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 las ers 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 sola r zenith angles between 85 degrees and 91 degrees. Regression identified wh ich pairs of instruments were most consistent, and so which could be used a s universal comparison instruments. For O-3, regression slopes for the whol e campaign agreed within 5% for most instruments despite the use of differe nt cross-sections and wavelength intervals, whereas similar agreement was o nly achieved for NO2 when the same cross-sections and wavelength intervals were used and only one half-day's data was analysed. Mean fractional differ ences in NO2 from a comparison instrument fall within +/-7% (I-sigma) for m ost instruments, with standard deviations of the mean differences averaging 4.5%. Mean differences in O-3 fall within +/-2.5% (I-sigma) for most instr uments, with standard deviations of the mean differences averaging 2%. Meas urements of NO2 in the cell had similar agreement to measurements of NO2 in the atmosphere, but for some instruments measurements with cell and atmosp here relative to a comparison instrument disagreed by more than the error b ars.