A novel, practical observation system for measuring tropospheric carbon
dioxide (CO2) concentrations using a non-dispersive infrared analyzer
carried by a small helium-filled balloon (CO2 sonde) has been
developed for the first time. Vertical profiles of atmospheric CO2 can
be measured with a 240–400 m altitude resolution through regular onboard
calibrations using two different CO2 standard gases. The standard
deviations (1σ) of the measured mole fractions in the laboratory
experiments using a vacuum chamber at a temperature of 298 K were
approximately 0.6 ppm at 1010 hPa and 1.2 ppm at 250 hPa. Two CO2
vertical profile data obtained using the CO2 sondes, which were
launched on 31 January and 3 February 2011 at Moriya, were compared
with the chartered aircraft data on the same days and the commercial
aircraft data obtained by the Comprehensive Observation Network for TRace
gases by Airliner (COTRAIL) program on the same day (31 January) and 1 d before (2 February). The difference between the CO2 sonde data
and these four sets of in situ aircraft data (over the range of each balloon
altitude ±100 m) up to the altitude of 7 km was 0.6±1.2 ppm
(average ±1σ). In field experiments, the CO2 sonde
detected an increase in CO2 concentration in an urban area and a
decrease in a forested area near the surface. The CO2 sonde was shown
to be a useful instrument for observing and monitoring the vertical profiles
of CO2 concentration in the troposphere.
Introduction
Atmospheric carbon dioxide (CO2) is one of the most important
anthropogenic greenhouse gases for global warming. Certain human activities,
such as fossil fuel combustion, cement production, and deforestation, are the
major cause of atmospheric CO2, making the global average concentration
of atmospheric CO2 increase from 280 ppm before the Industrial
Revolution to 400.0 ppm in 2015 (World Meteorological Organization, WMO,
2016). Over the last 10 years, the average rate of atmospheric CO2
increase is measured at 2.21 ppm yr-1 (WMO, 2016). Atmospheric CO2
is measured by ground-based stations and ships using the flask sampling and
continuous instrument methods such as non-dispersive infrared absorption
(NDIR) (Tanaka et al., 1983; Hodgkinson et al., 2013) and cavity ring-down
spectroscopy (CRDS) (Winderlich et al., 2010). A network of ground-based
Fourier transform spectrometers (FTS) that record the direct solar spectra
in the near-infrared spectral region (Total Carbon Column Observing Network,
TCCON) is used to observe the column-averaged mole fraction of CO2 in
dry air (total column XCO2) (Wunch et al., 2011). These observations
have provided extensive information regarding the distribution and temporal
variation of CO2 in the atmosphere (Pales and Keeling, 1965; Conway et
al., 1988; Komhyr et al., 1989; Tans et al., 1989; Conway et al., 1994).
Moreover, atmospheric CO2 measurement data are useful for estimating
CO2 fluxes at the surface through inverse modeling (Gurney et al., 2004;
Baker et al., 2006). Due to the limited number of observation sites and the
limitations of their altitudinal range, a large degree of uncertainty in the
current estimates of the regional CO2 sources and sinks is noted
(Gurney et al., 2002). More atmospheric CO2 measurements are needed to
reduce the uncertainties in CO2 flux estimation using an inverse
modeling.
To address the issues with insufficient CO2 observational data,
satellite remote sensing techniques have been used to investigate the
CO2 distribution on a global scale (Chédin et al., 2002; Crevoisier
et al., 2004; Dils et al., 2006). The Greenhouse Gases Observing SATellite
(GOSAT), which measures the short wavelength infrared (SWIR) spectra of
sunlight reflected by the earth's surface with a Fourier transform
spectrometer and obtains the total column XCO2, has been in operation
since early 2009 (Yokota et al., 2009; Yoshida et al., 2011; Morino et al.,
2011). Since 2014, the Orbiting Carbon Observatory-2 (OCO-2) satellite has
also measured the IR spectra of the surface-reflected sunlight with a
diffraction grating spectrometer and obtains total column XCO2
(Eldering et al., 2017). However, these satellite observations provide only
nadir total column XCO2 and do not measure the vertical distributions
of CO2 concentrations, as the observed spectra of the surface-reflected
sunlight do not provide enough information to determine the vertical
distributions. Furthermore, the satellites overpass a specific earth-based
target once several days only at about noon in the solar time because of
their sun-synchronous orbits.
The altitude distributions of CO2 concentrations have been measured
using other techniques. For instance, tall towers measure vertical profiles
of CO2 near the ground (Bakwin et al., 1992; Inoue and Matsueda, 2001;
Andrews et al., 2014). CO2 vertical profiles up to 10 km near the
airports have been observed by the equipment installed by the commercial
airlines, such as the Comprehensive Observation Network for TRace gases by
Airliner (CONTRAIL program) (Machida et al., 2008; Matsueda et al., 2008).
Measurements by equipment installed on chartered aircrafts have also been
undertaken, which include the High-performance Instrumented Airborne
Platform for Environmental Research (HIAPER), the Pole-to-Pole Observations
(HIPPO) program up to 14 km at altitudes spanning the Pacific from
85∘ N to 67∘ S (Wofsy et al., 2011), the NIES/JAXA
(National Institute of Environmental Studies and Japan Aerospace eXploration
Agency) program at an altitude from 2 to 7 km (Tanaka et al., 2012), and the
NOAA/ESRL Global Greenhouse Gas Reference Network Aircraft Program (Sweeney
et al., 2015). Although these aircraft measurements provided the vertical
profiles of CO2 concentrations, vertical profile measurements using the
commercial airlines are limited around the large airports, and the frequency of
the measurements using chartered airplanes is often limited by their
relatively high cost. The continuation and expansion of airborne measurement
programs for CO2 and related tracers are expected to enhance the
estimation of the global carbon cycling greatly (Stephens et al., 2007).
Atmospheric CO2 observations using balloons, to select specific
locations unless prohibited or restricted by aircraft flight paths, are
useful for solving the issues associated with the sparseness of CO2
vertical data. Balloon-borne observations of stratospheric CO2 were
previously conducted by other studies. For instance, stratospheric air
sampling was conducted using cryogenic sampler onboard balloons once a year
from 1985 to 1995 over the northern part of Japan (Nakazawa et al., 1995).
