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Measurement of air ions
Environment International, Vol. 12, pp. 109-113, 1986
0160-4120/86 $3.00 + .IX)
Printed in the USA. All fights reserved.
Copyright©1986Pergamon Journals Ltd.
MEASUREMENT OF AIR IONS M. Lehtim~.ki Technical Research Centre of Finland, Occupational Safety Engineering Laboratory, P.O. Box 656, SF-33101 Tampere, Finland
G. Graeffe Tampere University of Technology, Physics Laboratory, P.O. Box 527, SF-33101 Tampere, Finland (Received 22 May 1985; Accepted 27 August 1985)
A modified method for the simultaneous measurementof positive and negative air ions has been developed. A single aspiration condenser has been used in constructing a compact instrument for the studies concerning the behaviour of small ions in indoor air. The principle of the ion measuring technique is discussed. The constructionof the ion meter is described and possible problems of ion measurements are commented. Examples of the measurementresults have been included.
Introduction
Tammet, 1970). The principle o f the method used in the present modification is shown in Fig. 1. The basic idea o f this method is the simultaneous measurement o f both positive and negative ions with a single aspiration condenser. Traditionally, ion meters have been constructed with separate condensers for both polarities.
Because o f radioactive and cosmic radiation the air always contains varying concentrations of both positive and negative ions. The molecular cluster ions, which are formed by numerous ion molecule reactions, are usually called small ions. The interest in air ions has mainly been aroused by the possible effects o f ions on human beings, animals, and plants (Krueger and Reed, 1976). A widely expressed argument is that negative ions have beneficial effects while positive ions are assumed to be harmful. Air ion concentration can be regarded as an indicator o f air quality. The concentration o f natural air ions is dependent on two important factors, radioactivity o f the air and the concentration o f aerosol particles. On the other hand, artificial air ionization can be used in removing particles (Whitby and Liu, 1966) and radon decay products (Bigu, 1983) f r o m the indoor air. Thus the studies with air ions are connected to the research o f indoor air quality. The well-known technique o f measuring the concentration o f air ions is the aspiration condenser method. This method is based on the measurement o f the weak electric currents generated by ions drifting from the sample air flow to the measuring electrodes. Various modifications o f the aspiration method have been discussed b y several authors (e.g., Israel, 1970;
Theoretical The theory o f ion meters is well known, and therefore only the essential parts are presented in this paper. The determination o f ion concentrations is based on the measurement o f the electric current generated by the continuous flow o f ions to the measuring electrodes of the aspiration condenser. The electric field is used to drive the ions from the sample air flow to the electrodes. The mathematical form for the electric current Ip generated by positive ions is (Israel, 1970)
I,, = eF[J,"*oL(k)dk
+
(l/ko)
I~°kf~(k)dk],
(1)
where e is the elementary unit charge, F is the sample air flow, k is the ion mobility (i.e., ion drift velocity/ field strength), ko is the critical mobility o f the aspiration condenser, and fp(k) = dp/dk is the mobility distribution o f positive ions. The corresponding expression can be written for 109
110
M. Lehtirn~ikiand G. Graeffe
-4, ~+: i
U/2 Ip =
U/2
e~:
I ~
=eFn
=
EM electrometer Fig. 1. Principle of the ion meter.
