ACCURACY OF MEASUREMENTS OF TOTAL OZONE BY A SAOZ GROUND-BASED ZENITHSKY VISIBLE SPECTROMETER

During a 2-week intercomparison of ground-based zenith sky visible spe ctrometers in September 1994 at Camborne, United Kingdom (50 degrees N , 5 degrees W), ozone profiles were measured by electrochemical cell ( ECC) sondes during 11 twilight periods. We use these profiles and a ra diative transfer model to calculate separate air mass factors (AMFs) f or each twilight period. We examine ozone data from one of the spectro meters of the Systeme d'Analyse par Observation Zenithale (SAOZ) desig n and we show that these separate AMFs give very straight Langley plot s, except at solar zenith angles exceeding 90 degrees. Total ozone cal culated using these AMFs, by a variety of commonly used procedures, ag rees with the total ozone calculated by vertically integrating the son de profiles, with mean differences of 0 to 7 Dobson units (DU), depend ing on the method, and standard deviations of 9 to 11 DU (1 sigma). To tal ozone calculated using the best procedure (i.e., averaging twiligh t values), which is not sensitive to errors in the gradients of AMFs, gave excellent agreement whether using separate AMFs or fixed climatol ogical AMFs. We analyse the variance of the data set and several sourc es of systematic error in the measurements. We also illustrate from an example during the campaign that such analyses are pointless in the p resence of a strong jet stream, which can give rise to changes in ozon e during the course of the day that are large enough to invalidate the Langley plot.