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Comparison between the SDI and electrical methods for the measurement of fine aerosols
J. Aerosol Sci., Vol. 20, No. 8, pp. 1505-1508, Printed in Great Britain.
1989.
0021-8502/89 $3.00 + 0.OO Pergamon Press plc
COMPARISON BETWEEN THE SDI AND ELECTRICAL METHODS FOR THE MEASUREMENT OF FINE AEROSOLS
D. BOULAUD, C. COMPER, M. DIOURI* Laboratoire de Physique et M~trologie des A~rosols IPSN/DPT/SPIN/LEPA Commissariat g l'Energie Atomique BP 6, 92265 FONTENAY-AUX-ROSES CEDEX, France
i. INTRODUCTION This new inertial and diffusional spectrometer ("Spectrom~tre Diffusionnel et Inertiel" : SDI 2000) has been designed to cover a 2000 fold range in particle size from 0.0075 to 15 ~m, and to define the mass distribution corresponding to the real aerodynamic behavior of the aerosol examined. To achieve this, we chose to characterize the aerosol in terms of its inertial behavior, for particles larger than 0.3 pm, and its diffuslonal behavior, for smaller particles. To this end, we combined a cascade impactor in series with a diffusion battery comprising parallel pipes. To determine the mass distribution of an aerosol, particles are selected first on the basis of inertia, by collection on the different levels of the impactor, and then diffusion, by collection on filters downllne from the pipes where the aerosol diffuses. Operation of cascade impactors is now quite advanced and we selected the Andersen Mark II model. The nominal flow rate of this instrument is set at 28.3 I/min, which fixes the flow through each pipe, once the number of pipes in the diffusion battery is chosen. The choice of the type of diffusion battery suitable for use with the SDI 2000 essentially depends on three types of constraints : i. knowledge of the diffusion laws, 2. the risk of clogging of the diffusive medium during sampling of highly concentrated aerosols, 3. the cost of the diffusive medium and the building of the diffusion battery. The two most commonly used diffusion batteries consist of cylindrical or parallelepiped pipes and wire screen. We chose a granular bed of spherical glass beads. This is a medium commonly used in problems of gas clean-up, but to our knowledge has never been used as a diffusive medium in a diffusion battery. This medium allows constraints 2 and 3 to be dealt with effectively, and requires an experimental study of the laws of diffusion of an aerosol in a granular bed to deal with constraint i / i /. 2. DESCRIPTION AND OPERATION OF THE SDI 2000 A complete description of the SDI 2000 has been published elsewhere / 2, 3 /. Given below is a brief description of its main components and outline of its operation. Figure i is a schematic representation of the SDI 2000. The inertia unit comprises an eight-stage Andersen Mark II cascade impactor with operational cut-off diameters in the 0.35 to 7.5 pm range for a nominal flow rate of 28.3 i/mln. The diffusion unit consists of six parallel pipes 20 cm long and 4 cm in diameter containing granular beds of different depths. Beads diameter varies between i and 5 mm depending on the desired collection efficiency. The sixth pipe is empty and serves as a reference. Downline, six filters collect particles that have passed through the impactor and the different granular beds. The total flow rate corresponds to the nominal flow rate of the impactor, and the flow rate in each pipe is maintained constant by the use of critical orifices. The duration of sampling and the volume of the air sample depend on the concentration of the aerosol and on the method of analysis used to determine the mass deposited on the collection surfaces of the impactor, and on the filters placed downline from the diffusion battery. These data can be used to calculate the particle size distribution of the aerosol, employing an algorithm based on a numerical inversion treatment of Fredholm's equation using the non linear iterative method introduced by S. TWOMEY / 4 /. 3. PERFORMANCE OF THE SDI 2000 The study of the performance of the SDI 2000 focused essentially on the influence of the collection surface in the impactor on the capture of the smallest particles. The test * Laboratoire de Physique des A4rosols et de Transfert des Contaminations, Paris XII et Universit~ d'OUJDA, Maroc
1505
Universlt~
1506
D° BOULAUD et al.
