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An alternative method for obtaining the optical properties of monodisperse spherical non-absorbing aerosol using a cell-transmissometer
1. Aemso/Se’. Vol. 29, Suppl. 1. pp. S3914392, 1998 by Elsevier Science Ltd. All rights resewed Printed in Great Britain 0021-85WY98 $19.00 + 0.00
Q 1998 Published
Pergamon
AN ALTERNATIVE METHOD FOR OBTAINING THE OPTICAL PROPERTIES MONODISPERSE SPHERICAL NON-ABSORBING AEROSOL USING A CELL-TRANSMISSOMETER MARCOS A. PE%LOZA
OF
M.
University of Essex. Institute for Environmental Research. Central Campus Wivenhoe Park. Colchester, Essex CO4 3SQ. England, U. K. and Universidad de Los Andes. Fat. Ciencias. Dpto. Fi’sica. M&da. Venezuela KEYWORDS: Laser transmissometer. Spherical monodisperse aerosol. Ammonium Sulfate. I. INTRODUCTION One of the traditional experimental methods used to estimate the optical properties of aerosols is the method in which the extinction and scattering coefftcients are simultaneously measured with an extinction cell in combination with an integrating nephelometer (Horvath, 1993). The difference between the two measured quantities is the absorption coefficient. For slightly absorbing aerosols this difference is very small so that this method has to be applied with a high degree of accuracy and precision. Therefore correction of the truncation error of the integrating nephelometer or special calibration which corrects for this error has to be carry out. In addition forwardscattering in the extinction cell has to be considered. Based on an experimental method, suggested by Wu and Lim (1987) to perfom absorption and scattering measurements on a liquid crystal, an alternative method for directly obtaining, the optical properties of a monodisperse spherical non-absorbing aerosol using only a cell-transmissometer is presented; it consequently eliminates the use of the integrating nephelometer and therefore the complication of corrections. II. METHOD This method involves the linearisation of the Lambert-Beer law using an expansion series of its exponential form; this is given by,
where T is the transmission, d is the path length, cs.h is the absorption coefficient, cru: is the extinction coefficient at a specific wavelength. Under conditions that these coefficients are small and o& << 0% as for non-absorbing aerosol, quadratic or higher order terms in o&-,can be ignored; thus eq. (1) is given by, 1 - T = (&,,+&)d - (1 /2)(Oar,+0,32&
(2)
Considering a monodisperse spherical aerosol of radius r, the absorption coefficients are respectively given by, Gab=
rc?NQ,r,
(3-1)
and
osc = x?NQsc,
and scattering
(3-2)
where N is the aerosol number concentration and Q.,band QScare respectively the Mie absorption coefficient and the Mie scattering efficiency. Substitution of eqs. (3) into eq. (2) yields, s391
Abstracts
S392 (1 -
of the 5th International
Aerosol Conference
1998
T)/N = -( 1/2)n2r4 &QsE(QsC+ 2Q,i,)N + rr?d(Qsc + Qah)
(4)
A plot of (1 - T)/N vs N gives a straight line with parameters given by, I(intersect) = niZd(Qsc + Qab) (5-l)
and a(gradient) = -(l/%)rc2r4dQ,(Q=
+ 2Qab). (S-2)
Solving the equation system (5) for Qlt, and Qsc, the efficiency coefficients are obtained provided that measurements of T, using a cell-transmissometer, and N can be made. The appropriate solutions in this case are, Qsc = [I - (I2 - 2a)“2]/~r2d III. EXPERlMENTAL
(6-l)
and
Qab= (VrrIzd, - QE .
(6-2)
AND RESULTS
A first attempt to test experimentally this method has been achieved using a 632.8 nm He-Ne laser beam cell-transmissometer, for monodisperse spherical aerosol of ammonium sulfate. Examination by scanning electron microscopy (SEM) of a sample taken on a Make1 membrane filter (45 vrn pore size), determined that this aerosol was spherical-shaped. The SEM analysis showed that the aeirosol was quite monodisperse with an average radius of 114 nm with a geometric standard deviation of 1.15. The transmissometer has a cell of length d = 1 m and a volume of approximately 0.0155 m”. The aerosol was generated from a dilute solution with a concentration of 0.01 g/cm3 using a constant output atomiser (TSI model 3076), at a bubbler rate of 3.9 Vmin for dry and filtered air supply flow. For about 3 min the aerosol was pumped into the cell. Subsequently measurements of the mass concentration were undertaken using an airborne Figure 1 particle monitor (Casella model AMS95OIS). T----l Previous measurements of this parameter with the same aerosol showed that the variation of the mass concentration within the cell has a decay (Fig. 1). The particle concentration was calculated using a value of 1. 10x10-‘4g/particle. The results are shown in Fig.2. A linear relationship has been found according to eq (4). Figure 2 However in calculating the efficiencies by applying eqs.(5) the values were overstimated in _ comparison with those predicted by the Mie theory. Presumahly it is due to the fact that no corrections were made for the forwardscattering.Therefore to improve the present method corrections for this effect will have to be made (Deepak and Box, 1978) REFERENCES
Deepak, A. and M. A. Box. (1978). Appl. Opt., 17: 2900-2908. Hovarth, H. (1993). Atmos. Environ., 27A: 293-3 17. Wu, S-T and K-Ch. Lim. (1987). AppZ. Opt., 26: 1722-1727.
