Geology Reference
In-Depth Information
NO
2
þ
O(
3
P)
NO
3
þ
hn
-
(2.56)
having a lifetime in the region of 5 s for overhead sun and clear sky
conditions. Further, NO
3
will react rapidly with NO
NO
3
þ
NO
-
NO
2
þ
NO
2
(2.57)
which can have significant daytime concentrations in contrast to the
night time, where away from strong source regions, the NO concentra-
tions should be near zero.
The nitrate radical has a range of reactivity towards VOCs. The
nitrate radical is highly reactive towards certain unsaturated hydrocar-
bons such as isoprene, a variety of butenes and monoterpenes, as well as
reduced sulfur compounds such as dimethylsulfide (DMS). In the case of
DMS, if the NO
2
concentration is 60% that of DMS then NO
3
is a more
important oxidant than OH for DMS in the MBL.
27
In general, NO
3
abstraction reactions of the type
NO
3
þ
RH
-
HNO
3
þ
R
(2.58)
are relatively slow, with the alkyl radical reacting with oxygen under
atmospheric conditions to form a peroxy radical. In the case of RH being
an aldehyde, acyl products will form acylperoxy radicals (R
CO
O
2
),
potential sources of peroxyacylnitrates. In contrast, the reaction of NO
3
with alkenes occurs by an addition mechanism, initiating a complex
chemistry involving nitrooxy substituted organic radicals, which can
either regenerate NO
2
or produce comparatively stable bifunctional or-
ganic nitrate products.
28
For example, the products derived from the
reaction of NO
3
with propene in the presence of O
2
and NO
x
include
CH
3
CHO, HCHO, 1,2-propanedioldinitrate (PDDN), nitroxyperoxyp-
ropylnitrate (NPPN) and a-(nitrooxy)acetone (See Figure 15).
3
The
reaction channel that produces the nitrated acetones also yields peroxy
radicals, leading to the potential for a night-time source of OH, either by
reaction (HO
2
þ
O
3
) or by the direct reaction of the peroxy radical with
NO
3
.ForthereactionofNO
3
with propene the initial addition can take
place at either end of the double bond, viz
NO
3
þ
CH
2
QCHCH
2
þ
M
CH
2
CHCH
2
(ONO
2
)
þ
M (2.59)
-
NO
3
þ
CH
2
Q
CHCH
2
þ
M
-
CH
2
CH(ONO
2
)CH
2
þ
M (2.60)
The reaction can then proceed by the mechanism shown schematically in
Figure 15. The ratio of final products is dependent on the structure of
the individual alkenes. In general,
for branched alkenes,
there is
