Modern methods for determining aerosol size distribution

Journal of Electrostatics, 23 (1989) 371-379 Elsevier Science Publishers B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s

}IODERN ~ETHODS

FOR DETERI~ENING AEROSOL

371

SIZE DISTRIBUTION

K.P. Koutzenogii Institute of Chemical 630090, USSR Aerosols

Kinetics

are usually realized

gas-suspended by particle

of aerosol

size distribution,

/2-11/.

ring size distribution of Whitby, /12/.

of 0 . 0 0 1 - I 0 0 ~ m

the difficulties

of real aerosols.

The first mode of particle ~m

and is related

tions in the gas-to-dispersed zes the particle coagulation

fraction

of atmospheric

According

to the model distribution ranges

to the photochemical

phase.

aero-

connected with measu-

size distribution

The second mode

from

transformacharacteri-

of 0.3 - 0.7 ~ m, which is formed

aging of highly-dispersed

on of coarse-dispersed

diameter

determined

this question is under permanent

it is a complex system v~th three-mode

0.03 to 0.05

system of

clouds are greatly

Let us consider an example

sols in order to conceive

i{ovosibirsk,

as a heterogeneous

solid or liquid particles

/I/. As the properties attention

and Combustion,

particles,

spectrum fraction.

by

and sedimentati-

The modal diameter

of

the largest aerosol fraction lies commonly within 10 - 20 ~ m. These particles result from soil and watererosion. The mass distribution nitude.

of three fractions

This implies

tions within 7 - 8 orders distribution activity)

(e.g.

Since beside

condensation

the range

the

size

size distribu-

and ice-forming

of the concentrations

to create a single method

range of parameters.

Consequently,

about aerosol

ning the setups

phase

and methods with dispersed

Those belonging

phase

mine not only the microscopical 0304-3886/89/$03.50

aerosols,

isolation.

A whole

© 1989ElsevierSciencePublishersB.V.

by combiset of ap-

may be divided apto the first

class

the apparatus

are applied

characteristics

whole

the most reliable

As a rule,

isolation

a

is obtained

principles.

for studying

into two clasmes.

the dispersed

covering

at present

size distribution

based on different

paratus widely employed proximately

the concentra-

by more than 3 - 4 orders.

It is difficult information

by the order of mag-

to fully describe

aerosols.

properties

should be determined,

is increased

include

is needed

of atmospheric

tion other aerosol

are comparable

that the method for measuring

to deter-

but aerosol

par-

372

title composition as well. On the other hand, the technique

of

particle precipitation is often accompanied by substantial distortions in a sample composition.

Thus, preferable are the flow

methods in which the errors introduced by measuring devices, are less. The Table lists the setups and methods which are most popular in measuring aerosol size distribution.

The same Table indi-

cates the ranges of sizes and particle number concentrations

in

which the values to be found are measured to the best precision. TABLE ~ethods for determining aerosol size distribution Hame of the method

Aerosol diameter ( ~ m )

Counting concentration

( cm-3 ) I~ethods with dispersed phase isolation I. Filters

10 - 3 _ 102

10 - 4 _ 108

2. Cascade impactors

10 - 2 _ 102

10 - 4 _ 108

3. Centrifugal

separators

5x10 - 2 -

50

10 - 2 -

10 -3- 5 I 102

4. Thermoprecipitators 5. Sedimentators 6. Static counters of condensation and crystallization nuclei

10 - 3 -

1

5×107

10 - 108 10 -3- 106 5

-

104

Flowing methods I. Photoelectric counters 2. TV counters and devices using image-technique

7~IO -2- 30

10 -3- 106

2xiO -2_ 102

10 -4 _ 108

3. Electrical analyzers

10 -2- 20

10

4. ~lowing counters of condensation and crystallization nuclei

10 -3- 0.5

10 -4- 5~105

5. Diffusion batteries

10 -3 - 0.2

6. Pulsed photography and holography 7. Laser Doppler spectrometer

I 0.5

- 102 - 15

- 5~I05

10 -3- 106 5 10 -2- 5~I05 10 -3- 5×104

It is seen that among the first-group methods,

filters involve

the widest range of values. V~en measuring mass concentration by precipitate

on the filter automatically,

the method sensitivity

is insufficiently high. Besides, a detailed analysis of the particle size distribution of precipitate samples on the filters is