Q 1998 Published
Pergamon
AN ALTERNATIVE METHOD FOR OBTAINING THE OPTICAL PROPERTIES MONODISPERSE SPHERICAL NON-ABSORBING AEROSOL USING A CELL-TRANSMISSOMETER MARCOS A. PE%LOZA
OF
M.
University of Essex. Institute for Environmental Research. Central Campus Wivenhoe Park. Colchester, Essex CO4 3SQ. England, U. K. and Universidad de Los Andes. Fat. Ciencias. Dpto. Fi’sica. M&da. Venezuela KEYWORDS: Laser transmissometer. Spherical monodisperse aerosol. Ammonium Sulfate. I. INTRODUCTION One of the traditional experimental methods used to estimate the optical properties of aerosols is the method in which the extinction and scattering coefftcients are simultaneously measured with an extinction cell in combination with an integrating nephelometer (Horvath, 1993). The difference between the two measured quantities is the absorption coefficient. For slightly absorbing aerosols this difference is very small so that this method has to be applied with a high degree of accuracy and precision. Therefore correction of the truncation error of the integrating nephelometer or special calibration which corrects for this error has to be carry out. In addition forwardscattering in the extinction cell has to be considered. Based on an experimental method, suggested by Wu and Lim (1987) to perfom absorption and scattering measurements on a liquid crystal, an alternative method for directly obtaining, the optical properties of a monodisperse spherical non-absorbing aerosol using only a cell-transmissometer is presented; it consequently eliminates the use of the integrating nephelometer and therefore the complication of corrections. II. METHOD This method involves the linearisation of the Lambert-Beer law using an expansion series of its exponential form; this is given by,
where T is the transmission, d is the path length, cs.h is the absorption coefficient, cru: is the extinction coefficient at a specific wavelength. Under conditions that these coefficients are small and o& << 0% as for non-absorbing aerosol, quadratic or higher order terms in o&-,can be ignored; thus eq. (1) is given by, 1 - T = (&,,+&)d - (1 /2)(Oar,+0,32&
(2)
Considering a monodisperse spherical aerosol of radius r, the absorption coefficients are respectively given by, Gab=
rc?NQ,r,
(3-1)
and
osc = x?NQsc,
and scattering
(3-2)
where N is the aerosol number concentration and Q.,band QScare respectively the Mie absorption coefficient and the Mie scattering efficiency. Substitution of eqs. (3) into eq. (2) yields, s391
Abstracts
S392 (1 -
of the 5th International
Aerosol Conference
1998
T)/N = -( 1/2)n2r4 &QsE(QsC+ 2Q,i,)N + rr?d(Qsc + Qah)
(4)
A plot of (1 - T)/N vs N gives a straight line with parameters given by, I(intersect) = niZd(Qsc + Qab) (5-l)
and a(gradient) = -(l/%)rc2r4dQ,(Q=
+ 2Qab). (S-2)
Solving the equation system (5) for Qlt, and Qsc, the efficiency coefficients are obtained provided that measurements of T, using a cell-transmissometer, and N can be made. The appropriate solutions in this case are, Qsc = [I - (I2 - 2a)“2]/~r2d III. EXPERlMENTAL
(6-l)
and
Qab= (VrrIzd, - QE .
(6-2)
AND RESULTS
A first attempt to test experimentally this method has been achieved using a 632.8 nm He-Ne laser beam cell-transmissometer, for monodisperse spherical aerosol of ammonium sulfate. Examination by scanning electron microscopy (SEM) of a sample taken on a Make1 membrane filter (45 vrn pore size), determined that this aerosol was spherical-shaped. The SEM analysis showed that the aeirosol was quite monodisperse with an average radius of 114 nm with a geometric standard deviation of 1.15. The transmissometer has a cell of length d = 1 m and a volume of approximately 0.0155 m”. The aerosol was generated from a dilute solution with a concentration of 0.01 g/cm3 using a constant output atomiser (TSI model 3076), at a bubbler rate of 3.9 Vmin for dry and filtered air supply flow. For about 3 min the aerosol was pumped into the cell. Subsequently measurements of the mass concentration were undertaken using an airborne Figure 1 particle monitor (Casella model AMS95OIS). T----l Previous measurements of this parameter with the same aerosol showed that the variation of the mass concentration within the cell has a decay (Fig. 1). The particle concentration was calculated using a value of 1. 10x10-‘4g/particle. The results are shown in Fig.2. A linear relationship has been found according to eq (4). Figure 2 However in calculating the efficiencies by applying eqs.(5) the values were overstimated in _ comparison with those predicted by the Mie theory. Presumahly it is due to the fact that no corrections were made for the forwardscattering.Therefore to improve the present method corrections for this effect will have to be made (Deepak and Box, 1978) REFERENCES
Deepak, A. and M. A. Box. (1978). Appl. Opt., 17: 2900-2908. Hovarth, H. (1993). Atmos. Environ., 27A: 293-3 17. Wu, S-T and K-Ch. Lim. (1987). AppZ. Opt., 26: 1722-1727.