Page 52 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology En001c-14 May 7, 2001 18:25

Aerosols 275

cloud behavior, particle sampling, the description of their The estimated rate of particle injection into the air,
depositional behavior, and their removal from industrial which characterizes the global aerosol burden, is given in
gas streams. Table I. This table represents a compilation from investi-
gators who have tried to estimate the relative contributions
to the atmospheric aerosol. From this survey, the natural
B. Natural Phenomena
contributions far exceed emissions from human activities
Aerosols are readily observed in nature. The atmospheres on a global basis, but locally this is undoubtedly reversed,
of planets of the solar system are rich in suspended par- especially in parts of North America and Europe. From the
ticulate matter, as in interplanetary and interstellar space. table, the “best estimate” suggests that about 13% origi-
The wealth of visual experience in observing the plan- nate with human activity, while the remainder is assigned
ets depends on gases and particles concentrated in their to natural sources. The importance of particles from at-
atmospheres. The variety of color and opacity of atmo- mospheric chemical reactions of gases is also shown from
spheres is a direct result of light absorption and scat- data in the table. More than 13% of the estimated particle
tering from particles as well as their suspending gases. burden comes from the secondary processes. Noting that
Individual particle clouds are frequently identifiable in the rates are dominated by large particles in soil dust and
planetary atmospheres. They show the broad features of sea salt, the secondary fraction is much more important
atmospheric motion as giant swirls, veils, streaks, and if these sources are not considered. In addition, it readily
puffs. The best known planetary aerosols are those of the can be seen that the secondary material should be domi-
earth. The earth’s atmosphere is rich in suspended parti- nated by particulate sulfur, present as sulfate, and perhaps
cles. Their presence has been observed and reported in the organic carbon on the basis of these estimates. Indeed,
literature for centuries. Yet only since the early 1960s has sulfate is a universal constituent of atmospheric particle
scientific instrumentation become available to character- populations as is carbon.
ize atmospheric aerosols in great detail. The enormous quantities of particles injected into the
Airborne particles in the earth’s atmosphere probably earth’s atmosphere are mixed and aged by processes in the
were recognized first in relation to sea spray drift or dra- air to create a very diverse and complicated mixture. The
matic events associated with volcanic eruptions and forest mix varies greatly with geographic region and with alti-
or brush fires. However, the haze associated with sea spray tude, but also has some remarkably common physical and
and blowing soil or pollen dusts also contributes large chemical features. The presence of suspended particles in
quantities of particulate material to the atmosphere. Only the earth’s atmosphere provides for a variety of natural
in recent years has the significance of the contribution to phenomena and represents an important part of aerosol
the earth’s air burden of extraterrestrial dust and the in situ science. Particulate matter in the air exerts an influence on
production of particles by atmospheric chemical reactions the transfer of electromagnetic radiation through the atmo-
become known. The latter is of particular interest in that sphere. This manifests itself in changes in visibility and
the oxidation products of sulfurous and nitrogenous gases coloration as a result of light scattering and absorption. A
and certain hydrocarbon vapors are prolific producers of wealth of sky color, shadow, and haziness, which provides
small particles. Thus, the “breathing” of traces of gases a varied and often beautiful setting both for natural objects
from natural biological chemistry in soils such as hydro- and for architecture, is a direct result of the influence of
gen sulfide or ammonia, and pinene or similar vapors from suspended particles interacting with visible light.
vegetation, actually contributes substantially to the atmo- Changes in the transfer of radiation in different layers
spheric aerosol content. The direct transfer of particles of the atmosphere are the crux of the atmospheric en-
to the air is often called primary emissions. The mate- ergy storage process. Aerosol particles also play a role in
rials produced from atmospheric chemical processes are distributing solar energy throughout the atmosphere and
termed secondary contributions. consequently in affecting climate. A distinctly different
Added to the natural aerosol-forming processes are the function of aerosol particles in the atmosphere involves
emissions from human activities. With the industrializa- the formation of clouds of condensed water. Suspended
tion and urbanization of increasingly large geographic ar- particles basically provide the nuclei for the condensa-
eas,substantialquantitiesofparticulatematterareemitted. tion of moisture and for the nucleation of ice crystals in
The expansion of agriculture has also enhanced the sus- supercooled clouds. Thus, in a sense, aerosols provide a
pension of dust either directly by cultivation or indirectly skeleton through which are derived, with water vapor, rain
by deforestation and temporary overproduction, resulting clouds and precipitation. The opportunity then presents it-
in soil erosion. Pollutant gases, including sulfur dioxide, self for both weather and climate modification by injection
nitrogen oxides, and certain reactive volatile organic com- of particulate matter into the air.
pound (VOC) vapors, also represent substantial potential The interaction between aerosol particles and clouds
for particle production in the air. recently has led to an important theory about the