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Ionometric counters
Atmospheric Environment Pergamon Press 1971. Vol. 5, pp. 137-145. Printed in Great Britain.
IONOMETRIC COUNTERS VOLKER A. MOHNEN Associate Professor, Department of Atmospheric Science and Senior Research Associate, Atmospheric Sciences Research Center, State University of New York at Albany, Albany, New York, U.S.A. (Received 23 December 1969)
Almtraet--A differentialion chamber has been developedfor the continuous and non-destructive measurement of aerosol concentrations down to sizes as small as 10-6 cm radius. In a brief outline instruments based on the same principle are discussed. AIRBORNE particles of any size larger than approximately 10 -6 cm radius can be detected by their influence on the output current of an ion chamber arranged to collect the small ions produced by a low level radiation source in the gas stream containing the particles. In the absence of the particles, almost all of the ions are collected, resulting in maximum output current of a magnitude determined by the strength of the radiation source and the ionization properties of the gas stream. With aerosol particles present, some of the small ions will diffuse onto the particles. These charged particles ('large ions') have at least two orders of magnitude lower mobility; therefore, they are not collected by the weak electrical field applied to the ion chamber and are carried away with the gas stream. The resulting decrease in the output current of the ion chamber is a function of the particle concentration and particle size. This measuring principle was developed more than 50 yr ago when VON SCHWEIOLER (1918, 1919) derived the basic theory for the qualitative understanding of small ion physics; the so-called "Schweidler Method I" (VON SCHWEIDLER, 1924)and "Schweidler Method I r ' (VON SCHWEIDLER, 1927) played an important role in aerosol physics until about 1935, and well-known scientists, such as HESS (1929) and Hoc,G (1934) applied them to measure the lifetime of small ions, which is a direct indicator of the aerosol concentration present. GREINACHER (1922, 1938) was the first one who adapted the ion chamber technique for measuring dust concentration. In his paper entitled "On a differential ionometer and its application for electrical measuring of dust contents in air" he describes two ion chambers connected together to a differential electrometer. The air in both chambers is ionized by a Radium preparation placed in the chambers. If dustfree air is present in both chambers, ionization current is compensated; when air containing dust is blown into one of the chambers, the compensation is disturbed. Greinacher's arrangement is used in a modernized version as a technical ionic fire detector (MEILI, 1952; SOULIER,1950), and our own aerosol detector is also based principally on his design. The reasons for the ion chamber technique not being considered in the past as a precision instrument for measuring aerosol concentration are several. (1) No electrometer units were available that were rugged enough to be used outside of the laboratory, worked continuously and were sensitive enough to register environmental pollution levels. 137
138
v.A. MOHNEN
(2) No procedure was available for calibrating the instruments with known aerosol concentrations in various submicron size ranges. (3) Although van Schweidler developed the basic theory for the behavior of small ions and their interactions with particles, it was by no means advanced enough to predict quantitatively a relationship between variation of ion chamber current and particle concentration. This relationship supposes the knowledge of an attachment coefficient for small ions on particles as a function of size and concentration. BRICARD (1949, 1962) and Focas (1947, 1963) started out about 20 yr ago to establish a modern concept of aerosol charging mechanism, and since then various improvements have been published; for example, MOHNEN(1966), BAUST(1967), and WITTBV-LIu (1967). The difficulties mentioned above vanished in recent years and various instruments have been described in the scientific literature based on the ionometric principle. HASENCLEVF.Rand SmGMA~ (1960) developed a dust counter, which is illustrated in FIG. 1. This instrument was further improved and a second ion chamber has been added separated from the first one by a sedimentation-diffusion box in order to measure the concentration in a predetermined size range by MOHNEN and SmGMANN (1964). A weak cobalt 60 source provided bipolar ions. :
i
i
[
Aerosol
inlet ....
----=:=~
Heater ~ ~ - i
I
Double I ~ l absolute filters -.q
_~jmmim~-_, o
~-~, ~
Voltage
i
~=
1' [
:__~_~
,r__¢
.,~__
FI
~
c~ 3
.q rksc r ee n u n i t - - _ _ ~ = (Z) Heater
" o
o
~
~_~,
_~.
o~ ~
-
|-Screen
4
~
U Voltage ]e
3
Heuter '2 ~ -
i - -~l~ . . . . . . . . . . . .
