Comparison of ground-based Brewer and FTIR total column O3 monitoring techniques [Discussion paper]

We compare the currently most precise, ground-based total O3 measurement techniques: Brewer and FTIR. We give an overview of the similarities and the differences between the measurements and the retrieval approaches of both experiments. We compare coincident measurements performed at the Atmospheric Observatory of Izaña from 2005 to 2007 and demonstrate that, if the properties of the instruments are well characterised, the scatter between both experiments is as small as 0.5%. This is in agreement with the theoretical predictions and confirms empirically that both techniques are able to monitor total O3 amounts with a precision of better than 0.4%. However, we found systematic differences between both techniques of around 4.5%, which we think are mainly due to discrepancies between the applied UV and infrared spectroscopic parameters