We present observing system simulation experiments (OSSEs)
to evaluate the impact of a proposed network of ground-based
miniaturized laser heterodyne radiometer (mini-LHR) instruments that measure
atmospheric column-averaged carbon dioxide (XCO2) with a 1 ppm
precision. A particular strength of this passive measurement approach is its
insensitivity to clouds and aerosols due to its direct sun pointing and
narrow field of view (0.2∘). Developed at the NASA Goddard Space Flight
Center (GSFC), these portable, low-cost mini-LHR instruments were designed
to operate in tandem with the sun photometers used by the AErosol RObotic
NETwork (AERONET). This partnership allows us to leverage the existing
framework of AERONET's global ground network of more than 500 sites as well as
providing
simultaneous measurements of aerosols that are known to be a major source of
error in retrievals of XCO2 from passive nadir-viewing satellite
observations. We show, using the global 3-D GEOS-Chem chemistry transport
model, that a deployment of 50 mini-LHRs at strategic (but not optimized)
AERONET sites significantly improves our knowledge of global and regional
land-based CO2 fluxes. This improvement varies seasonally and ranges
58 %–81 % over southern lands, 47 %–76 % over tropical lands, 71 %–92 %
over northern lands, and 64 %–91 % globally. We also show significant added
value from combining mini-LHR instruments with the existing ground-based
NOAA flask network. Collectively, these data result in improved a posteriori
CO2 flux estimates on spatial scales of ∼10 km2,
especially over North America and Europe, where the ground-based networks are
densest. Our studies suggest that the mini-LHR network could also play a
substantive role in reducing carbon flux uncertainty in Arctic and tropical
systems by filling in geographical gaps in measurements left by ground-based
networks and space-based observations. A realized network would also provide
necessary data for the quinquennial global stocktakes that form part of the
Paris Agreement.
Introduction
Two recent satellite instruments have made significant contributions to
globally characterizing XCO2: the Japanese Greenhouse gases
Observing SATellite (GOSAT) or “IBUKI”
launched in 2009 (Kuze et al., 2009) and the Orbiting Carbon Observatory-2 (OCO-2; Crisp et al., 2017;
Eldering et al., 2017) launched in 2014. Both the Fourier transform
spectrometer (FTS) in GOSAT and the grating spectrometer in OCO-2 have
multiple viewing geometries (nadir, glint, and target) to observe absorption
of XCO2, but OCO-2 offers significant improvements in global surface
coverage. While GOSAT and OCO-2 have made important advances in observing
greenhouse gases from space, any uncharacterized systematic errors can
compromise the accuracy of their data (Wunch et al., 2017) and limit the
utility of such data sets for inferring surface flux distributions
(Basu et al., 2013). Ground-based networks of
accurate and precise XCO2 measurements such as Total Carbon Column Observing Network
(TCCON)
(Wunch et al., 2017) therefore play an important role in helping to validate these space-borne
missions. We describe how we can improve knowledge of the carbon cycle by
establishing a network of low-cost, portable mini-LHR (miniaturized laser
heterodyne radiometer) instruments that measure XCO2 to fill in gaps
left by existing column ground-based networks and space-borne observations.
These instruments can be quickly deployed (to be collecting data within a
few hours) and can run autonomously in the field with little or no
maintenance over a period of months or years.
Ground-based, broad spectral column measurements of XCO2 from the TCCON
FTS network have been used to minimize
regional systematic errors and serve as a gold standard to validate
satellite measurements. In 2010, the TCCON FTS instruments reported an
accuracy of ∼1 ppm due to bias errors from uncertainties in
spectroscopic parameters (Wunch et
al., 2010). They resolved this limitation at five of their sites by tying
their column-averaged dry-air mole fractions to the World Meteorological
Organization (WMO) in situ trace gas measurement scales using aircraft profiles
and indicated that they planned to eventually perform similar calibrations
at the remainder of their sites. While TCCON products are well
characterized, the majority of the 32 TCCON sites are in the Northern
Hemisphere, leaving important monitoring gaps in regions where our knowledge
of the drivers of carbon cycling is uncertain (Shuur et al., 2008; Commane
et al., 2017; Saunois et al., 2016; Le Quéré et al., 2016).
