AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus PublicationsGöttingen, Germany10.5194/amt-9-2303-2016Addition of a channel for XCO observations to a portable FTIR spectrometer
for greenhouse gas measurementsHaseFrankfrank.hase@kit.eduFreyMatthiashttps://orcid.org/0000-0003-0664-6817KielMatthäushttps://orcid.org/0000-0002-9784-962XBlumenstockThomasHarigRolandKeensAxelOrphalJohannesKarlsruhe Institute of Technology (KIT), Institute for Meteorology and
Climate Research (IMK-ASF), Karlsruhe, GermanyBruker Optik GmbH, Ettlingen, GermanyFrank Hase (frank.hase@kit.edu)25May2016952303231322December201518January201628April201612May2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://amt.copernicus.org/articles/9/2303/2016/amt-9-2303-2016.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/9/2303/2016/amt-9-2303-2016.pdf
The portable FTIR (Fourier transform infrared) spectrometer EM27/SUN,
dedicated to the precise and accurate observation of column-averaged
abundances of methane and carbon dioxide, has been equipped with a second
detector channel, which allows the detection of additional species,
especially carbon monoxide. This allows an improved characterisation of
observed carbon dioxide enhancements and makes the extended spectrometer
especially suitable as a validation tool of ESA's Sentinel 5 Precursor
mission, as it now covers the same spectral region as used by the infrared
channel of the TROPOMI (TROPOspheric Monitoring Instrument) sensor. The
extension presented here does not rely on a dichroic, but instead a fraction
of the solar beam is decoupled near the aperture stop of the spectrometer
using a small plane mirror. This approach allows maintaining the
camera-controlled solar tracker set-up, which is referenced to the field stop
in front of the primary detector. Moreover, the upgrade of existing
instruments can be performed without alterating the optical set-up of the
primary channel and resulting changes of the instrumental characteristics of
the original instrument.
Introduction
The ground-based solar absorption FTIR (Fourier transform infrared)
technique is capable of providing highly reliable measurements of
column-averaged CO2 and CH4 abundances. The TCCON (Total Carbon
Column Observing Network; Wunch et al., 2011) is one of the established
references for the validation of greenhouse gases measuring space sensors
(Reuter et al., 2011; Wunch et al., 2011; Butz et al., 2011; Heymann et al.,
2015; Inoue et al., 2016) and has also been used for e.g. model studies of
greenhouse gases sources (Messerschmidt et al., 2013) and observation of
local sources (Lindenmaier et al., 2014). Recently, several investigators
demonstrated that portable low-resolution FTIR spectrometers still allow
surprisingly precise and accurate measurements of column-averaged greenhouse
gas abundances (Petri et al., 2012; Gisi et al., 2012; Frey et al., 2015).
Such devices are a promising supplement to the TCCON, for performing
measurements at remote sites, for mobile applications (Klappenbach et al.,
2015), and for observations of dedicated sources and sinks on regional and
smaller scales (Hase et al., 2015).
In this work, we describe an instrumental extension of the system introduced
by Gisi et al. (2012), comprised of a portable FTIR spectrometer based on a
pendulum interferometer design and a camera-controlled solar tracker for
ensuring proper alignment of the line of sight on the solar disc centre
(Gisi et al., 2011). This spectrometer is available today as a standard
device from the Bruker Optics company in Ettlingen, Germany, under the model name
EM27/SUN and is used by many working groups. The observing strategy follows
the same approach demonstrated by the TCCON: the near-infrared spectral bands of
the greenhouse gases under study are co-recorded with the 1.27 µm band of
molecular oxygen. Thereby, the column-averaged dry-air mole fractions of a
target species can be derived from the ratio of the target species column
divided by the molecular oxygen column. This process takes advantage of our
accurate knowledge of the dry-air mole fraction of molecular oxygen and
helps to reduce various error sources (Wunch et al., 2011). In addition,
the TCCON uses DC-coupled interferograms for improving the precision of the
measurements and for quality flagging (Keppel-Aleks et al., 2007), which is
also used for the portable spectrometers. The main difference is that the
spectral resolution of the portable spectrometer is much lower: it provides
double-sided interferograms with a maximum optical path difference (OPDmax)
of 1.8 cm, while the TCCON applies 45 cm or higher OPDmax. An instructive
example of application is provided by Hase et al. (2015), where a set of five
EM27/SUN spectrometers has been used for detecting the carbon-dioxide-enriched
plume generated by the major city Berlin. The current EM27/SUN
spectrometer offers a spectral coverage of about 5500–11 000 cm-1,
slightly broader than the 6000–11 000 cm-1 coverage used for the
prototype described by Gisi et al. (2012). This still conservative choice has
been made to avoid any compromise of measurement accuracy of the primary
target species XCO2 and XCH4. Nevertheless, it would be highly
desirable to add the capability of observing XCO, which is a valuable tool
for the characterisation of sources connected to observed XCO2
enhancements (Wunch et al., 2009). Moreover, an extension of the spectral
coverage of the current EM27/SUN including the 2.3 µm region used by
TROPOMI (TROPOspheric Monitoring Instrument) for the observation of carbon monoxide and methane would qualify the
mobile spectrometer as a validation instrument for the Sentinel 5 Precursor
mission. Here, we introduce an enhancement of the EM27/SUN by adding the
capability of measuring XCO. In Sect. 2 we provide a summary of the basic
design considerations, in Sect. 3 we describe the practical implementation
of the extension, in Sect. 4 we present the characteristics of lamp and
atmospheric solar spectra recorded with the dual-channel prototype, and in
Sect. 5 we demonstrate the performance of the novel set-up based on
retrievals of atmospheric observations. Section 6 provides a summary of this
study and an outlook towards planned future activities.