Balloon-borne near-infrared tunable diode laser spectrometers have been
developed to provide in situ data for CO2 in the stratospheric
atmosphere (Durry et al., 2004; Joly et al., 2007; Ghysels et al., 2012).
Furthermore, two in situ CO2 analyzers adopting the NDIR technique,
using a modified commercial detector for stratospheric measurements, have
been developed for deployment on the NASA ER-2 aircraft and on a balloon
(Daube et al., 2002). These balloon-borne instruments described above were
specially designed to measure CO2 concentrations in the stratosphere.
Observation of the CO2 vertical distribution in the troposphere is
essential because the uncertainties in the estimated fluxes, using the
inverse method, can be attributed to the inaccurate representations of the
atmospheric processes in transport models. Misrepresentation of vertical
mixing by the transport models, particularly inside of the boundary layer,
which is the layer closest to the ground where CO2 is taken up and
released, is one of the dominant causes of the uncertainty in CO2 flux
estimation (Stephens et al., 2007; Ahmadov et al., 2009). Recently, the
observation of tropospheric CO2 was conducted, using a lightweight
unmanned aerial vehicle, such as a kite plane, with a commercial NDIR
instrument. CO2 profiles were observed in and above the planetary
boundary layer up to 2 km to investigate the temporal and spatial variations
of CO2 (Watai et al., 2006). A passive air sampling system for
atmospheric CO2 measurements, using a 150 m long stainless-steel tube
called an AirCore, was developed (Karion et al., 2010). The AirCore mounted on
an airplane or a balloon ascends with evacuating inside of the tube to a
high altitude of 30 km at flight maximum, then collecting ambient air by
pressure changes along a decrease in altitude. The sampled air in the tube
is analyzed with a precision of 0.07 ppm for CO2 indicated as 1
standard deviation in the laboratory, and the vertical profile of CO2 is
obtained.
In the present study, we have developed a practical CO2 sonde system
that can measure in situ CO2 vertical profiles in the atmosphere from
the ground to altitudes up to about 10 km with a 240–400 m altitude
resolution by using a small-sized balloon. Although the sonde system is
thrown away after every flight due to the difficulties associated with
recovery, the sonde systems are easily prepared with a relatively low cost.
We have tested the sonde flight experiments more than 20 times in Japan. The
CO2 sonde developed has the following advantages, compared with other
measurement techniques described above: (1) its cost of operation is low and
flight permission is easy to obtain from the authorities as compared
with the aircraft observations; (2) the CO2 sonde can be easily carried
to the launch sites since the instrument is light; (3) a limited amount of
power is required for the operation; (4) it can generally be launched at any
time; and (5) the meteorological data are obtained simultaneously with
CO2 profile data. In this study, the design of our novel CO2 sonde
and the results of the comparison experiments with aircraft measurements are
described. The target accuracy and precision in the measurements with the
CO2 sonde are below about 1 ppm CO2 mole fraction in the
atmosphere of 400 ppm CO2, preferable for carbon cycle studies (e.g.,
Maksyutov et al., 2008). The developed CO2 sonde system attained
virtually all the targets from the ground to an altitude of about 10 km.
Inai et al. (2018) measured vertical profiles of the CO2 mole fraction in
the equatorial eastern and western Pacific in February 2012 and
February–March 2015, respectively, by using our novel CO2 sondes which
are described in this report. They found that the 1–10 km vertically
averaged CO2 mole fractions lie between the background surface values
in the Northern Hemisphere (NH) and those in the Southern Hemisphere (SH)
monitored at ground-based sites during these periods. Their study showed
that the combination of CO2 sonde measurements and trajectory analysis,
taking account of convective mixing, was a useful tool in investigating
CO2 transport processes.
Materials and methodsDesign of the CO2 sonde
Many severe restrictions are noted for the operation of balloon-borne
CO2 sondes. First, the weight of the CO2 sonde package should be
less than about 2 kg, based on the legal restriction by the US FAA (Federal
Aviation Administration) and by the Japanese aviation laws for the payload
weight of 2.721 kg for unmanned free balloons. Balloon systems heavier than
the above-regulation weight are not useful for the frequent flights because
the flight permission from the authorities is much more difficult to obtain
and the additional safety requirements are more expensive. The balloon
system is thrown away in the ocean after each flight due to a long-distance
transportation (100 km or more to the east) by strong westerly winds in the
upper atmosphere of a mid-latitude area. This is done to avoid the accidents
associated with falling onto urban areas, resulting in high recovery
costs. Therefore, the cost of the CO2 sonde system should be low for
frequent observations. The non-recovery system implies that every instrument
should perform consistently.
In this study the NDIR technique was adopted for a detection of CO2
concentrations. The NDIR CO2 measurement techniques have been widely
used in many places such as WMO/GAW (Global Atmosphere Watch) stations. Our
target instrumental accuracy and precision of approximately 1 ppm are less
stringent than those of the ground-based instruments (±0.1 ppm) used
at the WMO/GAW stations (WMO, 2016). However, the surrounding conditions for
the instrument are substantially severe during the flight experiments, as
the pressure changes from 1000 to 250 hPa and the surrounding temperature
changes from 300 to 220 K during flights from the surface to an altitude of
10 km in about 60 min.
In the NDIR technique for CO2 measurements, the IR emission from a
broadband wavelength source is passed through an optical cell and two
filters, and then the light intensities are detected by two IR detectors.
The one optical filter covers the whole absorption band of CO2 around
4.3 µm, while the other covers a neighboring non-absorbed region around
4.0 µm provided that the chosen active and reference channel filters
do not significantly overlap with the absorption bands of other gas species
present in the application (Hodgkinson et al., 2013).
The Beer–Lambert law is expressed by Eq. (1), defining the light intensity
in the absence of CO2 in the cell as I0 and the light intensity
in the presence of CO2 in the cell as I:
II0=exp(-εCL),
where C is the CO2 concentration in molec. cm-3, L is the
optical path length in centimeters, and ε is the absorption
cross section in cm2 molecule-1. Using the relationship of
C=XP(kBT)-1, where X is the CO2 mole fraction and P is
the pressure of dehumidified ambient air, and the approximation of exp(-εCL)=1-εCL under the condition of
εCL≪1, Eq. (1) is rewritten as
(I0-I)P=XI0εLkBTC,
where Tc is the sample air temperature in the sensor cell and kB is
the Boltzmann constant. Equations (1) and (2) hold for monochromatic light
only and Eq. (2) only holds for small absorptions. Although the NDIR
analyzer exhibits nonlinear absorption due to the saturation of strong
absorption lines, it is known to have a good linearity within a certain
concentration range (Galais et al., 1985), and Eq. (2) may be used
correspondingly. In our analyses of the balloon data, Eq. (2) was used only
for the interpolation between the low and high mole fractions of the
in-flight calibration gases to obtain the ambient CO2 mole fractions.
