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Mobility of small atmospheric ions in the air from the ground at Uppsala
Journal of Atmospheric and Terrestrial Physics, 1953, Vol. 4, pp. 106 to 111. Pergamon Press Ltd., London
Mobility of small atmospheric ions in the air from the ground at Uppsala R.
SIKSNA
I n s t i t u t e of H i g h T e n s i o n R e s e a r c h , U n i v e r s i t y of U p p s a l a , Sweden
(Received 4 July 1953) ABSTRACT B y c o u n t i n g t h e ions in t h e air below a s h e e t covered over t h e g r o u n d , u s i n g a simple a s p i r a t i o n condenser, it is s h o w n t h a t t h e s m a l l ions m u s t be d i v i d e d w i t h r e s p e c t to t h e i r mobilities into t w o fractions, w h i c h m a y be a t t r i b u t e d to t h e initial s m a l l ions a n d t h e aifected s m a l l ions. T h e ions c o u n t e d were f o r m e d b y t h e r a d i o a c t i v e s u b s t a n c e s e x h a l e d f r o m t h e soil. I t w a s o b s e r v e d t h a t t h e m o b i l i t y distrib u t i o n of t h e ions d e p e n d e d on t h e speed w i t h w h i c h t h e air w a s s u c k e d t h r o u g h t h e i n t a k e pipe to t h e counter.
1.
INTRODUCTION
In the layers of atmosphere near the ground the small ions are mainly produced by radioactlive substances. The content of the radioactive substances is higher t h a n normal in the surroundings of this Institute (NoRINDER et al., 1952). An increase has also been observed in the concentration of small ions caused by accumulation of emanation exhaled from the soil during quiet summer nights (NoRI~DER and SIKS~A, 1952). During measurements of the mobility of ions present in the open air under such conditions (NORI~DER and SIKS~A, 1953), the idea arose t h a t it might b e possible to observe an enhanced concentration of ions in the air near the ground b y covering the ground and so preventing the exhaled emanation from dispersing in the higher layers of the atmosphere. And in fact, when the ground was covered with an impermeable screen the ion concentration was considerably higher in the air sucked from the space between the ground and the sheet t h a n it was in the open air. The mobilities of these ions were investigated, in accordance with the view indicated in ( N o R I N D E R a n d SIKSI~-A, in press), t h a t the ions commonly called small atmospheric ions consist of initial small ions and affected small ions. 2.
MEASURING ARRANGEMENT
The place for measuring was t h a t used earlier when measuring atmospheric ions at this Institute (NoRINDER and SIKSNA, 1952; NORINDER a n d SIKSNA, 1953). The measuring devices were placed in an observation hut (Fig. 1). The measuring apparatus was the same as used for determining the mobility of small ions in the open air during summer nights (NoRINDER a n d S[KSNA, 1953): an ion-counter IV with the Weger aspiration condenser. This counter was connected with the suction box B. An Ebert counter E was connected with the suction box in order to maintain a suitable air-flow through the box. The air was sucked into the suction box through a 3 m long pipe P, which consisted partly of a brass tube of 3 cm diameter and partly of a flexible metal tube wrapped with insulating tape to prevent leakage. The suction pipe was partly buried in the earth, so t h a t its open end came out a little above the ground, as shown in Fig. 1. Over the open end of 106
Mobility of small atmospheric ions in the air from the ground at Uppsala t h e pipe was placed a rectangular sheet of zinc 175 X 270 cm, and with the edges bent down so t h a t a 10 cm high closed space was built close to the grass-covered ground. The ions counted came from this space. In order to determine the losses of ions during passage of air t hr ough the pipe, the latter could be bent so t h a t its open end came under the protecting box on the roof of the hut, as shown in Fig. 1. B y closing and opening the slides Sp and Su it was possible to suck the air supplying ions for counting from the open air at the level of the roof alternately through the pipe P and through the wide opening T. For determining t he mobility of the ions present, the m et hod of periodical alternation of chosen limiting mobilities of the ion-counter was used ( N O R I N D E R a n d SIKSNA, 1953).
t~
: E~S E
I i
II I i
II
s !
Fig. 1.
ii'=-: !
