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 vary greatly in humid environments (sea fog, water mist, haze, etc.). These microphysical quantities directly affect the transmission and scattering characteristics of laser. 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, and remote sensing detection.

Based on the various possibilities of aerosol particles existing in humid environments, the physicochemical properties of aerosol particles, including their shapes (sphere, oblate spheroid, prolate spheroid, and irregular), size distributions, complex refractive indices, densities, aspect ratios, their distribution models, and hygroscopicity parameters, are all taken into consideration in this work. Therefore, a scattering model of the ensemble of complex externally mixed aerosol particles is presented. Based on the presented complex aerosol scattering model, the influences 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, are numerically analyzed at typical incident laser wavelengths (0.78, 0.905, 1.064, 1.55, and 2.1 μm).

In order to verify and demonstrate the rationality of the complex aerosol scattering model presented in this work, this model is compared with the scattering model of maritime pollution aerosol in optical properties of aerosols and clouds (OPAC). The results show that the optical properties of these two different aerosol scattering models vary similarly with wavelengths, although differences exist, but they are relatively small. Therefore, the influences of MR on the optical properties of the ensemble of complex internally mixed aerosol particles are analyzed. The influences of RH on the optical properties of the ensemble of complex internally mixed aerosol particles are also analyzed. 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 with MR, single scattering albedo and asymmetry factor are more sensitive to RH. Significant differences in the sensitivity to RH and wavelength between linear and circular polarization properties are observed at different scattering angles. The backscattering coefficient is found to be inversely proportional to the lidar ratio, and the backscattering coefficient and the lidar ratio are both 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 work 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 work only focuses on external mixing. Therefore, the optical properties of the ensemble of complex internally mixed aerosol particles under different RHs will be discussed in the future.