NAT'L INST. OF STAND & TECH

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NBS SPECIAL PUBLICATION 412

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards
NATIONAL BUREAU OF STANDARDS

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LiL BliRi:.\U OF STANDARDS

AEROSOL MEASUREMENTS

The Proceedings of a Seminar on

Aerosol Measurements

May 7, 1974

Wayne A. Cassatt and

Rosemary S. Maddock, editors

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

> ' Washington, D. C. 20234

Sponsored by

The National Bureau of Standards

Washington, D. C. 20234

and

The Food and Drug Administration

Washington, D. C. 20204

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued October 1974
Library of Congress Catalog Number: 74-600164

National Bureau of Standards Special Publication 412

Nat. Bur. Stand. (U.S.), Spec. Publ. 412, 193 pages (Oct. 1974)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1974

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

(Order by SD Catalog No. CI 3. 10:41 2). Price $2.65

Stock Number 0303-01341
ABSTRACT

Papers followed by verbal discussions were given in a seminar and

workshop sponsored by the National Bureau of Standards and the Food and

Drug Administration to define the state of development of aerosol measuring instruments. The instruments discussed were based upon a variety

of operating principles including laser light scattering, optical imaging, Doppler shift, electromobility, piezoelectric effect, and beta-ray

absorption. Two review papers were given which described other phenomena upon which aerosol measurements are based. The general summary includes a table which lists the specifications of the instruments discussed to illustrate the range of capabilities available in this field.

Discussion among seminar attendees revealed that many questions remain

to be answered before the more difficult aerosol measurements problems

can be solved. For example, in the analysis of very dense aerosols

questions arise concerning coincidence losses or agglomeration effects

that may result from collisions between particles as they are drawn into the measuring volume. Volatilization or condensation effects may

alter the size distribution if the measurements are made late in time.

Finally, variations in particle shape or index of refraction can alter

the instrument response and cause difficulties in interpretation.

Key Words: Aerosol instrument performance; aerosol measuring instruments; beta-ray absorption; Doppler shift; electromobility; laser light scattering; optical imaging; piezoelectric

effect

i i i
FOREWORD

The use of aerosols has found wide popularity in most life styles

of our population. Toxic aerosols present special problems which

require accurate measurement of airborne particles and droplets so that

their effect on health can be evaluated.

Meaningful monitoring of aerosol particles usually must be

performed in real time and at the site of exposure. Rigorous and

exacting measurement conditions must be met. Thus, new and innovative

methods and instrumentation are being developed to meet the rigorous

monitoring criteria necessary to safeguard health.

The dramatic increase in the number of food, drug and cosmetic

products now consumed as aerosol spray products coupled with their

measurement needs have prompted the Food and Drug Administration and the

National Bureau of Standards to co-sponsor this Seminar on Aerosol

Measurements to better define the needs and to stimulate the development

of new methodology for making rapid and reliable aerosol measurements.

Philip D. LaFleur, Acting Chief

Analytical Chemistry Division

iv
PREFACE

This seminar was organized to provide an opportunity for

representatives of instrument manufacturers, research groups and

I government agencies to evaluate the latest developments in

j instrumentation for making rapid measurements on aerosols. The

incentive behind this effort is the realization that the popular uses of

aerosols and the accompanying exposures are rapidly rising in frequency

and importance. Over two billion spray cans are used each year to apply

paints, insecticides, deodorants, oven cleaners and a host of other

useful but toxic materials. The need to maintain quality control in

spray applications and to monitor any potential hazards demands an ability to measure aerosol particles accurately and reproducibly.

The particle sizes and concentrations encountered in aerosol sprays

place such unusual demands on the measuring techniques that conventional

instruments appear limited in their ability to provide the required

dynamic range, resolution and accuracy. This workshop was directed,

therefore, to a discussion of a representative selection of the newer

aerosol instruments that are under development or have been completed

recently.

It is hoped that these proceedings will serve both to illustrate

some of the latest approaches to aerosol instrument design and to

stimulate the development of additional new concepts in the future.

Wayne A. Cassatt, Coordinator

Seminar on Aerosol Measurements

v
CONTENTS

Page

Abstract iii

Foreword iv

Preface v Summary viii

A Review of the Methods for the Particle Size Analysis

of Aerosol Spray Can Droplets 1

R. Davies and J. D. Stockham

Light Scattering by Single Aerosol Particles 13

Milton Kerker

Light Scattering Methods for the Characterization of

Particulate Matter in Real Time 21

C. C. Gravatt

Flow Apparatus for the Characterization of Aerosols 33

Madhav B. Ranade

360° Scattering Diagrams from Individual Aerosols in a Flowing Stream 41

Thomas R. Marshall, Charles S. Parmenter,

and Mark Seaver

Active Scattering Aerosol Spectrometry 57

Robert G. Knoll enberg

Rapid Measurement of Droplet Size Distributions by

Optical Heterodyne Spectroscopy 65

Ilan Chabay

Measurements of Aerosol Size Distributions with a Laser Doppler Velocimeter (LDV)

William J. Yanta

vi
An Optical Transform Technique for Measuring the

Size Distribution of Particles in Fluids 89

Albert McSweeney

Sizing Aerosols in Real Time by Pulsing UV Laser

Machine 97

Geoffrey A. Hotham

Rapid Respirable Mass Measurement 127

Lawrence Doemeny, George Carson,

and Bruce Almich

Particulate Mass Measurement by Piezoelectric

Crystal 137

Raymond L. Chuan

Development, Calibration and Application of Size

Distribution Instruments at the University of

Minnesota 149

V. A. Marple

List of Attendees 177

Authors Index 179

Key Word Index 180

vii
SUMMARY

The papers recorded in these proceedings describe several aerosol

measuring instruments based upon a variety of operating principles. The

physical phenomenon underlying these principles include laser light

scattering, optical imaging, Doppler shift, electromobility

,

piezoelectric effect, and beta-ray absorption. Two review papers are included which describe several other phenomena upon which measurements

are based. Instruments designed around the first three principles have

capabilities for detecting and sizing indivdual aerosol particles, but

they have definite limitations regarding the concentrations and count

rates that they can handle. Instruments based upon the latter three

principles are much more adept at handling high concentrations but it is

difficult, if not impossible, to detect and to size individual particles

with them. Obviously, the principle to be employed and the specific

design of the final instrument will depend upon the measurement

requirements to be met. Those devices that rely upon the interaction

between light and individual particles can be quite different in basic

design and performance. Some measure intensity at one or two selected

angles, others analyze the intensity pattern over large solid angles,

and still others measure focused optical images. In all, some eight

different instruments of this type were discussed here.

In order to compare these instruments and their performances in a convenient fashion, they have been listed along with their

specifications in table 1= This is by no means intended to be a complete list of the aerosol instruments on the market or under

construction in various laboratories around the country. Rather, it is

meant to be indicative of the state of the development in this field.

Discussions among seminar attendees revealed that many questions

remain to be answered before the more difficult aerosol measurement

problems can be solved. For example, in the analysis of very dense

aerosols questions arise concerning coincidence losses or agglomeration

effects that may result from collisions between particles as they are drawn into the measuring volume. Volatilization or condensation effects

may alter the size distribution if the measurements are made late in

time, the lack of size resolution or sensitivity may prevent the

detection of useful information. In some cases the lack of instrument

portability or the length of time required for data processing may

constitute serious handicaps. Finally, variations in particle shape or index of refraction can alter the instrument response and cause difficulties in interpretation.

The developers of the instruments described in this seminar have

quite successfully attacked and solved one or more of these problems

with respect to aerosol spray measurements. On the other hand, it would

appear that much remains to be done to produce an optimum device,

especially if it is to be useful for general applications under a variety of conditions.

Wayne A. Cassatt, Coordinator

Seminar on Aerosol Measurements

viii
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ix

X NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 412

Aerosol Measurements, Proceedings of a Seminar on Aerosol Measurements

Held at NBS, Galthersburg, MD, May 7, 1974 (Issued October 1974).

A REVIEW OF THE METHODS FOR THE PARTICLE SIZE

ANALYSIS OF AEROSOL SPRAY CAN DROPLETS

R. Davies and J. D. Stockham

I IT Research Institute

10 West 35th Street

Chicago, Illinois 60616

ABSTRACT

This paper reviews the methods that have been used for sizing

droplets, sprays, and mists, but places particular emphasis on the

methods that are applicable to the sizing of aerosol spray can droplets.

The methods reviewed include photographic imaging, collection,

deposition and size reconstruction, momentum transfer, pulse counting,

hot wire anemometry, electrical mobility, optical scattering, and

interference.

From the various techniques discussed in each class, laser

holographic imaging is considered the best approach, but the method has

an extremely high capital cost. Light scattering methods are the second

choice, but these, too, have a relatively high capital cost. If capital

cost is an important factor, then methods such as photographic imaging

and impaction can be usefully employed.

Key words: aerosol instrumentation; aerosol measurements; aerosol

review; aerosols; aerosol scattering; aerosol sizing; droplet sizing;

droplet imaging; electrical mobility; impaction; momentum methods;

particle imaging; particle measurements; particle scattering; particle

size determinations; particulates; sprays.

1. INTRODUCTION

In 1973, 1,900 million aerosol can products were produced in

Europe, and the U.S.A. contribution to atmospheric pollution from spray

can sources was estimated to be 5.4 ton/day/10° population [1]. Though

the total emission from this source only represents a minor source of

pollution, the nature of the emission has recently caused some concern. Reportedly, the fluorinated hydrocarbons used as a propel 1 ant have

produced toxic effects on the hearts of animals [2] and vinyl chloride,

used in some products, has caused cancer of the liver [3]. Also, with

the advent of the Occupational Safety and Health Administration (OSHA)

regulations, the inhalation hazard of particulates from spray can

1
products is now coming under scrutiny, and such agencies as the EPA,

NIOSH, and FDA are taking a close look at aerosolized product effects.

However, to determine the effect of these particulates, vapors, and

gases on animals during controlled experiments, and to predict their

effect on human life, the physical properties of the spray have to be

known. One important physical property of the spray that controls its

deposition behavior in the lungs is the particle size and size

distribution. In consequence, it is the purpose of this paper to

briefly review the methods that have been used successfully to measure

the particle size and size distribution of spray in general. From this

review, useful methods that can be employed for spray can analysis are suggested.

The information presented here has been abstracted Prom a 200

reference article presently being prepared by the author for publication

elsewhere [4]. In addition, some material has been drawn from the series

of four articles published by the author, on "Rapid Response Methods of

Size Analysis" [5-8].

2. METHODS OF MEASURING DROPLET SIZE

Generally, the methods can be grouped in the categories below.

1. photographic imaging methods

2. collection and deposition methods

3. momentum transfer methods

4. pulse counting methods

5. hot-wire anemometry methods

6. electrical mobility methods

7. optical methods

2.1 Photographic Imaging

Photographic methods were first used by Mache in Germany at the

turn of the century. Recent uses of the method have been by Mueller and

Sims [9] and Itagaki and Meline [10]. Meline used a 4" x 5" press camera with a long focal length and large shutter aperture, producing a sharp plane of focus in the droplet field.

Webster [11] used imaging systems to record high velocity droplets

in flight. Similar systems have been used by Knoll enberg [12], and

Cannon [13]. The latter author used the method for measuring cloud

droplets. Stockham, et at. [14] developed an automatic spray analyzer

for Holloman Air Force Base, which imaged the droplets on a TV tube

(fig. la). A comparison with conventional photographic methods was shown to be good (fig. lb). A similar system was developed by Spraying

Systems Inc. [15].

The method is more fully discussed in references 4 and 5.

2
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b. Data reported by Stockham et al, [14].

3
2.2 Collection Methods

In these methods, the particles are removed from the fluid by ' various physical mechanisms and deposited onto surfaces or into ; immiscible liquids. Sizing is then performed on the deposit.

j Separating mechanisms have included gravity and centrifugal ' sedimentation, horizontal elutriation, impaction, thermal and ! electrostatic precipitation. An example of a centrifugal collector is

the Goetz Spectrometer (fig. 2), and an example of a liquid droplet | impactor is the Brink impactor (fig. 3). ,

Figure 2. The Goetz Spectrometer.

Collection surfaces have included flour [16], absorbent paper [17],

coated screens sensitized paper nonporous surfaces [20], and liquids

[21]. The particles themselves are sometimes dyed [22], and sometimes

frozen and sized by microscope [23] or by sedimentation [24], Dissolved

tracers are sometimes used, and the size reconstructed from the particle

deposit [25]. For further information on this method see reference 4.

4
Figure 3. The Brink Impactor.

2.3 Momentum Transfer

This method is useful for volatile droplets of large size. Three

types of methods have been used to measure size from momentum

measurements. The size of craters in magnesium oxide coated surfaces

was used by May [26]. The magnitude of the impact of a drop striking a microphone was used by Cooper [27], and the vibration of a piezoelectric

crystal was used by Chuan [28]. A commercial version of this instrument

manufactured by Celesco, is described later today in the paper by R.

Chuan.

2.4 Pulse Counting

This method was reported by Pye [29], who used it for sizing oil

droplets in fuel burners. The method is not considered applicable for

spray can analysis. For more information, see reference 4.

2.5 Hot-Wire Anemometry

The output voltage fluctuations of a hot wire anemometer are a function of both the cooling of the wire by the air stream, and the

evaporation of the droplets that impact upon the wire. The use of

signal filters enable these effects to be separated, and size to be

determined [7].

5
2.6 Electrical Mobility Methods i

These methods determine the charge-weight relationship of the i droplets. Mobility analyzers precipitate particles in an electric field

of varying voltage, and measure the current due to the charges on the

particle surfaces. The various instruments are discussed in references

4 and 7. Commercial instruments include those produced by ThermoSy stems [7].

2.7 Optical Methods

Optical methods that have been used to measure droplet size have

included:

a. Tyndall Spectra

b. Single Particle Scattering

c. Multi -Particle Scattering

d. Laser Scattering

e. Laser Holography

f. Laser-Radar

g. Laser-Interferometry

When a light beam passes through a monodispersed aerosol which is viewed

at an angle to the incident light against a dark background, the

presence of particles is observed by the occurance of Tyndall spectra

due to the scattered light. The Owl is an instrument that uses this

principle to measure size [30]. With this device, the spectral colors

are observed using a rotating telescope. With white light, size can be

deduced from the number of red bands that appear in the spectrum.

Light scattering from single particles has been very widely used,

and over the last 20 years, many variations in particle counting

instruments have been developed. These are discussed in detail in

references 4 and 5.

Very recently, Gravatt [31] at NBS developed a light scattering

counter for the EPA in which a simultaneous measurement of the light

intensity at two small angles was performed. The ratio of these

intensities was then related to size.

Use of the intensity ratio method at two angles is also made with

multi -scattering particle systems . With the Bryce-Phoenix light

scattering photometer, this technique, termed the polarization ratio

method, is applied.

Other photometer systems using multiple particle scattering have

been built and tested by Ranade [32] and Cooke and Kerker [33]. These

systems were designed to measure the kinetic behavior of aerosol

particles in flowing gas streams.

6
The above light scattering devices use conventional optics, but

other systems have been developed that measure the scattering from

particles in a laser beam. Schleusener [34] developed a gas laser for

sizing particles; Harris and Morse [35] developed a laser system for

measuring fog droplet sizes. A recent device in this class was developed by Gucker et at, [36] and is the subject of the paper by

Parmenter in this symposium. Knollenberg also later discusses laser

scattering devices developed at Particle Measuring Systems Inc. in

Boulder, Colorado.

Laser holographic methods have also found application in sizing

droplet particles. Horn et al. [37] described a spray droplet analyzer,

and offers a laser holographic service through Laser Holography Inc.

This afternoon, Hotham describes the use of this system for sizing

sprays from deodorant, hairspray, and pesticide spray cans. Optronics

International offers a similar service, as do Laser Photographic

Laboratories in Chicago. Hathaway [38] recently published data taken by

this latter system on deodorant spray cans, and showed the effect of

evaporation on the size distribution at distances of 1", 6", and 12"

from the nozzle. The use of laser holography for sizing water drops was explored by Stigliani [39], who developed a forward scatter system at

the Illinois Water Survey.

For meteorological studies, laser-radar methods have found

application. Herman et at. [40] have reported on a system for measuring

atmospheric aerosols, and Pasqualucci [41] has described the use of

Doppler radar systems for rainfall measurement. Laser heterodyne

spectroscopy, which measures a distribution of Doppler shifts, has been

used by Gollub et al. [42] to measure the size of falling water droplets

in a diffusion cloud chamber. The principle of this system and some size data are reported by Chabay in this symposium. The Doppler shift

principle is also described today by Yanta, who has used it to measure

size distributions of particles accelerated through a nozzle. In this

work, the particle velocities were determined using the scattered light

spectrum from the particles passing through an interference pattern.

3. COMPARISON OF RESULTS FROM DIFFERENT DEVICES

To give some estimate of the problems associated with droplet

sizing, a comparative study on stearic acid aerosols was performed at

IIT Research Institute. Some of the results are given in figure 4.

For this comparison, the methods using Tyndall spectra, single

particle scattering, electrical mobility, particle collection, and image

reconstruction were employed. The data shows that a mean size of 0.9 ym

was recorded by the Owl from Tyndall spectral measurements. This agreed

fairly well with both the value of 0.84 ym obtained by the collection,

microscope measurement and particle reconstruction method, and the value

of 0.8 ym recorded by the mobility analyzer. The optical particle

counters showed greater variability, which was mainly attributed to a

7

561-452 0 - 74 -2
1.0

0.1

MEAN SIZE OWL 0.9 A WHITBY AEROSOL ANALYSER 0.80

0 MICROSCOPE 0.84

OPTICAL PARTICLE COUNTERS * IITRI 1.08 V BAUSCH a LOMB 1.22 A ROYCO 1.40 • aiMET 1.30 AEROSOL CONCN 30,000 p/CU.FT * WITH 10% DILUTION

I I I I I I I _1_L

20 40 60 80 95

Cum. Frequency % Less Than Size

99.5

Figure 4. Comparative data taken on optical particle

counters and the Whitby Aerosol Analyzer.

coincidence problem. With 30,000 particles/cubic feet, the Bausch and

Lomb, Climet and Royco counters gave mean sizes ranging from 1.22-1.4

um. In contrast, the IITRI counter, operating with a 10:1 dilution

system, gave a value of 1.08 ym.

In comparing these results, some correlation with the response time

of the sizing methods should be reported. For example, the Owl was situated on the exit of the aerosol generator, and measured the aerosol

at time = 0. In contrast, the optical particle counters, the Whitby

Aerosol Analyzer, and the electrostatic precipitator were connected to

an 800 cu ft aerosol chamber, and measured the aerosol several minutes

after generation. Allowing 5 minutes for homogenization of the aerosol

after the completion of aerosol generation, all the instruments began

their sampling period, at a time, t, = 5 min. The optical particle

counters then recorded the size distribution during a 1 minute sampling

period, the Whitby Aerosol Analyzer recorded the size distribution

during a 4 minute sampling period, and the electrostatic precipitator

collected aerosol over a 20 minute sampling period.

It is important to note that the collection and the mobility

analyzer results at t = 25 and t = 9 minutes, respectively, both

indicated lower mean values than the Owl at time =0. As the standard

deviation of the distribution by microscope was less than 1.2, the

aerosol can be termed monodispersed, and the Owl data should be

reliable. This suggests that the analyses at time = 9 min and time = 25

min shows indication of slight size contraction of the aerosol. Though

not considered an evaporative aerosol, stearic acid will undergo slight

8
size changes of this kind. In contrast, evaporative aerosols, such as solvent droplets, would have shown wide differences, depending on the

method, and time factor.

Though very brief, this data illustrates the point that droplet

sizing data will be a function of the sizing method employed, the

instrument used, the sampling time, and the characteristics of the

aerosol. In consequence, in recommending a method for aerosol spray can analysis, the FDA, NIOSH, and EPA should consider this fact critically

before proposing any specified technique to manufacturers and users of

consumer products.

4. POSSIBLE METHODS FOR SPRAY CAN ANALYSIS

From the number of methods described in section 2, a few can be se- lected for use in spray can analysis. In general, the factors that have

been taken into account in the selection of the following methods have

been (1) size range of application, (2) speed of analysis of

replication, and (3) cost of the method.

For consumer products, a size range of 1-300 ym is considered wide

enough to cover most spray can droplet size distributions. For

particles of this size and of high volatility, a maximum imaging and

sensing time of 2 seconds is desired.

On this basis, laser holographic methods are considered the best

for most applications. The method is fast, requires no aerosol

confinement, and functions independent of the droplet composition. When

coupled with image analysis equipment, the method is even better.

However, the cost of such a total system is estimated to be $60,000. By

using a manual image analysis method, the capital cost can be reduced to

$25,000. With light scattering equipment, the analysis is again fast,

but the aerosol does have to be confined, and the signal is dependent on the refractive index of the droplet. Further, to measure the size

distribution, the aerosol must be dilute. A light scattering system

capable of sizing aerosol sprays is estimated to cost around $10,000.

If low capital cost investment is a factor, then methods such as photographic imaging and impaction can be considered. With photographic

imaging, the camera is the only investment if sizing is manually performed on the photograph. With impactors, sizing can be performed by a reconstruction of the droplet size from the size of the deposit on each

stage, providing that the concentration of the solute is known.

Alternatively, sensitized paper can be used as the deposition surface

with an impactor of the Lundgren type. With this method, sizing would

be manually performed by measuring the spots on the paper. For these

low capital cost methods, the collection time is fast, but the

analytical time is long and slow. Such devices would have a capital

cost on the order of $1,000.

9
5. CONCLUSIONS

This brief review highlights the more important methods of sizing i droplets, sprays, and mists. It specifically reviews the methods of

application to aerosol spray can analysis, and recommends some techniques that are most suited to this purpose. i

Of these, the most technically attractive is laser holographic

imaging, but this involves a major capital cost investment. Light | scattering systems are less expensive, but are also less technically

attractive. Low capital cost methods that are useful include

photographic imaging and impaction, but these are analytically slow and

have high labor costs.

In selecting a method for the analysis of spray from aerosol cans,

the capital cost-labor cost factor has to be resolved. If capital cost

is no problem, then laser holographic imaging is considered the best

choice.

REFERENCES

Marchesani, V. J. et al., J.A.P.C.A., 20 (1), 19, 1970.

Harris, W. S., Arch. Int. Med., 131, 162, Jan. 1973.

Environmental News, Wed., April 17, 1974.

Davis, R., "Review of Methods for Sizing Droplets, Sprays, and i Mists," IITRI Literature Review to be published.

Davies, R., Proc. Internat. Conf. Particle Technology, IITRI,

Chicago, August, 1973.

| Davies, R., American Laboratory, Part I, Dec. 1973.

j Davies, R., ibid., Part II, Jan. 1974.

Davies, R., ibid.. Part III, Feb. 1974.

[

! Mueller, E. and Sims, A., U.S. Army Electron. Command Report No

EC0M-0032F (AD 645218).

Meline, H., Test Eng., Sept. 1968. I Webster, J. M., J. Photogr. Sci . , 19_, 38, 1971.

Knollenberg, R. G., J. App. Met., 9, 86, 1970.

Cannon, T. W., J. App. Meterology, 9, 104, 1970.

^

Stockham, J. et at., J. Physics E. (Sci. Inst.), 4, 557, 1971.

10

; 1

; 2

; 3

; 4

: 5

; 6

; 7

; 8

; 9

:io

ir

12:

13:

14'
Spraying Systems Inc., N. Avenue, Wheaton, 111.

Bentley, W., Mon. Weather Review, U.S.W.B., 32, 450, 1904.

Defant, A., K Acad. Wiss. Math. Naturwiss., 584, 1905.

Mt. Washington Obs. Res. Report, Jan. 1951.

Farlow, N. H., J. Coll. Sci . , ri_, 184, 1956.

May, K. R., J. Sci. Instrum., 22_, 187, 1945.

Delavan Corporation Literature, West Des Moines, Iowa.

Vooren, P. H. and Meyer, P. B., Am. Ind. Hyg. J., 134, Feb. 1971.

Schtinauer, G., Staub, 27_ (9), 7, 1967.

Tanasawa, Y. and Hiroyasu, H., Tech. Rep., Tohuku Univ., 27, 67,

1962.

Preining, 0. et at., J. Coll. Int. Sci., 23, 458, 1967.

May, K. R., J. Sci. Inst., 27_, 128, 1950.

Cooper, B. F., Australian J. App. Sci., 2j 43, 1951.

Chuan R. L., Aerosol Sci., 1_, 111, 1971.

Pye, J. W., J. Inst, Fuel, 253, May 1971.

Green, H. L. and Lane, W. R., Particulate Clouds , Van Nostrand,

1964.

Gravatt, C. C, J.A.P.C.A., 28, 12, Dec. 1973.

Ranade, M. B., Ph. D. Thesis, Illinois Inst. Technology, March

1974.

Cooke, D. D, and Kerker, M., J. Coll. and Int. Sci., 42 (3), 535,

1973.

Schleusener, S. A., Pow. Tech., 2^, 364, 1968.

Harris, F. S. and Morse, F. L», J. Opt, Soc, 59 (11 ), 1518, 1969.

Gucker, F. T. et al. , Aerosol Science, 4, 389, 1973.

Horn, J. et al., D.D.C. TISIA-2 Report 20 (AD 459 583), U.S. Dept.

Commerce

.

n
[38] Hathaway, Aerosol Age, 20, Sept. 1973.

[39] Stigliani, D. J. et al., J. Opt. Soc. America, 60 (8), 1059, 1970.

[40] Herman, B. M. et at., J. Atmos . Sci . , 28, 763, 1971.

[41] Pasqualucci, F., Proc. 15th Conf. Radar Meteorology, Champaign,

111., Oct. 1972.

12
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 412

Aerosol Measurements, Proceedings of a Seminar on Aerosol Measurements

Held at NBS, Gaithersburg, MD, May 7, 1974 (Issued October 1974).

LIGHT SCATTERING BY SINGLE AEROSOL PARTICLES

Milton Kerker

Clarkson College of Technology

Potsdam, New York 13676

ABSTRACT

Although angular light scattering is a sensitive measure of particle size, sensitivity is rapidly lost when a mixture of particulates

covers a range of particle sizes. However, instrumentation for studying

scattering by single particles can overcome this limitation. This will

be illustrated with reference to scattering by single fibers and

spheres.

Key words: aerosol fibers; light scattering; particle size measurements; particulates; refractive index.

I shall touch upon three aspects of the art of particle size analysis by light scattering. These are (i) difficulties associated with

the inversion of light scattered by dispersions, (ii) our experience

with scattering by single particles, and (iii) some recent calculations

of the response curves for commercial particle counters. Background

material may be found in reference 1.

I. THE PROBLEM OF INVERSION

Light scattering by particles which are comparable in size to the

magnitude of the wavelength of light is strongly dependent upon particle

size, and it is for this reason that light scattering can provide a sensitive technique for particle size analysis. However, for a dispersion, the light signals are averaged over the particle size distribution. This has the effect of washing out the information and makes inversion of the light scattering-data to obtain the particle size distribution increasingly difficult as the distribution broadens.

This effect is illustrated in figure 1, which is based upon

calculations for refractive index m = 1.43. The ordinate p represents

the ratio of the intensity of the parallel polarized component of the

13
Figure 1. Polarization ratio p plotted against scattering

angle 0 for afv] = 5.0 and oo = 0.100, 0.125, 0.145, 0.200,

0.250, and 0.300. Inset shows distribution curves for

Oo = 0.100 and 0.300 when X = 0.546 m.

scattered light to that of the vertical component. This is plotted

against the scattering angle for the size distributions illustrated in

the inset. These are logarithmic distributions with a modal optical

size a = 5 (ratio of particle circumference to wavelength) and breadth

parameters Oo = 0.100, 0,125, 0.145, 0.200, 0.250, 0.300. (See ref. 1,

p. 361).

For Oo = 0.100, the curve shows the typical oscillations characteristic of a narrow distribution. However, this structured character is

obliterated as the distribution becomes broader. It is the structure in

these curves, acting as a kind of fingerprint, that permits the precise

14
determination of the size distributions from the light-scattering data.

As the distribution becomes increasingly broader, it becomes increasingly difficult to obtain a unique solution. This is the dilemma. Narrowly dispersed colloidal suspensions are highly contrived laboratory preparations. Most systems encountered both in the laboratory and in nature

are too polydisperse for particle size analysis by light scattering or

by any other technique which depends upon inversion of such results. In

this size range, oo = 0.20 seems to be an upper limit.

On the other hand, quite accurate results can be obtained for a sufficiently narrow distribution. This is illustrated in figure 2,

1.75

1.50

P

1.25

1.00

o 0.43

0.54

A 0.57

• 6 yxm

6 jjLTn

8

j

40 70 100 130

e

Figure 2. Experimental polarization ratio p versus scattering angle e obtained at three wavelengths,

X = 0.436 ym, 0.546 ym, and 0.578 ym for a dibutylphthalate aerosol.

15
which gives experimental results for a di butyl phthal ate aerosol obtained

at three wavelengths, X = 0.436, 0.546, and 0.578 ym. Because the size

parameter a depends on wavelength, the light-scattering curves for the same system differ in the indicated manner at the different wavelengths.

Yet, internal consistency requires that inversion of these data yield

the same distribution of particle radii. The corresponding particle

size distributions plotted in figure 3 show that whereas the scattering

curves are quite different, the distributions calculated from these two

sets of data are in excellent agreement.

10.0

uj 5.0

c

().436 /i.m

).546^m

).578 fJLm

ifA\\

il \\

1

/r

li

1—W~

I 4

f 4

n\

1

1

If

IS

il

f

\\\

\\\

\)

\ \

\

/

li //

/J

k

0.1 0.2 0.3

V

0.4

a (jJLm)

Figure 3. Particle size distributions corresponding

to the data in figure 2.

16
II. RESULTS FOR SINGLE PARTICLES

The potential of the light-scattering method can be illustrated by some results obtained with single fibers obtained a number of years ago

[1].

The physics of scattering by long circular cylinders is quite analogous to that by spheres, and so the potentialities can be appreciated

if we consider the case illustrated in figure 4. The ordinate is the

polarization ratio and as in figure 2 this is plotted against the scattering angle. The circles show the experimental results for a particular silica fiber with unpolarized light of 546 nm. These are compared

with theoretical calculations shown as smooth curves for six combinations of the refractive index and the size parameter.

The results are striking. The best fit between the experiments and

theory is for m = 1.46 and a = 4.00 as depicted in the lower center

panel of figure 4. When a = 3.98 or 4.02, the theoretical results

differ from the experiments as shown. Similarly, there are differences

between theory and experiment for a= 4.00, but with m = 1.45 or 1.47.

In effect, as these diagrams show, this represents a determination of

the size of this fiber to an accuracy of about 0.25 percent. This is

remarkably accurate. For example, size determination in this range by

electron microscopy is hardly better than about 5 percent.

Figure 4. Polarization ratio p plotted against scattering

angle e for a cylinder illuminated at perpendicular incidence. Comparison of experimental results (circles)

and theory (smooth curves) for a silica fiber at X

= 0.546 ym.

17
With the availability of a commercial single particle light

scattering photometer*, it has become possible to make measurements of

dispersions, a single particle at a time. We have determined the

particle size distribution of a sample of polystyrene latex as obtained

particle by particle using this instrument.

The size distribution made up of 331 single particle runs is shown

in figure 5 as a histogram of percent frequency versus size parameter a. The maximum frequency is at a = 4.79 with 90 percent of the particles

between 4.58 and 4.90. The remaining particles are grouped discretely

in the ranges a = 3.0 to 3.3 and a = 3.72 to 4.32.

The two smaller populations have not been reported before. It is

not surprising that they would not be detected by conventional light

scattering. For example, we have calculated the light-scattering rad16

12

%

8

4

• 1 1 1 I i i 1 1 I II a 3.00 3.40 3.80 4.20 4.60

Dia.(nm) 490 556 621 686 752

Figure 5. Histogram of particle sizes of a polystyrene latex

obtained by light scattering from 331 single particles.

*Differential II Light Scattering Photometer manufactured by Science

Spectrum, P. 0. Box 303, Santa Barbara, Calif.

18
I iances that would be obtained for the distribution depicted in figure 5

and then have inverted these results on the assumption that the system

1 was a logarithmic distribution. This gave a modal size aw = 4.75

I instead of 4.90 and Oo = 0.02. Thus, the smaller class of particles

I affects the estimate only slightly, and if there were not some a priori

I information concerning them, there would be no reason, on the basis of

the light-scattering data, to suspect their presence. Accordingly,

light scattering from the assembly of particles cannot be used to detect

the presence of these smaller particles.

i We also determined the size distribution of this latex in the con1 ventional manner using the liquid suspension. The results for the average diameter are compared in table 1 with determinations in other laboratories.

Table 1. Comparison of average (or modal) diameters

of Dow Polystyrene Latex.

Method Diameter

nm Particle by particle light scattering (this work)

a) Major population group 784

b) All particles 764

Hydrosol light scattering (this work) 797

Hydrosol light scattering, Kratohvil and Wallace 773

Electron microscopy, Dow Chemical Company 796

Electron microscopy, Robbins and Jizmagian 777

Electron microscopy, Davidson et at. 765

The differences are probably within the experimental errors of the

techniques. The principal advantage of the single particle techniques

(both light scattering and electron microscopy) is that no a priori

assumptions are required regarding the form of the distribution function. Indeed, the negative skew of the main population group in figure

5 and the presence of the two classes of smaller particles could not be

predicted nor can their presence be confirmed using conventional lightscattering techniques.

The failure heretofore to observe the smaller particles calls for

further investigation, using both light scattering and electron microscopy. Also, much work remains to be done to assure the lack of bias in

the sampling procedure for the single particle light scattering; otherwise, the question of the skewness of the distribution cannot be re- solved definitively. In any case, the prospect for applying light scattering to size distributions broader than any heretofore amenable to

light-scattering analysis appears to be a reasonable prospect.

