Environmental Engineering Reference
In-Depth Information
E
XAMPLE
6.23 G
LOBAL
T
EMPERATURE
C
HANGE WITH
CO
2
I
NCREASE IN THE
A
TMOSPHERE
The pre-industrial era partial pressure of CO
2
was 280 ppmv. If a doubling of CO
2
in
the atmosphere changes the atmospheric temperature by 3
◦
C, what is the partial pres-
sure of CO
2
for an observed temperature change of 1
◦
C?
Δ
T
=
1
◦
C,
Δ
T
∗
=
3
◦
C,
P
=
280 ppmv. Hence
1
3
=
0.693
ln
P
CO
2
,
1
280
which gives
P
CO
2
=
352 ppmv.
6.3.3.3
Ozone in the Stratosphere and Troposphere
In this section, we will explore the chemical kinetics of atmospheric systems that
involve interactions between several species.An interesting example is the chemistry
of ozone, in both the upper stratosphere and the lower atmosphere (troposphere).
The troposphere extends up to 16 km and the stratosphere extends up to 50 km above
the surface of the earth. The pressure in the stratosphere decreases exponentially
with increasing altitude. This has interesting consequences for chemical reactions
in the stratosphere. Reduced pressure leads to reduced reaction rates. The reduced
temperature in the upper troposphere impacts reactions with high activation energies;
the rates of these reactions are lowered. Thus the lower troposphere is a far more
reactive region than the upper troposphere.
The major constituents of the atmosphere are nitrogen and oxygen. Generally these
are not the dominant reactive species. Trace species such as ozone, hydroxyl radical,
CO
2
,SO
2
,CH
4
,NO
x
, and CFCs are the ones that significantly impact atmospheric
chemistry. We have already seen that OH
•
is the most reactive species and appropri-
ately termed the “atmospheric detergent.” It is typically present at mixing ratios of
1to4
10
−
14
, despite the fact that the atmosphere contains 21% molecular oxygen
by volume. The GHG (CO
2
,CH
4
, and CFCs) were discussed in the previous section.
SO
2
and NO
x
participate in the acidity of the atmosphere and were also discussed in
an earlier section.
From the viewpoint of exploring different reaction kinetics in the stratosphere
and troposphere, ozone is a good choice. The total mass of ozone in dry air is
∼
3.3
×
10
12
kg. It has a maximum concentration of
×
g/m
3
in the upper stratosphere at
about 30 km height. In the troposphere it varies between 60 and 100
∼
500
μ
g/m
3
in clean
air. The significance of ozone in the stratosphere lies in its ability to shield the earth
from the harmful effects of the sun's UV radiation. However, in the lower troposphere
it is an undesirable species since it leads to the formation of smog. The U.S. federal
regulations on air quality stipulate that ozone concentration greater than 235
μ
g/m
3
is harmful and can cause breathing problems and eye irritations. Several cities in the
United States have low air quality due to nonattainment of ozone levels.
Let us consider the formation and dissipation of ozone in the upper stratosphere.
μ
At
∼
30 km height, ozone forms via dissociation of molecular oxygen into O atoms
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