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5
Atmospheric Chemistry
5.1 Introduction
In recent years, there has been an increasing interest in atmospheric chemistry. Since the
oceans are in intimate contact with the atmosphere and may act as a source or sink for
atmospheric gases, it is appropriate to briefly examine this area of science. The early interest in the atmosphere was related to the formation of photochemical smog in various cities.
Smog is formed by complicated interactions of unburned hydrocarbons from automobiles
and power plants and nitrogen oxides. The sun provides the necessary energy to furnish reactive species. More recently, interest in atmospheric chemistry has focused on the
formation of acid rain (HNO 3 and H 2 SO 4 ) from the oxidation of NO X and SO 2 gases that
result from the oxidation of fossil fuels. The decrease in the ozone layer resulting from the
use of chlorofluorohydrocarbons (CFCs) has also prompted new interest in atmospheric
chemistry. Interest has also been focused on the increasing concentration of gases that can
absorb infrared (IR) energy. These gases (CO 2 , CH 4 , etc.) contribute to the warming of the
atmosphere or the so- called greenhouse effect.
Since many of the chemical reactions of interest occur near the surface of the earth, it
is important to briefly examine the various layers of the atmosphere. The atmosphere can
be divided into four layers (see Figure 5.1): the troposphere (0 to 10 km), the stratosphere
(10 to 50 km), the mesosphere (50 to 100 km), and the thermosphere (100 to 1000 km). These
layers are marked by changes in the temperature of the atmosphere. From the surface to
10 km, the temperature decreases to a minimum at the tropopause. In the stratosphere,
the temperature increases to a maximum at the stratopause. In the mesosphere, the temperature decreases to a minimum at the mesopause. In the thermosphere, the temperature
increases again as the atmosphere diminishes. The changes in the temperature of the
atmosphere are related to the concentration of gases and the chemical reactions that occur
at various levels of the ocean. The height above the surface of the earth is similar to the
depth of water in the oceans. Since both fluids are compressible, changes in the height
or depth can change the temperature. The effect is easier to characterize in the atmosphere
since the gases behave in a near- ideal manner and can be approximated using the ideal gas
equation. The change in pressure with height z is related to the density ρ and acceleration
of gravity g (dp/ dz = –ρz).
For an ideal gas, the change in the adiabatic temperature with height can be determined
(dT/ Dz = –Mg/ Cp, where M is the molecular weight and Cp is the heat capacity). This
equation gives a change in the temperature of –9.8°/km. The decrease in the temperature
of the atmosphere from the surface to the tropopause is due to this adiabatic cooling. The
increase in the temperature between the stratosphere and the mesosphere, which goes
through a maximum at the stratopause, is the result of the adsorption of energy by ozone
5
Atmospheric Chemistry
5.1 Introduction
In recent years, there has been an increasing interest in atmospheric chemistry. Since the
oceans are in intimate contact with the atmosphere and may act as a source or sink for
atmospheric gases, it is appropriate to briefly examine this area of science. The early interest in the atmosphere was related to the formation of photochemical smog in various cities.
Smog is formed by complicated interactions of unburned hydrocarbons from automobiles
and power plants and nitrogen oxides. The sun provides the necessary energy to furnish reactive species. More recently, interest in atmospheric chemistry has focused on the
formation of acid rain (HNO 3 and H 2 SO 4 ) from the oxidation of NO X and SO 2 gases that
result from the oxidation of fossil fuels. The decrease in the ozone layer resulting from the
use of chlorofluorohydrocarbons (CFCs) has also prompted new interest in atmospheric
chemistry. Interest has also been focused on the increasing concentration of gases that can
absorb infrared (IR) energy. These gases (CO 2 , CH 4 , etc.) contribute to the warming of the
atmosphere or the so- called greenhouse effect.
Since many of the chemical reactions of interest occur near the surface of the earth, it
is important to briefly examine the various layers of the atmosphere. The atmosphere can
be divided into four layers (see Figure 5.1): the troposphere (0 to 10 km), the stratosphere
(10 to 50 km), the mesosphere (50 to 100 km), and the thermosphere (100 to 1000 km). These
layers are marked by changes in the temperature of the atmosphere. From the surface to
10 km, the temperature decreases to a minimum at the tropopause. In the stratosphere,
the temperature increases to a maximum at the stratopause. In the mesosphere, the temperature decreases to a minimum at the mesopause. In the thermosphere, the temperature
increases again as the atmosphere diminishes. The changes in the temperature of the
atmosphere are related to the concentration of gases and the chemical reactions that occur
at various levels of the ocean. The height above the surface of the earth is similar to the
depth of water in the oceans. Since both fluids are compressible, changes in the height
or depth can change the temperature. The effect is easier to characterize in the atmosphere
since the gases behave in a near- ideal manner and can be approximated using the ideal gas
equation. The change in pressure with height z is related to the density ρ and acceleration
of gravity g (dp/ dz = –ρz).
For an ideal gas, the change in the adiabatic temperature with height can be determined
(dT/ Dz = –Mg/ Cp, where M is the molecular weight and Cp is the heat capacity). This
equation gives a change in the temperature of –9.8°/km. The decrease in the temperature
of the atmosphere from the surface to the tropopause is due to this adiabatic cooling. The
increase in the temperature between the stratosphere and the mesosphere, which goes
through a maximum at the stratopause, is the result of the adsorption of energy by ozone