Atmospheric Enwonmeni Vol. 16. No. 5. PP. 955-958.1982 ooo4698 I !SZ~O_WSS-w $03.00/O

Printed in Great Britain. t 1982 Pergamon Press Ltd.

PARTICLE SIZE SENSITIVITY OF CONDENSATION NUCLEUS COUNTERS

DAVID SINCLAIR

Environmenta Measurements Laboratory, U.S. Department of Energy. New York, NY 10014. U.S.A.

Abstract-The counting efficiency of condensation nucleus counters as affected by aerosol particle size has been studied by many investigators. Literature on this subject was reviewed and it was found that the size for 100;” efficiency varies from 0.01 to 0.09 pm depending on the type of counter, the nature of the particle and the investigator. The minimum detectable diameter varies from 0.0014 to 0.005~m. Measurements made at the Environmental Measurements Laboratory (EML) showed that the efficiency of the Pollak and the TSI falls to about 15 ;b at diameters of 0.005 pm. The General Electric and the Environment One do not show this effect, at least when compared with the EML continuous flow counter.

The lower limit of particle size and the variation of counting efficiency with particle size observable in condensation nucleus (CN)counters have been studied theoretically and experimentally by many investi- gators. The lower limit, the minimum size of foreign particle on which vapor of a given supersaturation will condense, may be calculated from the Kelvin equation. This equation shows that the minimum size varies inversely and exponentially with the supersaturation of the vapor.

In most CN counters, su~r~turation is obtained by intermittent adiabaticex~nsion,and hencecooling, of previously humidified aerosol. By contrast, there are two continuous-flow counters in which supersatu- ration is obtained by passing the aerosol over a heated pool of alcohol and then through a cold chamber. In all counters the presence of aerosol is detected by observ- ing light either scattered or transmitted by the liquid droplets condensed on the aerosol particles. The adiabatic expansion method has the advantage that supersaturation occurs uniformly throughout the aerosol volume, while the continuous-cooling type must rely on adequate mixing of alcohol vapor and aerosol with cold air.

Theory and experiment do not usually agree as to minimum size; the minimum size observed in the adiabatic type CN counters is usually greater than the size calculated from the Kelvin equation. In continuous-cooling or continuous-flow counters there is no simple way to calculate the minimum size to be expected, since the supersaturation depends on the amount of mixing, wall losses, etc.

Most expansion type CN counters use an adiabatic expansion ratio of about 1.22 to obtain a temperature drop of about 16” C. This corresponds to a supersatu- ration of 200”,b, that is, 200”,; above 100% relative humidity, or ~turation. The theoretical minimum

diameter for this condition is 0.002 pm, but the experimental value is usually greater.

One investigator (Pedder, 1971, 1974) found close agreement between theory and experiment (Table 1). He observed the spontaneous growth of radiolytic aerosol in a closed chamber containing filtered lab- oratory air. The initial size observed with a Pollak counter and diffusion battery was 0.002 pm diameter. He also found that the minimum size for 1007; counting efficiency was 0.01 pm.

Preliminary measurements reported by Walter and Jaenicke (1973) gave a minimum diameter of 0.005 pm in a CN counter developed by Verzar (1953).

In a remarkable experiment Hart et al. (1973) and Schmidt (1975) used a special type of CN counter in which the effects of an “adiabatic” expansion of 1-2 s duration could be observed. They found minimum diameters of sulfate aerosol of 0.002-0.01 pm depend- ing on the overpressure or expansion ratio. The diameter of 0.01 pm corresponds, from the Kelvin equation, to an overpressure of 1.04.