aerosol chosen was that produced by a uranine generator used for control tests on high-efficiency filters according to the standard AFNOR NFX-44011. Our reference measurement involves the use of glass plates coated with grease as collection surfaces in the impactor / 2 /. Figure 2 indicates an example of a measurement, and shows a histogram generated from the data by the method of TWOMEY and fitted a log-normal law from the resulting mean mass diameter and the geometric standard deviation. The truncated histogram for particles larger than 0,3 ~m is due to the fact that in the uranine aerosol generator there is a cascade of two pipes which ensures the capture of large droplets. The reproducibility of these measurements is very good since for more than ten identical tests variation in mean mass diameter and in geometric standard deviation did not exceed 5 %. The effect of the impactor collection was studied using five types of surface : glass plates and metal plates, greased or ungreased, and filters. A coating of grease on the collection surfaces is recommended for capture of large particles (> i ~m) which cannot therefore rebound or be re-entrained / 2 /. In the case of finer particles (< I ~m), this effect was still apparent since the mean mass diameter determined with ungreased plates was always smaller. This reduction in mean mass diameter was small (about i0 %) but significant. As can be seen from figure 3, the largest effects were noted with filters. In this case, the changes induced in the mean mass diameter and geometric standard deviation were about + 30 % and + 40 %, respectively. The probable explanation of this phenomenon is that since the filters were uneven and permeable to air, they captured a large fraction of the finest particlesj which were therefore counted in the impactor rather than in the diffusion battery. It is therefore highly advisable to use greased plates of glass or metal, the minor deviation between these supports doubtless being due to a small difference in the distance separatin H the acceleration stage and the collection surface. 4. COMPARISON OF THE SDI 2000 WITH INSTRUMENTS BASED ON ELECTRICAL PROPERTIES OF AEROSOLS In this comparison, the above aerosol generator was adjusted to provide a range of particle size distributions. The SDI 2000 was compared with the EAA (Electrical Aerosol Analyser) and the DMPS (Differential Mobility Particle Sizer). The main results of this comparison are shown in figures 4 and 5, which represent respectively the comparisons of mean mass diameter and the mass produced by the generator per unit time. It can be seen then from figure 4 that the diameters determined by the electrical instruments were always smaller, the deviation becoming considerable when the diameter measured by the SDI exceeded 0.5 ~m. For diameters of about 0.2 ~m, the deviation was small, but increased at higher values determined by the SDI. The main reason for this increasing deviation is doubtless due to the fact that larger particles were not counted by electrical instruments, particularly for the DMPS, because of the presence of an impactor at the sample inlet. This hypothesis seems to be confirmed by comparison of the measurements of mass generated per unit time (figure 5). Indeed, when agreement was very good for the lowest masses generated (in the absence of large particles), the deviation increased, particularly for the DMPS, at higher values (presence of increasingly large particles). REFERENCES / i DIOURI M., BOULAUD D., MADELAINE G. (1986) Collection of fine particles by granular bed . In Aerosols Formation and Reactivity, p. 842, Pergamon Press. / 2
DIOURI M. (1987) Contribution g l'~tude du comportement a~rodynamique des a~rosols. Mise au point du SDI 2000 Rapport CEA-R-5412, Th~se d'~tat de l'Universit~ PARIS XII.
/ 3
BOULAUD D., DIOURI M. (1988) A new inertial and diffusional device 927-930.
/ 4
(SDI 2000). J. of Aerosol Science, 19,
(7),
TWOMEY S. (1977) In introduction to the mathematics of inversion in remote sensing and indirect measurements. Elsevier, Amsterdam.
M e a s u r e m e n t of flne a e r o s o l s
A B [ D £ F
l A------.-
D
1507
' cascade impactor • granular beds • filter holders • volume with 6 critical orifices pump • f[owmeter
-
Figure 1 - Schematic r e p r e s e n t a t i o n of the SDI
G r e a s e coated glass p l a t e s Mass mean diameter = .2087 I~m Geometric standard deviation = 1.717
2.S
2.0
-
O O
1.S "= 1 . 0 0.5uu".^
,
.001
,
,
.....
I
.01
.,
,
,
.1 Diameter (~m)
,
. . . . . . .
1
Figure 2 - Example of results o b t a i n e d w i t h the SDI
,
......
I
1508
D. B O U L A U D et al.
2.5 ~
2.0 o
5lass (grease) D.M.M. : .201 pm S.D. = 1.72 I
~' ~=~
1,5
=
1.0
I Mefa[ (grease) I - I D,M.M. = .189 Pm I ! S D : 183 !
0.5
f-- ............
I
i Filter i D.M.M. = .3 pm
i i
I S,D..._3..O1...... ]
o o
,
.......
/'~ f \\ / \.~ ///f'-~'\ I//'
,
.001
"~ " \
.JOF
\
"~.
,
,01
.I
I
Diameter (IJm)
Figure 3 - Influence of c o l l e c t i o n surface in impactor
Oiamefers comparison
Comparison of generated mass (mglh)
"~ ~ ~ ~
150J o EAA
~.
125 ,4
s
100
o
3
|
7s
E..z
~
so
2:
,1
l
i
T
~
.2
,3
.4
.5
J
r
~
~
~_ -
M.M.D SOl (l~m)
Figure 4 - C o m p a r i s o n of m e a n mass diameters o b t a i n e d by the SDI and by e l e c t r i c a l m e t h o d s
"
0
qm4R 1
25
. I
50
~.
~1
75 SOl
!