Design considerations for the XCO extension
The most straightforward approach for an XCO extension is obviously the
replacement of the standard indium gallium arsenide (InGaAs) diode covering 5500–11 000 cm-1 by
a detector element offering extended spectral coverage. However, as the FTIR
technique reconstructs the irradiated spectrum by performing a Fourier
transformation of the measured interferogram, it is susceptible to
characteristic interfering influences which degrade the recorded
interferogram. Periodic sampling errors, non-linear detector response, or
radiation which is reflected back into the interferometer and modulated
twice before detection (double passing) can all generate parasitic signals
in the spectral domain. If the spectrum is confined to a sufficiently narrow
region, the spectral perturbations can be detected (and corrected if
necessary) by the characteristic out-of-band artefacts they create, or they
can be tolerated if the parasitic spectral signal does not overlap with the
real spectral signal. Increasing the spectral bandwidth not only
significantly increases the noise level of the spectrum but is – more
seriously – possibly harmful as well due to an insufficient level of control of the
interfering influences mentioned above. The application of an extended
InGaAs diode has been investigated (Hedelius et al., 2016) but resulted
in a significant dependence of XCO2 and XCH4 on the overall signal
level, probably due to a non-linear detector response. This characteristic,
which damages the reliability of the EM27/SUN primary data products, is
highly undesirable and is not observed with the standard detector element.
An alternative approach is the use of an additional detector element for
widening the covered spectral region. This avoids the aforementioned
problems but requires (1) the use of a sandwich detector element,
(2) alternating observations with different detectors, or (3) the
distribution of the optical beam for feeding two separated detector elements
at a time.
A sandwich detector comprised of two sensors with different spectral
coverage is a delicate item of limited availability and considerably higher
cost than a pair of separate detectors. Moreover, the increased number of
stacked substrate interfaces promotes the occurrence of optical resonances,
which generate undulations in the spectral domain (“channelling”), a highly
undesirable feature from the viewpoint of quantitative spectral analysis of
crowded spectral scenes as provided by the atmosphere.
The second option of alternating observations is accompanied by the drawback
that species recorded in the two channels are not recorded simultaneously;
it reduces the duty cycle of the measurement; and it requires an additional
moving optical element in the detector branch of the spectrometer, which
gives rise to further risks, such as variable misalignment or complete
failure of the unit.
Therefore, the third approach seems most promising. This approach has also
been realised in the TCCON spectrometer operated by KIT near Karlsruhe:
here, a dichroic allows simultaneous observation of the shortwave part of
the spectral region (covering O2, CO2, and CH4) together with
either the longwave part (covering HF, N2O, and CO) for achieving the
same spectral coverage as the standard extended InGaAs diode used by other
TCCON sites or a spectral section in the mid-infrared spectral region as
observed by spectrometers of the NDACC (Network for the Detection of
Atmospheric Composition Change). The set-up uses the same InGaAs detector as
the EM27/SUN for the shortwave and a liquid-nitrogen-cooled indium antimonide (InSb) detector
for the longwave part of the spectrum. Further details and results achieved
with this set-up are provided by Kiel et al. (2016a, b).
The remaining drawback of the approach is that a specific dichroic is
required, which complicates the alignment of the detector branch and
generates undulations of the spectral sensitivity. The presence of
undulations requires special attention in the processing of the spectra
(Kiel et al., 2016a). In the case of the Karlsruhe high-resolution set-up,
any loss of the interferometric etendue (product of supported beam area and
covered solid angle) permitted by the hardware configuration is undesirable
in the InSb branch, because the NDACC type of measurements is performed with
narrow optical filters at very high spectral resolution. Therefore, the
choice of using a dichroic of good efficiency seems justified.
Schematic drawing of the detector branch of the extended
EM27/SUN. The camera is located above the drawing plane and images the
primary field stop. For clarity, the camera position has been shifted to the
right in the drawing; it actually is located above the off-axis mirror.
From the viewpoint of the EM27/SUN extension, the situation differs, as the
interferometric etendue is deliberately limited by an adjustable iris acting
as an aperture stop. The detector uses only a small fraction of the etendue
supported by the interferometer and solar tracker hardware, and a loss of
signal can easily be compensated by a slight adjustment of the iris
aperture. Therefore, in this case a wave front division seems the most
appropriate approach. If finally this wave front division is executed near
the aperture stop of the interferometer (defined by an adjustable iris), the
characteristics of the existing detector branch remain unimpaired, and with
proper geometrical arrangement of the extension set-up even the functionality
of the camera-controlled solar tracker, which references the solar image to
the position of the field stop aperture in front of the existing detector,
is maintained.
Technical set-up of the prototype
Figure 1 shows a schematic sketch of the partial beam decoupling and the
longwave detector branch. Behind the adjustable iris aperture that defines
the aperture stop of the system, an off-axis paraboloidal mirror is located,
which offers 127 mm effective focal length and generates a solar image on
the field stop in front of the existing detector element. Physically, the
field stop is realised by a circular hole of 0.60 mm diameter in a thin disc
of stainless steel. For partial decoupling of the beam, a plane mirror of 10×20 mm2 size has been added. This mirror is located directly behind
the off-axis paraboloid and accepts about 40 % of the incoming converging
beam. The deflection angle between the residual parent and reflected partial
beam amounts to about 25∘, and the deviation is chosen along the
horizontal, so that the second detector element can be mounted next to the
original detector.
Close-up of the plane mirror used for decoupling of the secondary
beam. (a) (left side) shows the support arm which carries the mirror.
