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the channel. The UV radiation was directed to channel 1 (214
314 nm) and channel
-
2 (309
404 nm). The visible light passed to channel 3 (392
605 nm) and channel 4
-
-
(598
information caused
by the absorption of atmospheric species which is used to retrieve their concen-
trations. SCIAMACHY operated in three observation modes: nadir, limb and solar/
lunar occultation. In limb viewing geometry, the line of sight was pointed tan-
gentially to the Earth
790 nm). These measured radiances contain
'
'fingerprint'
'
-
field of view of 110 km horizontally and
2.6 km vertically at the tangent point. Limb measurement sequence started from
3 km below the horizon (0 km after October 2010) and continued with a vertical
scan up to around 93 km. At each tangent height, SCIAMACHY performed a
horizontal scan with a total swath of about 960 km. The tangent height (TH) step
between subsequent horizontal scans is around 3.3 km. During the vertical scan,
SCIAMACHY keeps the same signal integration time for all THs while the scat-
tered limb radiance decreases exponentially. This results in a strong decrease of S/N
ratio in the measured spectra with increasing TH.
To evaluate ozone pro
'
s surface, with a
les retrieved from SCIAMACHY limb measurements,
ozone sonde data from stations which belong to the World Ozone and Ultraviolet
Radiation Data Centre (WOUDC) are used. This study is focused on the results
obtained at high latitudes of the Northern Hemisphere. Only stations which provide
more than 30 measurements from January 2003 to December 2012 are selected for
comparisons. These are Alert, Eureka, Ny-Alesund, Sodankyla, Lerwick and
Resolute. The locations of the stations are shown in Fig.
1
. Coincident SCIAM-
ACHY limb pro
le. The geographic
distance between the ozone sonde station and the footprint centre of the co-located
SCIAMACHY measurement is required to be within 5
les are selected for each ozone sonde pro
in
longitude. A maximum time difference of 24 h is allowed. Since a larger attenuation
in a longer optical pathway results in a lower signal, the coincident limb pro
°
in latitude and 10
°
les
with a solar zenith angle larger than 80
are rejected. In general, an altitude range
between 15 and 30 km is selected for the comparisons. This choice is justi
°
ed by
larger uncertainties of the current limb retrieval below 15 km and increasing
uncertainty in the ozone sonde data above 30 km. The latter is mostly caused by the
decaying pump ef
ciency at lower pressures (Johnson et al.
2002
). For each par-
ticular pro
first cloud-free level of
SCIAMACHY limb data or from 15 km whichever is higher and ended at either the
ozone sonde burst height or at 30 km whichever is lower.
For the partial column comparison, both SCIAMACHY limb ozone data and raw
ozone sonde measurements are integrated to calculate the stratospheric partial
ozone column (SPO) C
strato
:
le pair, the integration was done either from the
z
i
X
i¼TH
max
N(z
i
1
Þþ
N(z
i
Þ
C
strato
¼
ð
z
i
1
Þ
;
2
i
tph
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