Improvements to the accuracy of zenith-sky measurements of total ozone by visible spectrometers

Visible spectra of the zenith-sky were recorded at Faraday in Antartica (65°S) during 1990. Line-of-sight ozone amounts were deduced by fitting laboratory cross-sections to the spectra. At the lattitude of Faraday, visible spectra are more useful than the u.v. wavelengths used by the Dobson spectrophotometer, because they permit observations of ozone throughout the winter when the sun is too low in elevation for useful measurements with the Dobson. In order to find the accuracy of the visible-light system for the determination of medium- and long-term trends in ozone, Langley plots of the ozone amounts were examined to determine the stability of the offset to the zero of the ozone measurement. The plots revealed changes which correlated with shifts in wavelength of the spectrometer. This allowed the use of wavelength shift as a surrogate for offset, fortunate because the use of Langley plots to determine the offset on an individual day from zenith-sky spectra at high latitudes is unreliable. The standard deviation of the ratios of total ozone measured on the same day at Faraday by the Dobson spectrophotometer and the visible spectrometer was 10% using offsets calculated from wavelength shifts. This is 4% less than the standard deviation using a straight-line fit to the offsets. Although common for Dobson measurements of ozone in the u.v., such Langley-plot analyses are, so far, unusual for visible spectrometers, but are essential for future trend determination, even by systems that are inherently more stable.

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