The mirror itself is seen from the back. On the left, parts of the circular
mounting of the adjustable iris are seen. Behind the contours of the plane
mirror, the larger off-axis paraboloid is located. Above the mirror, the
camera used for the camera-controlled solar tracker is located.
(b) (right side) offers a different perspective: below the camera, the front
side of the plane mirror is seen. The frame of the iris is seen from the
back.
The solar image is formed on a secondary aperture stop (0.80 mm diameter) in
front of the additional detector element. The detector element used is a
windowless extended InGaAs diode (cut off 4000 cm-1) offering a 1 mm2
sensitive area. In a gap between the secondary field stop and the
diode a wedged Germanium long-pass filter is mounted, which shields the
extended InGaAs diode from the spectral section already covered by the
primary detector.
Figures 2–4 show close-up photographs of the dual-detector prototype.
Figure 2 shows the small plane mirror just in front of the off-axis
paraboloid. The mirror is glued with a two-component epoxy resin adhesive to
its aluminium support. The bonding layer has been chosen to be thick and
flexible enough to avoid deformation of the glass mirror due to temperature
changes. The support structure allows fine adjustment of the direction of
the beam reflected towards the secondary field stop. Figure 3 shows the
image of an artificial source on the primary and secondary field stop.
Figure 4 shows the detectors from the top. While the primary detector and
preamplifier unit remain in the standard detector box, a short cable is used
for operating the secondary detector outside of its box, which houses the
preamplifier unit only. The secondary detector is mounted on a support
structure, which in turn is solidly fixed to the primary detector box.
In the front, the image of an artificial source covering about the
same angle as the solar disc is seen on the primary field stop. The support
for the secondary field stop is mounted on the primary detector. The
secondary field stop itself is realised as a 0.8 mm hole in the aluminium
sheet. The image of the source on the secondary field stop is seen to the
upper right from the centre of the image. The final alignment of the image
position with respect to the position of the secondary field stop is
performed by fine adjustment of the plane mirror.
This image provides a top view of the detector units. The primary
detector remains in its standard detector box; the supporting structure for
the secondary field stop is mounted on the box of the primary detector. The
secondary detector and the Ge filter are accommodated in a separate holder,
which can be adjusted laterally with respect to the field stop.
The alignment of the secondary field stop is performed by using an artificial
external light source. The solar tracker is used for conveniently centring
the image of the source on the primary field stop; the evaluation is
performed with the camera of the solar tracker. The fine adjustment of the
source image on the secondary field stop is performed by aligning the plane
mirror; the evaluation is performed by eye with a magnifier. Finally, the
position of the unit containing the wedged Germanium filter and the
secondary detector element is adjusted with respect to the secondary field
stop by searching for maximum signal level.
Overview of measurement days before and after modification of the
spectrometer. The number of spectra used in the comparison is indicated. The
modification of the prototype was performed before 12 October 2015. NIR denotes
spectra recorded in the near infrared for the TCCON analysis, MIR denotes
mid-infrared observations of the fundamental band of CO as used for the
NDACC analysis.
DateNumber of spectra recordedNumber of spectra recordedNumber of spectra(JJMMDD)with the EM27/SUN selectedwith the EM27/SUN usedrecorded with the 125HRfor modification (S/N0039)as a reference (S/N0037)spectrometer (NIR/MIR)150518368369n/a150521282276n/a150702465477n/a150703472473n/a150706342344n/a150710338330n/a150831387400n/aModification performed 151012206Not operated86/415102618620140/315110513617372/41511108311433/215111110310720/2151116796371/3160318322297124/7Characteristics of spectra recorded with the dual-channel prototype
Figure 5 (upper panel) shows a lamp spectrum recorded with the prototype.
Both the primary and secondary channels are essentially free from channelling
(we estimate the upper limit for the peak-to-peak amplitude in the primary
channel to be 0.0005, in the secondary channel to be 0.0002), and the desired
separated spectral bandpass for each channel is achieved: the low-wavenumber
limit of the secondary detector results from the cut-off of the diode; its
high-wavenumber limit is defined by the Ge filter. Similarly, the
low-wavenumber limit of the primary detector is shaped by the diode cut-off; the
high-wavenumber limit is due to a Schott RG 830 long-pass filter glass that
acts as an entrance window of the spectrometer. Therefore, both detectors
observe through this optical element, and a further extension of the concept
presented here towards lower wavenumbers would in addition to a suited
detector element require a replacement of the entrance window (because the
glass-based window becomes nontransparent at a wavelength of about 3 µm).
Finally, also a beam splitter exchange would be needed, because the
standard EM27/SUN is equipped with a Quartz substrate beam splitter.
Lamp spectrum (top) and solar spectrum (bottom) recorded with the
dual-channel prototype. The primary detector covers the spectral section
observed with the standard EM27/SUN; the secondary detector covers the
4000–5500 cm-1 region.
Figure 5 (lower panel) shows a raw solar spectrum recorded with the
prototype. This spectrum reveals that the transition region between both
channels coincides nicely with the opaque region created by the strong water
vapour absorption between the atmospheric window regions H and K (we here
refer to the established convention used in infrared astronomy: the H band
covers 1.5–1.8 µm; the K band covers 2.0–2.4 µm). Close
inspection of the zero baselines reveals a minor out-of-band offset of the
order of 0.015 % for the primary detector branch and no indication of
out-of band signal for the secondary detector branch (below 0.0025 %). The
1σ signal-to-noise ratio in the peak of the primary channel is
13 000; the signal-to-noise ratio of the secondary channel reaches 20 000.