19
III. RESPONSE CALCULATIONS FOR LIGHT-SCATTERING

PARTICLE COUNTERS I

We have recently completed response calculations for five commercial light-scattering aerosol particle counters which take into account

the emissive power of the light source, the spectral sensitivity of the

| phototube, and the specific geometrical factors for each instrument. \ Earlier calculations had been published for two of these instruments, i but these earlier calculations had not considered each of these factors ' appropriately. The results will be published elsewhere [3]. Here we i wish only to state that there is a strong dependence of the response i upon both the real and the imaginary part of the refractive index and, i for a given refractive index, a multivalued response in the submicrometer range for three of the five instruments. Certainly these results

indicate that particle size determination with any of these instruments

will be highly precarious whenever the refractive index of the particles

is unknown. Furthermore, the hazards of venturing to extend the use of

| these instruments for particle size analysis of nonspherical particles

are quite unpredictable. ;

REFERENCES

[1] Kerker, M., "The Scattering of Light and Other Electromagnetic Radiation," Academic Press, New York, N.Y., 1969.

[2] Cooke, D. D. and Kerker, M., J. Colloid Interface Sci

.

42, 150

(1973).

j [3] Cooke, D. D. and Kerker, M., Appl . Optics . Submitted for publica- I ti on .

j

I

I

20
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 412

Aerosol Measurements, Proceedings of a Seminar on Aerosol Measurements

Held at NBS, Gaithersburg, MD, May 7, 1974 (Issued October 1974),

LIGHT SCATTERING METHODS FOR THE CHARACTERIZATION OF

PARTICULATE MATTER IN REAL TIME

C. C. Gravatt

National Bureau of Standards

Washington, D.C. 20234

ABSTRACT

This paper presents a brief overview of new light scattering

methods for the rapid characterization of particulate matter in air. An

instrument has been developed which determines the size distribution of

particulate matter in air in essentially real time by a forward lobe

light scattering method. The basic concept involves the simultaneous

measurement of the intensity of light scattered by a single particle at

two small scattering angles. The ratio of the two intensities is a direct measure of the size and is fairly independent of the index of

refraction of the particle. Numerical solutions of the Mie equations

for spheres have indicated that the sizing error by this method is not

greater than ±15% for the range of particle sizes from 0.2 to 4 ym for

essentially all possible indices of refraction. In addition, it appears

feasible to extend the lower limit of size determination to 0.05 um by

the measurement of a single forward lobe intensity. Also, a technique

has been developed which permits some degree of chemical

characterization of particles, and which has been employed in a smoke

detector capable of distinguishing between fire-produced and nonf ireproduced aerosols.

Key words: aerosol sizing; aerosol spectrometer; chemical characterization of particles; fire produced particles; laser light scattering by

aerosols; particle size measurements; particulates refractive index;

smoke detector.

I. INTRODUCTION

The National Bureau of Standards, as part of its Measures for Air

Quality Program (MAQ), has been investigating rapid, light scattering

methods for the characterization of particulate matter. These

investigations have primarily dealt with particles in air with diameters

in the respirable size range (0.05 ym to 5 ym) although the techniques

described below can be modified to operate over other size ranges and in

other media, such as water. A single particle light scattering

instrument has been developed which is based on a forward-lobe-ratio

technique and which covers the particle diameter range from 0.2 ym to 4

21
ym. Theoretical calculations for spheres have shown that the particle

size error, due to variations in index of refraction, is approximately

±15% for essentially all possible materials. This technique has been

incorporated into a field compatible instrument constructed for and

partially supported by the EPA, which will be part of the Regional. Air

Pollution Study (RAPS) to get comparative data among various particle

sizing instruments. In addition, a method has been developed which

permits some degree of chemical characterization of particles,

specifically, a determination of whether a particle is carbon like,

metal like, or something else. This method has been incorporated into a smoke detector which can distinguish between fire-produced and nonf ireproduced aerosols.

Light scattering methods provide a useful way of sizing particulate

matter in air in essentially real time and without the necessity of

collection of the sample. In addition, the size range of most interest

for lung interaction, 0.05 ym to 5. ym, is relatively easily detected by

light scattering if moderate power lasers are employed as sources. However, the scattered intensity is not only a function of the particle

size but depends rather strongly on the particle index of refraction.

As a result, in those instances where a wide and unknown range of

indices of refraction may exist in the sample, the light scattering

method may give erroneous size values. It is found that errors on the

order of factors of 2 or greater are not uncommon when sizing particles

which have indices of refraction differing from that of the calibration

species [1]. It was first pointed out by Hodkinson [2] that the forward

lobe region of the particle scattering spectra is least affected by the

index of refraction and provides a convenient method of sizing particles

to a moderately high degree of accuracy.

II. PARTICLE SIZING

An instrument has been developed which limits the error due to

unknown particle index of refraction to approximately ±15% for

essentially all possible types of aerosols. The basic concept of this

method involves the measurement of the intensity of light scattered by a single particle at two small scattering angles and the calculation of

the ratio of these two intensities. The intensity ratio is directly

related to the size of the particle and is essentially independent of

the index of refraction of the particle. The intensity ratio method is

also attractive since it eliminates or minimizes complications resulting

from a number of experimental phenomena, such as source intensity

variations and nonuniform particle velocities. Since this instrument

and its theoretical evaluation has been discussed in several

publications only a brief summary is given below [1,3,4].

Figure 1 provides an introduction to the concepts used for both

particle sizing and chemical characterization. This figure shows the

scattered intensity, in arbitrary units, as a function of scattering

angle, 0° to 180°, for a particle diameter of 1.0 ym and an incident

wavelength, X, of 514.5 nm. The incident beam is assumed to be linearly

polarized and the scattered intensity is measured in the plane of

22
0 20 40 60 80 100 120 140 160 180

Scattering Angle, degrees

Figure 1. Scattered intensity as function of scattering

angle in the plane of polarization, n = index of re- fraction. X = wavelength of incident radiation.

polarization. The two particles are one with an index of refraction of

n=1.55-0.0i, representing a nonabsorbing material with an index similar

to many earth dusts, and the second a carbon particle, with index of

1.96 - 0.66 i. It can be seen that the forward lobe for these two

materials are very similar in shape even though the intensity differs by

a factor of two or so. The similarity of shape of the forward lobe for

widely varying indices of refraction is the phenomenon employed for

particle sizing. Specifically, a ratio of two intensities in the

forward lobe is measured. The angular ratio of 20°/10° is effective

over the particle range 0.1 ym to 1.5 ym, the ratio 10°/5° is effective

over the range 0.2 ym to 3.5 ym and the ratio 5°/2.5° is effective over

the range 0.5 ym to 10 ym if the incident wavelength is approximately

500 nm. Shorter and longer wavelengths decrease and increase these

ranges respectively. Also, it can be seen that the region from 30 to

90° differs by about one order of magnitude for these two materials.

This effect is found to be generally true of absorbing and nonabsorbing

materials and permits the partial chemical characterization of

particles

23

561-452 O - 74 - 3
Figure 2 is a block diagram of the optical and electrical sytem

used in the original instrument. The EPA instrument differs slightly in

ways which are given below.

Total Number

Figure 2. Block diagram of forward-lobe-ratio particle sizing

instrument.

The light from a CW laser, either argon ion or helium-neon, is

focused to a small point by a lens, and the sample blown through this

point. A flow rate is chosen such that a particle takes approximately

10 ys to pass through the beam. When a 2 mW He-Ne laser is used as the

source, it is found that the smallest particle which can be sized

effectively is approximately 0.4 ym in diameter due to insufficient

scattered intensity for smaller particles. However, when the source is an argon laser of 10 mW or higher power, particles can be sized to the

limit of resolution of the ratio method, 0.2 ym for the angles of 10°

and 5°. Smaller particles than this are "seen" and, therefore, counted

by the system, but the ratio is relatively insensitive to particle sizes

below 0.2 ym. The EPA instrument employs an air cooled 10 mW argon ion

laser as source.

The light scattered by a particle is measured by the light

collector consisting of three parts and operating as a fixed, two angle,

annular iris detector. The innermost section has a hole in its center

to allow the main beam to pass through and an outer conical shape at an angle of 5° relative to the optical center. The second section has an inner conical shape of 5° and an outer conical shape of 10°. The outer

most section has an inner conical shape of 10°. Optical fibers are positioned in the rear of the collector system and these fibers are grouped in two bundles, one each for the scattering angles of 5° and

10°. The output ends of the fiber optic bundles are placed close to the

photocathodes of two photomultiplier tubes. When a particle passes

24
through the light beam a pulse will be produced in the output of each

photomultiplier. The height of these pulses is proportional to the

scattered light intensity and the width is the time of flight of the

particle through the beam. The optical system of the EPA instrument is

similar except the fiber optics have been replaced by a series of

lenses, mirrors, and baffles.

The active optical region is defined by the diameter of the laser

beam, 0.14 mm, and the diameter of the sample stream 0.1 mm, and it has

a volume of 1.5 x 10-5 cm^. Therefore, concentrations as high as 2 x 10^ particles/cm3 can be measured without coincidence effects being

larger than 1% of the total counts. At higher particle densities the

sample stream can be diluted to reduce the concentration to an acceptable level and thus prevent coincidence losses caused by two or more particles being in the beam at the same time. The scattered light

is detected by the two photomulti pliers , and level detectors in a coincidence circuit are employed in each output channel to define that

period of time during which a particle is "seen" by both detectors.

During this time the scattered intensity at each angle is integrated in

analog circuits (in the EPA instrument the peak intensity is detected).

At the completion of the passage of the particle through the beam an output pulse is generated whose height is proportional to the ratio of

the two integrated intensities. These pulses are counted either in a multichannel analyzer or a minicomputer to determine the particle size

distribution.

It is important to note that by taking a ratio of the two scattered

intensities, most experimental complications are eliminated. There is no uncertainty caused by particles taking different times to pass

through the light beam, or by passing through different parts of the

beam, or by variation in intensity of the laser. In addition, it is

possible to design the logic and timing system so that in the event that

two particles are in the beam at the same time, no data are taken.

The performance of this instrument has been tested on a number of

particulate samples. The primary calibration has been performed using

polystyrene latex sphere samples of accurately known diameter. In

addition to the latex sphere samples, this system has been tested with a variety of materials having wide ranges of indices of refraction and

nonspherical shapes. These samples include zinc cadmium sulfate

(fluorescent), tungsten, iron oxide, carbon black, titanium dioxide,

puff ball spores, kaolin, and silica. Since these materials have not

been accurately characterized as to their size distribution by an independent method, it is impossible to reach quantitative conclusions,

but from the reported data on mean size or size range it has been found

that this ratio instrument is not making large errors due to index of

refraction or shape. It is estimated that the total size error,

assuming a calibration performed with polystyrene latex spheres, is no greater than 20%.

25
Numerical solutions of the Mie equations for spheres have been

performed to evaluate theoretically the optimal parameters and the

inherent error in the forward lobe ratio method [4,5]. These

calculations were performed for essentially all possible indices of

refraction that might be expected in ambient air and other special types

of particulate samples such as fire-produced aerosols. Specifically,

the real component of the index of refraction was investigated over the

range 1.33 to 2.0 and the imaginary component investigated over the

range 0.0 to 4.0. This range of indices will cover most inorganic and

organic materials, metal ores, earth dusts, carbons, and the metals.

The results of these calculations have shown that the inherent error in

the particle size, due to index of refraction variation in the sample,

is no greater than 15% for the following combination of particle size

ranges and angular ratios, assuming the particles are spherical. The

angular ratio of 20°/10° is effective over the particle range 0.1 ym to

1.5 ym, the ratio of 10°/5° over the range 0.2 ym to 3 ym and the ratio

5°/2.5° over the range 0.5 ym to 10 ym if the incident wavelength is

approximately 500 nm. Shorter and longer wavelengths decrease and

increase these ranges respectively. The effect of angular aperture was found to be small, and moderate apertures of approximately 0.5° are advantageous in that they increase the scattered intensity and do not

degrade the size sensitivity. Various ratios were investigated, and it was found that those with a value of 2 are as effective as any other

ratio and lead to the most synmetrical calibration curve.

Figure 3 shows the theoretical error envelope for the intensity

ratio of I10/I5 unpolarized incident radiation. Also shown is the

response curve for particles with index of refraction of 1.50 - O.Oi.

The error envelope incloses all response curves and can be used to

determine the theoretical error of forward lobe ratio instruments. In

general, the individual response curves are not monotonic functions, and

therefore, in those situations in which the particle index of refraction

is known, the forward lobe ratio technique may not be the most accurate

method to employ. Also note that beyond a diameter of 3.0 ym (a = IID/A

= 19) the ratio envelope increases. To prevent substantially higher

errors due to this effect, an intensity cutoff is employed to prevent

the recording of a ratio for particles larger than this cutoff.

We have investigated theoretically several ways of decreasing the

size error. The most promising of these is to use a mul tiwavelength

laser as a source, especially one that has two widely-spaced lines. For

example, an argon-krypton laser, with blue, green, and red wavelengths,

would have an error of approximately 3% rather than the 15% error for a single wavelength. It appears that it is important that the source contain two or more widely spaced lines rather than having a continuous

wavelength distribution as in incandescent sources.

An extension of the theoretical studies described above [4,5] has

indicated that particles in the diameter range 0.05 ym to 0.2 ym which

are below the resolution of the ratio method (although they can be

detected by the system) can be sized by a single forward lobe intensity

or by a difference of two forward lobe intensities. The theoretical

26
Diameter (^xm) for X = 514.5 nm.

Intensity Ratio

Figure 3. Error envelope for particle sizing instrument.

Intensity ratio is for I (10°)/I (5°), unpolarized incident beam and no detector aperture, a = n(diameter)/X

cutoff by intensity at a = 19.

error of these techniques is approximately 25% for all possible indices

of refraction. However, the use of a single intensity or a difference

of intensities will produce experimental errors which are not of concern

in the ratio method. One possible way of eliminating the majority of

the experimental errors is to normalize the intensity data by the time

of flight of the particle through the laser beam. The time of flight is

relatively easily determined and serves as a measure of the main beam

intensity incident on each particle.

III. CHEMICAL CHARACTERIZATIONS

In many situations some degree of chemical characterization of

particulates would be advantageous, especially if it could be

accomplished without collection and subsequent chemical analysis of a sample. As an outgrowth of the theoretical studies described above,

which are concerned with a study of particle sizing methods, a technique

was discovered which seems to offer a way of determining some information related to the chemical composition of each particle as it

passes through the laser beam. The information that can be obtained is

a function of the imaginary component of the particle's index of

refraction.

27
Figure 4 shows how chemical information can be deduced. The

quantity I, represented as the integral over 40" to 70° of the scattered

intensity in the plane of polarization is shown as a function of

particle size for a series of absorbing and nonabsorbing materials. I can also be considered as the output of an intensity detector in the

polarization plane with an aperture from 40° to 70°. For particles

smaller than 0.3 ym, it is found that I is insensitive to chemical

character but for larger particles the discrimination is quite good.

The band represented by k=0 is the envelope region for all values of the

real component of n but with no absorption character.

A fairly complete analysis of this effect has shown that the

angular region from 40° to 70° is the most effective region to employ

for discrimination among materials [6].

Particle Diameter, /um

Figure 4. Detection of absorbing and nonabsorbing particles,

k = 0 region is for wide range of real components of index

of refraction.

28
By employing the particle sizing instrument described above and

adding to it an optical system which collects the light scattered by

each particle over the angular region of 40° to 70° in the plane of

polarization it should be possible to classify those particles which are larger than 0.3 ym as to whether they are nonabsorbing, carbon-like, or metallic. In somes cases, it also appears that some discrimination as to the type of metal might be possible.

In order to test the ability to distinguish between absorbing and

nonabsorbing particles, a smoke detector based on a measurement of the

intensity in the polarization plane from 40° to 70° was constructed.

The primary method now employed for the detection of fire-produced

aerosols involves monitoring a signal related to the total particle

density. This technique has a disadvantage in that certain nonfire

produced aerosols may have total particle densities high enough to cause confusion. These aerosols might be cigarette smoke, industrial dusts or fumes, or "hair spray" aerosols.

The smoke detector instrument contained a fairly straightforward

optical system. Light from an incandescent source was polarized,

collimated and impinged on an aerosol. Light scattered by the aerosol was monitored over two regions. One of these, I„ , was over the angular

range from 40° to 70° in the plane of polarization. This quantity

should be relatively large for nonabsorbing aerosols and should be less

for aerosols containing absorbing materials. The second region was perpendicular to the polarization plane and over the angular range 80°

to 100°. This intensity, Ii , can be shown to be approximately

proportional to the total number of particles in the beam regardless of

whether they are absorbing or not. For display purposes, these two

outputs have been applied to the vertical and horizontal axes of an oscilloscope, and the traces for a series of materials shown in figure

5. The length of the line is arbitrary and is related to how many

particles are in the beam at any one time. As can be seen, I„ is large

relative to Ij. for nonabsorbing materials such as steam, cigarette

smoke, and freon containing aerosols. However, for metal dusts and

carbon containing materials I„ is small relative to Ix. Thus, a smoke

detector which went into alarm on the slope of the I,, /Ij. curve rather

than the total particle density could distinguish between fire-produced

and nonfire-produced aerosols.

REFERENCES

[1] Gravatt, C. C, "Real time measurement of the size distribution of

particulate matter in air by a light scattering method," J. Air

Pollution Control Association, 23:12, 1035-1038 (Dec. 1973).

[2] Hodkinson, J. R., "Particle sizing by means of the forward scattering lobe," Appl. Opt . 5:5, 839-844 (May 1966).

29
Figure 5. Smoke detector response. I„ and Ij. are defined in

text. Length of line is arbitrary.

[3] Gravatt, C. C, and Allegrini, Ivo, A new Light Scattering Method

for the Determination of the Size Distribution of Particulate

Matter in Air, Proceedings of the 3rd International Clean Air

Congress , p. C3 (1973)"^

[4] Greenough, M. L., and Gravatt, C. C, EAP Final Report on Particle

Sizing Instrument (in preparation).

[5] Gravatt, C. C, and Allegrini, Ivo, Particle Sizing by Forward Lobe

Techniques (in preparation).

[6] Gravatt, C. C, and Allegrini, Ivo, The Characterization of

Particulates as to Their Optical Absorbing Properties (in

preparation)

.

30
DISCUSSION

GEORGE SINNOTT: In using two wavelengths to decrease the size do

you mean that one would take the two wavelength scatterings separately?

GRAVATT: No. The laser is in the all-line mode so there will be some eight lines emitted by the laser--the three prominent ones are the

448.0, 514.5 and 640.0 nm. These calculations are done weighing these

intensities appropriately. It is assumed that all the ligtit is

collected and it results in a smoothing of the ripples of the response

curve shown for a single wavelength.

RONALD NELSON: Going back to your calculation of the ratio as a function of the particle size in the envelope, my question is, when it

gets down to the practical business of calibrating, how does one choose

the center of that envelope for the real particle calibration?

GRAVATT: Let's go back to figure 3 and it will probably be easier

to see what is involved. For instance, we have calculated a large

number of these curves to make up the envelope. There is one for

polystyrene latex, particles the index of those is 1.59. Using that

curve and the instrument output for a number of polystyrene latex

samples it is possible to transform the data from a relative scale to an absolute size scale. It is a problem to generate monodisperse aerosols,

however. There is not a large number of well standardized or well

calibrated latex or aerosol standards. So calibrating any particle

sizing instrument is going to be somewhat difficult.

NELSON: I think that this ought to be considered as to how does one define this. I think that maybe we ought to advocate using the full

width of this envelope as opposed to maybe half-width because if you are going to pick half it implies you know where the middle of it is and

that is not necessarily the case.

GRAVATT: Well, as far as stating errors is concerned, one can report the full width or the half width. I do not feel that it is

difficult to define where the middle of the curve is. It does not seem

to me to be a problem that the particular calibration samples employed

do not lie on the center line. However, since neither NBS or EPA have

done an extensive calibration of the instrument I will have to withhold

final comment until we get some experience and data. I personally feel

that the most important problem in calibration is that of generating

monodisperse aerosols.

ROBERT KNOLLENBERG: Did you say the mean particular size is 0.3

micrometers?

GRAVATT: The minimum resolvable particle size for a ratio if 10°

to 5° is .3 ym. You can detect particles down to about 0.1 urn. With 3

1/3 orders of magnitude gain in the electronics, if the large particle

cutoff is set at approximately 3 ym.

31
KNOLLENBERG: Do you know roughly what percentage of the total

light scattering you collect?

GRAVATT: No, I don't know it exactly although I can calculate it.

I would estimate that we collect several percent.

KNOLLENBERG: You are collecting a large percentage then.

GRAVATT: The EPA instrument has 1° aperatures and this probably

collects a few percent of the total scattered intensity.

NELSON: I guess it is about 1 percent.

GRAVATT: It may be 1 percent.

WILLIAM DORKO: Is this calculation dependent upon the structure of

the particle such as spherical?

GRAVATT: All calculations are for spherical particles. It is

stated that the forward lobe is least sensitive to particle shape. Dr.

Kerker could probably make a better comment to that than I can. Hodgkinson also felt that way, but we have not done any calculations.

We've tested nonspherical particles in the instrument, and they fit

within the calibration curve. That's not really proof, it's only a suggestion. This is certainly an area where much more work is needed.

32
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 412

Aerosol Measurements, Proceedings of a Seminar on Aerosol Measurements

Held at NBS, Gaithersburg, MD, May 7, 1974 (Issued October 1974).

FLOW APPARATUS FOR THE CHARACTERIZATION OF AEROSOLS

Madhav B. Ranade

Fine Particles Research

IIT Research Institute

10 West 35th Street

Chicago, Illinois 60616

ABSTRACT

A flow apparatus has been developed to enable the size measurement

of liquid aerosols to be measured under defined conditions with the help

of a light scattering technique. This apparatus was used to study the

evaporative behavior of submicron aqueous aerosols during transportation

in air. The apparatus is currently being used to characterize large (~

50 ym) aerosol particles under sedimentation. Portable versions of the

flow apparatus are presently under development to characterize acid mist

droplets, and to investigate the characteristics of therapeutic aerosols.

Key words: aerosol size measurements; aerosol spectrometer; aerosol

sprays; condensation on aerosol droplets; evaporation of aerosol droplets; laser light scattering by aerosols; therapeutic aerosols.

I. INTRODUCTION

An aerosol system is characterized by the size distribution of the

particles, concentration of the particles, and the physical nature of

the particles. Physical properties of aerosols are strongly dependent

on the particle size characteristics. Sampling and measurement techniques for liquid aerosol systems must be chosen with a regard to physical conditions. In order to obtain a representative sample from an aerosol system, changes in the temperature, pressure, and flow field

during sampling must be minimized.

Volatile liquid aerosols pose a special problem, as the particle

size may change significantly during sampling. Examples of such aerosols are the therapeutic aerosols and aerosol can sprays. In the therapeutic applications, the drug is dispersed in the form of aqueous solution. In the aerosol cans, the product is dispersed by spraying with a volatile solvent in the form of droplets up to 100 ym in diameter. The

particle size of the final residue may vary from submicronic to a few

micrometers.

33
The processes of evaporation and condensation are fast. As an example, the time for complete evaporation of a 1 ym water droplet is

less than one second.

Due to these unstable characteristics of volatile droplets, techniques for in situ monitoring of the aerosols are needed. These techniques must not disturb the aerosol. The choice of techniques to meet

these requirements is limited. Optical visualization techniques such as in situ microscopy, photography, and holography are possible only for

aerosols with particle size above 5 to 10 ym. Techniques based on angular light scattering and light extinction are useful for the submicron

aerosols. The light extinction technique requires long path lengths

which are not practical in the laboratory. Thus, techniques based on angular light scattering offer the only choice available for monitoring

volatile liquid aerosols ranging from submicron to 50 ym in particle

size.

In the flow apparatus described here, the aerosol is monitored by

light scattering in a controlled environment and can be used to study

changing size characteristics of the aerosol.

II. FLOW APPARATUS

The volatile aerosol is injected isokinetically in a moving laminar

uniform velocity air stream (fig. 1). The axial distance from the aerosol injection point is directly related to the age of the aerosol. If

r

u

AIR u

U AEROSOL

P' U. P

AIR

u

x=o

U

Figure 1. Aerosol jet injected isokinetically in a uniform

laminar air stream.
the vapor concentration of the volatile component is higher in the air

stream than in the aerosol stream, the vapor will diffuse to the aerosol

stream and the aerosol particles will grow by condensation. When the

vapor concentration gradients are reversed, the aerosol particles will

evaporate; and if the vapor concentrations are the same, the particles

will be stable.

The flowing aerosol can be conveniently monitored by a light

scattering technique at various axial positions, and since the axial

position is directly related to the age of the aerosol, information

about the rate of evaporation and condensation can be obtained.

An apparatus based on this principal was developed for studying the

evaporation of 1 ym sodium chloride solution aerosols under various

humidities.* A three-inch diameter wind tunnel was constructed for this

study (fig. 2). A specially designed entrance section was used to

obtain a uniform laminar air flow in a three-inch diameter glass test

TRAVERSING PROBE SUPPORT

PHOTOMETER

3"DIA.PIPE

30

AEROSOL

24 15

EXIT SECTION TEST SECTION ENTRANCE

SECTION

AIR

IN

Figure 2. Wind tunnel for evaporation experiments.

section. The humidity of the air stream could be controlled by the

mixing of a dry and a saturated air stream. The aerosol was generated

from an aqueous sodium chloride solution (1% NaCl). The geometric

number mean diameter of the aerosol particles was 1 ym and a Og of 1.8.

A traversing 90° light scattering photometer (fig. 3) was used to

monitor the aerosol at various positions along the axis of the test

section.

The aerosol stream at the injection point is saturated with water

vapor. When the outer air stream was also saturated, no change in photometer response was observed. When the humidity in the outer stream

was less than 100 percent, evaporation of particles was observed. The

diffusion of the water vapor from the aerosol stream to the outer air

stream was shown to control the size change of the aerosol. Using the

*Dynamic Properties of Submicron Aerosols, M. B. Ranade, Ph. D. Thesis,

Illinois Institute of Technology, Chicago, 1974.

35
Figure 3. Traversing 90° light scattering photometer.

diffusion theory and the Maxwell evaporation law, the size of the

particles at various axial positions was predicted by computer

calculations. The light scattering response at these axial positions

was compared with the values calculated from the Mie Theory. As seen in

figure 4, good agreement between the theory and the data was observed.

This apparatus has been modified for studying the evaporation of

larger particles (up to 50 ym). The aerosol flows vertically downward

and sedimentation of large particles out of the aerosol stream is

avoided.

A portable version of the apparatus was constructed with a one-inch

diameter glass tube (fig. 5). In this system, the aerosol can be pulled

through the apparatus. If the air in the sample is used for sheath

(outer air stream), the aerosol will be stable during observation.

Other light scattering geometries and techniques may be used for best

sensitivity in various size ranges. Auxiliary apparatus for

humidifi cation and flow control are mounted on a trolley.

36
Air Stream Relative Hvimidity, 5%

X , cm

.

Figure 4. Evaporation of aqueous sodium chloride solution aerosol.

Figure 5. Portable flow apparatus.

At IITRI, we have set up a system for the monitoring of the liquid

aerosols (fig. 6). The Cascade impactor is used with nonvolatile or volatile aerosols containing a solute. A differential mobility

analyzer, developed by Dr. Earl Knutson, is used to obtain the size

distribution of submicron residual aerosols too small for the impactor.

A condensation nuclei counter is used to monitor the number

concentration of the residual aerosol. The aerosol sample is pulled by

a vacuum pump. A part of the sample air is filtered and used as a sheath air. The photometer is mounted on a motorized track utilizing a helical screw for traversing along the glass tube.

37
Liquid Aerosols

Impactor

Microscope

Image

Analysis

TP.S.D.

Sheath

Air

^Photometer

Flow

Apparatus

Differential

Differential

Mobility Analyzer

Figure 6. IITRI facility for characterization of volatile aerosols.

III. SUMMARY

A facility for the characterization of liquid volatile aerosols in

the size range of 0.5 to 50 ym has been established at IITRI. The heart

of the facility is a flow apparatus. This apparatus was originally developed to monitor the evaporation of micron size aqueous aerosols.

We feel that the facility will be a convenient means for the characterization of volatile liquid aerosols such as the therapeutic aerosols and the aerosols from spray cans.

SUMMARY OF THE MEASUREMENT TECHNIQUE

The flow apparatus was originally constructed as a research apparatus to study the evaporation of micron size aqueous aerosols. The aerosol was injected isokinetically in the middle of a laminar uniform ve- locity air stream. The aerosol was monitored by traversing 90° light

scattering photometer at various axial positions, which were related to

38
the age of the aerosol. The photometric data was reduced by the Mie

scattering theory on a computer and showed good agreement with the theory for the evaporation of aerosols developed for this study.

The apparatus and a portable version of the same are being adapted

to characterize volatile aerosols. The particle size measurement range

is 0.5 to 50 ym, and the concentration range is 102 to 10^ particles/

cm3. The aerosol is monitored in a controlled atmosphere without disturbance. The aerosol sampling rate is of the order of 1 to 5 liters/

minute.

The apparatus is currently under development and final specifications are yet unavailable. However, the cost of the apparatus is ex- pected to be quite low, as most of the parts are simple and easily

available.

DISCUSSION

ILAN CHABAY: Was most of the work you did with monodisperse

systems?

RANADE: We worked with polydispersed systems which had a standard

deviation of about 1.6.

CHABAY: What sort of size range do you use?

RANADE: Essentially one micrometer.

GEORGE SINNOTT: Did you build your own mobility analyzer?

RANADE: Yes, Dr. Earl Knutsen built it. He has a differential

mobility analyzer with a current measuring system.

MILTON KERKER: I have two questions. What is the holdup time and

what is the concentration? And the second question, how do you handle

the evaporation? Do you have to worry about the Kelvin effect and also

I assume you consider the evaporation from a single particle. How do

you handle a situation where you have a collection of particles?

RANADE: In response to the first question, the holdup time is the

order of one to two seconds. That is a rather short time for

coagulation to take place. The concentration is the order of 105 to 10^

per cubic centimeter which is not very high for micrometer size

particles. Now for the second question, the Kelvin effect has been

taken into account in the theoretical equation. One is essentially

concerned with the vapor pressure of the water in equilibrium with the

droplets. This gives rise to a solution effect and a Kelvin effect and

both have been taken into account in solving the equations for the

particle size.

39

561-452 0 - 74 -4
KERKER: There was an Aerosol Conference in Swan Sea last year, and

the question of evaporation figured very centrally. As I recollect some people found that evaporation rates were much lower than they would have

calculated from a simple hydrodynamic treatment of a single particle.

Some people attributed this to impurities on the surface, but the thing

that impressed me more than this was the fact that nobody seemed to come

up with even a rough idea as to how the evaporation rate for a single

particle would compare to that for a particle in an aerosol mixture. I wonder if you have any thoughts on that.

RANADE: Yes, in an aqueous aerosol, evaporation is contributing

water vapor to the system so the humidity is changing. For my aerosol

at one micrometer size and at a concentration of 10^ the contribution to

the humidity wasn't very large. I found that it could be treated the same as single particles. Now if you had a denser system, one would

have to build something like a cell model. I don't know whether that is

justified or not, but a cell model means that instead of assuming that

the particle is surrounded by an infinite media, we consider an average

interparticle distance and solve the equations accordingly. You can show for this concentration the correction for evaporation will be very

low.

KERKER: There is one thing that always bothers me with water

aerosols. You say the evaporation doesn't contribute very much to the

humidity, conversely you need fantastic humidity control in order to

define just what is happening with respect to evaporation.

RANADE: Yes, depending on which end of the scale you are working.

For example, my calculations were based on 10^ particles per cc. Assuming complete evaporation of one micrometer particles, the

contribution to the relative humidity was one percent. On the other

hand when the relative humidity exceeds 85 percent the growth curve is

very steep, and a small change can be quite important.

REG DAVIES: Dr. Morton Corn at Pittsburgh has been doing some work

with sulphuric acid aerosols in the range of 95 percent relative

humidity. Here he observes an enormous change in growth corresponding

to a change of 1/2 percent relative humidity, so this control of

humidity is an important factor.

WENDELL ANDERSON: I think the same phenomenon was reported 20 years

ago by LaMer.

DAVIES: We've done a literature search on this and there have been

quite a number of reports about size growth versus humidity, especially

i'or things like sodium chloride aerosols in lungs which have been used

to investigate lung humidity.

CHABAY: We did some work on cloud droplet growth and we saw some rather strange results at very low levels of supersaturation. The exact

values of supersaturation were extremely critical in determining growth

rate of the droplets falling through a temperature gradient.

40
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 412

Aerosol Measurements, Proceedings of a Seminar on Aerosol Measurements

Held at NBS, Gaithersburg, MD, May 7, 1974 (Issued October 1974),

360° SCATTERING DIAGRAMS FROM INDIVIDUAL AEROSOLS IN A FLOWING STREAM*

Thomas R. Marshall, Charles S. Parmenter, and

Mark Seaver

Department of Chemistry

Indiana University

Bloomington, Indiana 47401

ABSTRACT

Nearly complete 360° scattering diagrams have been obtained from

each of a large number of individual aerosols which stream in an air

sheath through a laser beam. A computer analysis of diagrams from a "monodisperse" aerosol (polystyrene spheres with mean diameter near 1.2

ym) shows that both diameter and refractive index can be determined from

each scattering diagram to compile a statistical characterization of the

aerosol. Diameter precision is about 0.4 percent and the refractive

index can be determined to within 0.7 percent.

The method is clearly a practical approach to routine characterization of aerosol sprays containing spherical particles whose diameters

range from about 0.3 ym to at least 20 ym. The refractive index determination will reveal differences between particle composition and bulk

composition of the parent material as well as composition changes during

aging of aerosol sprays.

