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SO
2
+
O
−→
SO
3
,
(6.160)
SO
3
+
H
2
O
−→
H
2
SO
4
.
There are two absorption bands for SO
2
: one is a weak band at 384 nm, which
gives rise to the excited triplet state of SO
2
, and the other is a strong absorp-
tion at 294 nm, which gives rise to a higher energy excited singlet state. These
reactions are slow and do not account for the observed rates of SO
2
oxidation
in the atmosphere (
0.01-0.05 h
−
1
)
. The oxidative process in the gas phase is
driven by the abundantly available highly reactive free radical species OH
•
in the
atmosphere.
≈
OH
•
−→
SO
2
+
..................
−→
H
2
SO
4
(6.161)
Oxidation of SO
2
to sulfate in atmospheric moisture is catalyzed by species such
as ozone, hydrogen peroxide, metal ions (Fe(III), Mn(III)), and nitrogen (N(III) and
NO
2
)
. The rate constants and rate expressions for several of these reactions have been
studied by Hoffmann and co-workers and are given in Table 6.9. For pH values
<
4,
the predominant oxidizer is H
2
O
2
, whereas for pH
=
5, O
3
is an order of magnitude
more powerful as an oxidizer. Only at pH
≥
5 is the catalyzed oxidation by Fe and
Mn of any significance.
The following example is to illustrate how the change in pH of an open system
can be computed as the sulfate content in the atmosphere increases. The problem
can be extended to realistic atmospheric conditions, and described by Seinfeld and
Pandis (2006).
TABLE 6.9
Oxidation of SO
2
to Sulfate by Different Species
in the Atmosphere
d [
S (IV)
]
d
t
Oxidizer
r
=
k
0
SO
2
·
H
2
O
+
k
1
HSO
3
+
k
2
SO
2
3
O
3
w
Ozone
k
H
+
HSO
3
1
+
K
H
+
⎞
⎠
H
2
O
2
w
Mn(II)
k
2
[Mn(II)] [HSO
3
]
Fe(II)
k
=[
Fe(III)
][
SO
2
−
3
⎛
⎝
Hydrogen peroxide
]
Note: k
0
=
2.4
×
10
4
L/mol s,
k
1
=
3.7
×
10
5
L/mol s,
k
2
=
1.5
×
10
9
L/mol s,
k
=
7.5
×
10
7
L/mol s,
K
=
13 L/mol,
k
2
=
3.4
×
10
3
L/mol s,
k
=
1.2
×
10
6
L/mol s.
Source:
From Hoffmann, M.R. and Calvert, J.G. 1985.
Chemical Transforma-
tion Modules for Eulerian Acid Deposition Models. Volume II. The
Aqueous-phase Chemistry
. Boulder, CO: National Center for Atmo-
spheric Research.
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