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The microphysical quantities (particle shape, composition, size, density, complex refractive index, size distribution model, aspect ratio, hygroscopic parameter, etc.) of the ensemble of complex externally mixed aerosol particles in humid environments (sea fog, water mist, haze, etc.) vary greatly. These microphysical quantities directly affect the laser transmission and scattering properties. Due to the optical properties (extinction coefficient, absorption coefficient, backscattering coefficient, phase function, etc.) of the ensemble of complex externally mixed aerosol particles directly determine the propagation properties of laser signals in the atmosphere, as well as the intensity and shape of echo signals. Therefore, studying the optical properties of the ensemble of complex externally mixed aerosol particles in humid environments is of significant importance for engineering applications such as autonomous driving, mapping, remote sensing detection, and more.
Based on the various possibilities of aerosol particles existing in humid environments, the diversity of physicochemical properties of aerosol particles, including their shape (sphere, oblate spheroid, prolate spheroid, and irregular), size distribution, complex refractive index, density, aspect ratio and its distribution models, as well as hygroscopicity parameters, etc., was fully taken into consideration in this paper. Therefore, a scattering model of the ensemble of complex externally mixed aerosol particles was presented in Section 2. Based on the presented complex aerosol scattering model, the influence of different mixing ratios (MR), and relative humidity (RH) on the optical properties, such as extinction coefficient, single scattering albedo, scattering phase matrix, asymmetry factor, backscattering coefficient, lidar ratio, and linear depolarization ratio, were numerically analyzed at typical laser wavelengths incident (0.78mm, 0.905 mm, 1.064 mm, 1.55 mm, and 2.1 mm).
In order to verify and illustrate the rationality of the complex aerosol scattering model presented in this paper, it was compared with the scattering model of maritime pollution aerosol in OPAC in Section 3.1. The results show that the optical properties of these two different aerosol scattering models vary similarly with wavelength, although differences exist; overall, the differences are relatively small. Therefore, the influences of MR on the optical properties of the ensemble of complex internally mixed aerosol particles were analyzed in Section 3.2. The influences of RH on the optical properties of the ensemble of complex internally mixed aerosol particles were also analyzed in Section 3.3. The numerical results indicate that the extinction coefficient and phase function P11 exhibit strong sensitivity to both the MR and RH. As RH increases, the extinction coefficient, and the forward scattering of P11 also increase. Compared to MR, single scattering albedo and asymmetry factor are more sensitive to RH. Significant differences in the sensitivity of linear, and circular polarization properties to RH and wavelength are observed at different scattering angles. The backscattering coefficient is found to be inversely proportional to the lidar ratio, and both the backscattering coefficient and lidar ratio are sensitive to MR and RH. It is observed that RH has a more pronounced effect on the linear depolarization ratio, while the influence of MR is weaker. The complex scattering model presented in this paper further expands the study of aerosol optical properties and provides theoretical support for studying engineering applications involving lasers in different RHs environments. It is worth emphasizing that this paper only focuses on external mixing. Therefore, the optical properties of the ensemble of complex internally mixed aerosol particles under different RHs will be the focus of future research. -
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