With a 120 mm long absorption cell, the absorption intensity is
approximately 3 % at 400 ppm CO2 with our CO2 NDIR system, i.e.,
εCL≈0.03, and the approximations of exp(-εCL)=1-εCL are well fitted. The values of I4.0-I4.3 were used instead of
I0-I to obtain the CO2 mole fraction values in
the NDIR measurements, where I(4.0) and I(4.3) were the signal
intensities at the 4.0 µm wavelength for background measurements and
the 4.3 µm wavelength for CO2 absorption measurements,
respectively. Thus, the value of I4.0-I4.3/P is proportional to the CO2 mole fraction
X in the optical cell. The proportionality constant is usually determined by
the measurements of the standard gases. In the NDIR measurements at the
ground WMO/GAW stations, carbon dioxide mole fractions are referenced to a
high working standard and a low working standard and are determined by the
interpolations of the signals with the two standards, and the calibrations
with the two standard gases are carried out every 12 h (Fang et al., 2014).
System configuration of the CO2 sonde system
A schematic diagram and photograph of the CO2 measurement instrument
are shown in Fig. 1. The CO2 sonde has three inlets installed for
ambient air and two calibration gases with mesh filters (EMD Millipore,
Millex-HA, 0.45 µm pore size) to remove the atmospheric particles.
Three solenoid valves (Koganei, G010LE1-21) were used to switch the gas flow
to the CO2 sensor. A constant-volume piston pump with a flow rate of
300 cm3 min-1 (Meisei Electric co., Ltd.), which was originally
used for ozonesonde instruments, directed the gas flows from the inlets
through the solenoid valves into a dehumidifier, a flow meter, and a
CO2 sensor. The absolute STP (standard temperature and pressure)
flow rate decreased with a decrease in pressure. Since the exit port of the
CO2 sensor was opened to the ambient air, the pressures of dehumidified
outside air and calibration gases in the absorption cell were equal to the
ambient pressure during the flight. Next to the pump, the gases were
introduced into a glass tube filled with the magnesium perchlorate grains
(dehumidifier) installed upstream of the CO2 sensor to remove the water
vapor. Fabric filters were installed on both ends of the dehumidifier, and a
mesh filter was installed downstream of the dehumidifier to prevent the
CO2 sensor from the incursion of magnesium perchlorate grains to the
optical cell.
(a) Schematic diagram of the CO2 measurement package, where
F1 and F2 represent the band-pass filters at wavelengths of 4.0 and
4.3 µm, respectively. The outlet port of the CO2 sensor is opened
to ambient air. Details of the system are described in the text. (b) Photograph of the inside of the CO2 sonde package. The components were
placed in a specially modeled expanded polystyrene box.
The infrared absorption cell consisted of a gold-coated glass tube, a light
source, and a photodetector. The light source (Helioworks, EP3963) consisted
of a tungsten filament with a spectral peak intensity wavelength of
approximately 4 µm. The light from the source passed through a
gold-coated glass tube (length 120 mm and inside diameter 9.0 mm). The
commercial CO2 NDIR photodetector (Perkin-Elmer TPS2734) had two
thermopile elements, one of which was equipped with a band-pass filter at a
wavelength of 4.3 µm for the measurement of the CO2 absorption
signal, whereas the other was equipped with a band-pass filter at a
wavelength of 4.0 µm for the measurement of the background signal. The
signals from the sensors were amplified by an operational amplifier and
converted to 16 bit digital values by an A/D convertor. The signal
intensities of the detectors at 4.0 and 4.3 µm without CO2 gas
were set to the equal levels by adjusting the amplification factors in the
laboratory. The electric power for the CO2 sensor, pump, and valves was
supplied through a control board using three 9 V lithium batteries, lasting
for more than 3 h during the flight. The control board connected to the
components regulated the measurement procedures, such as switching the
solenoid valves and processing the signal. As shown in Fig. 1, the
measurement system has an expanded polystyrene box molded specially to
settle the optical absorption cell, electronic board, pump, battery, and
other components.
Calibration gas package
Under the wide ranges of temperature and pressure conditions, the CO2
sensor signal was unstable, and the calibration of the CO2 sensor only
on the ground before launch was insufficient to obtain the precise values of
the CO2 concentrations. To solve this problem, an in-flight calibration
system was incorporated into the CO2 sonde. A calibration gas package
was attached to the CO2 sonde for the in-flight calibration, as shown
in Fig. 2. The calibration gas package consisted of two aluminum bags coated with
polytetrafluoroethylene (PTFE) (maximum volume: 20 L), containing
reference gases with low (∼370 ppm) and high (∼400 ppm) CO2 concentrations. In each bag, ∼8 L (STP) of the
reference gas was introduced from standard CO2 gas cylinders just
before launch. Since the gas bags were soft, their inner pressures were
equal to the ambient air pressures during the balloon flight. The gas
volumes in the bags increased with the altitude during the ascent of the
balloon due to a decrease in the ambient pressure, while the reference gases
were consumed during the calibration procedures. The optimum amounts of gas
in the bags were determined by both the ascending speed of the balloon and
the consumption rate to avoid the bursting of the bags and exhaustion of the
gases. The CO2 concentrations of the reference gases in the bags were
checked by the NDIR instrument (LICOR, LI-840) before launching. Thereafter,
approximately 6 L of the reference gas was left in each bag for a subsequent
in-flight calibration. The change in the CO2 mole fraction in the bags
was less than 1 ppm over a 3 d period, which was negligible over the
observation time during the balloon flight. All measurements were reported
as dry-air mole fractions relative to the internally consistent standard
scales maintained at Tohoku University (Tanaka et al., 1987; Nakazawa et al.,
1992).
