Optics of water cloud droplets mixed with black-carbon aerosols

Aerosol optics model for black carbon applicable to remote sensing, chemical data assimilation, and climate modelling

Aerosol optics models are an integral part of of climate models and of retrieval methods for global remote sensing observations. Such large-scale environmental applications place tight constraints on the affordable model complexity, which are difficult to reconcile with the considerable level of detail that is needed to capture the sensitivity of optical properties to morphological aerosol characteristics. Here, we develop a novel core-grey-shell dimer model and demonstrate its potential for reproducing radiometric and polarimetric properties of black carbon aerosols. The depolarisation is mainly sensitive to the relative size of the monomers, while the optical cross sections depend on the core-shell partitioning of black carbon. The optimum choice of these parameters is fairly stable across particle sizes and soot volume fraction, as is demonstrated by comparison with a more realistic coated aggregate model.

The role of biomass burning states in light absorption enhancement of carbonaceous aerosols

Carbonaceous aerosols, which are emitted from biomass burning, significantly contribute to the Earth's radiation balance. Radiative forcing caused by biomass burning has been poorly qualified, which is largely attributed to uncertain absorption enhancement values (Eabs) of black carbon (BC) aerosols. Laboratory measurements and theoretical modelling indicate a significant value of Eabs; but this enhancement is observed to be negligible in the ambient environment, implying that models may overestimate global warming due to BC. Here, we present an aggregate model integrating BC aerosol ensembles with different morphologies and mixing states and report a quantitative analysis of the BC Eabs from different combustion states during biomass burning. We show that the BC Eabs produced by flaming combustion may be up to two times more than those produced by smouldering combustion, suggesting that the particle morphology and mixing state of freshly emitted BC aerosols is an important source of the contrasting values of Eabs. The particle morphology of freshly emitted BC aerosols is widely assumed to be bare in models, which is rare in the ambient environment and leads to small estimates of Eabs by field observations. We conclude that the exact description of freshly emitted carbonaceous aerosols plays an important role in constraining aerosol radiative forcing.

Si quantum dots enhanced hydrogen bonds networks of liquid water in a stimulated Raman scattering process

Stimulated Raman scattering (SRS) of silicon quantum dots (Si QD) water solutions of different sizes (2 and 5 nm) are investigated using Nd:YAG laser. Since strong and weak hydrogen bonds are formed by the charge transfer between water molecules and Si QDs, two SRS peaks of OH stretching vibrations of Si QDs solutions are observed in the forward direction. Simultaneously, characteristic feature peaks related to the interaction between OH groups and excess electrons are obtained in the backward SRS of 2 nm Si QDs solutions. The excess electrons induce a strong electrostatic field, leading to the transformation from water to an ice-VIII structure.

Scattering and Radiative Properties of Morphologically Complex Carbonaceous Aerosols: A Systematic Modeling Study

This paper provides a thorough modeling-based overview of the scattering and radiative properties of a wide variety of morphologically complex carbonaceous aerosols. Using the numerically-exact superposition T-matrix method, we examine the absorption enhancement, absorption Ångström exponent (AAE), backscattering linear depolarization ratio (LDR), and scattering matrix elements of black-carbon aerosols with 11 different model morphologies ranging from bare soot to completely embedded soot–sulfate and soot–brown carbon mixtures. Our size-averaged results show that fluffy soot particles absorb more light than compact bare-soot clusters. For the same amount of absorbing material, the absorption cross section of internally mixed soot can be more than twice that of bare soot. Absorption increases as soot accumulates more coating material and can become saturated. The absorption enhancement is affected by particle size, morphology, wavelength, and the amount of coating. We refute the conventional belief that all carbonaceous aerosols have AAEs close to 1.0. Although LDRs caused by bare soot and certain carbonaceous particles are rather weak, LDRs generated by other soot-containing aerosols can reproduce strong depolarization measured by Burton et al. for aged smoke. We demonstrate that multi-wavelength LDR measurements can be used to identify the presence of morphologically complex carbonaceous particles, although additional observations can be needed for full characterization. Our results show that optical constants of the host/coating material can significantly influence the scattering and absorption properties of soot-containing aerosols to the extent of changing the sign of linear polarization. We conclude that for an accurate estimate of black-carbon radiative forcing, one must take into account the complex morphologies of carbonaceous aerosols in remote sensing studies as well as in atmospheric radiation computations.

Light Absorption Enhancement of Black Carbon Aerosol Constrained by Particle Morphology

The radiative forcing of black carbon aerosol (BC) is one of the largest sources of uncertainty in climate change assessments. Contrasting results of BC absorption enhancement ( E_abs) after aging are estimated by field measurements and modeling studies, causing ambiguous parametrizations of BC solar absorption in climate models. Here we quantify E_abs using a theoretical model parametrized by the complex particle morphology of BC in different aging scales. We show that E_abs continuously increases with aging and stabilizes with a maximum of ∼3.5, suggesting that previous seemingly contrast results of E_abs can be explicitly described by BC aging with corresponding particle morphology. We also report that current climate models using Mie Core-Shell model may overestimate E_abs at a certain aging stage with a rapid rise of E_abs, which is commonly observed in the ambient. A correction coefficient for this overestimation is suggested to improve model predictions of BC climate impact.

