and 2.24) constitute a cycle with no net chem-
istry. The PSS expression is sometimes expressed as a ratio, viz
j
2
:
2
½
NO
2
k
2
:
24
½
NO
½
O
3
f
¼
ð
2
:
26
Þ
If ozone is the sole oxidant for NO to NO
2
then f
¼
1. This situation
often pertains in urban areas where NO
x
levels are high and other
potential oxidants of NO to NO
2
such as peroxy radicals are suppressed.
In the presence of peroxy radicals, Equation (2.25) has to be modified as