"
The apparatus used: W small ion counter with a Weger condenser; E Ebert ion counter;
B suction box; P suction pipe; S sheet covering the ground; T opening to the open air; C protection box; S,,, S~, S,,, SE slides; VC vacuum cleaners. 3.
RESULTS
The concentration of ions in the air sucked from the space below the zinc screen was considerably higher t h a n t h a t in the open air. With the given measuring ar r an g emen t it was highest, c. 10 times higher than in the open air, when air was slowly sucked into the suction box (q) ---- 400 cm3/sec) only t hrough the measuring counter W. Whe n the air was sucked into the suction box with the E b e r t counter as well, the concentration was lower, but even in this case it was 2-3 times greater t h a n in the open air, even if the air-flow through the suction box was the m a x i m u m us ed - - 5 4 0 0 cma/sec. There was no indication t h a t the air with the greater concentration of ions might be exhausted from the space below the sheet, so t h a t the concentration might decrease with time. Although variations of concentration were observed, as shown in Figs. 3 and 4, this was not due to exhaustion of the supply of ions. These variations with time were not investigated in detail, because if t h e y are to be determined more precisely, the screen must also be made so t h a t the air exhaled from the ground is not diluted with other air not containing so 107
R. SIKSNA
70 /
I .... V .....1"
l----[ Negative , ions i 0 !
I ___+m
30
-i
Positive ions
!_a
i
!
I I
20~ i
-
~
I 0 ~
i
t
..... t
-~I
i
i
,
I
0 Fig. 2.
J
I
I
-~---
i
2
3
-V4
i¢ ,/sec I 5
6
Losses of concentration of ions due to passage of the air through a pipe 30 m m d i a m e t e r and 3 m long at different suction speeds (~.
0
1
2k
cm s e d ' : V o l t c m ' '
0
1
2
0
1 0
I 0
I 0
I
2
Fig. 3. The n u m b e r of positive ions in the air from the covered ground at chosen limiting mobilities k , of the counter and w i t h an air-flow of 400 cma/sec t h r o u g h the suction box. The distributiol~ function f(k) of the ions with respect to the mobilities.
108
Mobility of small atmospheric ions in the air from the ground at Uppsala
m a n y ions. R e d u c t i o n of t h e c o n c e n t r a t i o n of ions when the air-flow is more rapid m a y be explained b y reference t o t h e loss of ions during passage of air t hrough the pipe. The variation o f this loss w i t h t h e s p e e d o f air-flow t h r o u g h the pipe is shown in Fig. 2, which gives results of measurements when the open end of the suction pipe was placed on t he roof, and the concentration of ions in the open air was measured b y passing t h e air al t er na t e ly t h r o u g h the pipe P and the opening T. F r o m approx. ¢ ~ 0.8 1/sec the air-flow in the pipe is turbulent, and this fact m a y influence the loss of ions. As shown in Fig. 2, the loss of negative ions is greater t h a n t h a t of positive ions, which m a y be explained b y the higher mobility of negative ions. Because of t he com pa rat i vel y low concentration of ions in the
0
1 2 k crn seE' :Volt cn~'
0
1 2
0
1 0 1 0 1 0 1 2
Fig. 4. The number of negative ions in the air from the coveredground at chosenlimiting mobilities k, of the counter and with an air-flow of 400 cm3/sec through the suction box. The distribution function f(k) of the ions with respect to the mobilities. open air, there was a large r a n d o m error in the measurements of ion losses with the ar r an g emen t mentioned, especially at lower suction rates. In Figs. 3 and 4 the curves of the n u m b e r of ions measured at the chosen limiting mobilities k, of the counter condenser are shown for the air sucked from the space below the sheet at ¢ ~ 400 cm3/sec. T hough a r a n d o m variation m a y be seen, the numbers of ions counted at the three lower k, values--0.14, 0.31, 0.54 cm sec-1/volt c m - l - - c o u l d be assumed to be upon one curve, and m a y therefore be t a k e n to represent the saturation at these limiting mobilities. For the first two measuring periods, averages of the measured num be r s of ions were found for each limiting mobility k , , and with the aid of the n = F ( 1 / k , ) and n ---- ¢ ( k , ) characteristics the distribution function f(/c) with respect to the mobilities was computed. For the t hi rd 109
R. Sz~rs~(A
measuring period, the greater variations in the figures made it necessary to compute the mobility distribution of ions for four particular instants from the intersection of vertical straight lines with the curves giving the number of ions counted. As m a y be seen in the figure.s all the f(/c) obtained are similar to t h a t obtained for ions in the open air (NORINDER a n d SrKSNA, 1953). Furthermore, as in t h a t case, no a t t e m p t has been made to distinguish a finer structure of the distribution function. The mobility distribution of the present ions is as follows: (1) The occupied range of mobilities is comparatively wide. I t begins at c. 0.4-0-5 cm sec-1/volt cm -1 for positive ions, and at c. 0.6-0.8 cm sec-1/volt cm -1 for negative ions, and reaches c. 2 cm sec-1/volt cm -1 for positive ions. The upper limit of mobility for negative ions is usually c. 2.3-2.5 cm sec-1/volt cm -1. This fact is not shown in ,
4o0
•
f(k)
(+)
5000
¢
cm-sec .
