Near-Infrared Lunar Absorption Spectroscopy for the Retrieval of Column Averaged CO2 and CH4 - Towards an Interdisciplinary Approach in Earth System Science - page 78

The rather strict network policy assures inter-comparability between the different
sites and provides a valuable dataset for satellite validation and model comparisons
(Wunch et al. 2011 ).
At our measurement site in Ny- lesund, Svalbard, Norway (78 55 0 23 00 N,

11 55 0 22 00 E), during Polar Night the sun is permanently below the horizon from
October to March and solar absorption measurements are not possible. Conse-
quently there is no information from solar absorption FTIR measurements of the
polar atmosphere during winter. However, during full moon, the moon can be used
as a substitute infrared light source above the atmosphere, as shown by Notholt
( 1994 ) in the middle infrared region. However, the much lower intensity of the
moon light in the near-infrared results in a lower signal, which we mitigate by using
a new detector with increased sensitivity. In Fu et al. ( 2014 ) it was shown that a
similar detector setup can be used to measure spectra of sunlight re

Å

fl ected by the

ground from a mountaintop site.
In this article we will first present the proof of concept for using a thermo-

electrically cooled InGaAs detector to measure near-infrared (NIR) lunar spectra
and retrieve column averaged mole fractions of trace gases during a full moon
period at our site in Bremen (53 6 0 13 00 N, 8 50 0 58 00 E). Example spectra taken
with the new instrument setup are shown and we conclude with the presentation of

first retrievals of total column Carbon Dioxide and Methane (denoted xCO 2 and
xCH 4 ).

2 Ground-Based Column Averaged Trace Gas Retrieval

There are two fundamentally different methods to assess the atmospheric compo-
sition and its change over time: In situ and remote sensing techniques. In situ
measurements are able to observe the desired quantity with high precision and
accuracy and high temporal resolution. However, they can only obtain localised
measurements, which might be in

uenced by small scale processes and distur-
bances. This can be circumvented by installation of the in situ instrument on a
moving platform (e.g. a balloon or an aircraft). Unfortunately this can mostly only
be done on a campaign basis. The alternative is to use remote sensing. In case of
ground-based remote sensing of trace gases in the atmosphere, we are able to
retrieve information about the whole atmospheric column. Limitations are the
inherent dependency on clear sky conditions and the

fl

fixed location of the instru-
ment. Figure 1 shows a visualisation of the different measurement approaches and
highlights the viewing geometry of the ground-based FTIR setup.
Several trace gases absorb sunlight in the near infrared spectral region and the
incident solar radiation is therefore attenuated by the presence of the gases

'

mol-
ecules. The absorption lines are unique in their spectral position and can be non-
ambiguously associated with the target gas. The individual line shapes and inten-
sities are then used to retrieve the amount of gas along the lightpath.