negative ions as well, simply by changing the polarity o f the electric current and by using mobility distribution of negative ions. The expression for the critical mobility ko has the form ko = F l n ( R / r ) 2IIIU
(2)
The parameters in this equation denote to the radii o f outer and inner electrodes (R and r), the length o f the collecting section (I), and the voltage between the electrodes (U). All ions with a mobility higher than ko are collected but the ions whose mobility k is lower than the critical value are captured only in proportion to k / k o . If the critical mobility is low enough all the ions are captured and the currents Ip and I, have the simple forms lp = e F p ,
(3)
I,, = - e F n ,
(4)
trometer amplifiers from external disturbances are of great importance. Also the stability of the collecting voltage must be good. The schematic o f the aspiration condenser used in the present ion meter is shown in Fig. 2. As voltage supplies, floating at the inputs of the electrometers, small batteries are used because of their high voltage stability and the easiness o f construction. The voltage supplies and collecting electrodes, as well as all other parts connected to the electrometers' inputs, are carefully shielded against external electric fields. The leakage currents are eliminated by using guard ring electrodes in the condenser and by using triaxial cables between the voltage supplies and the collecting electrodes. The dimensions o f the aspiration condenser are R = 3.8 cm, r = 3.2 cm, and l = 8.0 cm. The net voltage between the electrodes is 45 V and the sample air flow generated by a small axial fan 5.2 L/sec. The value o f the critical mobility, calculated by using Eq. (2), is 0.4 cm2/V sec. This value is low enough to secure all the small ions being collected by the aspiration condenser. This can also be seen in the currentvoltage characteristics of the condenser. These curves, shown in Fig. 3, were determined in particlefree air by varying the condenser voltage in the range of 0 - 3 0 V. As can be seen in the figure, the increase in the voltage causes the ion current to reach a constant value which is an indication o f a 100% collection
3.8cm ~,~ o ) ~ m
where p and n are the concentrations of positive and negative ions, respectively. These are the basic equations used in determining the relationship between the ion currents and the corresponding ion concentrations. Also, it must be taken into account that this is the primary method and other methods which could be used, e . g . , in calibrating the ion meters, are not available. Construction of the Ion Meter The principle o f the ion meter is simple; however, when constructing a practical measuring instrument, some important aspects are worth noticing. The sensitivity of the ion meter is dependent on the minimum electric current that can be reliably measured. This is why the elimination o f leakage currents and the careful shielding o f the measuring electrodes and elec-
ILIC|il
ATd'~DC
(.1
o
o~
]
i
FAN 5.2 L/s Fig. 2. Schematicof the aspiration condenser.
Measurement of air ions W
111 ;
by the instrument are too high. In the case o f high radon concentrations it is advantageous to use a measuring technique with the fan " o f f " most o f the time and " o n " for short measuring periods only. The differences between the readings o f these two cases give the actual ion concentrations. In the case o f high concentration o f charged particles another problem may arise. Despite o f the low mobility o f the charged particles it is possible that a small part o f them drift to the electrodes. To minimize the effects o f charged particles, the collecting voltage must be kept as low as possible.
•
dk
1.0
A
A
I-Z
ILl ..~ O rt uJ N m il
< 0.5 rr
o Z
• neg
Experimental
0
--
•
I
I
I
I
0
5
10
15
20
25
30
VOLTAGE, V Fig. 3. Current-voltage characteristic of the aspiration condenser. Particle-free air.
efficiency. Furthermore, it can be noticed that the current, after having reached the plateau, does not decrease despite the increase in the voltage. This property can be regarded as an indication o f the negligible loss o f ions in the entrance o f the aspiration condenser. The electrometers o f the ion meter are constructed by using commercial electrometer amplifiers and the high value resistors. The lowest current values which can be reliably measured are in the range o f 0 . 0 0 5 - 0 . 0 1 pA, which means that the detection limit of the ion meter is approximately 10 ions/cm 3. In the conditions where, e . g . , rapid and strong variations in the air temperature occur, it is possible that the currents due to the small movements in the metal insulator interfaces can cause serious errors. In room air conditions the effects o f these phenomena are normally negligible. High relative humidity can lead to a reduction in the resistance o f the high value resistors, since the glass envelope o f the resistor starts to conduct. In order to eliminate these problems the electrometers have been installed into a airtight box containing a small amount of water-absorbing material. In addition to the zero instability caused by the strong temperature variations, there are other possible sources o f error. W h e n measuring ions in the case o f high radon concentration, it is possible that the alpha radiation from radioactive ions collected on the electrodes generates a small background ion current. This leads to an increase in the zero level o f the ion meter, and consequently the concentration values measured
