ESSDEarth System Science DataESSDEarth Syst. Sci. Data1866-3516Copernicus PublicationsGöttingen, Germany10.5194/essd-9-977-2017The Total Carbon Column Observing Network site description for Lauder, New ZealandPollardDavid F.dave.pollard@niwa.co.nzhttps://orcid.org/0000-0001-9923-2984SherlockVanessaRobinsonJohnDeutscherNicholas M.https://orcid.org/0000-0002-2906-2577ConnorBrianShionaHisakoNational Institute of Water and Atmospheric Research Ltd (NIWA), Lauder, New ZealandSchool of Chemistry, University of Wollongong, Northfields Ave, Wollongong, NSW, 2522, AustraliaBC Consulting Limited, Martinborough, New ZealandDavid F. Pollard (dave.pollard@niwa.co.nz)7December20179297799230June20172November201725October201712July2017This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://essd.copernicus.org/articles/9/977/2017/essd-9-977-2017.htmlThe full text article is available as a PDF file from https://essd.copernicus.org/articles/9/977/2017/essd-9-977-2017.pdf
In this paper we describe the retrievals of
atmospheric trace gases from near-infrared, high-resolution solar absorption
spectroscopy measurements at the Lauder atmospheric research station in New
Zealand and submitted to the Total Carbon Column Observing Network (TCCON)
archive.
The Lauder site (45.034∘ S, 169.68∘ E,
370 ma.s.l.) is located within a sparsely populated region of the
South Island of New Zealand and is sheltered from the prevailing wind
direction by the Southern Alps, which gives the site a high number of
clear-sky days and an air mass that is largely unmodified by regional
anthropogenic sources. The Lauder TCCON archive consists of data from two
instruments: a Bruker IFS 120HR from June 2004 to February 2010 and a Bruker
IFS 125HR from February 2010 to present. The bias between the two instruments
is assessed to be 0.068 % for CO2. Since measurements using the
IFS 125HR began, the SD about the hourly mean has been better than 0.1 %
for 96.81 % of CO2 column retrievals.
The retrievals have been calibrated against in situ airborne measurements
to correct for biases and provide traceability to the World Meteorological
Organization (WMO) scales with an accuracy of 0.1 % for CO2.
The Lauder TCCON time series of retrieved dry-air mole fractions of
CO2, CH4, N2O, HF, H2O, HDO and CO are
available from the TCCON data archive.
The DOIs are
10.14291/tccon.ggg2014.lauder01.R0/1149293 for the IFS 120HR data
10.14291/tccon.ggg2014.lauder02.R0/1149298 for the IFS 125HR data.
Instrument information.
DateInstrumentDetectorsSpectral rangeLSE correction methodData processing versionTCCON archive name20 Jun 2004–28 Feb 2011120HRExtendedInGaAs3900–9000 cm-1NoneGGG2014R01lauder0102 Feb 2010–present125HRExtendedInGaAs + Si3900–15 000 cm-1Si-basedGGG2014R01lauder02Introduction
Atmospheric trace gases that absorb infrared radiation have an important
influence on the Earth's climate due to the greenhouse effect. Accumulation
of some of these species, most notably carbon dioxide (CO2) and
methane (CH4), are causing changes to the global radiation budget,
which in turn is modifying the climate system. In order to understand the
direct and indirect influences this process is having on the climate, it is
important to understand the global distribution, sources and sinks of these
gases.
One method for measuring the total column abundance of these gases is high-resolution solar absorption spectroscopy using Fourier transform
spectrometers (FTSs) in the near-infrared spectral region. The Total Carbon
Column Observing Network TCCON, consists of
a geographically dispersed network of such instruments designed to provide
consistent and precise retrievals of a number of the atmospheric species. For
CO2 column measurements, demonstrated that
a precision of better than 0.25 % is necessary to improve understanding
of the carbon cycle while showed that a precision of better
than 0.1 % would allow the strength of the Northern Hemisphere carbon
sink to be assessed. The TCCON has adopted these values as measurement
precision targets for the network with an expected site-to-site bias of less
than 0.2 % .
In this paper we describe the TCCON site at Lauder, New Zealand
(45.034∘ S, 169.68∘ E, 370 ma.s.l.), and the time
series of column-averaged dry-air mole fractions (DMFs, denoted by
Xgas) of CO2, CH4, nitrous oxide (N2O),
hydrogen fluoride (HF), water vapour (H2O and HDO) and carbon
monoxide (CO), which have been retrieved using the 2014 version of the GGG
processing software and are archived on the TCCON data
archive . This time series spans from
2004 to the present and represents the longest TCCON dataset in the Southern
Hemisphere.
TCCON data from Lauder have been used extensively to support satellite
missions, such as the Greenhouse Gases Observing Satellite
GOSAT, and the Orbiting Carbon
Observatory 2 OCO-2,, by providing data for retrieval
validation and algorithm development
e.g., as well as for
carbon cycle studies and model validation
e.g..
Site details
The Lauder atmospheric research station, operated by New Zealand's National
Institute of Water and Atmospheric Research (NIWA), is located in a sparsely
populated rural area on the South Island of New Zealand. The station is
2 km north of the settlement of Lauder (population of approximately
50) in a broad valley with mountain ranges on three sides. Prevailing
westerly winds have to cross the Southern Alps before reaching the site, and
this orographic drying leads to annual rainfall of around 400 mm
evenly distributed throughout the year, resulting in a semi-arid climate with
a high number of clear-sky days.
Land around the site is predominantly used for irrigated livestock grazing or
seasonal cropping or it is dry scrub. Farming practices in the area have
shifted in recent years from low-intensity sheep and cattle grazing to more
intensive dairy farming with a corresponding change from flood to spray
irrigation. Some upland farming takes place in the region, which sometimes
requires austral springtime tussock burn off. Plumes from this biomass
burning may affect in situ measurements at the site on a small number of days
each year.
In the region, most domestic heating is by wood burning. The nearest major
population centre is the township of Alexandra, with a population of
approximately 5000, located 40 km southwest of the site. Although
air quality in this and other settlements in the region suffers from the
inefficient burning of wood, in most cases these plumes are capped by an
inversion layer and drain away from the research station, rarely affecting
measurements. There are no other major anthropogenic sources of greenhouse
gases or pollutants in the region.
Southern New Zealand is exposed to strong westerly flows that have had little
or no interaction with significant land masses. Consequently, the atmospheric
composition observed at the site can be considered representative of the
atmospheric background state .
