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 Antarcti ca (65-degrees-S) during 1990. Line-of-sight ozone amounts were deduce d by fitting laboratory cross-sections to the spectra. At the latitude of Faraday, visible spectra are more useful than the u.v. wavelengths used by the Dobson spectrophotometer, because they permit observation s 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 examine d to determine the stability of the offset to the zero of the ozone me asurement. The plots revealed changes which correlated with shifts in wavelength of the spectrometer. This allowed the use of wavelength shi ft as a surrogate for offset, fortunate because the use of Langley plo ts to determine the offset on an individual day from zenith-sky spectr a at high latitudes is unreliable. The standard deviation of the ratio s of total ozone measured on the same day at Faraday by the Dobson spe ctrophotometer and the visible spectrometer was 10% using offsets calc ulated from wavelength shifts. This is 4% less than the standard devia tion using a straight-line fit to the offsets. Although common for Dob son measurements of ozone in the u.v., such Langley-plot analyses are, so far, unusual for visible spectrometers, but are essential for futu re trend determination, even by systems that are inherently more stabl e.