Key words: aerosol light scattering; aerosol size measurements; aerosol

spectrometer; aerosol sprays; laser light scattering by aerosols; re- fractive index; scattering diagrams; 360° scattering by particles.

INTRODUCTION

Resolved 360° light scattering diagrams from individual particles

contain sufficient information to determine the size, the index of re- fraction, and, in principle, the shape of regular aerosol particulates.

Such data offers an attractive route for characterization of aerosols,

but unfortunately an experimental barrier exists. Most instruments

capable of these measurements require levitation of a particle for

periods of at least minutes while 360° scans are made [1-5], so that

*Contribution No. 2389 from the Chemistry Department, Indiana University. This work has been supported by NSF Grant GP 38275.

41
acquisition of data on statistical numbers of particles in an aerosol is

not routinely practical.

Professor Frank Gucker and his colleagues at Indiana University

have developed an instrument that obviates this problem [6,7]. They

have reduced data collection times by four orders of magnitude so that

360° diagrams can be made while particles stream through a laser scattering beam.

In this report we describe the initial calibrations and trials of

this instrument using spherical particles of about one ym diameter. Our

first results show clearly that the instrument represents a practical

approach for statistical characterization of an aerosol comprised of

spherical particles of unknown diameter (0.5 to 20 ym) and unknown

refractive index.

The Instrument

Details of the instrument's optical design and mechanical construction are given elsewhere [7]. A schematic of its operating principle is

given in figure 1. In short, an air stream containing aerosol particles

intercepts a He-Ne laser beam at one of the focii of an elliptical

mirror which directs a 360° slice of scattered light to a photomultiplier at the other focus. Angular discrimination is introduced by

a 5° rotating aperture between the scattering plane and the photomultiplier. A nearly complete 360° scattering diagram is obtained in

about 20 msec (7° are missing either side of 0° and 180°). The particle

is effective stationary in the laser beam during this time. Scattering

diagrams are displayed on an oscilloscope and can also be stored on magnetic tape by an interface with an XDS Sigma II computer.

EXPERIMENTAL SCATTERING DIAGRAMS

All of the data described in this report have been obtained from

polystyrene spheres in Dow aerosol sample LS-1028-E. This is a "monodisperse" aerosol with mean particle size reported by the manufacturer

to be 1.099 ± .0059 ym and with a refractive index of 1.59.

Figure 2 shows an experimental scattering diagram obtained from a single particle as it streamed through the laser beam. The diagram is

plotted from data in computer storage. The mirror symmetry of the scattering pattern is especially significant because it proves to be a sen- sitive indication of an accurate scattering diagram.

A test of the reproducibility of the data is offered in figure 3

where four scattering curves from the same particle are compared. These

curves were obtained in successive scans as the particle traversed the

1.5 mm cross section of the laser beam. Such reproducibility is typical

of the instrument. In a size analysis by computer (see below), these

four curves were as$igned to particles with diameters 1.180, 1.180,

42
AEROSOL

Figure 1. A schematic of the 360° scattering instrument. An aerosol

stream intercepts a He-Ne laser beam ("vertical" polarization) at one of two foci of an ellipsoidal mirror. Light scattered in the

"horizontal" plane is intercepted by a segment of that mirror and

directed to a photomultiplier at the second focus. A rotating

aperature allows only a 5" segment of the scattered light to be

detected at any given time by the photomultiplier. The aperature

rotates at about 3000 rpm so that a 360° scan is completed in 20

msec. The photomultiplier signal comprising a 360° scattering

diagram if fed through a logarithmic amplifier to a computer for

storage and processing.

43
DEGREE

Figure 2. A scattering diagram obtained from the instrument described

in figure 1 as plotted from computer storage. This diagram is from

a polystyrene sphere about 1.190 ym in diameter. The signal is

blocked for 7° either side of 0° (forward) scattering and 7° either

side of 180° scattering. The relative intensity scale is logarithmic and accurate at intensities - -0.2. Below that value, limited

bandwidth of the log amplifier has distorted the data.

44
>o.o eo.o tD.o ai.o so.a loo.o isg.q mo.o leo.a isq.q eoa.o eza.o smo.o eea.o eao.a 300.0 3zo.o 3^0.0 3Eo.a DEGREE

Figure 3. Four experimental scattering diagrams from the particle.

These were obtained sequentially as the particle was in transit

across the laser beam.

45
1.180 and 1.175 ym. The noise in these diagrams is thus sufficiently

low to allow size determinations to within about 0.5 percent.

COMPARISON WITH THEORY

A comparison of the experimental scattering diagram with a theoretical Mie scattering diagram is shown in figure 4. The theoretical

curve was computed for a particle of 1.190 ym diameter using a program

provided by Milton Kerker. The index of refraction was set at 1.59.

The latter value is taken from the manufacturer's statement. The size

was chosen by trial as the most satisfactory visual fit to the data.

The mismatch between the curves at relative intensities below -0.2 is

due to the restricted low-signal bandwidth of the log amplifiers

servicing the photomul tipler.

0.0 ?0.0 tO.O 60.0 80.0 100.0 1?0.0 110.0 160.0 180.0 500.0 gJO.O 2M0.Q 260.0 2B0.0 300.0 3S0.O SHO.O 3B0.0 DEGREE

Figure 4. A comparison of theoretical and experimental scattering

curves. The experimental curve is taken from figure 2 and the

theoretical curve is computed for a particle with 1.190 m diameter and m = 1.59. The theoretical curve is degraded to correspond to experimental data with the 5° aperture and blanked-out

at 0° ± 7° and 180° ± 7°. Mismatch between the curves at relative intensities below -0.2 is due to distortion of the experimental curve during amplification of the photomul tiplier signal.

46
Such visual comparisons form one method of size determination. The

fit in figure 4 is typical in quality to all comparisons we have made

using particles from Dow aerosol LS-1028-E. Visually, one can sense distinct preferences for a "best" trial fit only when size increments

used for the calculated curves become somewhat greater than 0.01 ym.

Thus it is probably accurate to say that visual comparisons of

theoretical with experimental diagrams in this size range can yield

particle size to within about 0.015 ym or 1.3 percent.

These points are illustrated in figure 5 where the experimental

curve of figure 2 is compared with less optimal theoretical curves. By

restricting attention to relative intensities greater than -0.2 (where

the data are valid), one can see that in this size range the sensitivity

to particle size appears predominantly in the 90° sectors adjacent to

backward scattering.

d'eGReF

"° " '"' "°° DEGREE

Figure 5. Comparisons of the experimental curve in figure 2 with

theoretical curves calculated for particle sizes departing from

the best fit (D = 1.190 ym) displayed in figure 4. Theoretical curves are shown by dotted lines. The upper curves show decrements from optimal fit by analogous changes; 0.015 ym (left)

and 2 x 0.015 ym. The assumed diameter D is shown for each com- parison, and the number to the right of the diameter describes

the error of fit (least mean squares) for intensities > -0.2.

A choice of particle size from the five comparisons in figures

4 and 5 is seen by this criterion as well as by visual fit to be

1 .190 ym.

47
COMPUTER ANALYSIS OF DATA

Particle Size

Such comparisons as described above are impractical by dint of

tedium for routine sizing of enough particles to yield a statistically

significant analysis. A computer has been used to automate the

analysis. The computer first checks the mirror symmetry of the

experimental diagram. Nonspherical or improperly measured aerosols are revealed by asymmetric scattering. The symmetric diagrams are then

compared to a library of theoretical curves representing various

particle sizes. A least mean square analysis chooses the best fit and

hence determines the particle size.

A check of the computer analysis has been made by comparison of

visual and computer size determinations on a sample of 95 particles

using 0.015 ym size increments. The agreement was satisfactory.

Computer fitting also yields a more precise determination of size.

Computer fitting can discriminate between alternate choices of

theoretical curves differing by only 0.005 ym. This is about 0.4

percent of particle size and represents the practical resolution of our apparatus in this range.

A computer analysis of Dow aerosol sample LS-1028-E was made with

data collected from 180 particles. Diagrams from 63 of these were computer rejected on the basis of asymmetric scattering. (The rejected

data are diagrams of dust, multiple particles, excessive noise,

particles exiting from the beam during recording of diagrams and so forth.) The remaining diagrams on 117 particles were explored by

computer for best fit to theoretical diagrams with trial increments of

0.005 ym and an index of refraction m = 1.59. The subsequent listing of

particle diameters is plotted in figure 6.

This analysis yielded a mean diameter of 1.188 ± 0.007 ym. The mean diameter is about ten percent larger than the value reported by the

manufacturer, but it agrees well with the values reported in seven other

explorations of this standard. The data are summarized in table 1.

As a side issue, we show in figure 7 a comparison of the experimental curve from a 1.190 ym diameter particle (close to the mean diameter)

with the theoretical scattering curve for the mean particle size given

by the manufacturer (1.099 ym). There is hardly a danger of confusing

the issue with 360° scattering curves.

Particle Size and Index of Refraction

The most intriguing promise of this method of aerosol characterization lies in its ultimate ability to define particle shape and index of

refraction in addition to the size measurements described above. No

steps have yet been taken to observe scattering from nonspherical parti

48
(D) = 1.188 //m ±.007 //m

VAn r

1.150 1.160 'l.17o' 1.180 'l.20o' Particle Diameter (ywm)

1.210 .220

Figure 6. The number distribution of particles sizes in Dow aerosol

sample LS-1028-E determined by computer comparison of experimental

360° scattering diagrams with calculated Mie diagrams. The refractive index was assumed to be 1.59 and the assumed diameters for

calculated curves were varied in 0.005 ym increments.

49
Table 1. Reported values of analysis of Dow aerosol standard

sample LS-1028-E.

Reference Method m.d.±S.D. (ym)

Dow

Heard et al . (1970)^

Stober and Flachsbart

(1971

Portsendorfer and Heyder

(1972)^

Phillips et al. (1970)^

Stober and Flachsbart

(1971)^

Wims and Meyers (1972)^

Gucker et al . (1973)^

This work

Electron microscope

Electron microscope

Electron microscope

Electron microscope

Scattering from single particle

Spiral centrifuge

Scattering from solution

Scattering from single particle

Scattering from single particle

1.099±0.0059

1.16 ±0.016

1.26 ±0.032

1.172±0.027

1.200±0.010

1.22 ±0.018

1.20 ±0.012

1.196±0.0065

1.188±0.007

^M.J. Heard, A.C. Wells, and R.D. Wiffen, Atmospheric Environment 4. 149

(1970).

'^D.T. Phillips, P.J. Wyatt, and R.M. Berkman, J. Colloid Interface Sci . 34, 159 (1970).

^J. Porstendorfer and J. Heyder, J. Aerosol Sci . 3_» (1972).

^W. Stober and H. Flachsbart, J. Aerosol Sci . 2, 103 (1971).

^A.M. Wims and M.E. Meyers, Jr., J. Colloid Interface Sci . 39, 447 (1972).

f

F.T. Gucker, Jiri Tuma, H.-M. Lin, C.-M. Huang, S.C. Ems and T.R.

Marshall, J. Aerosol Sci. 4, 389 (1972).

50
tO.a 60. D 80.0 100.0 i!>0.0 IHO.O 160.0 tftl.O 200 0 ?20.0 SMOO ?b0.0 ?80.0 -JOO.O 1?0.0 310.0 3etl.O DEGREE

Figure 7. A comparison of the experimental scattering curve (solid

line) for a 1.190 ym particle (near the mean diameter of particles

Dow aerosol LS-1028-E) with a curve calculated for diameter 1.099

ym. The latter is the mean size reported by the manufacturer.

cles, but the initial explorations of simultaneous size and index of re- fraction determinations are at hand.

We have taken the experimental scattering diagrams of seven particles from Dow aerosol LS-1028-E and searched for the best computer fits

to theoretical curves in which both m and D were varied parametrically.

Diameters, D, were incremented by 0.005 ym (0.4%) and the refractive

index, m, was incremented by 0.01 (about 0.9%).

Computer selections of m and D for these particles are summarized

in table 2. The index of refraction was found to be constant for six of

these particles and was chosen as an adjacent increment for the seventh.

Thus the technique is successful in the recognition of an assembly of

particles with uniform index of refraction but dispersion in size.

The fitting procedure is illustrated in table 3 which displays a matrix of least mean square deviations between theoretical trials and an experimental curve for particle No. 7 (which is also the curve in figure

2). Better fits are represented by smaller numbers. This is the only

51
Table 2„ Characterizations of particles in Dow aerosol LS-1028-E by

computer fit of calculated Mie diagrams to 360° experimental

diagrams.

Particle Index of refraction Diameter

m D

1 1.59 1 .190

2 1.59 1.190

3 1.59 1.190

4 1.59 1.195

5 1.59 1.195

6 1.59 1.195

7 1.58 1.195

^m and D were varied parametrically in trial Mie diagrams with increments

of 0.01 and 0.005 ym respectively.

Table 3. Least mean square deviations between the experimental

scattering diagram of the particle in Figure 2 and

calculated Mie diagrams for various values of m and D.

Best fit for this particle is seen to be m = 1.58 and

D = 1 .95 is in ym.

D 1.55 1.56 1.57 1.58 1.59 1.60 1.61 1.62 1.63

1.170 17. 13. 8.3 5.1 3.1 2.3 2.4 3.0 4.0

1.175 14. 9.6 5.9 3.4 2.0 1.7 2.0 2.8 3.9

1.180 11. 7.0 4.0 2.2 1.3 1.3 1.8 2.7 4.0

1.185 8.4 5.0 2.7 1.4 .93 1.1 1.8 2.9 4.4

1.190 6.3 3.5 1.8 .93 .77 1.2 2.1 3.4 5.2

1.195 4.6 2.5 1.3 .75 .85 1.5 2.6 4.3 6.6

1.200 3.5 1.9 1.02 .81 1.2 2.1 3.6 5.7 8.7

1.205 2.7 1.5 1.03 1.1 1.8 3.0 5.0 7.8 12.

1.210 2.2 1.4 1.3 1.7 2.7 4.5 7.1 11. 16.

one of seven diagrams so far explored in which the refractive index was found to be other than 1.59. Even in this case, however, the choice between m=1.58 and 1.59 is ambiguous. Trial curves for each fit almost

equally well. Deviations from the region of optimal m and D quickly be- come large, and our experience has been that a unique choice of m and D is generally derived.

Figure 8 shows the matrix of table 3 plotted as a contour map.

52
VIEWING ANGLE

THETfl =50.00

PHI = 70.00

VIEWING ANGLE

THETR = 50.00

PHI =: 290.00

Figure 8. Three dimensional perspective views of the curve fits listed

in table 3. The curve deviations (Z coordinate) have been inverted

so that the best fit appears as a contour peak. The coordinate

origins are x = 1.140 ym (diameter D) and y = 1.530 (refractive

index m). The contour lines are incremented by 0.0025 ym (D) and by

0.003 (m). D and m have been expanded relative to table 3 so that

each ranges to about four percent away from the optimum value. The

viewing angle is described by THETA from the Z axis and PHI from the

X axis.

SUMMARY

These trials using one micrometer polystyrene spheres demonstrate

that the instrument provides a practical route towards aerosol

characterization. 360° scattering diagrams from individual aerosols in

a flowing stream can be obtained at about 5° resolution with noise

sufficiently low so that both particle size and refractive index can be

independently determined. Size can be established to within 0.5 percent

and index of refraction to within about 0.7 percent. The data analysis

by computer provides accurate statistical pictures of aerosols.

Furthermore, particle sizing by 360° scattering diagrams is not

dependent on advance calibrations of the instrument as is required by

many other methods.

The method offers some distinct advantages for aerosol spray characterization. Individual size measurements compile a statistical char53
acterization of an aerosol with relatively high precision, and most importantly, without prior assumptions about size distributions. An

accurate picture of real size distributions should emerge. This is particularly useful for measurements of aerosols which are rapidly aging.

Refractive index measurements can be useful for studies of aerosols in

which composition changes may occur during formation and aging. Solvent

volatilization can be followed during aerosol aging, and component partitioning during aerosol formation can be recognized.

Extension to other size aerosols have not yet been completed, but

initial efforts with 0.5 ym particles show no barriers for that size

range. Visual comparisons of calculated curves indicate that the present instrument will allow sizing of 0.3 ym particles to within about two

percent. On the other end of the scale, the apparatus can be set to

size 20 ym particles to within at least one percent, and it is probable

that the range can be extended to somewhat larger particles without

great difficulty.

REFERENCES

[1] Gucker, F. T. and Egan, J. J., J. Colloid Sci

16, 68-84 (1961).

[2] Gucker, F. T. and Rowell, R. L. , Disc. Faraday Soc. 30, 185-191

(1960).

[3] Phillips, D. T., Wyatt, P. J., and Berkman, R. M., J. Colloid

Interface Sci. 34, 159-162 (1970).

[4] Breuer, H., Gebhart, J., and Robock, K., Staub-Reinh. Luft 30, No.

10, pp. 25-31 (English Translation) (1970).

[5] Blau, H. H., McCleese, D. J., and Watson, D., Appl . Optics 9, 2522-

2528 (1970).

[6] Tuma, J. and Gucker, F. T., Proc. 2nd Int. Clean Air Congr. , p. 463

(Academic Press, New York, N.Y., 197T).

[7] Gucker, F. T., Toma, J., Lin, H.-M., Huang, C.-M., Ems, S. C, and

Marshall, T. R., J. Aerosol Sci. 4, 389 (1973).

54
DISCUSSION

MILTON KERKER: I noticed that your polystyrene distribution was negatively skewed. It was asymmetric with relatively more particles of

the smaller size than you get from a Gaussian distribution. We found

the same thing. This is opposite to what people normally accept as a natural distribution, i.e., a positively skewed logarithmic

distribution. It will be interesting to see how that holds up under

further work and whether the asymmetry has some statistical information

to impart concerning the formation of these particles.

PARMENTER: We would, of course, feel much more secure had we measured as many particles as you did. However, these results are from

about 100 particles, so I'm sure the trend is correct. I've no reason

to believe that this distribution is in error.

KERKER: The other thing is that we also tried to fit the size with

the refractive index, and I would have to go back and look at our data

again. I think they were within the same ballpark. We did not consider

this a successful way of getting the refractive index. That is, we

found also that if you perturb the refractive index by about .01, you

got a size that was different by an amount greater than you would

normally be willing to accept. This is something, of course, that would

be interesting to pursue.

PARMENTER: We are finding the opposite now. We feel that

generally the computer can make a distinct choice of combinations.

KERKER: We were strictly using visual estimates. One could use a method of steepest descent or something to come into the well and find

out where you are, but we really didn't do anything like that. We just

kind of eyeballed these

PARMENTER: We were unable to do this by eye alone. The index of

refraction is more difficult to see by eyeballing, so your experience is

consistent with ours.

KERKER: It's worth looking at more precisely. Finally, what criterion did you use as a measure of fit between experimental and computer

results? We just used the least squares type.

PARMENTER: That is exactly what we are using.

ROBERT KNOLLENBERG: It appears that once your particle size is as you say up to about 20 microns, you have several more points to fit in

your set-up. Is this not correct? It appears that you have a problem

in resolution if you are to acquire all the data counts in 20

milliseconds.

PARMENTER: We have 5° resolution in the optics, and the resolution

on the data acquisition is about 1°.

55

561-452 0 - 74 -5
KNOLLENBERG: It is true your number of data goes up as your

| particle size increases? I

PARMENTER: It would have to go up as the particle size increases.

That's clear.

j EDGAR ETZ: When you do these computer fits, I believe you said you I analyze the sum of the squares of the deviations of the experimental , values from the calculated curves. You must have an estimate of the ! uncertainty in the experimental scattering intensities because you might j come up with an equation that fits the experimental curve much better

than is actually warranted. i

PARMENTER: We fit the noise in other words?

ETZ: Yes. Consequently, you get so many different solutions--some } are real and some are not real. You come up with more than one minimal i value in this standard deviation. I

! PARMENTER: We haven't yet made a careful analysis of it. I might j say that we have worried about it and made a preliminary exploration of

it in the following way. You saw the four curves which came from the same particle. That represents very typical noise--at least in this

particle range. We let the computer look at each one of those curves individually to tell us what the particle was. The computer decided

that the particle was identical in three of those cases and in the

fourth case the computer decided that the particle was different in size

by 5/1000 of a micron--about 0.4 percent which is a neighboring size

increment. So this indicates that noise is not going to introduce a serious error in this analysis.

GREG ROSASCO: You said you used a logarithmic amplifier? Would

pulse counting techniques and then a computer conversion to logarithmic

output help you in your sensitivity?

PARMENTER: I think it probably would. We are in the process now of considering how to build a better instrument, and this is one of the

things we have to consider. The problem that we would encounter with

pulse counting techniques is pulse pile-up. I don't know if this is

going to be severe or not. We haven't yet gone through it carefully.

It would be great if it works because that would eliminate several other

problems

SINNOTT: For about $500 you can buy a phototube that has a two

microsecond pulse pair resolving time.

56
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 412

Aerosol Measurements, Proceedings of a Seminar on Aerosol Measurements

Held at NBS, Gaithersburg, MD, May 7, 1974 (Issued October 1974).

ACTIVE SCATTERING AEROSOL SPECTROMETRY

Robert G. Knollenberg

Particle Measuring Systems, Inc.

5469 Western Avenue

Boulder, CO 80301

ABSTRACT

An active scattering aerosol spectrometer is one that uses the

active open cavity of a laser as the source of particle illumination.

The interferometric aspects of the oscillating radiation illuminating

the particles produces both forward and backward scattered radiation at

all collecting angles. This, coupled with the fact that the collecting

optics solid angle can be considerably greater than one steradian, and

an extremely intense source, results in a system with an extremely high

sensitivity, fully capable of sizing particles several hundred Angstroms

diameter using solid state silicon detectors.

Key words: aerosol light scattering; aerosol sizing; aerosol spectrometer; cloud droplet measurements; interferometer; laser imaging of particles; laser light scattering by aerosols; particle size measurements.

I've decided to change the content of my presentation considerably

in light of the presence of the FDA and their interest in aerosol can products, but I will cover the active scattering spectrometry at the end

of the presentation. What I'm going to do is outline several techniques

that we use to design instruments which we build for various customers.

These range from imaging systems to single particle extinction. With

particle scattering we have two types of techniques: one we call

classical scattering and the other we call active scattering.

Let me mention briefly that the type of imaging system we use essentially involves a col lima ted light field where we project the

shadows of particles and onto a photodetector array (fig. 1). At one

time we did use fiber optics with photomultiplier tubes; today we use photodiode arrays. This type of system is one that we designed for use

in sizing droplets from aircraft and has a size range at aircraft speeds

of about 5 to 5,000 micrometers. Not within one instrument mind you, the

instrument itself typically has 32 elements so that we can resolve one part in 32. The system has been designed in two standard probes, one covering 10 to 320 micrometers and a larger probe that covers about 200

to 4500 micrometers. I'm going to pass around some photographs so that

57
OBJECT PLANE

( Point of True Focus)

IMAGE PLANE

jPABTICLE (Moves into Plone ol Oiogroml

GEOMETRIC SHADOW

Diverges Due To Diffraction )

window

<:

SPATIAL

FILTERING PLANE

occulttfd diod*s

HE-NE //

GAS LASER

Figure 1. Schematic of imaging system designed to size droplets with

airborne equipment.

you can see what the instrument looks like (fig. 2). These instruments

have been mounted and used on 17 different aircraft.

I would say that even in the case of aerosol particles if we could

use imaging techniques we would. In fact, one of the things we looked

at for the Johnson Space Center recently was a possibility of extending

Figure 2. Photograph of imaging system instrument used on aircraft.

58
an imaging system such as this down to submicrometer particles. The

question is what we do about the fundamental limitations. What we chose

to do was to accept working in a vacuum, and to use a CRT gun and focus

it down to an electron beam on the order of one micrometer dimension.

We can keep it col lima ted to a field of something like 5 mm. We then

diverge the beam much in the same way the lens refocuses the particles.

This system is one that we have proposed, but has never been built. It

would appear to have the merits of an imaging system and yet have the

resolution on the order of 0.1 micrometer.

One might say that this would not have applications in the area of,

let's say, atmospheric pressure. This really isn't true because you can operate the CRT gun in a scanning mode as you would with a scanning

electron microscope. If you have ever used one you notice you can in

fact light the thing up at subatmospheric pressure. If you AC couple

you can separate out the signal. The limitation actually comes through

the bandwidth limitation produced by the scan of the particles. Now a tenth of a micrometer at one meter per second gives you a transit time

of 0.1 microsecond, so right now the problem appears to be stymied

because of the problem of scan times in the CRT. This is a technique

that you could use to image down to around 0.1 of a micrometer if one

had the capability of slowing down the particles. Given this, an electron beam equivalent technique that could be used for larger

particles at an optical wavelength.

Below 5 micrometers it turns out that we have two types of systems

to choose from; both extinction and scattering. In extinction system

one typically focuses a laser beam down with an f/100 lens to something

around 100 micrometers in diameter. Actually, one can go smaller to 30

micrometers diameter. You can look at the cross section of the beam and

match that with the extinction cross section. If you calculate what you

have to have in signal-to-noise ratio to resolve it, it's not easy. If

you want to resolve 1 micrometer with a 100 micrometer beam diameter

your signal to noise requirement is 10,000:1. In addition to that being

almost impossible, you have to have a large dynamic range. But we have

set up extinction systems to resolve down to as low as 1 micrometer with

a 30 micrometer diameter beam which lowers this requirement to 10^ or

more. This can be used. The advantage of going to extinction is that

extinction cross sections are a little better behaved, let's say, than

what our scattering cross section is, and of course the difference is

that we are not quite as refractive index sensitive. We do not

commercially build this kind of instrument. We have built two but they

have been for special applications where the source concentrations have

been very high.

In terms of classical scattering systems, there is a technique

using a focused laser beam. The beam is dumped on a spot on a lens and

a light is collected by the detector. The scattering light from the

particle will typically pass through in this direction. With this kind

of a system we have operated at aircraft speeds with a resolution of 0.5

micrometers. This instrument is refractive index sensitive. It is

designed for water droplets for which the refractive index is known, and

59

/
we calibrate typically with glass beads and then shift a calibration

curve. If you use it in the range of collecting angles from something

like 7 to 20° you can get a monotonic calibration curve.

The last technique I wanted to describe which was proposed as the

subject of discussion here is the active scattering technique. This

technique is one we have been working on for a couple of years. It involves the use of an open cavity laser (fig. 3). Initially, one might

say, what is the advantage of going to an open cavity laser? It turns

out there are several advantages. Our initial intent was simply to

gather more light. We found that several things actually happened. One

is, of course, that if you use highly reflective mirrors on both ends of

the laser you can achieve an energy density of somewhere between 400 to

600 times that which we get from putting in a standard dump mirror in

the laser. The reason for this is that it typically takes 99 percent

coupling internally for a helium neon laser to even lase. So you are using a one percent dump mirror. That means that at any time internal

to the cavity there is probably 200 times the energy density that exists

outside. Instead of using a 1 percent dump mirror, we use a 99,990

percent reflectivity mirror on both ends.

Scattering

Signol Pholodetector Module

Curved

Mirror 0 % T —-----—

He-Ne Hybrid Later

Spike

Filter

Porticle Plane

Reference

Phofodetector Module

Note: Mirrors are reversed for large particle size ranges.

Figure 3. Schematic of active scattering technique system using an open cavity laser.

There are several things that happen here. We are trying in this case to collect light that has been scattered through angles from about

3 to 30°. It turns out that this is not a simple scattering case, but we are dealing with light that is actually traveling in both directions.

So we also collect light that is being scattered from 177 to 150°. We

have taken the classical Mie solutions over the collecting angles for

the forward and backward components and superimposed these solutions.

It turns out this is only the first approximation to the true solution

which is solving the scatter equation in a standing wave which to my

60
I

knowledge has not been done. We have set up our own program to solve

the boundary conditions or changes in boundary conditions. We are about

ready to work out new theoretical curves. We do know that it is not

correct to simply superimpose the two solutions. I don't know why, but

I suspect it's because the boundary conditions here are not quite the same. In any case in this kind of a system you typically collect 20 to

30 percent of the light scattering. Two other things I should say that

are different. One is that we are dealing with about 103 to lO'^ watts

per centimeter square. This is a very hot, very intense beam. Now you

can reverse the cavity configuration. As I have it drawn here this is a flat mirror and this is a curved mirror. Now if I reverse them and put

the flat here and the curved up here I can in turn change the dimensions

of the beam, the size being typically something on the order of 200 to

300 micrometers at the flat mirror. I would like to point out two other

things that happened. When we first started using this system, we noticed that the pulses coming out of our pulse height detector

sometimes looked like this. Of course my engineers say it's

oscillation, but what kind of oscillation is it? It turns out that what

was happening was that particles would drift through at a slight angle.

This is a built-in interferometer, so we get automatically out of the

system a Doppler modulated pulse. We thought at first that this would

be useful as a Doppler velocimeter as an intrinsic measure of velocity,

so we could have a true measurement of flow rate through the beam.

However, there is another point. The reason that the pulse is

modulated, of course, is because the forward part of the Mie scatter in

this region, here, is being heterodyned with the backscatter portion.

As you know, once you get down to the Rayleigh particle, these two are

equal in amplitude. Once the particle is very large, this one is very

strong. So the modulation depth is also a measure of size. It also

turns out that we have done some studies and the number of waves across

the pulse is a function independent of the speed. It has to do with

path length through the beam. Why is that important? Typically, unless we run a high order mode we are dealing with a Gaussian beam, we would

like to know where particles go through the beam without having to put

jets into the system. We've developed circuits that can do this on a pulse width rejection basis. However, it appears now that we can actually have this feature here as well. I would point out that simple logical steps remain.

We don't right now use the modulation depth as a size measurement,

we simply use pulse height measurements. It is a near forward scatter

system and it is also a near backward scatter system with a high collection efficiency. Our typical sensitivity in terms of converted photocurrent is one microamp of signal per micrometer squared of scattering

cross section. Anyone knows that if you run that through a one megohm

resistor you have a one volt pulse. Now what it means basically is that

we do not use, in this kind of a system, photomul tiplier tubes; we use solid state detectors. It gets around the familiar problem that we've

always had with drift associated with photocathode sensitivity, particularly when it's hit hard, and this is a lot of current for any photocathode. So we have a system in which we collect scattered light using

61
'1

\

a photodiode. At the moment there are two limitations to this system,

j If you were to use an Argon laser you could go up another factor of 100 ! or even 1,000 in intensity. Calculations show that at 500 Angstroms, a particle with unit adsorbtivity and a heat of vaporization equal to that

of water will evaporate in that beam, so you cannot arbitrarily increase

the energy density at will. Another limitation, is due to the molecular ! ^ scattering from a gas. This gives rise to a large background noise com- , ponent with which you must compare your particle scattering intensity. I So with these two limitations in mind, we usually cut off any system of j this type around 500 Angstroms. It is a highly sensitive system, and , s gives us sensitivity simply by providing a very intense beam. '

This system actually was conceived not for routine aerosol measure- I ments, but for measurements in a vacuum. Of course it has certain ad- | vantages in a vacuum. One is that we have designed optical systems that j can locate that particle both in terms of depth of field and where it i passes through the beam radially. We can design an extremely open sys- ' tem. In fact, we have designed a system with an open area of some 6 | square inches. This photograph here shows a system with two heads that | was used in tests at the Marshall Space Flight Center. That system had

a sensitivity of about 0.1 micrometer. Actually, it had two size

ranges, 0.1 to 1.5 and 1 to 29.

|

i' The other thing I would like to point out is that because this is a col lima ted system, you can, within the limitations of the optics, obtain I a depth of field of something like ±5 mm without an appreciable change

within your scatter function. In an ordinary system where you have a

| highly focused illumination, you go through quite a change in intensity,

so you have a very small path in which you allow particles to interact, i It's not true in this case. You can accept information over quite a I path length.

One other thing I wanted to point out is that we call the probes

optical array spectrometers. If you take each one of those diode

elements, connect it with its own data storage register, that is a shift \ register, you can achieve a 2-D imaging system. We've done that now and

I'll just pass around some images of particles that we've taken with a 2-D spectrometer hooked up in that fashion. Some of these you'll see

' have bright spots in the center and that's an indication of

transparency. We have designed that system primarily to determine I crystalline structure in clouds, again for aircraft applications, and [ its limitation there is on the order of about 250 knots, and about 20 ^ micrometers resolution. Again, in any imaging system (it makes no

| difference what kind) it has somewhere a bandwidth limitation that's

tied to resolution. With moving particles, you only have so much time

f to resolve a certain linewidth. 1

62
DISCUSSION

GEORGE SINNOTT: I wonder if the problem you refer to in calculating forward and backward is not simply that in this case the forward and

backward waves have a phase relation to each other, so you can't simply

add up the intensities?

KNOLLENBERG: No, we add the complex waves. We've made some progress. We've taken a slide and put a particle on it and looked at the

scattering functions and they are not the same, not what you would calculate.

SINNOTT: And is not the modulation from the particle coming simply

passing through the standing wave of the laser?

KNOLLENBERG: Yes, that's Doppler shift. You have to have some angle into the axis of the beam. This would be hard to measure with

small cosine angles.

GEOFFREY HOTHAM: You mentioned the very intense beam inside the

cavity. Can you measure aerosols with it without them vaporizing?

KNOLLENBERG: Oh yes. The reason the 500 Angstroms is a cut-off is

because your surface to mass ratio goes as 1/r so the problem becomes more severe the smaller the particle.

HOTHAM: How small can you go?

KNOLLENBERG: We would predict by this that you would get into

trouble somewhere below 500 Angstroms. This turns out to be a limitation you would place on any kind of system that was that intense.

ILAN CHABAY: Is there any chance of you getting any radiation

pressure in terms of motion on a particle beamed at that kind of energy

density?

KNOLLENBERG: That's possible.

CHABAY: This question is related in some sense. I wasn't quite

clear that I understand your statement about the heterodyning. Are you

expecting to get information on particle size from the heterodyning?

