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Influence of an absorbing core of a particle on aerosol light scattering characteristics
~ Aerosol Sci Vol. 3 I, Suppl. 1, pp. $291-$292, 2000
Pergamon www.elsevier.com/locate/jaerosci
P o s t e r S e s s i o n I. Atmospheric aerosols: Optical properties
INFLOENCE OF AN ABSORBINGCORE OF A PARTICLE ON AEROSOL LIGHT SCATI'ERING CHARACTERISTICS L. WIND and W.W. SZYMANSKI Institute of Experimental Physics, Aerosol Laboratory, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria
Keywords: light absorption, scattering, layered particles, atmospheric aerosols INTRODUCTION A significant source of uncertainty in description of the actual influence of aerosols on atmospheric radiative processes are light scattering properties of ambient aerosols. Variation of aerosol optical characteristics depends strongly on the particle size and its composition. In atmospheric aerosols soot and dust are the only significant light-absorbing components in large quantities. Soot is particularly optically efficient affecting light transmission even when present as a small fraction of the mass. When enclosed into a transparent material the combined absorption properties can. substantially increase (Horvath, 1993). Soot itself is rather not hygroscopic, however it could serve as condensation nuclei and consequently alter scattering characteristics of a final droplet aerosol. In this contribution we investigate light propagation in nearly monodisperse aerosols consisting of water droplets formed on non-absorbing and absorbing particles. Experiments are performed in a cloud chamber. For theordical modeling the Mie theory of light scattering (Bohren and Huffman, 1983) a .dapted to scattering from layered particles has been applied. METHODS AND RESULTS Experiments are performed in well-defined aerosols. Nearly monodisperse droplet clouds are generated in an expansion cloud chamber allowing optical investigation of laser beam transmission and scattering. The expansion cloud chamber utilizes the heterogeneous nucleation and condensation of supersaturated vapor on condensation nuclei to form a droplet cloud. Nuclei particles (DOP or carbon) with diameters determined by an electrostatic classification are introduced into the chamber together with water vapor saturated air. Consequently, an adiabatic expansion causes a rapid pressure decrease in the chamber resulting in a supersaturation of the vapor and formation of a droplet cloud. Each droplet is assumed to consist of one nuclei particle within a water drop. This method enables a generation of a reproducible model cloud containing droplets with known sizes and known optical properties. Corresponding theoretical modeling is undertaken assuming that a spherical core (nuclei) particle is centered inthe external water droplet. Figure 1 shows the light intensity scattered atan angle of 29° from a droplet with an external diameter of 2 Itm but with a core varying in size and refractive index. It is evident that an absorbing core particle already with a volume of about 2% of the external droplet affects remarkably the scattering characteristics of the whole compound. It can be seen this is a highly non-linear effect depending oni the complex refractive index of the core particle. In ease when the volume of the core carbon particle (m=2.0-0.i) amounts to about 15% of the external droplet, the influence of its presence at this scattering angle is minimized. Experimental data obtained so far for scattering from PSL spheres with diameters of 90 nm and 450 nm immersed in a water droplet shows reasonable agreement with theory. Figure 2 presents the overall change in the phase function of a particle containing varying absorbing cores coated with water in comparison with a homogeneous water droplet and a spherical carbon particle. The strong scattered intensity variation depending on the particle composition and the scattering angle is evident. The negligence of these effects in efforts with regard to the quantification of the impact of atmospheric aerosols on radiative transfer might substantially contribute to the uncertainty of modeling results. Further investigations are in progress and will be reported.
S291
$292
Abstracts of the 2000 European Aerosol Co111brcncc
300
!. . . . . . . . . ," ,
,
¢.-I
~d m.
2oo
i
Scattenng angSe: 2 9 °, Waveleng~: 633 nm
J
homogeneous droplet
e-
I
I
._=
Iransparent core
rCI)
-
100
1
t '
x
-
m = 1.59 -
al~orbir~corem=
al~orbing core
/
,
0.0
0i
1.6-0.1i m = 2 . 0 - 1.0i
experimental data
I
t~ O 09
-
m = 1 . 3 3 - 0i
m = 1 . 5 9 - 0i
J
0.5 1.0 1.5 Diameter of core particle [um]
2.0
Fig. 1. Scattered light intensity from a water droplet of 2 pan in diameter containing different core particles.
9O
.°
I°°i
+/ _m m
/
\ 30
0,1~ 0,01 1E-3
~=
t°
o,oi 0,I
m = 1.33-0.0i, Dp:l.2 ~tm . . . . . . m,~,: 2.0-1.0i, Dc:1.0 p.m . . . . m,:=.o= 1.6-0.1i, Dc=l.0 pm m = 2.0-1.0i, Dp=I.2 ~tm
33O
~
lO 100
270
Fig. 2. Changes of the phase f~mcti(mdependingon the size of the absorbing core particle. ACKNOWLEDGEMENTS This work was financially supported by the Fonds zur FSrderung d ~ wissemchafllichen Forschung (Proj. Nr. P14090PHY). REFERENCES Horvath, H., Atm~pheric light absorption: A review, Atmos. Environ., Part A, 27:293 (1993). Bohren, C.F. and D.R. Huffman, Absorption and Scattering of Light by Small Particles, John Wiley & Sore, New York (1983).