The NASA mini-LHRs are designed to be deployed in tandem with AErosol RObotic
NETwork (AERONET) sun
photometers (Holben et al., 1998), taking advantage of
their sun trackers. This partnership provides a pathway for establishing a
global network of mini-LHRs by leveraging AERONET's network of over 500 sites and
offers a simultaneous measure of aerosol optical depth (AOD) that is a
necessary input for satellite retrievals (Butz et al., 2009). Similar to
TCCON, mini-LHRs can collect data during breaks in cloud coverage, thereby
offering the potential for new data products in formerly under-represented
regions such as the Amazon Basin, southern Asian monsoon areas, and the
Arctic. These vulnerable geographic regions are not well covered by OCO-2
and GOSAT. Here, using numerical experiments, we simulate a strategic (but
not optimized) deployment of 50 mini-LHR instruments to AERONET sites and
evaluate how this increase in measurement density impacts knowledge of
regional and global carbon fluxes.
Mini-LHR instrument configuration
The mini-LHR is a ground-based, passive, sun-viewing instrument that
observes trace gases in the atmospheric column. It has been under
development at the NASA Goddard Space Flight Center (GSFC) since 2009 (Melroy
et al., 2015; Clarke et al., 2014; Wilson et al., 2014; Wilson and McLinden, 2012),
and while earlier versions exclusively measured
XCO2, the current version observes both XCO2 and XCH4.
Current challenges associated with our understanding of emissions of CH4
(Wolf et al., 2017) will result in a different network design. The
mini-LHR has been tested at altitudes ranging from sea level to 3400 m
and in climates that include tropical, subtropical, and temperate
zones, extending to just below the Arctic Circle, and has shown consistent
precisions of 1 ppm in XCO2 and 10 ppb in XCH4
for hourly data products. Figure 1 shows a mini-LHR monitoring XCO2 and XCH4
over thawing permafrost
at a remote site in the Bonanza Creek Research Forest near
Fairbanks, Alaska. The goal of these field tests was to both improve the
quality of the data product as well as to test the durability of commercial
components that were intended for indoor lab use.
The mini-LHR (right) is portable and can be deployed to remote
locations where larger TCCON installations are not possible due to the fragile
ground conditions. Shown here are the mini-LHR monitors XCO2 and XCH4
alongside an eddy-covariance tower (left) in a collapse-scar bog permafrost
site in Alaska.
The mini-LHR measures XCO2 by scanning the CO2 absorption feature
near 1.61 µm. Figure 2 shows the current configuration of the system, and
Table 1 lists key system parameters. Sunlight is collected with a
fibre-coupled, 0.2∘ field-of-view collimator that is non-invasively
connected to an AERONET sun tracker. Once collected, sunlight is modulated
with a fibre switch, superimposed with infrared laser light from a
distributive feedback laser in a single-mode fibre coupler, and then mixed
in a fast photoreceiver and InGaAs
detector to produce an radio frequency (RF) beat signal. The
RF receiver separates RF and direct current (DC) outputs, and the RF signal is amplified,
filtered, and then detected with a square-law detector. The resulting signal
is measured with a lock-in amplifier referenced to the fibre switch
frequency as the laser scans across an absorption feature. A microprocessor
controls the laser scanning and data collection. The mini-LHR has spectral
sampling resolution of ∼0.013 cm-1, which is 15 times
higher than GOSAT (∼0.2 cm-1), 20 times higher than
OCO-2 (∼0.3 cm-1), and slightly higher than TCCON
(∼0.02 cm-1). Individual scans of the CO2 feature
are collected at 2 min intervals throughout the day during sunlight hours
when clouds are not present and averaged into hourly data products.
Parameters used to calculate mini-LHR averaging kernels for the Bonanza
Creek site. The molecules analysed in the 72-layer atmosphere included
H2O, CO2, O3, N2O, CO, CH4,
O2, and N2, and the level of variability was 0.05.
ParameterValueUnitsInstrument lower wavelength1.61137µmInstrument upper wavelength1.641165µmInstrument resolution at FWHM0.000003µmInstrument's data SNR500dA priori variance5%Zenith angle range0-60∘Latitude64+42.055/60.0∘ NLongitude360-(148+18.763/60.0)∘ EDate and time of observations to extract MERRA values30/05/2016, 19:15 UTdd/mm/yyyy, hh:mm
Schematic of a mini-LHR. Sunlight is collected with collection optics
that are non-invasively connected to the AERONET sun tracker. Sunlight is then
modulated with a fibre switch, superimposed with infrared laser light from a
distributive feedback laser in a single-mode fibre coupler, and mixed in a fast
photoreceiver and InGaAs detector to produce a radio frequency (RF) beat signal.
In the RF receiver (custom), a bias tee separates RF and DC outputs. The
RF signal passes through a gain stage and is then detected with a square-law
detector. The signal is measured with a lock-in amplifier that is referenced
to the modulation frequency as the laser scans across an absorption feature.