Screen
i---~-~ Needle valve
fl
Plus to minus current converter
: l',,f'~ Aerosol outlet F I G . 1.
continuous operation
'if ~]
q--
Electrometer
[---] Recorder
Ion counter after HASENCLEVERSIEGMANN(1966).
Another apparatus for the recording of condensation nuclei has been reported by SIKSNA (1961). Air under examination is mixed in a mixing chamber with small ions
Ionometric Counters
139
produced by a tritium ion generator (unipolar ions). The presence of condensation nuclei is measured by the decrease of the concentration of small ions recorded with an aspiration ion counter in comparison to the concentration recorded when filtered air is sucked through the mixing chamber. The experimental arrangement used is shown in Fie. 2. SKALA (1966) designed an ion chamber detector for the purpose of monitoring thermally produced particulates occurring in various kinds of gas environments.
:iI FIG. 2. Ionometrie counter for recording condensation nuclei: G, tritium ion-generator; Fx, filter for air through the ion generator; Fz, air filter for admixing filtered air; $1, $2, Sa, $4, stop-cocks; M, mixing chamber; C, aspiration condenser for measurement of air ions; E, electrometric valve; A, d.c. amplifier; W, d.c. recorder; R, rotameter; V, vacuum cleaner. From SIKSNA(1961). Tests have been conducted on the thermal particulate properties of various plastic materials in a hydrogen atmosphere at pressures up to 4 atmospheres. Skala's detector is presented in FIe. 3.
:
L-~'
L .....................................
~Pressurecontainer
X" t---7-3 cm---t ~z4
....__
i ~
FIG. 3. Skala's ion counter, SKALA(1966).
To
r
140
V.A. MOI-INEN
A commercially available trace gas detector (Mine Safety Appliances, Bulletin No. 0706-3) was developed recently using the same principle as discussed here. Various gases can be "converted" into aerosol particles which are then detected in a differential ion chamber. This idea is first mentioned in a British patent issued in 1933 to Paul Malsallez. The single ion chamber operates at maximum output current (saturation current I,) in aerosol-free air, and the presence of particulate matter is indicated by the decrease AI of this maximum current, as illustrated in FIG. 4. The abscissa shows the voltage applied to the chamber, and the ordinate represents the resulting ion current for a fixed radioactive preparation. Note that Alapproaches zero when the voltage(v) increases, which means that the ions generated within the chamber do not interact any longer with the aerosol present. Of course, the voltage necessary to bring A1 to zero increases with increasing aerosol concentration. Therefore, the
f
/ f
J
I
I0
/
/
]
i~ r
l/
J t
i I/2 [,
i
j
Ol.,,,~
f
I i
I [
I0
i
20
I
50
40
I
50
[
60
i
70
80
90
I~
Uk.F
U in V
FIG. 4. Ion-current characteristic for the single chamber operational mode. H: voltage at half the saturation current Iz (according to VON SCHVCEIDLER,1918, 1919, 1924).
instruments should be operated at a fixed voltage, Uk,i, determined by the plateau of the aerosol-free current characteristic (see FIG. 4). The disadvantages of the "single chamber operational mode" are numerous, the first of which is that the saturation current (the reference for aerosol-free air) depends drastically on environmental conditions, such as pressure, temperature, humidity, gas composition, etc. These disadvantages are due not to the basic principle, but to the experimental arrangements. The nature of the ion chamber detector allows the measurement of particle concentrations without physically altering the gas containing the particles, such as the pressure and temperature cycles produced by an expansiontype nuclei counter. A pressure of a few centimeters of water is sufficient to produce the required flow, and since no moving parts are required, maintenance is not a problem. MOrrNEN and HOLTZ (1964) successfully eliminated any environmental influences by adding a reference chamber identical to the measuring chamber but kept always free of aerosols and by operating these chambers in the differential mode. A schematic
Ionometric Counters
141
diagram is given in Fig. 5. The aerosol counting system consists of two ion chambers (each 10,000 cm 3 volume). The first is the calibration chamber, which is free of aerosols. The second is the measuring chamber, which the aerosol is carried through with a slight air stream. A small amount of Cobalt 60 (0.8 /zCi, no special U.S.A.E.C. license required) produces in both chambers an equal, small ion density, and the small ions are collected by applying a low voltage of the same sign on both chambers. In the absence of aerosols in the measuring chamber, the resulting current is zero when both inner electrodes are connected over an electronic transducer, which converts the current from plus to minus. Any atmospheric influences as well as fluctuations of the voltage
Gehbuse \
Pr~parot
Aussenelektrode (Kammersponnung)
Filter netz
Teflon........... ....................