•
5400 cm3secI
(+)
(-)
(-)
I~///,
4000 -
v / l l i / / j
3 0 0 0 - - ~'//" . / / ,
-
/.//L
2000 ~
,ooo v / / ,
0
i 0
1
/.'/ ///
//'Z 2 0
2 0 1 2 0 k cm see 1 : Volt cn~I
1
2
Fig. 5. The distribution function f(k) of the ions with respect to the mobilitios at the suction speeds (I) = 400 cma/sec and (I) ~ 5400 cm3]sec through the suction box. the figures, but the columns for higher mobilities are adequately heightened in these cases. (2) The distribution function m a y be divided into two components: ions with lower mobilities are included in one, ions with higher mobilities in the other. In some cases, a structure m a y be detected in these two components. The component representing lower mobilities is higher t h a n t h a t representing the higher. For positive ions the component representing lower mobilities is wider. At the ends of the observation periods considered, the component for higher mobilities increases--especially in the case of negative ions, where a higher column m a y be observed. Similar distribution functions were obtained from other series of measurements, by using the saxhe air-flow ¢ ---- 400 cma/sec and another of q) ---- 5400 cmS/sec through the suction box, as shown in Fig. 5. When the airflow was greater, the number of ions was smaller, because of higher losses caused by the turbulent air-flow in the pipe; but the component representing higher mobilities was raised in this case for both the negative and the positive ions. I t will be seen t h a t the observed facts confirm the idea (NORINDER a n d SrKs~A, 1953) t h a t in the region of mobilities attributed to small atmospheric ions two 110
Mobility of small atmospheric ions in the air from the ground at Uppsala
kinds of small ions must be accepted: the initial small ions and the affected small ions. The proportion of initial small ions m a y increase if the total concentration of ions increases because of the formation of new ions, as shown in Figs. 3 and 4 during the last period when the total concentration increased. The proportion of initial small ions m a y also be greater if the air from the space between the ground and the sheet is transported to the counter with greater velocity (¢ -~ 5400 cm3/sec, Fig. 5), because in this case the ions formed b y the radioactive substances in the air conveyed through the pipe will enter the counter a shorter time after their formation. In order to obtain greater clarity on the problems dealt with, particularly concerning the shape of the distribution function, a more precise method of measuring ionic mobilities must be used.
Acknowledgments--The author is grateful to the Director of the Institute of High Tension Research of the University of Uppsala, Professor H. NORINDER, for providing him with facilities to carry out this work and for his interest in its progress. Thanks are also due to the Swedish Natural Science Research Council for placing means at the disposal of the Institute to perform this investigation. The author gratefully acknowledges the assistance of Mr. A. METNIEKS and Dr. H. A. WATSO~ in the observations and calculations.
REFERENCES NORINDER, H., METNIEKS, A. and SIKSNA, R.
1952
NORINDER, H. and SIKSNA, R.
1952
NORINDER, H. and SIKSNA, 1:~.
1953
R a d o n content of the air in the soil at Uppsala. Kungl. Svenska Vetenskapsakademiens Ark. f. geofys. 1, Nr 21, 571-579 Variations of the concentration of ions at different heights near the ground during quiet summer nights at Uppsala. Kungl. Svenska Vetenskapsakademiens Ark. f. geofys. 1, Nr 19, 519-541 Mobility of atmospheric small ions during summer nights at Uppsala. J. Atrnosph. Terr. Phys. 4, 93
111
Mobility of small atmospheric ions in the air from the ground at Uppsala R.