A microprocessor controls the laser scanning and data collection.
Data processing and retrieval
Averaged absorption scans are analysed to extract column mole fractions of
CO2 using custom analysis software developed at GSFC that is similar to
the approach used by TCCON. There are two main steps involved in processing
data: (1) simulating the spectra (mathematically simulating what the
mini-LHR observes in the atmosphere) and (2) fitting the simulation to the
data to extract the abundance of XCO2.
We simulate the spectra using the Planetary Spectrum Generator (PSG),
which is an online tool developed at NASA–GSFC (Villanueva et al., 2015, 2016)
for synthesizing Earth and planetary spectra (atmospheres and
surfaces) for a broad range of wavelengths (0.1 µm to 100 mm,
spanning UV to radio wavelengths)
from any observatory, orbiter, or lander. This is achieved by combining several state-of-the-art radiative
transfer models, spectroscopic databases, and planetary databases. The PSG
code includes refraction of sunlight through the atmosphere as well as a
computationally efficient scattering package that incorporates the latest
radiative transfer numerical methods (Villanueva et al., 2015; Smith et al.,
2009) and is parameterized for LTE (local thermodynamic equilibrium)
calculations. While scattering is not required for direct sun-viewing
measurements, the scattering package contains a treatment of aerosols; the
extinction portion of this treatment
is needed to properly model the continuum shape.
The PSG is operated remotely by employing a versatile online application
program interface (API). The API operates by sending a configuration file to
the PSG servers. Upon reception of the configuration file, the PSG computes and
returns the spectra.
Part of this simulation includes the Modern-Era Retrospective analysis for
Research and Applications, Version 2 (MERRA-2) data set, which provides
meteorological inputs (Reichle et al.,
2011; Rienecker et al., 2011) and provides a 72-layer model of the atmosphere. The
retrieval employs the MERRA-2 database to define the state and a priori values for the
atmosphere. We “perturb” the CO2 profile by a scaler, which is the
value that is actually being retrieved by the retrieval algorithm. MERRA
is the Modern-Era Retrospective Analysis for Research and Applications
database, which is the latest atmospheric reanalysis of the modern satellite
era produced by NASA's Global Modeling and Assimilation Office. MERRA has incorporated information from hundreds of orbiters and ground stations since
1980 and provides global three-dimensional analyses
of atmospheric parameters (e.g. temperature, abundance profiles, and aerosols). Specifically, our retrieval
works with the M2I3NVASM component, which provides assimilated
meteorological fields (pressure, temperature, water vapour, ozone, and water
ice clouds) from the surface to ∼80 km (72 layers), with a
cadence of 180 min and spatial resolution of ∼0.5∘ (576×361).
The values are further refined temporally and spatially
to a resolution of better than 1 km, employing the USGS-GTOPO30 topographic
maps and considering a hydrostatic equilibrated atmosphere within every bin.
Our code computes temperature (T) and pressure (P) abundances for Earth by
first selecting a set of six standard profiles based on season and
latitude: “Tropical”, “Midlatitude-Summer”, “Midlatitude-Winter”,
“Subarctic-Summer”, “Subarctic-Winter”, and “US-Standard”
(Anderson et al., 1986). These profiles provide abundances
for a myriad of species and basic temperature and pressure profiles. The
code then extracts P, T, O3, H2O, and water ice abundances from the
MERRA-2 database for this location and time. The MERRA-2 grid is described
on a coarse grid, and it does not contain fine elevation information;
therefore the GTOPO30 topography database (∼1 km resolution)
is also used to derive the exact elevation of the mini-LHR site location.
The information from MERRA-2 at a particular geolocation is then refined in
elevation, e.g. using scale heights, using this high-resolution topographic map.
Our code generates an initial configuration file that establishes the
location, date, and time of the measurement. Using this configuration file,
the code calls the PSG–API, and this returns all of the geometry parameters
(air mass, phase angle, etc.) and an a priori vertical profile based on the date and
location. Then, using this configuration file, the program goes into the
fitting routine that calls the PSG–API to calculate spectra by fitting the
CO2 abundance using an optimal estimation approach. The fit perturbs
the CO2 abundance and obtains a fit based on the Levenberg–Marquardt
algorithm, which is an iterative least-squares curve-fitting procedure.
Calibration and validation of mini-LHR data
Mini-LHR instruments periodically undergo a calibration–validation procedure
at NASA–GSFC to track performance and establish documented traceability of
column data products. In particular, we calculate and report measurement
precision, measurement error, and measurement bias, as defined by the
Vocabulaire International de Métrologie (VIM; Meaures, 2012).
