- Email: [email protected]
Spectroscopy of pyrotechnically generated aerosols
AerosoJs in science, medicine and technolog)
322
13
o °" o
o
0 2 ~* w,~ veloc,t'dm/s) Fig. 139b. Collecting efficiency of the entire rod as function of wind velocity.
CONCLUSIONS Dividing total suspended particulate matter in two size fractions and sampling them with adjusted devices lor each o f them turns out to be a proper method. Problems still arise in the size fraction, where both devices connect. Adcquat© sampling will be pcrformtxi best, when the cut-offdiameter dso o f both devices coincide. Figure 133 shows that they do, but both instruments are still slightly dependent on the external variables windspeed and turbulence in their collection ©fficicncy o f particles in the size range 3.0-10/~m. For larger particles simple corrections based on windspeed can he made. If not only sampling o f total suspended particulate matter is intended, but also a cut-offdiameter between the two size fractions according to r~,xnrmmndations o f ISO TC 146 ad hoc working group on size definitions for particle sampling for assessing possible health effects, is wanted, a shift of the 50 o~ cut-offdiameters of cyclone and Rotorod is needed. A shift to larger particles (10 or 15 ~m) particles for the thoracic fraction will be in favour of dimensions, loading characteristics and sampled volume for the Rotorod. In this case the cyclone will pose problems due to significant deviations in sampling efficiency at the inlet from one for particles in the range 0 < ~l < 1.0, with = collection efficiency o f the cyclone by centrifugal force. A shift to smaller particles (2.5-3.5 #m) will be in favour of the cyclone sampler. The subsexttmnt increase in rotorspeed, and/or narrowing of the rods have disadvantages in loading characteristics and handling of the Rotorod. Acknowledgement--The authors wish to thank Mr. H. C. P. Wegh for preparation of performing of the DEHS analysis and M J. J
H. Willems and J. Colijn for technical support and performing parts of the study.
REFERENCES Bclyaev, S. P. and 12vin, L. M. (1974) J. derosol ScL 5, 325. Davi~, C. N. and Subari, M. (1982) .L Aerosol Sci. 13, 59. Durham, M. D. and Lundgren, D. A. (1980) J. Aerosol SoL I1, 179. [SO TC 146 (1981) Am. Ind. Hy 8, Ass~ d. 42, A64. Lippmann, M. and Chan, T, L. (1979) Staub Reinh. Luft 39. 7. May, K. R. (1966) J. Scient. Instr. 43, 841. May, K. R. and Clifford, R. (1967) Ann. Occup. HyO. 10, 83. Noll, K. E. (1970) Atmos. Environ. 4, 9. Ranz, W. E. and Wong, J. B. (1952) lnd: Enqng. Chem. 44, 1371.
SPECTROSCOPY
OF
PYROTECHNICALLY
GENERATED
AEROSOLS
M. T~,SCHNER, B. GEORGI Gesellschaft fiir StraMen- und Umweltforschung mbH.. Abteilung fiir 6kologische Physik, Hannover, Herrenh/iuserstraBe 2, BRD and A. BERNER, G. REISCHL
Institut fiir Experimentalphysik der Universit~it Wien Strudelhofgasse 4. Wien, Austria Pyrotechnical aerosols are dense smokes which have been successfully used in studying the propagation or'aerosols in the lower atmosphere ( 1). The size range of these aerosols extends from a few hundredths of a micrometer up to 10 t~m or more. In the study presented here the size distributions of pyrotechnical smokes have been measured by several low pressure impactors in order to investigate the reproducibility of the aerosol producing process and the agreement o f the low pressure impactors. This question is not trivial as low pressure impactors are rather complex instruments. The pyrotechnic is a mixture of a plastic fuel. a chlorinated compound and dysprosium oxtde. Dy20~. which is added as a tracer. The material is a compact mass and has the form of a cylindrical rod. After incineration the