KNOLLENBERG: It would appear that for some submicrometer particles, that you can select the near forward scatter and let's say back

where the intensity peaks out at 180°. If you would ratio these two

then you would have a measurement of some function of size. The modulation depth of the beam is exactly this ratio. In other words, for small

particles this proportion rises if you have a higher modulation index.

63
CHABAY: But you don't get that actually. That arises simply from

the amplitude of the ratio of the forward and backward Mie amplitudes

rather than from the size of the Doppler shift itself, that is not from

the motion of the particle.

KNOLLENBERG: That's right. The amplitude is a function only of

size. I'm sorry the modulation index is a function only of size.

BILL YANTA: The spacing there is just due to velocity as it's

going across.

CHABAY: That was what I was trying to find out. Whether the

spacing or the depth itself is useful in getting the information.

KNOLLENBERG: The frequency, let's say, gives you velocity. If you

have particles that are very fast, you'll have a higher frequency. The

only thing I said was that the number of waves across the beam is

independent of velocity. So you can in a sense measure the beam width

for each particle.

64
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 412 tTA'°^r^"T"'r'^l

Proceedings of a Seminar on Aerosol Measurements Held at NBS, Ga ithersburg, MD, May 7, 1974 (Issued October 1974).

RAPID MEASUREMENT OF DROPLET SIZE DISTRIBUTIONS

BY OPTICAL HETERODYNE SPECTROSCOPY

I Ian Chabay

Department of Chemistry

University of Illinois

Urbana, IL 61801

ABSTRACT

A new technique which allows rapid, direct determination of

particle size distributions by measurement of the Doppler shift of laser

light scattered by falling particles will be discussed. The technique

has been successfully applied to the measurement of parameters associated with cloud droplet growth (from 1 to 10 micrometer radius) in a diffusion cloud chamber. Applications to other types of particles in

the size range 0.5-50 micrometers radius will be pointed out.

Key words: aerosol cloud chamber; aerosol light scattering; aerosol

size measurements; aerosol spectrometer; cloud droplet measurements;

Doppler measurements of particle size; laser heterodyne; laser scattering by aerosol particles; particle sizing.

I will describe first the general technique of determining particle

size distributions by optical heterodyne spectroscopy and outline the

method of measuring the spectrum. Then I'll briefly mention some re- sults obtained on cloud droplets which will serve as an illustrative

example. Finally, I'll comment on the limitations of this method and

what extensions to a broader range of problems in particle measurement

are possible.

The experiment is performed by allowing the particles to fall

through a laser beam which passes horizontally across a chamber containing the particles. The falling particles scatter light from the scattering volume which is defined and observed by apertures and a PMT.

Scattering is observed at a small forward angle in the vertical plane.

The frequency of the light scattered by the falling particles is Doppler

shifted from the frequency of the incident light. The magnitude of this

Doppler shift gives the instantaneous vertical velocity of the particles. These velocities in turn can be related to the particle sizes by means of the Stokes relation. Thus, the radii of the droplets passing

through the beam is given by the Doppler shift frequencies, without re- gard to intensity.

The calculated Mie scattering intensity for a specific scattering

angle and beam polarization provides the relative intensity value for

65
each particle size. One can obtain the particle size distribution simply by dividing the Doppler shift spectrum point by point by the relative

Mie scattering intensities. The particular power of this technique is

that the size distribution can be obtained unambiguously without any a priori assumptions of its form by using simultaneously the information

available on scattered intensity and on particle velocity at a single

angle of observation.

Actual Doppler shifts that must be measured can be as small as a few parts in 10^^. In order to achieve such high resolution, a stable

laser with a minimum of low-frequency noise should be used. In the initial experiments on this technique, an Ar^ ion laser was used. It was operated at 5145 K with about 100 mW power.

A part of the incident beam served as a local oscillator or reference signal after elastic scattering from walls and windows of the chamber into the collection apertures. The surface of the PMT acts as a mixer for the Doppler shifted radiation field and the unshifted reference field. Beat notes occur from mixing of the fields with different

frequencies and these beat notes appear as amplitude fluctuations in the

photocurrent. The photocurrent is the input to a real-time spectrum

analyzer which is followed by a digital spectrum averager. The net

result is that the information comes out as a plot of the power spectrum

of the photocurrent.

The information term in the power spectrum is given by

The first term on the right hand side is the local oscillator photocurrent. Angular brackets around the second term, i (r ), indicate a timeaveraged quantity, which in this case is the photocurrent due to Mie

scattering of a particle of radius r (and which has an associated

Doppler shift oo). The size distribution is given by the function,

As an illustration of the technique, I would like to discuss very

briefly some key results of our studies of cloud droplets [1,2]. This

work was done in the laboratory of Professor W. H. Flygare at the University of Illinois and was done in collaboration with Professor Jerry

Gollub of Haverford College.

A steady state, diffusion cloud chamber was used to produce the

droplets under desired conditions. The scattering due to droplets falling through the horizontal laser beam was measured at 7° downward in the

forward direction. An example of the spectra obtained is shown in figure 1. These three spectra were taken about 0.5 mm apart in the 4 cm

high chamber. Each spectrum is the averaged result of 128 scans which

required about 20 seconds total. The dip in the spectra at 530 Hz is

due to a minimum in the Mie scattering intensity which occurs for

66
Figure 1. Aerosol spectra collected at three

different heights in the cloud chamber.

particles of 4.24 ym radius. The size distribution changed as different

portions of the chamber were observed, but in the set of heights

represented by the spectra shown, 4.24 ym particles were present at each

height. Therefore, the dip is present throughout and, of course,

remained at the frequency characteristic of a particle of that size.

From spectra such as were just shown, the size distribution was easily found at any height in the chamber. The mean size as a function

of height was then used to determine growth curves for the droplets as a function of conditions in the chamber. These growth curves then were compared to theories of droplet growth.

67
The technique I've outlined here is directly applicable to particles with real or complex indices of refraction. The application is

completely straightforward for particles which are spherical and range

from 0.5 to 50 ym in radius. This range can be extended to smaller

particles by more complex (and less unambiguous) data analysis. The

lower limit on size is due to the fact that the velocity due to gravity

and that due to Browning motion become equal for particles of about 0.5

ym radius. Nonspherical particles can be treated by more complex

calculation of the falling velocity as a function of size parameters and

by calculation of the Mie scattering intensities for more complex

geometries

.

As I've indicated, this technique can be extended to a very broad

range of particle sizes and materials. Information on particle size,

size distribution, growth rates, and aggregation as a function of chemical and physical conditions can be readily obtained virtually in real

time by this powerful technique.

ACKNOWLEDGMENT

Supported in part by the National Oceanic and Atmospheric Administrations Grant No. 04-4-022-10 to Haverford College and National Science

Foundation Grant NSF-GH-33634 to the Materials Research Laboratory at

the University of Illinois.

REFERENCES

[1] J. P. Gollub, I. Chabay, and W. H. Flygare, Applied Optics 1_2, 2838

(1973).

[2] J. P. Gollub, I. Chabay, and W. H. Flygare, submitted to J. Chem.

Phys.

68
DISCUSSION

MADHAV RANADE: Is that 20 seconds your measurement time or total?

CHABAY: Twenty seconds is the total time to accumulate 128 averages. A single scan and display of a spectrum requires 150 milliseconds. The minimum period of observation of the photocurrent depends on the frequency range, i.e., on the bandwidth of the spectrum analyzer.

RANADE: Isn't it true that within the time of observation you will

have evaporation or growth of the water droplets in your cloud chamber?

CHABAY: Yes, but the point is that this is a steady state chamber.

As the particles fall through the chamber, you have a steady state con- dition at each height. That is, if growth occurs through the chamber at

known conditions of supersaturation as a function of height, the droplet

size distribution is constant at each height.

MILTON KERKER: You pointed to the limitation that can be caused by

the Brownian motion. What about convection? Isn't this a problem?

CHABAY: It's very definitely a problem. It's something that we struggled with in our first set of experiments. I think that it can be

eliminated, certainly for particles of over a 1/2 micrometer. I haven't

tried to work with particles smaller than that. My guess is that it can

be sufficiently minimized to allow work with even smaller particles,

where one would determine a net diffusion constant, rather than the size

distribution itself. One can measure the diffusion constant as an average quantity when Brownian motion is predominant over settling velocity.

I think convection can be controlled well enough to do that by proper

insulation of the chamber.

RONALD NELSON: How would you adapt this method to measurements of

aerosol in aerosol cans?

CHABAY: The system is essentially the same. Basically one needs

to have a very slow air flow into which the aerosol particles are introduced. A very simplified version would have a large scattering chamber

with aerosol can on top. You push the button of the can and aerosol

particles enter the chamber, fall through it, and are removed at the

bottom. Let me show you the present apparatus (fig. 2). Water saturated air flows in through an annulus at the top next to the heated

upper block, then down through the chamber to the exhaust annulus at the

bottom. The wire screens help make the air and particle flow more

uniform. If the entire chamber were kept at constant temperature and

aerosol were sprayed in at the top, the same kind of data could be collected for aerosols as has been for water droplets.

69
Resistive heater

Copper block

Incident beam

Water-saiurated paper

Fine wire screen-^

^-'C' Scattered beam

Aperture *

e

Fine wire screen

Copper block

Thermoelectric coolers

Saturated

air flow in

Incident beam

II _ "^F~1 Air flow out

Figure 2. Schematic of cloud chamber for producing aerosols and

proving scattering volume.

NELSON: Can you get absolute concentrations this way, with the

essentially unknown flow-rate?

CHABAY: The flow-rate is something that one can determine independently but the absolute concentration is not something that we were

trying to measure. We were really looking at the relative distribution

of sizes of particles rather than absolute numbers.

KERKER: Are you familiar with Joe Katz's chamber?

CHABAY : Yes

.

KERKER: Could this technique be used to get a finer handle on the

rate of nucleation?

CHABAY: That was included in our original plans, but as yet we haven't had time to pursue it. One could change the chemical nature of

the nuclei introduced with the inlet air, then observe the growth process as a function of the type of nuclei. This could complement the

work described in a recent paper by Mirabel and Katz on binary homogeneous nucleation and aerosol formation.

KERKER: We are getting a little off the topic, of course, but are these people actually counting drops?

CHABAY: Right.

KERKER: But are you actually able to follow the rate of growth?

CHABAY: That's correct.

70
KERKER: Howard Reiss of UCLA found that by using his cloud chamber

and using the theory of nucleation by Byer and Greissers he could study

kinetics of formation of droplets. What I'm thinking is that this heterodyne technique could give you a handle on growth rates.

CHABAY: I think so. We are just now publishing some growth curves from the data that we have collected, and I think that this could be ex- tended to a very wide range of systems. It gives you very clearly the

growth characteristics of the material.

KNOLLENBERG: In studying the nucleation of particles down to the

order of 0.02 micrometer, you are talking about a nucleation technique

with particles that would be limited to the order of one micrometer.

CHABAY: The point is that with this technique one cannot study directly the nucleation process for particles less than 1 micrometer,

let's say. However, what one can see is the effect of growth as a function of the much smaller nuclei.

KERKER: Reiss is counting the total number of grown particles, and

he is getting a rate of nucleation from that. He (Chabay) can go further back into the growth stage and actually follow the growth--not all

the way back, no.

CHABAY: No, I can only start at about 1/2 micrometer size particle

and work from there up to very large particles.

RANADE: They will have grown to a few micrometers in a matter of

seconds.

CHABAY: You see the point is that in this kind of chamber the time

scale for that actually to occur is irrelevant. I do not have to re- solve the time scale, I only have to resolve the height. How well I can resolve a given time difference is a question of the size of the droplets, how fast they are falling, and how well I can manipulate the beam

in terms of height-"the resolution in height of this arrangement. I should mention in regard to this, that at the moment we were using a beam of about 2 mm diameter as a matter of convenience. We just used

the output beam directly. One could focus that down to a much smaller

beam, and get a higher resolution of the distribution and also greater

detail in terms of the growth process.

CARY GRAVATT: Do you get instant readout on the particle distribution and how long does it take?

CHABAY: What you get is the spectra such as I showed you, which

are power spectra. In order to get a size distribution out of that, you

have to divide by the Mie scattering curve which is a fairly trivial

arithmetic operation. A better way to do it, which we didn't do because

we didn't have facilities to do it directly, but I hope to do it while

I'm here at the Bureau, is to do processing directly on-line with the

71

561-452 0 - 74 -6
computer. That is, one could do this whole thing on the computer and

immediately process the data so it would be a matter of fractions of a second after you have taken the number of scans desired that the size

distribution would appear.

SINNOTT: Another technique very similar to this is to set up an interference pattern in the scattering region and then watch the particle scattered light fluctuations.

CHABAY: Right. I think Dr. Yanta is going to be talking about

that.

SINNOTT: Do you have any remarks on the relationship of these two

techniques?

CHABAY: Maybe it's appropriate to wait until Dr. Yanta 's talk. I think generally the techniques are very similar. I know of no other

study which has put together both the intensity and the falling velocity

information to get the size distribution. You can certainly get the

velocity of the droplet by the laser Doppler velocimeter measurements.

The basic physics of the technique is the same. In our case, we do not

image a real fringe field in space and look at the particle moving

through the light and dark regions to determine the rate at which it

goes through the fringes. The process of heterodyning gives the same information on velocity, though.

72
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 412

Aerosol Measurements, Proceedings of a Seminar on Aerosol Measurements

Held at NBS, Gaithersburg, MD, May 7, 1974 (Issued October 1974).

MEASUREMENTS OF AEROSOL SIZE DISTRIBUTIONS WITH A

LASER DOPPLER VELOCIMETER (LDV)*

William J. Yanta

Naval Ordnance Laboratory

Silver Spring, MD 20910

ABSTRACT

A miniature wind tunnel has been built which together with the

Laser Doppler Velocimeter (LDV) has been used to determine aerosol size

distributions. In principle the LDV was used to measure the particle

lag of individual aerosol particles as they were accelerated through a small supersonic nozzle. The measured velocity lag was then used in

conjunction with numerical predictions to determine the particle size.

An optical owl was used to determine the mean of the size distributions.

The LDV measurements were in good agreement with the owl measurements.

Key words: aerosol sizing; aerosol spectrometer; aerosol sprays; Doppler measurements of particle size; droplet sizing; interferometer;

laser light scattering by aerosols; particle size measurements; particle velocity measurements.

INTRODUCTION

The Laser Doppler Velocimeter has been used for many years in

measuring flow velocities. The primary requirement for the LDV is that

micrometer size particles be present in the flow. The LDV then measures

the velocity of these particles. If one assumes the particles are moving at the same velocity as the fluid (either gas or liquid), then

the fluid velocity is inferred directly from the particle velocity.

However, in air, particles greater than one micrometer may not follow

the flow precisely, that is, the particle may lag the flow. This is

especially true in supersonic flow [3]. It is demonstrated in reference

3 that the particle lag can be predicted if the particle size is known.

Conversely, one can predict particle size if the particle lag is known.

This particle lag can be determined from velocimeter results (particle

speed) in a calibrated or known flow field from which the particle size

can be deduced. The particle lag can be induced by placing a particle

in a rapidly accelerating or decelerating flow field. A convenient

method of generating this type of flow field is with a supersonic

nozzle. Using the computational procedure described in reference 3, one

can readily predict a particle's velocity in a supersonic nozzle.

*This paper is a unification of references [1] and [2].

73
THEORY

A particle placed in an airstream will experience a force

p = i

p (u - V„)^ A Cn 0)

2 9 9 P P D

where p is the gas density; A , the cross-sectional area of the particle; ^Vp, the velocity of the^ particle; Uq, the velocity of the airstream; and Crj the drag coefficient. If we assume: one-dimensional

motion, a spherical particle and ignore potential force fields, then

the force is given by the mass times the acceleration

F - ^ 4 "A % - '/'O

or

dV„ ~ P„ Cfv r,

^P dx 4 p„ D„ ^^g >^ ^"^^ P P

Thus, one can completely predict the particle behavior in a flow field

by simply integrating the above ordinary differential equation. The

ease of integration will depend on the complexity of the drag

coefficient.

If one assumes the classical Stokes drag coefficient where

^0 = 1^ (^)

then a closed-form solution may be obtained. However, in order to take

into account the effects of Mach number and Knudsen number, the drag

coefficient given by the following equations [4] was used:

For M < 0.5

P

. 51 .IM

^0

^^ 0.256 m/c, .

0 s

74
where

24 +0.4+1.6e-0-028Re°-^2

0 s

M = V

Re, = -£-2-

for M > 0.5

Where Cq^q, Co^Pf^, A and n are given functions of M in table 1. The use of the 'above* drag equation in eq (3) necessitates a numerical

integration of eq (3). Thus, given the flow distribution through a particular nozzle configuration, one can calculate the particle's

velocity along the nozzle. This can be done for a range of particle

s i zes

.

75
Table 1. Parameters in the drag coefficient expression.

M A n

0.5 0.520 ~i on

7 .80 U . o 1 b u . f 1 u 0.6 0.551 0 . bO U . 'foU

0.7 0.586 o.d7 n 1 QO U. lo^ U . DUU

0.8 0.625 /I no 4.9^ n 1/11 U . 1 4 1 U . Of0

0.9 0.666 /I /I c 4 .40 r» Tin U. 1 1

u . oyu

1 .0 0.712 /I in 4. 1 U n non u. uyu n fi90

1 .2

r\ OAT 0.801 o cn U . Ubb u . 0/ u

1 .4 0.880 O "DO U . Ubb n son u . oyu

1 .6

A AO A 0.929 o no

c . yo n n/i Q u . / 1 u

1 .8

A AC C 0.955 o on ^.80 n n/i 7 n 71 u . / 1 0

2.0 A A^T 0.971 O CO

c . bo n n/i U. U4b n 790

O /I 2.4 1^ no 1 0. 981 O /I o r\ n/i u . / ^0

2.8 A A/T A 0.969 ^. Jb n n/i cc U. U4bb 0 79R U . / tO

3.2 0.949 o oo d . CO n n/i CO 0 7*50

4.0 0.919 ^ . 1 / U . UHO^l n 7'?o

5.0 0.91

o in n n/iKi U.U'l-O 1 U . / 00

6.0 0.910 2.05 0.0449 0.735

8.0 0.910 2.02 0.0448 0.740

10.0 0.910 2.00 0.0447 0.745

12.0 0.910 2.00 0.0447 0.745

EXPERIMENT i

i In order to determine the size distributions of aerosols an LDV : electronic system was used in conjunction with a Mach 5 wind tunnel.

| The optical arrangement used in this study is shown schematically f in figure 1. This system is commonly referred to as the dual scatter or , fringe system. The laser beam is split into two beams of equal

j intensity and then focused to a common volume (the measurement volume). 1 At the location where the two beams intersect constructive and ; destructive interference takes place thus forming about 50 to 100 I fringes (fig. 2). As particles pass through these fringes, the , particles will alternately scatter and not scatter light depending on \ 76 !
Figure 1. Schematic of laser Doppler velocimeter.

Figure 2. Optical fringe pattern.

whether the particle is in a constructive or destructive fringe. This

scattered light is then detected by a photodetector. The output of the

detector (as shown in fig. 3) is then electronically processed to

determine the Doppler frequency.

77
Figure 3. PM signal of particles crossing fringes.

The velocity V can then be calculated with the usual Doppler

equation

Thus, the velocity of an individual particle has been determined. Then

by sampling a large number of velocities (on the order of 4000 samples)

the size distribution may be determined.

The wind tunnel system is shown in figure 4. The tunnel consisted

of a gas supply (nitrogen), an aerosol source, nozzle, diffuser, dump

tank and a vacuum pump. The supply pressure was maintained at a nominal

one-atmosphere pressure. The general operation of the tunnel was as follows. The aerosol to be measured was placed in a storage tank. The

ball valve was shut off, the vacuum pump started, and the rest of the

system was evacuated to a pressure less than 1 mmHg. The valve was then

opened, drawing the gas containing the aerosol through the tunnel.

Measurements with the LDV were made 0.1 inch downstream of the nozzle

exit.

With the system shown, the flow would "breakdown" after

approximately 45 seconds or when the pressure in the dump tank was approximately 30 mmHg. This corresponds to a diffuser efficiency of

about 50 percent. After a sufficient number of data points had been

recorded, the ball valve was shut off and the system evacuated again.

Xf,D

V

= (7)

78
BALL VALVE

TEST

SECTION VACUUM

PUMP

DUMP

TANK

2 CU. FT. AEROSOL

GENERATOR

NITROGEN NOZZLE DIFFUSER

Figure 4. Schematic of miniature wind tunnel.

Oil -pumped nitrogen was used as the working gas in place of room

air for two discovered reasons. First, natural particulates were present in the room air and thus influenced the measured size

distributions. The second reason was the presence of water vapor in the

air. When the water vapor was expanded in the nozzle, heterogeneous

nucleation takes place [5], thus forming ice crystals which also

influenced the measured particle size distributions. Thus, in order to

avoid these problems, oil -pumped nitrogen was used from a pressurized

bottle. Since the mass flow through the tunnel was 0.0442

pounds/minute, no problem is encountered in maintaining this amount of

flow from a single nitrogen bottle.

In figure 5 are seen the details of the conical nozzle and

diffuser. The entrance angle of the nozzle was 45° (half angle). The

CM

-^0.737 1.50 ALL DIMENSIONS IN INCHES

(NOT TO SCALE)

Figure 5. Details of nozzle and diffuser.

79
throat was 0.052 inch diameter with a cylindrical throat length of 0.025

inch. The expansion half angle was 9°. The flow was expanded to Mach 5

at the exit of the nozzle where the area ratio, A/A*, was 25.

Following the principles outlined in reference 1, the velocities

for a range of particle sizes were computed at 0.100 inch downstream of

the nozzle exit and these are shown in figure 6. The computations were

made for nitrogen having a supply temperature and supply pressure of

530°K and 15 psia, respectively. The aerosol material was dioctyl

phthalate (OOP) having a specific gravity of 1.0.

Dp (Aim)

Figure 6. Particle lag at 0.100 inches downstream of nozzle.

80
THE EXPERIMENT

In order to generate particles having a suitable range of the mean

diameter, the aerosol generator described in reference 6 and shown in

figure 7 was used. The aerosol material is first atomized and the

resulting polydispersed aerosol is vaporized by heating in an externally

heated tube. The vapor upon condensation forms an aerosol which has a relatively narrow size distribution. The geometrical standard

deviation, Og, typically varies from 1.1 to 1.3 depending upon the mean

1 particle size. This type of generator has an advantage in that the mean

' size can be decreased by using a solution of DOP diluted with a volatile

1/8'yRIFICE

DRY COMPRESSED

AIR, 35 PSI

PRESSURE ^

GAGE

LITERS

3.5 MINUTE

FLOW

METER

ONE ORIFICE COLLISON

12

FLOW

DIVIDER MADE UP FROM

STANDARD I COPPER

I FITTING I

MADE UP F,ROM STANDARD COPPER FITTING

40 MESH SCREEN

HEATING TAPE, r'x2FT.

1192 WATTS WITH LINE VOLTAGE!

VARIABLE TRANSFORMER

- 2 CM I.D. PYREX GLASS TUBE

NO. 47 DRILL HOLE

LITERS

/ LITERS^ ^ '•^8 MINUTE

NEEDLE VALVE

DILUTING AIR

EXCESS

AEROSOL

AEROSOL OUT

Figure 7. Aerosol generator (Lui, Whitby, Yu).

81
solvent. In the present tests, methyl alcohol was used. The diameter

of the aerosol generated from the diluted solution is given by the

relation

Dp = Dp^ (C)^'' (8)

where Dpo is the diameter of the aerosol generated from 100 percent OOP

| and C is the concentration of DOP in the diluted solution. This has , been verified experimentally. For these tests four particle sizes were i chosen, Dp^, 0.75 Dp^, 0.50 Dp^ and 0.25 Dp^.

; The optics is the same as used in reference 1. An argon laser with |

1 watt of power at 5145 A was used. The optical volume was 0.015 i inch in diameter and 0.085 inch long. The data was processed by a Doppler Data Processor fabricated by Arnold Research Organization, Inc.

[7J. This type of processor is a burst-type counting system. The data 1 from the processor was recorded on a high-speed incremental tape

recorder and then processed on the Navy Ordinance Laboratory CDC 6400

computer. The recorder could record about 200 data points per second.

About 4500 data points were used to determine each distribution. In

order to obtain an independent calibration of the aerosol size, an optical "owl" described in reference [8] was used. The owl is capable

of determining only the mean diameter of an aerosol.

RESULTS

The results for the four velocity and aerosol size distributions in

histogram form are shown in figure 8. In order to obtain an accurate

mean and geometrical standard deviation, the data is replotted in figure

9. The mean diameters obtained with the LDV and owl are shown along

with the standard deviation in table 2. It is seen that there is

excellent agreement between the owl and the LDV. No diameter was determined with the owl for 0.25 D^ .

Table 2. Comparison of LDV and OWL mean diameters.

LDV OWL

MEAN MEAN

°Po 1.30 ym 1.30 ym 1.15

0.75 Dp^ 1.10 ym 1.09 ym 1.23

0.50 Dp^ 0.71 ym 0.765 ym 1.14

0.25 Dp^ 0.45 ym 1.22

82
1200 1600 2000 2400 0 0.5 10 15 2.0 Vp (FT/SEC) Dp (^m)

Figure 8. Particle velocity and particle size histograms.

83
00

to

99.9

99.8

99.5

99

9895

90

80

7060

5040

30

20

10

5

2

10.5

0.2

0.01

A Dpo (100% OOP)

O 0.75 (42.2% DGP)

0.50 Dpj (12.5% OOP)

O 0.25 DpQ (1.56% DGP)

1

0.05 0.1 0.5 1.0 DIAMETER , jam

5.0 10.

Figure 9. Aerosol size distributions

CONCLUSION

A bench-type supersonic wind tunnel was designed, built and

calibrated for use along with an LDV in determining aerosol size

distributions. Excellent results were obtained between the LDV and the

optical owl.

NOMENCLATURE

'D,C

'D,FM

Id

constant in eq (6) Hq

cross-sectional area of Re^

particle

concentration Ug

drag coefficient of V

particle ^ continuum drag X

coefficient

free molecule drag 6

coefficient

particle diameter

undiluted particle

Doppler frequency

Mach number

index of refraction

Reynolds number based on particle diameter

gas velocity

particle velocity

distance downstream of

nozzle throat

angle between focused

laser beams

wavelength of laser

radiation in a vacuum

dynamic viscosity of gas

density of gas

density of particle

84
REFERENCES

[1] Yanta, W. J., "Measurements of Aerosol Size Distributions With a Laser Doppler Velocimeter (LDV)," AIAA Paper 73-705, AIAA 6th Fluid

and Plasma Dynamics Conference, Palm Springs, California, 16-18

July 1973.

[2] Yanta, W. J., "A Miniature Wind Tunnel For Use With the Laser

Doppler Velocimeter in Determining Aerosol Size Distributions,"

Presented at International Short Course and Workshop on Laser

Velocimetry, Purdue University, 25-29 March 1974.

[3] Yanta, W. J., Gates, D. F., Brown, F. W., "The Use of a Laser

Doppler Velocimeter in Supersonic Flow," AIAA Paper 71-287, AIAA

6th Aerodynamic Testing Conference, Albuquerque, New Mexico, 10-12

March 1971.

[4] Cuddihy, W. F., Beckwith, I.E., Schroeder, L. C, "A Solution to

the Problem of Communications Blackout of Hypersonic Re-entry

Vehicles," Presented at the Anti -Missile Research Advisory Council

Meeting, Annapolis, Maryland, 22-24 October 1965.

[5] Stein, G. D., "Condensation of Ice Clusters by Homogeneous

Nucleation from the Vapor Phase," Ph.D. Dissertation, Yale

University, 1967.

[6] Liu, Y. H., Whitby, K. T., Yu, H. H. S., "A Condensation Aerosol

Generator for Producing Monodispersed Aerosols in the Size Range,

0.036 ym to 1.3 ym," Journal De Recherches Atmospheriques, pp. 397-

406, 1966.

[7] Lennert, A. E., Smith, F. H., Kalb, H. T., "Application of Dual

Scatter, Laser, Doppler Velocimeters for Wind Tunnel Measurements,"

International Congress on Instrumentation in Aerospace Simulation

Facilities, IEEE-71-C-33 AES157, June 1971.

[8] Liu, Y. H., Marple, V. A., Yazdani , H., "Comparative Size

Measurements of Monodisperse Liquid Aerosols by Electrical and

Optical Methods," Environmental Science and Technology, Volume 3,

Number 4, April 1969, pp. 381-386.

DISCUSSION

I LAN CHABAY: By way of comparison to the laser doppler and the

heterodyne techniques, the index of refraction does matter in the case that I'm talking about simply because you have to do the Mie scattering

calculations for the distribution, but in your case you do not.

YANTA: It enters in indirectly because I could bias some particles

if the index was such that they would not scatter much.

85
CHABAY: But what I mean is that what you measure as the particles

pass through is essentially the number of pulses that give you a signal

rather than the intensity so you are not dependent on the Mie scatter.

YANTA: That's right. Once you reach a certain threshold level to

make your electronics work, it doesn't make any difference if it's a half micrometer or a five micrometer particle as long as that signal is

above a certain threshold for your electronics.

GEORGE SINNOTT: That's the difference between doing single

particle work and scattering pattern analysis basically?

MILTON KERKER: He is using in place of a Mie calculation, a drag

calculation.

YANTA: That's right. It's the drag equation, that's how I have

determined the size of the particles. In other words, I've got to solve

the equation of motion for the lag.

SINNOTT: I understand that, but take the particle in the

interference region. Let's say you have 5 particles going through there

at the same time at different velocities. How does your electronics

handle that?

YANTA: There are several different ways. One is that when any

particle enters--basical ly you take eight cycles of the data information

and compute the frequency of the velocity. Now what happens if a particle either enters or leaves during that processing time, you get a phase shift in the signals and the electronics compares the first four

cycles against the second four cycles. If those two times do not agree

within a specific error usually like one and a half percent in the

Doppler period, either we reject that data point or take the last one.

CHABAY: In our case the density is so high that there is a coincidence of particles where we get multiple scattering. Only if you

get multiple scattering does it make a difference. If there are 4

particles falling at different rates, you see essentially 4 different

Doppler shift frequencies and that's independent of . . .

SINNOTT: You are taking a power structure. You see the

difference. If you tried to do a Fourier transform of both particles at

the same time going through there, by looking at both frequencies, then

you have a problem of having a comparative intensity.

MADHAV RANADE: If you have particle sizes in the range of about 10

to 20 micrometers, the drag is very high. Does that effect the droplet

chemistry and humidity?

YANTA: That's a good point. We are looking at that problem now.

In the use of dioctyl -phthalate the problem did not seem to occur there.

Now when we go into more volatile liquids with less viscosity and higher

density, it could be a problem.

86
KERKER: Is your illuminated region small enough so that you have

to worry about velocity profiles in the gas stream itself?

YANTA: That's why I go off-axis with my collecting optics.

Generally, my cross-beam volume is about an inch long, and my nozzle is

only a quarter of an inch wide. By going off-axis I only look at a very

small portion of that focal volume and the scan volume is 0.080 inch.

There is no velocity profile, it is essentially flat across the exit.

RONALD NELSON: Do you have any trouble with condensation of

' particles within this extension, do you pick up water?

YANTA: A good point. When I ran this last week, we were just

using room air and it was no problem. Last week a cold front went

through the Washington area and the air was quite dry, but in the dead

of summer when the water vapor is quite high it is a nuisance. I had to use a nitrogen source to make sure my gas was completely dried out.

NELSON: What is your sample flow-rate through this device? The

I cc's of air or whatever?

YANTA: It's about 13 1/2 cubic inches per second, but I'm only

looking at a portion of that also. I'm not sampling the entire exit.

There are about 4,000 samples in each one of those histograms. It takes

about 20 to 30 seconds to obtain that. It is relatively slow because

the slow part of the whole system was my tape recorder where I recorded

my velocity data points. That's the slow part. The electronics is

capable of around 5,000 per second, and we are thinking about buying a new set of electronics which is capable of a million data points per

second.

ROBERT KNOLLENBERG: It's nice to be able to eliminate the

necessity for something like that because one would like to operate at a reasonably high count rate, with lots of applications.

YANTA: I see no problem in going to something like at least

500,000 data points per second, and doing it in real time. Right now it

goes through the NOL computer. I put the data on tape and process it on the computer, and I have some ideas of how to do it on-line.

KNOLLENBERG: What is the dependence upon the density of the

particle?

YANTA: It is a direct linear dependence. The equations for the

drag coefficients are very general in these calculations. What I've

done -is to take a very elaborate set of drag coefficients, if you use

the Stokes Law you can solve these equations in a closed form. However,

inside the nozzle the static pressure is about 2 mmHg and the Knudsen

number is about 5, so we are starting to get into the problem of free

molecular flow. The drag goes down considerably.

87

561-452 O - 74 - 7
JOHN WEINNEGER: How did you sample the room air freshener? Did

you exit the spray into your holding tank? How much time after you put

the spray into the holding tank if that's the way you did it, did you

sample . . .

YANTA: The kind that we were using was about three or four inches

in diameter, about 2 feet long with a volume such that we could

effectively sample the entire tank in ten seconds. We pulsed the

aerosol spray into the tank, so we made sure that there were some particles in the tank at all times. The maximum would be ten seconds in

the inside of the tank.

WEINNEGER: How much time did it take you to get the histograms?

YANTA: About 30 seconds.

WEINNEGER: Total instrument time?

YANTA: Right. Total instrument time.

CHABAY: The LDV technique is similar to the optical heterodyne

method. An important distinction is that the heterodyne method uses intensity and Doppler shift information (and requires knowledge of the

index of refraction) to obtain a size distribution when any number of

particles are present in the scattering volume. The only limitation is

that multiple scattering be negligible. The LDV arrangement is

independent of the index of refraction, but cannot deconvolute the

intensity information if more than one particle is scattering at one

time.

