Study On The Light Scattering Characteristic Of Suspended Particulates In Atmosphere

In recent years,air pollution have become more and more serious in the northern and inland cities in China,and atmospheric suspended particulates have become more and more,which cause the dust and haze weather to be more frequent.Therefore,the study of the light scattering characteristics of atmospheric suspended particulates has important significance for understanding the origin of particulate matter and the mechanism that affects the atmosphere.When laser transmits in atmosphere,suspended particles can cause its serious attenuation,and affect the distance of Free Space Optical communication(FSO).Thus,the study of the light scattering characteristics of atmospheric suspended particulates has important theoretical significance for FSO system that heavily depends on the atmospheric environment.In this thesis,the single-scattering characteristics of different types and shapes of suspended particles were studied by using the Mie theory and the T-matrix method.The Monte Carlo method was used to study the multiple-scattering characteristics of the suspended particles.And the varying relationship of transmittance,transmission attenuation and "visibility"with the mass concentration under dust and haze conditions were analyzed.The specific research contents are as follows:(1)Taking the suspended particulate matter of dust,PM2.5,sea fog and land fog for example,assume they are spherical,the relationship of scattering efficiency factor,extinction efficiency factor,absorption efficiency factor and backscattering coefficient with the size parameters of these four kinds of suspended particles were studied by Mie theory,when the wavelength of incident light is 0.55?m.And the effect of size parameter on the scattering phase function of dust particles was analyzed.(2)The scattering phase function of the ellipsoid particles under different axial ratio was studied by using T-matrix method,which was compared with the scattering phase function of the spherical particle calculated by Mie theory.The scattering phase functions of dust and PM2.5 particles were studied by ellipsoid and columnar models,and the effect of particle orientation angle on the light scattering characteristics of suspended particles with different type and different shape was compared and analyzed.(3)The log-normal distribution was used to fit the particle size distribution of dust,and the conversion equation from particle mass concentration to particle number concentration was established.The normalized bulk scattering function of dust particles whose size satisfies log-normal distribution was studied by Mie theory.The attenuation coefficient in the waveband 0.2?m-10.6?m was calculated when the mass concentration of dust particles was 1199.4?g/m3(the corresponding number concentration of dust particles was 1.2892 × 109/11m3).When the wavelength of incident light is equal to 1.064?m,the varying relationship of the attenuation coefficient and the "visibility" with the mass concentration of dust particles was calculated by Monte Carlo method,which was compared with the result by Mie theory.The results show that:the attenuation coefficients at wavelengths of 6.5?m and 7.8?m are significantly lower than those at other wavelengths,which are 0.92dB/km and 0.73dB/km respectively,while the attenuation coefficients at wavelengths of 1.064?m and 10.6?m which was commonly used in FSO are 11.53dB/km and 2.86dB/km;due to the existence of multiple-scattering effects,the results calculated by Mie theory and Monte Carlo method have a big gap;when the laser wavelength is 1.064?m,and the mass concentration of dust particles is 3000?g/m3,the attenuation coefficient calculated by the Monte Carlo method is 14.06dB/km,and the "visibility"is 1.2km.(4)The effect of size distribution parameters of PM2.5 on transmittance,attenuation coefficient and "visibility" was analyzed.The varying relationship of transmittance and attenuation coefficient with PM2.5 mass concentration was studied by Mie theory and Monte Carlo method.The varying relationship of "visibility" with PM2.5 mass concentration was studied when the wavelength of incident light is 0.55?m,which was compared with the data measured in Beijing from 2014 to 2016.The results show that:the "visibility" calculated by the Monte Carlo method is more consistent with the measured data;when the laser wavelength is 0.55?m and the PM2.5 mass concentration is 500?g/m3,the transmittance calculated by the Monte Carlo method is 3.8%,the attenuation coefficient is 14.8 dB/km,and the "visibility" is 1.1km.