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Influence of carbon aerosol aggregation on its optical properties in the asymptotic model of fractal cluster
Pergamon
J. Aerosol Sci. Vol. 28, Suppl. 1, pp. $243-$244, 1997
PII:SO021-8502(97)O0169-9
01997 Elsevier Science Ltd. All rights reserved Printed in Great Britain
oo21-s5o~7$17.00+0.00
Influence of carbon aerosol aggregation on its optical properties in the asymptotic model of fractal cluster V.A.Babenko B.I.Stepanov Institute of Physics, Belarus Academy of Science, F.Scarina Prospect 68, Minsk 220072 ,Belarus
KEYWORDS carbon aerosol, fractal cluster, optical extinction An investigation of the optical properties of soot aerosol is of great importance for atmospheric optics inasmuch as the carbon particle define to a large extent the Earth's atmospheric radiation fluxes both in global and regional scale. The studies of optical characteristics of airborne carbon are of special interest in relation to the hypotheses of the "nuclear winter" as a result of atmosphere smoking from numerous fires. The submicron smoke particles belong to the condensational aerosol that above some threshold concentration associates into the aggregates which stochastic structure may be described on the basis of fractal concepts. The optical properties of the carbon aggregates for the 10.6 micron wavelength were investigated in [ 1 ] in the framework of earlier suggested the asymptotic model of fractal cluster [ 2 ]. In direct continuation of this work we have conducted the extensive computations for mass coefficients of extinction
fl = 3Qex, / 4 p R f ( where Oext - the cluster extinction efficiency factor computed by the asymptotic model, /9- bulk density of subparticle, R - the cluster outer radius, f - the filling factor averaged over cluster volume ) in the 0.4-14/am spectral interval and over a wide range of the cluster parameters. The cluster is modeled by the two-layer sphere with homogeneous core and radially inhomogeneous shell within which the effective complex refractive index drops by the power law meg = A r 8, where A,B - complex constants allowing to satisfactorily describe in asymptotic the true inhomogeneity profile. This model profile enables us to solve the diffraction problem in known special functions [ 3 ]. The radius of subparticles composed the cluster is varied from 2 to 10 nm, the same is the radius a of the small model particle core. The optical constants of the core are calculated by the dispersion formula. The optical constants on the external shell boundary are calculated by the Maxwell Garnett theory, and the whole cluster radius ranges from 50 nm to 100 micron. The principal structural parameter of aggregate is the fractal dimension D which value in our computations varies from 1.78 (cluster-cluster association ) to 2.45 ( cluster-particle association ).
$243
$244
Abstractsof the 1997EuropeanAerosolConference
The results of computations demonstrate that the clusters have the values of the mass extinction coefficients far above that of any bulk particles with distinction increasing with 2. For instance, the typical if- values for rather large ( R > 5 t t m ) clusters with medium density ( D = 2.00) are 45 - 60 m 2/g over all spectral range being studied, that 5-6 times as much as flm~ for the homogeneous bulk particles in the visible region and approximately two orders more than ]~M~ for ,~ = 14 p.m. A very strong dependence of fl on the fractal dimension D is observed, especially in IR-region. The mass coefficient fl decreases sharply with D. The value of D specifies the form of the curve fl ( R ) also. There is a broad maximum on the curve for the high density ( D > 2.00 ) clusters, however, a grows monotonically with R up to quite large values for the low density ( D <2.00 ) clusters. In the most realistic case ( D = 2.00 the value of flceases to depend on R and .,~ starting with some sufficiently large R. This behavior may be elementary explained on the basis of geometrical optics. In particular, the very simple relationship flo~,,, =l/ap holds true for the asymptotic value of the mass extinction coefficient at D = 2.00 that fairly well conforms with computations. By and large, the obtained results allow to reveal the parameter domains within which the most effective interaction of radiation with carbon clusters as against the bulk particles is realized. References
1. V.N.Kuzmin. Izvestija AN SSSR.Fizika atmosfery i okeana, 28, 953-957 ( 1992 ). 2. V.N.Kuzmin, O.V.Moroz, A.P.Prishivalko. Doklady AN SSSR, 302, 332 - 334 ( 1988 ). 3. V.A.Babenko, S.T.Leiko. Optika atmosfery i okeana, 4, 191-196 (1991).
