Accounting for the effects of nonideal minor structures on the optical properties of black carbon aerosols

Abstract

Abstract. Black carbon (BC) aerosol is the strongest sunlight-absorbing aerosol, and its optical properties are fundamental to radiative forcing estimations and retrievals of its size and concentration. BC particles exist as aggregate structures with small monomers and are widely represented by the idealized fractal aggregate model. In reality, BC particles possess complex and nonideal minor structures besides the overall aggregate structure, altering their optical properties in unforeseen ways. This study introduces the parameter “volume variation” to quantify and unify different minor structures and develops an empirical relationship to account for their effects on BC optical properties from those of ideal aggregates. Minor structures considered are as follows: the polydispersity of monomer size, the irregularity and coating of the individual monomer, and necking and overlapping among monomers. The discrete dipole approximation is used to calculate the optical properties of aggregates with these minor structures. Minor structures result in scattering cross-section enhancement slightly more than that of absorption cross section, and their effects on the angle-dependent phase matrix as well as asymmetry factor are negligible. As expected, the effects become weaker with the increase in wavelength. Our results suggest that a correction ratio of 1.05 is necessary to account for the mass or volume normalized absorption and scattering of nonideal aggregates in comparison to ideal ones, which also applies to aggregates with multiple minor structures. In other words, the effects of minor structures are mainly contributed by their influence on particle volume/mass that cannot be ignored, and a relative difference of approximately 5 % is noticed after removing the volume effects. Thus, accurate knowledge and evaluation of BC volume/mass are more important than those of the minor structures themselves. Most importantly, the simulations of optical properties of nonideal aggregates are greatly simplified by applying the empirical relationship because they can be directly obtained from those of the corresponding ideal aggregates, a volume/mass difference parameter, and the correction factor, i.e., 1.05, not the detailed minor structure information. We expect this convenient treatment to find wide applications for the accounting for the effects of nonideal minor structures on BC optical properties.

Constructing Shapes and Mixing Structures of Black Carbon Particles With Applications to Optical Calculations

“…Here we only generate one BC shape model for each D f value, so here our discussions are presented based on the given shape models. The MAC of the bare BC particles ranges in 5.8-6.4 m 2 g −1 with an average of 6.1 m 2 g −1 , which is consistent with the observed MAC (∼6-7 m 2 g −1 at 550 nm) of the thermo-denuded BC particles in the London urban environment (D. , but is slightly lower than the commonly cited MAC value of 7.5  1.2 m 2 g −1 at 550 nm (Bond & Bergstrom, 2006) and the modeled results of 8.1 m 2 g −1 at 550 nm (Teng et al, 2019). Figure 3i shows that the MAC of the bare BC particles slightly increases from D f = 1.6 to D f = 2.0 and then decreases from D f = 2.0 to D f = 2.6.…”

The Ångström Exponent and Single-Scattering Albedo of Black Carbon: Effects of Different Coating Materials

“…In this work, the heavily-coated BC can be commonly modeled by a shape of a coating sphere covering the BC aggregates [32,43,73], and the partially-coated BC was not considered in this work. In the study of Teng et al [74], the thinly-coated BC is recognized as a non-ideal minor structure, and an empirical relationship was proposed for optical cross-section, while AAE and EAE were not calculated, and BrC coatings were not considered. In this work, BC is assumed to be thinly-coated as a mass of BC to OC (BC/OC) is larger than 0.375, and is assumed to heavily-coated when BC/OC is less than approximately 0.167, where BC/OC are calculated using:…”

Specifying the light-absorbing properties of aerosol particles in fresh snow samples, collected at the Environmental Research Station Schneefernerhaus (UFS), Zugspitze

“…The WIBS4 discriminates fluorescing biological aerosol particles (FBAPs) by combining single-particle fluorescence signals from two excitation-emission wavebands with a low cross-sensitivity to inorganic, combustion, and mineral dust particles (Toprak and Schnaiter, 2013). The WIBS4 measurement of the 10 March 2017 snow sample supports the Figure 10.…”