Geology Reference
In-Depth Information
2.7 OZONE-ALKENE CHEMISTRY
Another potential dark source of HO
x
in the atmosphere, more partic-
ularly in the boundary layer, is from the reactions between ozone and
alkenes. The ozonolysis of alkenes can lead to the direct production of
the OH radical at varying yields (between 7 and 100%) depending on the
structure of the alkene, normally accompanied by the co-production of
an (organic) peroxy radical. As compared to both the reactions of OH
and NO
3
with alkenes the initial rate of the reaction of ozone with an
alkene is relatively slow, this can be offset under regimes where there are
high concentrations of alkenes and/or ozone. For example, under typical
rural conditions the atmospheric lifetimes for the reaction of ethene with
OH, O
3
and NO
3
are 20 h, 9.7 days and 5.2 months, respectively in
contrast, for the same reactants with 2-methyl-2-butene the atmospheric
lifetimes are 2.0 h, 0.9 h and 0.09 h.
The mechanism for the reaction of ozone with alkenes was first
suggested by Criegee
30
in the late 1940s and involves the addition of
ozone to form a primary ozonide, which rapidly decomposes to form a
vibrationally excited carbonyl oxide (Criegee intermediate) and carbonyl
products. The Criegee intermediate can then either be collisionally
stabilised by a third body (M), or undergo unimolecular decomposition
to products (see also Section 2.8). It is now widely believed that alkyl-
substituted Criegee intermediates can decompose via a vibrationally hot
hydroperoxide intermediate to yield an OH radical, along with another
radical species of the general form R
1
R
2
CC(O)R
3
, which is expected to
react rapidly with O
2
to form a peroxy radical (RO
2
) in the atmos-
phere.
31
Figure 18 shows a schematic representation of the ozone-alkene
reaction mechanism. The OH and peroxy radical yield is dependent on
the structure and mechanism of the individual alkene-ozone reaction.
32
Table 5 shows typical OH yields for the reaction of a range of anthro-
pogenic and biogenic alkenes with ozone. There is growing experimental
evidence
33
of the importance of ozone-alkene reactions as significant
oxidative sinks for alkenes and a general source of HO
x
.
2.8 SULFUR CHEMISTRY
Sulfur chemistry is an integral part of life, owing to its role in plant and
human metabolism. Sulfur compounds have both natural and anthro-
pogenic sources. In modern times, the atmospheric sulfur budget has
become dominated by anthropogenic emissions, particularly from fossil
fuel burning. It is estimated that 75% of the total sulfur emission budget
is dominated by anthropogenic sources with 90% of it occurring in the
