1. . COMPARISON OF BREWER AND DOBSON TOTAL OZONE Brewer and Dobson spectrophotometers are widely used for Total Ozone monitoring. In Arosa (Switzerland, 46.8N/9.68E 1820 m a.s.l.) there are two semi-automatic Dobson systems and three automatic Brewer systems, which measure total ozone continuously and quasi - simultaneously. The co-location of these two instruments allows a direct comparison for QUALITY CONTROL and for STUDYING THE DIFFERENCES BETWEEN THE TWO TYPES OF SPECTROPHOTOMETERS. Data (1995-2004) from the two types of sun spectrophotometers (Dobson 62, 101 and Brewer 40, 72, 156) are compared two by two and analysed using the following statistical model: O 3 Br - O 3 D = C +  + X T +  O 3 Br ; O 3 D : Brewer and Dobson Total Ozone C: allows a zero mean difference μ: linear term proportional to the air mass [Thomason et al., 1983] X T : influence of temperature  error Two distinct approaches for differences in temperature sensitivities of ozone wavelengths are considered: Approach 1 T eff : ozone effective temperature calculated from ozone and temperature sounding data from Payerne (200 km from Arosa). Approach 2 T p : simplified approach only using temperature at two altitudes (at 31 and 51 hPa given by the soundings of Payerne Aerological Station). Reduction of seasonal differences by model (Fig. 3): Annual Average : The contribution to the difference from the effective temperature is significantly larger than that from μ (Fig. 4, center plot): Seasonal Variability: The relative contribution to the difference from μ is larger in winter than in summer Dependence on the magnitude of TOZ: – TOZ<290 DU: agreement between the 2 instruments is better, with improved agreement in summer than winter. –TOZ>290 DU: the influence of atmospheric parameters is reduced in winter. Approach 2 Less precise. Further analysis: Attempt to generalize transfer function using other instruments at Arosa. METHOD Fig 2. Model of the molecular ozone cross-section from the Global Ozone Monitoring Experiment (satellite UV- visible spectrophotometer) database. Quasi-simultaneous direct sun measurements from the two types of spectrophotometers yield small, but characteristic seasonal variation in the differences of Total Ozone (Fig. 1). Causes of differences might include: Different wavelengths with different T sensitivities of ozone absorption (Fig. 2) Different retrieval algorithms Different fields of view Differing influence of scattered light Fig 4. Residual distribution of Total Ozone when applying different explanatory variables. Contact: barbara.scarnato@env.ethz.ch Fig 3. Time series of the residuals for B40 and D101(AD) after the model has been applied to the total ozone difference. MEASUREMENTS RESULTS Approach 1 Barbara Scarnato 1), Johannes Staehelin 1), René Stübi 2), Herbert Schill 2) 1) IAC/ETH, Zürich; Switzerland 2) MeteoSwiss, Aerological Station; Payerne Switzerland Dobson Brewer INTRODUCTION Difference of readings [DU] Density Fig 1. Time series for B40 and D101 (AD pair) and their difference. The coincidence criterion is 10 min.