- Email: [email protected]
Effect of multiple light scattering on optical concentration measurement in aerosols
J. Aerosol Sci., Vol. 26. Suppl 1, pp. $23-$24, 1995
Elsevier Science Ltd Printed in Great Britain 0021-8502/95 $9.50 + 0.00
Pergamon
EFFECT OF MULTIPLE LIGHT SCATTERING ON OPTICAL CONCENTRATION MEASUREMENT IN AEROSOLS D. Filipovicova, G.P. Reischl, R. Rudolf, W.W. Szymanski, A. Vrtala, P.E. Wagner Institut fiir Experimentalphysik der Universit~itWien Boltzmanngasse 5, A-1090 Wien, Austria
Keywords: multiple scattering, optical measurement Introduction
Light propagation in aerosols is a basis for many optical measuring methods. In optically tenuous aerosols single scattering can be assumed to describe the transmitted and scattered light fluxes. With increasing optical depth, caused by increasing path lengths or particle concentration or both, multiple scattering effects will increasingly affect the characteristics of light propagation. The aim of this work is to investigate the angular dependence of these effects in aerosols with concentrations up to some 107 cm-3. Experimental results are compared with numerical modeling based on approximations to the radiative transfer equation. Method
The aerosols were activated to condensational growth in supersaturated vapor. Measurements were performed over a fixed path length at different aerosol concentrations up to some 107 em3 which corresponded to optical depth up to about 5. Aerosols with well defined particle concentrations were achieved using turbulent jet mixing in high temperature tube furnaces in connection with precision dilution unit. Light fluxes scattered at different angles as well as transmitted light flux were simultaneously quantitatively monitored during a droplet growth. The observed light flux vs. time curves show a specific oscillatory structure in close agreement with corresponding light flux vs. droplet size curves calculated by means of the Mie theory (v. d. Hulst, 1957). Normalizing the scattered relative to the transmitted light flux and establishing a unique correspondence between the experimental and theoretical curves, the concentration of aerosol can be determined from the height of the observed light scattering extremum (CAMSMethod: Wagner, 1985). The transmitted light flux measured at times when the drop size is known from the scattered light curves allows an independent absolute concentration determination (EXT-Method: Szymanski and Wagner, 1990). The apertures in light flux measuring systems amount to 20 mrad in order to possibly suppress the influence of multiple scattering on the monitored signals. Since both methods depend on the precise knowledge of the transmitted light flux, numerical modeling of multiple scattering contribution the transmittance was performed based on the small-angle approximation (Zardecki et al.,1983) and paraxial approximation (Bissonnette, 1988) to the time-independent radiation transfer equation. Results and Discussion
Measurements were performed at a droplet radius of 0.65 um for angle of 0° (transmitted light flux), 15° and 45 °. For particle concentrations below about 10 6 cm3 very good agreement of both methods CAMS and EXT with the actual aerosol number concentration was found. With the increasing particle concentration multiple scattering related deviations from the actual concentration for concentration values obtained by both methods occur (Fig.l). In order to interpret this nonlinear behavior of the methods a detailed study of all light fluxes was performed. Results obtained show an angular dependence of multiple scattering effects with the $23
$24
D. FILIPOVICOVAet aL
light flux at 45 ° being less affected than at 15° . This difference is understandable because at sufficiently high concentrations an isotropic distribution of radiation will be achieved, i.e. some angular ranges will experience an increase and some a decrease of received scattered intensity relative to the single scattering conditions. It was observed that at the experimental conditions chosen the transmitted light flux deviated from the single scattering behavior predicted by BeerLambert law at concentrations beyond some 10~ cm3 caused by some scattered photons being re-scattered into the direction of the propagation of the primary beam. In order to be able to accurately describe the light transmission at higher concentrations a multiple scattering correction term to the Beer-Lambert law can be applied (Szymanski, 1992). Numerical modeling of the correction term suggests that the paraxial approximation is more suitable for the description of multiple scattering for the experimental conditions chosen than the small angle approach. The work is in progress but results obtained so far indicate that an optimization of the scattered light detection system and a precise measurement of the transmitted and scattered light fluxes should allow a direct optical concentration measurement independent of any empirical calibration in the aerosol number concentration range from about 102 em3 up to 10s cm3. 1,00E+08
d~ gd 1,00E+07
-
1,00E+06
-
z
Iso - P r o p a n o l o
45 °
0
16-2-95 0 1,00E+05 1,00E+05
I
I
1,00E+06 1,00E+07 Corrected Particle Number Concentration [ cm ^-3 ] []
CAMS
.
1,00E+08
EXT
Fig. 1. Comparison of concentrations measured by means of EXT and CAMS-Method (at 45 °) to the actual concentration.
Acknowledgment This work was supported in part by the Fonds zur FOrderung der wissenschaftlichen Forschung, Proj. Nr. P9421 and by the Jubil~iumsfondsprojekt Nr.4363 der Osterreichischen Nationalbank.
References Bissonnette, L.R., Appl. Opt. 27, 2478-2484 (1988). Szymanski, W.W. and Wagner, P.E.J. Aerosol Sci. 21,441-451 (1990). Szymanski, W.W., J. Aerosol Sci. 23, 425-435 (1992). Van de Hulst, H.C.,"Light Scattering by Small Particles, Wiley & Sons, N.Y. (1957). Wagner, P.E., J. Coll. Interface Sci. 105, 456-466 (1985). Zardeeki, A., Gerstl, S.A.W. and Embury, J.F., Appl. Opt. 22, 1346-1353 (1983).
