Page 68 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: FWD Revised Pages
Encyclopedia of Physical Science and Technology En001c-14 May 7, 2001 18:25

Aerosols 291

resolution in particle size with chemical composition. The as a sample must be done carefully to avoid preferential
ideal instrument operated at full capacity would measure withdrawal of particles of different sizes. Particle deposi-
and read out directly the particle distribution as a function tion on the walls of the sampling tube, as well as possi-
of size and chemical properties. Only recently have ana- ble reentrainment, must be minimized and accounted for.
lyzers employing mass spectroscopy bigun to realize this Care must also be taken to avoid condensation or chemi-
potential. In practice, a variety of instruments are avail- cal reactions in the sampling duct. Problems of this kind
able that report size distribution functions or integral aver- are especially severe in sampling high-temperature, moist
age properties of the distribution function. These include gases from a stack or moist gases from a chemical reac-
the single particle counter, which measures particles over tor. Condensation can be avoided with a probe heated to
discrete size ranges using differences in light-scattering the sampled gas temperature if the pressure difference has
properties with particle size, and devices such as the elec- been minimized. When pressure differences in the sampler
trical mobility analyzer, which measures particles in size are large, control of pressure may be important. Chemical
groups by counting charged particles in a given size range reactions on the wall of the sampling tube are often dif-
over a discrete time interval. This instrument depends on ficult to control but can be minimized using tubes lined
the fact that particles realize a unique equilibrium charge with inert coatings such as Teflon.
as a function of size. Finally, a series of devices integrates The ideal condition for sampling is one in which the
the distribution function and gives information about cer- gas particle suspension is drawn into the instrument at a
tain moments of the size distribution. These include: (1) speed nearly equal to that of the external flow. Ideally,
idealized total particle counters such as a nuclei counter sampling should be done isokinetically, or with the sam-
that relies on the nucleation of supersaturated vapor to pler inlet velocity equal to the mainstream velocity. Only
produce droplets visible in a light beam, (2) a particle in the isokinetic case will the inertial deposition at the
collector such as an impactor that segregates the sam- sampler tip be minimized and preferential size separation
ple by size over certain discrete size and time intervals be small during sampling.
through inertial forces, and (3) a total mass–chemical an-
alyzer such as a filter placed in an aerosol stream and later
submitted to the laboratory for gravimetry and chemical A. Inertial and External Forces
assay.
1. Electrical Charging and Particle Size
Once collected, particles can be sized by a variety of
means. Optical and electron microscopy are probably best Two charging methods have been adopted to develop
known and are quite reliable. Yet they involve tedious particle measurement devices. These involve diffusion
scanning of many samples to obtain sufficient counts to charging and contact charging. Three characteristics of
provide meaningful particle statistics. Microscopic tech- ion charging affect the usefulness of a diffusion charg-
niques are suitable for solid particles and for nonvolatile ing method for aerosol sizing by electric methods. First,
liquids. Volatility creates a significant uncertainty unless the relationship between electric mobility and particle size
the particles are trapped in a substrate that reveals a “sig- must be established. This basically provides a means of
nal” of the impacted particle. calculating the migration velocity of particles under the
Microscopy remains the principal standard method of influence of an electrical force, which in turn gives the ba-
particle sizing and of shape and morphological classifi- sis for locating a particle collection in an instrument. The
cation. Though often tedious and time consuming in its mobility versus particle diameter curves are single-valued
application, it remains a standard by which individual par- for bipolar diffusion and unipolar diffusion. This is not the
ticles can be classified with confidence and most particle case for the field charging. Second, the fraction charged
sizing methods are referenced. must be known. Particles that do not acquire a charge
during their passage through a charger cannot be influ-
enced by subsequent electric fields and therefore cannot
Sampling Design
be measured by electrical migration. The fraction charged,
There are many pitfalls in measuring the properties of in combination with aerosol losses, is the principal factor
aerosols. One of the most critical is sampling of particu- that limits the lower useful size detection of an instrument.
late matter without disturbing the aerial suspension. There Third, the discrete nature of electrical charge on a particle
are some optical devices that make measurements of an must be accounted for in the instrument output.
aerosolinsituwithoutdisturbance.However,mostdevices In principle, calculation can correct for this effect on a
requires that a small sample be taken from the gas–particle measured size distribution, but the methods have not been
suspension. Because of inertial forces acting on particles, evaluated yet. If measurements are made using only the
it can be deduced readily that siphoning part of the fluid singly charged particles, then the resolution is as good as