Results of atmospheric retrievals
To evaluate the instrument performance, the modified spectrometer was
operated in parallel to a standard EM27/SUN for 6 days of measurements from
mid-October to mid-November 2015 and on 18 March 2016. Before the
implementation of the extension, the selected spectrometer was operated
side by side with the same standard EM27/SUN on various occasions. This
allows us to check whether the modification of the spectrometer changed the
oxygen column derived from the primary channel. We assume that the oxygen
column is the most sensitive indicator for changes of the instrumental
characteristics (especially instrumental line shape, ILS), as instrumental
error sources tend to cancel out in the final column-averaged abundances of
the target gases. For this purpose, we selected 7 days of measurements
before the intervention was performed on the spectrometer and 6 days after
the intervention. Table 1 lists the measurements used in this study. The
upper part of Table 1 lists the measurements taken between mid-May and
the end of August side by side with the spectrometer used for the prototype
(before modification) and a standard EM27/SUN used as a reference. The lower
part of Table 1 summarises the measurements taken after implementation of
the dual-channel extension, the measurements taken with the standard
EM27/SUN reference (the same spectrometer as used previously), and the near-
and mid-infrared measurements taken with the high-resolution FTIR
spectrometer at Karlsruhe operated as part of the TCCON. The low-resolution
interferograms used in this study passed the quality check implemented in
our calibration routines (based on average DC level and DC variations during
recording), the TCCON data used passed the standard quality flagging of the
GGG software suite. For limiting air-mass-dependent effects, data recorded at
solar elevations below 15∘ have been discarded. Note that the TCCON
time series is somewhat sparser than for a typical TCCON site, due to
alternating recording of high-resolution mid-infrared spectra.
Spectrum simulated for TCCON resolution showing the relevant
absorption features in the spectral region under consideration.
The EM27/SUN side-by-side observations were performed on the roof terrace of
our institute, at a distance of less than 1 km from the TCCON spectrometer
site Karlsruhe. In addition, the TCCON spectrometer was operational during
all days of observations with the dual-channel EM27/SUN. Therefore, the
official TCCON XCO product derived from these observations can serve as the
best available reference for the true column-averaged CO abundances. In
addition to the standard operation mode, the high-resolution spectrometer
intermittently records double-sided interferograms with the same resolution
as applied by the EM27/SUN, which can be used to evaluate systematic
retrieval biases introduced by the significantly different resolution of the
TCCON spectrometer and the EM27/SUN.
For the pre-processing of the EM27/SUN raw data, our suite of CALPY routines
was used (Frey et al., 2015). For the analysis of the EM27/SUN we applied
the retrieval code PROFFIT (Hase et al., 2004). PROFFIT is in wide use in
the NDACC (e.g. Sepúlveda et al., 2014; Virolainen et al., 2014;
Mengistu Tsidu et al., 2015), is in very good agreement with the official
TCCON analysis (Dohe, 2013), and is also successfully applied for the
analysis of spectra recorded with the EM27/SUN (Gisi et al., 2012; Frey et
al., 2015). For the analysis of the low-resolution spectra, the methane,
water vapour, HDO, and CO a priori profiles were adopted from the TCCON
processor. Figure 6 shows the complex spectral region containing the CO
overtone band as recorded at high spectral resolution and denotes the
absorption contributions from the relevant absorbers. Figure 7 shows a
typical spectral fit for the spectral window 4210–4320 cm-1, which
is used for the CO analysis. Systematic fit residuals significantly larger
than the noise level of the spectrometer are evident, indicating that the
simulation of this crowded spectral scene shaped by numerous strong
absorption lines could be improved by further progress on spectroscopic data
(our fits are essentially based on HITRAN 2008; see Rothman et al., 2008).
The provision of an improved and consolidated set of spectroscopic data for
the spectral region under consideration here is an ongoing effort (e.g.
Galli et al., 2012; Scheepmaker et al., 2013;
http://seom.esa.int/page_project003.php).
Typical spectral fit for the selected CO fitting region (4210–4320 cm-1).
Interfering species are CH4, H2O, and HDO.
Before we discuss results derived from the new spectral channel, we
investigate whether the modifications performed on the prototype affected
the results for the oxygen column, which is derived from the existing
spectral channel. Figure 8 compares the oxygen column from the dual-channel
prototype before and after the intervention and the standard EM27/SUN used
as a reference. The agreement meets our design expectations; no detectable
offset as result of the dual-channel implementation is found. We therefore
assume that the implementation of the extension did not significantly affect
the behaviour of spectrometer's primary channel. Note that a comparison of
oxygen columns is a very sensitive test, as many instrumental errors tend to
cancel out in the final column averaged abundances of the target species.
Total column amounts of molecular oxygen measured with the
EM27/SUN used as a reference (data in black) and the prototype before and
after the implementation (before: red; after: green) of the secondary
channel. The implemented extension did not affect the characteristics of the
prototype. Upper panel: time series of the O2 column amounts as
recorded with the reference spectrometer and the prototype. Lower panel:
Mean daily O2 column amounts as recorded with the prototype as a function
of the values recorded with the reference spectrometer. The error bars shown
are the standard deviation of the intraday scatter. The calibration factors
before and after the modification of the prototype are in mutual agreement.
Comparison of methane column amounts as derived from the primary
and secondary spectral channel. Upper panel: time series of methane columns.
Lower panel: daily mean values of methane columns derived from the extended
detector as a function of the methane columns derived from the standard
detector. The error bars are the standard deviations of the intraday
scatter. A linear fit of the calibration factor suggests a systematic
spectroscopic offset of 1.013. There is no indication of a drift of the
instrumental characteristics of the extended detector results with respect
to the standard detector.
Next, we investigate the compatibility of CH4 columns derived from the
primary detector with the CH4 columns from the secondary detector.