Pergamon www.elsevier.com/locate/jaerosci
P o s t e r S e s s i o n I. Atmospheric aerosols: Optical properties
INFLOENCE OF AN ABSORBINGCORE OF A PARTICLE ON AEROSOL LIGHT SCATI'ERING CHARACTERISTICS L. WIND and W.W. SZYMANSKI Institute of Experimental Physics, Aerosol Laboratory, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria
Keywords: light absorption, scattering, layered particles, atmospheric aerosols INTRODUCTION A significant source of uncertainty in description of the actual influence of aerosols on atmospheric radiative processes are light scattering properties of ambient aerosols. Variation of aerosol optical characteristics depends strongly on the particle size and its composition. In atmospheric aerosols soot and dust are the only significant light-absorbing components in large quantities. Soot is particularly optically efficient affecting light transmission even when present as a small fraction of the mass. When enclosed into a transparent material the combined absorption properties can. substantially increase (Horvath, 1993). Soot itself is rather not hygroscopic, however it could serve as condensation nuclei and consequently alter scattering characteristics of a final droplet aerosol. In this contribution we investigate light propagation in nearly monodisperse aerosols consisting of water droplets formed on non-absorbing and absorbing particles. Experiments are performed in a cloud chamber. For theordical modeling the Mie theory of light scattering (Bohren and Huffman, 1983) a .dapted to scattering from layered particles has been applied. METHODS AND RESULTS Experiments are performed in well-defined aerosols. Nearly monodisperse droplet clouds are generated in an expansion cloud chamber allowing optical investigation of laser beam transmission and scattering. The expansion cloud chamber utilizes the heterogeneous nucleation and condensation of supersaturated vapor on condensation nuclei to form a droplet cloud. Nuclei particles (DOP or carbon) with diameters determined by an electrostatic classification are introduced into the chamber together with water vapor saturated air. Consequently, an adiabatic expansion causes a rapid pressure decrease in the chamber resulting in a supersaturation of the vapor and formation of a droplet cloud. Each droplet is assumed to consist of one nuclei particle within a water drop. This method enables a generation of a reproducible model cloud containing droplets with known sizes and known optical properties. Corresponding theoretical modeling is undertaken assuming that a spherical core (nuclei) particle is centered inthe external water droplet. Figure 1 shows the light intensity scattered atan angle of 29° from a droplet with an external diameter of 2 Itm but with a core varying in size and refractive index. It is evident that an absorbing core particle already with a volume of about 2% of the external droplet affects remarkably the scattering characteristics of the whole compound. It can be seen this is a highly non-linear effect depending oni the complex refractive index of the core particle. In ease when the volume of the core carbon particle (m=2.0-0.i) amounts to about 15% of the external droplet, the influence of its presence at this scattering angle is minimized. Experimental data obtained so far for scattering from PSL spheres with diameters of 90 nm and 450 nm immersed in a water droplet shows reasonable agreement with theory. Figure 2 presents the overall change in the phase function of a particle containing varying absorbing cores coated with water in comparison with a homogeneous water droplet and a spherical carbon particle. The strong scattered intensity variation depending on the particle composition and the scattering angle is evident. The negligence of these effects in efforts with regard to the quantification of the impact of atmospheric aerosols on radiative transfer might substantially contribute to the uncertainty of modeling results. Further investigations are in progress and will be reported.
S291
$292
Abstracts of the 2000 European Aerosol Co111brcncc
300
!. . . . . . . . . ," ,
,
¢.-I
~d m.
2oo
i
Scattenng angSe: 2 9 °, Waveleng~: 633 nm
J
homogeneous droplet
e-
I
I
._=
Iransparent core
rCI)
-
100
1
t '
x
-
m = 1.59 -
al~orbir~corem=
al~orbing core
/
,
0.0
0i
1.6-0.1i m = 2 . 0 - 1.0i
experimental data
I
t~ O 09
-
m = 1 . 3 3 - 0i
m = 1 . 5 9 - 0i
J
0.5 1.0 1.5 Diameter of core particle [um]
2.0
Fig. 1. Scattered light intensity from a water droplet of 2 pan in diameter containing different core particles.
9O
.°
I°°i
+/ _m m
/
\ 30
0,1~ 0,01 1E-3
~=
t°
o,oi 0,I
m = 1.33-0.0i, Dp:l.2 ~tm . . . . . . m,~,: 2.0-1.0i, Dc:1.0 p.m . . . . m,:=.o= 1.6-0.1i, Dc=l.0 pm m = 2.0-1.0i, Dp=I.2 ~tm
33O
~
lO 100
270
Fig. 2. Changes of the phase f~mcti(mdependingon the size of the absorbing core particle. ACKNOWLEDGEMENTS This work was financially supported by the Fonds zur FSrderung d ~ wissemchafllichen Forschung (Proj. Nr. P14090PHY). REFERENCES Horvath, H., Atm~pheric light absorption: A review, Atmos. Environ., Part A, 27:293 (1993). Bohren, C.F. and D.R. Huffman, Absorption and Scattering of Light by Small Particles, John Wiley & Sore, New York (1983).