kuf
lotion
:. :ij\, \~~
I j .................
Vorv~st~r ker
l
-
Teflonisolation ~ Mit~elelek~rode / (rnKr Vorverst~rker verbunden)
/ pr~ipar'ot
'" Gabl?ise
Kabel zurn ,, HouptverstiSrker
FIG. 5. Mohnen-Holtzdifferentialion chamber (Mom~N and HOLTZ,1968). supply have been thus eliminated. Furthermore, collecting the same sign of ions on both inner electrodes saves supplementary corrections regarding the different mobility of positive and negative small ions. The differential ion current is graphed as a function of the applied voltage in FIG. 6 (the dash-dot line). The solid line is the current-voltage plot for the reference chamber; the dashed line at the bottom is the current-voltage plot for the measuring chamber; and the dash-dot line, therefore, is the difference between the other two curves. The differential ion current starts at zero, approaches a maximum current (/"*max) at the voltage Umax and decreases assymptotically to zero again for large voltages depending on the aerosol concentration in the measuring chamber. A theoretical analysis of the differential curve shows several interesting features. (1) The aerosol concentration present in the measuring chamber is determined by the maximum ion current V'm,, (see FIG. 6). This measuring procedure is not appropriate for continuous operation, since it requires the recording of the complete ion current characteristic in order to obtain V*r,ax. (2) The total area beneath the differential ion current characteristic (see FIG. 6) is an accurate measure for the aerosol concentration present in the measuring chamber. Again, the complete ion current characteristic has to be recorded. (3) For continuous operation, a fixed voltage is applied, and the change in output current 1Mis recorded. The fixed voltage should be sufficient to saturate the calibration chamber (aerosol free chamber). Since the differential-ionometric counter as described here is mostly used in this continuous operation, it is appropriate to evaluate the basic theoretical relationship
142
V . A . MOHNEN
tl/2L 50
40
U,~ in V
•
I/2Is
_ \,
--
\\
Fie. 6. Ion current characteristic for differential operational modeZ(dash-dot curve). H i : voltage at half the saturation current measured in calibration eharnber; H2: voltage at hall' the saturation current measured in measuring chamber; Ha: voltage at maximum ion current in the differential mode ( ~ v*mx).
between the ion current measured (lu) and the aerosol concentration to be determined. Since the instrument design is such that ion recombination can be neglected within the two chambers against ion removal by the electrical field applied (residence time of bipolar ions is smaller than time for ion recombination), a simple relationship between the output current l u of the differential chambers and the aerosol concentration in the measuring chamber can be derived. Let no be the equilibrium ion density in the calibration chamber (no aerosols present, ions are only lost to the electrodes of the ion chamber). Let n be the equilibrium ion density in the measuring chamber, which contains the aerosol to be measured (ions are lost to the aerosol by bipolar ion diffusion and to the electrodes of the ion chamber), n can be determined from Rmax -dt
--
--
n
fl(R)f(R)dR,
(l)
Rmin
/3(R): attachment coefficient for bipolar ions to the aerosol particles of total concentration Z and radius R. Z is the total aerosol concentration as determined by
Ionometrie Counters
143
Rmax f6
Z = If(R)
dR.