The Tenth AnnuaL Conference of the Association for Aerosol Research
323
combustion is self-sustaining and produces a dense smoke by chemical reaction of the chlorinated compound and the dysprosia. In our experiments the aerosol is produced in a large chamber of about 50 m 3 volume. Four to six impactors were set up as close together as possible and at a distance of approx. 1.5 m from the aerosol source. The smoke was blown over the impactors by a fan from behind the source and the impactors collected the aerosol directly from the plume. The impactors are Aeras low pressure impactors of the type LP! 30/0.06 and LPI 25/0.015 with flow rates of 30 l/min and 25 l/min, and measuring ranges from 0.06/am a.e.d. (aerodynamic equivalent diameter) to 16/am a.e.d. and 0.015 #m a.e.d, to 16/am a.e.d., respectively. Other data are represented in Table 35. The impactor LPI 30/0.06 has been described in more detail in Berner and Liirzer (1980). In Fig. 140, four mass size distributions of the same smoke are represented. The distributions have been normalized to a standard flow rate of 301/min. All distributions exhibit two distinct modes, a fine particle mode inbetween stages 2 and 3 at 0.25/am a.e.d., and a coarse mode on stages 6 and 7 at roughly 4/am a.e.d. This bimodal structure is typical, as both modes occur more or less pronounced in all experiments. In the fine particle range the agreement of the impactors is almost perfect. The scatter of the data points may be attributed partly to the inhomogeneity of the aerosol, to the errors of evaluating the deposits and to systematic differences among the impactors. The LPI 30 impactors are measuring somewhat larger modal sizes in comparison to the LP125 impactors (LPI 30:0.261/am and 0.247/am, LP125: 0.238 /am and 0.237/am). The differences in the coarse particle size range are to be attributed to the material of the collection foils. The collection efficiency of polyethylene is substantially higher than that of aluminum, both materials having been used without a coating. This effect can be explained by assuming that contact charges are developing upon impact of the particles. The respective forces increase the collection efficiency. It is interesting to note that the unbounded and blown off material is obviously not transported to the subsequent stages, otherwise we could not expect to observe the agreement in the fine particle size range. In Fig. 141 the mass size distributions of three subsequent experiments are shown together with a dysprosium distribution. Each distribution is the average of two LPI 30 measurements with polyethylene foils. The total mass
Table 35. Data of low pressure imp,actors LPI 25/0.015 flow rate: 25 l/min at 20C measuring rate: 0.015 am a.e.d to 16/am a.e.d. Stage D i I/am] Di I/am]
- 1
0
1
2
3
4
5
6
7
8
9
0.015 0.021
0.03 0.042
0.06 0.087
0.125 0.18
0.25 0.35
0.50 0.71
1.0 1.4
2.0 2.8
4.0 5.7
8.0 11.3
16 --
LPI 30/0.06 flow rate: 30 l/rain at 20C measuring range 0.06 #m a.e.d, to 16/am a.e.d. Stage Di [#m] Di [l~m]
1
2
3
4
5
6
7
8
9
0.06 0.087
0.125 0.18
0.25 0.35
0.50 0.71
1.0 1.4
2.0 2.8
4.0 5.7
8.0 11.3
16 --
Di: Cut off sizes. D,: mean diameter.
3.103 --
103
/"
"\"\
/'f
t, O
5.i0 z
/
3.10 n
.!
