Rayleigh–Brillouin scattering profiles of air at different temperatures and pressures

Abstract

Rayleigh-Brillouin (RB) scattering profiles for air have been recorded for the temperature range from 255 to 340 K and the pressure range from 640 to 3300 mbar, covering the conditions relevant for the Earth's atmosphere and for planned atmospheric light detection and ranging (LIDAR) missions. The measurements performed at a wavelength of λ=366.8 nm detect spontaneous RB scattering at a 90° scattering angle from a sensitive intracavity setup, delivering scattering profiles at a 1% rms noise level or better. The experimental results have been compared to a kinetic line-shape model, the acclaimed Tenti S6 model, considered to be most appropriate for such conditions, under the assumption that air can be treated as an effective single-component gas with temperature-scaled values for the relevant macroscopic transport coefficients. The elusive transport coefficient, the bulk viscosity η(b), is effectively derived by a comparing the measurements to the model, yielding an increased trend from 1.0 to 2.5×10(-5) kg·m(-1)·s(-1) for the temperature interval. The calculated (Tenti S6) line shapes are consistent with experimental data at the level of 2%, meeting the requirements for the future RB-scattering LIDAR missions in the Earth's atmosphere. However, the systematic 2% deviation may imply that the model has a limit to describe the finest details of RB scattering in air. Finally, it is demonstrated that the RB scattering data in combination with the Tenti S6 model can be used to retrieve the actual gas temperatures.

A systematic study of Rayleigh-Brillouin scattering in air, N2, and O2 gases

“…Since it follows from previous studies, that the bulk viscosity exhibits a linearly increasing trend with the temperature 30,31 , as it is expected from theoretical considerations 42 , values of the bulk viscosity are derived for the different temperature settings for the experiments, for the three different gases. The thus obtained values for the derived bulk viscosities for air, N 2 and O 2 as a function of temperature (for further discussion see below) are employed to interpolate the values for the other settings in p − T parameter space as listed in Table II.…”

“…This background was firstly ascribed to broadband fluorescence of the cell windows in 29 , and was later understood as an influence of Raman scattering 31 . In order to investigate the influence of Raman scattering, most of which maintains a large frequency shift from the incident light, a high transmission (90%) narrowband (∆λ = 1 nm) bandwidth filter with its central wavelength at 403 nm is implemented in front the PMT in this study to block most of the Raman scattering.…”

“…Hence, this model can be directly applied to LIDAR applications for exploring the properties of the atmosphere, such as wind speed profile retrieval or local temperature measurements. Scattering profiles of air at atmospheric conditions in the U.S. Stan- 30,31 . Only for the cases of RB-scattering in air the uncertainties are explicitly specified.…”

“…Experimental measurements on RB scattering profiles of molecular gases, particularly of air as a gas mixture, are required to the highest possible signal-to-noise ratio to test the accuracy of the S6 model, which is proposed to be used in retrieval algorithms of LIDAR applications. Previous studies have proven that the Tenti S6 model is accurate to the 2% level for a number of molecules, temperatures and pressures, at a wavelength of 366 nm, and for values of the bulk viscosity η b to be used in the S6 model as derived from RB-scattering experiments [29][30][31] .…”

A Micropulse eye-safe all-fiber molecular backscatter coherent temperature lidar

“…1 shows the transition from the Kundsen (where the backscatter signal is largely dominated by the Rayeligh backscatter) to hydrodynamic region for different wavelengths and altitudes. As a result, due to the presence of a structured SRB backscatter Doppler spectrum in the hydrodynamic region [10] as well as the inherent CTL detection sensitivity, accurate estimation of temperature from a weak molecular backscatter is possible; The presence of narrowband Brillouin peaks provide a reliable measure of the atmospheric temperature [7]. As we will see in Sec.…”