SIKSNA
I n s t i t u t e of H i g h T e n s i o n R e s e a r c h , U n i v e r s i t y of U p p s a l a , Sweden
(Received 4 July 1953) ABSTRACT B y c o u n t i n g t h e ions in t h e air below a s h e e t covered over t h e g r o u n d , u s i n g a simple a s p i r a t i o n condenser, it is s h o w n t h a t t h e s m a l l ions m u s t be d i v i d e d w i t h r e s p e c t to t h e i r mobilities into t w o fractions, w h i c h m a y be a t t r i b u t e d to t h e initial s m a l l ions a n d t h e aifected s m a l l ions. T h e ions c o u n t e d were f o r m e d b y t h e r a d i o a c t i v e s u b s t a n c e s e x h a l e d f r o m t h e soil. I t w a s o b s e r v e d t h a t t h e m o b i l i t y distrib u t i o n of t h e ions d e p e n d e d on t h e speed w i t h w h i c h t h e air w a s s u c k e d t h r o u g h t h e i n t a k e pipe to t h e counter.
1.
INTRODUCTION
In the layers of atmosphere near the ground the small ions are mainly produced by radioactlive substances. The content of the radioactive substances is higher t h a n normal in the surroundings of this Institute (NoRINDER et al., 1952). An increase has also been observed in the concentration of small ions caused by accumulation of emanation exhaled from the soil during quiet summer nights (NoRI~DER and SIKS~A, 1952). During measurements of the mobility of ions present in the open air under such conditions (NORI~DER and SIKS~A, 1953), the idea arose t h a t it might b e possible to observe an enhanced concentration of ions in the air near the ground b y covering the ground and so preventing the exhaled emanation from dispersing in the higher layers of the atmosphere. And in fact, when the ground was covered with an impermeable screen the ion concentration was considerably higher in the air sucked from the space between the ground and the sheet t h a n it was in the open air. The mobilities of these ions were investigated, in accordance with the view indicated in ( N o R I N D E R a n d SIKSI~-A, in press), t h a t the ions commonly called small atmospheric ions consist of initial small ions and affected small ions. 2.
MEASURING ARRANGEMENT
The place for measuring was t h a t used earlier when measuring atmospheric ions at this Institute (NoRINDER and SIKSNA, 1952; NORINDER a n d SIKSNA, 1953). The measuring devices were placed in an observation hut (Fig. 1). The measuring apparatus was the same as used for determining the mobility of small ions in the open air during summer nights (NoRINDER a n d S[KSNA, 1953): an ion-counter IV with the Weger aspiration condenser. This counter was connected with the suction box B. An Ebert counter E was connected with the suction box in order to maintain a suitable air-flow through the box. The air was sucked into the suction box through a 3 m long pipe P, which consisted partly of a brass tube of 3 cm diameter and partly of a flexible metal tube wrapped with insulating tape to prevent leakage. The suction pipe was partly buried in the earth, so t h a t its open end came out a little above the ground, as shown in Fig. 1. Over the open end of 106
Mobility of small atmospheric ions in the air from the ground at Uppsala t h e pipe was placed a rectangular sheet of zinc 175 X 270 cm, and with the edges bent down so t h a t a 10 cm high closed space was built close to the grass-covered ground. The ions counted came from this space. In order to determine the losses of ions during passage of air t hr ough the pipe, the latter could be bent so t h a t its open end came under the protecting box on the roof of the hut, as shown in Fig. 1. B y closing and opening the slides Sp and Su it was possible to suck the air supplying ions for counting from the open air at the level of the roof alternately through the pipe P and through the wide opening T. For determining t he mobility of the ions present, the m et hod of periodical alternation of chosen limiting mobilities of the ion-counter was used ( N O R I N D E R a n d SIKSNA, 1953).
t~
: E~S E
I i
II I i
II
s !
Fig. 1.
ii'=-: !
"
The apparatus used: W small ion counter with a Weger condenser; E Ebert ion counter;
B suction box; P suction pipe; S sheet covering the ground; T opening to the open air; C protection box; S,,, S~, S,,, SE slides; VC vacuum cleaners. 3.