CH4 is the main absorber in the 4200–4320 cm-1 region, followed
by H2O and HDO. The CO overtone band is weak in comparison to the
signatures of the other species, so CH4 provides a more sensitive
handle for revealing any instrumental issues. Figure 9 shows the CH4
columns from both channels of the prototype. The two data sets are in mutual
agreement on the 0.1 % level. There is a systematic offset between the two
time series (scaling factor 1.0133±0.0024), very likely due to residual
inconsistencies of the spectroscopic line intensities. The standard
deviation of the ratio of both time series is 0.104 %. It should be noted
that a misalignment between the primary and secondary field stop would induce
systematic differences as a function of azimuthal viewing angle. Indeed, the
comparison of the two methane products provides a tool for detecting a
misalignment of the secondary field stop, because a variation induced by our
limited capability to simulate the spectral scene will be symmetric around
local noon, whereas the field stop misalignment will differ between morning
and afternoon as a consequence of the rotation of the image on the field stop
as a function of azimuthal viewing angle (Reichert et al., 2015). At a solar
elevation angle of 20∘ a 0.1∘ displacement of the solar
disc would create an error of up to 0.5 % in dry-air mole fractions
derived from the secondary channel (solar elevation 10∘: up to
1 %), so this effect can easily be detected using the two methane data
products and subsequently be applied for establishing a correction for XCO
if required. However, we do not observe such a suspicious discrepancy
between the two time series.
Apparent air mass dependency of the XCO data observed with the
dual-channel prototype. For the air mass correction, we applied a second-order
polynomial fit, choosing 25∘ as the neutral point. For
establishing the correction, spectra taken between 15 and 33∘
SEA have been taken into account. This choice results in the functional form
(1+0.0027×(SEA-25∘)-0.00007×(SEA-25∘)2), wherein SEA denotes the solar elevation angle (solid curve). For
estimating the uncertainty introduced by the air mass correction when
comparing the prototype XCO results with the TCCON, the linear term of the
fit has been increased to 0.0034 (overcorrection, dashed curve) and
decreased to 0.0022 (undercorrection, dotted curve).
Finally, we investigate the XCO time series derived from the prototype.
Figure 10 displays measurements recorded in the 15–33∘ range of
solar elevation angles; the daily mean values have been scaled in order to
remove the day-to-day variability. Obviously, the results are affected by an
air mass dependency. The presence of such an artificial air mass dependency is
a frequent problem created by our inability to simulate the observed
spectral scene perfectly. We therefore apply a second-order polynomial
(shown in Fig. 10) to remove this artefact. We have located the neutral
point of this correction at 25∘ solar elevation angle, which is near
the average solar elevation angle of the complete data set. Figure 10 in
addition shows a strong and a weak variant of the correction, which result
in significantly poorer fits to the observed data. These two ad hoc
modifications will be used in the comparison with the TCCON reference
data set for estimating the level of uncertainty introduced by the air mass
correction. The TCCON also requires the aid of empirical air mass corrections
for carbon monoxide and other target species.
Figure 11 presents the XCO data as derived from the dual-channel EM27/SUN
prototype in comparison to the official TCCON data product. The air mass
correction clearly improves the agreement with the TCCON reference (as seen
in the upper panel of Fig. 11). The XCO results deduced from the
dual-channel prototype are in agreement with the TCCON reference data set
(within the statistical uncertainty of the calibration factor). The strong
and weak variants of the air mass correction do somewhat affect the
calibration factor of the prototype XCO. However, the resulting deviations
from unity are still not significant on the 2σ level of the
statistical uncertainty. Note that apart from the empirical air mass
correction and the TCCON calibration factor derived from aircraft
measurements (Kiel et al., 2016b) no adjustments on the XCO derived from the
prototype have been applied. The precision of an individual XCO measurement
can be estimated from differences of successive measurements and amounts to
0.35 ppb.
XCO deduced from the dual-channel prototype and the TCCON site
Karlsruhe. Upper panel: time series of XCO. Lower panel: XCO values measured
with the prototype as a function of the collocated TCCON XCO values (AMC
denotes air mass correction). The error bars are the standard deviations of
the intraday scatter. In the upper panel, all available TCCON values have
been taken into account, whereas for the calculation of the statistics shown
in the lower panel the TCCON measurements have been restricted to the
observing hours of the prototype, because the intraday variability is not
negligible in the case of XCO. As indicated by the calibration factors, the
choice of the air mass correction does not critically impact the level of
agreement found between the prototype and the TCCON XCO observations. The
lower panel of the figure in addition shows XCO results derived from
high-resolution mid-infrared spectra (open diamonds), analysed as described
by Kiel et al. (2016b) (the calibration factor of 0.954 recommended by Kiel
et al. has been applied to the mid-infrared data to match with the
calibration of the TCCON).
XCO intraday variability as observed during 18 March 2016, by
the TCCON spectrometer and the dual-channel prototype.
The vertical sensitivity of the low-resolution spectrometer differs somewhat
from the TCCON sensitivity. This is a consequence of differing spectral
resolutions and introduces a smoothing error component in the comparison.
Sensitivities for both the low-resolution spectrometer and TCCON are shown
in the work by Hedelius et al. (2016), indicating that the CO sensitivity of
the low-resolution spectrometer is even superior to that of the TCCON. In
this study – focused on an instrumental validation – we minimised the
systematic smoothing error contribution by using the same atmospheric CO
a priori profiles as the TCCON in our spectral analysis. An in-depth
quantification of the smoothing error has been performed by Kiel et al. (2016b).
