Retrieval of total ozone abundances from the ultraviolet region of spectra recorded with an ultraviolet-visible spectrometer

A method is described for deriving total ozone abundances from zenith-sky intensities measured by an UV-visible spectrometer, known as the System of Analysis of Observations at Zenith (SAOZ). Total ozone abundances are determined by comparison of intensity ratios measured at two wavelengths in the UV region with ratios computed with a radiative transfer model. The wavelength pair 320-307 nm was used in this study. Spectra recorded by the SAOZ spectrometer in Oslo (60 degrees N) from the beginning of May 1995 to the end of August 1995 were analyzed, and the results were compared with total ozone measured with a Brewer instrument. The relative difference in derived ozone abundances for the whole period, including days with thin and medium-thick cloud covers, is -0.18 +/- 1.46%. We study the effects of clouds and varying ground albedo on the derived total ozone. Clouds result in an overestimation of the derived total ozone. The error increases with the optical depth tau of the cloud from approximately 2% for an optically thin cloud (tau = 0.5) to approximately 10% for a thick cloud (tau = 50). The ratio between measured intensities at 550 and 350 nm, the so-called color index, can be used as a measure of the cloud optical depth for thin and medium-thick clouds. The effects of thin and medium-thick clouds on the derived ozone abundances can be compensated for by use of an empirical relationship found between the measured color index and the error in the inferred ozone abundances caused by cloud scattering. We also study the influence of changes in the ground albedo and in the ozone profiles on derived total ozone values. (C) 1997 Optical Society of America.