Perrin et al. (1977) observed the initiation of radiolytic aerosol by actinon (219Rn) in atmospheric air. They found that the minimum diameter detectable by the General Electric (GE) counter was O.~l4~.~19~m. These sizes correspond to super- saturations of 374 and 215 “/,, respectively. The former value exceeds the critical supersaturation for water, 3407; (Twomey, 1977). at which homogeneous nu- cleation of water vapor occurs. It also exceeds the theoretical value produced by the adiabatic expansion in our GE counter. This has varied over the years from 263 to 215 “/,. corresponding to an overpressure of 203 and 178 mm of Hg, respectively. By extrapolation of their experimental curves they found a “critical” diameter of 0.001-0.0015 pm for the initiation of the radiolytic aerosol. These sizes correspond to very high su~r~turations, 785 and 327 y,, respectively.

Cooper and Langer (1978) measured the perform-

955

References t. ‘ounter

Pedder (1974) Pedder (1971) Walter and Jaenicke

(1973)

Hart er u[. (1973) Schmidt (1975) Perrin rr ol. (1977) Cooper and Langer

(1978) O’Connor (1975)

Hollander and Schumann (1979)

Lm and Kim (1977)

POkik

Pollak Verzar (calcu

lated) Special Special

GE GE

E!One-100 Pollak E/one portable

E/One- 100 Pollak

E/One-100 GE

Haaf and Jaenicke (1977) Verzar Bricard et al. (1976) CFC Agarwall and Sem (1980) TSI-CFC

Madelaine and Metayer (1980) TSI-CFC

‘i,,,,li’. .‘, :

j’ I, d‘i

\jd i,iIl,) Ijlrnl Marerkdi

!I.002 Rad~oiytrc 0.01 100” 0 Chromturn

Ci.tU5 0.002

0.002 0.01 Sulfate 0002~0.01 Photochemical

0.0014 0.0019 0.001 -0.0015 Radiolvtic 0.03. 100 % 0.05 IO0 “I

0.0025 0.0025

AgI, NaCl, paraffin

Room aerosol Room aerosol

0.0016

0.04 100”~ 0.008 27’;;

0.0032 3‘ 0.002

0.09- IO0 ‘%; 0.005 lo”:,

o.o-5Of.7, 0.01 70% O.Oll9Ol%; 0.0480 ‘% 0.04- 90 ‘;; 0.04- 95 I’/ / 0

* Minimum detectable diameter. t Diameter for indicated efficiency. $ Diameter to start condensation.

ante of the GE and the Environment One-100 coun- ters using Agl. NaCl and parafhn aerosols. They found that the minimum diameter for 100”,, counting ef- ficiency was 0.03 pm for the GE counter and 0.05 pm for the Environment One counter.

O’Connor (1975) used the Pollak counter to cali-

brate the Environment One portable counter and the

Environment One-100 counter. He found that the

minimum diameters observable by these counters were 0.0025 and 0.0016 pm, respectively. These values are based on the assumption that the minimum diameter observable by the Pollak is 0.0025 pm, calculated from the Kelvin equation.

Other studies of the Pollak and Environment One-

100 counters were made by Hollander and Schumann (1979). They made aerosols by spray-drying solutions of various salts such as LiCl and KCI. They found, for example, critical diameters of 0.001 pm for LiCl and 0.01 pm for KCl. These sizes were derived from calculations based on the concentration and droplet size of the salt solutions used.

Studies of the GE counter by Liu and Kim (1977) showed that the minimum diameter for lOO’>,, ef- ficiency was 0.04 pm and the efficiency was 27 7, for 0.008 pm particles. These measurements were made on NaCl particles from spray-dried solutions. The size 01 the particles was measured by their electrical mobility.

0.001 0.01

Room aerosol Lic‘l KC-I

NaCl

H,SO,

NaC‘l

v,o, Monodisperse

HN, Monodisperse NaCl Monodisperse

V,G, Monodisperse

HzSG, Monodisperse NaCl Monodisperse

0.002 ii i bl pm

0.002 0 04 nm 0.001 0111 pm Monod)~perse

Polydisperse Polydirpersc

Polydrsperse Pnlydisperse

Monodisperse

Polydisperse Not given

Monodisperse

The Verzar (1953) counter was used by Haaf and Jaenicke (1977) to study the photo-oxidation products of SO,. They found that the minimum observable size of “dry” H,SO, droplets was 0.0032 pm diameter.