We estimate measurement precision (standard deviation) by routine laboratory
calibrations. In the calibration procedure, the mini-LHR instrument scans a
NIST (National Institute of Standards and Technology) traceable atmospheric
mixture of gases (NIST Traceable Reference Material Program for Gas
Standards, 10.6028/NIST.SP.260-126rev2013) in a 36 m Herriot absorption cell. The NIST traceable
atmospheric mixture of gases fulfils the criteria of a measurement standard
with a negligible measurement uncertainty. The calibration gas standards are
specified in the Code of Federal Regulations (CFR) to calibrate instruments
used to monitor regulated emissions. The standard deviation of the points in
the scan provides an estimate of precision, while regular calibrations track
any measurement bias (systematic measurement error).
To assess accuracy of the mini-LHR during its development, two
short-duration side-by-side comparisons were completed between the mini-LHR
and TCCON stations at Park Falls, Wisconsin, in 2012 and at Caltech in
Pasadena, California, in 2014. Sample scans from these comparisons are shown in
Fig. 3 with fits from the PSG retrieval. Data from 2014 showed a significant
improvement in agreement with the TCCON CO2 measurements because it was
possible to collect more scans within a shorter timeframe; the 2012 data are
the average of three scans collected over the period of 1 h and the
2014 data are the average of five scans collected over the period of a half
hour. A longer-duration side-by-side comparison is planned with the TCCON
located at the NASA Armstrong Flight Research Center (AFRC). In addition to
laboratory calibrations, potential bias between mini-LHR instruments will
also be addressed by regularly comparing a “standard” mini-LHR that is
co-located at the NASA–AFRC TCCON with all other mini-LHR instruments. TCCON
FTS instruments measure column CH4 and CO2 at the same wavelengths
but at lower resolution than the mini-LHR. While TCCON has a well-documented
history of characterization, we refer to this as an “estimate” of
measurement error due to differences in resolution and because there are
known biases between TCCON sites (1 % for CO2 at US sites and
1.1±0.2 % at European sites; Wunch et al., 2010).
Side-by-side comparisons of mini-LHR and TCCON instruments
(a) at Park Falls on 16 September 2012 and (b) at Caltech on
7 March 2014. Mini-LHR data are shown in blue with the PSG retrieval fit in
orange. The 2012 data are the average of three scans collected over the period
of an hour, and the 2014 data are the average of five scans collected over the
period of a half hour. The resulting XCO2 value for the mini-LHR and
the nearest corresponding value for TCCON are shown in the inset.
For passive satellite observations, scattering from clouds and aerosols is
known to be a significant source of retrieval error for XCO2 (Mao
and Kawa, 2004; Aben et al., 2007; Uchino et al., 2012; Yoshida et al., 2013).
This is primarily because these are nadir-pointing instruments that view
sunlight reflected on a portion of ground that is illuminated by direct
sunlight as well as scattered sunlight from clouds and aerosols.
In contrast, ground-passive measurements have a narrow field of view
(FOV) and point directly at the sun. The TCCON at Park Falls, Wisconsin, for example, has
an FOV of ∼0.14∘, and mini-LHRs have an FOV
of ∼0.2∘ (compared to the sun which has a field of view
of ∼0.5∘). Because the FOVs of these instruments are
narrower than those of the sun, their light collection optics do not accept
the scattered light outside of this FOV. Consequently, the mini-LHR and
TCCON are mainly impacted by extinction, resulting in lower levels of
sunlight reaching these ground instruments and lower signal-to-noise levels.
Solar intensity variations that impact TCCON are corrected by dividing the
interferograms by the unmodulated DC signal (Keppel-Aleks et al., 2007). For the mini-LHR,
transmittance scans are corrected for extinction by dividing scans by a
fitted baseline that tracks fluctuations in solar irradiance.
Theoretical potential of network to improve knowledge of regional carbon fluxes
We use numerical experiments to provide an upper limit on the theoretical
potential of the proposed network on reducing the uncertainty of regional
carbon flux estimates. The approach we take is to define a closed-loop
experiment, often called observing system simulation experiments (OSSEs), in
which we define the true atmospheric state using a global 3-D model of
atmospheric chemistry and transport driven by true surface fluxes. This true
atmospheric state is then sampled as it would be by the mini-LHRs (e.g.
time, location, and vertical sensitivity). We then generate a complementary
set of model values that are generated from an independent surface flux
inventory, including differences in the magnitude and distribution of
fluxes; we use this independent inventory as our a priori for the OSSEs. We infer
the a posteriori fluxes from measurements using an ensemble Kalman filter.