88
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 412

Aerosol Measurements, Proceedings of a Seminar on Aerosol Measurement;

Held at NBS, Gaithersburg, MD, May 7, 1974 (Issued October 1974).

AN OPTICAL TRANSFORM TECHNIQUE FOR MEASURING THE

SIZE DISTRIBUTION OF PARTICLES IN FLUIDS*

Albert McSweeney

Experiment Station

George Institute of Technology

Atlanta, GA 30332

ABSTRACT

Real-time inversion of the data obtained from the total diffraction

pattern due to particles in a relatively large volume yields a measure

of the size distribution of the particles. A significant advantage of

this technique is that the sample volume may be large, removing the re- quirement for piping the sample through small tubes. This technique may

be applied to particles in gases, in liquids, or on microscope slides.

Key words: aerosol light scattering; aerosol sizing; aerosol spectrometer; laser light scattering by aerosols; optical transform; particle

size measurements.

I. INTRODUCTION

The word "fluids" was used in the title of this paper to indicate

that the technique to be described is applicable to the measurement of

particle size distributions either in air or in liquids. Our first work

in this area was centered on measuring the concentration of sediment

suspended in water [1], and our present effort is directed toward the

measurement of large particles in the atmosphere. The size range of

interest is from 5 to 100 micrometers diameter.

II. TRANSFORM TECHNIQUE

The optical transform technique is based on the use of a coherent

optical processing system [2] to measure the light intensity in the

diffraction pattern due to a random group of particles. The light intensity as a function of radius is then inverted to yield a measure of

the size distribution of the particles [3].

The optical system (fig. 1) consists of a laser to serve as an intense source of coherent radiation. Lens L-| and Aperture Ai constitute

a pinhole spatial filter for removing undesired modes from the laser

radiation pattern. Lens Iz recollimates the filtered radiation, forming

89
LASER 01

T

OPTICAL FOURIER TRANSFORM SYSTEM.

Figure 1. Schematic of the optical transform technique system.

a beam whose diameter may be several times larger than that of the

original. Aperture A2 limits the beam diameter and transmits the uniformly intense center section. The Fourier transform lens, L3, has been

corrected to yield diffraction patterns that are independent of the

position of objects in the input region. Each particle in a group

located in the laser beam will yield a diffraction pattern centered on the optic axis. The resultant intensity pattern is the linear superposition of the intensity patterns from all the particles provided the

number of particles is large and their positions are random. Photographs of the optical system are shown in figure 2.

For particles of circular cross section, the resultant diffraction

pattern will be circularly symmetric. If the particles are of

noncircular cross section, the diffraction pattern will still display

circular symmetry provided there is a large number of particles and they

are randomly oriented.

In either case the optimum detector for measuring the radial distribution of the light intensity is a detector that shows circular

symmetry. Two types of circularly symmetric detector arrays have been

used by the author. One is a commercially available photodiode array

(fig. 3) and the other a meticulously arranged fiber-optic array [4]

(fig. 4) which can be used with either photodiodes or photomul tiplier

tubes. A third arrangement utilizing a rotating mask with slits and a photomul tiplier tube has been described in Applied Optics [5].

After the radial distribution of light intensity has been measured,

it must be inverted to yield a measure of the particle size distribution. The inversion process has been reduced to a matrix multiplication in which a column matrix representing a set of light intensity

data is multiplied by a rectangular inversion matrix to yield a column

representing the particle size distribution. The inversion matrix may

be derived from sets of light intensity measurements made on known

monodisperse samples covering the size range of interest [3]. A

comparison of the eigenvalues associated with an inversion matrix and

90
Figure 2. Two views of the optical transform technique setup

in the laboratory.

91
Figure 3. A commercially available photodiode array is shown in

the top photograph; the circular symmetry of the array is shown

in the enlargement (bottom).

92
Figure 4. Diagram of a fiber-optic array circularly symmetric

detector used with either photodiodes or photomul tipl ier tubes.

the noise in the measured data yields an estimate of the number of

independent pieces of information in the inversion process LoJ.

After an inversion matrix has been derived, it may be applied to

measured data essentially in real-time with the electronic arrangement

indicated in figure 5. The output of each detector is preamplified,

sampled, and held while a multiplexer feeds each signal in sequence to

an analog-to-digital converter. The digitized data are then read into a computer where they are multiplied by the inversion matrix stored in the

computer.

93
S & H

MULTIPLEXER

A/D I

CONVERTER

COMPUTER

CONTROL CIRCUITRY

Figure 5. Schematic of electronic arrangement used for applying

the derived inversion matrix to measured data.

III. RESEARCH EFFORT

Our present work, sponsored by EPA, is directed toward demonstrating that the technique described here can be successfully applied to the

measurement of large particles in the atmosphere.

The technique is being tested with both theoretical calculations

and experimental data. On the theoretical side, we are trying to determine the best size-resolution to be expected over a given range of

particle sizes. Experimentally, we have measured the diffraction patterns produced by pinhole appertures in order to compare the theoretical

values with experimental measurements under ideal conditions.

In the next phase of the program, an inversion matrix will be derived from measured data. In order to accomplish this, a set of mono- disperse samples of known properties is required. The first set of

samples will consist of photographic transparencies with opaque circular

spots representing spherical particles. Samples with "particle" sizes

covering the range from 17 to 104 micrometers diameter are being

produced. This size range is compatible with the Recognition Systems

photodiode array dimensions, the focal length of the highly corrected

Fourier transform lens, and the wavelength of the laser radiation.

After the technique has been successfully demonstrated in this size

range, a shorter focal length lens will be substituted for the highly

corrected lens to change the particle size interval to 5 to 25 micrometers diameter. At that point the technique will be tested with

samples of particles in liquids and in air.

94
IV. REFERENCES

[1] McSweeney, A., Turbidity Instrumentation: A Fiber-Optic System for

Measuring Sediment Concentration by Optical Fourier Transformation,

Final Report, OWRR Project No. A-027-GA, July 1971.

[2] Goodman, J. W., Introduction to Fourier Optics , McGraw-Hill, New

York (1968). [3] Anderson, W. L. and Beissner, R. E., Counting and

Classifying Small Objects by Far-Field Light Scattering, Applied

Optics 10, 1503, July 1971.

[4] McSweeney, A. and Rivers, W. , Optical -Fiber Array for Measuring

Radial Distributions of Light Intensity for Particle Size Analysis,

Applied Optics 1J_. 2101, Sept. 1972.

[5] Cornillault, J., Particle Size Analyzer, Applied Optics 1J_, 2669

Feb. 1972.

[6] Twomey, S. and Howell, H. B., Some Aspects of the Optical Estimation of Microstructure in Fog and Cloud, Applied Optics £, 2125,

Dec. 1967.

*This project has been financed in part with federal funds from the Environmental Protection Agency under Grant Number R802214-01. The con- tents do not necessarily reflect the views and policies of the Environmental Protection Agency, nor does mention of trade names or commercial

products constitute endorsement or recommendation for use.

DISCUSSION

MILTON KERKER: This presupposes that the intensities are going to

be monotonic in size, which is okay except where you have a one-to-one

relationship when you convert your intensity to size.

MCSWEENEY: No. This is analogous to Fourier transform

spectroscopy where— I 'm not sure how to explain the answer to your

question. Perhaps I don't understand the question.

KERKER: You're measuring the distribution of light intensity.

McSWEENEY: I think the answer is that we are not limiting

ourselves to the central part of the diffraction pattern. The system

should work when we are measuring particles of the sizes that give more

than one lobe in the diffraction pattern. So what's happening is that

the shape of the diffraction pattern is changing across the detector

area as a function of size. So it's not really a monotonic function of

size, and what it's analogous to is resolving the total diffraction

pattern into the number of patterns similar to this as is done in

Fourier transform spectroscopy. The question of resolution comes in

95
even though we are making 32 light intensity measurements with this

commercially available detector array. Of course you don't end up with

32 size intervals, and one of the things we are trying right now to use

is the eigenvector analysis to determine the number of subintervals that we can resolve, and the upper and lower limits of each subinterval in a j given size range. Now we've built a fiber optic array that consists of ' 168 annular rings concentric with the center fiber. Now this would give I us about 5 times the spatial resolution in the light intensity

measurements that we could make and, analogous to Fourier transform I spectroscopy, by sampling at a much finer interval in- the diffraction

pattern we could measure size distributions that consisted of larger

particles. As we improve the rate of sampling in this plane we increase

the size range we can measure. By increasing the diameter of the

bundles, we could go down lower in size.

I LAN CHABAY: How is this detector array related to the width

distribution that you are able to deconvolute from your data? What

limits the width of the polydispersion in the sample?

McSWEENEY: It's both the widths of the annular rings in the

detector and the diameter of the detector array, as well as, of course,

the focal length of the transform lens and the wavelength of light used.

CHABAY: Within the particle size range that you can measure, let's

say you are looking at a polydisperse system, can you resolve the fine

changes in distribution?

McSWEENEY: I hope so. This is, of course, one of the tests we

will have to use on the systems. Right now we are getting to the point

of taking the data to derive an inversion matrix from experimental data,

so we are still working partially on the eigenvector analysis to

determine the maximum resolution that we can achieve with the equipment

that is available now.

CHABAY (added comment): This Fourier Transform (FT) technique

offers a faster method of doing classical light scattering measurements

due to its multiplex advantage. The information obtained is intensity

as a function of spatial position rather than intensity as a function of

angle (for each angle studied one at a time) as in classical scattering.

Thus, FT data contain the same amount of information as do classical

methods and the same problems of data inversion are involved.

96
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 412

j' Aerosol Measurements, Proceedings of a Seminar on Aerosol Measurements

j Held at NBS, Gaithersburg, MD, May 7, 1974 (Issued October 1974),

SIZING AEROSOLS IN REAL TIME

BY PULSING UV LASER MACHINE

Geoffrey A. Hotham

Laser Holography Inc.

Santa Barbara CA 93108

ABSTRACT

A new laser machine will be described which utilizes a pulsing UV

laser to produce "instantaneous" images of aerosol droplets on a TV screen in real time. With this instrument it is possible to measure

from 0.3 to 1000 micrometers.

Key words: aerosol impact studies; aerosol sizing aerosol spectrometer;

aerosol sprays; laser holography; laser imaging of particles.

LASER MACHINE

What you will see will be a video tape that depicts a new laser

machine that presents spray-drop pictures in real time. Figure 1 shows

the pulsing laser machine on the left interfaced with automatic scanner

on right. Instantaneous pictures of spray drops are presented while

they are happening [1,2].

The spray is simply left on and a high speed laser is continuously

pulsed through the spray producing instantaneous spray-drop pictures on the TV screen.

The new system uses a continuously pulsing laser, requires no film

or wet developing or any reconstruction. This laser machine produces

spray-drop pictures in real time. They are recorded in an instant

replay recorder. The pictures are replayed and sized by a reticule on the face of the monitor.

This machine is useful for sizing the aerosols of hairsprays, oven cleaners, deodorants, air fresheners, insecticides and all household

goods. The quantity of respirable particles in aerosol sprays may be

readily determined. Inhalation toxicity may be predicted. Such things

as size distribution, agglomeration, coalescence, evaporation, air

pollutants may be obtained and studied in real time.

The size range capability of this machine is from .3 micrometers to

10,000 micrometers. Size distribution is plotted in steps as follows:

97
MONITOR

LASER

I

Figure 1. Pulsing laser machine and automatic scanner.

.3 micrometers to 1 micrometer; 1 micrometer to 5 micrometers; 5 micrometers to 10 micrometers; 10 micrometers to 20 micrometers; and so on.

The velocity of measurable particles is stationary to 1,000 feet

(300 meters) per second. The types of particles measurable include all

aerosols, liquid droplets, powders, particulates, tubular structures,

crystals, touching particles, doughnut constructions, hollow spheres and

balloons

LASER SET-UP

This is the laser set-up (fig. 2). I will identify the various

parts; the laser, the spray, the lens, the camera, and the monitor.

Next, I will turn the spray on. The can may be moved in and out to

study drop sizes at different positions from the nozzle (figs. 3 and 4).

We also offer an automatic scanner that interfaces with this laser

machine providing size distribution while the spray is going on; i.e.,

in real time. Counts may be made at rates of 5,000 particles per second

to as high as 10,000 particle counts per second (fig. 1).

Size distribution is shown in real time by way of nixie tubes displaying accumulated totals in each size range.

VIDEO PRESENTATION

Most of my presentation today will be made via video tape. You

will see video pictures of aerosol particles in flight. Also, I will

98

CAMERA

SPRAY
I

Figure 2. Schematic of laser setup.

MONITOR

LASER

Pulsing UV Laser -

variable from 1 shot

per minute to 1000

pulses per second. Typical repetition

rate is 60 pps.

Aerosol spray particles

INSTANT REPLAY RECORDER

SPRAY LENS

I

Quartz

optical system

Consisting of UV sensitive vidicon

and memory storage

tube

Shows magnified

images of aerosols while the spray is going on

Stores images of aerosols. Can be replayed in slow motion to show stop action pictures of aerosols in flight.

Figure 3. Pulsing laser machine showing laser, spray, camera, monitor

and instant replay recorder. Instantaneous particle pictures are generated on the monitor while the spray is going on. These pictures are stored in an instant replay recorder and may be played

back in slow motion. Size of the particles is determined by eye

by the reticle on the face of the monitor. Size range measurable

0.3 to 10,000 micrometer.

99
MONITOR

LASER SPRAY

AUTOMATIC

SCANNER

CAMERA \

• • • • ••

•. •• •

o o

o o

raBEDDD

PARTICLE COUNT

INSTANT REPLAY RECORDER

ACCUMULATOR

DISPLAY 6789 4308 602 137 27 5 1 3 2

0.3-1 1-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100

SIZE RANGE - MICROMETER

Figure 4. Pulsing laser machine and automatic scanner snowing enlarged

view of accumulator display of particle counts. Size distribution

in real time scans images of particles and automatically sizes them

in real time while the spray is going on. Counts of particles in

various size ranges are registered in the accumulator display. Capable of sizing particles and counting them at rates of 10,000 particles per second. Accumulated counts are shown in the accumulator

display while the spray is going on.

show excerpts from various contracts that I have performed with this

equipment.

To start with I will show some large drops recorded by this machine. These pictures will show their size, shape, and outline their

breakup (fig. 5).

WATER SPRAY DROPLETS

What you see now are some water drops being sprayed from a pneumatic nozzle. We are looking at the spray pattern 4 inches (-10 cm)

downstream or 4 inches (-10 cm) from the nozzle. Those particles are quite large--l inch (-2.5 cm) on the monitor represents 200 micrometers.

We're looking at the general spray pattern at 4 inches downstream.

The laser is fired through the spray, the camera is generating those

spray-drop pictures, and they are being recorded in the instant replay

recorder.

100
Figure 5. Water drops sprayed from a pneumatic nozzle

„

These droplets are fairly round; most of them are round, some of

them are odd-shapped—el 1 iptical and dumbbell shaped (fig. 6).

We'll move the camera nearer to the nozzle and see the actual

breakup of the water jet into discrete particles.

Figure 6. Water droplets are mostly round in shape--

some are elliptical and dumbbell shaped.

101
BREAKUP OF WATER JET

The camera and laser have been moved to look at that point where

the liquid jet breaks up into discrete particles; it is at the rupture

point of the liquid (fig. 7).

Figure 7. Rupture point of the liquid—the liquid

jet breaks up into discrete particles.

We are looking at the filaments and ligaments of liquid just prior

to breakup.

The laser is being fired at 10 pulses per second, and each one of

those pulses is capturing a picture of the breakup of the particle.

The width of the laser pulse is 10 nanoseconds so it, for all intents and purposes, makes a stop-action picture of the particles at that

instant (fig. 8).

You can see that the jet is broken up into discrete particles at

this point. This is about 1 inch (-2.5 cm) downstream. Those are the

discrete particles now in the stream.

We are moved downstream and we are looking for the actual rupture

point and this is it, and this is where the jet is breaking up into

discrete particles (fig. 9). (I'll play that in slow motion.) That is

frame by frame and that is the liquid right at its final point before it

breaks up into discrete particles.

102
Figure 8. Stop action picture of the particles at the

instant the laser pulse width is 10 nanoseconds.

i

Figure 9. Actual rupture point of the liquid.

103-

561-452 0 - 74 -8
Those are the filaments of the liquid just prior to breakup (fig.

10). The jet is moving from left to right and a few milliseconds later

each one of those filaments will be broken into discrete particles.

The jet is going from left to right and the laser is fired normal

through the axis of the spray and we're looking at the end of the liquid

jet (fig. 11). This is the actual mechanics of the breakup of a liquid

jet into discrete particles.

Figure 10. Filaments of the liquid just prior to breakup.

Figure 11. Looking at the end of the liquid jet.

104

*
CALIBRATION

Calibration is performed with a microscope reticule placed in the

plane of the spray prior to spraying. It is recorded in the instant

replay recorder. It is then removed from the optical bench. Its image

may be superimposed over the spray particles during spraying.

TIME GENERATOR

A time generator is added to the system to provide a record of the

date of the test for legal reasons: month, day and year (fig. 12).

It also provides time: hours, minutes and seconds (and tenths of

seconds if necessary) for studying such things as evaporation, decay,

agglomeration, etc.

I0yu.m

Figure 12. Time generator added to system.

ANTIPERSPIRANT. IMPACTION.

This next frame (fig. 13) will show an antiperspirant sprayed onto

a 5-micrometer target strand. That strand is 5 micrometers in diameter

and you'll see particles impacting onto that strand. That is the first

impaction. The spray was directed at that strand from 10 inches (-25

cm) away. There'll be another spraying in a few seconds. This is the

replay recorder replaying those scenes that we just saw. In a few

seconds you'll see the first hitting of the particles and you'll see particles flying around those impaction strands. In a few seconds

you'll see the first hitting of the antiperspirant. Target strand is 5

micrometers

105
Figure 13. Antiperspirant sprayed onto a 5 micrometer target strand.

Those are the particles and that is the first impaction. On playback we see the first hitting of the particles. Rolling the recorder

reel back we are looking at the first arrival of the antiperspirant.

The next scenes are all taken from the instant replay recorder. The

laser is being fired through that scene and as the particles impact you

will see them coming on the screen (fig. 14). That's an antiperspirant.

Figure 14. Frame showing particles coming into screen

as they impact with laser pulse.

106
ANTIPERSPIRANT. SIZE.

The strand is 5 micrometers and if you remember from the calibration, the smallest division in the calibration reticule was 10 micrometers, so that gives you an idea of the size of the flying particles.

The magnification is 2400X.

That is a second spraying and you can see the buildup and agglomeration onto other particles (fig. 15). That shows the buildup and

impaction of an antiperspirant

.

Figure 15. Buildup and agglomeration onto other

particles (second spraying).

DEODORANT. AT THE NOZZLE

The next spraying is a deodorant. The photo shows the spray at the

nozzle (fig. 16).

107
Figure 16. Frame showing spray at the nozzle.

j

DEODORANT. FALL OUT i

I These are some large drops that fall out along the bottom of the

j spray (fig. 17). They, of course, reduce the efficacy of the formulation.

Figure 17. Large drops fall out along the bottom

of the spray.

108
This scene is taken 8 inches (-20 cm) downstream from the nozzle

(fig. 18). It shows the particles in free flight. It can be seen from

the reticule that all the particles are quite small—all below 10

micrometers in size.

Figure 18. Frame taken 20 cm downstream from nozzle

shows particles in free flight.

DEODORANT. REAL TIME SIZE DISTRIBUTION .

This information may be sent directly to the Automatic Scanner, and

size distribution is obtained immediately. Counts of particle size are made at rates of 5,000 counts per second. The accumulated totals are shown in real time on the Accumulator Display. Hence, size distribution

is obtained in real time while the spray is going on.

Accumulated total counts of particles are displayed in size ranges

as follows: 0.3-1 micrometer, 1-5, 5-10, 10-20, 20-30, 30-40, up to 100

micrometers or higher. See the drawing outlining the Automatic Scanner

and accumulated display of particle counts for real time size

distribution (fig. 3) [3,4].

AUTOMATIC SCANNER

This scanner automatically sizes only the in-focus particles. Outof-focus particles appear grey and have diffraction patterns around

them. The scanning circuitry senses diffraction rings and does not

count them. Similarly the sensitivity of the scanner is set so that it

misses the grey out-of-focus particles. It only sizes and counts the

solid black dots or images of particles in focus.

109
Since the scanner is looking at typically 60 frames per second

where there are usually about 100 particles per frame in focus, the

statistical sampling is indeed reliable and representative.

The question is often asked, "What is the depth of focus of this

system?" Careful measurements have been made on this. The depth of

focus for a 100 micrometer particle has been determined to be ±2 mm.

IN SITU

This laser machine records aerosol spray patterns in situ--in place

while they are happening. It records all the particles in the sceneall of the small particles in the 0.3 to 10 micrometer range and all of

the large particles, 10-20-30 micrometer size range, up to as high as 10,000 micrometers.

Most light scattering particle sizing machines use isokinetic sampling. This is, they suck in or draw in by vacuum flow only the small

fine particles that are left suspended in the air. These machines measure only the airborne particles drawn into them by vacuum. They usually

do not "catch" droplets above 30 or 40 micrometers. Certainly the 80

and 90 micrometer sized particles of a hair spray which go as high as 130 or 180 micrometer jam up the small entrance orifice of these

isokinetic-sampling light-scattering instruments that sample particles

one at a time. There is also the problem in these airborne sampling machines of impaction at the orifice as mentioned above, together with the

probability of coalescence and agglomeration of particles such as an oven cleaner as they are drawn together down the entrance orifice.

These orifices have to be small to try to sample particles one at a time

and they are usually on the order of a few millimeter in inside

diameter.

This laser machine photographs the flying particles in flight— situ—and records all the particles in the spray, large and small.

Thus, it does not rely on sampling particles one at a time— it

records all particles, not just the fine airborne particles; it does not

deposit, impinge, impact or contact any of the particles during a measurement.

EVAPORATION

The next sequence shows a large drop of pure alcohol evaporating

(fig. 19). It acts in a similar way to a hairspray; except that in a hairspray after the alcohol has evaporated, there is a residue left on the hair. This residue acts as a binder and supports the human hair.

no
a. large drop of pure alcohol b. four seconds later

e. one second later, complete evaporation

Figure 19. A large drop of pure alcohol evaporating.

Ill
OVEN CLEANER. HOLLOW SPHERES COLLAPSING.

We're going to another lens system and I want to show the calibration— 1 division equals 100 micrometers. We'll be spraying with a household oven cleaner and you'll see a particle impacting at 3 o^clock

(fig. 20a).

There it is there--it's expanding slowly, absorbing moisture from

the atmosphere and it'll collapse in a few seconds due to this water

absorption and the residual effect of the expanding freon inside the

sphere (fig. 20b).

That's the collapsed particle. It's a liquid droplet. It started

as a hollow balloon (fig. 20c)-

a. particle at 3 o'clock b. expanding slowly

c, collapsed particle

Figure 20. Series of frames showing particle of household oven cleaner spray impacting at 3 o'clock.

112
OVEN CLEANERS.

Most oven cleaners sh^ ^ jp as large particles— hollow spheres or balloons 100 to 200 micromete. ..i size and larger (fig. 21). However,

together with these large balloons there is always an abundance of small

fine particles in the respirable range as shown impacted on the 2

micrometer strand below the large balloon. They are all very fine—all

in the 1-2-3-4-5 micrometer size range. Using the 2 micrometer strand

as a reference you can see that they are all certainly under 10

micrometer. They are in the respirable range [5-8].

Figure 21. Large particles (oven cleaner).

RESPIRABLE PARTICLES

Considerable information is available on micro-organisms and their

relation to respiratory infections. The amount of particulate material

and bacteria retained in the respiratory system is largely dependent

upon the size of the inhaled particles. Particles larger than 10

micrometer are efficiently trapped in the upper respiratory tract and

are removed by ciliary action or some other means. Particles in the 1 to 10 micrometer range which reach the alveoli are usually retained [5-

8].

This has been substantiated by a variety of methods and by examination with the Electron Microscope of the lung of humans after death [9-

11].

113
There has been a great deal of work published on this subject. A

brief description of some of these diseases follows with references.

• Asbestosis is given as an illustration [12-14].

• The results of breathing coal dust have long been known [15].

• Many aerosols are toxic [11].

• Silicosis is well known [14-16].

• Beryllium has become a problem [17].

• Farmers are now known to suffer from respiratory ailments—farmer 's lung [18].

• Those working with sugarcone have been known to develop bagassosis

or bagasse worker's lung [19].

• Inhaled cadmium has toxic effects [20-22].

• There are many synergistic effects of inhaled particles [23].

• Various chemicals, if inhaled, have disastrous effects [24,25].

• Cotton workers and those working in cotton textile mills [26-28].

• Dust and aerosol inhalation [29,30].

• Certainly cigarette smoking is damaging to the respiratory tract

and has been so stated by The Surgeon General [31,32].

• There is an interesting relation between human shock from stress

and inhalation [33].

• Finally, the toxic effects of aerosol propel 1 ants on the heart is

carried in the Archives of Internal Medicine [34].

A further list is included of some other lung diseases that are new

to the author. They were discussed in some detail at the Postgraduate

Course on Lung Disease, American College of Chest Physicians, Chicago,

Illinois, which the author attended, March 19-21, 1973.

• Inhalation of particles of bagasse, the moldy residue of sugarcane

found at sugarcane processing plants.

• Farmer's lung and thresher's lung caused from the inhalation of

particles formed in moldy hay.

• Mushroom grower's disease.

• Lung damage amongst paprika workers, coffee bean dust, particles

from roof thatch, cheese washing processing plants, flour mills,

redwood sawdust, cork dust and maple bark.

• Pidgeon breeder's disease caused by breathing airborne particles of

dried pidgeon excrement.

114

i
SUSPENDED PARTICLES

This machine is also* useful for studying the trajectory of sus- pended particles floating in the air. You'll see particles in this se- quence falling and reversing their direction (fig. 22). We just saw one

at 1 o'clock—there's another at 12. This sequence is shown on the

monotor from the instant replay video recorder. You can see the particles falling and then reversing and going upward again. This shows the

trajectory of suspended particles in the air.

Reduction in size due to evaporation may be observed during flight.

Also, the actual path of flight is shown and recorded. Hence the actual

impaction of insecticide particles on insects may be studied frame by

frame

Figure 22. Trajectory of suspended particles in the air.

AGGLOMERATION

This next sequence will show an agglomeration (fig. 23). If you

look at the particles on the right side of the screen at approximately 3

o'clock, you'll see them collide with flying particles agglomerating and

coming together to form one large blob as shown in the last frame.

These frames are 17 milliseconds apart. They were rolled back in slow

motion from the Instant Replay Recorder.

This scene shows the three large particles on the right. They are

the initial particles and will be impacted as shown in the following

sequence.

115
Figure 23. Sequence showing an agglomeration of three large particles

(time between frames is 17 millisecond).

116
Two additional particles have collided with the top particle. A

second particle has appeared inbetween.

They are all solidifying into one conglomerate.

They have pulled together.

And now form one mass.

It is now in the final form. A blob.

INSECTICIDE IMPACTION ON LIVE INSECT.

The next sequence is a live insect and we're looking at the setal

hair protruding from the integument. The insect is on the right and it

is alive. The spray is coming from the left and you will see the

agglomeration of the small particles of insecticide hitting the tip of

that setal hair (fig. 24).

The setal hair is approximately 20 micrometers diameter at its base

and 2 micrometers at the very end or at the tip. Now, there are the

particles impacting at the tip of the setal hair. The uppermost setal

hair you'll see has very small fine particles impacting at the very end

of the setal hair. Those particles are impacting at the very tip and

are very small. They are 1, 2 and 3 micrometers in size. The tip of

the setal hair is 2 micrometers. You can use that as a reference and

Figure 24. Frame from sequence showing insecticide

impaction on live insect.

117
get an idea of the size of the particles impacting at the tip. These

particles only hit at the tip of the setal hair. They agglomerate at

the tip and flow down the setal hair to the skin of integument of the

insect. That is the mechanics of an insecticide spray hitting an insect.

CONCLUSION

This presentation was made via video tape and shown on a TV monitor. In most every case moving particles in flight were displayed and

their dynamic activities were shown.

All of the photographs were taken from "stills" from the Instant

Replay Recorder used in this presentation. The recorder was stopped and

the picture was held still on the TV monitor. Polaroids were taken of

the held scene. These stills are depicted in this technical report.

SUMMARY

I have shown excerpts from various contracts that I have performed

with this machine. The machine presents spray drop pictures in real

time. These pictures are stored in an Instant Replay Recorder and may

be played back in slow motion frame by frame to study impaction, evaporation, agglomeration, droplet breakup, shape and size distribution.

Size distribution may be obtained in real time by adding the automatic Scanner. Very fast particle counts are realized, i.e., 10,000

counts per second.

The machine is useful for evaluating the size of all aerosols, such

as hair sprays, anti perspi rants , deodorants, air fresheners, insecticides, asbestos particles, coal cust, atmospheric pollution and the

1 i ke

REFERENCES

[1] Hotham, G. A., "Sizing Aerosols by Laser Holography," Aerosol Division Technical Seminar, Chemical Specialties Manufacturers

Association Conference, Washington, D.C., December 4, 1973.

[2] Hotham, G. A., "Particle Sizing by Laser Holography," Society of

Experimental Stress Analysis, Los Angeles, Calif., May 14, 1973.

[3] Irani, R. R., and Callis, C. F., "Particle Size: Measurement, Interpretation, and Application," John Wiley & Sons, Mar. 1963.

[4] Dal la Valle, J. M., "Micrometrics , The Technology of Fine Particles," Pitman & Sons.

[5] Lourenco, R. V., "Inhaled Aerosols", Archives Internal Medicine,

Vol. 131, Jan. 1973.

118
[6] Goldberg, I. W. , and Lourenco, R. V., "Deposition of Aerosols in

Pulmonary Disease," Archives Internal Medicine, Vol. 131, Jan.

1973.

[7] Lourenco, R. V., Klimek, M. F., and Borowski , C. J., "Deposition

and Clearance of 2u Particles in the Tracheobronical Tree of Normal

Subjects--Smokers and Nonsmokers," Journal of Clinical

Investigation, Vol. 50, 1971.

[8] Pierce, A. K., and Sanford, J. P., "Bacterial Contamination of

Aerosols," Archives Internal Medicine, Vol. 131, Jan. 1973.

[9] Davis, M. J. 6., "Electron-Microscope Studies of Asbestosis in Man

and Animals," Annals New York Academy Science, Vol. 132, pp. 98-

111, 1965.

[10] United States Department of Health, Education, and Welfare:

Occupational Characteristics of Disabled Workers, by Disabling

Condition: Disability insurance benefit awards made in 1959-1962 to men under age 65, Washington, D.C. Government Printing Office, 1967

(PHS Publication No. 1531).

[11] Allison, A. C, Harington, J. S., Birbeck, M., et al.. Observations

on the cytotoxic action of silica on macrophages. Inhaled Particles

and Varpous II. Edited by C. N. Davies, Oxford, Pergamon Press,

1967, pp. 121-131.

[12] Knox, J. E., Holmes, S., Doll, R., and Hill, I.D., "Mortality from

Lung Cancer and Other Causes Among Workers in an Asbestos Textile

Factory," British Journal Industrial Medicine, Vol. 25, pp. 293-

303, 1968.

[13] Biological Effects of Asbestos, Ann. New York Academy of Science,

Vol. 132, 1-765, 1965.

[14] Murphy, R. L. H., Asbestosis in pipe coverers engaged in new ship

construction. Thesis, Harvard University School of Public Health,

Boston, Mass., 1968.

[15] Morgan, W. K. C, "The prevalence of coal workers' pneumoconiosis,"

Amer. Rev. Resp. Dis., 98, 306-310, 1968.

[16] Trasko, V. M., "Silicosis, a Continuing Problem," Public Health

Rep. 73, 839-846, 1958.

[17] Tepper, L. B., Hardy, H. L., Chamberlin, R. I., "Toxicity of

Beryllium Compounds," Amsterdam, Elsevier Publishing Company, 1961.

[18] Barbee, R. A., Callies, Q., Dickie, H. A., et al., "The Long-Term

Prognosis in Farmer's Lung," Amer. Rev. Resp. Dis., 97, 223-231,

1968.

119

561-452 0 - 74 -9
[19] Nicholson, D. P., "Bagasse Worker's Lung," Amer. Rev. Resp. Dis.,

97, 546-560, 1968.

[20] Paterson, J. D., "Studies on the Toxicity of Inhaled Cadmium. III.

The Pathology of Cadmium Smoke Poisoning in Man and in Experimental

Animals," J. Industr. Hyg. Toxicol., 29, 294-301, 1947.

[21] Spolyar, I. W., Keppler, J. F., Potter, H. G., "Cadmium Poisoning

in Industry: Report of Five Cases, Including One Death," J.

Industr. Hyg. Toxicol., 26, 232-240, 1944.

[22] Buxton, R. St. J., "Respiratory Function in Man Casting Cadmium

Alloys. Part II. The Estimation of the Total Lung Volume, its

Subdivisions, and the Mixing Coefficient," British Journal Industrial Medicine, Vol. 13, pp. 36-40, 1956.

[23] Dalhamn, T., Strandberg, L., "Synergism Between Sulphur Dioxide and

Carbon Particles: Studies on Adsorption and on Ciliary Movements in

the Rabbit Trachea in Vivo," Air Water Pollut., 7, 517-529, 1963.

[24] Peters, J. M., Murphy, R. L. H., Pagnotto, L. D., et al., "Acute

Respiratory Effects in Workers Exposed to Low Levels of Toluene

Diisocyanate (TDI)," Archives Environmental Health (Chicago), 16,

642-647, 1968.

[25] Gandevia, G., "Studies of Ventilatory Capacity and Histamine Response During Exposure to Isocyanate Vapour in Polyurethane Foam

Manufacture," British Journal Industrial Medicine, 20, 204-209,

1963.