A commercially available continuous-flow counter is made by Thermo-Systems. Inc.. St. Paul, MN. It is based on a development by Bricard, Delattre. Madelaine and Pourprix (1976). In this device aerosol

flows through a porous tube wet with butyl alcohol heated to 35’ C and thence through a chamber cooled thermoelectrically to 10 C. Light scattered by the issuing alcohol drops is measured photoelectrically from one drop at a time at low concentrations or from many drops simultaneously at high concentrations. Agarwal and Sem (1980) report that the measurement efficiency of 0.005 pm diameter condensation nuclei is lo’:,,.

Madelaine and Metayer (1980) found a range of efficiencies in the TSI continuous-flow counter which depended on the particle diameter and the chemical composition of the aerosol.

EXPERIMENTAL MEASUREMENTS

At the Environmental Measurements Laboratory (EML) we have developed a continuous-flow counter (CFC) which

Particle size sensitivity of condensation nucleus counters 957

uses ethyl alcohol (Sinclair and Hoopes, 1975). The CFC contains an ethanol pool heated to 30” C. Aerosol flows over this pool and hence through a cold chamber cooled thermo- electrically to - 10°C. Measurements are made on the light transmitted through a 30.5 cm path length in the fog. Under these temperature conditions the theoretical vapor pressure ratio is 14: 1. This would cause self-nucleation if the mixing were uniform and complete, but it is evident that much alcohol vapor is lost to the cold chamber walls and there is a significant time lag in cooling the vapor-aerosol mixture. Fortunately, the process is aided by the fact that the diffusion coefficient of ethanol is 0.0994, while the thermal diffusivity is 0.179.

We have studied the performance of the CFC with submicron aerosols of silver and gold generated in an induction furnace, carnauba wax from a tube furnace, spray- dried NaCl and AgC1, and room aerosol. The CFC was calibrated with the GE, Environment One-100, and the Pollak condensation nucleus counters. Both the GE and Environment One were found to require occasional recali- bration. The calibrations of the Pollak and the CFC, if properly operated, do not change. (The concentration cali- bration and relative performance of condensation nucleus counters will be considered in detail in a subsequent paper.)

Particle size of the silver and gold was measured with the CFC and the Pollak in conjunction with the portable diffusion battery, previously described (Sinclair, Countess, Liu and Pui, 1979). (See ‘Table 2 for additional information.) The CFC was designed to have the constant flow required by diffusion batteries. The GE and Environment One were also used with the diffusion battery to measure particle size, but they both require pulsation dampers which remove many of the smallest particles. Exact knowledge of the concentration calibration is not necessary for particle size measurements since they are based on relative values of the penetrations through the different lengths of the diffusion battery.

Figure 1 shows calibration curves of the CFC made with the General Electric, Environment One and the Pollak for different particle sizes and materials. The upper curve (0) was obtained with the General Electricand the Environment One for all sizes from 0.005 to 0.2 pm, and with the Pollak for diameters of 0.02-0.2 pm. The 0.02&0.2~m particles were spary-dried NaCI, thermally generated carnauba wax, and silver metal and gold metal. The curves for 0.01 pm ( x ) and for 0.005 wrn (0) were obtained with the Pollak. These particles were silver metal.