We use version 9.02 of the GEOS-Chem model of atmospheric chemistry and transport
(http://geos-chem.org, last access: 24 April 2019) driven by GEOS-5-analysed meteorological fields that
include a simulation of atmospheric CO2 that has been evaluated with a
range of ground-based, aircraft, and satellite observations (Feng et al.,
2009, 2011, and 2016). For our experiments, we use the
model at a horizontal resolution of 4∘ latitude × 5∘ longitude
for an arbitrary year, which is 2014 in our experiments. We use monthly
ODIAC fossil fuel emissions (Oda and Maksyutov, 2011), monthly ocean biosphere fluxes
(Takahashi et al., 2009), and weekly biomass burning emissions from the Global Fire Emission Database
(GFEDv3; van der Werf et al., 2010).
Distribution of land biosphere CO2 fluxes (1014 molec cm-2 s-1)
for January (a, c, e) and July (b, d, f) in our study year,
described on the 4∘ latitude × 5∘ longitude GEOS-Chem
model grid. (a, b) show the CASA model that we used as our a priori,
(c, d) show output from the ORCHIDEE land surface model that we use
to define the true state, and (e, f) show the difference between
ORCHIDEE and the CASA model. The cyan triangles represent observation sites
from (1) current and future TCCON networks (a, b), (2) the enhanced
mini-LHR network (c, d), and (3) the subset of the NOAA ground-based network (e, f).
Currently, it is common practice to assume that most of the uncertainty in
atmospheric CO2 stems from natural fluxes, so other sources are
typically assumed to be well described by existing inventories (e.g. Gurney
et al., 2003). While this practice is slowly being challenged by the
community, we retain these assumptions for the purpose of our theoretical
calculations. To define our true atmospheric state, we use three-hourly land
biosphere fluxes from the ORCHIDEE land surface model (Krinner et al., 2005);
in a separate model calculation, to define our a priori state, we use three-hourly
land biosphere fluxes from CASA (Olsen and Randerson, 2004). Figure 4
shows that there are significant seasonal differences in the magnitudes and
distributions of ORCHIDEE and CASA land biosphere CO2 fluxes so that
our OSSE provides a rigorous test of the theoretical data.
For the purposes of inter-comparability of impacts of different data, we
have ignored any source of systematic error in the different measurements or
the transport model that links a priori information to 4-D atmospheric mole
fractions of CO2. Even sub-parts-per-million levels of uncharacterized systematic
error in atmospheric measurements will significantly compromise our ability
to infer unbiased regional CO2 flux estimates.
Our retrieval simulation requires a matrix of averaging kernels (A)
that describes the sensitivity of the retrieved state vector x^ (in
this case, a vector describing the vertical profile of CO2 described
over n atmospheric layers) to the “true” state vector x, for
different values of solar zenith angle throughout the day. Using the
standard convention, upper-case and lower-case emboldened variables denote a
matrix and vector, respectively, and superscripts -1 and T denote matrix
inverse and transpose operations, respectively. The averaging kernel is
calculated as (Rodgers, 2000; Liuzzi et al., 2016)
A=∂x^∂x=Sa-1+KTSϵ-1K-1KTSϵ-1K,
where Sa is the a priori error covariance matrix (size
n×n),
Sϵ is the measurement error covariance
matrix (size m×m, where m denotes the length of the radiance vector), and
K is the matrix of weighting functions that describes the derivative of
the radiance with respect to a change in the CO2 profile (size m×n).
The matrix of averaging kernel is used here to describe the instrument
sensitivity to changes in CO2 so that we can simulate mini-LHR XCO2
column measurements in GEOS-Chem. For simplicity, we assume that
Sa and Sϵ are diagonal
and represent the square of the background variability of CO2
concentration in each atmospheric layer and the square of the instrument noise, respectively.
The sum of the rows of A corresponds to summing the retrieval
sensitivities to the CO2 in each atmosphere layer and describes the
sensitivity of the atmospheric column to a change in atmospheric CO2 in
the vertical profile, i.e. the column averaging kernel a. In the
ideal case, each summed row would be close to unity. Figure 5 shows that for a
variety of solar zenith angles, a >0.8 in the troposphere but
falls off quickly in the stratosphere, consistent with TCCON averaging
kernels (Wunch et al., 2011).
Averaging kernel (AK^) values for CO2 for
different values of solar zenith angles. Calculations for CO2 have
been done assuming a mini-LHR SNR of 500 (although in reality SNR varies with
solar zenith angle (SZA) and is highest near an
SZA of 0) and assuming a background variability of 5 %.