Photograph of the CO2 sonde developed in this study before
launching. (a) The CO2 measurement package as shown in Fig. 1, (b) GPS sonde,
and (c) calibration gas package.
Since the gas exit port of the optical absorption cell was opened to the
ambient air, the cell pressure was equalized with the ambient pressure for
measuring both the ambient air and two standard gases. During the
balloon-borne flights, the temperatures inside the CO2 sonde package
were measured with thermistors. The temperature inside the CO2 sonde
package gradually decreased by approximately 5 K, from 298 K on the ground
to 293 K at an altitude of 10 km during the flights. Probably due to the
polystyrene box and the heat produced by the NDIR lamp, pump, and solenoid
valves, the temperature inside the sonde package remained virtually constant in
spite of low ambient temperatures at high altitudes (∼220 K). Within
one measurement cycle time (160 s) with the standard gases, the temperature
change was less than 0.4 K in the sonde package. The temperatures of the
sample gas in the tube just before the inlet of the CO2 NDIR cell were
also measured using a thin wire thermistor, commonly used for ambient
temperature measurement in GPS sonde equipment with a quick response time
(shorter than 2 s). The gas temperature change was negligible at the valve
change timings between the standard gas and ambient air (<0.1 K).
The result indicated that the gas temperatures were relatively constant
after passing through the valves, pump, dehumidifier cell, and piping for
both the standard gases and ambient air.
The performances of the CO2 sonde instruments were checked before the
balloon launching since the CO2 sonde systems were not recovered after
the launch experiments were performed. For about 60 min before the launch,
the values of I4.0-I4.3/P were measured with the valve cycles (each step 40 s, total 160 s)
for two standard gas packages (∼370 and ∼400 ppm) for calibration and one intermediate concentration gas package
(∼385 ppm) as a simulated ambient gas sample.
Total sonde system
The CO2 sonde was equipped with a GPS radiosonde (Meisei Electric co.,
Ltd., RS-06G). The balloon carried the instrument packages in the altitude
with measuring CO2 and meteorological data (GPS position and time,
temperature, pressure, and humidity). The CO2 sonde transmitted those
data to a ground receiver (Meisei Electric co., Ltd., RD-08AC) at 1 s
intervals, and thus it was unnecessary to recover the CO2 sonde after the
balloon burst. Figure 2 showed an overall view of the CO2 sonde
developed in this study, which consisted of a CO2 measurement package,
a calibration gas package, a GPS radiosonde, a balloon, and a parachute. The
total weight of the CO2 sonde was 1700 g, including the GPS radiosonde
(150 g), CO2 measurement package (1000 g), and calibration gas package
(550 g). The dimensions of the CO2 measurement package were width (W)
280 mm × height (H) 150 mm × depth (D) 280 mm. The size of
the calibration gas package was W 400 mm ×H 420 mm ×D
490 mm.
The CO2 sonde system was flown by a 1200 g rubber balloon (Totex). The
ascending speed was around 4 m s-1 by controlling the helium gas amount in
the rubber balloon and checking the buoyancy force. In practice, it was
difficult to precisely control the ascending speed of the balloon, and the
actual resulting speeds were in the range of 3–5 m s-1. This
corresponds to the height resolution of approximately 240–400 m for the
measurements of the CO2 vertical profiles.
Ascending speed slower than 3 m s-1 can lead to a collision with a
nearby tree and building, resulting in equipment falling in the urban areas.
With faster ascending speeds, the altitude resolution of the measurements
decreased, the standard gas bag became full, and the pressure inside the
gas bags became higher than the ambient pressure because of the lower
ambient pressures at higher altitudes. The high pressure inside the gas bag
resulted in the fast flow speed in the optical absorption cell of NDIR,
which shifted the signal values for the pressurized gas sample. Since
pressure relief valves for the bags did not work at low pressures at high
altitudes, we did not use the pressure relief valve for the standard gas
bags. When the ascending speed was low, the standard gas bags became empty
since they were consumed by the in-flight calibration procedures during the
long ascending time. Since the measurements after the over-pressurization or
the exhaustion of the reference gas bag are useless, this technical problem
determines the upper limit (10 km) of altitude for the measurements in this
study. Based on our experiences, this problem generally occurred at an
altitude above approximately 10 km. A prototype of the CO2 sonde is
available from Meisei Co. Ltd. (Isesaki, Japan) for about USD 4500.
Data processing procedures
Since the surrounding conditions of the sonde change significantly during
the ascending period, the NDIR measurement system is calibrated with the two
standard gases at every altitude. However, since the balloon-borne
instrument is only equipped with one NDIR absorption cell and the balloon
ascends continuously, it is not possible to measure the ambient air sample
and the two standard gases at the same time and at the same altitude.
Therefore, the measurement cycle during the flights consisted of the
following steps: (1) low concentration standard gas, (2) ambient air, (3) high concentration standard gas, and (4) ambient air. The measurement time
for each step was 40 s. At switching timings of the valve cycles, the signal
became stable within 10 s, and the averages of residual 30 s period signals
were used for the calculation of the CO2 mole fractions. Since the gas
exit port of the NDIR optical absorption cell was opened to the ambient air,
the cell pressure was equalized with the ambient pressure. During the period
of the 40 s gas change, the pressure would change about 2 % when the
ascending speed of the balloon was 4 m s-1. The temperature of the
ambient air and standard gas samples at the inlet port of the optical cells
was measured and found to be constant during each cycle of the calibration
procedure.
Figure 3 shows an example of the raw data obtained from the CO2 sonde
experiment. Figure 3 presents the plots of the values of I4.0-I4.3/P against the altitude, where
I(4.0) and I(4.3) are the signal intensities at the wavelength of 4.0 µm for background measurements and the 4.3 µm wavelength for
CO2 absorption measurements, as obtained by the NDIR CO2 sensor on
the balloon, and P is the ambient atmospheric pressure obtained by the GPS
sonde data and pressure measurements on the ground.
Raw data obtained by the CO2 sonde launched on 26 September 2011 at Moriya, Japan. The vertical axis is the difference between the 4.0 and 4.3 µm signal intensities divided by the ambient pressure.
The black line indicates the observation results during the balloon flight
with calibration cycles. The red circle indicates the 30 s average values in
each step of the calibration. The red curve indicates the cubic spline fitting
curves for the observation points of the 30 s average values of the same
standard gas. The small black dots on the cubic spline curves indicate the
estimated values for the standard gases at the ambient gas measuring timing,
which is used for the interpolation to determine the ambient air
concentrations.