/
i0 z
(A) /~' y, / /. A
L
-I
I
0
~
o L•I 3 0 / 0 , 0 6 ,
PE foils
• LPI 3 0 / 0 , 0 6 ,
PE foils
•
I
i
LPI 2 . 5 / 0 , 0 1 5 ,
AI foils
LPI 25/0,015,
AI foils
I
2
I
3
I
4
I
.5
L
6
I
7
I
8
Sto(~e number
Fig 140. Mass size distributions of a pyrotechnical aerosol measured with four low pressure impactors. (Particle sizes corresponding to stage numbers are reported in Table 35J
324
Aerosols in science, medicine and technology
3.,o~L-
~'/.t--';"
F
"~
/
I : C Moss 1T : ~ M a s s o Oyspros,um ~ : e Mass
/
3JO; i ,
.__.~" ./
/
"
J
F ] -i
0
I
2
3
4
5
6
7
8
Stage number Fig. 141. Mass size distributions and dysprosium size distribations of three pyrotechnical aerosols. (Particle sizes corresponding to stage numbers are reported in Table 35.1
Table 36. Dysprosium concentration of impaetor samples in % of total mass per impactor stage Stage
- 1
0
1
2
3
4
5
6
7
8
~o Dy
2.25
31.4
36.8
52.6
56.3
62.4
70.8
75.3
72.1
65.7
varies considerably, mainly because of the errors in setting up the experiments and by the inhomogeneity of the plumes. But the size distributions, the Dy distribution included, agree fairly well, especially in the fine particle range. The modal sizes are 0.254 #m in exl~iment L 0.259 #m in II and 0.265/am in III. The standard deviation of these sizes amount to 2.1 ~g of the average stze, indicating a fairly stable production of the fine aerosol. Some of the impaetor samples have been investigated for dysprosium by neutron activation analysis. These size distributions, an example of which is represented in Fig. 141, correspond well with the mass size distributions. However, the dysprosium content of the particles varies with particles size (see Table 36). From the smallest particles. which are almost free of dysprosium, the Dy concentration increases. In the fine particle mode the Dy concentration amounts to 54 ~ of the particle mass. This value might indicate that DyCI 3 is the main constituent of the particles. but this compound would account for 82 ~g of the mass only. In the coarse mode the Dy concentration rises up to 75 ~o, and here the main constituent could be Dy203, accounting for 85 % of the mass. On the other hand the coarse particles should have collected some small particles by coagulation, and we would expect some DyC13 on the coarse particles. Splitting the amount of Dy between these compounds and assuming that no other material is on the particles, the coarse particles would consist of 55 ~o D Y 2 0 3 and 45 % DyCI 3. The difference in chemical composition could indicate the independent formation of the fine and coarse particles. The fine particles form from the vapours of DyCI3, which is formed during combustion. They are very small and coagulate rapidly forming larger a88r¢sates. So the fine particle mode observed here represents obviously the accumulation mode of the DyCIs condensation aerosol. The formation of the coarse particles cannot he fully explained. We observed that the fuel is partly melting during combustion. Gaseous reaction products may become trapped within the liquid material and upon disruption will give rise to the formation of larger particles. The chemical problem that these particles or the residues hereof are mainly composed of Dy203, cannot be explained without imposing too many assumptions on the combustion process.
REFERENCES Berner, A. and Liirzer, Ch. (1980) Mass size distribution of traffic aerosols at Vienna, J. Phys. Chem. 84, 2079.
MASS
SIZE
DISTRIBUTIONS
OF TRAFFIC
AEROSOLS
OF VIENNA
A. BERNER a n d G. REISCEIL lnstitut fiar Experimentalphysik, Universitat Wien Strudlhofgasse 4, Vienna. Austria
Mass size distributions of atmospheric aerosols can be measured directly by special low pressure impactors (Whitby. 1978). It has been demonstrated that the distributions near a street in the city definitely exhibit a modal structure with
The Tenth Annual Conference of the Association for Aerosol Research
325
three modes corresponding to the coarse mode, the accumulation mode and the nucleation mode of the trimodal model which has been introduced by Whitby (1978) for atmospheric aerosols. Recently, other measurements have been performed by using a low pressure impactor with a measuring range from 0.015/am a.e.d. (aerodynamic equivalent diameter) to 16/am a.e.d. This impactor was mounted on the front bumper of a passenger car and the aerosols were collected while driving the car on roads with heavy traffic. A series o f 40 measurements was performed. Several of the mass size distributions show a very pronounced trimodal structure with three distinct modes at 0.08/~m a.e.d., 0.6/am a.e.d, and roughly 4/am a.e.d. These modes correspond very well with former results (Whitby, 1978). However, the other size distributions show one or two modes. In the unimodal mass size distributions the accumulation mode predominates, and the other modes appear as humps on the slopes of the distributions. In the bimodal distributions the accumulation mode is nearly missing. Some typical distributions, which are almost completely covered by the measuring range of the impactor, are represented in Fig. 142.