First-principles modeling of electromagnetic scattering by discrete and discretely heterogeneous random media

A discrete random medium is an object in the form of a finite volume of a vacuum or a homogeneous material medium filled with quasi-randomly and quasi-uniformly distributed discrete macroscopic impurities called small particles. Such objects are ubiquitous in natural and artificial environments. They are often characterized by analyzing theoretically the results of laboratory, in situ, or remote-sensing measurements of the scattering of light and other electromagnetic radiation. Electromagnetic scattering and absorption by particles can also affect the energy budget of a discrete random medium and hence various ambient physical and chemical processes. In either case electromagnetic scattering must be modeled in terms of appropriate optical observables, i.e., quadratic or bilinear forms in the field that quantify the reading of a relevant optical instrument or the electromagnetic energy budget. It is generally believed that time-harmonic Maxwell's equations can accurately describe elastic electromagnetic scattering by macroscopic particulate media that change in time much more slowly than the incident electromagnetic field. However, direct solutions of these equations for discrete random media had been impracticable until quite recently. This has led to a widespread use of various phenomenological approaches in situations when their very applicability can be questioned. Recently, however, a new branch of physical optics has emerged wherein electromagnetic scattering by discrete and discretely heterogeneous random media is modeled directly by using analytical or numerically exact computer solutions of the Maxwell equations. Therefore, the main objective of this Report is to formulate the general theoretical framework of electromagnetic scattering by discrete random media rooted in the Maxwell-Lorentz electromagnetics and discuss its immediate analytical and numerical consequences. Starting from the microscopic Maxwell-Lorentz equations, we trace the development of the first-principles formalism enabling accurate calculations of monochromatic and quasi-monochromatic scattering by static and randomly varying multiparticle groups. We illustrate how this general framework can be coupled with state-of-the-art computer solvers of the Maxwell equations and applied to direct modeling of electromagnetic scattering by representative random multi-particle groups with arbitrary packing densities. This first-principles modeling yields general physical insights unavailable with phenomenological approaches. We discuss how the first-order-scattering approximation, the radiative transfer theory, and the theory of weak localization of electromagnetic waves can be derived as immediate corollaries of the Maxwell equations for very specific and well-defined kinds of particulate medium. These recent developments confirm the mesoscopic origin of the radiative transfer, weak localization, and effective-medium regimes and help evaluate the numerical accuracy of widely used approximate modeling methodologies.

Direct demonstration of the concept of unrestricted effective-medium approximation

The modified unrestricted effective-medium refractive index is defined as one that yields accurate values of a representative set of far-field scattering characteristics (including the scattering matrix) for an object made of randomly heterogeneous materials. We validate the concept of the modified unrestricted effective-medium refractive index by comparing numerically exact superposition T-matrix results for a spherical host randomly filled with a large number of identical small inclusions and Lorenz-Mie results for a homogeneous spherical counterpart. A remarkable quantitative agreement between the superposition T-matrix and Lorenz-Mie scattering matrices over the entire range of scattering angles demonstrates unequivocally that the modified unrestricted effective-medium refractive index is a sound (albeit still phenomenological) concept provided that the size parameter of the inclusions is sufficiently small and their number is sufficiently large. Furthermore, it appears that in cases when the concept of the modified unrestricted effective-medium refractive index works, its actual value is close to that predicted by the Maxwell-Garnett mixing rule.

Effects of morphology on the radiative properties of internally mixed light absorbing carbon aerosols with different aging status

Light absorbing carbon aerosols play a substantial role in climate change through radiative forcing, which is the dominant absorber of solar radiation. Radiative properties of light absorbing carbon aerosols are strongly dependent on the morphological factors and the mixing mechanism of black carbon with other aerosol components. This study focuses on the morphological effects on the optical properties of internally mixed light absorbing carbon aerosols using the numerically exact superposition T-matrix method. Three types aerosols with different aging status such as freshly emitted BC particles, thinly coated light absorbing carbon aerosols, heavily coated light absorbing carbon aerosols are studied. Our study showed that morphological factors change with the aging of internally mixed light absorbing carbon aerosols to result in a dramatic change in their optical properties. The absorption properties of light absorbing carbon aerosols can be enhanced approximately a factor of 2 at 0.67 um, and these enhancements depend on the morphological factors. A larger shell/core diameter ratio of volume-equivalent shell-core spheres (S/C), which indicates the degree of coating, leads to stronger absorption. The enhancement of absorption properties accompanies a greater enhancement of scattering properties, which is reflected in an increase in single scattering albedo (SSA). The enhancement of single scattering albedo due to the morphological effects can reach a factor of 3.75 at 0.67 μm. The asymmetry parameter has a similar yet smaller enhancement. Moreover, the corresponding optical properties of shell-and-core model determined by using Lorenz -Mie solutions are presented for comparison. We found that the optical properties of internally mixed light absorbing carbon aerosol can differ fundamentally from those calculated for the Mie theory shell-and-core model, particularly for thinly coated light absorbing carbon aerosols. Our studies indicate that the complex morphology of internally mixed light absorbing carbon aerosols must be explicitly considered in climate radiation balance.