Categorization of atmospheric suspended particles using degree of linear polarization parameter data of sun-photometer

Aerosols are solid or liquid particles suspended in the earth's atmosphere, which enter the earth's atmosphere from both natural and human sources. The wind blowing in the deserts, the evaporation of oceans and seas, the eruption of volcanoes, and the burning of forests and pastures are natural sources, and the burning of fossil fuels and the change of the earth's surface cover are human sources of their production. Aerosols can be classified into four types: dusty, marine, urban-industrial, and biomass burning particles. Due to the significant temporal and spatial changes of aerosols and for properly understand their climatic effects, we need to use long-term measurements of satellites and ground-based instruments. Measurements made from space and ground have allowed us to have a detailed view of the properties and effects of different types of atmospheric particles. Ground-based remote sensing is one of the powerful methods for determining the optical and physical properties of atmospheric aerosols.
The sun-photometer (SPM) is a spectrometer that records the intensity of the sun's radiation usually in four wavelength channels of 440, 675, 870, and 1020 nm in two modes of measuring the sun and the sky with a limited viewing angle of 1.2 degrees during the day. Spectral aerosol optical depth, columnar water vapor, Angstrom exponent, single scattering albedo, polarized phase function, the real and imaginary refractive index of aerosols, and degree of linear polarization of sunlight are characteristics of atmospheric particles that are extracted from SPM measurements.
There are different methods for classifying aerosols using data extracted from SPM measurements. One of the most common methods is to use aerosol optical depth data (a measure of the amount of atmospheric suspended particles) in terms of the Angstrom exponent (a qualitative measure of the dimensions of atmospheric particles). By combining other parameters obtained from the mode of the sun and the sky of the SPM, such as the aerosol optical depth, Angstrom exponent, particle size distribution, and refractive index, atmospheric aerosols can be classified. Our aim in this article is to investigate the ability of the degree of linear polarization parameter to classify the atmospheric particles. The degree of linear polarization measures the linear polarization of sunlight scattered by atmospheric particles (molecules and aerosols). For this purpose, the data of four sites of Banizoumbou, Beijing, El-Arenosillo, and Minsk, which have dusty, urban-industrial, marine, and biomass-dominant particles, respectively, were selected from the AERONET (AErosol RObotic NETwork) data.
This paper uses three parameters of aerosol optical depth, Angstrom exponent, and degree of linear polarization extracted from SPM data. The results show that the maximum value of the degree of linear polarization (standard deviation) at the wavelength of 870 nm for dusty (Banizoumbou), urban-industrial (Beijing), marine (El-Arenosillo) and biomass (Minsk) aerosols are equal to 0.14 (0.05), 0.35 (0.10), 0.47 (0.08) and 0.37 (0.08) respectively. Therefore, the parameter of the degree of linear polarization is able to separate dusty, urban-industrial, and marine atmospheric particles from each other. However, biomass particles overlap a lot with urban-industrial aerosols and cannot be separated from each other.