Geoscience Reference
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blackbodies. Although the interferometer acquires an uncalibrated spectrum every
2 s, averaging reduces the radiometric noise in the measurements. The instrument
contains two calibration blackbodies, one at 60
◦
C and one at ambient temperature,
which allow for a self-calibration of the instrument (Feltz et al.
2003
). The high
spectral resolution of an interferometer allows for significantly improved accuracy
and vertical resolution of retrieved temperature and humidity profiles. AERI can
measure up to a height of about 3 km above ground. The vertical resolution is
100-200 m.
3.7 FTIR
The Fast Fourier Infrared (FTIR) absorption spectroscopy (Bacsik et al.
2004
)in
its active version is a bistatic method operating a heat source (broad-band infrared
radiation source, called glo(w)bar, see Fig.
3.14
) and a spectrometer based on a
Michelson interferometer. Typical path lengths are several hundreds of metres. In
a passive version the sun or another well-known heat source can be used. Passive
FTIR emission spectroscopy analyses the emission of gases in front of a cooler
background. The infrared detector of a FTIR needs cooling down to 77 K with
liquid nitrogen or by the operation of a Joule-Thompson cooling system.
The interferometer in the FTIR spectrometer changes the incoming frequency-
dependent radiation intensity
I
(
ν
) into its Fourier transform
Î(x)
. The following
relations apply:
)
I
(
x
)
+∞
=
1
/
(2
π
I
(
ν
) cos (2
πν
x
)d
ν
(3.25)
−∞
and
+∞
I
(
x
) cos (2
I
(
ν
)
=
πν
x
)d
x
.
(3.26)
−∞
The great advantage of an interferometer compared to a spectrometer using a
split or grate is the high light intensity, which reaches the detector. This lowers the
signal-to-noise ratio by a factor of about 200. This advantage is also known as the
Jacquinot advantage (Griffiths
1983
). In order to convert the interferograms back
into spectra (3.26) must be solved. An efficient and fast method to solve (3.26) was
developed by Cooley and Tuckey (
1965
). The modern computer technology today
permits a rapid online calculation of spectra from interferograms.
Further advantages of the FTIR spectroscopy are that all frequencies are observed
simultaneously (at a grate spectrometer they are observed subsequently), which
speeds up the observation up to a factor of 500 (multiplex or Fellget advantage
(Fellgett
1951
; Compton et al.
1990
)). FTIR spectroscopy offers a high spectral res-
olution (
f
100 000, Hase and Fischer (
2005
). These advantages allow for a
simultaneous detection of many trace gases along the measurement path.
/
f
∼
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