The present ion meter, shown in Fig. 4, has been used for several years in studies dealing with the behaviour o f air ions in indoor air. In this paper exampies of the results from the measurements in controlled laboratory conditions are presented. These measurements have been carried out in a test room with a volume o f 25 m 3. During these studies simultaneous measurements o f ions, radon, and aerosol particles have been carried out. The concentration of radon has been measured by using a traditional air ionization chamber (Janka and Lehtimaki, 1982). The measurement o f particle concentration has been carried out with the aid of an optical particle size analyzer (PMS Las x). The test room is constructed in such a way that the concentrations o f radon and aerosol particles can be carefully controlled. As the radon source, 226RaC12salt dilution (activity 1 mCi) has been used. Nearly monodisperse particles have been generated by using a condensation-type aerosol generator. The relationship between the concentration o f radon and air ions in the particle-free air is shown in Fig. 5. This result indicates that the equilibrium ion concentration in particle-free air is proportional to the square root o f the radon concentration. This is the case for high radon concentrations. When the radon concentration is low, the effect o f the background radiation starts to dominate. A typical relationship between the ion concentrations and the concentration o f aerosol particles is shown in Fig. 6. This result clearly indicates that the ion concentration is strongly dependent on the particle concentration. Furthermore, it can be noticed that in particle-free air the concentration o f positive ions equals that o f negative ions. The unequal concentrations are connected to the presence o f aerosol particles. The purpose o f the results shown in Fig. 6 is to demonstrate how the concentrations o f ions decrease as the particle concentration increases. It must be emphasized that the concentration curves are quite different for aerosols with different particle sizes. The importance of the particle size will be more thor-
112
M. Lehtimiiki a n d O. Graeffe
Fig. 4. Photograph of the ion meter. 5-104 ,•E
oughly discussed in a separate paper (Lehtim~iki, 1986). An e x a m p l e of the ion concentrations in the presence o f artificial ion sources is shown in Fig. 7. These results have been determined in a test r o o m by using an unipolar corona ion generator. The m e a s u r e m e n t s have been carried out for both positive and negative ionization voltages. T h e concentration of air ions for different corona voltages were measured at a distance o f appr. 1.3 m f r o m the corona electrode. It must be noticed that the results in Fig. 7 correspond to two
o
z"
o
2.1d
i,z
"' U
°"e~wo.*f'"
104
Z
0 0 5"103 z 0
4 l Jl~
~
-O" 4 )
0
.~.q,o~,-e-"
2.10~ 20
i
50
~
,
,,n
t
100
=
200
|
n
n
500
, , , ,
1000
RADON CONCENTRATION, Bq m"3
Fig. 5. The relationship between the concentrations of ions and radon. Particle-free air.
(9|
2.10 5
E
i 4000
0
i
?
I
• o
neg pos t ~
• neg
2 3ooo
"0 Z 0 (J
2000
z
oQ.qm-
z
2 10001
1 05
O;-"
Z
o
"i, °%-o
0
"°'~.o. o
"~.11.0
"'0-.0.0. 0 "I)'41'I) .11 "<)0'''0~ " "OttO-Q•'" "•t) 0
0
0
i
IZ
i
w
|
o poe
Eu
E Z
I
I
I
200
400
t
0 600
PARTICLE CONCENTRATION, cm -3
Fig. 6. Example o f the relationship between the concentrations of ions and aerosol particles. D O P particles, average particle size 1.7 I~.
t
2
l
t
4
i
t
6 CORONA
I
I
8
I
10
V O L T A G E , kV
Fig. 7. Ion concentration as a function of corona voltage. (Note: during the positive air ionization the concentration of negative ions is near zero and vice versa.)
Measurement of air ions
different conditions. During the positive air ionization the concentration of negative ions is practically zero and vice versa. Acknowledgements--This work has been supported by the Academy of Finland and the Ministry of Trade and Industry. The contributions of K. Janka and E. M~cinen in constructing the electronics of the ion meter are sincerely appreciated.
References Bigu, J. (1983) On the effect of a negative ion-generator and a mixing fan on the plate-out of radon decay products in a radon box, Health
113
Phys. 44, 259-266. Israel, H. (1970) Atmospheric electricity, vol. 1. Israel Program for Scientific Translations, Jerusalem. Janka, K. and Lehtimgki, M. (1982) Method of eliminating the effect of decay products in continuous measurement of 222Rn, Rev. Sci. lnstrum. 53, 523-527. Krueger, A. P. and Reed, E. J. (1976) Biological impact of small air ions, Science 193 1209-1213. Lehtimiki, M. (1986) Air ions and indoor air quality, to be published. Tammet, H. F. (1970) The aspiration method for the determination of atmospheric-ion spectra. Israel Program for Scientific Translations, Jerusalem. Whitby, K. T. and Liu, B. Y. H. (1976) The electrical behaviour of aerosols, in Aerosol Science, C. N. Davies, ed., pp. 59-86. Academic Press, London.