ii
Rmln
A theoretical expression for fl(R) has been derived for example in Morton (1966). Assuming a mean value fl(R) for the size distributionf(R), equation (1) becomes dn dt
--
--
(2)
nfl(R)Z
It is known that (3)
[fl(R)Z] - ~ = ta,
represents the'mean lifetime of ions which are attaching to aerosols. dn dt
n ta
ldn =_
1
n
ta
(2')
dt,
(4)
no
lnn--lnno
(5)
=--ts/to
t, denotes the mean time for ion removal by the electrical field applied to both chambers (residence time of ions in the chambers). (a 2 tS
__ b 2 ) l n a
b
(6)
2MUk,~
where a, b = the outer and inner electrode radius respectively of the cylindrical ion chambers M = mobility of small ions, 1.5 cm 2 V - i s - 1 Uk,t = voltage applied to saturate the calibration chamber The measuring chamber alone will record a current I proportional to n, while the calibration chamber records a saturation current Is proportional to no. With equation (5), the difference current IM is then given by
(7)
IM = & - - I
tal J
In 1 . . . . .
,. ta
^.~. 5/3--E
(8) (9)
144
V.A. MOHNEN
The final relationship between the total aerosol concentration Z and the output current 1M of a differential ionometric counter is given by [with equations (3) and (9)] Z --
1
ts/ (R)
In 1 -
(10)
As mentioned above fl(R) is a very complicated function which describes the attachment of bipolar ions to any mono- or polydisperse aerosol with known size distribution R. As an approximation, it can be shown that
fl(R) ---- const./~
(11)
where /~ denotes the average radius of the given aerosol (mean arithmetic radius). Equation (10) then becomes Z./~ --
1 In (1 --/_M], const.ts Is ]
(12)
and, for values of IM/Is < 0.5
Z R --
IM const, t~ Is
(13)
We thus have a linear scale up to very high concentrations. A calibration of any ionometric counter principally supposes some knowledge of either the mean attachment coefficient or the mean arithmetic radius of the aerosol size distribution. If the latter is constant, then the total aerosol concentration can be determined accurately. Otherwise, the recorded current IM only indicates changes of the quantity Z/~. If a recording of Z can be obtained by another independent measurement, the ionometric counter would then provide information of the change in R of the aerosol. We have operated the differential ionometric counter for the measurement of atmospheric aerosol, with the assumption that/~ is constant or does not vary by more than 10 per cent. Another interesting feature of the differential ionometric counter is its capability of changing sensitivity by just changing the voltage applied to the chamber. It can be easily seen by combining equations (6) and (13) that Z _ _
Ukri
~
/M
o
Increasing the fixed voltage Uk,l by a factor of two would cause the output current l u to drop by one-half. We have, therefore, a simple way of adapting the instrument to high concentrations of particles or of maintaining linearity (IM/ls < 0.5). Furthermore, we are able to check the instrument for proper operation at any desired time by applying a high voltage (about 10 times Uk~) to the ion chambers. The output current l u should then decrease to zero, as can be seen in Fig. 6. Finally, in FIG. 7 we see photographs of the complete laboratory system with all options included necessary for discontinuous (according to number 1 and number 2 above) and continuous operation. The standard system for continuous operation is much less voluminous and
FJ6.7. Photograph of the complete apparatus.
(Facing p. 144)
lonometric Counters
145
includes only the i o n chamber, the electrometer a n d a strip chart recorder. O n e o f these i n s t r u m e n t s has been operating n o w for one year c o n t i n u o u s l y as a m o n i t o r for outside aerosol at o u r b e n c h m a r k station (Cloud Physics L a b o r a t o r y , Schenectady, New York). Acknowledgements--This work has been supported in part by the Office of Naval Research, Contract No. 0001469C0043.
