Experimental determination of Rayleigh scattering cross sections at 408 nm

Accurate knowledge of the Rayleigh scattering cross sections, σ Rayl , of atmospheric gases as a function of wavelength is required for radiate transfer calculations and also for the calibration of optical systems e.g. high finesse optical cavities. The cavity ring-down spectroscopy (CRDS) is a highly accurate absolute method which enables experimentally determined σ Rayl to be compared with σ Rayl n-values calculated for atmospheric gases using knowledge of the refractive index. In this study, CRDS has been applied to measure light extinction and to retrieve highly accurate σ Rayl of nitrogen, N 2 , oxygen, O 2 , argon, Ar, carbon monoxide CO, carbon dioxide, CO 2 , nitrous oxide, N 2 O, methane, CH 4 molecules and synthetic air (SA) at 408 nm. Experiments using step pressure changes and pressure ramps in three optical cavities were used to retrieve σ Rayl . These agree on average within 0.6% with the calculated σ Rayl n-values for CO 2 , N 2 O and CH 4 and within 2.4%, 1.2%, 2.2%, 1.5%, and 4.1% for N 2 , O 2, SA, Ar and CO respectively. For N 2 O and CH 4, these results confirm experimentally the inaccuracies resulting from extrapolating the refractive index and King correction factors from other wavelength regions to the region around 408 nm, reported elsewhere.