Rayleigh-Raman-Mie Lidar Detection Study Of Optical Parameters Of Tropospheric Aerosols

In this article,the optical properties of tropospheric aerosols over the northern suburb of Nanjing were measured by Rayleigh Raman Mie lidar.Firstly,the Mie scattering channel signal is processed to remove the background noise.It is found that it is reasonable to take the average value of the echo signal of the clean atmosphere above 10km as the background noise.Then,two parameters which will have a great impact on the Fernald inversion:the extinction coefficienta_a(z _c)of aerosol particles in the clean atmosphere and the lidar ratio S1 are analyzed and compared.It is found that the value ofa_a(z _c)has little impact on the final inversion results;the value of S1 has a great impact on the inversion results,and the larger the value is,the smaller the extinction coefficient is.When using Raman channel signal to retrieve aerosol optical parameters,the original data are denoised by fast Fourier transform and wavelet transform,and the results show that the effect of wavelet denoising is better.Then the two parameters of wavelet de-noising:adaptive threshold and soft hard thresholding are compared and analyzed.It is found that the fixed threshold has the best de-noising effect,and the soft thresholding is better than the hard thresholding.Based on Mie scattering echo data,the profile of extinction coefficient can be obtained by inversion.Combined with weather map,the height of cloud layer and aerosol layer and their changes over a period of time can be found;the low concentration aerosol layer at low altitude may diffuse and dilute under the effect of turbulence.By comparing the aerosol extinction coefficient retrieved by Fernald and Raman method,it is proved that Raman method is more accurate because it does not need to assume the radar ratio in advance.However,the real radar ratio calculated by Raman method can greatly improve the inversion accuracy of the Fernald method,the average lidar ratio can be replaced by the Fernald method,and the inversion effect of the average lidar ratio is general,while the inversion result of assuming that the lidar ratio is constant is the worst.The vertical distribution profiles of aerosol extinction coefficient,backscattering coefficient and radar ratio under different weather conditions were obtained and analyzed by using Raman scattering signal inversion.The results show that the aerosol extinction coefficient,backscattering coefficient and radar ratio obtained by the inversion are larger than those of the atmospheric model when there is no cloud on a clear day,indicating that the aerosol distribution in the northern suburb of Nanjing is uniform.When there are clouds above,the aerosol extinction coefficient and backscattering coefficient have maximum values,and the corresponding height radar ratio is low.When there are no clouds and a lot of aerosols over the sky,the extinction coefficient and radar ratio have large peaks,while the backscattering coefficient is at a low level.Finally,combining with wavelet covariance transformation and aerosol extinction coefficient time series analysis,it is found that the height of boundary layer in daytime is generally higher than that at night,and the height of atmospheric boundary layer is higher than that in cloudy or cloudy days when the weather condition is clear.When there is cloud layer over the sky,it will affect the observation of the height of atmospheric boundary layer on the time series diagram of aerosol extinction system.