Abstract
Abstract. Vibrational Raman scattering from nitrogen is commonly used in aerosol lidars for evaluation of particle backscattering (β) and extinction (α) coefficients. However, at mid-visible wavelengths, particularly in the daytime, previous measurements have possessed low signal-to-noise ratio. Also, vibrational scattering is characterized by a significant frequency shift of the Raman component, so for the calculation of α and β information about the extinction Ångström exponent is needed. Simulation results presented in this study demonstrate that ambiguity in the choice of Ångström exponent can be the a significant source of uncertainty in the calculation of backscattering coefficients when optically thick aerosol layers are considered. Both of these issues are addressed by the use of pure-rotational Raman (RR) scattering, which is characterized by a higher cross section compared to nitrogen vibrational scattering, and by a much smaller frequency shift, which essentially removes the sensitivity to changes in the Ångström exponent. We describe a practical implementation of rotational Raman measurements in an existing Mie–Raman lidar to obtain aerosol extinction and backscattering at 532 nm. A 2.3 nm width interference filter was used to select a spectral range characterized by low temperature sensitivity within the anti-Stokes branch of the RR spectrum. Simulations demonstrate that the temperature dependence of the scattering cross section does not exceed 1.5 % in the 230–300 K range, making correction for this dependence quite easy. With this upgrade, the NASA GSFC multiwavelength Raman lidar has demonstrated useful α532 measurements and was used for regular observations. Examples of lidar measurements and inversion of optical data to the particle microphysics are given.
Highlights
Atmospheric aerosols influence the radiative budget of the earth, and the corresponding forcing depends on the vertical distribution of particle scattering properties (IPCC, 2013; Guan et al, 2010)
The spectral dependence of particle extinction can be neglected when pure-rotational Raman scattering is used; for vibrational scattering, which is characterized by a significant wavelength shift of the Raman component, the ambiguity in the choice of the Ångström exponent is an additional source of uncertainty in the βa and αa calculations
Vibrational Raman measurements are characterized by a significant frequency shift of the return signal from the stimulating one, introducing uncertainties due to www.atmos-meas-tech.net/8/4111/2015/
Summary
Atmospheric aerosols influence the radiative budget of the earth, and the corresponding forcing depends on the vertical distribution of particle scattering properties (IPCC, 2013; Guan et al, 2010). The spectral dependence of particle extinction can be neglected when pure-rotational Raman scattering is used; for vibrational scattering, which is characterized by a significant wavelength shift of the Raman component, the ambiguity in the choice of the Ångström exponent is an additional source of uncertainty in the βa and αa calculations This uncertainty was considered before for measurements acquired at 351 nm (Whiteman, 2003a, b). The parameters of the fine and the coarse mode of real aerosols may exhibit strong variability (Dubovik et al, 2002); still the choice of typical values for this simulation permits us to estimate the effect of the spectral dependence of the Ångström exponent on backscattering and extinction calculation. It is desirable to eliminate the uncertainties related to the assumption of the A value through the use of rotational Raman scattering instead of vibrational scattering, decreasing the frequency shift of the inelastically scattered component
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