Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 338

Stratosphere

Solar
energy

Reflected radiation

Re-radiated to space

Re-radiated
To space

CO 2 , CO 4 , H 2 O, N 2 O

Attenuated radiation

Infra red radiation
emitted from earth

Radiated to
surface

Atmosphere

Heat reflected
or radiated
from surface

Earth

FIGURE 6.38 Solar energy absorbed and reflected in the earth's atmosphere.

responsible for this. During the pre-industrial era, the CO 2 concentration in the atmo-
sphere remained fairly constant. In 1850 the average concentration was 270 ppmv.

Since then the concentration has steadily increased. In 1957 it was 315 ppmv and
in 1992 it was 356 ppmv. Figure 6.39 shows the concentration of CO 2 observed at
the Mauna Lao observatory in Hawaii. The oscillations in CO 2 concentration reflect
the seasonal cycles of photosynthesis and respiration by the biota in the northern
hemisphere. Note the steady increase in CO 2 . This increase is attributed to the burn-
ing of fossil fuels (coal and oil), and is therefore strictly anthropogenic in origin
(Figure 6.40). Human intervention through deforestation of tropical forests also plays
a role by reducing photosynthesis that fixes CO 2 from the atmosphere. The increase
in CO 2 emissions worldwide increases the capacity of the earth's atmosphere to trap
the outgoing radiation. The other trace gases such as CH 4 , CO, NO x , and CFCs also
perform a similar function, since their concentrations have also increased steadily
with time (see Table 6.11). Presently, CO 2 contributes the largest toward the green-
house effect. Figure 6.41 indicates the radiative forcing effects of various greenhouse
gases (GHG) as predicted by the most recent evaluation by the International Panel on
Climate Change (IPCC, 2007).
The ability of CO 2 to initiate the greenhouse effect has been contemplated as far
back as 1896 by the Swedish chemist SvänteArrhenius (of theArrhenius equation for
rate constant in Chapter 5). But only in the past decade has the prospect of a global
greenhouse effect gained attention and been recognized to have serious repercussions
within the lifetime of the present generation. Recently, however, some good news
on this front has emerged. Globally the rate of input of CFCs into the atmosphere
has slowed down. This can be traced to the worldwide ban on its production and the
implementation of the Montreal Protocol.

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