0160-4120/86 $3.00 + .IX)
Printed in the USA. All fights reserved.
Copyright©1986Pergamon Journals Ltd.
MEASUREMENT OF AIR IONS M. Lehtim~.ki Technical Research Centre of Finland, Occupational Safety Engineering Laboratory, P.O. Box 656, SF-33101 Tampere, Finland
G. Graeffe Tampere University of Technology, Physics Laboratory, P.O. Box 527, SF-33101 Tampere, Finland (Received 22 May 1985; Accepted 27 August 1985)
A modified method for the simultaneous measurementof positive and negative air ions has been developed. A single aspiration condenser has been used in constructing a compact instrument for the studies concerning the behaviour of small ions in indoor air. The principle of the ion measuring technique is discussed. The constructionof the ion meter is described and possible problems of ion measurements are commented. Examples of the measurementresults have been included.
Introduction
Tammet, 1970). The principle o f the method used in the present modification is shown in Fig. 1. The basic idea o f this method is the simultaneous measurement o f both positive and negative ions with a single aspiration condenser. Traditionally, ion meters have been constructed with separate condensers for both polarities.
Because o f radioactive and cosmic radiation the air always contains varying concentrations of both positive and negative ions. The molecular cluster ions, which are formed by numerous ion molecule reactions, are usually called small ions. The interest in air ions has mainly been aroused by the possible effects o f ions on human beings, animals, and plants (Krueger and Reed, 1976). A widely expressed argument is that negative ions have beneficial effects while positive ions are assumed to be harmful. Air ion concentration can be regarded as an indicator o f air quality. The concentration o f natural air ions is dependent on two important factors, radioactivity o f the air and the concentration o f aerosol particles. On the other hand, artificial air ionization can be used in removing particles (Whitby and Liu, 1966) and radon decay products (Bigu, 1983) f r o m the indoor air. Thus the studies with air ions are connected to the research o f indoor air quality. The well-known technique o f measuring the concentration o f air ions is the aspiration condenser method. This method is based on the measurement o f the weak electric currents generated by ions drifting from the sample air flow to the measuring electrodes. Various modifications o f the aspiration method have been discussed b y several authors (e.g., Israel, 1970;
Theoretical The theory o f ion meters is well known, and therefore only the essential parts are presented in this paper. The determination o f ion concentrations is based on the measurement o f the electric current generated by the continuous flow o f ions to the measuring electrodes of the aspiration condenser. The electric field is used to drive the ions from the sample air flow to the electrodes. The mathematical form for the electric current Ip generated by positive ions is (Israel, 1970)
I,, = eF[J,"*oL(k)dk
+
(l/ko)
I~°kf~(k)dk],
(1)
where e is the elementary unit charge, F is the sample air flow, k is the ion mobility (i.e., ion drift velocity/ field strength), ko is the critical mobility o f the aspiration condenser, and fp(k) = dp/dk is the mobility distribution o f positive ions. The corresponding expression can be written for 109
110
M. Lehtirn~ikiand G. Graeffe
-4, ~+: i
U/2 Ip =
U/2
e~:
I ~
=eFn
=
EM electrometer Fig. 1. Principle of the ion meter.
