Geology Reference
In-Depth Information
A useful quantity with which to compare this with is the reciprocal of
the lifetime calculated from the sum of the first order loss processes
involving NO
3
and N
2
O
5
t
ð
NO
3
Þ
1
X
i
k
ð
NO
3
þ
HC
i
Þ
½
HC
i
þ
k
ð
NO
3
þ
DMS
Þ
½
DMS
þ
k
het
ð
NO
3
Þ
þ
k
I
½
H
2
O
þ
k
II
½
H
2
O
2
þ
k
het
ð
N
2
O
5
Þ
:
K
2
:
54
½
NO
2
ð
2
:
62
Þ
where the pseudo first-order loss rates over the i reactive hydrocar-
bons are summed and k
I
and k
II
are the first- and second-order
components with respect to H
2
O of the reaction
N
2
O
5
þ
H
2
O
-
2HNO
3
(2.63)
k
het
(NO
3
) and k
het
(N
2
O
5
) are the heterogeneous loss rates for these
species and K
2.54
is the equilibrium constant for reactions (2.53 and
2.54). Figure 17 shows the observed lifetime of NO
3
in the MBL at
Mace Head, Ireland
29
segregated by arrival wind sector. Mace Head
experiences a range of air masses from clean marine air to European
continental outflow. The measured NO
3
lifetime varies from 2 min to
4 h (see Figure 17). An assessment of the parameters controlling NO
3
atmospheric lifetime (cf. Equation (2.61)) highlights that, under the
conditions encountered the lifetime of NO
3
chemistry is very sensitive
to DMS and NMHC chemistry in clean marine air. However, in
more polluted air the terms involving the indirect loss of N
2
O
5
either
in the gas-phase with H
2
O or through uptake on aerosol tend to
dominate.
From the preceding discussion it can be seen that the involvement of
NO
3
chemistry in gas-phase tropospheric chemistry has potentially six
significant consequences.
28
(i) The radical can control NO
y
speciation in the atmosphere at
night (via reaction (2.54)).
(ii) Nitric acid can be formed, by hydrolysis of N
2
O
5
, as a product of
a hydrogen abstraction process or indirectly via the NO
3
-medi-
ated production of OH, which can react with NO
2
(reaction
(2.23)) to produce nitric acid.
(iii) Primary organic pollutants can be oxidised and removed at night.
(iv) Radicals (HO
x
and RO
2
) produced by NO
3
chemistry can act as
initiators for chain oxidation chemistry.
