Small and large particle limits of the asymmetry parameter for homogeneous, spherical particles

Abstract

The aerosol single scattering asymmetry parameter g is an important intensive particle parameter in determining aerosol direct radiative forcing and radiation pressure. For homogeneous spherical particles and a complex refractive index that is independent of wavelength, g is solely dependent on the size parameter x, the ratio of the particle circumference to the wavelength, and the complex refractive index m of the particle. Here, we explore this dependency for the small and large particle limits with size parameters much smaller and much larger than one. We show that in the small particle (Rayleigh) regime, g is proportional to x2, and give the proportionality factor as a function of the complex m. In addition, we separate regions in the complex refractive index space where the asymmetry parameter is negative (dominantly backscattering) and where it is positive (dominantly forward scattering). In the large particle limit, the geometric optics regime, g converges to a constant value that is a function of m.