negative ions as well, simply by changing the polarity o f the electric current and by using mobility distribution of negative ions. The expression for the critical mobility ko has the form ko = F l n ( R / r ) 2IIIU
(2)
The parameters in this equation denote to the radii o f outer and inner electrodes (R and r), the length o f the collecting section (I), and the voltage between the electrodes (U). All ions with a mobility higher than ko are collected but the ions whose mobility k is lower than the critical value are captured only in proportion to k / k o . If the critical mobility is low enough all the ions are captured and the currents Ip and I, have the simple forms lp = e F p ,
(3)
I,, = - e F n ,
(4)
trometer amplifiers from external disturbances are of great importance. Also the stability of the collecting voltage must be good. The schematic o f the aspiration condenser used in the present ion meter is shown in Fig. 2. As voltage supplies, floating at the inputs of the electrometers, small batteries are used because of their high voltage stability and the easiness o f construction. The voltage supplies and collecting electrodes, as well as all other parts connected to the electrometers' inputs, are carefully shielded against external electric fields. The leakage currents are eliminated by using guard ring electrodes in the condenser and by using triaxial cables between the voltage supplies and the collecting electrodes. The dimensions o f the aspiration condenser are R = 3.8 cm, r = 3.2 cm, and l = 8.0 cm. The net voltage between the electrodes is 45 V and the sample air flow generated by a small axial fan 5.2 L/sec. The value o f the critical mobility, calculated by using Eq. (2), is 0.4 cm2/V sec. This value is low enough to secure all the small ions being collected by the aspiration condenser. This can also be seen in the currentvoltage characteristics of the condenser. These curves, shown in Fig. 3, were determined in particlefree air by varying the condenser voltage in the range of 0 - 3 0 V. As can be seen in the figure, the increase in the voltage causes the ion current to reach a constant value which is an indication o f a 100% collection
3.8cm ~,~ o ) ~ m
where p and n are the concentrations of positive and negative ions, respectively. These are the basic equations used in determining the relationship between the ion currents and the corresponding ion concentrations. Also, it must be taken into account that this is the primary method and other methods which could be used, e . g . , in calibrating the ion meters, are not available. Construction of the Ion Meter The principle o f the ion meter is simple; however, when constructing a practical measuring instrument, some important aspects are worth noticing. The sensitivity of the ion meter is dependent on the minimum electric current that can be reliably measured. This is why the elimination o f leakage currents and the careful shielding o f the measuring electrodes and elec-
ILIC|il
ATd'~DC
(.1
o
o~
]
i
FAN 5.2 L/s Fig. 2. Schematicof the aspiration condenser.
Measurement of air ions W
111 ;
by the instrument are too high. In the case o f high radon concentrations it is advantageous to use a measuring technique with the fan " o f f " most o f the time and " o n " for short measuring periods only. The differences between the readings o f these two cases give the actual ion concentrations. In the case o f high concentration o f charged particles another problem may arise. Despite o f the low mobility o f the charged particles it is possible that a small part o f them drift to the electrodes. To minimize the effects o f charged particles, the collecting voltage must be kept as low as possible.
•
dk
1.0
A
A
I-Z
ILl ..~ O rt uJ N m il
< 0.5 rr
o Z
• neg
Experimental
0
--
•
I
I
I
I
0
5
10
15
20
25
30
VOLTAGE, V Fig. 3. Current-voltage characteristic of the aspiration condenser. Particle-free air.
efficiency. Furthermore, it can be noticed that the current, after having reached the plateau, does not decrease despite the increase in the voltage. This property can be regarded as an indication o f the negligible loss o f ions in the entrance o f the aspiration condenser. The electrometers o f the ion meter are constructed by using commercial electrometer amplifiers and the high value resistors. The lowest current values which can be reliably measured are in the range o f 0 . 0 0 5 - 0 . 0 1 pA, which means that the detection limit of the ion meter is approximately 10 ions/cm 3. In the conditions where, e . g . , rapid and strong variations in the air temperature occur, it is possible that the currents due to the small movements in the metal insulator interfaces can cause serious errors. In room air conditions the effects o f these phenomena are normally negligible. High relative humidity can lead to a reduction in the resistance o f the high value resistors, since the glass envelope o f the resistor starts to conduct. In order to eliminate these problems the electrometers have been installed into a airtight box containing a small amount of water-absorbing material. In addition to the zero instability caused by the strong temperature variations, there are other possible sources o f error. W h e n measuring ions in the case o f high radon concentration, it is possible that the alpha radiation from radioactive ions collected on the electrodes generates a small background ion current. This leads to an increase in the zero level o f the ion meter, and consequently the concentration values measured
