Scattering by Nonspherical Particles of Size Comparable to a Wavelength: A New Semi-Empirical Theory and Its Application to Tropospheric Aerosols

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  • 1 Space Science Division, NASA Ames Research Center, Moffett Field, CA 94035
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Abstract

We propose an approximate method for evaluating the interaction Of randomly oriented. nonspherical particles with the total intensity component of electromagnetic radiation. When the particle size parameter x (the ratio of particle circumference to wavelength) is less than some upper bound xO (∼5) theory is used. For x>xO the interaction is divided into three components: diffraction, external reflection and transmission. Physical optics theory is used to obtain the first of these components; geometrical optics theory is applied to the second; and a simple parameterization is employed for the third. The predictions of this theory are found to be in very good agreement with laboratory measurements for a wide variety of particle shapes, sizes and refractive indices. Limitations of the theory are also noted.

As an application of the theory, we consider the influence of the shape of tropospheric aerosols on their contribution to Earth's global albedo. Irregularly shaped tropospheric particles generally have larger single-scattering albedos and smaller scattering asymmetry factors than their equal volume spherical counterparts. Hence, for a fixed optical depth, the former cause a larger increase in the global albedo than the latter. Explicit calculations of the contribution of tropospheric aerosols to the global albedo for a variety of values of the particles’ imaginary index of refraction and shape parameters indicate that size able differences occur between cases involving nonspherical particles and ones involving their spherical counterparts.

Abstract

We propose an approximate method for evaluating the interaction Of randomly oriented. nonspherical particles with the total intensity component of electromagnetic radiation. When the particle size parameter x (the ratio of particle circumference to wavelength) is less than some upper bound xO (∼5) theory is used. For x>xO the interaction is divided into three components: diffraction, external reflection and transmission. Physical optics theory is used to obtain the first of these components; geometrical optics theory is applied to the second; and a simple parameterization is employed for the third. The predictions of this theory are found to be in very good agreement with laboratory measurements for a wide variety of particle shapes, sizes and refractive indices. Limitations of the theory are also noted.

As an application of the theory, we consider the influence of the shape of tropospheric aerosols on their contribution to Earth's global albedo. Irregularly shaped tropospheric particles generally have larger single-scattering albedos and smaller scattering asymmetry factors than their equal volume spherical counterparts. Hence, for a fixed optical depth, the former cause a larger increase in the global albedo than the latter. Explicit calculations of the contribution of tropospheric aerosols to the global albedo for a variety of values of the particles’ imaginary index of refraction and shape parameters indicate that size able differences occur between cases involving nonspherical particles and ones involving their spherical counterparts.

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