We also calculate the number of degrees of freedom (DOF) of the retrieval as
the trace of A, which estimates the number of independent pieces of
information that can be derived from retrieval. In the case of the column
XCO2 retrieval, this should be close to or higher than 1; values less
than 1 indicate the influence of a priori information
(Camy-Peyret et al., 2017). We find that with an
signal-to-noise ratio (SNR)
value of 500 and an assumed a priori variance of 5 %, for CO2,
concentrations result in a DOF between 0.88 and 2.10.
To infer regional fluxes of CO2 from the measurements, we use an
established ensemble Kalman filter approach (Feng et al., 2009,
2016, 2017). For brevity, we refer the reader to Feng et al. (2009) for a detailed description of the approach and its application within
GEOS-Chem. We adopt a uniform 50% a priori uncertainty and assume a conservative
1.5 ppm for individual measurement and model transport errors. To
characterize the impact of the mini-LHR measurements on the a priori knowledge we
use a metric that describes how uncertainty of fluxes has reduced after the
a priori has been informed by the following measurements:
γ=1-Scii/Sdii, where
Scii and Sdii denote the
diagonal elements of the a posteriori and a priori CO2 flux error covariance matrices,
respectively. A larger value for γ denotes a larger scientific
impact of the observations. We also report comparisons between true, a priori, and
a posteriori CO2 fluxes over key geographical regions. Together, our use of two
independent land biosphere flux inventories, the γ metric and the
inter-comparison of fluxes provide a transparent theoretical assessment of
the LHR data to quantify geographical fluxes of CO2.
We performed three experiments to estimate true CO2 fluxes using the following:
(1) TCCON measurements with their current measurement configuration (Fig. 4),
(2) mini-LHR measurements collected at an enhanced number (50) of sites, and
(3) the combined data sets from mini-LHR measurements and selected surface flask
sites to study the added value of mini-LHR data to the existing NOAA Earth
System Research Laboratory ground-based network of mole fraction data that
is commonly used to infer regional CO2 fluxes (e.g. Peylin et al.,
2013). We conservatively assume a mini-LHR measurement precision of 1.5 ppm
in our experiments; however, the current precisions for the mini-LHR and
TCCON data products are <1 ppm for 1 h data products (Wunch
et al., 2010; Messerschmidt et al., 2011; Wilson et al., 2017; Melroy et al.,
2015). Instrument biases were not included in these OSSE runs because our
focus is the relative performance of different ground-based remote-sensing networks.
Table 2 lists the proposed enhanced distribution of mini-LHR instruments.
These sites were initially chosen to target regions where AERONET sites
already existed and where there are gaps in the existing in situ measurements, TCCON
measurements, and satellite observations. Consideration was also given to
accessibility, acknowledging evolving political environments. Consequently,
the enhanced network has not been optimized to minimize carbon flux uncertainties.
Table 3 reports the TCCON sites used in our numerical experiments. We
simulate TCCON XCO2 observations using the same approach as we use for the
mini-LHR instruments. For TCCON, we use CO2 averaging kernels from the
latest TCCON XCO2 retrievals of version GCC2014 (Wunch et al., 2011).
Table 4 shows the surface flask sites used in our joint assimilation
experiment. They are a subset of the NOAA ground-based network and are
chosen mainly based on their data continuity in recent years (i.e.
2009–2016). In our experiments, we use the real availability of the flask
data in the compiled surface data set (GLOBALVIEW-v3.2) while simulating
the observation values by sampling model surface CO2 concentrations at
the observation location and time.
Figure 6 compares the annual mean deviation between true (ORCHIDEE) and a posteriori
fluxes inferred from the TCCON, the enhanced mini-LHR network, and the
mini-LHR and NOAA flask observations. We acknowledge that the primary purpose
of TCCON is to provide a ground truth for satellite observations, which are not
optimized for surface flux estimation. Nevertheless, we find that the
current TCCON network can generally reduce the systematic bias between the
a priori and the true state, particularly over the northern mid-latitude land region that
reflects observation coverage. As expected, the uneven and coarse coverage
over tropical and southern land regions results in a large-scale dipole
effect between tropical South America and other tropical and extratropical
regions, e.g. Australia. Sensitivity experiments (not shown) show that the
performance of the mini-LHR instruments distributed using the current TCCON
measurement configuration is comparable with the TCCON instruments, despite
larger assumed random errors. We find this is primarily due to the
comparable role of instrument and atmospheric transport model errors
(1.5 ppm), particularly at the 4∘ latitude × 5∘ longitude horizontal
resolution employed here. Using finer-scale meteorology could very well
reduce model error, but knowledge of this error is poorly defined, with no
robust quantitative method currently available.
