Particle size determination in absorbing aerosols by photon correlation spectroscopy

~ Pergamon

PII: SOO21-8502(96)OO339-4

J. A"otOl sa; Vol. 27. Suppl. I, pp. SS3l-SS3<4, 1996 Copyrighl 0 1996E10evier Science Lid Printed in Oreal Britain. All righu reserved 0021-&502/96 SI5.OO + 0.00

PARTICLE SIZE DETERMINATION IN ABSORBING AEROSOLS BY PHOTON CORRELATION SPECTROSCOPY R. WEBER and G. SCHWEIGER

Ruhr-Universitat Bochum, Maschinenbau, Laseranwendungstechnik und MeBsysteme, 44780 Bochum, Germany KEYWORDS Dynamic Light Scattering, Photon Correlation Spectroscopy, Particle Size Determination, Flowing Aerosol, Soot INTRODUCTION Photon correlation spectroscopy (PCS) was shown to be a usefull measurement technique for particle size analysis in laminar flowing aerosols with transparent particles of submicrometer size [1,2]. In this work the PCS has been applied for particle size determination in absorbing aerosols with particle concentrations below I08cm-3. The measured autocorrelation functions are a superposition of a DC-component and two separate decays, one in the short the other in the long time region of the autocorrelation function. The theory of a laminar flowing monodisperse aerosol with particle number fluctuations in the scattering volume was applied for determination of a mean particle diameter. The results show a significant dependence on the incident laser power. Furthermore, the first decay of the measured autocorrelation function (ACF) shows a curvature similar to that which would be expected in cases of high aerosol flow velocities. The second decay remains constant for all measurements. This decay is an effect of particle number fluctuations in the scattering volume . It indicates the mean transit time of the particles and therefore the mean translational particle velocity through the scattering volume. METHOD The measurements were performed with a standard pinhole PCS spectrometer (AL V) in a laminar flowing soot aerosol under room conditions. As light source we used an Ar+-Laser (A..=514,5nm) in the power range between 0.2-IW, which was focussed down to a beam waist radius of about l2Ollm . The aerosol was generated by dispersing dry soot (Flame Soot 101, Degussa) with a rotating brush disperser (RGB 100, Pallas) into an air stream. Before flowing laminar into the scattering volume the soot aerosol passed through an impactor to remove huge particle aggregates. In the scattering volume a mean count diameter of approximately 400nm was measured with a differential mobility particle sizer (DMPS, TSI Inc.). Additional SEM measurements of particles impacted in the scattering volume confirm the existence of soot particles in the size range measured with the DMPS. RESULTS A typical ACF measured at a laser power of 200mW is shown in Fig. 1. It is presented together with a fitted theory ACF for a laminar flowing monodisperse aerosol. The data are plotted without the DC-component of the ACF. The residuals stress a systematic deviation S533

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Abstracts of the 1996 European Aerosol Conference

of the measurement from the theory in the short time range. They indicate a curvature of the measured ACF in the opposite direction of what is expected from theory for a Brownian motion system in low macroscopic motion. Fig. 2 showsthis deviation in detail. The effects of the incident laser power on the particle diameterand translational particle velocity through the scattering volume, determined by fitting the theory ACF for a laminar flowing, monodisperse aerosol on the measurements, are shown in Fig. 3 and Fig. 4. While the particle diameter drops down with increasing laser power the translational particle velocity through the scattering volume remains constant for all measurements. - measurement

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ACKNOWLEDGEMENTS The financial support of AG Solar NRW and MWF (ministry for science and research of NRW), respectively, is gratefully acknowledged. We thank Degussa for providing the soot. REFERENCES [1] Weber, R., Rambau, R., Schweiger, G., and Lucas K. (1993) Analysis of a flowing aerosol by correlation spectroscopy: concentration, aperture, velocity and particle size effects. 1. Aerosol Sci. 24, 485-499. [2] Weber, R., and Schweiger, G. (1995) Determination of particle size distribution in flowing aerosols by photon correlation spectroscopy. 1. Aerosol. Sci. 26 Supp//, 529S30.