373

time-consuming. Consequently, together with filters, various cascade impactors are also very popular. At atmospheric pressure, the cascade impactors deposit effectively the particles exceeding 0.3 ~ m. To deposit the smaller ones, the pressure in precipitator is reduced. Thus, to deposit the aerosols of about O.O1 ~ m, one must produce vacuum of several mm of Hg. Therefore, this method cannot be used to investigate the liquid drops from the substance with high pressure

of

saturated vapours. The second group involves photoelectrical counters, and during the last few years the apparatus employing TV and image technique are under intensive development. A characteristic feature of the last decade studies is

the

creation of measuring-computing complexes containing several different setups. The structure of two of the most developed complexes is presented in ~ig. I (a

and

b). The first includes electric analy-

zer, photoelectrical counter, and condensation ~Iplifier (or flowing counter of condensation nuclei). One would think if to measure only the size distribution of O.O1-10 ~ m particles, it will be sufficient to combine electric aerosol analyzer and photoelectric counter. According to a simplified theo~j of electric analyzer /13/, the characteristics of a given setup may be estimated from the size dependence of the electric mobility and aerosol particle charge. The latter are described by the following relations:

7=

j.e-C

6 fr C =~ + r~"~[1.25 +0.42e×p (-0.87-~-)] Here

Z

is the particle mobility,

mentary charges on a particle,

~

(2)

] - is the number of ele-

- is the particle radius, ~

-

is the gas viscosity, I - is the length of gas molecule path, @ - is the electron charge, In the second formula of relation (I), the particle radius is given in microns.

(I) and (2) show that for large p a r t i c l e ( ~ < 1 )

the mobility is practically independent of aerosol particle size.

374 a)

Fphotoelectric counter

aerosol

I

electric separator

Icondensation I amplifier Size range

t

0.01

<

d -< I0 ~ m

Range of number aerosol concentrations

eroo @

b)

] 1

10 -3

_

2

o

2xi0 cm -~

1 ooee ooot I

l diffusion screen[ battery

t condensation amplifier

]

Size rauge 0.001 < Concentration range Pig. I.

d < 30 ~ m I - 106 cm -3

Block-scheme for measuring complexes to determine microphysical characteristics.

At atmospheric pressure, this condition is fulfilled with ~ O . 5 m which defined the upper limit of sizes taken using the electric analyzer. The minimum particle size depends on the fraction of charged aerosol particles. This value for particle more than 0 . 0 1 ~ m is appron:imated by the Boltzman law:

NJ_-exp(_ j2"@2) N

Z.rKT

N j is the particle concentration with charge J , N is the number concentration of aerosol particles. ~rom formula (3) it follows that at d = 0.01 ~ m the fraction of charges is only O.OO1 of the nuraber concentration. To ~:eliably measure these ~a~?ticles, the ylU[,lber C O L I O @ I ~ b r a b2. O z l ......

"

"

]:!liSt

375

exceed 106 cm -3. The size distribution of particles less 0.01 ~ m

than

is difficult to measure by using the electric analyzer

due to a sharp drop in the fraction of charges with decreasing size. Besides,

the variations in particle

diameter from O.O1 to

1 ~tm cause the strong changes in the range of the reliably measured number concentrations. V,'ith the minimum size, the number concentration range amounts to 106 - 107 cm -3, at the m~:imum to 30 - 3xi03 cm -3. The photoelectric counter is employed to measure the aerosol particles exceeding 0.3 ~ m. It covers the range of 0.3 - 10 ~ m to 102 cm -3.

particle

size at number concentrations from 10 -3

To widen the ra~ze of the number concent_aomons and to increase the sensitivity in /14,15/, a differential electric separator with condensation ~unplifier has been proposed. the area of measuring-computing

9'ig. 2

complex: (crosshatched)

the scheme using the differential electric separator, tion amplifier,

presents

and photoelectric

based

on

condensa-

couo:~ter /16/.

q

I

10 5 I

complex "a" 0

[

10 .3

0 4~

I

w I

complex "b" ¢P 0

101

0

~o

10 -I

10-3 0 . O01

0.01

0.1

.0

10

100

Particle diameter, ~ m Fig. 2. The measurement

ravage of complexes.

