Optical properties of smoke particles: modeling and interpretation

Atmospheric aerosols of both natural and anthropogenic origin have a significant impact on the Earth's climate and on human health by degrading air quality. To study the influence of aerosols on climate and to monitor air pollution, it is important to know the characteristics of aerosol particles. In the case of smoke particles, their microphysical, chemical and optical properties are complex and strongly depend on the combustion sources, the aging process and the meteorological conditions.

This study is devoted to the analysis of optical properties of the particles that can be adopted as a model of smoke particles. The main attention has been paid to the analysis of the spectral dependence of the backscattering linear depolarization ratio (LDR), in particular, the strong wavelength dependence of the LDR, that is observed for some biomass burning events in lidar measurements. Among other light scattering models of smoke particles that can describe such spectral dependence and reproduce the optical properties of smoke particles, the most realistic model in the form of fractal-like clusters of carbonaceous spherules (monomers) was selected. The analysis was carried out on the basis of the created database of optical characteristics of fractal-like clusters calculated for a certain set of cluster parameters differing in their structure, size and morphological characteristics of individual monomers. Our results show that the LDR is a complex function of various factors such as particle size and shape, refractive index, fraction and composition of the core and monomer coating material, etc., and for a certain choice of morphological characteristics, a fractal-like model can reproduce the observed spectral dependence of the LDR. In the framework of such a model, the values of the LDR measured by lidars can be used to estimate the monomer size of the cluster particles, more precisely, the product of the monomer size and the real part of the refractive index of the monomer.

The integration of fractal aggregate morphology into aerosol models can improve the retrieval accuracy of microphysical and chemical properties of smoke particles. And combined analysis of Sun photometer and lidar measurements can provide complementary information for clarifying morphological characteristics of the monomers, such as size, refractive index, and cluster structure and size distribution.