Distribution of annual mean residuals that describe the true state minus
a priori and a posteriori CO2 fluxes (1014 molec cm-2 s-1),
described on the 4∘ latitude × 5∘ longitude
GEOS-Chem model grid.
List of 50 selected AERONET and NASA sites with mini-LHR installed in the
future for the OSSE study.
Distribution of annual mean root-mean-square errors between the true
state and a priori and a posteriori CO2 fluxes
(1014 molec cm-2 s-1), described on the 4∘
latitude × 5∘ longitude GEOS-Chem model grid.
TCCON stations used in this OSSE study.
SiteLatitudeLongitudeSiteLatitudeLongitude(∘)(∘)(∘)(∘)Arrival Heights, Antarctica-77.8166.7Karlsruhe, Germany49.18.4Anmyeondo, South Korea36.5126.3Lamont, OK, USA36.6-97.5Ascension Island-7.9-14.3Lauder, New Zealand-45.0169.7Bialystok, Poland53.223.0Los Alamos, NM, USA35.87-106.32Bremen, Germany53.18.9Ny Alesund, Spitsbergen, Norway78.911.9Burgos, Philippines18.5120.7Orleans, France48.02.1Caltech, CA, USA34.1-118.1Paris, France48.82.4Darwin, Australia-12.4130.9Park Falls, WI, USA45.9-90.3Darwin, Australia-12.5130.9Reunion Island-20.955.5Dryden, NY, USA35.0-117.9Rikubetsu, Japan43.5143.8East Trout Lake, Canada54.4-105.0Saga, Japan33.2130.3Eureka, Canada80.1-86.4Sodankyla, Finland67.426.6Garmisch, Germany47.511.1Tsukuba, Japan36.1140.1Harwell, Oxfordshire, UK51.6-1.32Wollongong, Australia-34.4150.9Hefei, China31.90118.67Yekaterinburg, Russia57.0459.55Izana, Tenerife, Spain28.3-16.5Zugspitze, Germany47.411.0
A posteriori fluxes inferred from the enhanced mini-LHR network significantly improve
agreement with true fluxes compared to TCCON measurement configuration,
particularly over tropical land regions. Annual mean deviations are
generally smaller than 2×1013 molec cm-2 s-1 and vary on
the grid scale throughout the year for many continental regions. We find
that including the NOAA flask data helps with reducing these variations, in
particular over North America and Europe, where the coverage by the surface
data is densest (Fig. 6). Figure 7 compares the root-mean-square error (RMSE)
for our three inversion experiments relative to the true state.
First, all our inversions show much smaller deviations compared to the a priori
values. The enhanced mini-LHR network performs significantly better over
tropical lands such as tropical South America and tropical North Africa.
True, a priori, and a posteriori monthly CO2
fluxes (GtC yr-1), described over large-scale geographical regions for
our study year. Vertical lines and the grey shading
denote uncertainties associated with the a priori fluxes and the a posteriori
fluxes inferred from the mini-LHR and NOAA in situ flask networks, respectively.
Annual mean root-mean-square errors (RMSEs; GtC yr-1) associated
with (red) a priori and a posteriori flux estimates inferred
from (green) TCCON and (blue) mini-LHR CO2 columns and from the (black)
combined mini-LHR–NOAA in situ flask networks.
Figures 8 and 9 summarize the agreement between a priori, true, and a posteriori CO2 fluxes
from our three inversions for hemispheric-scale land regions. TCCON broadly
reproduces true fluxes, but the current TCCON configuration is insensitive
to some geographical regions (e.g. tropical South America and North Africa),
as expected. Fluxes inferred from data from the mini-LHR network,
independently and in combination with NOAA flask data, are closer to the
true state over most parts of the world, e.g. north land summer months, as
expected. Figures 8 and 9 also show that on the large spatial scales we have
studied, the NOAA flask data provide a modest amount of additional
information to the mini-LHR network. This suggests that a ground-based
remote-sensing network that provides calibrated, high-frequency observations
of column CO2 has comparable performance with the existing in situ network on
larger continental spatial scales.
Theoretical improvement in the knowledge of CO2 fluxes for a
nominal (a, c, e) January and (b, d, f) July as determined by
the γ factor, defined in the main text, for the (a, b) mini-LHR,
(c, d) TCCON, and (e, f) combined mini-LHR–NOAA
in situ flask measurement networks.
Figure 10 shows that the mini-LHR enhanced network of 50 sites results in
global and significant improvements in our knowledge of CO2 fluxes.
