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Optical characterization of size separated aerosol particles of different composition and morphology with a polar nephelometer

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1993 Pure Appl. Opt. 2 471 (http://iopscience.iop.org/0963-9659/2/5/008) View the table of contents for this issue, or go to the journal homepage for more

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Pure Appl. Opt. 2 (1993) 471-88. Printed in the UK

Optical characterization of size separated aerosol particles of different composition and morphology with a polar nephelometer Aniello Mennellat and Franco Prodit: T ENIRICERCHE SPA, V. Maritano 26 (20097) S. Donato Milanese, Milan, Italy $ FlSBAT-CNR, V. Castagnoli, Bologna, Italy and Dipartkento di Fisica dell’Universiti, Ferrara, Italy Received 20 November 1992, in final form 2 March 1993 Ahtrack. A new technique for the determination of the complex refractive index of aerosol particles hasbeen developed. The particles are deposited on a membrane filter by means of an aerosol inertial spectrometer, in order to have k e d and size-separated particles. The filter is then made transparent by exposure to acetone vapours and the sample inspected with a laser beam. Scattering data are then analysed and the complex refractive index is obtained. Data and results are presented for a numbei of materials.

1. Iotroductioo

The knowledge of optical characteristics of aerosol particles is of great importance both for industrial and environmental purposes. It is known [l] that, depending on the complex refractive index of the particles, a variation in the aerosol loading of the atmosphere could lead either to an increase or to a decrease of the mean global temperature of the earth. Light scattering techniques have often been used to determine the complex refractive index of airborne particles [i, 31 performing field or laboratory measurements and inverting scattering data taking advantage of Mie theory. According to Mie theory (in the hypothesis of a linearly polarized monochromatic incident EM wave) the intensity scattered by a sphere of radius r and complex refractive index m at a given scattering angle 8 is:

where 8 is the scattering angle, $ is the angle of polarization o f the incident field, m = m, im, is the complex refractive index of the particle, d is the distance between the particle and the optical sensor, A: is the wavelength, x = 2m/L is the Mie parameter, r is the particle radius, k = 2n/% is the wavenumber and j(8,$, m,,mi) = i, sin’ 4 + i, cos’ @ (where i l , 2 = /Sl,21z) is the scattering phase function [4]. If we have, in the scattering volume, a polydispersed population of spherical particles with radii in the range [rl, rz], in the hypothesis of single and independent scattering,

+

0963-9659/93/050471+ 18 $7.50

0 1993 IOP Publishing Ltd

471

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A Mennella and F Prodi

the scattered intensity is given by

where n(r) dr is the number of particles with radii in the range [r, r + dr]. From (1.2) the importance of knowing the function n(r) and the shape of the particles in order to invert scattering data and obtain the complex refractive index m is evident; in fact, there is still little understanding as far as the imaginary part is concerned, owing principally to the following problems: (i) unknown sue distribution; (ii) variable optical characteristics of the particles in the scattering volume; (iii) particle motion originated by convection and Brownian diffusion; (iv) multiple scattering effects. In order to avoid these problems we have taken advantage of a new technique that combines ordinary light scattering measurements with inertial separation and sampling of aerosol particles [S, 61.

2. Experiment 2.1. Aerosol sampling by inertial separation

In order to obtain a sample containing sue-separated aerosol particles the inertial spectrometer INSPEC has been used [7]; the operating principle is shown in figure l(a). The apparatus consists of a channel of rectangular cross-section with a 90” bend; clean air is drawn through the channel, with a thin aerosol sheath injected through a nozzle close to the inner wall. The outer wall after the bend is covered by a membrane filter through which the air is sucked. Due to the bend the particles leave the streamlines and deposit onto the filter at a distance from the bend which is a unique function of their aerodynamic diameter. The fine particle fraction remains in the original streamline and is captured on the same filter which then contains the complete size information of the aerosol. A cross-section of the sampling head is shown in figure l(b); more details about INSPEC may be found in the cited paper. An example of the instrument performance is given in figure 2, which shows a typical deposition pattern made by distinct bands; inside each band there are almost monodispersed particles.