As well as being part of TCCON, the Lauder station is a member of the Network
for the Detection of Atmospheric Composition Change (NDACC), the Baseline
Surface Radiation Network (BSRN), Global Atmosphere Watch (GAW) and the GCOS
(Global Climate Observing System) Reference Upper-Air Network (GRUAN).
Other greenhouse gas measurements are also made at the site; these are
summarised in Sect. .
Major events.
DateInstrumentDetailsImpact30 Mar 2004120HRFirst NIR measurements20 Jun 2004120HRStart of TCCON archived data30 May 2006120HRChange to using single scans instead of co-added interferograms and from 20 to 40 kHz scan rate1 Apr 2009120HRLaser change1 Aug 2009125HRInstrument installed2 Feb 2010125HRStart of TCCON archived data28 Feb 2010120HREnd of TCCON archived data17–25 Mar 2010125HRShear alignment7–14 Apr 2010125HRIR beam steer alignment1–7 Jul 2010125HRAlignment using Hase and Blumenstock method5 Oct 2010–20 Jan 2011125HRTiming errorsXair uncertainty increased20 Jan 2011125HRLaser sampling board replaced18 Mar 2011125HRNew CaF2 beam splitter installed4–11 May 2011125HRAlignment optimised20 Apr 2012125HRLaser replacedGhosts minimised 24 Apr 201212 Sep–24 Oct 2012125HRTiming errorsXair uncertainty increased21 and 24 Oct 2013125HRMeasurements affected by smoke fromAustralian bushfires.Data excluded from archive12 Sep–24 Oct 2012125HRTiming errorsXair uncertainty increased09 Mar–30 Jun 2014125HRTiming errorsXair uncertainty increased28 Sep–1 Oct 2014125HRTiming errorsXair uncertainty increased5 Jan 2015125HRLaser replacedGhosts minimised28 Jan 2015125HRLaser replacedGhosts minimised20 Oct–16 Nov 2015125HRControl electronics upgrade and various firmware updatesSome data lost30 Mar 2016125HRSwitch from scan rate of 20 to 10 kHzInstrumentation
Two FTS instruments have been used to make
TCCON measurements at Lauder (Table ); the
chronology of these instruments and major changes to them are listed in
Table .
Both instruments are accommodated in a dedicated, air conditioned and
temperature-stabilised laboratory. Both have dedicated solar trackers,
protected by autonomous covers, which close automatically in the event of
rain or high winds.
Bruker IFS 120HR
From June 2004, following the installation of an indium gallium arsenide
(InGaAs) detector, an existing Bruker 120HR FTS (referred to as lauder01 in
the TCCON archive) was used in a time-sharing mode to make routine
measurements of solar absorption in the mid-infrared (MIR, for the NDACC) and
near-infrared (NIR, for the TCCON). Acquisition of spectra in the two
spectral regions (MIR and NIR) requires a beam splitter exchange,
necessitating daily venting and re-evacuation of the instrument (and loss of
observation time for the original MIR measurement programme). This was the
operational TCCON instrument from 20 June 2004 until 28 February 2010. From
20 June 2004 to 30 May 2006, each retrieval consisted of 10 co-added
interferograms measured at a scanner velocity of 20 kHz. After this
period retrievals were made from individual interferograms measured at
40 kHz.
A prototype active tracking system was installed in the IFS 120HR solar
tracker telescope in 2006, which adjusted the position of a 45∘
mirror immediately before the instrument input port based on feedback from
quadrant diodes. The in-house-developed NIWA360 tracker as used by the IFS
125HR instrument was not installed for the IFS 120HR until 2011, after the
end of the lauder01 dataset.
After installation of the IFS 125HR instrument (see below) 1 month of
measurements using both instruments have been released to the TCCON archive
to allow users to assess any biases between instruments. An assessment of
bias between instruments is also made in Sect. .
Measurements in TCCON mode are still made periodically (approximately
monthly) with this instrument to provide an independent check of the primary
TCCON instrument. These data are not released to the TCCON archive.
Airborne calibration flights at the Lauder TCCON site.
CampaignFlightDateAlt. rangeTCCON instrumentSpecies comparedHIPPO 1RF 721 Jan 20090.7–14.6 km120HRCO2, CH4, CO and N2OHIPPO 2RF 510 Nov 20090.8–13.1 km120HRCO2, CH4, CO and N2OHIPPO 3RF 66 Apr 20100.6–12.9 km125HRCO2, CH4, CO and N2OHIPPO 5RF 828 Aug 20112.7–13.7 km125HRCO2, CH4, CO and N2OHIPPO 5RF 930 Aug 20112.8–14.1 km125HRCO2, CH4, CO and N2OAirCore19 Sep 20150.7–16.0 km125HRCO2 and CH4AirCore211 Sep 20151.2–21.0 km125HRCO2 and CH4Bruker IFS 125HR
In 2009 NIWA purchased a Bruker IFS 125HR spectrometer dedicated to TCCON
measurements (lauder02 in the TCCON data archive). This instrument began
routine TCCON measurements in February 2010.
Active tracking for this instrument is achieved using the in-house-developed
NIWA360 system, which directs a small amount of the incoming solar radiation
through a telescope and onto quadrant photo diodes, which steer the tracker
to keep the solar image centred on the FTS input aperture to within 10 %
of the radial extent of the solar disc, an accuracy of better than
0.025∘. The NIWA360 system has been used for the IFS 125HR since
installation.
As with most TCCON sites, this instrument was originally supplied with
a faulty ECL03 laser sampling board, resulting in spurious “ghost” signals,
which interfere with the spectral fitting. The electronics board in the
Lauder IFS 125HR was upgraded to an ECL05 board in January 2011 and the ghost-to-parent ratio (GPR) has since been monitored regularly and minimised
whenever necessary. A full discussion on the treatment of this issue is given
in Sect. .
Apart from some early testing between February and October 2010 when some
measurements were taken at scanner velocities of 10 and 40 kHz, the
majority of the IFS 125HR time series have been taken at 20 kHz.
Following the installation of new control electronics in October 2015,
measurements were transitioned to 10 kHz at the end of March 2016.
