Page 74 - 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 297

The methods listed yield the concentrations either of 3. Continuous Methods
water-soluble ions measured in terms of certain oxidized
Continuous air monitoring for trace contaminants in am-
states or of elements. For example, materials appear-
bient air has developed extensively since the mid-1960s
ing as sulfate and nitrate may include lower oxidation
as a result of stimulation from new air pollution mea-
states, but the methods basically do not distinguish among
surement requirements. Workers expect that similar needs
them. The metal elements found are either oxides or
will develop as certain chemical constituents of partic-
soluble salts. The actual composition of the material is
ulate material are identified as factors in human health
indeterminant, but workers have deduced the composi-
effects. Techniques for the continuous chemical charac-
tion suspected to be present by a material balance, com-
terization of particulate matter are slow in coming be-
bined with knowledge of the origins of the particulate
cause the amounts of material sampled are small, of-
material.
ten below the detection limits of instrumentation. In all
cases so far, either a precollection method like filtra-
2. Microscopic Techniques tion or a special detector of high sensitivity has been
required.
The hope of chemical characterization of individual parti-
Flame photometry has promise for the measurement
cles or even the surface nature of individual particles has
of sodium, lead, and potassium. An application to mea-
led investigators to apply new microtechniques to aerosol
surement of sodium and alkali metals has been reported.
research early in their development. Early methods em-
The continuous measurement of sulfur-containing parti-
ployed the electron microscope for the identification of
cles has received considerable attention. The motivation
particles, which were captured on a reactive substrate. The
for observation of sulfur-containing particles comes from
collection surface was selected to provide an indicator of
concern about the potential hazard posed by sulfate in the
a specific compound. Frank and Lodge in 1967 found that
atmosphere.
sulfuric acid particles could be identified from a satellite
Practical methods also have been reported for semicon-
ring structure after collection on a silicon surface. Later,
tinuous measurement of nitrate and carbon in particles
Bigg, in 1974, used specific chemical surface coatings to
from ambient air. For example, an instrument for nitrate
bring about colored chemical reactions, which could be
monitoring uses collection of particles on an impactor sur-
identified microscopically. These methods are generally
face, followed by flash volatilization and determination
semiquantitative in terms of mass concentration but give a
of the nitrate present using a chemiluminescence tech-
numberdensityestimateandaroughparticlesizeestimate,
nique. Ion chromatographs also have been adopted for
especially if they are used for qualitative identification for
semicontinuous determination of gaseous and particulate
some years.
nitrate. Real-time carbon analyzers also are available, one
Electron microscopy has been used for some years. The
of which uses differential thermal analysis of impactor-
scanning electron microscope has been used in conjunc-
collected material.
tionwithenergy-dispersedX-ray(EDX)fortheanalysisof
Animportantadvanceincontinuousanalyzersusesboth
single particles. Microprobe techniques include: (1) AEM
particle size data and single-particle chemical composi-
(auger electron microscopy), (2) ESCA (photoelectron
tion. These instruments employ a method of rapid depres-
spectroscopy), (3) SIMS (secondary ion microscopy), (4)
surization of the aerosol that produces a particle beam and
EPMA (electron probe micro analysis), (5) MOLE (laser
irradiation of particles to generate ions that are analyzed
Raman microprobe analysis), and (f) LAMMA (laser mi-
by mass spectroscopy. The single particle analyzers have
croprobe mass analysis). AEM, ESCA, and SIMS are
been employed in atmospheric research recently but have
surface-sensitive methods that provide knowledge of sur-
not reached the stage where they are used routinely in air
face properties but are difficult to extrapolate to hetero-
monitoring.
geneous materials. EPMA gives information related to
bulk properties with a greater penetration depth; MOLE
may also be useful in this respect. LAMMA has only
recently become available as a research device but will VI. INDUSTRIAL GAS CLEANING
undoubtedly be used more extensively in fine-particle
characterization. Theenvironmentalcontrolofparticlesuspensionsfromin-
Ingeneral,microprobeanalysismethodshavebeencon- dustrial practice is an important aspect of aerosol technol-
sidered semiquantitative, but EPMA has been promising ogy.Controlofindustrialaerosolsisdonebothinthework-
for quantitative studies. With further improvement and in- place to preserve safe conditions and at the exit exhaust
vestigation, these microprobe techniques will be useful for stack to minimize the pollution of ambient air. In gen-
thecharacterizationofsurfaceandbulkparticleproperties. eral, a variety of regulations govern industrial operations.