Environmental Engineering Reference
In-Depth Information
N-N distance. The LUMO is an antibonding 3
ˀ
character is strengthened by the fact that oxygen has a higher electronegativity
than nitrogen [
10
]. The N
2
O molecule exhibits three fundamental absorp-
tions with superimposed rotational fine structure in infrared spectroscopy, an asym-
metric stretch (
ˀ
molecular orbital, whose
+
)at
1284.91 cm
-1
, a bend (
588.77 cm
-1
,and
ʣ
ν
1
¼
ʠ
ν
2
¼
)at
+
)at
2223.76 cm
-1
. Both stretches (
ʣ
ν
3
¼
a symmetric stretch (
ν
3
)alsoappear
in the Raman spectrum, and these properties provided initial proof that the linear
structure of the molecule was asymmetric. Nitrous oxide liquefies at 183.7 K
and solidifies at 170.8 K. It is well soluble in water with a mole fraction of 4.4
10
-4
at 298 K, so that solvated nitrous oxide is readily accessible as an enzyme
substrate [
11
]. Hydration of N
2
O in water could potentially yield hyponitrous acid
(H
2
N
2
O
2
),aweakacidwithp
K
1
¼
ν
1
and
11.5, but the equilibrium for
this reaction is far on the side of N
2
O, which is consequently the dominant species
in aqueous solution.
7.2 and p
K
2
¼
2.2 Nitrous Oxide, the Greenhouse Effect,
and Ozone Depletion
In the atmosphere, nitrous oxide appears as a trace gas with a current concentration
of about 325 ppb, and it can be traced through climate history by analyzing air
entrapped in polar ice cores. Interestingly, the present concentration is higher than
at any time during the last 45,000 years [
12
]. From low levels during the last glacial
period (the W¨rm ice age in the European Alpine region) the atmospheric concen-
tration of N
2
O increased to about 275 ppb and remained largely constant for the last
10,000 years. It started to rise in the 19th century, concomitant with the onset of
industrialization, and is increasing since, at an average annual rate of 0.8 ppbv [
13
].
Due to its chemical stability, the atmospheric half-life of N
2
O is estimated to be
120 years, so that any increase in production will lead to an atmospheric accumu-
lation that persists for centuries to come [
14
].
The problematic aspect of this increase is that N
2
O is a potent greenhouse gas
that absorbs sunlight reflected from the Earth's surface with approximately the
300-fold efficiency of CO
2
[
14
,
15
]. The control and reduction of N
2
O emissions
thus should be an imminent aspect of global climate policies, but while attempts are
undertaken to regulate and limit CO
2
emissions, the impact of nitrous oxide has
long gone unnoticed. Only in 2009, the gas has been designated the dominant
anthropogenic emission of the 21st century [
16
], and public awareness is rising.
Most critically, nitrous oxide not only acts as a greenhouse gas, but is also involved
in complex atmospheric chemistry.
N
2
O is the predominant source of stratospheric NO
x
that react with and deplete
the reservoir of ozone, O
3
. However, in conjunction with the notorious ozone-
depleting chlorofluorocarbons (CFC) the effect of NO
x
may even be beneficial, as
chlorine oxide radicals generated through the decomposition of CFCs react with
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