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Abstract:
Retrieving the column of an absorbing trace gas
from spectral data requires that all absorbers in the corre-
sponding wavelength range are sufficiently well known. This
is especially important for the retrieval of weak absorbers,
whose absorptions are often in the 10
−
4
range.
Previous publications on the absorptions of the oxygen
dimer O
2
–O
2
(or short: O
4
) list absorption peaks at 328 and
419 nm, for which no spectrally resolved literature cross sec-
tions are available. As these absorptions potentially influence
the spectral retrieval of various trace gases, such as HCHO,
BrO, OClO and IO, their shape and magnitude need to be
quantified.
We assume that the shape of the absorption peaks at
328 and 419 nm can be approximated by their respec-
tive neighbouring absorption peaks. Using this approach
we obtain estimates for the wavelength of the absorption
and its magnitude. Using long-path differential optical ab-
sorption spectroscopy (LP-DOAS) observations and multi-
axis DOAS (MAX-DOAS) observations, we estimate the
peak absorption cross sections of O
4
to be
(
1
.
96
±
0
.
20
)
×
10
−
47
cm
5
molec
−
2
and determine the wavelength of its
maximum at 328
.
59
±
0
.
15 nm.
For the absorption at 419
.
13
±
0
.
42 nm a peak O
4
cross-section value is determined to be
(
5
.
0
±
3
.
5
)
×
10
−
48
cm
5
molec
−
2
.