2.2. Filter diaphanization

During this phase, the filter with the collected particles is fixed in a metal frame and made transparent by means of exposure to acetone vapours. It is important to expose the filter for a time long enough to be sure of the complete optical transparency of the filter itself. Exposure to dust has been avoided by making use of a glove box. Once made transparent the filter is removed from the frame and it is then suitable for light scattering measurements, for optical microscope and SEM observations too. Some care must be taken in choosing the aerosol material; some organic particles, in

413

Optical characterization of aerosol particles

~~~~

Figure 1. The inertial spectrometer sampling head.

101

INSPEC: ( a ) working

principle; (h) cross-section of the

Figure 2. Typical deposiiion p;illcm u l thc incrlial specllomclei.

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Figure 3. SEM micrograph of carnauba wax particles gcneratcd by means or the aerosol generator MAGE.

fact, might he destroyed by acetone vapours. In these cases (with latex and carnauba wax particles) we have transferred the deposit from the filter to a cover glass, by means of simple superposition and finger pressing. In our work we have inspected the following materials: (i) latex particles (spherical-2.5 pm diameter); (ii) montmorillonite clay (non-spherical-polydispersed); (iii) carnauba wax particles (spherical-1 pm diameter); (iv) glass beads (spherical-50 pm diameter);) (v) Y S Z (yttrium zirconium nickel oxide) particles (spherical-polydispersed). Figures 3-6 show some examples of optical microscope and SEM observations of the tested materials laying o n the filter already treated in acetone vapours. Some conclusions may he drawn. (i) It is possible to control the surface density of particles in order to he sure that single scattering hypothesis is verified. (ii) The particles lay on the surface of the filter. This fact will he discussed in section 3 as far as the filter effects are concerned. (iii) The monodispersion inside each hand is good, so that an initially polydispersed sample can be separated in different narrow size ranges. A size distribution function inside each band could he obtained from micrographs of the sample even if this would lead to a considerable increase of the computing time during the scattering data analysis.

2.3. Aerosol generation Aerosols of tested materials have been produced following different ways according to the nature of the material itself. Carnauba wax particles have been produced (already monodispersed) by means of the generator MACE (monodisperse aerosol generator) from solid material. This

Optical characterization of aerosol particles

Figure 4.

SE\{

415

micrographs olmnntmonllonilc clay particles rel:mve to three different size bands.

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A Mennella and F Pro&

Figure 5.

SEM

micrographs of YSZ particles relalive to three different size bands.

Optical characterization of aerosol particles

477

Figure 6. Deposition of glass beads (50 pm diameter; photograph obtained through the optical microscope).

generator is a modified Sinclair-La Mer generator [SI which can generate aerosol particles from liquids and solid materials which have a melting point not over 250 "C. Latex and YSZ particles have been aerosolized from water suspensions by means of a De Vilhiss nebulizer connected to a silica gel desiccator to obtain the dry aerosol. Montmorillonite clay, instead, has been aerosolized from the dry powder by means of a cyclone, while glass beads have been directly filtered from water suspension. Because of their large diameter (50 bm), in fact, it has not been possible to use the inertial spectrometer for sampling.

2.4. The polar nephelometer

The experimental set-up of the polar nephelometer for the light-scattering analysis is shown in figure 7. All the samples have been analysed by means of an HeNe (A = 632.8 nm, P = 20 mW) laser while YSZ and carnauba wax particles have also been inspected with a n Ar+ laser at 488 and 514.5 nm wavelength ( P = 30 mW).

Figure 7. Schematic diagram of the light-scattering experiment set-up.

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SCREW TO

TIGHTEN THE FILTER

FIXTEEFILTER

Figure 8. Scheme of the sample holder used in our experiments,

