Polarization and ring effect influences upon stratospheric DOAS measurements

The problems of the stratospheric ozone decline during the last decades is the subject of scientific efforts to discover the main factors influencing the processes leading to its disappearance. The exact understanding of chemical-physical processes in the Earth's atmosphere needs high-quality representative measurements for time-series and modelling study. The present paper deals with the problems of the polarization characteristics of DOAS instruments and the corresponding requirements for instrumental orientation relative to the scattered plane. To avoid the polarization effects upon the measurement's accuracy the instrument has to be ''unpolarized''. A fiber optic and an appropriate diffraction grating are used to do this. Experimental measurements have been carried out to check the possibility to use a fiber optic bundle for DOAS applications. The results obtained from this experiment show that the tested fiber bundles do not completely destroy the polarization state of the incoming radiation. At the fiber exit the degree of the polarization reaches up to 10-15% in the measured spectral intervals. On the other hand, accordingly to some of the existing methods, to remove Ring effect from the measured zenith spectra, the DOAS instrument used has to be strongly ''polarized''. This achieved by means of a polarizer inserted into the instrument. The assumption is that, applying such kind of measurements makes it possible to separate the Raylegh from the Raman scattering. The latter e is assumed as unpolarized, which is in conflict with the theory. A new method based on the deconvolution is offered by us as a possible way of overcoming the problems with the Ring effect.