VALIDATION OF GROUND-BASED VISIBLE MEASUREMENTS OF TOTAL OZONE BY COMPARISON WITH DOBSON AND BREWER SPECTROPHOTOMETERS

Comparisons of total column ozone measurements from Dobson, Brewer and SAOZ instruments are presented for the period 1990 to 1995 at seven s tations covering the mid-and the high northern latitudes, as well as t he Antarctic region. The main purpose of these comparisons is to asses s, by reference to the well established Dobson network, the accuracy o f the zenith-sky visible spectroscopy for the measurement of total ozo ne. The strengths and present limitations of this latter technique are investigated. As a general result, the different instruments are foun d to agree within a few percent at all stations, the best agreement be ing obtained at mid-latitudes. On average, for the mid-latitudes, SAOZ O-3 measurements are approximately 2% higher than Dobson ones, with a scatter of about 5%. At higher latitudes, both scatter and systematic deviation tend to increase. In all cases, the relative differences be tween SAOZ and Dobson or Brewer column ozone are characterised by a si gnificant seasonal signal, the amplitude of which increases from about 2.5% at mid-latitude to a maximum of 7.5% at Faraday, Antarctica. Alt hough it introduces a significant contribution to the seasonality at h igh latitude, the temperature sensitivity of the O-3 absorption coeffi cients of the Dobson and Brewer instruments is shown to be too small t o account for the observed SAOZ/Dobson differences. Except for Faraday , these differences can however be largely reduced if SAOZ AMFs are ca lculated with realistic climatological profiles of ozone, pressure and temperature. Other sources of uncertainties that might affect the com parison are investigated. Evidence is found that the differences in th e air masses sampled by the SAOZ and the other instruments contribute significantly to the scatter, and the impact of the tropospheric cloud s on SAOZ measurements is displayed.