The photomultiplier (PMT), connected to a photon counter, is fixed on a goniometer connected to a computer-controlled stepping motor. The sample holder is fixed on a rail which allows the sample to move transversally with respect to the laser beam in is comparable order to choose a specific size band. The width of each band (1.4") with the diameter of the laser spot; the particles are faced to the laser beam. In figure 8 the sample holder is shown in detail. It consists of a frame with an open side which allows the PMT to measure the scattered light in the whole range 15"-165". A particular tightening screw avoids filter folding. It is worth noting that measurements in the range 75"-115" are not suitable for data analysis because in that angular region the single scattering hypothesis is no longer verified. In our case measurements have been performed with parallel polarized incident light in a range between 30" and 60" (with 1" angular step) which has been seen to be less sensitive to experimental errors, non-sphericity and filter effects. All the run is computer controlled; at each angle the PMT performs six measurements of the number of incident photons per unit time and returns the mean value according to Poisson's statistics. In order to subtract the filter contribution from the scattered light, on each filter a measurement with the beam inspecting an empty zone has been done. The aim to determine the complex refractive index of aerosol particles justifies the choice to perform measurements with only one polarization state. The experimental data obtained this way, in fact, contain enough information for the numerical inversion. Moreover, the presence of only one polarization state minimized the CPU time when calculating the Mie functions.

3. Data analysis

To analyse scattering data some hypotheses must be made about our system. (i) Single and independent scattering by particles; (the hypothesis may be verified by inspecting the filter by means of a microscope and checking if the particles are sufficiently far away from one another). Thanks to the possibility of controlling the surface particle distribution it could in principle be possible to measure the critical distribution beyond which this hypothesis is no longer verified.

Optical characterization of aerosol particles

479

(ii) The particles are considered as spheres. This is a critical point in the use of Mie theory to invert scattering data in order to obtain the complex index of refraction of non-spherical particles; at this regard it is known that iE m, x 1 or if mi is slightly different from zero, Mie theory can often safely be used to invert scattering data [SI, especially in the forward region 0" 5 8 560" [lo]. Anyway it is possible. in principle, to use other models dealing with particular shapes (for example fibres) that cannot be simply modelled by means of Mie theory, thanks to the possibility to characterize the shape of the particles deposited onto the filter. (iii) The filter does not interact as far as the boundary conditions of the EM field are concerned (this condition may be true if the particles are not embedded too deeply in the filter and if care is taken to wait long enough during the diaphanization phase). In order to satisfy this condition it is necessary for the particle diameter to be smaller than the mean filter pore diameter: in our experiments we used an 0.2 pm Nuclepore@filter which prevented particles from sinking into the filter matrix. (iv) The field scattered by the filter linearly superimposes onto the one scattered by the particles (this approximation is reasonably good if the surface covered by the particles is much less than the total spot surface). (v) The field across the filter damps following the Bouger-Lambert law (3.1) -dl=lTdx where -dl is the intensity decay across the length dx and T is the optical extinction coefficient of the filter. s being the filter thickness we have that the output intensity at a gjven angle 8 is related to the input one by the relation

where H = TS. The relation between PMT output M ( 8 ) and the complex refractive index m = m, + imi of N, identical spherical particles is therefore

where x = 2nr/J. is the Mie parameter (r is the particle radius and A is the wavelength), 19is the scattering angle, 4 is the polarization angle with respect to the scattering plane, C = c~plo/(k2d')with c = PMT calibration constant, N,,= number of particles in the inspected zone, 1, = incident light intensity, k = wavenumber 2 4 2 and d = distance sample PMT. The unknown quantities of the problem are, therefore, H,C, m,, mi; the particle radius r is obtained by microscope observations. Our method is based on the following steps: (i) write the x2 function; (ii) obtain H and C as functions of m, and m,; (iii) minimize the xz function in the m,, mispace to obtain the desired solution. 3.1. Derivation of H and C

Taking the natural logarithm of (3.3) we obtain:

(3.4)

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the 1' function may thus be written as

Solving the system

(3.6) we can easily obtain H and C as (3.7)

where

:(-)

s,=

i=,

1 , C0S(Si)

J

and NA is the number of scattering angles.

3.2. Data processing