40
/./'~\
g 30 Q. ~E
20
/o
-i
/
•
I
L
~
,
i
L
0
i
2
3
4
5
i
t i i 1
6 ÷ 8 -~o
,'
5 ~ 4 5
Stoge
~ i
o/°
e~o
\
•
7 g -,o
number
Fig. 142. Mass size distributions of traffic aerosols. (Particle diameters corresponding to stage numbers are: 0.021 ,am a.e.d. ---stage 1; 0.042 ,am a.e.d. - - s t a g e 0; 0.087 ,am a.e.d. - - s t a g e 1; 0.18 ,am a.e.d. ---stage 2; 0.35 ,am a.e.d. - - stage 3; 0.71/zm a.e.d. - - stage 4:1.4 ,am a.e.d. - - stage 5; 2.8 ,am a.e.d. - - stage 6; 5.7 # m a.e.d. - - stage 7; 11.3 # m a.e.d. - - stage 8. A.e.d.: aerodynamic equivalent diameter.)
The mathematical decomposition o f these size distributions into three logarithmic normal distributions reveals a strong correlation between the concentrations o f the nucleation and coarse modes, but a weak correlation between the accumulation mode and either one o f the other modes. This result, which still may pertain to the limited series of measurements, is strongly supporting two hypotheses: (1) The nucleation mode and major parts of the coarse mode o f traffic aerosols have a common source or at least a common location of sources. Nucleation mode particles are generated by the cars and coarse mode particles are generated by redispersing road dust and tail pipe deposits. Additionally, coarse particles may be generated directly by combustion. (2) The local traffic at Vienna is not the direct source of the accumulation mode particles, or at least the main part hereof. The concentrations of the accumulation mode appear to be correlated with the weather situation. They are specifically low, when fairly clean air masses are quickly moved into the local area from the Atlantic ocean or the north polar region. They increase when the air mass becomes charged with pollution during the travel over industrialized areas, and the concentrations are highest in stagnant air masses when the local sources gradually increase the aerosol load.
REFERENCES Berner, A. and Liirzer, Ch. (1980) J. Phys. Chem. 84, 2079. Whitby, K. T. (1978) Atmos. Environ. 12, 135.
PRACTICAL
COMPARISON
OF THREE
CASCADE
IMPACTORS
P. BOESCH Analytical Chemistry Department. Pavilion des Isotopes University of Geneva. CH-121] Geneva 4. Switzerland
INTRODUCTION
In order to compare the distributions and the concentrations of several metals obtained with different impactors, three lov, volume impactors were sampling the air of the city of Geneva, side by side, on the roof of a three storey
322
13
o °" o
o
0 2 ~* w,~ veloc,t'dm/s) Fig. 139b. Collecting efficiency of the entire rod as function of wind velocity.
CONCLUSIONS Dividing total suspended particulate matter in two size fractions and sampling them with adjusted devices lor each o f them turns out to be a proper method. Problems still arise in the size fraction, where both devices connect. Adcquat© sampling will be pcrformtxi best, when the cut-offdiameter dso o f both devices coincide. Figure 133 shows that they do, but both instruments are still slightly dependent on the external variables windspeed and turbulence in their collection ©fficicncy o f particles in the size range 3.0-10/~m. For larger particles simple corrections based on windspeed can he made. If not only sampling o f total suspended particulate matter is intended, but also a cut-offdiameter between the two size fractions according to r~,xnrmmndations o f ISO TC 146 ad hoc working group on size definitions for particle sampling for assessing possible health effects, is wanted, a shift of the 50 o~ cut-offdiameters of cyclone and Rotorod is needed. A shift to larger particles (10 or 15 ~m) particles for the thoracic fraction will be in favour of dimensions, loading characteristics and sampled volume for the Rotorod. In this case the cyclone will pose problems due to significant deviations in sampling efficiency at the inlet from one for particles in the range 0 < ~l < 1.0, with = collection efficiency o f the cyclone by centrifugal force. A shift to smaller particles (2.5-3.5 #m) will be in favour of the cyclone sampler. The subsexttmnt increase in rotorspeed, and/or narrowing of the rods have disadvantages in loading characteristics and handling of the Rotorod. Acknowledgement--The authors wish to thank Mr. H. C. P. Wegh for preparation of performing of the DEHS analysis and M J. J
H. Willems and J. Colijn for technical support and performing parts of the study.
REFERENCES Bclyaev, S. P. and 12vin, L. M. (1974) J. derosol ScL 5, 325. Davi~, C. N. and Subari, M. (1982) .L Aerosol Sci. 13, 59. Durham, M. D. and Lundgren, D. A. (1980) J. Aerosol SoL I1, 179. [SO TC 146 (1981) Am. Ind. Hy 8, Ass~ d. 42, A64. Lippmann, M. and Chan, T, L. (1979) Staub Reinh. Luft 39. 7. May, K. R. (1966) J. Scient. Instr. 43, 841. May, K. R. and Clifford, R. (1967) Ann. Occup. HyO. 10, 83. Noll, K. E. (1970) Atmos. Environ. 4, 9. Ranz, W. E. and Wong, J. B. (1952) lnd: Enqng. Chem. 44, 1371.