RESULTS
The concentration of ions in the air sucked from the space below the zinc screen was considerably higher t h a n t h a t in the open air. With the given measuring ar r an g emen t it was highest, c. 10 times higher than in the open air, when air was slowly sucked into the suction box (q) ---- 400 cm3/sec) only t hrough the measuring counter W. Whe n the air was sucked into the suction box with the E b e r t counter as well, the concentration was lower, but even in this case it was 2-3 times greater t h a n in the open air, even if the air-flow through the suction box was the m a x i m u m us ed - - 5 4 0 0 cma/sec. There was no indication t h a t the air with the greater concentration of ions might be exhausted from the space below the sheet, so t h a t the concentration might decrease with time. Although variations of concentration were observed, as shown in Figs. 3 and 4, this was not due to exhaustion of the supply of ions. These variations with time were not investigated in detail, because if t h e y are to be determined more precisely, the screen must also be made so t h a t the air exhaled from the ground is not diluted with other air not containing so 107
R. SIKSNA
70 /
I .... V .....1"
l----[ Negative , ions i 0 !
I ___+m
30
-i
Positive ions
!_a
i
!
I I
20~ i
-
~
I 0 ~
i
t
..... t
-~I
i
i
,
I
0 Fig. 2.
J
I
I
-~---
i
2
3
-V4
i¢ ,/sec I 5
6
Losses of concentration of ions due to passage of the air through a pipe 30 m m d i a m e t e r and 3 m long at different suction speeds (~.
0
1
2k
cm s e d ' : V o l t c m ' '
0
1
2
0
1 0
I 0
I 0
I
2
Fig. 3. The n u m b e r of positive ions in the air from the covered ground at chosen limiting mobilities k , of the counter and w i t h an air-flow of 400 cma/sec t h r o u g h the suction box. The distributiol~ function f(k) of the ions with respect to the mobilities.
108
Mobility of small atmospheric ions in the air from the ground at Uppsala
m a n y ions. R e d u c t i o n of t h e c o n c e n t r a t i o n of ions when the air-flow is more rapid m a y be explained b y reference t o t h e loss of ions during passage of air t hrough the pipe. The variation o f this loss w i t h t h e s p e e d o f air-flow t h r o u g h the pipe is shown in Fig. 2, which gives results of measurements when the open end of the suction pipe was placed on t he roof, and the concentration of ions in the open air was measured b y passing t h e air al t er na t e ly t h r o u g h the pipe P and the opening T. F r o m approx. ¢ ~ 0.8 1/sec the air-flow in the pipe is turbulent, and this fact m a y influence the loss of ions. As shown in Fig. 2, the loss of negative ions is greater t h a n t h a t of positive ions, which m a y be explained b y the higher mobility of negative ions. Because of t he com pa rat i vel y low concentration of ions in the
0
1 2 k crn seE' :Volt cn~'
0
1 2
0
1 0 1 0 1 0 1 2
Fig. 4. The number of negative ions in the air from the coveredground at chosenlimiting mobilities k, of the counter and with an air-flow of 400 cm3/sec through the suction box. The distribution function f(k) of the ions with respect to the mobilities. open air, there was a large r a n d o m error in the measurements of ion losses with the ar r an g emen t mentioned, especially at lower suction rates. In Figs. 3 and 4 the curves of the n u m b e r of ions measured at the chosen limiting mobilities k, of the counter condenser are shown for the air sucked from the space below the sheet at ¢ ~ 400 cm3/sec. T hough a r a n d o m variation m a y be seen, the numbers of ions counted at the three lower k, values--0.14, 0.31, 0.54 cm sec-1/volt c m - l - - c o u l d be assumed to be upon one curve, and m a y therefore be t a k e n to represent the saturation at these limiting mobilities. For the first two measuring periods, averages of the measured num be r s of ions were found for each limiting mobility k , , and with the aid of the n = F ( 1 / k , ) and n ---- ¢ ( k , ) characteristics the distribution function f(/c) with respect to the mobilities was computed. For the t hi rd 109
R. Sz~rs~(A
measuring period, the greater variations in the figures made it necessary to compute the mobility distribution of ions for four particular instants from the intersection of vertical straight lines with the curves giving the number of ions counted. As m a y be seen in the figure.s all the f(/c) obtained are similar to t h a t obtained for ions in the open air (NORINDER a n d SrKSNA, 1953). Furthermore, as in t h a t case, no a t t e m p t has been made to distinguish a finer structure of the distribution function. The mobility distribution of the present ions is as follows: (1) The occupied range of mobilities is comparatively wide. I t begins at c. 0.4-0-5 cm sec-1/volt cm -1 for positive ions, and at c. 0.6-0.8 cm sec-1/volt cm -1 for negative ions, and reaches c. 2 cm sec-1/volt cm -1 for positive ions. The upper limit of mobility for negative ions is usually c. 2.3-2.5 cm sec-1/volt cm -1. This fact is not shown in ,
4o0
•
f(k)
(+)
5000
¢
cm-sec .