The Pollak curves for smaller particle sizes are lower because the Pollak is less efficient for small sizes. The Pollak has less supersaturation than the CFC and many small particles diffuse to the walls during the 30s waiting period between compressing the sample and expansion. Calculations (Sinclair, 1975) show that 40 % of 0.005 pm particles and 20 I%: of 0.01 pm particles diffuse to the walls during this period. We

O-GE, E/I 0 005-O 2pm

Pollak 0 02 -0 2 pm

x -FQllak 0 01 /em l -Pollak 0 005pm

doom

01 I I I I I 0 20 40 60 80 100

J

CFC % light tronsmlsslon

Fig. 1. Calibration curves of Pollak and continuous-flow condensation nucleus counter showing decrease in efficiency

of Pollak with particle size.

have not observed inefficiency in the CFC. probably because of the high supersaturation. Also there is excess alcohol vapor available when the concentration is high. On the other hand, there is a fixed amount of water vapor in the Pollak, determined by the adiabatic expansion ratio. This accounts, we think, for the convergence of the curves for all particle sizes at the lower concentrations.

The above results may be compared with those of Pedder (1971,1974) who found 100% efficiency in the Pollak for sizes > 0.01 pm. He was also able to observe 0.002 pm diameter particles, but the efficiency is not stated. The calculations show that about 80% of this size would diffuse to the walls during the 30s waiting period. O’Connor (1975) was able to observe 0.0025 pm in both the Pollak and the Environment One, but no efficiency is given.

Comparison with the EML-CFC and the E/l and Pollak does not show a decrease of efficiency with particle size in the

Table 2. Aerosol charactertstics

Material Diameter Size distribution Concentration

(pm) (pm) (lo3 cm-j)

Silver metal* 0.07 * 0.005 230 Silver metal 0.01 * 0.001 140 Silver metal 0.005 * 0.001 50 Gold metal* 0.04 * 0.005 200 Sodium chloridet 0.025 1.95)) 100 Carnauba wax+ 0.225 1.1)) 500

* Method of measurement -diffusion battery and nucleus counter. t Method of measurement- electrical aerosol analyzer (Liu and Pui. 1975). $ Method of measurement-active scattering aerosol spectrometer

(Knollenberg and Luehr, 1975); and electron microscope. $ Count median. II Geometric standard deviation.

GE counter as found by I_lu and Km? (lY77), and Cooper and Langer (1978). The overpressure IS higher m the GE (1.23-1.27) than in the Pollak (1.21) so that the GE should be more efficient than the Pollak. Our observationa are more m accord with those of Perrin <‘I 01. (1977) who found that the minimum observable diameter in the GE was about CI 002 irm. although no efficiency is giv-en

The continuous-flow C’N counter produced by l‘hermo Systems, Inc., uses butyl alcohol at a vapor pressure ratio of 5.7: 1. much less than the 14: 1 in the EML-CFC. Agarwal and Sem (1980) report IO’ (, efficiency at 0.005 pm. WC observed about 15 ‘II, efficiency in their counter at 0.008 pm and below. This is probably caused by the low supersatu- ration and by wall losses. They also report a loss of efficiency beginning at 0.09 pm which seems high. We prefer the use of ethanol even though it absorbs water, which butanol does not. We have found no change in the calibration with as much as 25” 0 water in the ethanol

SUMMARY AND CONCLUSIONS

The performance of condensation nucleus counters may vary for several reasons:

, s

and hygroscopicity of the aerosol particles .- the more

(a) The counting efficiency decreases with decreas- ing particle size. This deficiency is caused primarily by

hygroscopic the less the influence of particle size.

too low supersaturation of condensible vapor. The effect is also influenced bv the chemical comoosition

(b) The overall calibration of the counter may vary. This is oarticularlv true of automatic counters having

Haaf W. and Jaenicke R. (IY77! Determmatr,rn t:l ti#, smallest particle size detectable in condensation nucieu3 counters by observation of the coagulation of SO. phoro oxidation products. J. Aerosol SC i 8, 441 456

Hart E. J.. Schmidt K. H. and k.:sudrvao K. :y I iti?:ii Condensation nucleus discrimrnator making aptlcai measurements in fog: A tool for c:nvlrrmmentai -~:<;earch Science 180, 1064 1067.

