Nonsphericity of dust‐like tropospheric aerosols: Implications for aerosol remote sensing and climate modeling

Abstract

The nonsphericity of dust‐like tropospheric aerosols causes us to question the applicability of using conventional Mie theory to compute their radiative properties. In this paper we compare T‐matrix computations of light scattering by polydispersions of randomly oriented nonspherical aerosols and Mie computations for equivalent spheres. We demonstrate that even moderate nonsphericity results in substantial errors in the retrieved aerosol optical thickness if satellite reflectance measurements are analyzed using Mie theory. On the other hand, the use of Mie theory for nonspherical aerosols produces negligible errors in the computation of albedo and flux related quantities, provided that the aerosol size distribution and optical thickness are known beforehand. The first result can be explained by large nonspherical‐spherical differences in scattering phase function, while the second result follows from small nonspherical‐spherical differences in single‐scattering albedo and asymmetry parameter. Our results demonstrate that no cancellation of errors occurs if one consistently uses Mie theory in the retrieval algorithm and then in computing the albedo for the retrieved aerosol optical thickness.

Impact of dust aerosols on the radiative budget, surface heat fluxes, heating rate profiles and convective activity over West Africa during March 2006

“…Indeed, author's performed optical calculations by using mixtures of oblate and prolate spheroids and concluded that difference in SSA and g are lower than 10%. This result is in accordance with those reported by Mishchenko et al (1995) and Bellouin et al (2004).…”

Mie simulations as an error source in mineral aerosol radiative forcing calculations

“…For τ , reliable remote sensing measurements are often available, whereas g and ω are typically simulated by use of the SPA in conjunction with additional information on the aerosol size distribution and refractive index. Several studies have confirmed that ω is insensitive to particle shape (Mishchenko et al, 1995(Mishchenko et al, , 1997Kahnert et al, 2002;Nousiainen and Vermeulen, 2003;Kahnert, 2004), whereas it strongly depends on m. Earlier investigations concluded that g can be modelled by the SPA (Mishchenko et al, 1995), which would justify the use of the SPA in climate forcing simulations. However, recent investigations indicate that the earlier models may have underestimated the impact of aerosol non-sphericity (Kahnert et al, 2005), and that the error in g caused by the SPA can equal that related to the uncertainty in m (Kahnert and Nousiainen, 2006b).…”

“…The uncertainty in the refractive index m and, to a lesser extent, the uncertainty in the size distribution of the aerosols are commonly regarded as the most important sources of error (Myhre and Stordal, 2001). It has been generally assumed that computing the aerosols' optical properties using the Mie theory for spherical particles -henceforth referred to as the spherical particle approximation (SPA) -causes negligible errors in radiative forcing simulations (Mishchenko et al, 1995). The validity of this notion is re-evaluated in this paper.…”

Particle shape and internal inhomogeneity effects on the optical properties of tropospheric aerosols of relevance to climate forcing

“…An alternative way, which we followed in this paper, is to use the equations derived by Middleton [1952] For each of the three approximations, a different approach is followed to calculate the particle phase functions. In the homogeneous sphere case, a simple-stable iterative scheme is used to calculate the appropriate Ricatti-Bessel functions and then the formation of the amplitude functions and of the phase function [Kerker, 1969 [Mishchenko, 1991[Mishchenko, , 1993Mishchenko et al, 1995Mishchenko et al, , 1996] to allow computations for large particles. NEPH3 uses this improved version of the Tmatrix method to calculate the phase functions of randomly oriented oblate and prolate spheroids.…”