Geology Reference
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aerosol or HNO 3 at night and potentially lead to the generation of a
night-time source of HO x . Figure 16 provides a simplified summary of
the relevant night-time chemistry involving the nitrate radical.
One important difference between NO 3 chemistry and daytime OH
chemistry is that NO 3 can initiate, but not catalyse, the removal
of organic compounds. Therefore its concentration can be suppressed
by the presence of fast-reacting, with respect to NO 3 , organic com-
pounds.
Reactive lifetime with respect to different oxidants
Different oxidants react at different rates in the atmosphere. Further,
the average concentrations of the main oxidants can vary. For the
following table of data, calculate the relative lifetime with respect to
reaction with OH and NO 3 , given that the average [OH] E 10 6
molecule cm 3 and the [NO 3 ] E 10 9 molecule cm 3 .
k(OH) (cm 3
molecule 1
s 1 )
k(NO 3 )
(cm 3 mole-
cule 1 s 1
t OH
(days)
t NO 3
(days)
Compound
8.5 10 15
o 1 10 19
CH 4
1361
115,740
2.7 10 13
8 10 18
C 2 H 6
43
1446
8.5 10 12
2 10 16
C 2 H 4
1.3
58
1.1 10 10
4.5 10 11
2 10 4
(CH 3 ) 2 C Q C(CH 3 ) 2
0.1
The data nicely illustrate that lifetime is a product of rate and the
concentration of the radical species.
Case Study III - Evidence for the role of NO 3 in night-time oxidation
chemistry from experimental based studies. 27,28 Significant NO 3 con-
centrations have been detected over a wide range of atmospheric
conditions, indicating a potential role for NO 3 over large regions of
the atmosphere. 28 The atmospheric lifetime of NO 3 can be estimated
using the steady-state approximation (cf. PSS) to be
½ NO 3
k 52 ½ NO 2 ½ O 3
t ð NO 3 Þ¼
ð 2 : 61 Þ
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