. i
100
125
•
i
150
Figure 5 - C o m p a r i s o n of g e n e r a t e d mass by unit of time as m e a s u r e d by the SDI and by e l e c t r i c a l methods
1989.
0021-8502/89 $3.00 + 0.OO Pergamon Press plc
COMPARISON BETWEEN THE SDI AND ELECTRICAL METHODS FOR THE MEASUREMENT OF FINE AEROSOLS
D. BOULAUD, C. COMPER, M. DIOURI* Laboratoire de Physique et M~trologie des A~rosols IPSN/DPT/SPIN/LEPA Commissariat g l'Energie Atomique BP 6, 92265 FONTENAY-AUX-ROSES CEDEX, France
i. INTRODUCTION This new inertial and diffusional spectrometer ("Spectrom~tre Diffusionnel et Inertiel" : SDI 2000) has been designed to cover a 2000 fold range in particle size from 0.0075 to 15 ~m, and to define the mass distribution corresponding to the real aerodynamic behavior of the aerosol examined. To achieve this, we chose to characterize the aerosol in terms of its inertial behavior, for particles larger than 0.3 pm, and its diffuslonal behavior, for smaller particles. To this end, we combined a cascade impactor in series with a diffusion battery comprising parallel pipes. To determine the mass distribution of an aerosol, particles are selected first on the basis of inertia, by collection on the different levels of the impactor, and then diffusion, by collection on filters downllne from the pipes where the aerosol diffuses. Operation of cascade impactors is now quite advanced and we selected the Andersen Mark II model. The nominal flow rate of this instrument is set at 28.3 I/min, which fixes the flow through each pipe, once the number of pipes in the diffusion battery is chosen. The choice of the type of diffusion battery suitable for use with the SDI 2000 essentially depends on three types of constraints : i. knowledge of the diffusion laws, 2. the risk of clogging of the diffusive medium during sampling of highly concentrated aerosols, 3. the cost of the diffusive medium and the building of the diffusion battery. The two most commonly used diffusion batteries consist of cylindrical or parallelepiped pipes and wire screen. We chose a granular bed of spherical glass beads. This is a medium commonly used in problems of gas clean-up, but to our knowledge has never been used as a diffusive medium in a diffusion battery. This medium allows constraints 2 and 3 to be dealt with effectively, and requires an experimental study of the laws of diffusion of an aerosol in a granular bed to deal with constraint i / i /. 2. DESCRIPTION AND OPERATION OF THE SDI 2000 A complete description of the SDI 2000 has been published elsewhere / 2, 3 /. Given below is a brief description of its main components and outline of its operation. Figure i is a schematic representation of the SDI 2000. The inertia unit comprises an eight-stage Andersen Mark II cascade impactor with operational cut-off diameters in the 0.35 to 7.5 pm range for a nominal flow rate of 28.3 i/mln. The diffusion unit consists of six parallel pipes 20 cm long and 4 cm in diameter containing granular beds of different depths. Beads diameter varies between i and 5 mm depending on the desired collection efficiency. The sixth pipe is empty and serves as a reference. Downline, six filters collect particles that have passed through the impactor and the different granular beds. The total flow rate corresponds to the nominal flow rate of the impactor, and the flow rate in each pipe is maintained constant by the use of critical orifices. The duration of sampling and the volume of the air sample depend on the concentration of the aerosol and on the method of analysis used to determine the mass deposited on the collection surfaces of the impactor, and on the filters placed downline from the diffusion battery. These data can be used to calculate the particle size distribution of the aerosol, employing an algorithm based on a numerical inversion treatment of Fredholm's equation using the non linear iterative method introduced by S. TWOMEY / 4 /. 3. PERFORMANCE OF THE SDI 2000 The study of the performance of the SDI 2000 focused essentially on the influence of the collection surface in the impactor on the capture of the smallest particles. The test * Laboratoire de Physique des A4rosols et de Transfert des Contaminations, Paris XII et Universit~ d'OUJDA, Maroc
1505
Universlt~
1506
D° BOULAUD et al.