Auxiliary measurements
In order to satisfy the requirements of a TCCON station, and to carry out
retrievals, a number of auxiliary measurements of meteorological parameters
are required. A NIWA climate monitoring station is sited at Lauder,
approximately 220 m from the FTS instruments. This includes
measurements of
pressure: Vaisala PTB100A ±0.3hPa.
temperature and humidity: Vaisala HMP155D ±0.3∘C, ±3.5 %.
wind speed and direction: Vector A101M anemometer
(±0.1<10ms-1, ±2 % rest of range) and Vector W200P vane (±2∘).
Other instruments present at the site
The atmospheric research station at Lauder hosts a wide range of atmospheric
remote sensing and in situ instrumentation. With respect to atmospheric
greenhouse gas measurements, these include in situ FTS measurements of
CO2, CO, CH4 and N2O,
non-dispersive infrared (NDIR) measurements of CO2 using a LI-COR LI-7000
instrument and flask samples for subsequent measurements of CO2, CO,
CH4, N2O and stable isotopologues of CO2, which are
collected between 15:00 and 16:00 NZST when the wind speed is above
5 ms-1 in order to sample a well mixed boundary layer
. The in situ measurements at Lauder are described
more fully in Appendix A of . The site also hosts lidar
measurements of ozone and aerosols.
Data processing
The TCCON uses a common processing and retrieval system known as GGG
currently version GGG2014 described by in order to
minimise biases across the network. This section will describe the data
processing as it applies to the IFS 125HR data. The processing is broadly
similar for IFS 120HR except that for the IFS 120HR the Silicon (Si) detector
is not used, only forward scan interferograms are recorded and in principle
there is no requirement to correct for laser sampling errors because the IFS
120HRs did not contain the defective laser sampling boards.
The IFS 125HR records direct-current-corrected, dual channel (InGaAs and Si),
forward and reverse scan interferograms throughout the day whenever there are
clear skies. After each day of measurements, the interferograms are assessed
visually by the operator to check for data quality to remove any periods of
total cloud contamination. The raw interferograms are then processed by the
I2S (interferogram-to-spectrum) software package, which corrects for solar
intensity fluctuations , applies a phase correction
and a laser sampling error (LSE) correction
(Sect. below), and then computes the spectra using a fast
Fourier transform . The causes and mitigation of the LSE
are described later in this section.
The resulting spectra are analysed using the GFIT non-linear, least-squares
fitting algorithm, which uses molecular absorption coefficients, a priori
estimates of gas mole fraction profiles, profiles of temperature, pressure,
and humidity and the calculated solar viewing geometry to calculate
theoretical absorption spectra. The atm.101 spectral line list
used is based on the HITRAN2012 database
with exceptions as described in
. The solar line list used is that of
.
The profiles of temperature, pressure and humidity are interpolated to the
site location from the National Centers for Environmental Prediction (NCEP)
reanalysis data for local noon.
A priori profiles of all retrieved species other than water vapour are
generated using a set of empirical functions that are optimised to fit
observations from a number of different in situ, aircraft and balloon, and
satellite sources as a function of latitude, longitude and season, as well as
taking account of long-term variability. The GGG2014 a priori profiles can
be generated using a stand-alone Fortran program .
GFIT then compares the theoretical and measured spectra in a number of
independent micro-windows and minimises the RMS difference between them by
iteratively scaling the assumed gas profiles and fitting spectroscopic
characteristics such as continuum level, tilt and some degree of curvature,
and frequency shifts.
In order to reduce the sensitivity of the measurement to variations in
surface pressure and atmospheric water vapour, TCCON reports the
column-averaged dry-air mole fractions (DMFs or Xgas), calculated
by dividing the column of gas by the column of dry air inferred from the
co-retrieved O2 column multiplied by the assumed dry-air mole
fraction of O2:
Xgas=VCgasVCO2⋅0.2095.
This scaling by the retrieved O2 column also reduces the effect of
systematic instrumental biases such as timing or pointing errors.
The DMF of dry air, Xair, is a special case given by
Xair=VCairVCO2⋅0.2095-XH2O⋅mH2Omairdry,
where mH2O and mairdry are the mean
molecular masses of water (18.02 gmol-1) and dry air
(28.964 gmol-1), XH2O is the retrieved DMF of water
vapour and VCair is calculated from the surface pressure,
Ps.
VCair=Ps{g}⋅mairdryNa,
where {g} is the column-averaged acceleration due to gravity and
Na is Avogadro's constant.
In the case of a perfectly accurate measurement, the value of
Xair would be unity. Due to spectroscopic limitations the actual
value is approximately 0.98 for all TCCON sites and varies by approximately
1 % with solar zenith angle. Deviations from the characteristic values
for Xair are generally indicative of erroneous behaviour in the
measurement and retrieval system such as tracking errors or fitting to an
incorrect air mass as described in Sect. . However, these
errors will generally cancel out in the calculation of DMFs of individual
gases as the error will be present for both the gas and O2.
As detailed elsewhere , the TCCON
XCO2 data exhibit non-physical symmetric variability with
changing air mass. The data presented here have the TCCON standard air mass
correction of -0.0068 applied; however, this value is derived from
a limited time series of data from three sites only. Calculating the
corresponding value from the uncorrected Lauder XCO2 data yields
an air mass correction of -0.0095±0.0021; applying this value to the
Lauder XCO2 time series generates a seasonal cycle amplitude
that is ±0.6 µmolmol-1 smaller. Users for whom the
seasonal variability is critical can circumvent this issue by only selecting
XCO2 values corresponding to a restricted range of solar zenith
angles (SZAs) that is consistent throughout the entire year, e.g. only using
measurements between 35 and 55∘ SZA, at which the air mass correction is
smallest.
A network-wide bias correction based on airborne measurements to provide
traceability to WMO trace gas scales as described in with
updated values from , listed in the second column of
Table , is also applied. The Lauder-specific
calibration campaigns are described in Sect. of this article.
Time series of LSE calculated using the I2S algorithm for the IFS 125HR
spectra. For 20 kHz data only every 10th data point is plotted for
clarity. Vertical lines show significant changes or adjustments to the
instrument.
Laser sampling error correction
The IFS 125HR FTS makes use of a helium–neon (HeNe) metrology laser running
parallel to the measured solar beam in the interferometer to allow accurate
measurement and control of the optical path difference by counting
interference fringes. A fault with the original ECL03 laser amplifier board
supplied with this instrument meant that the metrology laser was not sampled
correctly at the zero crossing point between the peak and trough of the
resulting sine wave . As a result, some of the
spectral information above the Nyquist frequency of 7899 cm-1 was
folded below it and vice versa, causing features known as ghosts.
