Single scattering albedo of homogeneous, spherical particles in the transition regime

Aerosol single scattering albedo (SSA) is the most important intensive particle parameter controlling aerosol direct radiative forcing. For homogeneous, spherical particles and a complex refractive index independent of wavelength, the SSA is solely dependent on size parameter (ratio of particle circumference and wavelength) and complex refractive index of the particle and can be accurately calculated with Mie theory. Here, we explore this dependency for particles of intermediate size in the transition or peak regime between the Rayleigh scattering and geometric optics regimes. We show that in the transition regime, low-frequency oscillations (the interference structure) of SSA as function of size parameter occur for relatively small imaginary parts of the refractive index and are caused by interference between transmitted and diffracted components of the scattered light. Anomalous diffraction theory (ADT) semi-quantitatively describes this behavior of the SSA as function of size parameter and complex refractive index. While ADT accurately gives the size parameters of the interference peaks, ADT amplitudes only approximate exact Mie results. A significant improvement in agreement with Mie theory can be obtained with modified ADT (MADT) that adds a parameterization for the physics neglected in ADT, namely internal reflection/refraction, photon tunneling, and edge diffraction.