REFERENCES BRICA~ J. (1949) 3". geophys. Res. 54, (1) 39-52. B~CAROJ. (1962) Geol. pura appl. 51, 237-242. BAUSTE. (1967) Z. Phys. 199, 187-206. FucHs N. A. (1947) Isv. Acad. Nauk USSR 11,341. Fucns N. A. (1963) Geofi~. pura appl. 56, 185-193. GREINAC~RH. (1922) Bull. Schweiz. elektrot. Ver. 8, 356-365. Gl~INACnERH. (1938) Swiss Patent No. 195 697. HASENCLEVERD. and SIEGMANNH. CHR (1960) Staub 20, 212. HESS V. F. (1929) Gerl. Beitr. Geophys. 22, 256-314. HOGGA. R. (1934) Gerl. Beitr. Geophys. 41, 1-32. LIu B., WHITBYK.T. and Yu H. H. S. (1967) 3. Colloid interface Sci. 23, 367. MEILI E. (1952) Bull. S.E.V. 23, 933-939. MOHNENV. (1966) BMwF--Research Report K66-11. MOHNENV. and HOLTZP. (1968) APCA 3". 18, (10) 667. MOHNENV. and SIEGMANNH. CHR. (1964) Staub 24, 256-261. SIKSNAR. (1961) Geof pura appl. 50, 23-36. SKALAG. F. (1966) J. Rech. Attn. 2, No. 2-3. SOULIEgA. (1950) Rev. gen. Electr. 59, 367-372. VON SCI-IWEIDLERE. (1918) Wien. Bet. 127, 953-967. VON SCHWEIDLERE. (1919) Wien. Ber. 128, 947-955. "CONSCHWEIDLERE. (1924) Wien. Bet. 133, 23. VON SCHWEIDLERE. (1927) Phys. Z. XXVIII.
IONOMETRIC COUNTERS VOLKER A. MOHNEN Associate Professor, Department of Atmospheric Science and Senior Research Associate, Atmospheric Sciences Research Center, State University of New York at Albany, Albany, New York, U.S.A. (Received 23 December 1969)
Almtraet--A differentialion chamber has been developedfor the continuous and non-destructive measurement of aerosol concentrations down to sizes as small as 10-6 cm radius. In a brief outline instruments based on the same principle are discussed. AIRBORNE particles of any size larger than approximately 10 -6 cm radius can be detected by their influence on the output current of an ion chamber arranged to collect the small ions produced by a low level radiation source in the gas stream containing the particles. In the absence of the particles, almost all of the ions are collected, resulting in maximum output current of a magnitude determined by the strength of the radiation source and the ionization properties of the gas stream. With aerosol particles present, some of the small ions will diffuse onto the particles. These charged particles ('large ions') have at least two orders of magnitude lower mobility; therefore, they are not collected by the weak electrical field applied to the ion chamber and are carried away with the gas stream. The resulting decrease in the output current of the ion chamber is a function of the particle concentration and particle size. This measuring principle was developed more than 50 yr ago when VON SCHWEIOLER (1918, 1919) derived the basic theory for the qualitative understanding of small ion physics; the so-called "Schweidler Method I" (VON SCHWEIDLER, 1924)and "Schweidler Method I r ' (VON SCHWEIDLER, 1927) played an important role in aerosol physics until about 1935, and well-known scientists, such as HESS (1929) and Hoc,G (1934) applied them to measure the lifetime of small ions, which is a direct indicator of the aerosol concentration present. GREINACHER (1922, 1938) was the first one who adapted the ion chamber technique for measuring dust concentration. In his paper entitled "On a differential ionometer and its application for electrical measuring of dust contents in air" he describes two ion chambers connected together to a differential electrometer. The air in both chambers is ionized by a Radium preparation placed in the chambers. If dustfree air is present in both chambers, ionization current is compensated; when air containing dust is blown into one of the chambers, the compensation is disturbed. Greinacher's arrangement is used in a modernized version as a technical ionic fire detector (MEILI, 1952; SOULIER,1950), and our own aerosol detector is also based principally on his design. The reasons for the ion chamber technique not being considered in the past as a precision instrument for measuring aerosol concentration are several. (1) No electrometer units were available that were rugged enough to be used outside of the laboratory, worked continuously and were sensitive enough to register environmental pollution levels. 137
138
v.A. MOHNEN
(2) No procedure was available for calibrating the instruments with known aerosol concentrations in various submicron size ranges. (3) Although van Schweidler developed the basic theory for the behavior of small ions and their interactions with particles, it was by no means advanced enough to predict quantitatively a relationship between variation of ion chamber current and particle concentration. This relationship supposes the knowledge of an attachment coefficient for small ions on particles as a function of size and concentration. BRICARD (1949, 1962) and Focas (1947, 1963) started out about 20 yr ago to establish a modern concept of aerosol charging mechanism, and since then various improvements have been published; for example, MOHNEN(1966), BAUST(1967), and WITTBV-LIu (1967). The difficulties mentioned above vanished in recent years and various instruments have been described in the scientific literature based on the ionometric principle. HASENCLEVF.Rand SmGMA~ (1960) developed a dust counter, which is illustrated in FIG. 1. This instrument was further improved and a second ion chamber has been added separated from the first one by a sedimentation-diffusion box in order to measure the concentration in a predetermined size range by MOHNEN and SmGMANN (1964). A weak cobalt 60 source provided bipolar ions. :
i
i
[
Aerosol
inlet ....