During installation the LSE was minimised for a scanner velocity of
40 kHz, resulting in a small LSE at a rate of 20 kHz used for
the majority of operational measurements.
Following the discovery of this fault, Bruker supplied all TCCON sites with
replacement ECL05 laser amplifier boards, which allowed the zero level (i.e.
the level at which the electronics identify the zero crossing point) to be
adjusted. The rectified electronics board was installed in the Lauder IFS
125HR in January 2011. Since this time, regular (approx. every 3 months)
checks have been conducted to check for the presence of ghosts and to
minimise them by adjusting the zero level if necessary. This is achieved by
taking measurements of a tungsten lamp, placing a band pass filter
(5500–6600 cm-1) between the interferometer and detector, and
measuring the signal aliased at 9800 cm-1. If any signal is
detected, the zero level is iteratively adjusted in order to minimise
it.
The 2014 release of the GGG processing software I2S algorithm makes use of
the spectra recorded using the Si detector, which is wholly contained in the
upper half of the alias, to derive the magnitude of the LSE and allow the
spectra of both detectors to be resampled, removing the ghost signal. This
method is described fully in .
The time series of retrieved modulation efficiencies at
45 cm optical path difference (a) and maximum phase
error (b) for the Bruker IFS120HR and IFS125HR instruments at
Lauder, based on lamp measurements of a 10 cm HCl cell, indicating
the improved stability of the IFS125HR instrument line shape over time.
Figure shows the time series of LSE determined using the I2S
algorithm over the period of measurements made using the IFS 125HR. Periods
of instrument alignment are denoted by the shaded vertical bands whilst other
notable instrument changes are indicated by the various vertical lines. The
solid vertical line in early 2011 represents the change of laser amplifier
board and subsequent improvement in LSE. Also of note is the change in LSE
performance with individual metrology lasers, with the performance of the
unit installed between April 2012 and January 2015 being poorer and less
stable than other devices as indicated by the relative scatter in the derived
LSEs.
Instrument stabilityInstrumental line shape
The performance and alignment of both instruments is monitored through
monthly retrievals of the instrumental line shape (ILS) from spectra obtained
from a lamp source with a reference hydrogen chloride (HCl) cell, using the
LINEFIT 14 software . The retrieved ILS describes the
deviation from a theoretical ideal ILS, which is assumed by the GFIT
retrieval code, in terms of the modulation efficiency (ME) and phase error,
both of which vary as a function of optical path difference (OPD) of the
interferometer. In the following discussion, ME will refer to the modulation
efficiency at the maximum OPD of 45 cm used for TCCON measurements.
Figure shows a time series of the retrieved ME and
maximum phase error for both instruments. For the IFS 125HR, the mean of the
ME is 0.9995 with a SD of 0.0106 indicating the stability of the instrument
optical alignment over time. This value is significantly better than 4 %
deviation in ME, which demonstrated was necessary in order to
achieve the XCO2 retrieval accuracy required by the TCCON. The
results for the IFS 120HR show more scatter and less stability, with a mean of
1.0124 and SD of 0.0256. Regular ILS retrievals from lamp spectra have been
carried out since August 2010.
IFS 125HR timing errors
Due to the instability of the IFS 125HR instrument's internal clock, there
have been a number of occasions when timing errors have been noted in the
retrieved data. These are listed in Table . The errors
have generally occurred when updates to the operating system or experimental
set-up have prevented the instrument control computer from periodically
resetting the time in the instrument's firmware. Such errors cause the
retrieval scheme to attempt to fit the spectra to an incorrect air mass due
to small differences in the calculated and true SZA. At the magnitude at
which these errors occurred, the effect on the retrieved Xgas
values is negligible as these are scaled by the retrieved column of
O2 in order to remove such instrumental artefacts. However, the
values of Xair that are retrieved and used as a diagnostic and
quality indicator within TCCON are perturbed, as in this case it is the
theoretical column of dry air that is scaled by the retrieved O2
column. By artificially introducing a timing error into retrievals that are
known to have accurate timing, it has been shown that for a timing error of
60 s and a SZA less than
85∘, the perturbation of XCO2 is less than 0.02 %,
compared to 1 % for Xair. Therefore, for the 2014 TCCON data
release, data subject to these errors have been included without any attempt
to correct them.
Time series of Xair
As noted previously, for an ideal retrieval, Xair would be unity,
but due to spectroscopic limitations there is a TCCON wide bias and SZA
dependence. These are, however, stable both temporally and across the
network. Thus, Xair is a useful diagnostic of the quality of
measurements, with retrievals deviating more than 1 % from the nominal
value of 0.98 indicating systematic errors, e.g. the timing errors mentioned
above, or deviations from an ideal ILS.
Time series of IFS 125HR Xair. Data affected by timing
errors are shown in green and defined in Table . For
values not affected by timing errors, only every 10th data point is plotted
for clarity.
Time series of hourly means of Xair(b) and SDs
about them (a) retrieved from measurements made with both FTS
instruments at Lauder.
Figure shows the time series of Xair retrievals
from the IFS 125HR instrument that have satisfied the data quality criteria
for release to the TCCON archive. It can be seen that most of these values
fall within 1 % of 0.98 (indicated by the dashed lines); however, there
are some periods with values outside of these limits, which have been
identified as having timing errors. These periods are highlighted in green
and defined in Table .
In Fig. we show the hourly mean and SD (upper panel)
of Xair values for the entire Lauder TCCON dataset covering both
instruments.
There is a discontinuity in the SD of the hourly mean time series
corresponding to a change from performing retrievals using 10 co-added
interferograms sampled at a scanner velocity of 20 kHz to single
scans at 40 kHz in May 2006. The discontinuity in both the mean and
SD that occurs in early 2010 indicates the change from the IFS 120HR to the
IFS 125HR and demonstrates the improvement in performance with the newer
instrument.
Comparison of airborne calibration scale factors, derived from
Lauder flights, with TCCON network-wide values.
The accuracy of TCCON measurements is limited by the available spectral line
parameters to around 2–3 %. However, because of the use of a common
retrieval system across the network, the resulting bias is consistent for all
retrievals and sites.
In order to align TCCON measurements with the accepted WMO gas standard
scale,
a number of in situ, airborne profile measurements have been made at many
of the TCCON sites, including Lauder, to derive a network-wide scaling factor
for a subset of retrieved species . In this section we
compare the network wide scaling factors with those derived for the two
Lauder instruments.