The authors compared TCCON results with data derived from
mid-infrared spectra (with a comparable degree of mismatch of vertical
sensitivities) and found smoothing error contributions of a few per cent due
to different a priori profile choices and vertical sensitivities. We have
included the mid-infrared observations of XCO in the lower panel of Fig. 11,
because these results support our explanation of the remaining
discrepancies being generated by different vertical sensitivities of the
TCCON and the prototype measurements: the mid-infrared observations of XCO
tend to be low compared to the TCCON when the prototype results are low and high
when the prototype results are high.
The demonstrated performance of the prototype indicates that the design is
well suited for source attribution (Wunch et al., 2009; detected intraday
XCO enhancements of up to 30 % in the Los Angeles Basin) and satellite
validation (the TROPOMI accuracy and precision targets are 15 and
10 %, respectively). We expect that the upcoming set of improved
spectroscopic data will further improve the reliability of XCO derived from
low-resolution spectra (ideally rendering the air-mass-dependent correction
obsolete).
On 18 March 2016, a larger intraday variability of XCO occurred, which has
been nicely sampled by both the TCCON spectrometer and the prototype. The
results are shown in Fig. 12, indicating that the prototype can detect XCO
intraday variability in the sub-per-cent range.
Summary and outlook
The portable EM27/SUN spectrometer is dedicated to measurements of
column-averaged abundances of carbon dioxide and methane with sufficient
quality for climate research. In this work, we have described a dual-channel
extension of this device, which can be added in a straightforward manner
without interfering with the standard set-up of the spectrometer. The second
channel uses an extended InGaAs detector element and a wedged Ge filter to
define a spectral bandpass beyond the spectral coverage of the standard
spectrometer. It is fed via a small plane mirror which decouples parts of
the beam towards a secondary detector element. This approach avoids
interference with the concept of a camera-controlled tracker referenced to
the field stop and conserves the spectral coverage and characteristics of the
primary detector. The secondary detector allows the simultaneous measurement
of XCO, which is a very useful tool for source characterisation, while the
use of a cooled detector element can be avoided. In the second part of this
work, we performed a preliminary validation of the set-up by verifying that
the dual-channel prototype maintains the characteristics of the standard
spectrometer and by investigating the XCO data product. We showed that, due
to the fact that methane is the primary absorber in the spectral window used
for the carbon monoxide analysis, the comparison of CH4 columns derived
from either the primary or the secondary channel can be used as a diagnostic
tool for detecting a residual misalignment of the secondary field stop or
other instrumental issues.
We plan a further in-depth characterisation of the dual-channel EM27/SUN
prototype introduced here. The spectrometer is foreseen to participate in
the TCCONcomp campaign funded by ESA. This campaign is scheduled for 2017;
is led by University of Bremen, Germany, and BIRA (Royal Belgian Institute
for Space Aeronomy), Belgium; and involves several partners contributing
various promising approaches for the Sentinel 5 Precursor validation. The
aim of TCCONcomp is to validate these techniques with respect to the TCCON
reference instrument operated by FMI (Finnish Meteorological Institute) at
the Sodankyla station, Finland.
We would like to mention that the secondary channel of the extended EM27/SUN
also allows observation of HDO, opening up the possibility of also using the
device for observations of water vapour isotopic variability, although the
verification of the quality and information content of such data from
low-resolution spectra will require careful validation efforts (for prior
work based on high-resolution near-infrared spectra see Rokotyan et al.,
2014). The new spectroscopic data sets under preparation in support of the
Sentinel 5 Precursor mission will be a highly valuable ingredient for the
TCCON; the spectrometer presented here; and any other remote sensing devices working
in the 2.35 µm spectral region targeting methane, carbon monoxide, or
water vapour isotopic composition.
Acknowledgements
We acknowledge support by the ACROSS and MOSES research infrastructure of
the Helmholtz Association.
We acknowledge support by the Deutsche Forschungsgemeinschaft and Open Access
Publishing Fund of the Karlsruhe Institute of Technology.
We thank the anonymous referees for triggering numerous useful corrections
of the original manuscript.The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association.
Edited by: J. Notholt
ReferencesButz, A., Guerlet, S., Jacob, D. J., Schepers, D., Galli, A., Aben, I.,
Frankenberg, C., Hartmann, J.-M., Tran, H., Kuze, A., Keppel-Aleks, G.,
Toon, G. C., Wunch, D., Wennberg, P. O., Deutscher, N. M., Griffith, D. W. T.,
Macatangay, R., Messerschmidt, J., Notholt, J., and Warneke, T.: Toward accurate
CO2 and CH4 observations from GOSAT, Geophys. Res. Lett., 38,
2–7, 10.1029/2011GL047888, 2011.Dohe, S.: Measurements of atmospheric CO2
columns using ground-based FTIR spectra, Dissertation, Karlsruhe Institute
for Technolgy (KIT), Germany, 2013.Frey, M., Hase, F., Blumenstock, T., Groß, J., Kiel, M., Mengistu Tsidu, G.,
Schäfer, K., Sha, M. K., and Orphal, J.: Calibration and instrumental line shape
characterization of a set of portable FTIR spectrometers for detecting greenhouse
gas emissions, Atmos. Meas. Tech., 8, 3047–3057, 10.5194/amt-8-3047-2015, 2015.Galli, A., Butz, A., Scheepmaker, R. A., Hasekamp, O., Landgraf, J., Tol, P.,
Wunch, D., Deutscher, N. M., Toon, G. C., Wennberg, P. O., Griffith, D. W. T.,
and Aben, I.: CH4, CO, and H2O spectroscopy for the Sentinel-5 Precursor mission:
an assessment with the Total Carbon Column Observing Network measurements,
Atmos. Meas. Tech., 5, 1387–1398, 10.5194/amt-5-1387-2012, 2012.Gisi, M., Hase, F., Dohe, S., and Blumenstock, T.: Camtracker: a new camera
controlled high precision solar tracker system for FTIR-spectrometers,
Atmos. Meas. Tech., 4, 47–54, 10.5194/amt-4-47-2011, 2011.Gisi, M., Hase, F., Dohe, S., Blumenstock, T., Simon, A., and Keens, A.: XCO-2-measurements
with a tabletop FTS using solar absorption spectroscopy, Atmos. Meas. Tech., 5, 2969–2980, 10.5194/amt-5-2969-2012, 2012.Hase, F., Hannigan, J., Coffey, M., Goldman, A., Höpfner, M.,
Jones, N., Rinsland, C., and Wood, S.: Intercomparison of retrieval codes
used for the analysis of high resolution, ground-based FTIR measurements, J.
