Slant column measurements of O-3 and NO2 during the NDSC intercomparison of zenith-sky UV-visible spectrometers in June 1996

In June 1996, 16 UV-visible sensors from 11 institutes measured spectra of the zenith shy for more than 10 days. Spectra were analysed in real-time to determine slant column amounts of O-3 and NO2, Spectra of Hg lamps and lasers were measured, and the amount of NO;? in a cell was determined by each spectrometer. Some spectra were re-analysed after obvious errors were found. Slant columns were compared in two ways: by examining regression analyses against comparison instruments over the whole range of solar zenith angles; and by taking fractional differences from a comparison instrument at solar zenith angles between 85 degrees and 91 degrees. Regression identified which pairs of instruments were most consistent, and so which could be used as universal comparison instruments. For O-3, regression slopes for the whole campaign agreed within 5% for most instruments despite the use of different cross-sections and wavelength intervals, whereas similar agreement was only achieved for NO2 when the same cross-sections and wavelength intervals were used and only one half-day's data was analysed. Mean fractional differences in NO2 from a comparison instrument fall within +/-7% (I-sigma) for most instruments, with standard deviations of the mean differences averaging 4.5%. Mean differences in O-3 fall within +/-2.5% (I-sigma) for most instruments, with standard deviations of the mean differences averaging 2%. Measurements of NO2 in the cell had similar agreement to measurements of NO2 in the atmosphere, but for some instruments measurements with cell and atmosphere relative to a comparison instrument disagreed by more than the error bars.