The Lauder site was included in the flight schedule of each of the HIAPER
Pole-to-Pole Observations (HIPPO) campaigns , and in 2015
it was the base for an AirCore campaign. Details of
successful calibration flights are listed in
Table . Each flight consists of one or more
altitude profiles that have been averaged together to give measured profiles
of the mole fractions of each target species. In some cases operational
requirements meant that these profiles could not be performed directly over
Lauder and were instead carried out during the approach into, or climb out
of, Christchurch International Airport approximately 285 km to the
northeast. It is considered that for the target species, the spatial
variability is sufficiently small that these profiles can be considered
representative of the air mass at the TCCON site if the boundary layer is
discarded. In order to extend the profiles from their lowest level to the
surface, they are interpolated using mole fractions measured from surface
flask samples collected at the time of overflight at Lauder. Above the
maximum altitude attained during the flight, these profiles have been
supplemented using the TCCON a priori profiles.
In order to derive DMFs that can be directly compared to the TCCON
measurements, the airborne profiles are integrated following the method of
, as it was applied by , weighting with the
FTS retrieval averaging kernels to give a smoothed column-average DMF
xs:
xs=xa+A(xh-xa),
where xa is the a priori column DMF, xh is the DMF of
the merged aircraft profile and A is the column-averaging kernel.
Figure shows the resulting calibration curves for the
smoothed airborne DMFs and the mean of the corresponding day's FTS retrievals
for the four species compared. Uncertainties in the airborne measurements are
assigned as in , and those for the TCCON measurements are
the SD of the day's retrievals. Calibration scaling factors are then
calculated assuming a linear relationship, forced through the origin and
taking account of the uncertainties in both the smoothed aircraft and TCCON
values. The resulting scaling factors for the Lauder instruments and those
derived for the entire TCCON network in , updated in
, are listed in Table . These show
good agreement and the accuracy, based on the uncertainty in the fitted
linear relationship, following this correction is 0.1 % for
XCO2.
Comparison of retrievals from both instruments. Mean difference (IFS
120HR minus IFS 125HR) of daily mean values, their SD and standard error (SD
of the sample mean) expressed as a percentage, and the number of daily means
compared.
Airborne calibration curves for XCO2,
XCH4, XCO and XN2O with fitted linear
relationships derived for the TCCON network and Lauder-only intercomparisons
(forced through origin). The HIPPO 1 and 2 flights were compared to the IFS
120HR (open symbols); all others were compared to the IFS 125HR (filled symbols).
Time series of daily means of retrieved XCO2 from
measurements made with both FTS instruments at Lauder (a) and the
difference between them (b). Error bars in the upper panel indicate
the SD about the daily mean, while those in the lower panel are the sum in
quadrature of the SD about the daily means. The horizontal lines in the lower
panel show the mean of the daily mean differences and the SD and standard
error (SD of the sample mean) about it.
As Fig. but for XCH4.
As Fig. but for XCO.
Time series of hourly means of XCO2 retrieved from
measurements made with both FTS instruments at Lauder.
Time series of hourly means of XCH4 retrieved from
measurements made with both FTS instruments at Lauder.
Time series of hourly means of XCO retrieved from
measurements made with both FTS instruments at Lauder.
ResultsInstrument intercomparison
In order to assess any biases between the two instruments used to collect
TCCON data at Lauder, we have compared days on which measurements were taken
with both instruments between October 2009 and December 2012.
Figures to show in the upper panels
the daily mean retrieved values for XCO2, XCH4 and
XCO respectively. Error bars represent the SD about the daily
means. The lower panels show the difference (IFS 120HR – IFS 125HR) between
the daily means, and the error bars are the sum in quadrature of the SDs
about the daily means. Also plotted are the mean of the daily mean
differences and the associated SD and standard error. These values are listed
as percentages for all compared species in Table . For
XCO2 the mean bias is 0.068 % with a standard error of
0.015 %.
Time series of retrieved species
Here we present a subset of retrieval time series available in the Lauder
TCCON data archive. Figures to show the
hourly means of the retrieved Xgas, along with their SD in the
upper panels. These figures are made up of data from both of the Lauder
instruments with the change-over occurring on 1 February 2010.
Precision of Lauder XCO2 datasets compared to TCCON
targets.
The time series of XCO2 values is shown in
Fig. . Over the length of this time series XCO2
has been increasing by approximately 2 µmolmol-1yr-1.
There is a seasonal cycle due to summer drawdown, but it is smaller than in
the Northern Hemisphere . Since measurements using the
IFS 125HR began, the SD about the hourly mean has been better than 0.1 %
for 96.81 % of retrievals and better than 0.25 % for 99.96 % of
retrievals. The precision of the IFS 120HR was poorer, with only 14.95 %
of hourly SDs better than 0.1 and 95.93 % better than 0.25 %. The
precision of both instruments compared to the TCCON targets are shown in
Table .
Figure shows the XCH4 time series.
XCH4 has a smaller positive trend of approximately
5.6 nmolmol-1yr-1 and a seasonal cycle of similar magnitude
with a summer minimum due to oxidation by photochemically produced OH
radicals. The Lauder TCCON XCH4 time series starts around 2
years before the end of a period of stable atmospheric CH4
concentrations that has been observed in a number of different time series
(, and references therein) and this is also apparent in
the data shown here.
The time series of XCO shown in Fig. has
a small, negative trend and a stronger seasonal cycle with a summer minimum
due to OH oxidation and a springtime peak attributed to CO transported from
Southern Hemisphere biomass burning sources .
Figure shows the time series of the remaining retrieved
species.
Time series of hourly means of XH2O(a),
XHDO(b), XN2O(c) and
XHF(d) retrieved from measurements made with both FTS
instruments at Lauder.
The GGG2014 versions of the processed Lauder TCCON data are
available from the TCCON data archive .
The data DOIs are 10.14291/tccon.ggg2014.lauder01.R0/1149293 for the
IFS 120HR data and 10.14291/tccon.ggg2014.lauder02.R0/1149298 for the
IFS 125HR data.
The IFS 120HR data are available from 20 June 2004 to 28 February 2010, while
the IFS 125HR data cover the range 2 February 2010 to the present day, with
data becoming publicly available 180 days after collection.
