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.