Instrumental effects on the total ozone observations
The Dobson- and the Brewer spectrophotometers are sensitive for SO2. The Dobson spectrophotometer measures a cumulated O3 and SO2 total column. For that reason, the Dobson spectrophotometer overestimates the total ozone column. The Brewer is able to measure the O3 and the SO2 total columns separately, so we can take the error in the total ozone column of a Dobson spectrophotometer into account. With this knowledge, a homogenisation has been performed on the Dobson time series. Therefore, ground measurements of SO2 concentrations have been used to reconstruct the Dobson time series. In the next figure, the impact of SO2 on the measured total ozone column (dashed line) is shown. It should be noted that the contribution of SO2 after 2000 is virtually zero. The linear trend in total ozone for the time period 1971-2001 is negative: -4.945 DU/decade, ± 0.403 DU/decade .
Effect of the correction for SO2 on the Dobson time series (in cyan: uncorrected; in dark blue: corrected) in comparison with Brewer time series (in red).