Geology Reference
In-Depth Information
the NO/NO
2
partitioning is shifted to favour NO
2
, viz
½
NO
2
½
NO
¼ð
k
2
:
24
½
O
3
þ
k
2
:
19
½
HO
2
þ
k
2
:
21
½
RO
2
Þ=
j
2
:
2
ð
2
:
27
Þ
Though the radical concentrations are typically ca. 1000 times smaller
than the [O
3
], the rate of the radical oxidation of NO to NO
2
is ca. 500
times larger than the corresponding oxidation by reaction with O
3
.We
shall return to the significance of the peroxy radical catalysed oxidation
of NO to NO
2
when considering photochemical ozone production and
destruction. From the preceding discussion, it can be seen that the
behaviour of NO and NO
2
are strongly coupled through both photolytic
and chemical equilibria. Because of their rapid interconversion they are
often referred to as NO
x
.NO
x
, i.e. (NO
þ
NO
2
) is also sometimes
referred to as ''active nitrogen''.
Photostationary state
At what NO
2
/NO ratio will the PSS ratio be equal to 1 for midday
conditions (j
2.2
¼
1
10
3
s
1
) given that k
2.24
¼
1.7
10
14
cm
3
molecule
1
s
1
and that O
3
¼
30 ppbv.
Using the Equation (2.26)
f
¼
j
2
:
2
½
NO
2
k
2
:
24
½
NO
½
O
3
Converting O
3
from ppbv to molecule cm
3
, as 1 ppbv
¼
2.46
10
10
molecule cm
3
(at 251C and 1 atm) 30 ppbv
¼
7.38
10
11
molecule
cm
3
. Given that f
¼
1 then
½
NO
2
½
NO
¼
k
2
:
24
½
O
3
j
2
:
2
[NO
2
]/[NO]
¼
12.55
The extent of the influence of NO
x
in any given atmospheric situation
depends on its sources, reservoir species and sinks. Therefore, an
important atmospheric quantity is the lifetime of NO
x
. If nitric acid
formation is considered to be the main loss process for NO
x
(i.e. NO
2
),
then the lifetime of NO
x
(t
NO
x
) can be expressed as the time constant for
reaction (2.23), the NO
2
to HNO
3
conversion.
1
k
2
:
23
½
OH
1
þ
½
NO
½
NO
2
t
NO
x
¼
ð
2
:
28
Þ