Quant. Spectrosc. Ra., 87, 25–52, 10.1016/j.jqsrt.2003.12.008, 2004.Hase, F., Frey, M., Blumenstock, T., Groß, J., Kiel, M., Kohlhepp, R.,
Mengistu Tsidu, G., Schäfer, K., Sha, M. K., and Orphal, J.: Application of
portable FTIR spectrometers for detecting greenhouse gas emissions of the major
city Berlin, Atmos. Meas. Tech., 8, 3059–3068, 10.5194/amt-8-3059-2015, 2015.Hedelius, J. K., Viatte, C., Wunch, D., Roehl, C., Toon, G. C., Chen, J.,
Jones, T., Wofsy, S. C., Franklin, J. E., Parker, H., Dubey, M. K., and Wennberg, P. O.:
Assessment of errors and biases in retrievals of XCO2, XCH4, XCO, and XN2O
from a 0.5 cm-1 resolution solar viewing spectrometer,
Atmos. Meas. Tech. Discuss., 10.5194/amt-2016-39, in review, 2016.Heymann, J., Reuter, M., Hilker, M., Buchwitz, M., Schneising, O., Bovensmann, H.,
Burrows, J. P., Kuze, A., Suto, H., Deutscher, N. M., Dubey, M. K., Griffith, D. W. T.,
Hase, F., Kawakami, S., Kivi, R., Morino, I., Petri, C., Roehl, C., Schneider, M.,
Sherlock, V., Sussmann, R., Velazco, V. A., Warneke, T., and Wunch, D.: Consistent
satellite XCO2 retrievals from SCIAMACHY and GOSAT using the BESD algorithm,
Atmos. Meas. Tech., 8, 2961–2980, 10.5194/amt-8-2961-2015, 2015.Inoue, M., Morino, I., Uchino, O., Nakatsuru, T., Yoshida, Y., Yokota, T.,
Wunch, D., Wennberg, P. O., Roehl, C. M., Griffith, D. W. T., Velazco, V. A.,
Deutscher, N. M., Warneke, T., Notholt, J., Robinson, J., Sherlock, V., Hase, F.,
Blumenstock, T., Rettinger, M., Sussmann, R., Kyrö, E., Kivi, R., Shiomi, K.,
Kawakami, S., De Mazière, M., Arnold, S. G., Feist, D. G., Barrow, E. A., Barney, J.,
Dubey, M., Schneider, M., Iraci, L., Podolske, J. R., Hillyard, P., Machida, T., Sawa, Y.,
Tsuboi, K., Matsueda, H., Sweeney, C., Tans, P. P., Andrews, A. E., Biraud, S. C.,
Fukuyama, Y., Pittman, J. V., Kort, E. A., and Tanaka, T.: Bias corrections of GOSAT SWIR XCO2
and XCH4 with TCCON data and their evaluation using aircraft measurement data,
Atmos. Meas. Tech. Discuss., 10.5194/amt-2015-366, in review, 2016.
Keppel-Aleks, G., Toon, G. C., Wennberg, P. O., and Deutscher, N. M.:
Reducing the impact of source brightness fluctuations on spectra obtained by
Fourier-transform spectrometry, Appl. Optics,
46, 4774–4779, 2007.Kiel, M., Wunch, D., Wennberg, P. O., Toon, G. C., Hase, F., and Blumenstock, T.:
Improved retrieval of gas abundances from near-infrared solar FTIR spectra measured
at the Karlsruhe TCCON station, Atmos. Meas. Tech., 9, 669–682, 10.5194/amt-9-669-2016, 2016a.Kiel, M., Hase, F., Blumenstock, T., and Kirner, O.: Comparison of XCO abundances
from the Total Carbon Column Observing Network and the Network for the Detection
of Atmospheric Composition Change measured in Karlsruhe, Atmos. Meas. Tech. Discuss.,
10.5194/amt-2015-364, in review, 2016b.Klappenbach, F., Bertleff, M., Kostinek, J., Hase, F., Blumenstock, T.,
Agusti-Panareda, A., Razinger, M., and Butz, A.: Accurate mobile remote sensing of
XCO2 and XCH4 latitudinal transects from aboard a research vessel,
Atmos. Meas. Tech., 8, 5023–5038, 10.5194/amt-8-5023-2015, 2015.Lindenmaier, R., Dubey, M. K., Henderson, B. G., Butterfield, Z. T.,
Herman, J. R., Rahn, T., and Lee, S.: Multiscale observations of
CO2, 13CO2, and pollutants at Four Corners for emission
verification and attribution, P. Natl. Acad. Sci. USA, 111,
8386–8391, 10.1073/pnas.1321883111, 2014.Mengistu Tsidu, G., Blumenstock, T., and Hase, F.: Observations of precipitable
water vapour over complex topography of Ethiopia from ground-based GPS, FTIR,
radiosonde and ERA-Interim reanalysis, Atmos. Meas. Tech., 8, 3277–3295, 10.5194/amt-8-3277-2015, 2015.Messerschmidt, J., Parazoo, N., Wunch, D., Deutscher, N. M., Roehl, C.,
Warneke, T., and Wennberg, P. O.: Evaluation of seasonal atmosphere-biosphere
exchange estimations with TCCON measurements, Atmos. Chem. Phys., 13,