The values of I4.0-I4.3/P are proportional to the CO2 mole fraction X according to the
Beer–Lambert law as expressed by Eq. (2). By using the values of I4.0-I4.3/P, we can compensate
for the pressure change to determine the CO2 concentration. As shown in
Fig. 3, the differences in the I4.0-I4.3/P values between the low and high standard gases remained
relatively constant while ascending to the higher altitudes. However, the
I4.0-I4.3/P values for
each standard gas did not change linearly but sometimes displayed some
curvatures as shown in Fig. 3. This may be due to the differences between
the baseline drifts of the two sensors at 4.3 and 4.0 µm in the
NDIR detector. Since the measurements were performed alternately for the
standard gases and the ambient air with the NDIR cell and are not performed
simultaneously, the values for the standard gas signals at the time of the
ambient air measurement were estimated. Therefore, the cubic spline fitting
curves for the observation points of the 30 s average values (red circles in
Fig. 3) of the same standard gas were used to obtain the low and high
calibration points for the calculation of the mole fractions in the ambient
air. In Fig. 3, the cubic spline fitting curves are represented by the red
curves, and the estimated values for the standard gases at the ambient gas
measuring time are represented by the small black dots on the cubic spline
curves, which are used for the interpolation to determine the ambient air
concentrations. Linear line fitting between the standard gas values did not
work well because the connection lines of the values sometimes displayed
curvatures as shown in Fig. 3. Since there were in-phase fluctuations in the
I(4.0) and I(4.3) signals during the flights, the subtraction of I4.0-I4.3 could partly improve
the signal-to-noise ratios by canceling in-phase fluctuations with each
other.
Results and discussionLaboratory tests
Since the linear interpolation method for the I4.0-I4.3/P values was used to
determine the ambient air CO2 mole fractions in the balloon-borne
experiments, the deviations from the linear interpolation process were also
investigated. The measurements of various mole fraction gas samples in the
laboratory indicated that the linear interpolation error with the two
standard gas packages (∼370 and ∼400 ppm)
was less than 0.2 ppm in the range between 360 and 410 ppm. Figure 4 shows
the measurement results of the NDIR cell developed in this study at various
CO2 mole fractions. The outlet port of the NDIR system was connected to
the commercial CO2 instrument (LICOR, LI-840A) as a standard device,
and the two instruments simultaneously measure the sample gas at 1010 hPa.
The standard gases of 365 and 402 ppm were used for the calibration, and the
mixtures of the standard gases were used for the samples. This indicated the
values of I4.0-I4.3/P of
the system were proportional to the mole fraction of CO2. This type of
experiment could not be performed at low pressures, since we did not have a
standard device which can be operated under low pressures.
I4.0-I4.3/P values versus CO2 mole fraction, where I(4.0) and I(4.3)
are the signal intensities at the 4.0 µm wavelength for background
measurements and the 4.3 µm wavelength for CO2 absorption
measurements, obtained by the NDIR CO2 sensor, and P is the ambient
atmospheric pressure. CO2 mole fractions were measured with a standard
NDIR instrument (LICOR, LI-840A) connected to the balloon sensor in series.
The pressure while carrying out the measurements was constant at 1010 hPa.
Figure 5 shows the results of an experiment using a vacuum chamber in the
laboratory, where the flight pressure conditions were simulated and the
performances of the CO2 sonde instruments were evaluated. The
temperature inside the chamber was not controlled and was about 298 K. In
the actual flights, the temperature inside the sonde package did not change
more than 5 K. The CO2 sonde system and two standard gas packages were
placed in the vacuum chamber. The chamber was filled with the mole fraction
sample gas of 377.3 ppm before the pumping. The pressure of the chamber was
gradually and continuously decreased using a mechanical pump from 1010 hPa
(ground surface pressure) to 250 hPa (about 10 km altitude pressure) over 60 min, corresponding to a balloon ascending speed of 3 m s-1 in actual flights,
whereas the sample gas was slowly and continuously supplied to the chamber.
The values I4.0-I4.3/P
were measured for the two standard gas packages and the sample gas with the
valve cycles (each step 40 s, total 160 s) as described in the previous
section. The mole fractions of the sample gas in the chamber were calculated
by the interpolation of the signals for the two standard gases. The 30 s
signals 10 s after the valve changes were used for the interpolation
calculations to avoid the incomplete gas exchanges in the NDIR optical cell.
The black circle in Fig. 5 indicates the sample gas mole fraction obtained
from the linearly interpolated standard gas signals in each calibration
cycle. The vertical error bar in Fig. 5 indicates the square root of the sum
of squares for the standard deviations of the sample and standard gas
signals at each step. The errors in the CO2 mole fractions were
estimated to be 0.6 ppm at 1010 hPa and 1.2 ppm at 250 hPa using the
calibration cycles. The results in Fig. 5 indicated that the determination
of the sample gas concentration using the linear interpolation with the
standard gases was appropriate within the error, when the pressure
continuously decreased from 1000 to 250 ppm over 60 min.
Results of a chamber experiment of the CO2 sonde. Pressure in
the chamber was reduced from 1010 hPa (ground level pressure) to 250 hPa
(about 10 km altitude pressure) at a temperature of about 298 K. The black
circles indicate the value of the CO2 mole fraction of the sample air
in the chamber, which was obtained from the interpolation of the standard
gas values in each calibration cycle. Vertical error bars indicate the
square root of the sum of squares for the standard deviations of the sample and
standard gas signals at each step in the calibration cycle. The black dashed
line shows an average of all the values obtained for the sample gas. See the
text for more details.
When the CO2 sonde instrument was inclined and vibrated in the
laboratory, the fluctuations in the signals were observed. The quantitative
correlation between the signal fluctuation intensities and acceleration
speed, measured by a three-dimensional acceleration sensor, was investigated,
but no distinct correlation was detected. However, the in-flight calibration
system partly solved this problem by taking the signal difference of I4.0-I4.3 and also by measuring
alternately the two standard gases every 40 s during the balloon flights.