SPECTROSCOPY
OF
PYROTECHNICALLY
GENERATED
AEROSOLS
M. T~,SCHNER, B. GEORGI Gesellschaft fiir StraMen- und Umweltforschung mbH.. Abteilung fiir 6kologische Physik, Hannover, Herrenh/iuserstraBe 2, BRD and A. BERNER, G. REISCHL
Institut fiir Experimentalphysik der Universit~it Wien Strudelhofgasse 4. Wien, Austria Pyrotechnical aerosols are dense smokes which have been successfully used in studying the propagation or'aerosols in the lower atmosphere ( 1). The size range of these aerosols extends from a few hundredths of a micrometer up to 10 t~m or more. In the study presented here the size distributions of pyrotechnical smokes have been measured by several low pressure impactors in order to investigate the reproducibility of the aerosol producing process and the agreement o f the low pressure impactors. This question is not trivial as low pressure impactors are rather complex instruments. The pyrotechnic is a mixture of a plastic fuel. a chlorinated compound and dysprosium oxtde. Dy20~. which is added as a tracer. The material is a compact mass and has the form of a cylindrical rod. After incineration the
The Tenth AnnuaL Conference of the Association for Aerosol Research
323
combustion is self-sustaining and produces a dense smoke by chemical reaction of the chlorinated compound and the dysprosia. In our experiments the aerosol is produced in a large chamber of about 50 m 3 volume. Four to six impactors were set up as close together as possible and at a distance of approx. 1.5 m from the aerosol source. The smoke was blown over the impactors by a fan from behind the source and the impactors collected the aerosol directly from the plume. The impactors are Aeras low pressure impactors of the type LP! 30/0.06 and LPI 25/0.015 with flow rates of 30 l/min and 25 l/min, and measuring ranges from 0.06/am a.e.d. (aerodynamic equivalent diameter) to 16/am a.e.d. and 0.015 #m a.e.d, to 16/am a.e.d., respectively. Other data are represented in Table 35. The impactor LPI 30/0.06 has been described in more detail in Berner and Liirzer (1980). In Fig. 140, four mass size distributions of the same smoke are represented. The distributions have been normalized to a standard flow rate of 301/min. All distributions exhibit two distinct modes, a fine particle mode inbetween stages 2 and 3 at 0.25/am a.e.d., and a coarse mode on stages 6 and 7 at roughly 4/am a.e.d. This bimodal structure is typical, as both modes occur more or less pronounced in all experiments. In the fine particle range the agreement of the impactors is almost perfect. The scatter of the data points may be attributed partly to the inhomogeneity of the aerosol, to the errors of evaluating the deposits and to systematic differences among the impactors. The LPI 30 impactors are measuring somewhat larger modal sizes in comparison to the LP125 impactors (LPI 30:0.261/am and 0.247/am, LP125: 0.238 /am and 0.237/am). The differences in the coarse particle size range are to be attributed to the material of the collection foils. The collection efficiency of polyethylene is substantially higher than that of aluminum, both materials having been used without a coating. This effect can be explained by assuming that contact charges are developing upon impact of the particles. The respective forces increase the collection efficiency. It is interesting to note that the unbounded and blown off material is obviously not transported to the subsequent stages, otherwise we could not expect to observe the agreement in the fine particle size range. In Fig. 141 the mass size distributions of three subsequent experiments are shown together with a dysprosium distribution. Each distribution is the average of two LPI 30 measurements with polyethylene foils. The total mass
Table 35. Data of low pressure imp,actors LPI 25/0.015 flow rate: 25 l/min at 20C measuring rate: 0.015 am a.e.d to 16/am a.e.d. Stage D i I/am] Di I/am]
- 1
0
1
2
3
4
5
6
7
8
9
0.015 0.021
0.03 0.042
0.06 0.087
0.125 0.18
0.25 0.35
0.50 0.71
1.0 1.4
2.0 2.8
4.0 5.7
8.0 11.3
16 --
LPI 30/0.06 flow rate: 30 l/rain at 20C measuring range 0.06 #m a.e.d, to 16/am a.e.d. Stage Di [#m] Di [l~m]
1
2
3
4
5
6
7
8
9
0.06 0.087
0.125 0.18
0.25 0.35
0.50 0.71
1.0 1.4
2.0 2.8
4.0 5.7
8.0 11.3
16 --
Di: Cut off sizes. D,: mean diameter.
