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where I
0
ðkÞ
represents the initial intensity (extra-terrestrial solar spectrum), I
ðkÞ
the
intensity after passing through the atmosphere,
q
i
;
Ray
;
Mie
;
Ring
ð
s
Þ
are the number density of the absorbers and the scattering molecules and particles,
s the light path, and
r
i
;
Ray
;
Mie
;
Ring
ðkÞ
are the absorption and scattering cross sections,
which are speci
k
the wavelength,
c to each species. The so-called Ring effect is related to rotational
Raman scattering on air molecules, this produce wavelength shifts in the ultraviolet
and visible comparable to the width of Fraunhofer lines, which result in an apparent
filling-in of the Fraunhofer lines. In Eq.
1
and not taking into account any
dependence of the cross sections on distance (ds), we can exchange the integral
with the sum and de
ne the slant column (SC) as, Si
i
¼
R
q
i
ð
s
Þ
ds. The cross section
can be decomposed now into a component that varies
“
”
slowly
with wavelength,
0
r
i
, that shows a
r
i
, and another component,
“
fast
”
variation with wavelength
. The rapidly varying part
r
i
r
i
¼ r
i
þ r
0
i
is representative of narrow bands in the
0
absorption of trace gases, while
i
includes low frequency variations of the spec-
trum. The latter together with the Rayleigh scattering
r
, and Mie scat-
r
Ray
k
4
2
can be substituted by a low-order polynomial. With
these considerations we obtain, I
ðkÞ¼
I
0
ðkÞ
r
Mie
k
k
tering
0
k
;
h
i
.IfI
ðkÞ
P
i
r
i
ðkÞ
S
i
P
p
a
p
k
p
exp
,
I
0
ðkÞ
r
i
are known, the slant columns density, Si,
i
,
can be determined by a linear least-squares method. As slant column depends on
observation geometry, the vertical column (VC) is computed which is related with
the SC by the air mass factor (AMF). The AMF depends on different parameters
such as the priory trace gas pro
, and the absorption cross sections
le, temperature, air pressure, surface albedo, aer-
osol and ozone pro
les, and clouds, as well as solar zenith angle.
In this work, CHOCHO SCs have been retrieved from OMI measurements and
converted to VCs using AMFs computed by the radiative transfer model SCIA-
TRAN (Rozanov et al.
2005
) and assuming typical glyoxal pro
les as used in
(Wittrock
2006
). Brie
y, OMI is a nadir viewing spectrometer providing the
spectral coverage and resolution needed for DOAS retrievals of atmospherics trace
gases. The instrument measures the light scattered by the atmosphere and surface in
the ultraviolet and visible range. The spatial resolution of OMI is 13 km
fl
24 km at
nadir. The Aura satellite has an equator crossing time of 13:45 LT (ascending
node), and global coverage by OMI is achieved in 1 day (Levelt et al.
2006
).
CHOCHO is retrieved in the spectral window between 433 and 458 nm and a
polynomial of third order for removal of broad band signatures is used (Alvarado
et al.
2014
). Interfering species such as O
3
(Bogumil et al.
2003
), NO
2
(220 and
294 K) (Vandaele et al.
1998
), O
4
(Thalman and Volkamer
2013
), water vapour
(Rothman et al.
2005
), and liquid water (Pope and Fry
1997
) also absorb in the
CHOCHO
×
t. Moreover, a syn-
thetic ring spectrum (Vountas et al.
1998
) accounting for the effect of rotational
Raman scattering in the atmosphere is included in the retrieval.
fitting window and are therefore included in the
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