[26] Schrag, P. E., Gullett, A. D., "Byssinosis in Cotton Textile

Mills," Amer. Rev. Resp. Dis., 101, 497-503, 1970.

[27] Bouhuys, A., Lindell, S. E., "Release of Histamine by Cotton Dust

Extracts from Human Lung Tissue in vitro," Experientia, 17, 211-

212, 1961.

[28] Bouhuys, A., van de Woestijne, K. P., "Respiratory Mechanics and

Dust Exposure in Byssinosis," J. Clin. Invest., 49, 106-118, 1970.

[29] Widdicombe, J. G., Kent, D. C, Nadel , J. A., "Mechanism of Bronchoconstriction During Inhalation of Dust," J. Appl . Physiol., 17,

613-616, 1962.

[30] DuBois, A. B., Dautrebande, L., "Acute Effects of Breathing Inert

Dust Particles and of Carbachol Aerosol on the Mechanical

Characteristics of the Lungs in Man: Changes in Response after

Inhaling Sympathomimetic Aerosols," J. Clin. Invest., 37, 1746-

1755, 1958.

120
[31] Bouhuys, A., Schilling, R. S. F., van de Woestijne, K. P., "Cigarette Smoking, Occupational Dust Exposure, and Ventilatory Capacity," Archives Environmental Health (Chicago), 19, 793-797,

1969.

[32] Diehl, H. S., M.D., "The Surgeon General Reports on the Health

Consequences of Smoking," Tobacco and Your Health, 1965, 1967,

1969, and 1972 (Washington, D.C.).

[33] Schayer, R. W., "Relationship of Induced Histidine Decarboxylase

Activity and Histamine Synthesis to Shock from Stress and from

Endotoxin," American Journal Physiology, 198, 1187-1192, 1960.

[34] Harris, W. S., M.D., "Toxic Effects of Aerosol Propel 1 ants on the

Heart," Archives Internal Medicine, American Medical Association,

(Chicago), Jan. 1973.

DISCUSSION

ROBERT KNOLLENBERG: I have a question as to your depth of field on

a very small particle? One that becomes increasingly small?

HOTHAM: Typically the depth of field for a 100 micrometer particle

is plus or minus 2 mm. The optical system has been designed to have a sharp cut-off after this depth of field has been exceeded. So that

particles do not gradually drop out of focus but are cut off abruptly;

they are either in-focus and appear as black dots or they are out-offocus and are very grey and are not sensed by the computer. Since this

is programmed in by contrast setting in the memory holding tube, it

abandons all greys and retains only the in-focus black images. This

system works on sizing only the in-focus particles.

KNOLLENBERG: How do you make that discrimination?

HOTHAM: The computer logic rejects all particles that show up with

diffraction patterns around them, i.e., particles out-of-focus . Hence,

in summary, the Automatic Scanner sees only in-focus particles.

Many frames are being stored in the Automatic Scanner. Typically

60 frames or 100 to 1000 frames per second are being stored and scanned

so that the system is scanning many, many frames taken within the spray

and statistically the in-focus particles counted, several thousand per

second, represent a true count and size distribution of the particles in

the spray.

LAURENCE DOEMENY: What is the resolution of the instrument? Have

you looked at any fibers like asbestos?

HOTHAM: The smallest particles that I have looked at were some polystyrene spheres that were 980 Angstrom in diameter; they were fuzzy

but I could detect them.

121
We are quoting a minimum resolution of 3000 Angstrom which is 0.3 ' micrometers. I

DOEMENY: Have you looked at asbestos?

HOTHAM: No.

i VIRGIL MARPLE: What is the cost of this instrument?

j HOTHAM: The laser system is $25,000 and the computer that does the

| automatic scanning is $60,000.

j RONALD NELSON: How long does it take to scan an image? !

HOTHAM: Less than a millisecond. The laser machine is generating

video signals which are fed to the computer; each frame containing many I hundreds of particle images, is stored. It takes less than a

| millisecond to scan that stored frame and rack up in the registers the I counts in the different size ranges. Realistically you are talking

about counts of 10,000 particles per second.

JOHN WEINNEGAR: Have you ever looked at the breath that someone [ exhales after inhaling an aerosol, perhaps one that's been humidified?

| HOTHAM: Yes, tobacco smoke. The smoke was put into a rubber bulb

(dry) and then squeezed and put into the path of the laser and the

picture showed dry tobacco smoke particles. Secondly, the tobacco smoke t was inhaled and blown from the mouth into the laser, and you could see I the difference in the size of the particles exhaled. The exhaled | particles from the human lung were larger than those expelled from the

| dry rubber bulb. Hence, the exhaled particles had been humidified while i they were in the lung. i

REG DAVIES: With your particular system, and where you focus this

| onto the camera and then onto the screen, can this just be focused, say | for example in our case where we have a Quantimet 720 image analyzer | with a TV camera, can this be focused through our system? i

HOTHAM: I believe your question is, can the Laser Machine be made

| to work with the Quantimet 720? The answer is "yes." We sell both the ; Laser Machine and Image Analysis Computers together. They do interface.

| Thus you accomplish real time size distribution. |

DAVIES: When you interface, this computer scanner, which par- j ticular scanner do you use? Your own?

i HOTHAM: Right now I'm using the Bausch & Lomb Image Analyzer. i

DAVIES: In other words you focus onto the TV scanner to find ! space, and then use that to size on the screen.

122
HOTHAM: Yes, you focus into the spray or select a region of the

spray you wish to measure. Those spray images are held in the memory

tube and scanned by Image Analyzer or Automatic Scanner as we call it

and you get size distribution directly.

ILAN CHABAY: What is the amount of energy you are hitting these

particles with per pulse:

HOTHAM: It is 200 microjoules, peak power 2-50 KW, pulse width 10

nanoseconds, 1 to 1000 pulses per second.

I have studied the heart beat of small micro-organisms. The

Daphnia is one illustration. It is about a millimeter long. The heart

beat was measured under the influence of various narcotics, both

stimulants and depressants. The Daphnia was live and continuously

receiving the laser. The pulse width is so small, 10 nanoseconds, 10

billionths of a second, that it did no harm and the Daphnia biological

specimen swam away quite lively after the exposure.

CHABAY: You said something at some point about trajectory of

suspended particles in the air, and I couldn't see the particles.

HOTHAM: Yes, the particles came out quite small on the TV screen,

about 1/4 of an inch in diameter and were hard to see.

CHABAY: If I saw it correctly they were merely descending and then

rising again.

HOTHAM: This test was performed in an 8-ft high tower or settling

chamber. The particles were introduced at the top. Monodispersed

particles of 100 micrometers were used.

They fall with gravity until the counter flow of air from the

bottom balanced their flight. What you saw on the screen was the

floating point for the particles. At this point change in size and

shape during evaporation could be studied. It was a study in evaporation to determine the final shape.

These particles were composed of an experimental insecticide.

There was some concern by the customer that the insecticide particles

would wind up as crystalline particulates in the shape of frisbies.

Frisbie particles drift inordinately far, drifting from the intended

field to a neighboring crop belonging to another farmer.

In this test it was clearly shown that the particles did not go to

frisbies but remained as liquid drops.

Also, in reference to your question about the amount of energy

absorbed by aerosols from the laser and the possible effect it might

have on them, I would like to illustrate some observations I have made.

123
In looking at impacted aerosol liquid drops on those 5 micrometer

strands, I have found that if you select one drop and fire the laser

slowly, at say, once a minute and look at the drop you will see that it

does not change in size. But most important, when you speed up the

firing rate of the laser to say 100 pulses per second, also, the liquid

drop did not show any change in shape or size.

You can probably reason through this observation by remembering

that the laser beam cross-section is quite large, several millimeters

when it reaches the small particle, which in this case was 10 micrometers, so that the beam is spread out sufficiently not to cause damage

to the aerosol

CHARLES PARMENTER: Have you tried doping the liquid particles with

fluorescent dyes and looking at it by fluorescence?

HOTHAM: Yes, we have done that. The fluorescent particles change

the way the insecticide droplet acts. If you mix the fluorescent particles with the insecticide, it will change the density and character of

the droplet. These insecticide droplets with added fluorescent particles do not impact on setal hairs with the same efficiency as do un- mixed particles. I am only familiar with droplets of insecticide in the

1 to 20 micrometer size range that had added to them fluorescent

particles in the 1 to 5 micrometer range.

You are probably suggesting the use of the fluorescent dyes because

I am using a UV laser of considerable power hoping to use the

fluorescent effect to detect the size of the particles. Unfortunately

the Laser Machine as it is now constructed forms images of the particles

only; it does not sense light level or the fluorescent effect. The

machine could be made to sense light level by simply using a photodetector.

EDGAR ETZ: At this point do you prefer to size the particles in

flight or after they have impacted on a target strand?

HOTHAM: You can do it both ways. The short pulse of the laser, 10

nanoseconds, makes a stop action picture of the flying aerosol particle.

This frozen picture of the particle is sized by eye by the reticule on the face of the TV monitor. Or the frame may be stored in the memory

tube and sized automatically by the Automatic Scanner.

ETZ: How do they compare? What happens to the particle once it

makes contact with a surface? And depending on the nature of the

surface, doesn't that surface change the characteristics of the aerosol?

Is it reliable to size a particle of a liquid aerosol when it is on a surface?

HOTHAM: If you wish to obtain a true size distribution of the

spray in air as generated from a dispenser, then you should record the

particles in free flight. The Laser Machine is designed to operate this

124
way. It photographs the particles in free flight. The images are sized

to provide size distribution, either manually or by the Automatic

Scanner.

The particles in this case are in free flight and have not been

impacted.

In the case of the hair spray, I showed you particles hitting

target strands to simulate the hitting of human hair. Those pictures

showed the actual mechanics of impaction. This was also shown for a deodorant (liquid) and an antiperspirant (particulate). I wanted to

show the actual mechanics of impaction.

Certainly, to answer your question, a particle hitting a flat

surface is spread out and alters the shape of the aerosol. This Laser

Machine technique does not use impaction on flat surfaces to determine

size distribution.

Going back to the hair spray and the measurement in free flightjust as a reminder, the laser is fired through the spray perpendicular

to the axis of the spray. If you fire the laser through the axis of the

spray at say 8 inches downstream, you will get a different size

distribution from a measurement taken at say 12 inches downstream.

Also, measurements made on the outer edges of a spray will be entirely

different from those taken along the center line or axis of the spray.

In the case of a hair spray, you want to produce fairly large drops

in the 150 to 300 micrometer size range or larger. Once the hair has

been styled by a beautician, the object of the spray is to hold the hair

in that position. You do not want the spray to bring the hair fibers

together and wet them into a mat. The droplet ideally will strike two

hairs simultaneously, straddle them, evaporate and leave a supporting

residue to space the hairs apart. The other effect is for the droplet

to spread along the length of the hair fiber and around it, and

evaporate, giving it a strong sleeve support to maintain its form.

Again, it is the "shellac-like" residue that gives hair its support.

Unfortunately, accompanying these large drops in a hair spray are a great abundance of fine, small respirable particles in the 1 to 10

micrometer size range which are formed as satellites to the larger drops

which are formed, and which if inhaled remain in the lung for long

periods of time.

Ideally, then, you wish to produce a spray with many large drops

for a hair spray without the generation of a lot of respirable particles.

PETER PELLA: Is there any relationship between these and

antiperspi rants? I'm thinking of aluminum chloride that is generally

put in antiperspi rants. Do you see a residue of that?

125
Hotham: In a hair spray you want the liquid drop to hit the hair ' and leave a bonding or supporting residue. |

I In an antiperspirant you want most of the liquid to have evaporated 1 just before hitting the skin so that it won't feel cold, i.e., from

j evaporation on the skin after impacting. But you want the particle to

' be sufficiently wet to give some sticking qualities to the skin.

For best efficacy you would want the greatest surface area on the | drops per gram of antiperspirant expelled from the dispenser. Hence the i antiperspirant is usually generated as very small fine particles. This

produces the greatest antiperspirant effect and has the greatest

efficacy.

Unfortunately, these small particles, if they get down into the

lower lung, into the alveoli, they usually remain.

To answer your question about seeing aluminum chloride in an antiperspirant. Yes, you can see the residue at impaction. That is what I i showed in the film on anti perspi rants . You were shown the residue on the 5 micrometer strands. You were also shown flying particles of

antiperspirant that were round, liquid drops. At impaction, or shortly

after (microseconds or milliseconds later), the impacted particles

completed their evaporation and hung onto the strand as residues. You

can see this from their jagged and irregular shape. That was the residue of aluminum chloride. This has been substantiated by chemical analysis of the residue.

On the other hand, a deodorant flies as a liquid drop, impacts as a liquid drop and remains in the liquid form to spread over the skin.

Particles of deodorant in the 1 to 10 micrometer size range were shown

to you today just after the series on anti perspi rants

.

126
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 412

Aerosol Measurements, Proceedings of a Seminar on Aerosol Measurements

Held at NBS, Galth ersburg, MD, May 7, 1974 (Issued October 1974).

RAPID RESPIRABLE MASS MEASUREMENT

Lawrence Doemeny, George Carson and Bruce Almich

U.S. Department of Health, Education, and Welfare

Public Health Service

Center for Disease Control

National Institute for Occupational Safety and Health

Engineering Branch

Division of Laboratories and Criteria Development

1014 Broadway

Cincinnati , Ohio 45202

ABSTRACT

The respirable mass concept of dust sampling will be used to introduce the GCA Corporation, beta absorption particulate aerosol sampler.

NIOSH has extensively evaluated the GCA instrument as a coal dust monitor and is beginning to apply it to the sampling of other aerosols. The

evaluation included mass loading, accuracy and environmental simulations. Other sampling methods which look promising will be discussed.

Key words: aerosol size measurements; beta absorption; cascade impactor; coal dust monitor; dust inhalation hazards; environmental sampler;

respirable dust sampler.

I. RESPIRABLE MASS CONCEPT

Airborne particles having aerodynamic diameters less than 100 ym

present a potential inhalation hazard to the worker. The nature and

magnitude of the hazard depends upon the number of particles within a size range and primary mode of entry to critical organs. The fraction

of inhaled articles with aerodynamic diameters greater than 10 ym impact

or settle on the nasal and oral passages and are eliminated through the

gastrointestinal tract. The hazard from these and other particles

depend upon their physical, chemical and biological properties.

Generally, the fraction of particles less than 10 ym present the greater

potential health risk and this fraction has a probability of depositing

on the nonciliated passages of the respiratory system.

The deposition of inhaled particles has been studied theoretically

and experimentally. The currently accepted lung regional deposition

model is that arrived at by a special Task Group on Lung Dynamics to the

International Commission of Radiological Protection (ICRP) Committee 2

[1]. The ICRP model is based on a theoretical treatment using log127
normal dust distributions, and an anatomical model at several

ventilatory capacities.

Today several criteria and definitions exist for respirable dust.

Common to these definitions is the concept that the particles be sized

according to their aerodynamic diameter. These organizations mentioned

below have different criteria for respirable dust sampling. The criteria and size selective samples used to collect rejDresentative respirable dust levels are based on the lung depositions model. Figure 1 shows the criteria used by each organization and also the ICRP lung

deposition model. The British Medical Research Council (BMRC) curve is

the particle penetration resulting when a stacked parallel plate

horizontal -el utrai tor type sampler is used. This curve is theoretically

derived and is based on the gravimetric settling of particles. The BMRC

is the respirable mass fraction criteria used by England and the United

States when sampling coal dust in mines.

0123456789 10

AERODYNAMIC DIAMETER (micrometers)

Figure 1. Lung deposition model and respirable mass definitions.

128
The Los Alamos Scientific Laboratory (LASL-AEC) and the American

Conference of Governmental Industrial Hygienists (ACGIH) curves are identical except for particles with aerodynamic diameters less than 2

ym. Although the curves were developed from lung depositions studies

and not devised with size selective dust samplers in mind, small cyclone

inertial collectros closely approximate the curves. In the United

States the ACGIH curve is used as the respirable dust definition for all

mineral dusts (coal dust excepted) and would presumably apply to organic

particulate aerosols.

NIOSH and OSHA have favored evaluating dust exposure using portable

sampling equipment worn by the individual to obtain an eight-hour timeweighted average (TWA) breathing zone sample. These samplers include a battery operated pump and a sampling head consisting of a particulate

classifier and preweighed membrane filter. The most commonly used

particulate classifier is a lO-mn nylon cyclone which separates

particles aerodynamical ly according to the ACGIH criteria when operated

at 1.7 1pm. Particles passing through the cyclone are collected on a filter and weighed or could be chemically analyzed as the respirable

fraction.

NIOSH is interested in occupational aerosol spray exposures in

occupations where they are manufactured or used such as in beauty

schools and salons, spray painting, cleaning services, etc. A specific

pulmonary disease, thesaurosis, has been reported in the hair dressor

occupation and NIOSH has conducted some investigation of this problem.

These aerosol sprays present a unique problem to aerosol sampling

because particulate concentrations are extremely high and their physical

and chemical properties are unstable. The aerosol is a mul ticomponent

system consisting in part of a propellent, a volatile solvent, a fragrance and an active ingredient. Because of the volatility of some com- ponents the aerodynamic diameter of the aerosol and its composition is

changing with time. For OSHA compliance this is a staggering problem

because an eight hour TWA exposure is the hazard evaluation criteria but

an instrument capable of short rapid sampling is required to collect and

evaluate the sample with time dependent characteristics. Frequent

short-term samples might then be representative of an eight-hour TWA

exposure.

II. AN INSTRUMENT WHICH MAY BE CAPABLE OF MEASURING AEROSOL

SPRAY EXPOSURE

NIOSH has recognized the need for instruments which, although not

necessarily able to measure occupational compliance, or noncompliance

can quickly identify areas of greatest hazards concentration so compliance monitoring can be efficiently initiated. Under a NIOSH

contract the GCA Corporation has developed a portable, short-term

respirable dust monitor for application in "walk-thru" surveys in coal " mines. Although developed for coal dust sampling its use is not limited

to coal dust.

129
GCA (fig. 2) now has available one instrument which can be obtained

in one of four configurations (table 1). They are called respirable

dust monitors, RDM-101. Depending on the model the RDM's can evaluate

respirable dust concentrations ranging from 0.06 to 200 mg/rn^. The

range change is accomplished by altering the sampling times from 12

seconds up to 8 minutes. The RDM-101 is designed for short time, on the

spot measurements of mass concentrations of the respirable fraction of

the dust or, with the cyclone preclassifier removed, the total mass concentrations using the principle of beta-particle absorption. With

the cyclone removed the impactor is limited to the collection of

particulates with aerodynamic diameters less than 50 ym.

The cyclone and impactor combination comprise a two stage collection system. The first stage is the cyclone. The second stage consists

of the inlet nozzle impactor beta absorption assembly and mylar

impaction disk. The beta particle source is housed in the impactor

nozzle. As material impacts on the mylar disk, radiation is increasingly absorbed, decreasing the radiation reaching the Geiger tube beneath the disk. The transmission of beta particles depends almost en- tirely on the mass of material per unit area of absorber and the maximum

Figure 2. Respirable dust monitor with 10-minute nylon cyclone,

battery, charger, air flow calibration ratementer and carrying case. 130
Table 1. Respirable dust monitors.

Model Mass Range Sampling Time

RDM-101-0.2 10 to 200 mg/m^ 12 s RDM-101-1 1 to 50 mg/m^ 1 min

RDM-101-4 0.2 to 8 mg/m^ 4 min

RDM-101-8 0.06 to 5 mg/m^ 8 min

beta energy. For coal dust, composition has little effect because the

beta particle absorption cross section is proportional to the ratio of

atomic number (Z) to gram molecular weight (A). For large organic molecules containing mostly carbon, hydrogen, nitrogen, oxygen and some sulfur and phosphorus the error should be negligible. For methane particles (if they were to exist) the instrument would read 25 percent high

when calibrated for coal. Similarly, for hexane the RDM would read 16

percent high and as the ratio of carbon mass to hydrogen mass decreases,

the error decreases.

Beta radiation is attenuated exponentially according to the equati on

N/No = exp (-ys) (1)

where:

N and No are the initial and final beta counts produced by the

Geiger tube respectively.

y is the beta absorption coefficient.

s is average mass of material per unit area.

The mass concentration C is equal to:

P _ A (In No - In N) /«x ^ - y Q t

where:

A is the dust collection area.

Q is the air flow.

t is the effective sampling time.

131
The term A/uQt is constant set during calibration. The RDM's are completely automatic. By starting the sampling cycle the instrument

concurrently begins sampling the atmosphere and takes an initial beta | count. During the last few seconds of the cycle the final beta intensity is measured. The difference in the log of the two beta counts is

multiplied by the instrument calibration constant and displayed on a three digit LED in mg/m3 for ten seconds. The value can be recalled any

j time before the next cycle. The instrument is completely solid state i and battery operated. Our tests [2] have confirmed the measurement | accuracy to better than ±25 percent (95% coefficient of variation) for

j coal dust. For the RDM-101 the beta counts are taken during the initial

and final 20 seconds of the sample period. This value was selected by

minimizing the variance in the concentration with respect to the \ counting time and varies accordingly for each instrument model.

The beta absorption principle has been shown to be useful as an aerosol mass monitor by GCA's instrument and as a stack emission monitor

in an instrument sold by Lear-Seigler. The principle can be applied to

existing particulate samplers like the cascade impactors which classify i particulates into several aerodynamic diameter intervals. Such a unit j could then be calibrated to directly display total collected mass, I aerodynamic mass median diameter and geometric standard deviation, for

an assumed log-normal distribution.

III. FEW EXPERIMENTS WITH AEROSOL HAIR SPRAY

Hair spray was directed at an RDM-lOl from a distance of 60 cm for

the entire period of the cycle. Masses of approximately 10 mg/m^ and 5

mg/m-^ were obtained for "total" and respirable fractions respectively.

As mentioned above, the "total" sample is for aerosols with aerodynamic

diameters less than 50 ym. Literature values show respirable fractions

ranging from 0.5 percent up to 65 percent. Associated with the 65 percent respirable mass fraction were observed particles ranging from 1 to

3 urn [3] while the 0.5 percent value observed 50 percent of the particles with diameters greater than 35 ym [4]. It is difficult to assess

the conflicts in this data for the size distribution can vary between

products tested. However, the data does show a need for instrumentation

which is capable of rapid aerosol measurement to eliminate any ambiguity

or doubt in aerosol sizing.

IV. CONCLUSION

The principle of beta absorption as applied to aerosol mass measurements is viable for sampling high concentrations of aerosols.

The detection principle can be applied to existing aerosol sampling

equipment like Lundgren, Andersen and May type impactors for rapid size

determination by positioning a beta source and detector at each stage of

separation.

132
Another technique worth mentioning is a new concept in aerosol sizing described by Dr. Lawrence Hrubesh [5]. Dr. Hrubesh passes particles

through a tuned microwave cavity. The resonant frequency of the cavity

is changed by the presence of the particle. The change in frequency as measured on a spectrum analyzer is proportional to the volume of the

particle (eq 3)

6f^v(£' - 1)

where

f^ is the resonant frequency of the cavity

V is the volume of the sphere

e' is the complex permittivity of the dielectric (assumed lossless)

a, b and d are the cavity dimensions

Af is the change in resonant frequency of the cavity by the presence of the sphere.

The instrument in its present form is limited to particles greater

than 50 ym. Dr. Hrubesh and I will meet this summer to discuss methods

to extend the usefulness of the device into the respirable range.

REFERENCES

[1] Task Group on Lung Dynamics to the ICRP, Committee 2. Deposition

and Retention Models for Internal Dosimetry of the Human

Respiratory Tract, Health Phys. 1_2, 173 (1966).

[2] Almich, B. P., Solomon, M., and Carson, 6. A., "A Theoretical and

Laboratory Evaluation of a Portable Direct Reading Particulate Mass

Concentration Instrument." In final review before submission to

the American Industrial Hygiene Association Journal for

publication. Presented at the May 1974 meeting of the American

Industrial Hygiene Association, Miami Beach, Fla.

[3] Draize, G. H., Nelson, A. A., Newburger, S. H., and Kelley, E. A.,

"Inhalation Toxicity Studies of Six Types of Aerosol Hair Sprays."

Proceedings of the Scientific Section of the Toilet Goods Association, Number 31, May 1959.

133
[4] Brunner, M. J., Giovacchini, R. P., Wyatt, J. P., Dunlap, F. E.,

and Calandra, J. C. Pulmonary Disease and Hair Spray Polymers: A

Disputed Relationship. The_ Journal of the American Medical

Association , 184 , 851-857 (June 15, 1963).

[5] Hrubesh, L. W., A Simplified Microwave Instrument for Measuring

Particle Volumes and Densities, Rev. Sci . Inst. 45, 125-126 (Jan.

1974).

*Mention of company or product names is not to be considered as an endorsement by NIOSH.

DISCUSSION

REG DAVIES: When you mentioned that you do this by measuring the

respirable percentage of hair sprays or consumer products, are you re- lating the mass to the residue or to the droplet itself?

DOEMENY: The study was an introductory effort, therefore the re- sults should not be considered conclusive. We were attempting to get an idea as to what percentage of total aerosol was respirable. At the time

it would be difficult to say how much solvent evaporated from the sample

collected on the impaction disk.

DAVIES: So this figure of one-half percent to sixty-five percent

where you say you agree with the literature, is it based purely on the

residue or would it have had a little liquid in it?

DOEMENY: As in the previous answer, it is difficult to say what

fraction of solvent had evaporated. The presence of solvent in the

droplet does influence the droplet aerodynamic diameter. As solvent

evaporates the aerodynamic diameter changes. Because the aerosol piles

up on the impaction disk, evaporation would be slowed down. The

instrument reading would then include the solvent fraction if it were

present.

DAVIES: Therefore, you can be over estimating what is respirable

or inhalative because you are looking at something that is smaller?

DOEMENY: That is possible, but the literature value of 0.5 percent

seems too low. Remember, also, our samples were collected some distance

from the aerosol source so that the large particles may never have

reached the meter.

GEOFFREY HOTHAM: The sampling in that machine for the hair spray,

that's just airborne particles that you are sucking into the machine?

DOEMENY: Correct.

134
HOTHAM: What about the big particles that you get in the hair

spray, the 200 to 300 micrometer range?

DOEMENY: The instrument samples for particles in the range of 0.4

to 50 ym. The larger particles are not collected.

HOTHAM: Hair spray contains particles of 150 to 300 micrometers?

DOEMENY: As far as we know that is correct. However we are ultimately interested in the respirable mass fraction which would

generally be those less than 10 ym in diameter.

HOTHAM: When you make the measurements though, using the fraction

by mass, it's quite different than what you quoted.

DOEMENY: Yes. Under this setup particles larger than 50 ym are neglected and would then make my earlier statement on respirable mass fraction appear high.

HOTHAM: Because of the large particles that don't get into that

machine.

DOEMENY: That is correct.

EDGAR ETZ: How did you get the respirable fraction? Did you run

it with and without the cyclone?

DOEMENY: Yes.

ETZ: I'm trying to visualize your sampling situation. Whatever

concentration of airborne particles that you have in mind, you are going

to force a known volume of this atmosphere into an intake of your

sampling pump. Is there anything that goes on in the process of

sampling or channeling particles into the intake of your sampling device

that causes particles to agglomerate?

DOEMENY: I do not know of any definitive studies but it would seem

unlikely that agglomeration occurs in the cyclone sampler for the

concentrations, sizes and distributions frequently encountered.

ETZ: Does anyone else in the audience have any feeling for what

happens to solid airborne particulates as they are channeled into a small orifice?

DOEMENY: We have done concentration studies of cyclones with

aerosol concentration in the 2 mg/m^ and found no effect on the

concentration measurement.

ETZ: With varying concentrations at the orifice, what effect does

this have?

135

561 -452 O - 74 - 10
HOTHAM: I would like to answer your question from the work I've

done with the pulsing laser system. Sampling devices that carry in the

airborn particulates, suck in only the small particles. The small

orifice pulls in only the small particles and misses the large ones. The size distributions that you obtain from such a machine are therefore

distorted.

DOEMENY: With this instrument that is indeed possible. However in

most cases, especially for dry, stable aerosols, the fraction is

obtained from the cyclone classified sample and a total aerosol sample

that has been collected isokinetically on an open face filter.

HOTHAM: Didn't you say the size range only went up to 50

micrometers?

DOEMENY: I was speaking to other methods to obtain respirable mass

fractions, like the personal cyclone sampler a miner might wear.

HOTHAM: I'm saying the sampling leaves a little bit to be desired

in that the sampling method only takes in the small particles.

DOEMENY (added in proof): This is not correct. The cyclone was designed to sample respirable aerosols which it collects very well. The

larger particles are collected on the open face filter.

ETZ: I would think that depending on the dryness of the

atmosphere, these particles are charged and when they increase their

concentration you increase the chances of agglomeration.

DOEMENY: You don't increase their concentration by passing them

through the cyclone.

136
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 412

Aerosol Measurements, Proceedings of a Seminar on Aerosol Measurements

Held at NBS, Gaithersburg, MD, May 7, 1974 (Issued October 1974).

PARTICULATE MASS MEASUREMENT BY

PIEZOELECTRIC CRYSTAL

Raymond L. Chuan

Celesco Industries, Inc.

Costa Mesa, CA 92626

ABSTRACT

An adhesive-coated quartz crystal working as the impaction surface

capturing particulates from decelerating jets measures mass by frequency

change, to a sensitivity of 10^ hz gm" ' . The scheme is applied to a number of working instruments which measure continuously and directly

particulate mass concentration, and particulate mass distribution (by

using the devices in a cascade).

Key words: aerosol sizing; cascade impactor; particle detection by impaction; particle size measurements, piezoelectric crystal; quartz crystal microscope.

I. DESCRIPTION OF THE TECHNIQUE

The Quartz Crystal Microbalance (QCM) is a piezoelectric device

which senses the mass of particulate matter impacted aerodynamical ly on

an adhesive-coated quartz crystal through a number of small jets and

creates a decrease in the resonant frequency of the crystal. Two

crystals are placed close together in the Sensing Module, one acting as the impactor surface (The Sensing Crystal) and the other acting as the

Reference Crystal (and temperature compensator). See figure 1. The

reference oscillator is set at a slightly higher freguency than that of

the Sensing Oscillator (both operating near 10' Hz) so that a beat

frequency of about 10^^ Hz is produced in the mixer. A decrease in the

sensing frequency (due to a mass increase) then results in an increase

in the beat frequency.

The fundamental relationship between mass change and frequency

change for a nominal frequency of 10^ Hz is (from ref. 1):

in which Af is the frequency change in Hz, Am the mass change in gram

and A the active area in cm^ on the crystal where mass is added.

137
SENSING MODULE CONTROL MODULE

SENSING

CRYSTAL

REFERENCE

CRYSTAL

SENSING

OSCILLATOR

REFERENCE

OSCILLATOR

MIXER

FREQ. TO

VOLTAGE RATE

OUTPUTS 4

FREQ. SATURATION CONCENTRATION

ELECTRONIC SCHEMATIC

BY-PASS

SAMPLE

BY-PASS

VALVE

IMPACTION

JET

EXHAUST

BLOWER

REFERENCE XTAL.

SENSING XTAL

Figure 1. Sample air flow and electrical schematic.

Typically, the active area is about 0.4 cm^, resulting then in

^= 5.5 X 10^ Hz/g

Thus, a nanogram of mass addition will give rise to a frequency change

of about 0.5 Hz. The beat-frequency from the mixer is amplified and

converted to a voltage in the frequency-vol tage-converter whose output

is proportional to cumulated mass. The rate of change of the beat-frequency, for a fixed volume flow rate of sampling, is proportional to the

particulate mass concentration. The output of the frequency-voltage

converter is, therefore, differentiated in the rate converter whose

voltage output is then the mass concentration.

II. DISCRETE MASS MEASUREMENT

An early form of the instrument based on the QCM technique was re- ported in references 2 and 3 in which the ability of the device to measure the mass of discrete particulates was discussed. If the number

density of particles in suspension in air is small enough so that the

impactions of particles against the crystal are not coincident, the

individual masses can be measured.

138
The arrival of a particulate mass on the crystal results in a stepwise rise in the beat frequency. If the beat frequency is converted to

an analog voltage and then differentiated, the step-wise frequency rise

becomes a pulse whose heights is proportional to the mass of the

particulate.

With conventional circuitry it is possible to resolve a frequency

change, over a short time interval, of the order of 10"^ Hz, equivalent

to a mass of about 2x10'^ g. Actual test results to date have reached a mass resolution of about 1x10"! 1 g. This is the mass of an equivalent

sphere of diameter 2 microns with a mass density of 2 g/cm^.

Figure 2 shows an actual recording trace of a differentiated analog

signal from the beat frequency change. The five readily discernible

pulses represent discrete masses of from 5x10-11 to 6x10-10 g. These

are produced by particulates of mass density 3 g/cm3; thus the largest

pulse represents a single particulate of diameter 7.5 micrometers.

€. X lo "'® gm

3 gm cm MASS

JO- C

SOLID F 10 SECS

ARTICULATi

TIME

:S

Figure 2. Differential spectra for solid particulates.

If the particulate matter incident on the crystal is of volatile

material, some of the mass initially captured may be lost due to evaporation, which is also accompanied by some cooling of the crystal surface. The resulting response in the differentiated analog signal is a pulse followed by a negative pulse—the former due to the arrival of the

mass, and the latter due in part to mass loss and in part to cooling.

An example of this is shown in figure 3. The pulses here are produced

139
Figure 3. Differential spectra for volatile particulates.

by water droplets --the difference from the signals in figure 3 are quite

apparent. The largest pulse here represents a water droplet of diameter

13 micrometers.