Another variant of measuring-computing /17/. Its block-scheme

is depicted in Fig.

complex is proposed in lb. A screen diffusion

376

battery whose parameters rator of size particles of 0.3 - 30 ~ m tric analyzer

are available from 0.001

in /18/ serves

~m

to 0 . 2 ~

dis~meber are measured

by means

described

in /19/.

of the photoelec-

The particles

of less

~fm are enlarged with the help of a condensation standard

fog,

The range

described

size distribution

measurements

2 with a gorizontal

analyzer with optical

supplemented

than 0.3

s~plifier

of

in /20/.

of aerosol

complex is shown in Fig. electrlc

as a sepa-

m. ~he partic]e~

formation

with the photoelectric

hatch.

using

this

If the photo-

of measuring

range

counter with mechanical

is for-

matioi~ of ~Jnber range,

the lower range of the concentrations

measured may be widened

to 10 -3 cm -3.

7.~hen comparing complexes,

the characteristics

ring smaller particles realize

of two measuring-computing

we see that the second one has some advantages. in a diffusion

the potentialities

battery allows

of the condensation

sign of the flowing amplifier

described

detect even the aggregate

of molecular

of low-volatile

vapours

are readily

of 104 cm -3, case,

chosen when,

the growing

the drop diameter

when increasing particles

particles

substances.

the number

tion on the resolving importance.

are described

operation, the aerosol detected. The operation

at the n~aber amount

concentration

concentration

to 1.5 - 2 ~! m. In this

com~ter.

of the method

used is of

strength

even

up to 106 cm -~. These

by photoelectric

The characteristics

one to

Under stan-

at the exit will be about 0.5 ~ m

are easily recorded

The de-

size due to condensation

dard comditions of condensation amplifier particles of about 0.001 ~ m are reliably conditions

one to fully

swaplifier.

in /20/ permitted

of supersaturated

I~leasu-

The ques-

of the screen diffusion

great battery

by the followii~z relations:

-) (4)

Pe- U-6_D is is

the

the

screen

quantity

particle

of

diffusion,

_D=

fiber

diameter,

screens,

C

.C

~

-

is

Cl-

ls

the

coefficient

- is the Kanigem

the

screen

correction

step,

of

(see

[

aerosol

(2)).

-

377

From the above formulae one may see that if the passage factor (Ni/N) is less than 70 ~$ for aerosols less than 0.1 ~ m ,

a rela-

tive error in measuring sizes will be defined by the accuracy of measuring the passage. A more detailed consideration of the question on the resolving ability of the screen diffusion battery is presented in /21/. If the particle

charge was defined by its size

the efficiency of the separation in the electric analyzer would be defined only by the accuracy of establishing voltage on the condensator plate. The latter is sustained to a very high precision. Therefore,

a real resolving strength is defined by the law

of charge distribution on particles. Theoretical and experimental

data testify that the high reso-

lution may be attained by using the electric separator only aerosols with narrow size distribution. ther polydispersed systems,

for

Usually, vle deal with ra-

and, hence, high resolution can hard-

ly be obtained without very complex measuring technique. Further expansion of the potentialities of measuring apparatus for determining aerosol size distribution is related to the development of setups using TV and image technique./22-24/.

At pre-

sent the apparatus are available which allow one to measure the particles from 0.02 to 103 ~ m. Thus it is possible

to measure

both the sizes and velocities of particle motion. The quickness of action increased substantially.

For particles above 2 ~ m one

may measure not only sizes but the forms as well. The

range of

number concentrations meast~ed without preliminary dilution, widened due to simultaneous measurements The development

is

of several particles.

of laser Doppler spectrometer appeared to be pro-

mising /25,26/. Although the size range measured by this technique is not wide

(0.5 - 15 ~ m) it is of importance

that the au-

tomatic classification by aerodynamic dio~leter is performed

ir-

respective of aerosol particle shape. Despite great progress in the field of measuring aerosol size distributiom achieved during last years, the question om measuring the size of particles of irregular shape and inhomogeneous coraposi~ion is so far unsolved.

It can be thought that the prog-

ress in this direction will be connected with the application of lasers, opto-electron setups and computers.

378

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