•
5400 cm3secI
(+)
(-)
(-)
I~///,
4000 -
v / l l i / / j
3 0 0 0 - - ~'//" . / / ,
-
/.//L
2000 ~
,ooo v / / ,
0
i 0
1
/.'/ ///
//'Z 2 0
2 0 1 2 0 k cm see 1 : Volt cn~I
1
2
Fig. 5. The distribution function f(k) of the ions with respect to the mobilitios at the suction speeds (I) = 400 cma/sec and (I) ~ 5400 cm3]sec through the suction box. the figures, but the columns for higher mobilities are adequately heightened in these cases. (2) The distribution function m a y be divided into two components: ions with lower mobilities are included in one, ions with higher mobilities in the other. In some cases, a structure m a y be detected in these two components. The component representing lower mobilities is higher t h a n t h a t representing the higher. For positive ions the component representing lower mobilities is wider. At the ends of the observation periods considered, the component for higher mobilities increases--especially in the case of negative ions, where a higher column m a y be observed. Similar distribution functions were obtained from other series of measurements, by using the saxhe air-flow ¢ ---- 400 cma/sec and another of q) ---- 5400 cmS/sec through the suction box, as shown in Fig. 5. When the airflow was greater, the number of ions was smaller, because of higher losses caused by the turbulent air-flow in the pipe; but the component representing higher mobilities was raised in this case for both the negative and the positive ions. I t will be seen t h a t the observed facts confirm the idea (NORINDER a n d SrKs~A, 1953) t h a t in the region of mobilities attributed to small atmospheric ions two 110
Mobility of small atmospheric ions in the air from the ground at Uppsala
kinds of small ions must be accepted: the initial small ions and the affected small ions. The proportion of initial small ions m a y increase if the total concentration of ions increases because of the formation of new ions, as shown in Figs. 3 and 4 during the last period when the total concentration increased. The proportion of initial small ions m a y also be greater if the air from the space between the ground and the sheet is transported to the counter with greater velocity (¢ -~ 5400 cm3/sec, Fig. 5), because in this case the ions formed b y the radioactive substances in the air conveyed through the pipe will enter the counter a shorter time after their formation. In order to obtain greater clarity on the problems dealt with, particularly concerning the shape of the distribution function, a more precise method of measuring ionic mobilities must be used.
Acknowledgments--The author is grateful to the Director of the Institute of High Tension Research of the University of Uppsala, Professor H. NORINDER, for providing him with facilities to carry out this work and for his interest in its progress. Thanks are also due to the Swedish Natural Science Research Council for placing means at the disposal of the Institute to perform this investigation. The author gratefully acknowledges the assistance of Mr. A. METNIEKS and Dr. H. A. WATSO~ in the observations and calculations.
REFERENCES NORINDER, H., METNIEKS, A. and SIKSNA, R.
1952
NORINDER, H. and SIKSNA, R.
1952
NORINDER, H. and SIKSNA, 1:~.
1953
R a d o n content of the air in the soil at Uppsala. Kungl. Svenska Vetenskapsakademiens Ark. f. geofys. 1, Nr 21, 571-579 Variations of the concentration of ions at different heights near the ground during quiet summer nights at Uppsala. Kungl. Svenska Vetenskapsakademiens Ark. f. geofys. 1, Nr 19, 519-541 Mobility of atmospheric small ions during summer nights at Uppsala. J. Atrnosph. Terr. Phys. 4, 93
111