----=:=~
Heater ~ ~ - i
I
Double I ~ l absolute filters -.q
_~jmmim~-_, o
~-~, ~
Voltage
i
~=
1' [
:__~_~
,r__¢
.,~__
FI
~
c~ 3
.q rksc r ee n u n i t - - _ _ ~ = (Z) Heater
" o
o
~
~_~,
_~.
o~ ~
-
|-Screen
4
~
U Voltage ]e
3
Heuter '2 ~ -
i - -~l~ . . . . . . . . . . . .
Screen
i---~-~ Needle valve
fl
Plus to minus current converter
: l',,f'~ Aerosol outlet F I G . 1.
continuous operation
'if ~]
q--
Electrometer
[---] Recorder
Ion counter after HASENCLEVERSIEGMANN(1966).
Another apparatus for the recording of condensation nuclei has been reported by SIKSNA (1961). Air under examination is mixed in a mixing chamber with small ions
Ionometric Counters
139
produced by a tritium ion generator (unipolar ions). The presence of condensation nuclei is measured by the decrease of the concentration of small ions recorded with an aspiration ion counter in comparison to the concentration recorded when filtered air is sucked through the mixing chamber. The experimental arrangement used is shown in Fie. 2. SKALA (1966) designed an ion chamber detector for the purpose of monitoring thermally produced particulates occurring in various kinds of gas environments.
:iI FIG. 2. Ionometrie counter for recording condensation nuclei: G, tritium ion-generator; Fx, filter for air through the ion generator; Fz, air filter for admixing filtered air; $1, $2, Sa, $4, stop-cocks; M, mixing chamber; C, aspiration condenser for measurement of air ions; E, electrometric valve; A, d.c. amplifier; W, d.c. recorder; R, rotameter; V, vacuum cleaner. From SIKSNA(1961). Tests have been conducted on the thermal particulate properties of various plastic materials in a hydrogen atmosphere at pressures up to 4 atmospheres. Skala's detector is presented in FIe. 3.
:
L-~'
L .....................................
~Pressurecontainer
X" t---7-3 cm---t ~z4
....__
i ~
FIG. 3. Skala's ion counter, SKALA(1966).
To
r
140
V.A. MOI-INEN
A commercially available trace gas detector (Mine Safety Appliances, Bulletin No. 0706-3) was developed recently using the same principle as discussed here. Various gases can be "converted" into aerosol particles which are then detected in a differential ion chamber. This idea is first mentioned in a British patent issued in 1933 to Paul Malsallez. The single ion chamber operates at maximum output current (saturation current I,) in aerosol-free air, and the presence of particulate matter is indicated by the decrease AI of this maximum current, as illustrated in FIG. 4. The abscissa shows the voltage applied to the chamber, and the ordinate represents the resulting ion current for a fixed radioactive preparation. Note that Alapproaches zero when the voltage(v) increases, which means that the ions generated within the chamber do not interact any longer with the aerosol present. Of course, the voltage necessary to bring A1 to zero increases with increasing aerosol concentration. Therefore, the
f
/ f
J
I
I0
/
/
]
i~ r
l/
J t
i I/2 [,
i
j
Ol.,,,~
f
I i
I [
I0
i
20
I
50
40
I
50
[
60
i
70
80
90
I~
Uk.F
U in V
FIG. 4. Ion-current characteristic for the single chamber operational mode. H: voltage at half the saturation current Iz (according to VON SCHVCEIDLER,1918, 1919, 1924).