By analyzing a train of pulses it is possible to determine the

distribution of mass with respect to size, as seen in figure 4 (from

'ref. 2). The particulate material used is a lapping grit made of

aluminum oxide with mass density of 3.89 g/cm^. (A mass of 10-10

g

corresponds to a particle of 3.7 y diameter.) The distribution supplied

by the manufacturer is shown for comparison with that determined by the

QCM.

III. PARTICULATE MASS CONCENTRATION MEASUREMENT

With coincident arrivals of particles, some of which may also be

too small to be resolved discretely, the total particulate mass con- centration can be determined by measuring the cumulative mass change on the crystal with respect to time. The mass rate and the volume flow

rate then yield the mass concentration. Figure 5 shows the temporal

variation in the particulate mass concentration in a ventilated room when an aerosol hair-spray can is released for about 1 second, 2 meters

from the instrument. In a small, unventilated room the concentration is seen in figure 6 to remain quite high for some time after the hair-spray

application.

140
4

MASS ^'

(g)

0

Li

•

c

o —5—

"O" 9-

n MEASURED BY QUARTZ

" CRYST. INST. •b A DI

B"!STR]f MA

BUTION SUPPLII ^lUFACTURER

ID 0

o

k

(

0

p ERC;entAGE BELow MASS

50 90 99 99.99

Figure 4. Particulate mass distribution curve

obtained from a pulse height analysis of a train of pulses.

IV. POST SAMPLING ANALYSIS

Since the adhesive-coated crystal retains the impacted material

,

post sampling analysis can be performed; and such analysis can be performed nondestructively and without disturbing the material by the use

of scanning-electron-microscope (SEM), because the undisturbed crystal

can be placed directly in a SEM. Figure 7 (from ref. 5) shows a lOOX

SEM of an entire impaction area (taken at normal incidence), measuring

about 500 micrometers across, produced by exposing the crystal through a 400 -micrometer impaction nozzle to smoggy atmosphere for about 10

minutes. The measured concentration was 250 yg m"3. At an impaction

flow rate of 150 ml min-1, this means a total mass accumulation on the

crystal of about 0.38 yg. The symmetrical distribution of material,

with smaller particles spread to the outer regions, is quite apparent.

Figure 8 is a 10,000X SEM (at 10 degrees tilt) of a part of the

impaction area shown in figure 7.

Figure 9 is a 200X SEM of the material collected from the plume of

a solid-fueled rocket; while figures lO and 11 show enlargements of two

of the particles marked in figure 9. Particle 3 appears to be one of

several cubical shapes, presumably salt crystals; while particle 2,

along with many similar ones around it, appears to be spherical combustion product from the rocket motor. Particle 1 is quite complicated in

shape, and does not suggest anything readily identifiable. Results of

x-ray analysis of these particles and of a reference table salt crystal are shown in table 1.

141
to

O

«/) o

s-a

to

+-> c

(Us- MO

4-> rOt.

4->

O)o

o

o

t/)

(U

•I—

S-O.

C

o

•Jj <o

•r-s- • tC E

> o

o

(U s- E

•r- T3

I— CU

+-> fO LO •!-

(U C

S- (U 3 >

C7>

142
Figure 7. lOOX SEM of impaction area of smog sample.

/ 0,000

Figure 8. lO.OOOX SEM of impaction area of smog

sample (same as in fig. 7).

143
200 X

Figure 9. Impaction region from rocket

plume 200 X SEM.

\ooo X

Figure 10. Impaction region from rocket

plume 1 ,000 X SEM.

144
2000 X

Figure 11. Impaction region from rocket

plume 2,000 X SEM.

Table 1. Molal fractions of particle samples.

Particle

Element 1-1 1-2 2 3 Table Salt

Na 34.0 23.6 50.3 41 .0

Al 9.1 14.8 70.7 1.2 11.4

Si 11.0 18.0 2.9 0.8 6.5

An

S 15.9

CI 38.3 5.6 26.4 47.8 41.0

K 4.7 8.6

Ca 2.8 6.8

Ti 6.7

It is seen that particle 2, with a preponderant percentage of

aluminum, is probably a piece of aluminum oxide from the rocket motor.

The presence of chlorine without, however, any accompanying sodium (as

exhibited in other particles) suggests that the adsorption of hydrogen

chloride (part of the rocket combustion product) by the aluminum oxide.

From its shape and composition it is evident that particle 3 is salt—

and a purer salt, in fact, than the reference table salt. Particle 1-1

is apparently made up of 68 percent salt and a few other materials.

Particle 1-2, on the other hand, appears to be a different and much

smaller particle adhering to the larger particle, and has a fairly

complex makeup.

145
V. REFERENCES

[1] Sauerbray, G. Z., Verwandung von Schwingquarzen zur Wagung Dunner

Schlicht en und zur Mikrowagung, Z. Physik 155 , 206-222 (1959).

[2] Chuan, R. L., An Instrument for the Direct Measurement of Particulate Mass, Journal of Aerosol Science, Vol. 1, No. 2, 111-114

(May 1970).

[3] Chuan, R. L., Particulate Contaminant Measurement by Quartz Crystal

Microbalance, ASTM/IES/AIAA Space Simulation Conference, Sept.

1970.

[4] Chuan, R. L., Measurement of Particulate Pollutants in the Atmosphere, AIAA Journal, Vol. 10, No. 2, 1701-1704 (Dec. 1972).

[5] Chuan, R. L., Application of an Oscillating Quartz Crystal to

Measure the Mass of Suspended Particulate Matter, 165th National

Meeting of the American Chemical Society, Apr. 1973.

DISCUSSION

GEORGE SINNOTT: Is the negative overshoot a temperature effect?

CHUAN: It is a combination of negative mass rate and cooling. The

concentration trace is a rate of mass of the crystal, so with the

evaporation going on you actually get a negative. At the same time the

cooling has the same effect.

I LAN CHABAY: Is there any effect of build-up of matter on the

crystal surface?

CHUAN: Very definitely. You have to move your impactor site after

a certain amount of accumulation because we have found empirically that

is, for say particles of about one micrometer and down, when about 20

percent of the area that is available impaction area--about 20 percent

of that area is occupied then the probability of a late arriving particle hitting on top of an already occupied site increases so that your

response becomes nonlinear after that. Of course, with larger

particles, you can put on more before that happens. So empirically this can be established, and for most of these things it's used mainly for

diagnostic purposes of quick sampling where you can get information very

quickly and then in between runs you can move your impaction site which

can be done by rotating the nozzle or changing the crystal --a number of

ways one can do that, and then be ready for the next run.

GEOFFREY HOTHAM: You mentioned that the device can measure spatially the hair spray, etc. Aren't you just measuring the impacted particles?

146
CHUAN: When I say spatially, I only mean that in the sense you can move a cloud over the instrument or you can move the instrument through

a cloud, and look at the spatial distribution of the concentration. I'm

not talking about size distribution at any given point in time or space.

I didn't make that clear.

AL G0LDIN6: But your probability of coincidence impaction would be

pretty high and that would slow up your particle sizing when you are running with, say, an aerosol can.

CHUAN: I don't think it would be practical at all to use one device like this in the discrete mode to try to do pulse height analysis.

It would be all mixed up. One obvious extension of this would be to

cascade the device. We are in fact in the process of putting together

one ten-stage and one six-stage cascade where each stage records mass continuously so that you can get a continuous distribution. It will

work out to about a total mass concentration if you had a total mass concentration of the order of 100 micrograms per cubic meter then a reasonable central time to give you a good ten-step distribution would

be about two minutes. Higher concentrations then you can do almost

continuously.

EUGENE SCHEIDE: How does this instrument compare to the one built

by Thermal Systems that uses an electrostatic precipitator?

CHUAN: Well, they are basically the same thing. The reason we went the impactor route is because as an aerodynamisist that's the only

thing I can understand, and is tractable.

SCHEIDE: How does the collection efficiency compare? Do you have

any idea?

CHUAN: We have run a number of comparisons and calibrations. For

instance, we have done just direct optical comparisons. That is taking

the crystal out and photomicrographical ly examined it. We periodically

run it in parallel with the Hi-Vol for instance. It is one of the ref- erence methods and a comparison is usually quite good.

SCHEIDE: Do you achieve 100 percent collection?

CHUAN: That depends, of course, entirely on your impaction geometry. With the instrument which produces data that you see here it has

a 50 percent cutoff at 0.2 microns. In the cascade the theoretical

lowest cutoff is 0.05 micrometers at 50 percent. This is for mass density, not unit density.

MADHAV RANADE: These 0.05 micrometers, do they have to be operated

at reduced pressure?

CHUAN: Yes, it's about a quarter of an atmosphere in the tenth

stage.

147
VIRGIL MARPLE: Have you done any study on particle bounce? Problems you have?

CHUAN: Yes, that is why we have an adhesive on it to make sure we don't have bounce.

MARPLE: Do you check the adhesive to make sure you don't have

bounce?

CHUAN: Yes. We find that for particles, say general ly--this is

very rough--5 micrometers are larger and there is a tendency to bounce.

Not only that, but beyond about 5 or 10 micrometers, they actually are not mechanically bonded to the surface. They sit there and rattle. You

put the instrument on a very short time count in the high sensitivity

and if you run it without an adhesive and put some large particles on,

all you see is noise. You put the adhesive on it and you see it stays

there and shows the increase in mass.

148
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 412

Aerosol Measurements, Proceedings of a Seminar on Aerosol Measurements

Held at NBS, Galthersburg, MD, May 7, 1974 (Issued October 1974).

DEVELOPMENT, CALIBRATION AND APPLICATION OF SIZE DISTRIBUTION

INSTRUMENTS AT THE UNIVERSITY OF MINNESOTA

V. A. Marple

Mechanical Engineering Department

University of Minnesota

Minneapolis, MN 55455

ABSTRACT

Work with size distribution instruments has been in the areas of

development, calibration, and application. Instruments have been

developed in the area of particle size determination, such as the

electrical aerosol analyzer, and in the area of monodisperse particle

generation, such as the vibrating orifice generator and the differential

mobility analyzer. These aerosol generators, along with new techniques,

have been used to calibrate a variety of aerosol measuring instruments

such as optical particle counters, nuclei counters, and the electrical

aerosol analyzer. These instruments have then been used to study a range of particle types, from fairly ideal particles such as atmospheric

aerosols to very nonideal particles such as coal dust.

Key words: aerosol electrical analyzer; aerosol generators; aerosol

instrument calibration; aerosol size measurements; aerosol spectrometer;

aerosol sprays; nuclei counters; particle generators; particle size

measurements; particle size measurements by electromobility; particle

standards.

INTRODUCTION

In the past few years the Particle Technology Laboratory of the

Mechanical Engineering Department at the University of Minnesota has put

considerable effort into the study of aerosol analyzing instruments.

This work has been primarily in the areas of instrument development,

calibration of aerosol sizing instruments, and the application of these

instruments to field studies of real aerosols.

In the area of instrument development, four instruments will be

discussed here. These are a size distribution determining instrument,

the electrical aerosol analyzer; two aerosol generators, the vibrating

orifice monodisperse aerosol generator and the submicrometer aerosol

generator; and an eight-channel multichannel analyzer.

In the area of instrument calibration, the newly developed aerosol

generators have been used to calibrate aerosol analyzing instruments

such as the condensation nuclei counter, the electrical aerosol

149
analyzer, and the optical particle counter with ease and accuracy

previously not possible. Also, techniques have been developed to

calibrate optical particle counters on nonideal aerosols such as coal

dust particles.

By using these instruments and calibrations, several particle size

analyzing systems have been developed and used in field studies. These

systems include various numbers of sensors and data acquisition and

analyzing capabilities. Three of these systems will be described and some results presented.

In the following sections, the above items are discussed more

fully. For further details the reader is referred to the respective

references

.

DEVELOPMENT

Electrical Aerosol Analyzer (EAA)

(edited from Whitby and Liu, 1973)

The electrical aerosol analyzer is a size distribution measuring

instrument with in-situ measurement capabilities over the 0.003 to 1 ym

diameter range. The operating principle of the device is that of

electrical charging and mobility analysis, a principle first described

by Whitby and Clark (1966). Recent advances (Liu, Whitby and Pui , 1973)

in charger and mobility design and the use of all solid-state electronics have resulted in an improved instrument that is portable (about

30 kg in weight) and considerably more versatile.

Figure 1 is a schematic diagram of the instrument showing its major

components: the aerosol charger, the mobility analyzer, the current

sensor, and associated electronic and flow controls. The instrument

samples aerosols at the rate of 5 liters per minute with an additional

45 liters per minute of clean air needed to operate the mobility

analyzer.

In the instrument, the aerosol is first sampled into the charger

where the particles are exposed to unipolar positive ions and become

electrically charged. Figure 2 shows the relationship between the

electrical mobility and the size of the particles under different

charging conditions. It is observed that there is monotonic functional

relationship between particle mobility and size. Using this

relationship, the size distribution of the aerosol can be calculated

from the mobility distribution measured by the mobility analyzer.

The mobility analyzer shown in figure 1 is in the form of a cylindrical condenser with clean air and aerosol flowing down the tube

in a laminar stream. The charged aerosol particles are deflected

through the clean air core by the voltage applied on the center

electrode. For a given voltage on the center rod, particles above a certain critical mobility and size are precipitated, while those with

lower mobility and a larger particle size escape precipitation and are

150
j CLEAN AIR IN [><1 CHARGER

Figure 1. The portable electrical aerosol analyzer

(Liu, Whitby and Pui, 1973).

Figure 2. Relationship between electrical mobility

and particle size for diffusion charging (Liu,

Whitby and Pui, 1973).

151
sensed by the electrometer current sensor. By changing the voltage on the center rod and measuring the corresponding electrometer current, the

mobility and size distribution of the aerosol can be determined. The

standard operating condition of the instrument provides a total of 11 voltage steps dividing the size range of the instrument into 10 equal

geometrical intervals of four intervals per decade in particle size.

The size interval boundaries are located at 0.0032, 0.0056, 0.01,

0.0178, 0.032, 0.056, 0.1, 0.178, 0.32, 0.56, and 1.0 um. The complete

voltage sequence can be scanned in about two minutes, thus allowing a complete size distribution analysis to be made in the same time period.

The instrument has been interfaced to a mini -computer for complete

automated data acquisition.

The standard deviation of number, surface, and volume measurement

of the new instrument has been evaluated. The standard deviation of the

electrometer current (cj^i) ranges from a maximum of about 0.01 pa at

0.004 um to a low of about 0.001 pa at 0.4 ym. This corresponds to

standard deviations of ±57,000/cm3, ±0.4 ym^/cm^, ±0.003 ym^/cm^ at

0.0042 ym for number, surface, and volume respectively. At 0.75 ym the

corresponding standard deviations are ±0.6/cm"^, ±1 ym^/cm-^, and ±1.1

ym-^/cm^. It has been found that the variability is only significant for

number distributions below 0.02 ym and for the volume distribution above

0.5 ym. Therefore for most field work, data from the electrical analyzer has only been used for calculating sizes smaller than 0.422 ym.

An optical particle counter has been used to measure the distributions

for larger sizes.

The instrument has been used in a variety of laboratory and field

studies and has proven its usefulness in these applications. The

instrument is now available commercially from Thermo-Systems , Inc. (2500

Cleveland Ave. No., St. Paul, Minn. 55113).

Vibrating Orifice Monodisperse Aerosol Generator (VOMAG)

(edited from Whitby and Liu, 1973}

The calibration of aerosol measuring and sampling devices is

facilitated by the use of monodisperse aerosol. When the size or concentration of the aerosol (or both, depending on the application) are known to a sufficiently high degree of accuracy, the aerosol may be

referred to as an aerosol standard.

During the past few years our laboratory has devoted much effort to

the development of apparatus and procedures for generating monodisperse

aerosols of a primary standard quality. Two approaches have been used

for generating these primary standard aerosols, one involving the

controlled disintegration of a liquid jet by a vibrating orifice, and

the other the pneumatic atomization of a liquid and the electrostatic

classification of the polydisperse aerosol to be described in the next

section. The first method is applicable to larger particles over 0.5 ym

in diameter while the latter method is better suited for smaller

particles below 0.5 ym.

152
Figure 3 is a schematic diagram of the vibrating orifice monodisperse aerosol generator described by Berglund and Liu (1973). The

generator consists of a droplet generation and dispersion system and an aerosol dilution and transport system. A Krypton-85 radioactive

neutral izer of 10 millicurie activity is placed within the generator

drying column to neutralize the particle electrostatic charge incurred

during the droplet generation process.

ROTOMETER

ABSOLUTE

FILTER PRESSURE

AEROSOL

GENERATOR

SIGNAL GENENRATOR

VARIABLE TRANSFORMER BLOWER

DIFFERENTIAL PRESSURE GAUGE

DISPERSED

DISPERSION

AIR

Figure 3. Vibrating orifice monodisperse aerosol generator (Berglund and Liu. 1973). Top: schematic

of system. Bottom: generator head.

153
In the vibrating orifice generator, uniform liquid droplets are generated by forcing a liquid through a small (5 to 10 ym diameter),

vibrating orifice by a syringe pump at a predetermined rate of Qi cm^/s.

The orifice is vibrated by a piezoelectric ceramic at a predetermined

frequency of f Hz supplied by a signal generator. Within an appropriate

frequency range, the liquid jet is broken up into uniform droplets.

Since each cycle of the disturbance produces precisely one droplet, the

individual droplet volume is equal to

The uniform droplet stream is then injected axially along the center of

a turbulent air jet to randomize the particle motion and to prevent

particle collision and coalescence. The dispersed droplets are then

mixed with a much larger volume of filtered dry air to evaporate the

solvent and to obtain a stable aerosol of a particle diameter

where C is the volumetric concentration of the nonvolatile aerosol

material in the liquid solution.

Equation (2) shows that the diameter of aerosol particles produced

by the vibrating orifice principle can be calculated from the liquid

flow rate, Q] , the frequency, f, and the solution concentration, C.

Since these quantities can be easily measured to a high degree of

accuracy, the particle diameter also can be calculated to a high degree

of accuracy by means of eq (2). However, when dilute solutions (small

C) are used, the nonvolatile impurity in the solvent must be taken into

account in the calculation of particle diameter. Experiments (Berglund

and Liu, 1973) have shown that with proper precautions, the particle

diameter. Dp, can be calculated by means of eq (2) to a considerably

higher degree of accuracy than can be measured by the conventional

microscope techniques.

Table 1 is a summary of the operating conditions of the generator

for three orifice sizes, and figure 4 shows some typical particles

produced by the generator.

Theoretically the vibrating orifice generator can also produce

aerosols of a known particle concentration. The theoretical aerosol

concentration is given by

V, = Qi/f (1)

1/3

(2)

(3)

154
Table 1. Characteristics of the

aerosol generator.

vibrating orifice monodisperse

Diameter . , b of Particle"

1 i niii d Nomi nal Drool et^ c

MUiii 1 1 la 1 orifice frequency diameter range concentration

ym kHz ym ym parti cle/cm^

5 450 15 0.6 - 15 273

10 225 25 1.0-25 137

20 60 40 1.8-40 36

Continuously adjustable over an approximate 25% range by varing

.the frequency.

Obtainable by the solvent evaporation technique.

Theoretical concentration based on the nominal aerosol output

of 100 liters per minute.

Figure 4. Monodisperse particles produced by the vibrating orifice

generator (Berglune and Liu, 1973). a. 3.7 m diameter solid

methylene blue particles, b. 9.5 micrometer diameter liquid DOP

(di-octyl phthalate) particles collected on an oil -phobic slide

c. solid sodium chloride particles of 27.4 cubic micrometer

volume.

155

561-452 0 - 74 - 12
The actual aerosol concentration at the generator output is less than

the theoretical concentration due to particle loss in the drying and

neutralization chambers and in the transport system. However, the

operation of the generator is sufficiently stable so that after these

losses are determined, the aerosol concentration at the generator

output is known. Figure 5 (Liu, Berglund and Agarwal , 1973) shows the

measured aerosol concentration at the generator output expressed as a percentage of the theoretical concentration and as a function of

particle size. The actual concentration is seen to be about 81 percent

(±5%) of the theoretical concentration for particles smaller than 6 ym

diameter, and the actual concentration decreases steadily with

increasing particle size. This particular feature of the generator is

very convenient for determining the sampling efficiencies of aerosol

measuring and sampling devices. For instance, to determine the absolute

sampling efficiency of an optical particle counter it is necessary only

to apply the aerosol to the counter and to compare the counts calculated

from the known aerosol concentration and the sampling flow rate of the

counter. The generator has been used in a variety of studies including

the calibration of optical particle counters (Liu, Berglund and Agarwal,

1973), impactor calibration studies, and turbulent deposition studies

(Liu and Agarwal, 1973).

Figure 5. Output aerosol condensation of the vibrating

orifice aerosol generator as a function of particle

diameter (Liu, Berglund and Agarwal, 1973).

156
Submicrometer Aerosol Generator (SAG)

(edited from Whitby and Liu. 1973

For generating submicrometer aerosol standards below 0.5 ym, the

system shown in figure 6 is used. The device produces uniform particles

at concentration levels up to 10^ parti cles/cm^ by pneumatic atomization

and electrostatic classification. The system consists of a Collison

atomizer, a Krypton-85 radioactive neutral izer, a diffusion dryer, a differential mobility analyzer, and an electrometer current sensor.

COLUSON

ATOMIZER

Figure 6. Apparatus for generating submicrometer

aerosol standards (Liu and Pui, 1973).

In the apparatus a polydisperse aerosol is produced by the Collison

atomizer. The aerosol particles are then brought to a state of charge

equilibrium with bipolar ions produced by the ionizing beta radiation

from the Kr-85 source. Below a size of about 0.1 ym diameter, most of

the particles are either electrically neutral or carry one elementary

unit of charge with an electrical mobility of

1 = 300 eC/37r y cm^/V-s (4)

where e = 4.8 x 10' esu is the elementary unit of charge, C is the

dimensionless slip correction, y is the gaseous viscosity, and Dp (cm)

is the particle diameter. These singly charged particles can then be

separated into monodisperse fractions by means of the differential

mobility analyzer.

157
The differential mobility analyzer shown in figure 6 is in the form

of a cylindrical condenser with concentric electrodes. The inner

electrode is held at a high voltage and the outer tube is grounded.

Under a given set of operating conditions, charged particles in the

aerosol stream flowing along the outer tube are deflected through the

inner clean air core. If these particles have the appropriate

electrical mobility, they would arrive at the exit slit near the lower

end of the center electrode and be swept out by the air stream flowing

through the slit. The electrical mobility of the particles is given by

Zp = [Qq + 1/2 (Qg - Q^)] In (r2/r^) / 2 tt V L (5)

3 3

where Q (cm /s) is the total flow in the mobility analyzer, Qg (cm /s

is the aerosol flow at the inlet, Qg (cm^/s) is the sample aerosol flow

through the slit and at the generator output, and ri (cm) are the

outer and inner electrode radii, V (volts) is the applied voltage on the

center rod, and L (cm) is the distance between the aerosol inlet and the

exit slit.

The monodispersity of the aerosol at the generator output is determined by the ratio of flow (Qg + Qa)/Qo- In the experiments reported

by Liu and Pui (1973), the relative standard deviation of the aerosol

ranges from 0.04 to 0.08.

Since nearly all the particles classified by the differential

mobility analyzer were singly charged and the percentage of multiply

charged particles in the aerosol stream is small, particularly for small

particle sizes, the absolute concentration of the aerosol can be

measured by measuring the total current associated with the aerosol

particles,

I = qg e N (6)

The electrometer current sensor shown in figure 6 is used to measure

this current, from which the aerosol concentration, N, can be calculated

by means of eq (6)

Eight-Channel Multichannel Analyzer

Commercial multichannel analyzers (MCA's) are designed to provide a high resolution of the signals to be analyzed. Thus most MCA's have the

capability of at least 100 channels. However, in our laboratory, MCA's

have been used to analyze the signals from optical particle counters,

which, due to the limited size range of any one OPC (approximately 30 to

1 in particle size) and the desired resolution of the data, the data

from the MCA is sorted into just a few groups. Thus a new MCA is being

developed which has eight channels, making it unnecessary to group data

from a large number of channels. Also, this MCA has analog outputs

compatible with small analog data loggers.

158
Not only does this simplified MCA aid in data reduction, but it is

also relatively compact. One such unit is currently being equipped to

operate on batteries so that it can be used with a portable OPC (Royco

218) to make a portable OPC-MCA system.

Calibration of Condensation Nuclei Counters (CNC)

(by B. Y. H. Liu; edited from Whitby, 1974}

The condensation nuclei counter is an aerosol concentration measuring device with useful applications in aerosol, air pollution, and

meteorological research and in other scientific and technical work. The

instrument is capable of detecting very small particles (to about 20 A

in diameter) over a wide concentration range (from about 100 parti

cles/cm^ to 10' particles/cm^)

The Nolan-Pollak counter has been used for the past several decades

as a calibration standard for other nuclei counters. Recently we have

used the new submicrometer aerosol generator to perform a primary

calibration on the Nolan-Pollak counter (Liu, et al.^ 1974). The

results shown in figure 7 show that the Pollak calibration is in

remarkably good agreement with the new technique.

CALIBRATION

bJ<

0.1 I 10 100 AEROSOL CONCENTRATION (ELECTRICAL), Ng . lo' PARTICLES/cc

Figure 7. Calibration of the Nolan-Pollak condensation

nuclei counter against the electrical aerosol standard.

159
We have also used the new technique to calibrate automatic nuclei

counters such as the Environment/One and General Electric instruments

(Liu and Pui, 1973). A typical result for a factory-calibrated

Environment/One counter is shown in figure 8. It is seen from this

figure that the indicated count is nonlinear and is low by a factor of

2.5 up to about 100,000 particles/cm^ and by a factor of 6 at

concentrations of 500,000 parti cles/cm3.

140

1 1 1 1 1 1 1 1 1 1 1

120 CNC NO. 1 ( ENVIRONMENT/ONE, SERIAL NO. 149

)

' _

100

o

Dp, /im

.01

80

.016 .022

60

.032

.050 40

0 .075 .088

20

.150

n 1 1 1 1 1 1 1 1

1

1 1 0 50 100 150 200 250 300 350 400 450 500 550 6C ACTUAL NUCLEI CONCENTRATION, I0^( PARTICLES/cc

Figure 8. Response of the Environment/One condensation nuclei

counter as a function of particle size and concentration.

Calibration of the Electrical Aerosol Analyzer

(by D. Y. H. PuTTedited from Whitby, T9741

Because of the lack of a suitable aerosol standard, the calibration

of the electrical analyzers was difficult. The calibration constants

for the prototype unit were obtained using the earlier procedures of

Husar and Whitby (Whitby et al., 1972). These procedures consisted of

determining the space charge and diffusion loss experimentally.

Further, some theoretical calculation of the instrument has to be included. This method does not take into account the spreading effect of

the instrument. The detailed calibration of the EAA was thus limited

due to a lack of a primary aerosol standard.

The development of the submicrometer aerosol generator (SAG) enables us to perform a detailed calibration on the EAA. The SAG can produce uniform particles of a known size and concentration over a wide

range of values (Q.Ol ym to 0.5 ym in diameter at concentrations up to

10^ particles/cm^). The particle size can be calculated to an accuracy

of 2 percent and the concentration to an accuracy of 5 percent.

160
Therefore, it is particularly suitable for the primary calibration of

the EAA. The calibration is currently underway and has yet to be

completed. The outline of the calibration plan and the results of preliminary data are briefly described below.

It is required that the aerosol charge distribution be reduced to

Boltzmann's equilibrium before being introduced into the EAA.

Therefore, the singly charged particles from the SAG must first be

passed through a Kr-85 radioactive neutral izer in order that they may

attain the condition of charge equilibrium. The concentration must then

be determined by means of another instrument in place of the Faraday's

cage. The instrument used here for measuring concentration is an Environment/One Condensation Nuclei Counter (CNC). Its calibration was previously standardized against the SAG. Particles passing through the

neutral izer are first measured by the CNC before being introduced into

the EAA. Since the aerosol flow for the EAA was 4 1pm, 1 1pm of clean

air must be added to the 3 1pm of aerosol output from the SAG.

NaCl aerosols with diameters at the geometric midpoint of the standard size intervals of the EAA, namely, .024, .042, .075 ym, eto. , were

fed into the EAA. For each particle size a set of current vs. voltage

was obtained. Table 2 lists the data that we have obtained thus far.

The .042 ym diameter particles are generated by the vibrating orifice

generator (Berglund and Liu, 1973). Due to the large diameter reduction

(more than 20:1) for generating these small particles, the data may not

be accurate.

Note that for particles larger than .075 ym, the largest current

drop occurs in the size interval larger than the nominal size interval

of the EAA. This indicates that the presently used voltage for these

larger particle size intervals is in error and needs to be adjusted

later when the preliminary calibration is completed. The spread in

current seems to decrease as the particle diameter decreases. For the

.042 ym diameter, the current drop vs. size shows two peaks at .042 ym

and .133 ym corresponding to singly and doubly charged particles of the

chosen size.

The NaCl aerosol generated by the Colli son atomizer has a geometric

standard deviation of 2.65. It is therefore necessary to consider the

interference effects of double charged particles from the SAG. For this reason we plan to continue the calibration of the EAA with the

condensation aerosol generator (CAG) in place of the Colli son atomizer

in the SAG. With a geometric standard deviation of 1.3 for the CAG, the

interference effects can be ignored.

161
Table 2. Preliminary calibration data for the EAA.

I, 10" ampere.

EAA

Mean particle size (generated by SAG)-vim

D .-um

pi ^ .024 .042 .075 .133 .42

.0042 0 0 0 0 0

.0075 0 0 0 0 0

.01 33 .001 0 0 0 0

.0237 .029 0 0 0 0

.0422 .009 .10 0 0 U

.0/50 0 06 20 ) 02 A

u 133 0 .105 .27 ) 0

.237 0 .002 .042 .27 1 0

.422 0 .003 .025 .12 •O^Ma

.750 0 0 .016 .026 .018 )

Number

concentration 9 K 30 K 30 K 20.5 K .72 K

Preliminary calibration on the first production TSI Model 3030

EAA. The indicated particle sizes have been found to be larger

than the true sizes. This is primarily due to the fact that the

charger on this instrument was designed from theory and the precipitator voltages were not adjusted prior to calibration. A new voltage program chip is being designed to adjust the observed

sizes to agree with the true sizes.

Calibration of Optical Particle Counters (OPC)

Considerable work has been done in our laboratory on calibrations

with very monodisperse ideal particles (spherical oil droplets of known

refractive index) generated on the vibrating orifice monodisperse

aerosol generator. Work has also been done on calibration with nonideal

particles such as coal or rock dusts.

If the ideal particles from the Vibrating Orifice Monodisperse

Aerosol Generator (VOMAG) are used, the calibrations are relatively

simple. Figure 9 illustrates the pulse height spectrum, as recorded by

an MCA, of a counter exposed to aerosols produced by this method. In

figure 9(a) the aerosol is essentially monodisperse as indicated by the

162
3O

o

1500

1000 -

500

Figure 9. Pulse height spectra of an optical particle

counter exposed to aerosols from the vibrating orifice aerosol generator (Liu, Berglund and Agarwal

,

1973). a. under good dispersion conditions b. under poor dispersion conditions.

pronounced peak in the spectrum. In figure 9(b) additional peaks appear

which are produced by particles with two, three, or more times the

volume of the primary particle volume. These multiplet particles are obtained by deliberately operating the vibrating orifice generator under

poor dispersion conditions to increase the collisional frequencies

between the primary droplets. Figure 9(b) shows that for the specific

counter studied, the singlets and doublets with a volume ratio of 1:2 or

a diameter ratio of l:(2)'/3 or 1:1.15 are also resolved by the counter.

The triplets and quadruplets with diameter ratio of 1:1.10 are not

clearly resolved.

Although calibration is relatively simple for ideal aerosols shown

in figure 9, calibration with nonideal aerosols is considerably more

difficult. Such a calibration has been made on a Bausch and Lomb OPC to

size coal and rock dust particles.

The calibration was accomplished in two stages (Liu, Marple, Whitby

and Barsic, 1974). For particles smaller than 2.4 ym, monodisperse coal

dust was supplied to the OPC by using the differential mobility analyzer

identical to that used in the submicrometer aerosol generator. For

particles larger than 2.4 ym, a polydisperse coal aerosol was sized by

the OPC and the resulting MCA output compared to the particles, sized by

microscopy, which passed through the MCA.

In the first stage (<_2.4 ym calibration), monodisperse aerosols of

coal were generated by the system shown in figure 10. A small quantity

of finely ground coal is first aerosolized by means of the fluidi zed-bed

dust feeder and subsequently passed through the Kr-85 aerosol charge

neutral izer to reduce the particle electrostatic charge. The aerosol

163
Figure 10. System for generating monodisperse coal

aerosols for calibrating optical particle counters.

then enters a 0.2 cubic meter holding chamber where it is mixed with a much larger volume of clean filtered air. After a sufficient quantity

of coal dust has been generated and an adequate dust level has built up

in the chamber, the dust feeder is turned off. The aerosol from the

chamber is then passed through the differential mobility analyzer to

obtain particles within a narrow mobility and particle size range, which

are the calibration particles for the OPC. The value of the signal at

the mode of the distribution was used as the calibration for any

specific size.

In the second state (<_2.4 ym calibration), a polydisperse coal

aerosol within the appropriate size range was generated by means of the

f luidized-bed dust feeder as before, and the particles were sampled

directly into the OPC. The pulse height distribution was measured by

the MCA; simultaneously, these particles were collected by a membrane

filter after they had passed through the viewing volume. The collected

particles were subsequently measured by an optical microscope fitted

with a Portron reticle. The particle diameter was based on an equivalent circle having an area the same as the projected area of the

particle. Further analysis of the MCA and microscope data gave the

required calibration data in the large particle size range.

The resulting calibration from these techniques is given in figure

11. This shows the ideal Dioctyl Phthalate (OOP) calibration curve as well as the coal calibration. Note that for the larger sizes a coal

particle must be about five times as large as an ideal particle to give

the same signal level.