Hollander W. and Schumann G. ( 1979) The determmdtlon oi particle size detection efficiency of condensation nucleus counters by means of polydisperse aerosols produced from soluble salt solutions and the problem of particle shatter ing upon crystallization. .4rmosyh~:(~ Fnl~lrnnrui:~~;t 13, 1319 -1327.

Knollenberg R. G. and Luehr R. ri3?S) Open cater:, laser “active” scattering particle spectrometry. In Fmr P~;rti&.r (Edited by B. Y H. Liu) pp. 669 -696. Academic Press. New York.

Liu B. Y. H. and Pui D. Y. H. (1975) On the performance of the electrical aerosol analyzer. J. Aerosol Sci. 6, 249.-264.

Liu B Y. H. and Kim C. S. (197?) On the counting efficiency ofcondensation nuclei counters -~tmosph~~r~ Em ;ronmc’nr I I, 1097 1100.

MadelameG. and Metayer Y. (IYXU) %otc J A(,rot,~. \C I I I. 35x.

characteristics of aerosols with parucle sizes icsb than 0.01 pm using the Pollak condensation nucleus counter J. Ph.v.s. D.. Appl. Phys. 4, 531 53X.

Pedder M. A. (1974) Note on the smallest particles detected by condensation nucleus counters Armo.+ric Environ-

O’Connor D. T. (1975) The callbratlon 01 two mrdsurmg Instruments. one portable and one automatic for concen- trations of condensation nuclei. J. .4~rosol Ser. 6, 2.3 30.

Pedder M. A. (1971) The measurement of size and dlffususlon

complicated electronic circuits. The Pollak counter, me,lr 8. 1061 1062.

having the simplest design. has the most stable Perrin M. L.. Malgne J. P. and Madelame G. J. f IY77) Rayon

. . . critique des particules d’aerosols produites par radiolysc calibration. J Aerosol Sri. 8, 349 -353.

(c) The reoorted particle size efficiencv varies with SEhmldt K. H. (1975) A condensation nucleus size analvzer

the observer as well as with the type of counter, as is suitable for studying the kinetics of aerosol formanon. Inr

evident from Table 1. This is probably due to different J. them. Kincr. 19’15, 557 565.

methods of particle size measurement used by different Smclalr D. (1975) Comments on the paper: The cahbratlon

of two measuring instruments. one portable and one observers. In general, it appears that most CN counters automatic. for co&entrations of condinsation nuclei by

significantly undercount particles < 0.02 pm diameter. D T. O’Connor. J. .4erosol Sci 6. 484 485. Smclair D. and Hoopes G. S. (1975) A contmuous flow

REFERENCES

Agarwal J. K. and Sem G. .I. (1980) Continuous flow. smgle- particle-countingcondensation nucleuscounter. J. Aerosol Sci. 11, 343-357.

Bricard J., Delattre P., Madelame G. and Pourprix M. (1976) Detection of ultra-fine particles by means of a continuous flux condensation nuclei counter. In fine Parricles (Edited by B. Y. H. Liu) pp. 565-580. Academic Press, New York.

Cooper G. and Langer G. (I 978) Limitations of commercial condensation nucleus counters as absolute aerosol counters. J. Aerosol Sci. 9, 65 75.

condensation nucleus counter. J .4erosol Sci. 6, I ? Smclair D.. Countess R. J., Liu B. Y. H. and PUI D 1’. H.

(1979) Automatic analysis of submicron aerosols. In .4rrosol Merrsuremenr, Umversitk Presses of Florida. Gainesville, pp. 544.--563.

Twomey S ( 1977) Armo.~phww .4~wro/.~. Elsw~r Ne& York.

Verzar F. (1953) Kondensatlonskernzahler mlt auttbmatls- cher reglstrierung. Archs Me/. C;eophv.nk Bioklimat AS, 372.-376.

Walter H. and Jaemcke R. (1973) Remarks about the smallest particle size detectable m condensation nuclei counters. Armospheric flnrironmen/ 7. 939 944