xz function had to be minimized in the (mr, mi)space. This has been done making use of Powell's method [ll] which finds the minimum of a function of n variables without derivative information; this fact has been useful to overcome the difficulties of numerically calculating the gradient of the x2 function. Wiscombe algorithms [12] have been used for Mie scattering functions computations. Before running the program, the positions of the minima were determined for each sample by performing a visual analysis of the shape of the x2 function in a reasonably wide (mr, mi)window. Then the main program was run in order to identify the absolute minimum. In figure 9 two examples of the xZ function shape are shown (both in 3~ and top view). The surface structure, although quite complex, is roughly the same for each sample, with a chain of peaks steeply descending towards a plateau in which are located the minima. In order to obtain the complex refractive index from scattering data, the

Optical characterization of aerosol particles

48 1

Figure 9. Examples of visualization of the x2 function (data relative to Carnauba wax particles; I = 0.6 pm): ( a ) A = 514.5 nm, ( b ) top view of (a). (41. = 488 nm, ( d ) a close-up top view of (e).

4. Results and diseussion

All the results are summarized in tables 1 and 2. Some considerations may hence be drawn. (i) Carnauba wax particles are nearly transparent t o red light while the absorption increases at 514.5 and 488 nm wavelength; this is in agreement with the yellowish colour of the material, which denotes a high absorption of green and blue light.

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A Mennella and F Prodi

Figure 9. (Continued.)

Tabk 1. Complex refractive indices obtained

at

Material

Radius (pm)

Latex Glass beads Montmorillonite Clay

25 2.46

1.45

A: = 632.8 nm. Complex refractive index ( m )

+ 0.Oi 1.62 + O.Oi 1.63 - 2.0 x 10-3i

1.59

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Table 2 Comdcx reiractive indices obtained at different wavelengths. ~~

(A = 632.8 nm)

Mated

m

Carnauba wax (r = 0.6p) YSZ (7 = 0.48 pm)

1.67 - 0.Oi 2.66 - 6.0 x IO-%

m (1. = 514.5 mm)

m (. =I 488.0 nm)

1.62 - 1.62 x 1O-'i

1.65 - 2.39 x IO-'i m

m

-Theoretical cwe

Experimental values

0

30

35

45

40

A@

50

55

6a

(JW4

Figure 10. Comparison between experimental and theoretical phase functions; data relate to 2.9 pm latex particles (incident wavelength A = 632.8 nm).

(ii) YSZ particles show weak absorption to red light and high reflectivity to green and blue (according to their green colour). (ui) Glass and latex refractive indices fit quite well with tabulated ones (between 1.539 and 1.705 for glass [13] and 1.6 for latex [14]) even if latex refractive index is underestimated. The underestimation may be due to other filter effects which have not been taken into account. They might be: (i) part of the forward scattered light that is reflected in the backward direction, (ii) distortion of the scattered signal due to some filter structure that remains after diaphanizatioh, (iii) other non-linear effects. Figures 10-13 show the quality of tbe interpolations between data and the theoretical relation (3.3) calculated with the values of m, aild mi relative to the minimum of the xz function. From these graphs it is possible to obtain information about the optical characteristics of the sample. In figure 10 data relative to latex particles are well interpolated by the theoretical curve that shows the characteristic ripple structure, even if the ripples are smoothed in the forward direction due to the filter absorption. In this m e (as well as in the case of clay particles) there seems to be a shift between the experimental and theoretical phase

.-3-c

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1

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.E 0.6

4

--

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5 E

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Theoretical curve

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Experimental Values

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Angle (Degrees)

-Theoretical CUrVe

Experimental Values

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Angle (Degrees)

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I

'

Theoretical CurVe

Experimental values

I

04

30

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Angle (Degrees)

Figure 11. Same as figure 10 but with experimental data relating to montmorillonite clay particles. Data are presented for four direrent s i z bands 0. = 6328 nm).

Optical characterization of aerosol particles 0.16

485

,

(4

Band no20

-Theoretical Curve

30

35

40

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55

w

Angle (Degrees)

Figore 11. (Continued)