instruments should be operated at a fixed voltage, Uk,i, determined by the plateau of the aerosol-free current characteristic (see FIG. 4). The disadvantages of the "single chamber operational mode" are numerous, the first of which is that the saturation current (the reference for aerosol-free air) depends drastically on environmental conditions, such as pressure, temperature, humidity, gas composition, etc. These disadvantages are due not to the basic principle, but to the experimental arrangements. The nature of the ion chamber detector allows the measurement of particle concentrations without physically altering the gas containing the particles, such as the pressure and temperature cycles produced by an expansiontype nuclei counter. A pressure of a few centimeters of water is sufficient to produce the required flow, and since no moving parts are required, maintenance is not a problem. MOrrNEN and HOLTZ (1964) successfully eliminated any environmental influences by adding a reference chamber identical to the measuring chamber but kept always free of aerosols and by operating these chambers in the differential mode. A schematic
Ionometric Counters
141
diagram is given in Fig. 5. The aerosol counting system consists of two ion chambers (each 10,000 cm 3 volume). The first is the calibration chamber, which is free of aerosols. The second is the measuring chamber, which the aerosol is carried through with a slight air stream. A small amount of Cobalt 60 (0.8 /zCi, no special U.S.A.E.C. license required) produces in both chambers an equal, small ion density, and the small ions are collected by applying a low voltage of the same sign on both chambers. In the absence of aerosols in the measuring chamber, the resulting current is zero when both inner electrodes are connected over an electronic transducer, which converts the current from plus to minus. Any atmospheric influences as well as fluctuations of the voltage
Gehbuse \
Pr~parot
Aussenelektrode (Kammersponnung)
Filter netz
Teflon........... ....................
kuf
lotion
:. :ij\, \~~
I j .................
Vorv~st~r ker
l
-
Teflonisolation ~ Mit~elelek~rode / (rnKr Vorverst~rker verbunden)
/ pr~ipar'ot
'" Gabl?ise
Kabel zurn ,, HouptverstiSrker
FIG. 5. Mohnen-Holtzdifferentialion chamber (Mom~N and HOLTZ,1968). supply have been thus eliminated. Furthermore, collecting the same sign of ions on both inner electrodes saves supplementary corrections regarding the different mobility of positive and negative small ions. The differential ion current is graphed as a function of the applied voltage in FIG. 6 (the dash-dot line). The solid line is the current-voltage plot for the reference chamber; the dashed line at the bottom is the current-voltage plot for the measuring chamber; and the dash-dot line, therefore, is the difference between the other two curves. The differential ion current starts at zero, approaches a maximum current (/"*max) at the voltage Umax and decreases assymptotically to zero again for large voltages depending on the aerosol concentration in the measuring chamber. A theoretical analysis of the differential curve shows several interesting features. (1) The aerosol concentration present in the measuring chamber is determined by the maximum ion current V'm,, (see FIG. 6). This measuring procedure is not appropriate for continuous operation, since it requires the recording of the complete ion current characteristic in order to obtain V*r,ax. (2) The total area beneath the differential ion current characteristic (see FIG. 6) is an accurate measure for the aerosol concentration present in the measuring chamber. Again, the complete ion current characteristic has to be recorded. (3) For continuous operation, a fixed voltage is applied, and the change in output current 1Mis recorded. The fixed voltage should be sufficient to saturate the calibration chamber (aerosol free chamber). Since the differential-ionometric counter as described here is mostly used in this continuous operation, it is appropriate to evaluate the basic theoretical relationship
142
V . A . MOHNEN
tl/2L 50
40
U,~ in V
•
I/2Is
_ \,
--
\\
Fie. 6. Ion current characteristic for differential operational modeZ(dash-dot curve). H i : voltage at half the saturation current measured in calibration eharnber; H2: voltage at hall' the saturation current measured in measuring chamber; Ha: voltage at maximum ion current in the differential mode ( ~ v*mx).
between the ion current measured (lu) and the aerosol concentration to be determined. Since the instrument design is such that ion recombination can be neglected within the two chambers against ion removal by the electrical field applied (residence time of bipolar ions is smaller than time for ion recombination), a simple relationship between the output current l u of the differential chambers and the aerosol concentration in the measuring chamber can be derived. Let no be the equilibrium ion density in the calibration chamber (no aerosols present, ions are only lost to the electrodes of the ion chamber). Let n be the equilibrium ion density in the measuring chamber, which contains the aerosol to be measured (ions are lost to the aerosol by bipolar ion diffusion and to the electrodes of the ion chamber), n can be determined from Rmax -dt
--
--
n
fl(R)f(R)dR,
(l)
Rmin
/3(R): attachment coefficient for bipolar ions to the aerosol particles of total concentration Z and radius R. Z is the total aerosol concentration as determined by
Ionometrie Counters
143
Rmax f6
Z = If(R)
dR.