Conclusions
The Lauder TCCON dataset contains retrievals of XCO2,
XCH4, XN2O, XHF, XH2O,
XHDO and XCO from June 2004 to the present date using
two FTS instruments. Overlapping measurements were made using both
instruments and from these, the bias between them was assessed to be
0.068 % for XCO2. Regular monitoring of the ILS (monthly,
both instruments) and LSE (quarterly, IFS 125HR only) is carried out to
ensure the consistent performance of the instruments and the quality of the
retrievals and to achieve the TCCON target precision of 0.1 % for
XCO2 as well as consistency with the rest of the network.
Airborne calibration campaigns have shown that the scale factors required to
align the Lauder retrievals of XCO2, XCH4,
XCO and XN2O with WMO scales are consistent with
those used across the TCCON.
Timing errors have sporadically affected measurements taken with the IFS
125HR instrument, which have perturbed the retrieved values for
Xair by up to 2 %. However, due to the scaling by the
co-retrieved column of O2, the effect on retrievals of other gases
has been minimal, less than 0.02 % in the case of CO2 at SZAs
less than 85∘. In a future data release it is expected that these
timing errors will be retrospectively corrected.
BC was the instigator of the Lauder TCCON programme and was
the Lauder TCCON principal investigator (PI) until 2008. VS was involved in the early
development of the Lauder TCCON project and was the site PI from 2008 to 2014. VS was
responsible for the retrieval of the TCCON products from the Lauder spectra. JR is
responsible for the operation and alignment of the FTS instruments and developed the NIWA
solar trackers. ND analysed the air mass dependence and provided advice on data processing
and methods for carrying out airborne calibrations. HS maintains the GGG processing system
(developed by VS) and quality control of the TCCON data products. DP is the current TCCON
site PI for Lauder and wrote this paper. All authors have read and provided feedback
on the paper.
The authors declare that they have no conflict of
interest.
Acknowledgements
The Lauder TCCON programme is core-funded by NIWA through New Zealand's
Ministry of Business, Innovation and Employment with additional support from
the National Institute for Environmental Studies, Japan, GOSAT project.
Nicholas Deutscher is supported by an Australian Research Council –
Discovery Early Career Researcher Award DE140100178.
The authors would like to acknowledge the following: Colm Sweeney, Sonja
Wolter, Tim Newberger and Jack Higgs of NOAA in organising the AirCore
campaign at Lauder and the logistical support of Richard Querel, Wills
Dobson, Jocelyn Turnbull and Joel Wilson during the AirCore flights; Steve
Wofsy and Britt Stevens for providing calibrated HIPPO data; Paul Wennberg,
Debra Wunch, and Coleen Roehl of Caltech and Geoff Toon of JPL for technical
and hardware assistance over the years; and Dan Smale and Mike Harvey for the
useful and informative discussions and advice.
Edited by: David Carlson Reviewed by: two anonymous referees
ReferencesBergland, G. D.: A radix-eight fast Fourier transform subroutine for
real-valued series, IEEE T. Acoust. Speech, 17, 138–144,
10.1109/tau.1969.1162043, 1969.Brailsford, G. W., Stephens, B. B., Gomez, A. J., Riedel, K., Mikaloff
Fletcher, S. E., Nichol, S. E., and Manning, M. R.: Long-term continuous
atmospheric CO2 measurements at Baring Head, New Zealand, Atmos. Meas.
Tech., 5, 3109–3117, 10.5194/amt-5-3109-2012, 2012.Buchholz, R. R., Paton-Walsh, C., Griffith, D. W. T., Kubistin, D.,
Caldow, C., Fisher, J. A., Deutscher, N. M., Kettlewell, G., Riggenbach, M.,
Macatangay, R., Krummel, P. B., and Langenfelds, R. L.: Source and
meteorological influences on air quality (CO, CH4 & CO2)
at a Southern Hemisphere urban site, Atmos. Environ., 126, 274–289,
10.1016/j.atmosenv.2015.11.041, 2016.Chevallier, F., Deutscher, N. M.,
Conway, T. J., Ciais, P., Ciattaglia, L., Dohe, S., Fröhlich, M.,
Gomez-Pelaez, A. J., Griffith, D., Hase, F., Haszpra, L., Krummel, P.,
Kyrö, E., Labuschagne, C., Langenfelds, R., Machida, T., Maignan, F.,
Matsueda, H., Morino, I., Notholt, J., Ramonet, M., Sawa, Y., Schmidt, M.,
Sherlock, V., Steele, P., Strong, K., Sussmann, R., Wennberg, P., Wofsy, S.,
Worthy, D., Wunch, D., and Zimnoch, M.: Global CO2 fluxes inferred
from surface air-sample measurements and from TCCON retrievals of the
CO2 total column, Geophys. Res. Lett., 38, L24810,
10.1029/2011GL049899, 2011.Connor, B. J., Sherlock, V., Toon, G.,
Wunch, D., and Wennberg, P. O.: GFIT2: an experimental algorithm for vertical
profile retrieval from near-IR spectra, Atmos. Meas. Tech., 9, 3513–3525,
10.5194/amt-9-3513-2016, 2016.Crisp, D., Miller, C. E., and DeCola, P. L.: NASA Orbiting Carbon
Observatory: measuring the column averaged carbon dioxide mole fraction from
space, J. Appl. Remote Sens., 2, 023508, 10.1117/1.2898457, 2008.Crisp, D., Fisher, B. M., O'Dell, C., Frankenberg, C., Basilio, R.,
Bösch, H., Brown, L. R., Castano, R., Connor, B., Deutscher, N. M.,
Eldering, A., Griffith, D., Gunson, M., Kuze, A., Mandrake, L., McDuffie, J.,
Messerschmidt, J., Miller, C. E., Morino, I., Natraj, V., Notholt, J.,
O'Brien, D. M., Oyafuso, F., Polonsky, I., Robinson, J., Salawitch, R.,
Sherlock, V., Smyth, M., Suto, H., Taylor, T. E., Thompson, D. R., Wennberg,
P. O., Wunch, D., and Yung, Y. L.: The ACOS CO2 retrieval algorithm –
Part II: Global XCO2 data characterization, Atmos. Meas.
Tech., 5, 687–707, 10.5194/amt-5-687-2012, 2012.Deutscher, N. M., Griffith, D. W. T., Bryant, G. W., Wennberg,
P. O., Toon, G. C., Washenfelder, R. A., Keppel-Aleks, G., Wunch, D., Yavin,
Y., Allen, N. T., Blavier, J.-F., Jiménez, R., Daube, B. C., Bright, A.