3.103 --
103
/"
"\"\
/'f
t, O
5.i0 z
/
3.10 n
.!
/
i0 z
(A) /~' y, / /. A
L
-I
I
0
~
o L•I 3 0 / 0 , 0 6 ,
PE foils
• LPI 3 0 / 0 , 0 6 ,
PE foils
•
I
i
LPI 2 . 5 / 0 , 0 1 5 ,
AI foils
LPI 25/0,015,
AI foils
I
2
I
3
I
4
I
.5
L
6
I
7
I
8
Sto(~e number
Fig 140. Mass size distributions of a pyrotechnical aerosol measured with four low pressure impactors. (Particle sizes corresponding to stage numbers are reported in Table 35J
324
Aerosols in science, medicine and technology
3.,o~L-
~'/.t--';"
F
"~
/
I : C Moss 1T : ~ M a s s o Oyspros,um ~ : e Mass
/
3JO; i ,
.__.~" ./
/
"
J
F ] -i
0
I
2
3
4
5
6
7
8
Stage number Fig. 141. Mass size distributions and dysprosium size distribations of three pyrotechnical aerosols. (Particle sizes corresponding to stage numbers are reported in Table 35.1
Table 36. Dysprosium concentration of impaetor samples in % of total mass per impactor stage Stage
- 1
0
1
2
3
4
5
6
7
8
~o Dy
2.25
31.4
36.8
52.6
56.3
62.4
70.8
75.3
72.1
65.7
varies considerably, mainly because of the errors in setting up the experiments and by the inhomogeneity of the plumes. But the size distributions, the Dy distribution included, agree fairly well, especially in the fine particle range. The modal sizes are 0.254 #m in exl~iment L 0.259 #m in II and 0.265/am in III. The standard deviation of these sizes amount to 2.1 ~g of the average stze, indicating a fairly stable production of the fine aerosol. Some of the impaetor samples have been investigated for dysprosium by neutron activation analysis. These size distributions, an example of which is represented in Fig. 141, correspond well with the mass size distributions. However, the dysprosium content of the particles varies with particles size (see Table 36). From the smallest particles. which are almost free of dysprosium, the Dy concentration increases. In the fine particle mode the Dy concentration amounts to 54 ~ of the particle mass. This value might indicate that DyCI 3 is the main constituent of the particles. but this compound would account for 82 ~g of the mass only. In the coarse mode the Dy concentration rises up to 75 ~o, and here the main constituent could be Dy203, accounting for 85 % of the mass. On the other hand the coarse particles should have collected some small particles by coagulation, and we would expect some DyC13 on the coarse particles. Splitting the amount of Dy between these compounds and assuming that no other material is on the particles, the coarse particles would consist of 55 ~o D Y 2 0 3 and 45 % DyCI 3. The difference in chemical composition could indicate the independent formation of the fine and coarse particles. The fine particles form from the vapours of DyCI3, which is formed during combustion. They are very small and coagulate rapidly forming larger a88r¢sates. So the fine particle mode observed here represents obviously the accumulation mode of the DyCIs condensation aerosol. The formation of the coarse particles cannot he fully explained. We observed that the fuel is partly melting during combustion. Gaseous reaction products may become trapped within the liquid material and upon disruption will give rise to the formation of larger particles. The chemical problem that these particles or the residues hereof are mainly composed of Dy203, cannot be explained without imposing too many assumptions on the combustion process.
REFERENCES Berner, A. and Liirzer, Ch. (1980) Mass size distribution of traffic aerosols at Vienna, J. Phys. Chem. 84, 2079.