The temperature characteristics of the CO2 sensor were also
investigated by changing the sensor cell block temperature from 273 to 323 K
at the pressure of ∼1010 hPa, using a heater in the laboratory. The
laboratory experiment related to the temperature dependence suggested that
the measurement error is less than 0.2 ppm due to the temperature change
during one valve cycle (160 s) in the balloon-borne experiments.
In principle, the absorption intensities I0-I in the
NDIR measurements are proportional to the absolute CO2 concentrations
in the sample air in the absorption cell. Therefore, at higher altitudes
where the pressures were lower, the values of I4.0-I4.3 were smaller and the signal-to-noise
ratios decreased. The error of the CO2 mole fractions of 1.2 ppm at
250 hPa corresponds to an absolute CO2 concentration of 3.2×1013 molecule cm-3. The equivalent altitude for this value was 90 km with a CO2 molar fraction of 400 ppm. As described previously, the
purpose of CO2 balloon observations is to measure the CO2 mole
fraction within 1 ppm errors in the atmospheres around 400 ppm CO2. The
upper limit of the altitude for the observations with the developed CO2
sonde is considered to be ∼10 km. Furthermore, as described in Sect. 2.4, the problems of the vacancy or over-pressure in the standard gas bags
took place around 10 km altitudes, which resulted in large errors. This also
practically determines the upper altitude limit for CO2 sonde
observations.
Comparison with aircraft data
Two types of aircraft measurement data, the NIES/JAXA chartered aircraft and
the CONTRAIL data, were used for comparison with the CO2 sonde
measurement data. The NIES/JAXA chartered aircraft measurements were
conducted on the same days as the CO2 sonde observations (31 January and 3 February 2011). The chartered aircraft observations were
performed as a part of the campaign for validating the GOSAT data and
calibrating the TCCON FTS data at Tsukuba (36.05∘ N,
140.12∘ E) (Tanaka et al., 2012). The chartered aircraft data were
obtained using an NDIR instrument (LICOR LI-840) that had a control system
of constant pressure and had the uncertainty of 0.2 ppm. On both 31 January
and 3 February, the chartered aircraft measured the CO2 mole
fractions during descent spirals over Tsukuba and Kumagaya (Fig. 6). Because
the air traffic was strictly regulated near Haneda and Narita
international airports, the aircraft observations at altitudes above 2 km
over Tsukuba were prohibited. Therefore, the descent spiral observations
were conducted over Kumagaya at altitudes of 7–2 km and over Tsukuba at
altitudes of 2–0.5 km. Tsukuba is located approximately 20 km northeast of
Moriya, whereas Kumagaya is located approximately 70 km northwest of Moriya.
Flight paths of the CO2 sonde observations launched at Moriya
on 31 January (blue solid line) and 3 February (red solid line) 2011,
the CONTRAIL 11_060d data on 31 January 2011 (black solid
line) and 11_062d data on 2 February 2011 (black dashed
line) from Hong Kong to Narita, and the NIES/JAXA chartered aircraft
experiment on 31 January (green solid line) and 3 February (purple
dotted line). The altitudes of the flight paths are also indicated.
Seven profiles based on the CONTRAIL measurements, obtained during the
ascent and descent of aircrafts over Narita airport and with passage times
close to the CO2 sonde observations, were available within 2 d
after or before the dates of the CO2 sonde measurements (Table 1). The
CO2 sonde observations were conducted on 31 January and 3 February 2011 from Moriya. One set of CONTRAIL data, obtained from the flight from
Hong Kong to Narita (data set name: 11_060d), was available
on 31 January, but no CONTRAIL data were available for 3 February.
Therefore, the CONTRAIL data, obtained from the flight from Hong Kong to
Narita on 2 February (data set name: 11_062d), were used
for comparison with the 3 February CO2 sonde data. Figure 6 also
shows the CONTRAIL 11_060d and 11_062d flight
paths and the CO2 sonde launched at Moriya on 31 January and 3 February 2011. On 31 January the flight time of the CONTRAIL
11_060d over Narita airport and the launch time of the
CO2 sonde at Moriya were relatively close to one another. The flight
path of the CONTRAIL 11_062d data on 2 February 2011 was
close to that of the CO2 sonde on 3 February 2011, and both
observations were conducted in the early afternoon. The CONTRAIL data
referred to in the present study were obtained using the Continuous CO2
Measuring Equipment (CME) located onboard commercial airliners (Machida et
al., 2008; Matsueda et al., 2008). The typical measurement uncertainty
(1σ) of the CME has been reported as 0.2 ppm (Machida et al., 2008).
CONTRAIL flight data near to the CO2 sonde measurements on 31 January and 3 February 2011.
* Time for the CONTRAIL data represents the flight time in Japan
Standard Time at an altitude of 1 km over Narita airport. Time for the
CO2 sonde data represents the launching time at Moriya.
Figure 7 shows the vertical profiles of CO2 observed by the CO2
sonde at Moriya, the chartered aircraft at Kumagaya and Tsukuba, and the
CONTRAIL over Narita airport on 31 January 2011. The overall vertical
distribution of the CO2 sonde data resembled those of the
chartered aircraft. The vertical profiles of the CONTRAIL 11_060d flight on 31 January at the 5.3–6.8 km altitude range consisted of
the missing data because of the CME calibration period.
The CO2 vertical profiles obtained by the CO2 sonde
(circles connected with blue lines), NIES/JAXA chartered aircraft data (dots
connected with green lines), and the CONTRAIL data (diamonds connected with
black lines) on 31 January 2011.
Figure 8 shows the comparison of the CO2 vertical profiles obtained by
the CO2 sonde over Moriya, NIES/JAXA chartered aircraft over Kumagaya
and Tsukuba on 3 February 2011, and the CONTRAIL on 2 February 2011
over Narita. The shape of the vertical profile obtained by the chartered
aircraft on 3 February resembled that obtained by the CO2 sonde,
although the profile from the chartered aircraft was shifted to the lower
CO2 concentration side compared to that of the CO2 sonde.
The CO2 vertical profiles obtained by the CO2 sonde
(circles connected with red lines), NIES/JAXA chartered aircraft data (dots
connected with purple lines) on 3 February, and CONTRAIL data (diamonds
connected with black lines) on 2 February 2011.
Comparisons of the CO2 mole fractions between the balloon
CO2 sonde and NIES/JAXA chartered aircraft measurements on 31 January
and 3 February 2011.