V., Matross, D. M., Wofsy, S. C., and Park, S.: Total column CO2
measurements at Darwin, Australia – site description and calibration against
in situ aircraft profiles, Atmos. Meas. Tech., 3, 947–958,
10.5194/amt-3-947-2010, 2010.Deutscher, N. M., Sherlock, V., Mikaloff Fletcher, S. E., Griffith, D. W. T.,
Notholt, J., Macatangay, R., Connor, B. J., Robinson, J., Shiona, H.,
Velazco, V. A., Wang, Y., Wennberg, P. O., and Wunch, D.: Drivers of
column-average CO2 variability at Southern Hemispheric Total Carbon
Column Observing Network sites, Atmos. Chem. Phys., 14, 9883–9901,
10.5194/acp-14-9883-2014, 2014.Dupuy, E., Morino, I., Deutscher, N., Yoshida, Y., Uchino, O., Connor, B.,
De Mazière, M., Griffith, D., Hase, F., Heikkinen, P., Hillyard, P.,
Iraci, L., Kawakami, S., Kivi, R., Matsunaga, T., Notholt, J., Petri, C.,
Podolske, J., Pollard, D., Rettinger, M., Roehl, C., Sherlock, V.,
Sussmann, R., Toon, G., Velazco, V., Warneke, T., Wennberg, P., Wunch, D.,
and Yokota, T.: Comparison of XH2O retrieved from GOSAT
short-wavelength infrared spectra with observations from the TCCON Network,
Remote Sens., 8, 414, 10.3390/rs8050414, 2016.Griffith, D. W. T.,
Deutscher, N. M., Caldow, C., Kettlewell, G., Riggenbach, M., and Hammer, S.:
A Fourier transform infrared trace gas and isotope analyser for atmospheric
applications, Atmos. Meas. Tech., 5, 2481–2498,
10.5194/amt-5-2481-2012, 2012.Hase, F., Drouin, B. J., Roehl,
C. M., Toon, G. C., Wennberg, P. O., Wunch, D., Blumenstock, T., Desmet, F.,
Feist, D. G., Heikkinen, P., De Mazière, M., Rettinger, M., Robinson, J.,
Schneider, M., Sherlock, V., Sussmann, R., Té, Y., Warneke, T., and
Weinzierl, C.: Calibration of sealed HCl cells used for TCCON instrumental
line shape monitoring, Atmos. Meas. Tech., 6, 3527–3537,
10.5194/amt-6-3527-2013, 2013.Kalnay, E., Kanamitsu, M.,
Kistler, R., Collins, W., Deaven, D., Gandin, L., Iredell, M., Saha, S.,
White, G., Woollen, J., Zhu, Y., Chelliah, M., Ebisuzaki, W., Higgins, W.,
Janowiak, J., Mo, K. C., Ropelewski, C., Wang, J., Leetmaa, A., Reynolds, R.,
Jenne, R., and Joseph, D.: The NCEP/NCAR 40-year reanalysis project, B. Am.
Meteorol. Soc. , 77, 437–471,
10.1175/1520-0477(1996)077<0437:tnyrp>2.0.co;2,
1996.Karion, A., Sweeney, C., Tans, P., and Newberger, T.: AirCore: an innovative
atmospheric sampling system, J. Atmos. Ocean. Tech., 27, 1839–1853,
10.1175/2010JTECHA1448.1, 2010.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, 10.1364/AO.46.004774, 2007.Mertz, L.: Auxiliary computation for Fourier spectrometry, Infrared Phys., 7, 17–23,
10.1016/0020-0891(67)90026-7, 1967.Messerschmidt, J., Macatangay, R., Notholt, J., Petri, C., Warneke, T., and
Weinzierl, C.: Side by side measurements of CO2 by ground-based Fourier
transform spectrometry (FTS), Tellus B, 62, 749–758,
10.1111/j.1600-0889.2010.00491.x, 2010.Olsen, S. C. and Randerson, J. T.: Differences between surface and column
atmospheric CO2 and implications for carbon cycle research, J. Geophys.
Res.-Atmos., 109, D02301, 10.1029/2003JD003968, 2004.Rayner, P. J. and O'Brien, D. M.: The utility of remotely sensed CO2
concentration data in surface source inversions, Geophys. Res. Lett., 28,
175–178, 10.1029/2000GL011912, 2001.Rodgers, C. D. and Connor, B. J.: Intercomparison of remote sounding
instruments, J. Geophys. Res.-Atmos., 108, 4116–4229,
10.1029/2002jd002299, 2003.Rothman, L. S., Gordon, I. E., Babikov, Y.,
Barbe, A., Benner, D. C., Bernath, P. F., Birk, M., Bizzocchi, L.,
Boudon, V., Brown, L. R., Campargue, A., Chance, K., Cohen, E. A.,
Coudert, L. H., Devi, V. M., Drouin, B. J., Fayt, A., Flaud, J. M.,
Gamache, R. R., Harrison, J. J., Hartmann, J. M., Hill, C., Hodges, J. T.,
Jacquemart, D., Jolly, A., Lamouroux, J., Le Roy, R. J., Li, G., Long, D. A.,
Lyulin, O. M., Mackie, C. J., Massie, S. T., Mikhailenko, S.,
Muller, H. S. P., Naumenko, O. V., Nikitin, A. V., Orphal, J., Perevalov, V.,
Perrin, A., Polovtseva, E. R., Richard, C., Smith, M. A. H., Starikova, E.,
Sung, K., Tashkun, S., Tennyson, J., Toon, G. C., Tyuterev, V. G., and
Wagner, G.: The HITRAN2012 molecular spectroscopic database, J. Quant.
Spectrosc. Ra., 130, 4–50, 10.1016/j.jqsrt.2013.07.002, 2013.Schaefer, H., Fletcher, S. E. M., Veidt, C., Lassey, K. R., Brailsford, G. W.,
Bromley, T. M., Dlugokencky, E. J., Michel, S. E., Miller, J. B., Levin, I., Lowe, D. C., Martin, R. J., Vaughn, B. H., and
White, J. W. C.: A 21st-century shift from fossil-fuel to biogenic methane
emissions indicated by 13CH4, Science, 352, 80–84,
10.1126/science.aad2705, 2016.Schepers, D., Butz, A., Hu, H., Hasekamp, O. P., Arnold, S. G., Schneider, M., Feist, D. G., Morino, I.,
Pollard, D., Aben, I., and Landgraf, J.: Methane and carbon dioxide total
column retrievals from cloudy GOSAT soundings over the oceans, J. Geophys.