164
Figure 11. Calibration curve for the Bausch and Lomb

40-1 or 40-lA Dust Counter.

APPLICATION

Instrument Systems for Aerosol Size Distribution Measurement

(edited from Whitby and Liu, 19737"

Several aerosol size distribution measurement systems have been

built in our laboratory. Two of these systems were made mobile by

installing them in a semitrailer. One trailer was delivered to the Air

Resources Board of the State of California and the other was delivered

to the Environmental Protection Agency. Since both of these systems

were used to study atmospheric aerosols, the instrumentation and trailer

construction were similar.

Because of the broad range of sizes in most aerosols of interest,

and because of steep decreases in particle number with increasing size,

it is ordinarily not desirable to measure more than about one and one- half decades of size with any one instrument. Therefore, for our field

aerosol measurement work we have developed instrument systems which

combine a nuclei counter, an electrical counter, and up to three optical

165
counters in a single system. A mini -computer is used to control the

counters and to calculate a single size distribution from the different

instrument outputs.

Figure 12 is a schematic of a mobile air pollution laboratory which

we designed, built, and operated for the Air Resources Board of the

State of California in 1972 (Whitby et al., ]973). This used an Environment/One CNC, a Thermo-Systems , Inc. Model 3000 Whitby Aerosol

Analyzer for the 0.0075 to 0.42 ym range, and a modified Royco 245 for

the 5.6 to 38 ym range. It is interesting to note that total aerosol

volumes measured with this system are within ±30 percent of the volumes

calculated from total filter mass measurements. This indicates that

with proper calibration and operation these automatic instruments can achieve good absolute accuracy as well as providing ease of operation

and speed of measurement.

Figure 12. Schematic of a large mobile air pollution laboratory

constructed for the Air Resources Board of the State of California. This laboratory includes a comprehensive array of

automatic aerosol size analyzers, particle samplers, gas an- alyzer, and meterological instruments (Whitby et al.^ 1973).

Another system built in our laboratory was delivered to the Bureau

of Mines for the study of coal dust distributions in coal fragmentation

tests. This system was designed to measure the concentration and size

distribution dust in the 0.005 to 20 ym diameter range. As shown in

figure 13, three sensors are used in the system together with a data

acquisition system: the Whitby Aerosol Analyzer for size distribution

166
Bausch i Lomb

Optical Particle Sensor r

HP 5400 A Multichannel analyzer

HP 9125 A Calculator

Plotter

HP 9100 A

cal cul ator Buffer

HP extended memory

L _

HP 2570 A coupler and controller

Hewlett-Packard components

HP 2752 A Teleprinter

and Tape Punch

Aerosol IN

Whitby

Aerosol Analyzer

Crossbar Scanner

Digital Voltmeter

Frequency Meas . Kit

i

Quartz Crystal Mass Monitor

Aerosol IN

Figure 13. Block diagram of data acquisition and data reduction

system for the coal dust measuring system.

measurement in the 0.005 to 0.5 ym range, the Bausch and Lomb optical

particle counter for the 0.5 to 20 ym range, and a quartz crystal mass monitor sensor to measure the mass concentration of the coal dust.

The data acquisition system is comprised of several pieces of electronic equipment shown within the dashed area in figure 13. It is a Hewlett-Packard calculator-based system consisting of a multichannel

analyzer, a digital voltmeter with frequency measuring capability, a calculator, a teleprinter, a plotter, and a crossbar scanner connected

through a coupler/controller. The coupler/controller serves as the

master control for the entire system, but can then issue commands to

other pieces of equipment in the system. Since the mass concentration

can also be calculated from the size distribution data, the value can be

compared with the values determined directly by the quartz crystal mass monitor.

167
When the data acquisition and reduction system is used to determine

the size distribution of coal dusts, a "reset" or "start" button is

depressed on the coupler/controller. The coupler/controller then , instructs the WAA to begin operation and the multichannel analyzer to

begin accumulation of counts from the OPC. The WAA then steps through

its internal voltage sequence. At the same time, the MCA records j signals from the OPC for a predetermined length of time. After stepping j to a new voltage and after allowing about 15 seconds for the electrode ! current to settle to a steady value, the WAA sends a read signal to the

j coupler/ controller, which in turn commands the digital voltmeter to take

a current reading. This current reading is then stored in the

calculator's extended memory. The calculator is then instructed to

determine the size distribution from the WAA and the OPC data for

printout on the teleprinter or graphical display on the plotter.

The crossbar scanner enables the voltmeter to read data from various sources, such as the WAA, strain gages, temperature sensors,

humidity sensors, and other instruments whose signal needs to be re- corded by the same central data acquisition system.

The digital voltmeter includes a frequency measuring option which

enables the system to accept the frequency signals generated by the

quartz crystal mass monitor.

Summary of Current Knowledge of Fine Particles in the Atmosphere

(by K. T. Whitby, 1973)

Introduction

This brief paper is intended as a partial summary of current knowledge of fine particles gathered by the University of Minnesota Particle

Technology group both from their own laboratory research and collaborative field studies. Some conclusions from previously published

papers are repeated here, but data from these papers is generally

excluded. Some new and significant unpublished data is included.

Frequent reference is made to two recent papers, Whitby and Liu (1973)

and Whitby (1973), which present and discuss much of our most recent

knowledge about atmospheric aerosol size distributions and their

concentration.

Principal Conclusions About Atmospheric Aerosol

Particle Size Distributions

Following are some of the principal conclusions we have reached

concerning the nature of atmospheric aerosol size distributions. These

conclusions have been derived from field study data taken near the

earth's surface, predominantly in urban areas.

(1) The mass or volume distribution is usually bimodal with the

saddle point in the 1 to 3 ym diameter range (see fig. A). The particles larger than a few micrometers originate from natural or man-made

168
Fine Coarse

Mass

Particle diameter, ym

Figure A Atmospheric aerosol mass size distributions are usually

bimodal with about one-third of the total mass in the fine particle (or submicrometer) mode and two-thirds in the course particle mode.

mechanical processes. The mechanically produced particles are hereafter

called "coarse particles."

The particles smaller than a few micrometers arise predominantly

from condensation processes. These aerosols smaller than a few micrometers are called "fine particles." The predominant man-made source of

these fine particles is combustion or the condensation of chemical or photochemical reaction products on nuclei from combustion.

The fine particle size range may show two distinct surface area modes (see fig. B). The first mode in the vicinity of 0.02 ym diameter

results from the direct emission of combustion particles. The second

submicrometer mode in the 0.1 to 0.5 ym range is the result of either

the coagulation of primary particles or the condensation of reaction

products or water on primary particles.

Surface

.01

Combustion Nuclei Mode

.1 1 10 Particle diameter, ym

Figure B Atmospheric surface area size distributions may be trimodal near sources of combustion nuclei.

169
(2) There appears to be very little exchange of mass under most

conditions between the fine and coarse particle ranges in the atmosphere. This means that the origin and behavior of fine particles and coarse particles are different.

(3) Some typical values of the concentration of fine and coarse particles in the atmosphere derived from table 1 (Whitby, 1973) are as follows:

3 3 3 3

Clean air = 4 ym /cm (p = 1 g/cm ) and coarse particles = 9 yg/m

In moderately polluted urban areas, the fine particle concentration is

typically 50 yg/m^ and coarse particle concentrations are 120 vg/nr.

Because of the relative independence of fine and coarse particles,

both nonurban and urban sites have yielded widely varying ratios of fine

to coarse particles. On the average, however, the fine particles

constitute about one-third of the total aerosol mass.

(4) There appears to be a significant difference in the ratio of

fine to coarse background aerosol for California, Colorado, and the

Eastern part of the United States. The West Coast appears to be lower

in large particle concentration than does Colorado. A probable cause is

that Colorado is downwind of dry desert areas in which greater quantities of dust are suspended in the atmosphere and transported long distances .

(5) Because of the rapid coagulation rate between 0.01 ym size particles and particles in the 0.1 to 0.5 ym size range, the Aitken nuclei

count under heavily polluted conditions may be lower than when the air

is relatively clean. While the Aitken nuclei counter is not a good indicator of general pollution, it may be useful for indicating the proximity of sources of combustion aerosol.

(6) Chemical or photochemical aerosol formation mechanisms may or may not increase the atmospheric particle number depending upon the already existing surface area concentration. At surface area concentrations below about 500 ym^/cm^^, new nuclei may be formed and will appear

in the 0.01 to 0.02 ym size range. At surface area concentrations

greater than a few thousand ym^/cm^, the chemically or photochemical ly

converted material condenses primarily on particles in the size range

from 0.1 to 0.5 ym and may not form significant numbers of new nuclei.

For example, from some recent aircraft aerosol size distribution

measurements taken with the University of Minnesota electrical aerosol

analyzer in cooperation with EPA and MRI in St. Louis (September 1973),

it was observed that although there was a large amount of secondary

aerosol formed in a power plant plume, most of this aerosol mass was deposited on particles in the 0.1 to 0.5 ym size range.

170
On the other hand, evidence of secondary aerosol formation was observed in the inversions existing at 600 m (2000 ft) and at 2700 m

(9000 ft.) At 2700 m, because the existing surface area was small, this

secondary aerosol formed a mode at 0.02 ym.

(7) In a recent thesis, Lundgren (1973) reports the results of

measurements in which a special set of impactors was used to supplement

the Lundgren impactor to measure the mass distribution of aerosol up to

100 ym diameter. These data show that the mass distribution is bimodal

when measured directly. It also shows that particles up to an aerodynamic diameter of 100 ym or more exist under normal conditions in

the atmosphere.

Lundgren also found that perhaps 50 percent of the mass of the coarse particles consisted of organic materials such as insect fragments, fibers, and pollen. The remaining coarse particle mass was mineral matter.

Ambient Air Qual ity Standards

The fact that the origin and behavior of the fine and coarse particles in the atmosphere are independent has important implications

for the development of future air quality standards.

Perhaps there should be separate air quality standards for fine and coarse particles, and, correspondingly, separate methods for measuring

fine and coarse particles. A study of our data in urban areas also

suggests that a greater fraction of the fine particle mass than coarse particle mass is contributed by anthropogenic sources. It appears that

in areas where particulate pollution is of any consequence, better than

90 percent of the fine particle mass comes from anthropogenic sources whereas probably not more than one-half or two-thirds of the coarse

particles are anthropogenic. Thus the fine particles are a better

indicator of anthropogenic contributions to aerosol pollution than are the coarse particles. Total mass as measured by a high volume sampler

is of course a combination of the two. However, in general the fine

particles represent only one-third or one-half of the total mass. Thus

it seems that measurements of fine particle mass would be a better

indicator of anthropogenic particulate air pollution than total mass measured by a high volume sampler.

Classification and Names for Fine Particle Aerosol Levels

Fine particle aerosol concentrations vary greatly in background and

urban areas. It has become desirable to make an attempt to classify

areas of the world in terms of their fine particle aerosol concentration

and to assign names to these conditions and areas. Table 3 shows such a classification. Background conditions have been divided into four

categories and urban conditions into three.

The rationale behind this table and supporting data from field re- search has recently been presented by Whitby et at, (1974).

171

561-452 O - 74 - 13
Table 3. Background and urban aerosol parameters.

b . Volume Volume

Aitken

^ 4 -i (submicrometer) (total)

Type Number/cm xlQ" m" ym^/cm^ ym^/cm^

Background : Oceanic 100-400 .1 - .3 1-4

Clean continental 50-1000 .2-. 5 .5-2.5 2-10

Average continental 2-5 K .2 - .8 2.5 - 8 10-40

Urban district 5-15 K .8-3 8-30 20-60

Urban

Clean 50-1000 K .8-3 8-30 20-60

Average 100-200 K 3 35 70

Polluted 100-4000 K 3-15 30-150 100-300

REFERENCES

Berglund, R. N. and Liu, B. Y. H. (1973), "Generation of Monodisperse

Aerosol Standards," J. of Environmental Science and Technology 7:147-

153.

Liu, B. Y. H. and Agarwal , J. K. (1973), "Experimental Observation of

Aerosol Deposition in Turbulent Flow," submitted to of Aerosol

Science .

Liu, B. Y. H., Berglund, R. N., and Agarwal, J. K. (1973), "Experimental

Studies of Optical Particle Counters," submitted to of Atmospheric

Environment .

Liu, B. Y. H., Marple, V. A., Whitby, K. T., and Barsic, N. J. (1973),

"Size Distribution Measurement of Airborne Coal Dust by Optical Particle

Counters," Particle Technology Laboratory Pub. No. 183, University of

Minnesota.

Liu, B. Y. H. and Pui , D. Y. H. (1973), "A Sub-Micron Aerosol Standard

and the Primary, Absolute Calibration of the Condensation Nuclei

Counter," accepted by J. of Colloid and Interface Science .

Liu, B. Y. H., Pui, D. Y. H., Hogan, A. W., and Rich, T. A. (1974),

"Calibration of the Pollak Counter by Monodisperse Aerosols," submitted

to of Atmospheric Sciences .

Liu, B. Y. H., Whitby, K. T., and Pui, D. Y. H. (1973), "A Portable

Electrical Aerosol Analyzer for Size Distribution Measurement of Submicron Aerosols," presented at the 66th Annual Meeting of the Air

Pollution Control Association, Chicago, 111., June 24-28, 1973.

172
Whitby, K. T. (1974), "Summary Progress Report for EPA Grant No.

R801175, Generation and Decay of Small Ions and EPA Grant No. R800971

,

Sampling and Analysis of Atmospheric Aerosols," Particle Technology

Laboratory Pub. No. 228.

Whitby, K. T. and Clark, W. E. (1966), "Electric Aerosol Particle Counting and Size Distribution Measuring System for the 0.015 to 1 Micron

Size Range," Tellus 18:573-586.

Whitby, K. T., Clark, W. E., Marple, V. A., Sverdrup, G. M., Willeke,

K., Liu, B. Y. H., and Pui, D. Y. H. (1973), "Evolution of the Freeway

Aerosol," presented at the Symposium on Surface and Colloid Chemistry in

Air Pollution Control, ACS Annual Meeting, Chicago, 111., Aug. 29,

1973.

Whitby, K. T. and Liu, B. Y. H. (1973), "Advances in Instrumentation and

Techniques for Aerosol Generation and Measurement," presented at the

Harold Heywood Memorial Symposium, Loughborough, England, Sept. 17-18,

1973.

Whitby, K. T., Liu, B. Y. H., Husar, R. B., and Barsic, N. J. (1972),

"The Minnesota Aerosol Analyzing System," of Col loid and Interface

Science 39:136-164.

Whitby, K. T., "On the Multimodal Nature of Atmospheric Aerosol Size

Distributions," presented at the VIII International Conference on Nucleation, Leningrad, U.S.S.R., Sept. 24-28, 1973.

Lundgren, D. A., "Mass Distribution of Large Atmospheric Particles,"

Ph.D. Thesis, University of Minnesota (1973).

Whitby, K. T., Pui, D. Y. H., Sverdrup, G., and Clark, W. E., "Background Aerosol Size Distributions," presented at the 16th Annual Meeting

of the American Chemical Society, Los Angeles, Calif., Apr. 1, 1974.

DISCUSSION

GEORGE SINNOTT: It seems to me that in the calibration of your

electronic analyzer, you made the same measurement twice on a particle

with different results. You run through essentially a mobility analyzer

and run through the portable analyzer and get two complete sets. The

monodisperse aerosol is simply the results of the mobility analyzer?

MARPLE: Dr. Sinnott's question concerns the validity of the calibration procedure in which an aerosol size analyzer operating on the

principle of particle electrical mobility is calibrated by a monodisperse aerosol whose size is also determined from electrical mobility.

173
It is true that both the monodisperse aerosol generator and the

aerosol analyzer make use of the electrical mobility of the particles;

there are certain differences that should be noted.

In the monodisperse aerosol generator, particles carrying one elementary unit (4.8xl0"10 esu) of charge are used. Since the particle

charge is known precisely in this case, the particle size can be determined precisely by measuring the particle electrical mobility (see eq

4). Similarly the absolute particle concentration can be determined by

collecting the particles and measuring the current and flow (see eq 6).

On the other hand, the electrical aerosol analyzer makes use of a unipolar diffusion charger which places an unknown charge on the

particles. This charge must be determined experimentally as a function

of particle size. Further, the particle loss in the system due to

diffusion must also be determined experimentally. The sub-micron

aerosol generator provides an excellent source of uniform particles of

known size and concentration for these experimental purposes.

While the size and concentration of the monodisperse aerosols were calculated on the basis of electrical measurements, they have been

checked against an independent manual technique involving sampling the

particles, and sizing and counting them in the electron microscope.

Very good agreement has been obtained. We estimate the sizing and

concentration accuracies of the electrical method to be 2 percent and 5

percent respectively.

EDGAR ETZ: I noticed that your inorganic particles you produced were certainly not spherical. Are there ways of producing spherical

particles of inorganic substances? Have you tried to go this way?

MARPLE: The sodium chloride particles generated on the vibrating

orifice generator (fig. 4 (C) of paper) were not spherical. However,

the methylene blue particles (fig. 4 (a) of paper) are spherical.

ETZ: I would think that it has to do with the nature of the material that produces the particle.

MARPLE: Do you mean the shape of the particle?

ETZ: The shape of the particles coming from the aerosol.

MARPLE: The sodium chloride particles were crystals and the

methylene blue particles were spheres.

ETZ: You apply them to a high temperature flame, and in that

process none of them may come out as spheres.

MILTON KERKER: Dr. Etz, all of your colleagues from CI arks ton were

making particles from inorganic compounds.

ETZ: Out of ionic materials?

174
KERKER: Sodium chloride, vanadium pentoxide, silver chloride.

Yes, you are quite right, you can make them in a flame; or you can just

simply sublime them and cool them and you have beautifully spherical

particles.

SINNOTT: Dr. Kerker, was that the same size range you were talking

about--27 micrometers?

KERKER: On these were smaller, but I don't see why we couldn't.

LARRY DOEMENY: 27 micrometers?

KERKER: We haven't made them that large, yet.

MADHAV RANADE: We made some at the place I was working. We were

using a vibrating jet using molten liquids, molten sodium chloride, and

forcing it out under high pressure through a small orifice, and by that

method we could produce spherical particles.

ETZ: To what extent were they spherical? Were they crystallizing

on the surface of the sphere or what?

RANADE: It depends on the temperature. If they are hot enough,

the surface of the sphere solidified first, and essentially that is preserved, but if they cooled slowly, then they had a chance to develop

crystal linity. I don't exactly off-hand know what the temperatures

would have to be.

RAYMOND CHUAN: These aluminum oxide spheres coming out of a com- bustion process are very spherical. They go down to about 10-12 ym.

ETZ: Can you take time to describe briefly your coal dust generally? What makes the coal dust particle not an ideal particle to measure?

MARPLE: A nonideal particle as I described it is light absorbing,

and nonspherical . It is very irregular in shape.

ETZ: How did you produce it in your generator?

MARPLE: We used a fluidized aerosol generator. As shown in the

figure (fig. 10 of paper) the fluidized bed was obtained by placing

approximately 3 cm of bronze powder (100 ym diameter spheres) over a supporting screen in a 7.5 cm I.D. aluminum tube and flowing clean filtered air vertically upward through the column at the rate of 10 LPM.

The air was dried to avoid oxidation of the bed material. Finely ground

coal was placed in a small tube and introduced into the bottom of the

fluidized bed by a short burst of air. Microscopic examination of the

coal dust generated by this method showed that deagglomeration was thorough and complete even for submicron particles which are difficult

175
to deagglomerate by conventional methods such as aerodynamic impaction

or turbulent dispersion.

ETZ: Why did you use amorphous materials?

MARPLE: Coal particles were used on the calibrations material be- cause that is the material of the particles which the optical particle

counter was to measure. Generally, the optical particle counter calibration curve is dependent on the refractive index and shape of the

particles being sized. Our calibrations were made with coal dust produced in a ball mill so the particles were modular in shape. However,

when we examined coal dust as produced at a cutting tool we found that

the particles were spears, platelets, and very irregular in shape.

176
LIST OF ATTENDEES

i Speakers

Ilan Chabay

Department of Chemistry

University of Illinois

Urbana, IL 61801

j R. Chuan, Staff Assistant

! Celesco Industries Inc.

Environmental and Industrial Products

Costa Mesa, CA 92626

Reg Davies

IIT Research Institute

Chicago, IL 60116

Laurence Doemeny

Engineering Branch

Division of Laboratories and

Criteria Development

NIOSH

Cincinnati, OH 45202

Cary Gravatt

Office of Associate Director

for Programs

National Bureau of Standards

Washington, DC 20234

Geoffrey Hotham, President

Laser Holography Inc.

Santa Barbara, CA 93108

Milton Kerker, Dean

The School of Arts and Sciences

Clarkson College of Technology

Potsdam, NY 13676

Robert Knollenberg, President

Particle Measuring Systems, Inc.

5469 Western Avenue

Boulder, CO 80301

Albert McSweeney, Research Physicist

Experiment Station

Georgia -Institute of Technology

Atlanta, GA 30332

Virgil Marple

Department of Mechanical Engineering

125 Mechanical Engineering Building

University of Minnesota

Minneapolis, MN 55455

Charles Parmenter

Department of Chemistry

Indiana University

Bloomington, IN 47401

M. Ranade, Research Chemical Engineer

IIT Research Institute

Chicago, IL 60116

William Yanta

Aerophysics Division

Naval Ordnance Laboratory

Silver Spring, MD 20910

Other Attendees

Wendell Anderson, Head

FC Division

U.S. Naval Weapons Laboratory

Dahlgren, VA 22448

George Carson, Chief

Personal and Environmental Air

Measurements Section

Center for Disease Control, NIOSH

Cincinnati, OH 45202

Wayne Cassatt, Coordinator

Particle Technology Group

Surface Microanalysis Section

Analytical Chemistry Division

National Bureau of Standards

Washington, DC 20234

Robert Coon

Bureau of Foods

Food and Drug Administration

Washington, DC 20204

177
Heinz Eiermann, Acting Director

Division of Cosmetics Technology

Food and Drug Administration

200 C Street, SW

Washington, DC 20204

Edgar Etz

Particle Technology Group

Surface Microanalysis Section

Analytical Chemistry Division

National Bureau of Standards

Washington, DC 20234

Kurt Heinrich, Chief

Surface Microanalysis Section

Analytical Chemistry Division

National Bureau of Standards

Washington, DC 20234

William Kirchhoff

Measures for Air Quality Program

Institute for Materials Research

National Bureau of Standards

Washington, DC 20234

Albert Kolbye, Jr., Acting Director

Office of Sciences

Food and Drug Administration

200 C Street, SW

Washington, DC 20204

Philip LaFleur, Acting Chief

Analytical Chemistry Division

National Bureau of Standards

Washington, DC 20234

James McNesby

Measures for Air Quality Program

Institute for Materials Research

National Bureau of Standards

Washington, DC 20234

Bernard McNammarraa

Toxicology Division

Biomedical Laboratory

Edgewood Arsenal, MD 21010

I

I

I Ronald Nelson ! Chemistry and Physics Laboratory | Environmental Protection Agency | Research Triangle Park, NC 27709 i

Edmond Owens, Chief I Environmental Toxicology Branch

Toxicology Division

Biomedical Laboratory { Edgewood Arsenal, MD 21010

George Sinnott, Acting Chief

Program for Physics and Dynamics

Fire Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, DC 20234

George Thompson

Division of Cosmetics Technology

Food and Drug Administration

200 C Street, SW

Washington, DC 20204

Jack Weimer, Assistant Chief

Environmental Toxicology Branch

Toxicology Division

Biomedical Laboratory

Edgewood Arsenal, MD 21010

Gale Wyer, Director

Division of Chemistry

Bureau of Biomedical Science

Consumer Product Safety Commission

200 C Street, SW

Washington, DC 20204

Robert Williamson, Jr.

Naval Ordnance Laboratory

Silver Spring, MD 20910

178
AUTHORS INDEX

*Almich, B. . . . *Carson, G. . . . Chabay, I. . . . Chuan, R. L. Davies, R. . . . Doemeny, L. Gravatt, C. C.

Hotham, G. A.

Kerker, M. . . . Knollenberg, R. G.

Marple, V. A.

*Marshan, T. R. McSweeney, A.

Parmenter, C. S.

Ranade, M. B. *Seaver, M. . . . *Stockhani, J. D. Yanta, W. J.

*Co-author

179
Key Word Index

Entries were taken from the key words given by authors of

the papers printed herein. Reference is to the person who

presented the paper at the seminar. Although this index is

not exhaustive, it should serve as a subject guide.

Aerosol

cloud chamber

- Chabay

electrical analyzer

- Marple

generators

- Marple

impact studies

- Chuan

- Davies

- Hotham

instrument calibration

- Marple

light scattering

- Chabay

- Davies

- Gravatt

- Kerker

- Knoll enberg

- McSweeney

- Parmenter

- Ranade

review

- Davies

size measurements

- Chabay

- Chuan

- Davies

- Doemeny

- Gravatt

- Hotham

- Knoll enberg

- Marple

- McSweeney

- Parmenter

- Ranade

- Yanta

spectrometer

- Chabay

- Gravatt

- Hotham

- Knoll enberg

- Marple

- McSweeney

- Parmenter

- Ranade

- Yanta

sprays

- Davies

- Hotham

- Parmenter

- Ranade

- Yanta

therapeutic

- Ranade

Beta absorption

- Doemeny

Cascade impactor

- Chuan

Coal dust monitor

- Doemeny

Detection of particles

by impaction

- Chuan

Doppler measurements of

particle size

- Chabay

- Yanta

Droplet

cloud measurements

- Chabay

- Knoll enberg

condensation on

- Ranade

evaporation of

- Ranade

imaging

- Davies

-Hotham

- Knoll enberg

180
sizing

- Davies

- Hotham

- Yanta

Dust inhalation hazards

- Doemeny

Electrical mobility

- Davies

- Marple

Environmental sampler

- Doemeny

Fibers

- Kerker

Interferometer

- Knoll enberg

- Yanta

Laser heterodyne

- Chabay

Laser holography

- Hotham

Laser imaging of particles

- Davies

- Hotham

- Knoll enberg

Laser light scattering by

aerosols

- Chabay

- Davies

- Gravatt

- Knoll enberg

- McSweeney

- Parmenter

- Ranade

- Yanta

scattering diagrams

- McSweeney

- Parmenter

Momentum methods

- Davies

Nuclei counters

- Marple

Optical transform

- McSweeney

Particle

chemical characterization

- Gravatt

cloud chamber

- Chabay

electrical analyzer

- Marple

fire produced

- Gravatt

generators

- Marple

imaging

- Davies

- Hotham

- Knoll enberg

impact studies

- Chuan

- Davies

- Hotham

instrument calibration

- Marple

light scattering

- Chabay

- Davies

- Gravatt

- Kerker

- Knoll enberg

- McSweeney

- Parmenter

- Ranade

refractive index

- Gravatt

- Kerker

- Parmenter

review

- Davies

size measurements

- Chabay

- Chuan

- Davies

- Doemeny

- Gravatt

- Hotham

- Knoll enberg

- Marple

- McSweeney

- Parmenter

- Ranade

- Yanta

spectrometer

- Chabay

- Gravatt

- Hotham

181
- Knoll enberg

- Marple

- McSweeney

- Parmenter

- Ranade

- Yanta

standards

- Marple

therapeutic

- Ranade

360° scattering by

- Parmenter

velocity measurements

- Yanta

Particulates (see particles)

Piezoelectric crystal

- Chuan

Quartz crystal microscope

- Chuan

Respirable dust sampler

- Doemeny

Smoke detector

- Gravatt

182
NBS-1I4A (REV. 7-73) —U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NRS Special Pr.hlirafinn 41?

2. Gov't Accession

No.

4, TITLE AND SUBTITLE

AEROSOL MEASUREMENTS, PROCEEDINGS OF SEMINAR

ON AEROSOL MEASUREMENTS, MAY 7, 1974

3. Recipient's Accession No.

5. Publication Date October 1974

6. Performing Organization Code

7. AUTHOR(S)

W. A. Cassatt and R. S. Maddock, editors

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Bureau of Standards

Food and Drug Administration, Washington, D.C,

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Number: 74- 600164

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survev, merition it here.) Papers followed by discussions were given in a closed seminar and workshop sponsored by the National Bureau of Standards and the Food and Drug

Administration to define the state of development of aerosol measuring

instruments. The instruments discussed were based upon a variety of

operating principles including laser light scattering, optical imaging,

Doppler shift, electromobility , piezoelectric effect, and beta-ray absorp

tion. Two review papers were given which described other phenomena upon

which aerosol measurements are based. The general summary includes a

table which lists the specifications of the instruments discussed to

illustrate the range of capabilities available in this field. Discussion

among seminar attendees revealed that many questions remain to be answered

before the more difficult aerosol measurements problems can be solved.

For example, in the analysis of very dense aerosols questions arise concerning coincidence losses or agglomeration effects that may result from

collisions between particles as they are drawn into the measuring volume.

Volatilization or condensation effects may alter the size distribution if

the measurements are made late in time. Finally, variations in particle

shape or index of refraction can alter the instrument response and cause

difficulties in interpretation.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper

name; separated by semicolons

Aerosol instrument performance; aerosol measuring instruments; beta-ray

absorption; Doppler shift; electromobility; laser light scattering;

optical imaging; piezoelectric effect

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U.S. GOVERNMENT PRINTING OFFICE ; 1974 0 - 56I-452 USCOMM-DC 29042-P74
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NBS TECHNICAL PUBLICATIONS

PERIODICALS JOURNAL OF RESEARCH reports National Bureau

of Standards research and development in physics,

mathematics, and chemistry. Comprehensive scientific papers g-ive complete details of the work, including

laboratory data, experimental procedures, and theoretical and mathematical analyses. Illustrated with photographs, drawings, and charts. Includes listings of other NBS papers as issued.

Published in two sections, available separately:

• Physics and Chemistry (Section A)

Papers of interest primarily to scientists working in these fields. This section covers a broad range of physical and chemical research, with major emphasis on

standards of physical measurement, fundamental constants, and properties of matter. Issued six times a year. Annual subscription: Domestic, $17.00; Foreign,

$21.25.

• Mathematical Sciences (Section B)

Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design, numerical analysis, theoretical physics and chemistry, logical design and programming of computers and computer

systems. Short numerical tables. Issued quarterly. Annual subscription: Domestic, $9.00; Foreign, $11.25. DIMENSIONS/NBS (formerly Technical News Bulletin) —This monthly magazine is published to inform

scientists, engineers, businessmen, industry, teachers,

students, and consumers of the latest advances in science and technology, with primary emphasis on the work at NBS. DIMENSIONS/NBS highlights and reviews such

issues as energy research, fire protection, building

technology, metric conversion, pollution abatement,

health and safety, and consumer product performance.

In addition, DIMENSIONS/NBS reports the results of Bureau programs in measurement standards and techniques, properties of matter and materials, engineering standards and services, instrumentation, and automatic

data processing.

Annual subscription: Domestic, $6.50; Foreign, $8.25.

NONPERIODICALS Monographs—Major contributions to the technical liter- ature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and

industrial practice (including safety codes) developed

in cooperation with interested industries, professional

organizations, and regulatory bodies. Special Publications—Include proceedings of high-level national and international conferences sponsored by

NBS, precision measurement and calibration volumes,

NBS annual reports, and other special publications

appropriate to this grouping such as wall charts and

bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists,

engineers, chemists, biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides

quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a world-wide program coordinated by NBS. Program under authority

of National Standard Data Act (Public Law 90-396). See also Section 1.2.3. Building Science Series—Disseminates technical infor- mation developed at the Bureau on building materials, components, systems, and whole structures. The series presents research results, test methods, and performance criteria related to the structural and environmental functions and the durability and safety characteristics of building elements and systems. Technical Notes—Studies or reports which are complete

in themselves but restrictive in their treatment of a

subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under pro- cedures published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations.

The purpose of the standards is to establish nationally

recognized requirements for products, and to provide

all concerned interests with a basis for common understanding of the characteristics of the products. The

National Bureau of Standards administers the Voluntary Product Standards program as a supplement to the activities of the private sector standardizing

organizations. Federal Information Processing Standards Publications (FIPS PUBS)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The purpose of the Register is to serve

as the official source of information in the Federal Gov- ernment regarding standards issued by NBS pursuant

to the Federal Property and Administrative Services Act of 1949 as amended. Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717

(38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations). FIPS PUBS

will include approved Federal information processing standards information of general interest, and a complete index of relevant standards publications. Consumer Information Series—Practical information, based on NBS research and experience, covering areas

of interest to the consumer. Easily understandable language and illustrations provide useful background

knowledge for shopping in today's technological

marketplace.

NBS Interagency Reports—A special series of interim

or final reports on work performed by NBS for outside sponsors (both government and non-government). In

general, initial distribution is handled by the sponsor;

public distribution is by the National Technical Infor- mation Service (Springfield, Va. 22151) in paper copy

or microfiche form. Order NBS publications (except Bibliographic Subscription Services) from: Superintendent of Documents, Government Printing Office, Washington, D.C. 20402. BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey

bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service

(Publications and Reports of Interest in Cryogenics).

A literature survey issued weekly. Annual subscription: Domestic, $20.00; foreign, $25.00. Liquefied Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00. Superconducting Devices and Materials. A literature survey issued quarterly. Annual subscription: $20.00. Send subscription orders and remittances for the preceding bibliographic services to the U.S. Department

of Commerce, National Technical Information Service, Springfield, Va. 22151. Electromagnetic Metrology Current Awareness Service (Abstracts of Selected Articles on Measurement

Techniques and Standards of Electromagnetic Quantities from D-C to Millimeter-Wave Frequencies).

Issued monthly. Annual subscription: $100.00 (Special rates for multi-subscriptions). Send subscription order and remittance to the Electromagnetic Metrol- ogy Information Center, Electromagnetics Division, National Bureau of Standards, Boulder, Colo. 80302.
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