MASS
SIZE
DISTRIBUTIONS
OF TRAFFIC
AEROSOLS
OF VIENNA
A. BERNER a n d G. REISCEIL lnstitut fiar Experimentalphysik, Universitat Wien Strudlhofgasse 4, Vienna. Austria
Mass size distributions of atmospheric aerosols can be measured directly by special low pressure impactors (Whitby. 1978). It has been demonstrated that the distributions near a street in the city definitely exhibit a modal structure with
The Tenth Annual Conference of the Association for Aerosol Research
325
three modes corresponding to the coarse mode, the accumulation mode and the nucleation mode of the trimodal model which has been introduced by Whitby (1978) for atmospheric aerosols. Recently, other measurements have been performed by using a low pressure impactor with a measuring range from 0.015/am a.e.d. (aerodynamic equivalent diameter) to 16/am a.e.d. This impactor was mounted on the front bumper of a passenger car and the aerosols were collected while driving the car on roads with heavy traffic. A series o f 40 measurements was performed. Several of the mass size distributions show a very pronounced trimodal structure with three distinct modes at 0.08/~m a.e.d., 0.6/am a.e.d, and roughly 4/am a.e.d. These modes correspond very well with former results (Whitby, 1978). However, the other size distributions show one or two modes. In the unimodal mass size distributions the accumulation mode predominates, and the other modes appear as humps on the slopes of the distributions. In the bimodal distributions the accumulation mode is nearly missing. Some typical distributions, which are almost completely covered by the measuring range of the impactor, are represented in Fig. 142.
40
/./'~\
g 30 Q. ~E
20
/o
-i
/
•
I
L
~
,
i
L
0
i
2
3
4
5
i
t i i 1
6 ÷ 8 -~o
,'
5 ~ 4 5
Stoge
~ i
o/°
e~o
\
•
7 g -,o
number
Fig. 142. Mass size distributions of traffic aerosols. (Particle diameters corresponding to stage numbers are: 0.021 ,am a.e.d. ---stage 1; 0.042 ,am a.e.d. - - s t a g e 0; 0.087 ,am a.e.d. - - s t a g e 1; 0.18 ,am a.e.d. ---stage 2; 0.35 ,am a.e.d. - - stage 3; 0.71/zm a.e.d. - - stage 4:1.4 ,am a.e.d. - - stage 5; 2.8 ,am a.e.d. - - stage 6; 5.7 # m a.e.d. - - stage 7; 11.3 # m a.e.d. - - stage 8. A.e.d.: aerodynamic equivalent diameter.)
The mathematical decomposition o f these size distributions into three logarithmic normal distributions reveals a strong correlation between the concentrations o f the nucleation and coarse modes, but a weak correlation between the accumulation mode and either one o f the other modes. This result, which still may pertain to the limited series of measurements, is strongly supporting two hypotheses: (1) The nucleation mode and major parts of the coarse mode o f traffic aerosols have a common source or at least a common location of sources. Nucleation mode particles are generated by the cars and coarse mode particles are generated by redispersing road dust and tail pipe deposits. Additionally, coarse particles may be generated directly by combustion. (2) The local traffic at Vienna is not the direct source of the accumulation mode particles, or at least the main part hereof. The concentrations of the accumulation mode appear to be correlated with the weather situation. They are specifically low, when fairly clean air masses are quickly moved into the local area from the Atlantic ocean or the north polar region. They increase when the air mass becomes charged with pollution during the travel over industrialized areas, and the concentrations are highest in stagnant air masses when the local sources gradually increase the aerosol load.
REFERENCES Berner, A. and Liirzer, Ch. (1980) J. Phys. Chem. 84, 2079. Whitby, K. T. (1978) Atmos. Environ. 12, 135.
PRACTICAL
COMPARISON
OF THREE
CASCADE
IMPACTORS
P. BOESCH Analytical Chemistry Department. Pavilion des Isotopes University of Geneva. CH-121] Geneva 4. Switzerland
INTRODUCTION
In order to compare the distributions and the concentrations of several metals obtained with different impactors, three lov, volume impactors were sampling the air of the city of Geneva, side by side, on the roof of a three storey