Significant values of the error reduction γ are found over most of
North America and Eurasia as well as over South America and central and
southern Africa. There is a similar geographical distribution of
improvements during boreal summer months, but with larger values over North
America and Eurasia, including the northernmost latitudes. Similar
calculations for the TCCON network show comparable levels of improvement but
are more spatially limited, particularly over the Northern Hemisphere. We
show there is clear value in combining in situ flask data with the mini-LHR
network, with significant improvements in CO2 fluxes, particularly over
North American and Eurasia.
The mobility of mini-LHR sensors allows us to locate them in remote
environments where an AERONET site is already established. This includes, in
particular, tropical ecosystems where the physical environment is
challenging for large-scale instrument installations and at polar latitudes
where space-borne measurements are compromised because of low solar
illumination and low surface reflectance over snow or ice. This suggests that
the mini-LHR network could play a substantive role in an Arctic monitoring
network, particularly during spring and autumn months.
Our results demonstrate the complementarity of the mini-LHR and in situ flask
networks. Calibrating sensors from the TCCON and mini-LHR networks represent
additional observational constraints on CO2 fluxes that rival knowledge
inferred from the current in situ observation network over large-scale geographical
regions (e.g. North America and Eurasia) and outperform for other regions
(e.g. tropics). The in situ networks provide an invaluable record on the changing
carbon cycle by putting present-day changes in a historical context, whose
value is reduced if they are terminated.
Concluding remarks
The development of the mini-LHR technology is ongoing, but this computational
study already builds on a growing body of work that has characterized its
error budgets and its in-field performance (Wilson et al., 2013;
Clarke et al., 2014; Melroy et al., 2015).
With a modest deployment of mini-LHR instruments to 50 sites, numerical
experiments with the GEOS-Chem model indicate that the resulting XCO2
data products lead to improvements of carbon flux uncertainties ranging from
58 % to 81 % over southern lands, 47 % to 76 % over tropical lands,
71 % to 92 % over northern lands, and 64 % to 91 % globally. Because
mini-LHRs leverage AERONET's global network of more than 500 sites
worldwide, additional instruments can be rapidly added to target specific
areas of uncertainty, such as thawing permafrost emissions in the Arctic or
tropical ecosystems in the mid-latitudes. In addition to infrastructure,
co-location of these instruments provides a simultaneous measurement of
aerosol optical depth which is necessary for evaluating and correcting aerosol
scattering effects in XCO2 satellite retrievals and consequent
uncertainty in local and regional carbon flux estimates. Sun-viewing
mini-LHR instruments are not impacted by some of the issues that degrade the
quality of airborne or space-borne techniques that use reflected sunlight:
surface reflectivity (e.g. darkness and angular dependence), surface
roughness (sunlight path length), geolocation error, and scattering from clouds and aerosols. Together with the capability of measuring through gaps in cloud
cover and continuous observation during daylight hours, the mini-LHR surface
network in tandem operation with AERONET could provide full global and
seasonal observation coverage and offer a necessary validation product for orbital missions.
However, the modelling study is largely agnostic to the underlying
technology. Consequently, a similar result would be obtained, for example,
by using a network of Bruker EM27/SUN instruments after they have been
modified to withstand inclement weather and can run exclusively on solar
power. A growing network of inter-calibrated ground-based remote-sensing
units, as part of a global carbon measurement system, must strike a balance
between a diffuse network of gold-standard spectrometers (TCCON), a larger
network of intermediate (cost–performance) spectrometers (COCCON), and an
even larger network of cheaper, less precise autonomous spectrometers that
can be deployed in high-risk or high-reward environments where we remain data
poor (e.g. tropics).
Data availability
Code and data used in this study are available from the
authors upon request.
Author contributions
PIP and ELW conceived the study and wrote the paper. LF, with
support from JM, led the calculations. All co-authors provided comments on the paper.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
Development of the mini-LHR was supported by the NASA–GSFC Internal Research
and Development (IRAD) programme, the NASA–GSFC Science Innovation Fund (SIF),
and the NASA Interdisciplinary Science (IDS) programme (NNH127DA001N). Work at
the University of Edinburgh was partly funded by the NERC National Centre for
Earth Observation (NCEO). Paul I. Palmer gratefully acknowledges funding from
his Royal Society Wolfson Research Merit Award. We would also like to thank
Brent Holben and the AERONET team for their ongoing support and Laura Iraci and
Jim Podolske from NASA Ames Research Center (ARC) for support with TCCON
access and data at NASA–AFRC.
Review statement
This paper was edited by John Worden and reviewed by two anonymous referees.
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