J. Aer,,~JlSci. Vol. It), pp. 437 to 443. 0021-8502:79:0901 0437 ~02.tlO~ © PergamonPressLtd. 1979. Printedin Great Britain. SIZE MEASUREMENT OF NARROW DISTRIBUTION AEROSOLS BY THE POLARIZATION RATIO OF 90 ° SCATTERED LIGHT W. HINDS, M. W. FtRST and D. LEITH School of Public Health, Harvard University, Boston, MA 02115, U.S.A. (Received 8 February 1979) A~tr~ - The ratio of intensities of horizontal (in the scattering plane) and vertical polarization components of 90 ° scattered light is considered to be an accurate means for determining the particle size of monodisperse aerosols in the 0.1 to 0.4 tan size range. Numerical analysis and experimental results presented here show that polarization ratio depends strongly on particle size (~ D s'6) and that a polarization ratio Owl used with a calibration curve based on perfectly monedisperse aerosols yields a diameter of average polarization ratio which may be up to 18% larger than the CMD for aerosols that are not perfectly monodisperse but that meet the criteria for monodispersity suggested by Fuchs and Sutugin (1966), i.e. ag < 1.22. For precise measurement of log normally distributed aerosols, a0 must be _< 1.10 to ensure that the size measured with the Owl is within 5% of the CMD. INTRODUCTION The use of the polarization ratio, the ratio of intensities of the horizontal (in the scattering plane) and vertical polarization components of scattered light, as a means for determining particle size of monodisperse aerosols was first described by Sinclair and LaMer (1949). This technique came in association with the development of an instrument called the Owl, Sinclair (1950), for determining particle size by the angular position of spectral colors in the light scattered by aerosols. By using a split field eyepiece, top half polarized vertically and bottom half polarized horizontally, and a rotatable polarized disk (analyzer) between the eyepiece and the observer the Owl can be used to measure directly the polarization ratio of light scattered at any angle. The most common procedure is to set the eyepiece to view 90 ° scattered light, which may be filtered to restrict observations to certain wavelengths and rotate the analyzer disk through an angle, tk, until the light passing both halves of the eyepiece appears of equal intensity. At this condition the polarization ratio is given by, i2/i I = (tan ~b) 2, (1) where i2 and it are the intensity parameters for horizontal and vertical polarization, respectively. Sinclair (1950) published a series of curves for particles with different refractive indices which give the theoretical relationship between analyzer angle and particle size. Because these curves show a monotonic relationship between analyzer angle from 0 to 60° and particle diameter from 0.1 to 0.4 #m for particles with refractive indices from 1.33 to 2.00, 90° polarization ratio measurements can .be used to determine size for monodisperse particles within these limits. The method is simpleand rapid but is restricted to monodisperse aerosols and to this narrow range of sizes because of double value problems. Other investigators have sought to extend the capabilities of polarization ratio size measurement by taking measurements at several wavelengths, Heller (1963), Heller (1964) or at several scattering angles, Kerker et ai. (1964). Several authors, Sinclair and Lamer (1949), Sinclair (1950), Green and Lane (1964) and Mercer (1973) have cautioned that polarization ratio measurements should be restricted to monodisperse aerosols without quantifying the term monodisperse. This paper describes the effect of size spread on indicated particle size where 90° polarization ratio is used to measure aerosols that meet the criteria for monodispersity defined by Fuchs and Sutugin (1966), i.e. a relative standard deviation of 0.0 to 0.2, which corresponds to a geometric standard deviation (GSD) for a log normal distribution of 1.0 to 1.22. 437