Geology Reference
In-Depth Information
Table 3 Global turnover of tropospheric gases and fraction removed by reaction
with OH
Global emission rate(Tg yr
1
)
Removal by OH
a
(%)
Trace gas
CO
2800
85
CH
4
530
90
C
2
H
6
20
90
Isoprene
570
90
Terpenes
140
50
NO
2
150
50
SO
2
300
30
(CH
3
)
2
S
30
90
CFCl
3
0.3
0
a
Assuming mean global [OH]
¼
1
10
6
molecule cm
3
.
Source: After ref. 59.
A major component of the reactive hydrocarbon loading are the
biogenic hydrocarbons.
21
As previously indicated, the hydrocarbon
oxidation chemistry is integral to the production of ozone. Globally,
the contribution of NMHC to net photochemical production of ozone is
estimated to be about 40%.
23
There are a number of inorganic molecules such as NO
2
and SO
2
(See
Table 3) which are also lost via reaction with OH. A number of
halocarbons also exist that posses insubstantial tropospheric sinks and
have importance in the chemistry of the stratosphere (see Section 2.10).
The ultimate products of the oxidation of any hydrocarbon are
carbon dioxide and water vapour, but there are many relatively stable
partially oxidised organic species such as aldehydes, ketones and carbon
monoxide that are produced as intermediate products during this proc-
ess, with ozone produced as a by-product of the oxidation process.
Figure 10 shows a schematic representation of the free radical catalysed
oxidation of methane, which is analogous to that of a hydrocarbon. As
previously discussed, the oxidation is initiated by reaction of the hydro-
carbon with OH and follows a mechanism in with the alkoxy and peroxy
radicals are chain propagators and OH is effectively catalytic, viz
OH
þ
RH
-
R
þ
H
2
O
(2.35)
R
þ
O
2
þ
M
-
RO
2
þ
M
(2.36)
RO
2
þ
NO
-
RO
þ
NO
2
(2.37)
RO
-
carbonyl products
þ
HO
2
(2.38)
