Improvement of the bearing system of the Stöber aerosol spectrometer

Atmospheric

Enuironmunt,

Vol. 9, pp. 859-860.

Pergamon Press 1975. Printed in Great Britain.

TECHNICAL NOTE IMPROVEMENT OF THE BEARING’SYSTEM STOBER AEROSOL SPECTROMETER

OF THE

(First received 23 December 1974 and in final form 4 March 1975)

Abstract-Aerosol particles were generated in the bearing system of the Stober aerosol spectrometer. The equivalent atmospheric concentration of this aerosol was higher than the concentration of atmospheric aerosols reported in the literature. Moreover leakage occurred in the bearing system especially at high pressures at the clean air inlet and large su~tmospheric pressures at the suction side. The spectrometer was improved by introducing sealing rings in the bearing system.

INTRODUCTION

In a number of experiments carried out with the Stober aerosol spectrometer (Stober and Flachsbart, 1969) in which PSL aerosols were sampled, a deposit of particles with a deviating form appeared at a distance of 7-25 cm. The corresponding aerodynamic diameters of the particles are 7.0 and 1.0 m respectively. Because the clean air flow was thoroughly purified using a high efficiency aerosol filter before being introduced into the s~~ometer, the particles were obviously generated somewhere in the aerosol spectrometer. In order to check this assumption an experiment was carried out in which the aerosol sampling tube was closed. A deposit mainly consisting of liquid particles was obtained in this case (Fig. 1). affirming this assumption. Probably the particles were generated in the bearing system. The particle size dis~ibution n(De,) in terms of the aerodynamic dia. D,, was determined experimentally for D, = 1.4 p and amounted to about 11 (cm- 3 .prn- i). The equivalent atmospheric particle size distribution for D,, = 1.4 pm is obtained by multiplying the value of n(D,,) mentioned above by the ratio of the clean air flow rate to the aerosol flow rate. Under suitable experimental conditions these flow rates are 17.3 and 1.9 Lmin-’ respectively -and the equivalent atmospheric particle size dis~ibution for Dee = 1.4~ amounts to 100 (cm-3.q-‘). This value exceeds the corresponding values observed for Los Angeles smog aerosol (Whitby et al., 1972) and natural aerosol measured in Frankfurt/Main (Junge, 1963). The particle size distribution of the Los Angeles smog aerosol was measured with an optical counter for the size range 05-6.8 pm and was given in terms of the geometric dia. Dr A comparison between the n(D,,,)-value reported in this paper and the n(D,)-value given by Whitby et ai. is permitted because (a) both instruments were calibrated with PSL and (b) the difference between the aerodynamic and geometric diameter is negligible for PSL. The particle size distribution reported by Junge was measured with a cascade impactor. Although this was not stated, the diameter is probably given in terms of the Stokes dia. D, (Junge, 1953). A comparison between the n(D,,&value reported in this paper and the @Q-value given by Junge is also allowed because the oil particles have a density of nearly 1 g.cmm3. As the presence of background material interferes during the analysis of atmospheric aerosols (especially in case of liquids) attempts were made to eliminate the background aerosol. It was found ex~rimen~ly, that an aerosol filter in front of the laminator was an effective device for particle

Fig. 1. Deposit of background aerosol (particle size 1.4 pm; deposition distance 20 cm; incident illumination;

I

I

scale 25 pm. removal. Unfortunately the presence of the filter led to serious disturbances. In order to overcome the pressure drop of the filter (about 4cm Hg at a clean air flow of 75 1 .min- ‘) the pressure at the clean air inlet had to be increased. Leakage in the bearing system occurred as a result of this pressure increase and an accurate adjustment of the clean air flow was impossible under these circumstances. MODIFICATION

OF THE

BEARING

SYSFEM

A diagram of the modified bearing system* is given in Fig. 2. The connections between the clean air inlet (A) and the six capillaries (B) (only two are visible in Fig. 2) present in the shaft of the rotor were made completely airtight by the introduction of V-type sealing rings (C). Scaling rings (D) were also introduced in the exit chamber (E). The bearings present in the original design (type: 62042RS) were replaced by a smaller type (6004-2RS) in order * The reconstruction was carried out by Instrumentum to obtain sufficient space necessary for the introduction TNO, Delft, The Netherlands. of the sealing rings. 859

860

Technical note RESULTS I

After the reconstruction of the bearing system it was found that: (1) no more particles were generated in the bearing system, (2) no leakage occurred even in the case of high pressures (about 1Ocm Hg above atmosphere) at the clean air inlet and large subatmospheric pressures (about 15cm Hg) at the suction side of the instrument. Chemical-Laboratory TNO, Rijswijk(nf),

F.

&3EBURG

R. Roes

The Netherlands

REFERENCES

Junge Chr. E. (1953) Die Rolle der Aerosole und der gasfiirmigen Beimengungen der Luft im Spurenstofiaushalt der Troposphlre. Tellus 5. l-26. Junge Chr. E. (1963) Air Chemistry and Radioactivity, pp. 113-123. Academic Press, New York. StSber W. and Flachsbart H.’ (1969) Size-separating precipitation of aerosols in a spinning spiral duct. Environ. Sci. Technol. 3. 1280-1296.

Fig. 2. Modified bearing system. (A clean air inlet, B capillaries, C V-type sealing rings, D V-type sealing rings, E exit chamber).

Whitby K. T., Husar R. B. and Liu B. Y. H. (1972) The aerosol size distribution of Los Angeles Smog, J. Colloid Interface Sci. 39, 177-204.