Figure S2. Time series comparing the normalized mean ozone bias [(CAMS-obs)/obs in %] of the CAMS reanalysis with observations from MIPAS (blue), ACE-FTS v3.6 (green) and MLS v4.2 (red) for the period 2003 to 2019 in the (a) middle stratosphere (10-30 hPa averages) and (b) lower stratosphere (30-70 hPa averages) for the Arctic (latitude band 90-60⁰N).
Figure S3 shows relative stratospheric ozone biases from the CAMS reanalysis against ozone sondes averaged over the Arctic and at Ny-Ålesund. The ozone sonde data used for the validation were acquired from a variety of data centres: World Ozone and Ultraviolet Radiation Data Centre (WOUDC), Southern Hemisphere ADditional OZonesondes (SHADOZ), NDACC, and campaigns for the Determination of Stratospheric Polar Ozone Losses (MATCH). The precision of electrochemical concentration cell (ECC) ozone sondes is on the order of ±5% in the range between 200 and 10 hPa, between −14% and +6% above 10 hPa, and between −7% and +17% below 200 hPa (Komhyr et al., 1995). Larger errors are found in the presence of steep gradients and where the ozone amount is low. The same order of precision was found by Steinbrecht et al. (1998) for Brewer–Mast sondes.
Over much of the stratosphere (10-100 hPa) the biases of the CAMS reanalysis in the Arctic are small and mainly negative (below 5-10%). Larger relative biases are found above 10 hPa and below 100 hPa. An exception is the year 2004 when no profile data were assimilated in the CAMS reanalysis resulting in a reduced quality of the ozone analysis and larger biases with respect to the independent observations (Inness et al., 2019). The timeseries averaged over the Arctic stations shows increased positive biases after 2015. Such an increase in bias was not seen against the FTIR and satellite data in Figures S1 and S2 and is likely to be the result of changes in the ozone sondes. Stauffer et al. (2020) documented a drop-off in ozone values observed by ozone sondes at several stations after 2014-2016. The reason for this drop-off has to date not been identified. The sondes at Ny-Ålesund seem to suffer less from this problem in the stratosphere than other sondes that go into the Arctic mean and we will therefore use the Ny-Ålesund sondes further for validation of CAMS reanalysis profiles in Section 3 of the main paper.