118ATMOSPHERIC CHEMISTRY
INTRODUCTIONAtmospheric chemistry is a broadly based area of scientific
endeavor. It is directed at determining the quantities of vari-
ous chemicals in the atmosphere, the origin of these chemi-
cals, and their role in the chemistry of the atmosphere. Many
atmospheric chemists are involved in the development of
techniques for the measurement of trace quantities of differ-
ent chemicals in the atmosphere, in emissions, and in depo-
sitions. Other atmospheric chemists study the kinetics and
mechanisms of chemical reactions occurring in the atmo-
sphere. Still other atmospheric chemists are involved in the
development of chemical models of the processes occurring
in the atmosphere. Atmospheric chemists work closely with
other disciplines: engineers in characterizing anthropogenic
emissions; biologists and geologists in characterizing natural
emissions and in evaluating the effects of air pollution; physi-
cists in dealing with gas-to-particle conversions; and meteo-
rologists, physicists, computer scientists, and mathematicians
in dealing with model development. Atmospheric chemistry
plays a key role in maintaining the general well-being of the
atmosphere, which is extremely important for maintaining
the health of the human race.
In recent years, there has been a growing concern about a
number of atmospheric environmental problems, such as the
formation of photochemical oxidants, acid deposition, global-
scale effects on stratospheric ozone, the sources and fates of
toxic chemicals in the atmosphere, and urban and regional haze
issues and the presence and effects of fine particulate matter in
the atmosphere. These problems are affected by a wide vari-
ety of complex chemical and physical processes. Atmospheric
chemistry is the broad subject area that describes the interrela-
tionships between these chemical and physical processes.
The principal components of the atmosphere are nitro-
gen and oxygen. These molecules can absorb a portion of the
high-energy solar ultraviolet radiation present in the upper
atmosphere and form atoms. These atoms may react with a
variety of other species to form many different radicals and
compounds. For example, the short-wavelength ultravio-
let radiation present in the upper atmosphere can photolyze
molecular oxygen to form oxygen atoms. These oxygen
atoms may react with molecular oxygen to form ozone.
These reactions are only of importance at high altitudes,
where the short-wavelength ultraviolet radiation is present.
In the lower regions of the atmosphere, only light of wave-
lengths greater than about 300 nm is present. Table 1 lists therelative concentrations of a number of species present in the
atmosphere, near the Earth’s surface. The chemistry that is
most important at lower altitudes is initiated by a variety of
compounds or trace species, which are present in the atmo-
sphere at concentrations of much less than 1 ppm.
One of the most important reasons to understand atmo-
spheric chemistry is related to our need to understand and
control air pollution. The air-pollution system, shown in
Figure 1, starts with the sources that emit a variety of pollut-
ants into the atmosphere. Those pollutants emitted directly
into the atmosphere are called primary pollutants. Once these
primary pollutants are in the atmosphere, they are subjected
to meteorological influences, such as transport and dilution,
in addition to chemical and physical transformations to sec-
ondary pollutants. Secondary pollutants are those formed by
reactions in the air. The pollutants in the air may be removed
by a variety of processes, such as wet and dry deposition. An
ambient-air-monitoring program is used to provide detailed
information about the compounds present in the atmosphere.TABLE 1
Relative composition of the atmosphere near
the Earth’s surface
Species Concentration (ppm)N 2 780,840
O 2 209,460
H 2 O 35,000
Ar 9,340
CO 2 335
Ne 18
He 5.2
CH 4 1.7
Kr 1.14
H 2 0.53
N 2 O 0.30
CO 0.2
Xe 0.087
O 3 0.025
Source: Adapted from J. Heicklen (1976),
Atmospheric Chemistry, Academic Press, New York;
and R.P. Wayne (1985), Chemistry of Atmospheres,
Clarendon Press, Oxford.C001_008_r03.indd 118C001_008_r03.indd 118 11/18/2005 10:15:18 AM11/18/2005 10:15:18 AM