Intercomparison of total ozone data measured with Dobson and Brewer ozone spectrophotometers at Uccle (Belgium) from January 1984 to March 1991, including zenith sky observations

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Abstract
More than 7 years of quasi‐simultaneous observations of the total ozone amount measured with Dobson spectrophotometer 40 and Brewer spectrophotometer 16, both located in Uccle (50°48′N, 4°21′E), are compared. Direct Sun observations with both instruments show a significant drift relative to each other of 0.1% per year. On the other hand, the Brewer instrument measurements indicate that the SO2 amount at Uccle has had a strong downward trend of 8.9% per year over the same time period. This is confirmed by in situ measurements of SO2 density near the ground. If the SO2 trend is taken into account in the comparison between Brewer and Dobson instrument total ozone values, the two total ozone data sets show no drift with respect to each other. The question of SO2 tendency therefore deserves investigation in any trend analysis of Dobson instrument total ozone data. Quasi‐simultaneous zenith sky observations made with both instruments are also intercompared. Results indicate that the coherence between direct Sun and zenith sky data of the Brewer instrument is slightly better than that of the Dobson instrument.