Impact on the accuracy of aerosol and cloud properties derived from the oxygen bands by ignoring rotational Raman scattering

Kurzfassung

In a special issue of the Indian Journal of Physics of 31 March 1928, and in a letter to Nature, C. V. Raman presented results on an anomalous radiation from light scattered by transparent liquids at wavelengths shifted from the incident one. The same effect in crystal lattices was reported in the Soviet Union by Landsberg and Mandelstam. Inelastic Raman scattering is now commonly used to describe both observed effects.Raman scattering can be thought of as an inelastic two-photon process. Lower or higher energy transitions are observed together with elastically scattered Rayleigh radiation at the excitation wavelength. Depending on the energy states involved, electronic, vibrational and rotational Raman transitions can be distinguished. Each is characterised by intensity, phase function and cross-section. The cross-section of the rotational component is 2-to-3 orders of magnitude larger than that of the others and it is also observed on air molecules. This affects only molecules with asymmetric polarisability, as reported by Shefov in 1959 and by Grainger and Ring. They provide the trans-spectral photons that fill telluric absorption bands as solar Fraunhofer lines of stellar bodies and gaseous bands (oxygen B- and A-band), thereby reducing their absorption strength. Since RRS is proportional to optical path lengths and sensor spectral resolutions, it is expected that as the capabilities of space-based spectrometers improve, RRS will play an increasingly important role in those applications using gaseous absorption signature to diagnose properties of tropospheric components. In this study, we aim to characterise the errors introduced in the retrieval of optical and physical properties of scatterers when RRS is not included in the forward problem. We do this by generating measurements of the Sentinel-4 mission. It aims to provide hourly measurements over Europe of trace gases, aerosols and clouds. The latter are a key input for the former, which are retrieved in the UV-Vis at 0.5 nm resolution. In this case the RRS signature is flattened and it can be neglected. However, the retrieval of aerosol and clouds' properties use the oxygen B- and A-band at 0.12 nm, for which the filling is no longer negligible. As the RRS is an additive component, a practical application is its inclusion as a spectral correction in cases where the retrieved parameters' accuracy is not within the nominal mission requirements.