Elsevier Science Ltd Printed in Great Britain 0021-8502/95 $9.50 + 0.00
Pergamon
EFFECT OF MULTIPLE LIGHT SCATTERING ON OPTICAL CONCENTRATION MEASUREMENT IN AEROSOLS D. Filipovicova, G.P. Reischl, R. Rudolf, W.W. Szymanski, A. Vrtala, P.E. Wagner Institut fiir Experimentalphysik der Universit~itWien Boltzmanngasse 5, A-1090 Wien, Austria
Keywords: multiple scattering, optical measurement Introduction
Light propagation in aerosols is a basis for many optical measuring methods. In optically tenuous aerosols single scattering can be assumed to describe the transmitted and scattered light fluxes. With increasing optical depth, caused by increasing path lengths or particle concentration or both, multiple scattering effects will increasingly affect the characteristics of light propagation. The aim of this work is to investigate the angular dependence of these effects in aerosols with concentrations up to some 107 cm-3. Experimental results are compared with numerical modeling based on approximations to the radiative transfer equation. Method
The aerosols were activated to condensational growth in supersaturated vapor. Measurements were performed over a fixed path length at different aerosol concentrations up to some 107 em3 which corresponded to optical depth up to about 5. Aerosols with well defined particle concentrations were achieved using turbulent jet mixing in high temperature tube furnaces in connection with precision dilution unit. Light fluxes scattered at different angles as well as transmitted light flux were simultaneously quantitatively monitored during a droplet growth. The observed light flux vs. time curves show a specific oscillatory structure in close agreement with corresponding light flux vs. droplet size curves calculated by means of the Mie theory (v. d. Hulst, 1957). Normalizing the scattered relative to the transmitted light flux and establishing a unique correspondence between the experimental and theoretical curves, the concentration of aerosol can be determined from the height of the observed light scattering extremum (CAMSMethod: Wagner, 1985). The transmitted light flux measured at times when the drop size is known from the scattered light curves allows an independent absolute concentration determination (EXT-Method: Szymanski and Wagner, 1990). The apertures in light flux measuring systems amount to 20 mrad in order to possibly suppress the influence of multiple scattering on the monitored signals. Since both methods depend on the precise knowledge of the transmitted light flux, numerical modeling of multiple scattering contribution the transmittance was performed based on the small-angle approximation (Zardecki et al.,1983) and paraxial approximation (Bissonnette, 1988) to the time-independent radiation transfer equation. Results and Discussion
Measurements were performed at a droplet radius of 0.65 um for angle of 0° (transmitted light flux), 15° and 45 °. For particle concentrations below about 10 6 cm3 very good agreement of both methods CAMS and EXT with the actual aerosol number concentration was found. With the increasing particle concentration multiple scattering related deviations from the actual concentration for concentration values obtained by both methods occur (Fig.l). In order to interpret this nonlinear behavior of the methods a detailed study of all light fluxes was performed. Results obtained show an angular dependence of multiple scattering effects with the $23
$24
D. FILIPOVICOVAet aL
light flux at 45 ° being less affected than at 15° . This difference is understandable because at sufficiently high concentrations an isotropic distribution of radiation will be achieved, i.e. some angular ranges will experience an increase and some a decrease of received scattered intensity relative to the single scattering conditions. It was observed that at the experimental conditions chosen the transmitted light flux deviated from the single scattering behavior predicted by BeerLambert law at concentrations beyond some 10~ cm3 caused by some scattered photons being re-scattered into the direction of the propagation of the primary beam. In order to be able to accurately describe the light transmission at higher concentrations a multiple scattering correction term to the Beer-Lambert law can be applied (Szymanski, 1992). Numerical modeling of the correction term suggests that the paraxial approximation is more suitable for the description of multiple scattering for the experimental conditions chosen than the small angle approach. The work is in progress but results obtained so far indicate that an optimization of the scattered light detection system and a precise measurement of the transmitted and scattered light fluxes should allow a direct optical concentration measurement independent of any empirical calibration in the aerosol number concentration range from about 102 em3 up to 10s cm3. 1,00E+08
d~ gd 1,00E+07
-
1,00E+06
-
z
Iso - P r o p a n o l o
45 °
0
16-2-95 0 1,00E+05 1,00E+05
I
I
1,00E+06 1,00E+07 Corrected Particle Number Concentration [ cm ^-3 ] []
CAMS
.
1,00E+08
EXT
Fig. 1. Comparison of concentrations measured by means of EXT and CAMS-Method (at 45 °) to the actual concentration.
Acknowledgment This work was supported in part by the Fonds zur FOrderung der wissenschaftlichen Forschung, Proj. Nr. P9421 and by the Jubil~iumsfondsprojekt Nr.4363 der Osterreichischen Nationalbank.
References Bissonnette, L.R., Appl. Opt. 27, 2478-2484 (1988). Szymanski, W.W. and Wagner, P.E.J. Aerosol Sci. 21,441-451 (1990). Szymanski, W.W., J. Aerosol Sci. 23, 425-435 (1992). Van de Hulst, H.C.,"Light Scattering by Small Particles, Wiley & Sons, N.Y. (1957). Wagner, P.E., J. Coll. Interface Sci. 105, 456-466 (1985). Zardeeki, A., Gerstl, S.A.W. and Embury, J.F., Appl. Opt. 22, 1346-1353 (1983).