JAXA-NIES chartered aircraft (31 January 2011) JAXA-NIES chartered aircraft (3 February 2011) AltitudeBalloon CO2Aircraft CO2DifferenceAltitudeBalloon CO2Aircraft CO2Difference(m)a(ppm)(ppm)b(ppm)c(m)a(ppm)(ppm)b(ppm)c849399.05397.621.431324396.60394.452.151202398.16397.530.631612394.65393.031.621610398.00397.170.831917394.86394.100.762038396.50396.95-0.452223395.77393.542.232291398.03396.041.992539395.41393.951.452463396.54395.650.892867394.71395.11-0.402844393.44395.24-1.793215394.99392.992.003329395.45394.151.303543393.59393.070.523732393.51393.63-0.123764393.69393.400.284161395.47393.541.933938395.15393.112.044575394.62392.941.684169393.83392.681.154918393.24393.64-0.414458396.57393.513.065273392.41393.25-0.844750394.88393.691.195654393.02393.47-0.455047396.53394.012.536083391.87392.91-1.045214395.91393.452.466510392.76391.651.115383396.78393.583.20Average =0.425565395.83393.672.15SDd=1.165781395.18393.391.80rmse=1.206092391.75392.83-1.096287392.44392.420.026467393.67392.231.446639395.07392.422.656815394.00393.001.00Average =1.41SDd=1.00rmse=1.62
a Altitudes of the balloon-borne experiments using the in-flight
calibration with 40 s time intervals.
b Averaged values of the aircraft measurement results over the range of the
balloon altitudes ±100 m.
c Difference values of [balloon CO2] - [aircraft CO2].
d Standard deviation of the differences (1σ).
e Root mean square values.
Table 2 lists the comparisons of the CO2 mole fractions measured by the
balloon CO2 sonde and NIES/JAXA chartered aircraft on 31 January and
3 February 2011. The averaged values of the aircraft measurement over the
range of each balloon altitude ±100 m are listed in Table 2, since
the altitude resolution of the aircraft measurements is higher than that of
the CO2 sonde. From the 3 February measurements, the height of the
boundary layer around an altitude of 1 km was different between the CO2
sonde and the NIES/JAXA aircraft measurements, as shown in Fig. 8. Therefore,
the data below 1 km on 3 February are not included in Table 2. From the
data on 31 January, the averaged value of the differences between the
CO2 sonde and the NIES/JAXA aircraft was relatively small (0.42 ppm),
which corresponded to the bias of the measurements. The standard deviation
of the differences was 1.24 ppm. From the 3 February data, the bias was
large (1.41 ppm), whereas the standard deviation of the differences was not
so large (1.00 ppm), which corresponded to the similar but shifted vertical
profiles in shapes between the CO2 sonde and aircraft measurements as
shown in Fig. 8. The difference between the CO2 sonde data and the
NIES/JAXA chartered aircraft data on 3 February is nearly equal to the
difference between CONTRAIL data on 2 February and the NIES/JAXA chartered
aircraft data on 3 February.
Table 3 lists the comparisons of the CO2 mole fractions measured by the
balloon CO2 sonde and CONTRAIL aircraft, 11_060d on
31 January and 11_062d on 2 February 2011 up to the
altitude of 7000 m. The averaged values of the aircraft measurements over
the range of each balloon altitude ±100 m are listed in Table 3. The
biases between the CO2 sonde and the CONTRAIL aircraft results were
relatively small, 0.33 and 0.35 ppm, and the standard deviations of the
differences were 1.16 and 1.30 ppm for the results on 31 January and
3 February, respectively.
Comparisons of the CO2 mole fractions between the balloon
CO2 sonde measurements on 31 January and CONTRAIL aircraft CME on 31 January (11_060d) and between the CO2 sonde on 3 February and CONTRAIL on 2 February (11_062d) up to the
altitude of 7 km. The annotations are the same as Table 2.
From the comparison between the CO2 sonde data and the aircraft
(NIES/JAXA and CONTRAIL) data, it was found that the CO2 sonde
observation was larger than those of aircrafts by about 0.6 ppm on average.
The standard deviation of the difference from the CO2 sonde and
aircraft observations was 1.2 ppm (1σ). If the four sets of aircraft
measurement data obtained by the NIES/JAXA and CONTRAIL observations were
accurate within the published uncertainties, ignoring the differences in the
flight time and geographical routes, the measurement error of the CO2
sonde system was estimated from the standard deviations of all the
difference values in Tables 2 and 3. The estimated error value up to an
altitude of 7 km was 0.6±1.2 ppm for the CO2 sonde observation
with a 240 m altitude resolution and 3 m s-1 ascending speed. The root
mean square value (1.3 ppm) from all the difference values in Tables 2 and 3
indicated that the CO2 sonde could measure the CO2 vertical
profiles within 1.3 ppm on average compared to the aircraft observations. It
is noted that, although error estimation was conducted for the data up to an
altitude of 7 km due to the availability of the chartered aircraft data, the
CO2 sonde data above 7 km up to about 10 km were available. The measurement errors for
the data above 7 km are expected to be larger than the above estimation.
CO2 sonde observations over a forested area
Figure 9 shows the vertical profiles of the CO2 mole fraction,
temperature, and relative humidity obtained from the balloon-borne
experiments of the CO2 sonde at Moshiri (44.4∘ N,
142.3∘ E) on 26 August 2009. The launch site is in a rural area
of Hokkaido, Japan, and is surrounded by forests. The CO2 sonde was
launched at 13:29 LST and ascended with a mean vertical speed of
approximately 3 m s-1. The CO2 sonde reached an altitude of 10 km
after 56 min. The wind horizontally transported the CO2 sonde distances
of 10 and 21 km northeast when the CO2 sonde reached the altitudes
of 5 and 8 km, respectively. The CO2 sonde rapidly moved 52 km
southeast at an altitude of 16 km. Finally, the CO2 sonde reached an
altitude of 28 km before the balloon burst and the subsequent fall of the
sonde was directed by the parachute into the Sea of Okhotsk located 80 km
east of the launch site. The error bars for the CO2 mole fraction in
Fig. 9a were calculated from the deviation of the signal intensities from
the CO2 sensor during the 40 s measurement periods for the ambient air
and the two standard gases.