Res.-Atmos., 121, 5031–5050, 10.1002/2015JD023389, 2016.Sherlock, V.,
Connor, B., Robinson, J., Shiona, H., Smale, D., and Pollard, D.: TCCON data
from Lauder, New Zealand, 120HR, Release GGG2014R0,
10.14291/tccon.ggg2014.lauder01.R0/1149293, TCCON data archive, hosted
by the Carbon Dioxide Information Analysis Center, Oak Ridge National
Laboratory, Oak Ridge, Tennessee, USA, 2014a.Sherlock, V.,
Connor, B., Robinson, J., Shiona, H., Smale, D., and Pollard, D.: TCCON data
from Lauder, New Zealand, 125HR, Release GGG2014R0,
10.14291/tccon.ggg2014.lauder02.R0/1149298, TCCON data archive, hosted
by the Carbon Dioxide Information Analysis Center, Oak Ridge National
Laboratory, Oak Ridge, Tennessee, USA, 2014b.Steinkamp, K., Mikaloff Fletcher, S. E., Brailsford, G., Smale, D., Moore, S., Keller, E. D., Baisden,
W. T., Mukai, H., and Stephens, B. B.: Atmospheric CO2 observations and
models suggest strong carbon uptake by forests in New Zealand, Atmos. Chem.
Phys., 17, 47–76, 10.5194/acp-17-47-2017, 2017.Toon, G. C.: Telluric line list for GGG2014, available at:
10.14291/tccon.ggg2014.atm.R0/1221656, last access: 2 May 2014a.Toon, G. C.: Solar line list for GGG2014, available at:
10.14291/tccon.ggg2014.solar.R0/1221658, last access: 2 May 2014b.Toon, G. C. and Wunch, D.: A stand-alone a priori profile generation tool
for GGG2014, available at: 10.14291/tccon.ggg2014.priors.R0/1221661
(last access: 15 June 2016), 2014.Wofsy, S. C.: HIAPER Pole-to-Pole Observations (HIPPO): fine-grained, global-scale measurements of
climatically important atmospheric gases and aerosols, Philos. T. R. Soc. A,
369, 2073–2086, 10.1098/rsta.2010.0313, 2011.Wunch, D., Toon, G. C., Wennberg, P. O., Wofsy, S. C., Stephens, B. B.,
Fischer, M. L., Uchino, O., Abshire, J. B., Bernath, P., Biraud, S. C.,
Blavier, J.-F. L., Boone, C., Bowman, K. P., Browell, E. V., Campos, T.,
Connor, B. J., Daube, B. C., Deutscher, N. M., Diao, M., Elkins, J. W.,
Gerbig, C., Gottlieb, E., Griffith, D. W. T., Hurst, D. F., Jiménez, R.,
Keppel-Aleks, G., Kort, E. A., Macatangay, R., Machida, T., Matsueda, H.,
Moore, F., Morino, I., Park, S., Robinson, J., Roehl, C. M., Sawa, Y.,
Sherlock, V., Sweeney, C., Tanaka, T., and Zondlo, M. A.: Calibration of the
Total Carbon Column Observing Network using aircraft profile data, Atmos.
Meas. Tech., 3, 1351–1362, 10.5194/amt-3-1351-2010, 2010.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, Philos. T. R. Soc. A, 369, 2087–2112,
10.1098/rsta.2010.0240, 2011a.Wunch, D., Wennberg, P. O., Toon, G. C., Connor, B. J.,
Fisher, B., Osterman, G. B., Frankenberg, C., Mandrake, L., O'Dell, C.,
Ahonen, P., Biraud, S. C., Castano, R., Cressie, N., Crisp, D., Deutscher, N.
M., Eldering, A., Fisher, M. L., Griffith, D. W. T., Gunson, M., Heikkinen,
P., Keppel-Aleks, G., Kyrö, E., Lindenmaier, R., Macatangay, R.,
Mendonca, J., Messerschmidt, J., Miller, C. E., Morino, I., Notholt, J.,
Oyafuso, F. A., Rettinger, M., Robinson, J., Roehl, C. M., Salawitch, R. J.,
Sherlock, V., Strong, K., Sussmann, R., Tanaka, T., Thompson, D. R., Uchino,
O., Warneke, T., and Wofsy, S. C.: A method for evaluating bias in global
measurements of CO2 total columns from space, Atmos. Chem. Phys., 11,
12317–12337, 10.5194/acp-11-12317-2011, 2011b.Wunch, D., Toon, G., Sherlock, V.,
Deutscher, N. M., Liu, C., Feist, D. G., and Wennberg, P. O.: The Total
Carbon Column Observing Network's GGG2014 Data Version, available at:
10.14291/tccon.ggg2014.documentation.R0/1221662, last access: 16
December 2015.Wunch, D., Wennberg, P. O., Osterman, G., Fisher, B., Naylor, B., Roehl, C.
M., O'Dell, C., Mandrake, L., Viatte, C., Kiel, M., Griffith, D. W. T.,
Deutscher, N. M., Velazco, V. A., Notholt, J., Warneke, T., Petri, C., De
Maziere, M., Sha, M. K., Sussmann, R., Rettinger, M., Pollard, D., Robinson,
J., Morino, I., Uchino, O., Hase, F., Blumenstock, T., Feist, D. G., Arnold,
S. G., Strong, K., Mendonca, J., Kivi, R., Heikkinen, P., Iraci, L.,
Podolske, J., Hillyard, P. W., Kawakami, S., Dubey, M. K., Parker, H. A.,
Sepulveda, E., García, O. E., Te, Y., Jeseck, P., Gunson, M. R., Crisp,
D., and Eldering, A.: Comparisons of the Orbiting Carbon Observatory-2
(OCO-2) XCO2 measurements with TCCON, Atmos. Meas. Tech., 10,
2209–2238, 10.5194/amt-10-2209-2017, 2017.Yokota, T., Yoshida, Y.,
Eguchi, N., Ota, Y., Tanaka, T., Watanabe, H., and Maksyutov, S.: Global
concentrations of CO2 and CH4 retrieved from GOSAT: first
preliminary results, Sola, 5, 160–163, 10.2151/sola.2009-041, 2009.