J. Aerosol Sci. Vol. 28, Suppl. 1, pp. $243-$244, 1997
PII:SO021-8502(97)O0169-9
01997 Elsevier Science Ltd. All rights reserved Printed in Great Britain
oo21-s5o~7$17.00+0.00
Influence of carbon aerosol aggregation on its optical properties in the asymptotic model of fractal cluster V.A.Babenko B.I.Stepanov Institute of Physics, Belarus Academy of Science, F.Scarina Prospect 68, Minsk 220072 ,Belarus
KEYWORDS carbon aerosol, fractal cluster, optical extinction An investigation of the optical properties of soot aerosol is of great importance for atmospheric optics inasmuch as the carbon particle define to a large extent the Earth's atmospheric radiation fluxes both in global and regional scale. The studies of optical characteristics of airborne carbon are of special interest in relation to the hypotheses of the "nuclear winter" as a result of atmosphere smoking from numerous fires. The submicron smoke particles belong to the condensational aerosol that above some threshold concentration associates into the aggregates which stochastic structure may be described on the basis of fractal concepts. The optical properties of the carbon aggregates for the 10.6 micron wavelength were investigated in [ 1 ] in the framework of earlier suggested the asymptotic model of fractal cluster [ 2 ]. In direct continuation of this work we have conducted the extensive computations for mass coefficients of extinction
fl = 3Qex, / 4 p R f ( where Oext - the cluster extinction efficiency factor computed by the asymptotic model, /9- bulk density of subparticle, R - the cluster outer radius, f - the filling factor averaged over cluster volume ) in the 0.4-14/am spectral interval and over a wide range of the cluster parameters. The cluster is modeled by the two-layer sphere with homogeneous core and radially inhomogeneous shell within which the effective complex refractive index drops by the power law meg = A r 8, where A,B - complex constants allowing to satisfactorily describe in asymptotic the true inhomogeneity profile. This model profile enables us to solve the diffraction problem in known special functions [ 3 ]. The radius of subparticles composed the cluster is varied from 2 to 10 nm, the same is the radius a of the small model particle core. The optical constants of the core are calculated by the dispersion formula. The optical constants on the external shell boundary are calculated by the Maxwell Garnett theory, and the whole cluster radius ranges from 50 nm to 100 micron. The principal structural parameter of aggregate is the fractal dimension D which value in our computations varies from 1.78 (cluster-cluster association ) to 2.45 ( cluster-particle association ).
$243
$244
Abstractsof the 1997EuropeanAerosolConference
The results of computations demonstrate that the clusters have the values of the mass extinction coefficients far above that of any bulk particles with distinction increasing with 2. For instance, the typical if- values for rather large ( R > 5 t t m ) clusters with medium density ( D = 2.00) are 45 - 60 m 2/g over all spectral range being studied, that 5-6 times as much as flm~ for the homogeneous bulk particles in the visible region and approximately two orders more than ]~M~ for ,~ = 14 p.m. A very strong dependence of fl on the fractal dimension D is observed, especially in IR-region. The mass coefficient fl decreases sharply with D. The value of D specifies the form of the curve fl ( R ) also. There is a broad maximum on the curve for the high density ( D > 2.00 ) clusters, however, a grows monotonically with R up to quite large values for the low density ( D <2.00 ) clusters. In the most realistic case ( D = 2.00 the value of flceases to depend on R and .,~ starting with some sufficiently large R. This behavior may be elementary explained on the basis of geometrical optics. In particular, the very simple relationship flo~,,, =l/ap holds true for the asymptotic value of the mass extinction coefficient at D = 2.00 that fairly well conforms with computations. By and large, the obtained results allow to reveal the parameter domains within which the most effective interaction of radiation with carbon clusters as against the bulk particles is realized. References
1. V.N.Kuzmin. Izvestija AN SSSR.Fizika atmosfery i okeana, 28, 953-957 ( 1992 ). 2. V.N.Kuzmin, O.V.Moroz, A.P.Prishivalko. Doklady AN SSSR, 302, 332 - 334 ( 1988 ). 3. V.A.Babenko, S.T.Leiko. Optika atmosfery i okeana, 4, 191-196 (1991).