ii
Rmln
A theoretical expression for fl(R) has been derived for example in Morton (1966). Assuming a mean value fl(R) for the size distributionf(R), equation (1) becomes dn dt
--
--
(2)
nfl(R)Z
It is known that (3)
[fl(R)Z] - ~ = ta,
represents the'mean lifetime of ions which are attaching to aerosols. dn dt
n ta
ldn =_
1
n
ta
(2')
dt,
(4)
no
lnn--lnno
(5)
=--ts/to
t, denotes the mean time for ion removal by the electrical field applied to both chambers (residence time of ions in the chambers). (a 2 tS
__ b 2 ) l n a
b
(6)
2MUk,~
where a, b = the outer and inner electrode radius respectively of the cylindrical ion chambers M = mobility of small ions, 1.5 cm 2 V - i s - 1 Uk,t = voltage applied to saturate the calibration chamber The measuring chamber alone will record a current I proportional to n, while the calibration chamber records a saturation current Is proportional to no. With equation (5), the difference current IM is then given by
(7)
IM = & - - I
tal J
In 1 . . . . .
,. ta
^.~. 5/3--E
(8) (9)
144
V.A. MOHNEN
The final relationship between the total aerosol concentration Z and the output current 1M of a differential ionometric counter is given by [with equations (3) and (9)] Z --
1
ts/ (R)
In 1 -
(10)
As mentioned above fl(R) is a very complicated function which describes the attachment of bipolar ions to any mono- or polydisperse aerosol with known size distribution R. As an approximation, it can be shown that
fl(R) ---- const./~
(11)
where /~ denotes the average radius of the given aerosol (mean arithmetic radius). Equation (10) then becomes Z./~ --
1 In (1 --/_M], const.ts Is ]
(12)
and, for values of IM/Is < 0.5
Z R --
IM const, t~ Is
(13)
We thus have a linear scale up to very high concentrations. A calibration of any ionometric counter principally supposes some knowledge of either the mean attachment coefficient or the mean arithmetic radius of the aerosol size distribution. If the latter is constant, then the total aerosol concentration can be determined accurately. Otherwise, the recorded current IM only indicates changes of the quantity Z/~. If a recording of Z can be obtained by another independent measurement, the ionometric counter would then provide information of the change in R of the aerosol. We have operated the differential ionometric counter for the measurement of atmospheric aerosol, with the assumption that/~ is constant or does not vary by more than 10 per cent. Another interesting feature of the differential ionometric counter is its capability of changing sensitivity by just changing the voltage applied to the chamber. It can be easily seen by combining equations (6) and (13) that Z _ _
Ukri
~
/M
o
Increasing the fixed voltage Uk,l by a factor of two would cause the output current l u to drop by one-half. We have, therefore, a simple way of adapting the instrument to high concentrations of particles or of maintaining linearity (IM/ls < 0.5). Furthermore, we are able to check the instrument for proper operation at any desired time by applying a high voltage (about 10 times Uk~) to the ion chambers. The output current l u should then decrease to zero, as can be seen in Fig. 6. Finally, in FIG. 7 we see photographs of the complete laboratory system with all options included necessary for discontinuous (according to number 1 and number 2 above) and continuous operation. The standard system for continuous operation is much less voluminous and
FJ6.7. Photograph of the complete apparatus.
(Facing p. 144)
lonometric Counters
145
includes only the i o n chamber, the electrometer a n d a strip chart recorder. O n e o f these i n s t r u m e n t s has been operating n o w for one year c o n t i n u o u s l y as a m o n i t o r for outside aerosol at o u r b e n c h m a r k station (Cloud Physics L a b o r a t o r y , Schenectady, New York). Acknowledgements--This work has been supported in part by the Office of Naval Research, Contract No. 0001469C0043.
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