The present ion meter, shown in Fig. 4, has been used for several years in studies dealing with the behaviour o f air ions in indoor air. In this paper exampies of the results from the measurements in controlled laboratory conditions are presented. These measurements have been carried out in a test room with a volume o f 25 m 3. During these studies simultaneous measurements o f ions, radon, and aerosol particles have been carried out. The concentration of radon has been measured by using a traditional air ionization chamber (Janka and Lehtimaki, 1982). The measurement o f particle concentration has been carried out with the aid of an optical particle size analyzer (PMS Las x). The test room is constructed in such a way that the concentrations o f radon and aerosol particles can be carefully controlled. As the radon source, 226RaC12salt dilution (activity 1 mCi) has been used. Nearly monodisperse particles have been generated by using a condensation-type aerosol generator. The relationship between the concentration o f radon and air ions in the particle-free air is shown in Fig. 5. This result indicates that the equilibrium ion concentration in particle-free air is proportional to the square root o f the radon concentration. This is the case for high radon concentrations. When the radon concentration is low, the effect o f the background radiation starts to dominate. A typical relationship between the ion concentrations and the concentration o f aerosol particles is shown in Fig. 6. This result clearly indicates that the ion concentration is strongly dependent on the particle concentration. Furthermore, it can be noticed that in particle-free air the concentration o f positive ions equals that o f negative ions. The unequal concentrations are connected to the presence o f aerosol particles. The purpose o f the results shown in Fig. 6 is to demonstrate how the concentrations o f ions decrease as the particle concentration increases. It must be emphasized that the concentration curves are quite different for aerosols with different particle sizes. The importance of the particle size will be more thor-
112
M. Lehtimiiki a n d O. Graeffe
Fig. 4. Photograph of the ion meter. 5-104 ,•E
oughly discussed in a separate paper (Lehtim~iki, 1986). An e x a m p l e of the ion concentrations in the presence o f artificial ion sources is shown in Fig. 7. These results have been determined in a test r o o m by using an unipolar corona ion generator. The m e a s u r e m e n t s have been carried out for both positive and negative ionization voltages. T h e concentration of air ions for different corona voltages were measured at a distance o f appr. 1.3 m f r o m the corona electrode. It must be noticed that the results in Fig. 7 correspond to two
o
z"
o
2.1d
i,z
"' U
°"e~wo.*f'"
104
Z
0 0 5"103 z 0
4 l Jl~
~
-O" 4 )
0
.~.q,o~,-e-"
2.10~ 20
i
50
~
,
,,n
t
100
=
200
|
n
n
500
, , , ,
1000
RADON CONCENTRATION, Bq m"3
Fig. 5. The relationship between the concentrations of ions and radon. Particle-free air.
(9|
2.10 5
E
i 4000
0
i
?
I
• o
neg pos t ~
• neg
2 3ooo
"0 Z 0 (J
2000
z
oQ.qm-
z
2 10001
1 05
O;-"
Z
o
"i, °%-o
0
"°'~.o. o
"~.11.0
"'0-.0.0. 0 "I)'41'I) .11 "<)0'''0~ " "OttO-Q•'" "•t) 0
0
0
i
IZ
i
w
|
o poe
Eu
E Z
I
I
I
200
400
t
0 600
PARTICLE CONCENTRATION, cm -3
Fig. 6. Example o f the relationship between the concentrations of ions and aerosol particles. D O P particles, average particle size 1.7 I~.
t
2
l
t
4
i
t
6 CORONA
I
I
8
I
10
V O L T A G E , kV
Fig. 7. Ion concentration as a function of corona voltage. (Note: during the positive air ionization the concentration of negative ions is near zero and vice versa.)
Measurement of air ions
different conditions. During the positive air ionization the concentration of negative ions is practically zero and vice versa. Acknowledgements--This work has been supported by the Academy of Finland and the Ministry of Trade and Industry. The contributions of K. Janka and E. M~cinen in constructing the electronics of the ion meter are sincerely appreciated.
References Bigu, J. (1983) On the effect of a negative ion-generator and a mixing fan on the plate-out of radon decay products in a radon box, Health
113
Phys. 44, 259-266. Israel, H. (1970) Atmospheric electricity, vol. 1. Israel Program for Scientific Translations, Jerusalem. Janka, K. and Lehtimgki, M. (1982) Method of eliminating the effect of decay products in continuous measurement of 222Rn, Rev. Sci. lnstrum. 53, 523-527. Krueger, A. P. and Reed, E. J. (1976) Biological impact of small air ions, Science 193 1209-1213. Lehtimiki, M. (1986) Air ions and indoor air quality, to be published. Tammet, H. F. (1970) The aspiration method for the determination of atmospheric-ion spectra. Israel Program for Scientific Translations, Jerusalem. Whitby, K. T. and Liu, B. Y. H. (1976) The electrical behaviour of aerosols, in Aerosol Science, C. N. Davies, ed., pp. 59-86. Academic Press, London.