5103–5115, 10.5194/acp-13-5103-2013, 2013.Petri, C., Warneke, T., Jones, N., Ridder, T., Messerschmidt, J., Weinzierl, T.,
Geibel, M., and Notholt, J.: Remote sensing of CO2 and CH4 using
solar absorption spectrometry with a low resolution spectrometer,
Atmos. Meas. Tech., 5, 1627–1635, 10.5194/amt-5-1627-2012, 2012.Reichert, A., Hausmann, P., and Sussmann, R.: Pointing errors in solar absorption
spectrometry – correction scheme and its validation,
Atmos. Meas. Tech., 8, 3715–3728, 10.5194/amt-8-3715-2015, 2015.Reuter, M., Bovensmann, H., Buchwitz, M., Burrows, J. P., Connor, B. J.,
Deutscher, N. M., Griffith, D. W. T., Heymann, J., Keppel-Aleks, G.,
Messerschmidt, J., Notholt, J., Petri, C., Robinson, J., Schneising, O.,
Sherlock, V., Velazco, V., Warneke, T., Wennberg, P. O., and Wunch, D.:
Retrieval of atmospheric CO2 with enhanced accuracy and precision from
SCIAMACHY: Validation with FTS measurements and comparison with model
results, J. Geophys. Res., 116, 1–13, 10.1029/2010JD015047, 2011.Rokotyan, N. V., Zakharov, V. I., Gribanov, K. G., Schneider, M., Bréon, F.-M.,
Jouzel, J., Imasu, R., Werner, M., Butzin, M., Petri, C., Warneke, T., and Notholt, J.:
A posteriori calculation of δ18O and δD in atmospheric water vapour
from ground-based near-infrared FTIR retrievals of H216O, H218O,
and HD16O, Atmos. Meas. Tech., 7, 2567–2580, 10.5194/amt-7-2567-2014, 2014.Rothman, L. S., Gordon, I. E., Barbe, A., Benner, D. C., Bernath, P.
F., Birk, M., Boudon, V., Brown, L. R., Campargue, A., Champion, J.-P.,
Chance, K., Coudert, L. H., Dana, V., Devi, V. M., Fally, S., Flaud, J.-M.,
Gamache, R. R., Goldman, A., Jacquemart, D., Kleiner, I., Lacome, N.,
Lafferty, W. J., Mandin, J.-Y., Massie, S. T., Mikhailenko, S. N., Miller, C.
E., Moazzen-Ahmadi, N., Naumenko, O. V., Nikitin, A. V., Orphal, J.,
Perevalov, V. I., Perrin, A., Predoi-Cross, A., Rinsland, C. P., Rotger, M.,
Simeckova, M., Smith, M. A. H., Sung, K., Tashkun, S. A., Tennyson, J.,
Toth, R. A., Vandaele, A. C., and Vander Auwera, J.: The HITRAN 2008
molecular spectroscopic database, J. Quant. Spectrosc. Ra., 110, 533–572,
10.1016/j.jqsrt.2009.02.013, 2009.Scheepmaker, R. A., Frankenberg, C., Galli, A., Butz, A., Schrijver, H., Deutscher, N. M.,
Wunch, D., Warneke, T., Fally, S., and Aben, I.: Improved water vapour spectroscopy in
the 4174–4300 cm-1 region and its impact on SCIAMACHY HDO/H2O measurements,
Atmos. Meas. Tech., 6, 879–894, 10.5194/amt-6-879-2013,
2013.Sepúlveda, E., Schneider, M., Hase, F., Barthlott, S., Dubravica, D.,
García, O. E., Gomez-Pelaez, A., González, Y., Guerra, J. C.,
Gisi, M., Kohlhepp, R., Dohe, S., Blumenstock, T., Strong, K., Weaver, D.,
Palm, M., Sadeghi, A., Deutscher, N. M., Warneke, T., Notholt, J., Jones, N.,
Griffith, D. W. T., Smale, D., Brailsford, G. W., Robinson, J., Meinhardt, F.,
Steinbacher, M., Aalto, T., and Worthy, D.: Tropospheric CH4 signals as
observed by NDACC FTIR at globally distributed sites and comparison to GAW
surface in situ measurements, Atmos. Meas. Tech., 7, 2337–2360, 10.5194/amt-7-2337-2014, 2014.Virolainen, Y., Timofeyev, Y., Polyakov, A., Ionov, D., and
Poberovsky, A: Intercomparison of satellite and ground-based measurements of
ozone, NO2, HF, and HCl near Saint Petersburg,
Russia, Int. J. Remote Sens., 35, 5677–5697,
10.1080/01431161.2014.945009, 2014.Wunch, D., Wennberg, P. O., Toon, G. C., Keppel-Aleks, G., and
Yavin, Y. G.: Emissions of greenhouse gases from a North American megacity,
Geophys. Res. Lett., 36, 1–5, 10.1029/2009GL039825, 2009.Wunch, D., Toon, G. C., Blavier, J.-F. L., Washenfelder, R. A.,
Notholt, J., Connor, B. J., Griffith, D. W. T., Sherlock, V., and Wennberg, P. O.: The Total
Carbon Column Observing Network, Phil. Trans. R. Soc. A, 369, 2087–2112,
10.1098/rsta.2010.0240, 2011.