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Fig. 2 A solar (green) and a lunar (blue) absorption spectrum, both taken on July 21, 2013 with
the FTS in Bremen. Note the better signal-to-noise ratio of the solar spectrum, visible in the
blacked-out regions, e.g. at 5,400 cm
−
1
. The inset shows some of the CO
2
absorption lines of the
1.6
μ
m band
Part of the CO
2
band can be seen in the inset in Fig.
2
. Both spectra have a high
spectral resolution of 0.014 cm
−
1
in the standard and 0.08 cm
−
1
in the lunar case,
which allows the analysis of individual spectral lines. The lower resolution enables
a larger number of spectra to be taken during an equal amount of time, to improve
the signal-to-noise ratio. In case of the lunar spectra, 32 spectra were added,
compared to 2 in case of the solar measurements. This results in an overall inte-
gration time per spectrum of 6 min for lunar and 4 min for solar spectra.
4 xCO
2
and xCH
4
from Lunar Absorption Spectroscopy
We now apply the standard TCCON retrieval approach (as described in Sect.
2
)to
the (solar and lunar) spectra taken in Bremen in July 2013, i.e. we calculate for each
spectrum the column averaged dry air mole fraction of the two target gases xCO
2
and xCH
4
.
TheresultscanbeseeninFig.
3
, were values retrieved from solar spectra (green)
cover most of the days and those from lunar spectra cover most of the nights. Shown
are in both cases 30-minute-averages, i.e. arithmetic mean of all measurements within
30 min. The error bars are given by the standard deviation of that half-hourly mean.
Note that the error bars for the solar data are mostly too small to be seen on this scale.
As a
first measure of the precision of the lunar measurements, the mean standard
deviation of the 30 min averages is smaller than 4 ppm for xCO
2
and smaller than
18 ppb in case of xCH
4
. This can be compared to smaller than 0.3 ppm (xCO
2
)and
smaller than 1.4 ppb (XCH
4
) for the standard TCCON solar measurements.
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