functions. This could be an effect due to errors in the size parameter. While in the case of clay particles this can be ascribed to a certain polydispersion within each band (the INSPEC separates particles according to their aerodynamic diameter that can he different from the geometric one), it is not clear whether in the data relative to latex particles the shift is due to errors in the scattering data or in the determination of the particle diameter. In figure 11 (a)-@) data relative to clay particles of different size bands are shown. Here we may notice the characteristic smoothing of the experimental phase function due to non-sphericity effects as well as an increasing ripple structure with increasing particle size (as expected). Figure 12 shows data relative to glass beads. It is possible to notice the strong variability of the theoretical phase function due to a big Mie parameter (about 250). In this case it has been very difficult to interpolate experimental data because

30

35

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50

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Angle (Degrees)

Figure12 Same as figure 10 but with experimental data relating to glass beads

(I = 632.8 nm).

A MenneIlu and F Prodi

486

,

120000'

-Theomid Curve @ . 40000

B

0

Y

4'

Experimental

2qooo 0 30

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Angle (&grees)

-

looow

'

90000

.d

,80000 70000

..

60&

;50000 -- -Theoretical s0. 4owo

-

c .

z.

,'

.. 30000

20000 -.

-

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Experimental Values

I

10000

..

I

O* 30

35

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45

Angle (W?W 80000

-Theordid Curve

I 30

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Angle @-res)

Figure 13. Same as figure 10 but with experimental data relating to YSZ particles: (a) 6 = 632.8 nm. (b) 6 = 514.5 nm,(c) A = 488.0 nm.

Optical characterization of aerosol particles

487

the angular scale within which the phase function varies is much smaller than the angular step of the goniometer (1'). Anyway, even if the experimental data cannot reproduce the ripple structure of the phase function, the result of the interpolation is quite good and the obtained refractive index is consistent with tabulated data. Figure 13 (a)-(e) shows the experimental and theoretical phase function behaviour (relative to ysz particles) from weak absorption (figure 13(a)) to strong reflectivity (figure 13(b) and (c)). The disappearing of the ripple structure (that is characteristic when the mi is large and in the approximation of m = m) is here clearly visible.

5. Conclusions

The feasibility of light-scattering experiments to determine the complex refractive index of aerosol particles deposited on a filter and separated by size has been demonstrated. The complex reffactive index of a certain number of materials of spherical or isotropic shape has been determined taking into account some filter effects as linear scattering superposition and exponential damping of the forward scattering. The results thus obtained have been found to be in good agreement with already tabulated values (where present) and the spectral behaviour of mi at the tested wavelengths is the expected one. Refinements of the technique will be performed as far as the filter effects are concerned Further applications will be addressed to the study of complex refractive index of non-spherical particles (such as fibres) and to its application to the determination of optical characteristics of atmospheric aerosols.

Acknowledgments

We acknowledge the Physical Chemical Department in ENIRICERCHE for their contribution to SEM analysis. In the Colloidal Science Department in ENIRICERCHE we wish to acknowledge Dr F Gagliardi, Dr G Piro and Mr G Colombani for their contribution to the synthesis of YSZ particles and Dr G Calloni for his experimental contribution to the latest part of the work.

References [I] Reck R A 1974 A r m s . Enuironment 8 823-33 [Z] Tanaka M, Takamura T and Nakajima T 1982 1. Climate Appl. Mefeor. 22 1253-61 [3] Grams G W, BLiRord I H, Gillette D A and Russel B P 1974 J . Appl. Meteor. 13 459-71 [4] Kerker M 1969 The Scnfterlng o f l i g h t (New York Academic) [SI Prodi F, Levbani V and Casarini G 1986 Digest Int. C o d on Opticnl and Millimeter Wnue fiopngntion and Scattering in the Almosphere (Florence) pp 391-2 [6] Prodi F and Mennella A 1990 J . Aerosol Sei. 21 suppl. I 5539-42 [7l Prodi V, Melandri C, Tarroni G, de Zakcomo T and Formignani M 1919 Aerosol Sei. 101 411-19 [8] Prodi V 1972 Assessment of Airborne Particles ed P E Morrow and W Strober (Sprin&eleld, I L Thomas) p 169

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[9] Pinnick R G, Carroll D E and Hofmann D J 1976 Appl. Opt. 15 384-93 [lo] Wismmbe W 1 and Mugnai A 1986 Single Scattering Jrom Non-Spherical Chebysheo Particles: ~2 Compendium of Calculations (NASA Ret Publ. 1157) (NASIVGSFC, Greenbelt, MD) [ll] 1981 Handbook of Applicable Mathematics vol 3, ed R F Churchhouse (Chichester: Wiley) 1121 Wiscombe W J 1979 Mie Scattering Calculations: Advances in Technique and Fast,Vector Speed Compuler Codes NCAR Technical Note KCARjTN - 140 .t STR [I31 Lide D R (ed) 1990 Handbook $Chemistry om‘ Physics 71st edn (Cleveland, O H CRC) [I41 Drake R M and Gordon J E 1985 Am. 3. P h p 53 955-62