aerosol chosen was that produced by a uranine generator used for control tests on high-efficiency filters according to the standard AFNOR NFX-44011. Our reference measurement involves the use of glass plates coated with grease as collection surfaces in the impactor / 2 /. Figure 2 indicates an example of a measurement, and shows a histogram generated from the data by the method of TWOMEY and fitted a log-normal law from the resulting mean mass diameter and the geometric standard deviation. The truncated histogram for particles larger than 0,3 ~m is due to the fact that in the uranine aerosol generator there is a cascade of two pipes which ensures the capture of large droplets. The reproducibility of these measurements is very good since for more than ten identical tests variation in mean mass diameter and in geometric standard deviation did not exceed 5 %. The effect of the impactor collection was studied using five types of surface : glass plates and metal plates, greased or ungreased, and filters. A coating of grease on the collection surfaces is recommended for capture of large particles (> i ~m) which cannot therefore rebound or be re-entrained / 2 /. In the case of finer particles (< I ~m), this effect was still apparent since the mean mass diameter determined with ungreased plates was always smaller. This reduction in mean mass diameter was small (about i0 %) but significant. As can be seen from figure 3, the largest effects were noted with filters. In this case, the changes induced in the mean mass diameter and geometric standard deviation were about + 30 % and + 40 %, respectively. The probable explanation of this phenomenon is that since the filters were uneven and permeable to air, they captured a large fraction of the finest particlesj which were therefore counted in the impactor rather than in the diffusion battery. It is therefore highly advisable to use greased plates of glass or metal, the minor deviation between these supports doubtless being due to a small difference in the distance separatin H the acceleration stage and the collection surface. 4. COMPARISON OF THE SDI 2000 WITH INSTRUMENTS BASED ON ELECTRICAL PROPERTIES OF AEROSOLS In this comparison, the above aerosol generator was adjusted to provide a range of particle size distributions. The SDI 2000 was compared with the EAA (Electrical Aerosol Analyser) and the DMPS (Differential Mobility Particle Sizer). The main results of this comparison are shown in figures 4 and 5, which represent respectively the comparisons of mean mass diameter and the mass produced by the generator per unit time. It can be seen then from figure 4 that the diameters determined by the electrical instruments were always smaller, the deviation becoming considerable when the diameter measured by the SDI exceeded 0.5 ~m. For diameters of about 0.2 ~m, the deviation was small, but increased at higher values determined by the SDI. The main reason for this increasing deviation is doubtless due to the fact that larger particles were not counted by electrical instruments, particularly for the DMPS, because of the presence of an impactor at the sample inlet. This hypothesis seems to be confirmed by comparison of the measurements of mass generated per unit time (figure 5). Indeed, when agreement was very good for the lowest masses generated (in the absence of large particles), the deviation increased, particularly for the DMPS, at higher values (presence of increasingly large particles). REFERENCES / i DIOURI M., BOULAUD D., MADELAINE G. (1986) Collection of fine particles by granular bed . In Aerosols Formation and Reactivity, p. 842, Pergamon Press. / 2
DIOURI M. (1987) Contribution g l'~tude du comportement a~rodynamique des a~rosols. Mise au point du SDI 2000 Rapport CEA-R-5412, Th~se d'~tat de l'Universit~ PARIS XII.
/ 3
BOULAUD D., DIOURI M. (1988) A new inertial and diffusional device 927-930.
/ 4
(SDI 2000). J. of Aerosol Science, 19,
(7),
TWOMEY S. (1977) In introduction to the mathematics of inversion in remote sensing and indirect measurements. Elsevier, Amsterdam.
M e a s u r e m e n t of flne a e r o s o l s
A B [ D £ F
l A------.-
D
1507
' cascade impactor • granular beds • filter holders • volume with 6 critical orifices pump • f[owmeter
-
Figure 1 - Schematic r e p r e s e n t a t i o n of the SDI
G r e a s e coated glass p l a t e s Mass mean diameter = .2087 I~m Geometric standard deviation = 1.717
2.S
2.0
-
O O
1.S "= 1 . 0 0.5uu".^
,
.001
,
,
.....
I
.01
.,
,
,
.1 Diameter (~m)
,
. . . . . . .
1
Figure 2 - Example of results o b t a i n e d w i t h the SDI
,
......
I
1508
D. B O U L A U D et al.
2.5 ~
2.0 o
5lass (grease) D.M.M. : .201 pm S.D. = 1.72 I
~' ~=~
1,5
=
1.0
I Mefa[ (grease) I - I D,M.M. = .189 Pm I ! S D : 183 !
0.5
f-- ............
I
i Filter i D.M.M. = .3 pm
i i
I S,D..._3..O1...... ]
o o
,
.......
/'~ f \\ / \.~ ///f'-~'\ I//'
,
.001
"~ " \
.JOF
\
"~.
,
,01
.I
I
Diameter (IJm)
Figure 3 - Influence of c o l l e c t i o n surface in impactor
Oiamefers comparison
Comparison of generated mass (mglh)
"~ ~ ~ ~
150J o EAA
~.
125 ,4
s
100
o
3
|
7s
E..z
~
so
2:
,1
l
i
T
~
.2
,3
.4
.5
J
r
~
~
~_ -
M.M.D SOl (l~m)
Figure 4 - C o m p a r i s o n of m e a n mass diameters o b t a i n e d by the SDI and by e l e c t r i c a l m e t h o d s
"
0
qm4R 1
25
. I
50
~.
~1
75 SOl
!
. i
100
125
•
i
150
Figure 5 - C o m p a r i s o n of g e n e r a t e d mass by unit of time as m e a s u r e d by the SDI and by e l e c t r i c a l methods