Experimental determination of Rayleigh scattering cross-sections at 408 nm

Cavity ring-down spectroscopy (CRDS) has been applied to retrieve accurate Rayleigh scattering cross-sections, σRayl for a set of atmospheric gases at ~ 408 nm. Step pressure changes and pressure ramps were used in three different but similar optical cavities to measure light extinction by N₂, O₂, Ar, CO, CO₂, N₂O, CH₄ and synthetic air (SA). The experimentally determined σRayl were compared with σRayl values calculated for these atmospheric gases using knowledge of extrapolated literature values of the refractive index (n) and King correction factors measured in different spectral regions.

The difference between measured and calculated σRayl using the most recently published dispersion relationship for N₂O and CH₄ is ~ 0.6%. This confirms the improved accuracy of the new dispersion relationship. The calculated and measured σRayl agreed on average within 0.6%, 2.4%, 1.2%, 2.2%, and 1.5% for the gases CO₂, N₂ O₂, SA, and Ar at ~ 408 nm. However, for CO, the difference is 4.1%, i.e. more than twice the error. This implies that improved knowledge of the dispersion relationship for CO, and thus the refractive index and King correction factors, would be of value to calculate σRayl in the blue spectral region.