Environmental Engineering Reference
In-Depth Information
TABLE 6.10
Characteristic Times for S(IV) Oxidation in an Aqueous Droplet
Process
Expression for
τ
Typical Value
τ
g
=
R
2
/
4
D
g
2.5
×
10
−
6
s
Diffusion in the gas phase
2
)D
w
K
aw
Equilibrium at the air-water interface
τ
i
=
(
2
π
MRT/
α
0.15 s
H
+
]+[
HSO
3
]
)
]
−
1
10
7
s
Dissociation in the aqueous droplet
τ
d
=[
k
1
+
k
−
1
(
[
2
×
R
2
/
4
D
w
Diffusion in the aqueous phase
τ
a
=
0.025 s
Reaction within the aqueous droplet
τ
r
=−[
S(IV)
]
/
{
d
[
S(IV)
]
/
d
t
}
≈
1.5 s
Definitions of characteristic times:
τ
g
is the time to achieve steady-state concentration in the gas phase around the droplet;
τ
i
the time to achieve local equilibrium at the interface;
τ
d
the time to achieve equilibrium for the dissociation reaction;
τ
a
the time to achieve uniform steady-state concentration in the droplet; and
τ
r
the time to convet 1/
e
of the reactants to products.
Note: R
=
10
μ
m,
D
g
=
0.1 cm
2
/s,
k
1
=
3.4
×
10
6
s
−
1
,
k
−
1
=
2
×
10
8
mol/L s,
D
w
=
10
−
5
cm
2
/s,
M
K
aw
/
α
2
=
98, pH
=
4,
P
SO
2
=
1 ppbv,
P
H
2
O
2
=
1 ppbv.
Source:
From Seinfeld, J.H. 1986.
Atmospheric Chemistry and Physics
, p. 390, NewYork: Wiley.
6.3.3.2
Global Warming and Greenhouse Effect
Our atmosphere was, at one time, oxygen rich. The highly oxidative atmosphere
could not sustain early life forms. Gradually, the atmosphere became oxygen depleted
(nitrogen rich) and evolved into the one that we have today. The present atmosphere,
conducive to life, is believed to be sustained by a symbiosis between the biota and
the various atmospheric processes. This is the central theme of the so-called
Gaia
hypothesis
(Lovelock, 1979). In short, our planet is a gigantic experiment, whether
by design or by chance.
The composition of air is 78% (v/v) nitrogen and 20% (v/v) oxygen. All other
gases together constitute the remaining 2% (v/v) of our atmosphere; these are called
trace gases
. Of the trace gases, a few are of special relevance. Rare gases such as
argon do not vary in concentration to any measurable extent. Other gases such as
CO
2
, CO, NO
x
,CH
4
, and CFCs are variable. Even though these species are at trace
concentrations, they exert profound effects on the environment.
The atmosphere does not absorb incoming energy from the sun in the visible region
of the spectrum. The stratosphere absorbs only a part of the UV radiation. The cloud
and the earth's surface reflect a large fraction of the radiation. A portion of the sun's
energy is used to heat the surface of the earth, which reradiates heat in the infrared
region of the spectrum (4-100
m) is
absorbed in the atmosphere, mostly by water and CO
2
.Water drops in the atmosphere
also absorb energy in the region between 4 and 7
μ
m).A portion of the reradiated energy (13-100
μ
μ
m. It is in the window of 7-13
μ
m
that heat escapes freely into space (Figure 6.38).
The atmosphere acts as a greenhouse trapping moderate amounts of heat energy
that is reradiated from the earth's surface, and thus maintaining a comfortable aver-
age temperature that sustains life on earth. The atmospheric CO
2
and H
2
O are mainly
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