Encyclopedia of Environmental Science and Engineering, Volume I and II

AEROSOLS 25

r p is the particle density, m the viscosity and l is the mean

free path of the gas. The remaining quantities are defined in

Figure 8.

The value of the Stokes number at the 50 percent collection

efficiency for a given impactor geometry and operating condi-

tion can be found from the figure, and it follows that the cut-off

size, the size at 50 percent collection efficiency, is determined.

If impactors having different cut-off sizes are appropri-

ately connected in series, the resulting device is called a cas-

cade impactor, and the size distribution of aerosol particles

can be obtained by weighing the collected particles on each

impactor stage. In order to obtain an accurate particle size dis-

tribution from a cascade impactor, the following must be taken

into account: 1) data reduction considering cross sensitivity

between the neighboring stages, 2) rebounding on the impac-

tion surfaces, and 3) particle deposition inside the device.

Various types of impactors include those using multiple

jets or rectangular jets for high flow rate, those operating under

low pressure (Hering et al., 1979) or having microjets for par-

ticles smaller than about 0.3 m m and those having a virtual

impaction surface, from which aerosols are sampled, for sam-

pling the classified aerosol particles (Masuda et al., 1979).

TABLE 4

Methods of aerosol particle size analysis

Quantity to be

measured

Method or

instrument Media Detection

Approx size

range Concentration Principle

microscope gas number 0.5 mm

length electron microscope vacuum number 0.001

absorbed gas adsorption method,

BET

gas – 0.01 BET

area liquid

permeability permeability method gas – 0.1 Kozeny-

Carman’s

equation

volume electric resist. Coulter Counter liquid number 0.3 low

gravitational (individual)

ultramicroscope

gas number  1 low Stokes equation

settling (differential conc.) liquid mass  1 high Stokes equation

motion in fluid velocity (cumulative conc.) liquid mass  1 high Stokes equation

centrifugal (differential conc.) liquid area mass 0.05 high Stokes equation

settling velocity spiral centrifuge,

conifuge

gas number mass 0.05–1 high–low Stokes equation

inertial collection impactor, acceleration

method

gas mass number 0.5 high–low relaxation time

inertial motion impactor, aerosol

beam method

gas number 0.05 high–low in low pressure

diffusion loss diffusion battery and

CNC

gas mass number 0.002–0.5 high–low Brownian

motion

Brownian motion photon correlation liquid number 0.02–1 high

integral type (EAA) gas number (current) 0.005–0.1 high–low

electric mobility

differential type

(DMA)

gas number (current) 0.002–0.5 high–low

intensity of scattered

light

light scattering gas liquid number >0.1 low Mie theory

light diffraction gas liquid number 1 high–low

(Other Inertial Methods)

Other inertial methods exist for particles larger than 0.5

m m, which include the particle acceleration method,

multi-cyclone (Smith et al., 1979), and pulsation method

(Mazumder et al. , 1979). Figure 9 illustrates the particle

acceleration method where the velocity difference between

PARTICLE DIAMETER PULSE VOLTAGE PULSE VOLTAGETIME

PARTICLENUMBER FREQUENCY

SENSING

VOLUME LIGHT TRAP

INCIDENT BEAM

AEROSOL

θ

PHOTOMULTIPLIER

FIGURE 7 Measurement of aerosol particle size by an optical

method.

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