Use of rotational Raman measurements in multiwavelength aerosol lidar for evaluation of particle backscattering and extinction

摘要

Vibrational Raman scattering from nitrogen is commonly used in aerosollidars for evaluation of particle backscattering (β) and extinction(α) coefficients. However, at mid-visible wavelengths, particularlyin the daytime, previous measurements have possessed low signal-to-noiseratio. Also, vibrational scattering is characterized by a significantfrequency shift of the Raman component, so for the calculation of αand β information about the extinction Ångström exponent isneeded. Simulation results presented in this study demonstrate thatambiguity in the choice of Ångström exponent can be the a significantsource of uncertainty in the calculation of backscattering coefficients whenoptically thick aerosol layers are considered. Both of these issues areaddressed by the use of pure-rotational Raman (RR) scattering, which ischaracterized by a higher cross section compared to nitrogen vibrationalscattering, and by a much smaller frequency shift, which essentially removesthe sensitivity to changes in the Ångström exponent. We describe apractical implementation of rotational Raman measurements in an existingMie–Raman lidar to obtain aerosol extinction and backscattering at 532 nm. A2.3 nm width interference filter was used to select a spectral rangecharacterized by low temperature sensitivity within the anti-Stokes branchof the RR spectrum. Simulations demonstrate that the temperature dependenceof the scattering cross section does not exceed 1.5 % in the 230–300 Krange, making correction for this dependence quite easy. With this upgrade,the NASA GSFC multiwavelength Raman lidar has demonstrated useful α measurements and was used for regular observations. Examples oflidar measurements and inversion of optical data to the particlemicrophysics are given.