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Fig. 3 The retrieved column averaged dry-air mole fractions of CO
2
(left) and CH
4
(right) for 3
consecutive days in July 2013. Note the difference in scatter for the lunar absorption measurements
(blue) compared to the solar ones (green) due to the increased noise in the spectrum (see Fig.
2
)
Especially in the lunar case clear sky conditions are very important and times
with too dense cloud cover have been removed. Additionally the measurements of
lunar spectra are removed at times, when the solar zenith angle is smaller than 95
°
because the sky is too bright to ensure accurate pointing of the solar/lunar tracking
system.
The origin of the apparent draw down in both xCO
2
and xCH
4
on the early
morning of July 22 still has to be investigated but is considered an artifact and not
real at this point.
5 Conclusions
We presented the
first high resolution near-infrared lunar absorption spectra for the
retrieval of xCO
2
and xCH
4
from Bremen. Despite the decreased resolution of
0.08 cm
−
1
with an integration time of about 6 min, we observe an increase in scatter
by a factor of 10 compared to the solar absorption spectra. This in turn leads to a
lower precision (higher standard deviation of the mean) of the lunar measurements
in the order of one magnitude compared to the solar data. The daily cycle in mid-
latitudes is estimated in Keppel-Aleks et al. (
2011
) as peak-to-trough diurnal
amplitude of CO
2
of about 0.5
0.75 ppm. The seasonal cycle is more pronounced
in the mid and high-latitudes. Warneke et al. (
2005
) found a seasonal cycle
amplitude of CO
2
of 11 ppm for the Ny-
-
lesund site. In this study, the scatter of the
lunar retrievals (4 ppm) prevents a column averaged daily cycle from easily being
seen. Further studies and longer time series are needed to investigate if a daily cycle
can be derived from the lunar measurements.
Å
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