Profiles of (a)CO2 mole fraction, (b) temperature (solid
line) and potential temperature (dotted line), and (c) relative humidity
observed over a forest area, Moshiri in Hokkaido, Japan, by the balloon
launched on 26 August 2009 at 13:30 (LST). The black circles with error
bars in panel (a) represent the data obtained by the CO2 sonde.
The vertical temperature profile in Fig. 9b indicated the existence of three
inversion layers of the altitudes of approximately 2.0, 3.2, and 4.3 km. The
relative humidity from the ground to the first inversion layer at 2.0 km and
between the second and third inversion layers from 3.2 to 4.3 km was higher
compared with that observed from 2.0 to 3.2 and from 4.3 to 7.5 km. The
CO2 mole fraction was lowest near the ground (∼373 ppm) and
increased to approximately 384 ppm at an altitude of 4–5 km around the
third inversion layer before reaching a value of 387 ppm in the upper
troposphere (5–9 km). Significant decreases in the CO2 mole fractions
were observed in the two lower layers from the ground to 3.2 km. Considering
the clear weather on the day of the balloon experiment, these results are
explained by the uptake of CO2 near the surface by plants in the
forests through photosynthesis processes in the daytime hours and the
diffusion and advection of the air mass containing low CO2
concentrations at the upper altitudes.
Because the CO2 mole fraction for the vertical profiles near the
surface is critically important to estimating the flux around the
observation point, the vertical profile data taken by our CO2 sonde are
useful.
CO2 sonde observations over an urban area
Figure 10 shows the vertical profiles of the CO2 mole fraction,
temperature, and relative humidity obtained by the CO2 sonde at Moriya
(35.93∘ N, 140.00∘ E) on 3 February 2011. The
launching time was 13:10 LST and the sonde ascended with a mean vertical
speed of approximately 2.9 m s-1. Moriya is located in the Kanto region
and is 40 km northeast of the Tokyo metropolitan area. The launching site
was surrounded by the heavy traffic roads and residential areas. As seen in
Fig. 10a, high CO2 mole fractions were observed from the ground up to
an altitude of 1 km. The average CO2 volume mole fraction in this layer
was higher than that measured in the free troposphere approximately above 15 ppm. A small temperature inversion layer appeared at approximately 1 km, and
the maximum relative humidity was observed just below this inversion layer
(Fig. 10b and c). These results suggested that the CO2 emitted from
anthropogenic sources in and/or around the Tokyo metropolitan area
accumulated in the boundary layer at altitudes below 1 km.
Profiles of (a)CO2 mole fraction, (b) temperature (solid
line) and potential temperature (dotted line), and (c) relative humidity
observed over an urban area, Moriya near Tokyo, on 3 February 2011 at
13:10 (LST).
An analysis of Figs. 9 and 10 indicated that there were a clear local
consumption and emission of CO2 from the comparison of the levels of
CO2 concentration in the free troposphere, which suggested a decoupling
with the boundary layer and synoptic inversion layers (Mayfield and
Fochesatto, 2013). When a small increase in a column XCO2 value is
observed by a satellite, it is difficult to estimate which part of the
atmosphere is responsible for the increase in XCO2, the boundary layer
with strong CO2 emission in the nearby area, or the free troposphere.
Considering this fact, the vertical profile data obtained by the CO2
sonde around urban areas should provide more useful information than the
column-averaged observations obtained by the satellites and FTS measurements
to estimate the flux of anthropogenic CO2 emitted in and/or around
the urban areas.
Conclusion
The CO2 sonde is shown to be a feasible instrument for CO2
measurements in the troposphere. The laboratory test with a vacuum chamber
has shown the precision of the CO2 sonde at ∼1010 hPa for 0.6 ppm
and at ∼250 hPa for 1.2 ppm. Comparisons of the CO2 vertical
profiles obtained by the CO2 sonde with two types of aircraft
observations, the CONTRAIL and the NIES/JAXA chartered aircraft, were
carried out. The CO2 sonde and CONTRAIL data were consistent. The
CO2 sonde data on 31 January 2011 were in good agreement with the
chartered aircraft data on the same day, but the CO2 sonde data
observed on 3 February 2011 were larger by approximately 1.4 ppm, as
compared with the chartered aircraft data obtained on the same day from the
ground to an altitude of 7 km. The measurement errors of the CO2 sonde
system up to an altitude of 7 km were estimated to be 1.4 ppm for a single
point of 80 s period measurements with a vertical height resolution of
240–400 m. We conducted the field CO2 sonde observations more than 20
times in Japan and successfully obtained CO2 vertical profiles from the
ground up to altitudes of approximately 10 km.
Our results showed that low-cost CO2 sondes could potentially be used
for frequent measurements of vertical profiles of CO2 in many parts of
the world, providing useful information to understand the global and
regional carbon budgets by replenishing the present sparse observation
coverage. The CO2 sondes can detect the local and regional transport
evidence by determining CO2 concentrations in the air layer trapped
between elevated inversion layers. Also, the CO2 sonde observation data
could help improve the inter-comparison exercise for inverse models and for
the partial validation of satellite column integral data. In future, the
CO2 sonde data will be used for the validation of satellites and the
calibration of ground-based observations of sunlight spectroscopic
measurements for column values of CO2 concentration.
Data availability
The CO2 sonde and chartered aircraft data used in this paper are available on request to the corresponding author. The CONTRAIL CME CO2 data are available on the Global Environmental Database of the Center for Global Environmental Studies at the National Institute for Environmental Studies (10.17595/20180208.001, Machida et al., 2018).
Author contributions
MO, YM, TN, KS, and TS developed the CO2 sonde and conducted laboratory experiments and field observations with contributions of RI. TM, HM, YS, IS, OU, and TT obtained the CONTRAIL and chartered aircraft data for the validation of CO2 sonde. MO, YM, and TN analyzed data and prepared the manuscript with contributions of all of the other authors.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We would like to thank Noriji Toriyama, Masahiro Kanada, Hidehiko Jindo, Masayuki Sera, Hiroshi Sasago, Tomoyuki Ide, Shoichiro Takekawa, Masahiro Kawasaki, Gen Inoue (Nagoya Univ.), Masatomo Fujiwara, Yoichi Inai (Hokkaido Univ.), Shuji Aoki, and Takakiyo Nakazawa (Tohoku Univ.) for their assistance and useful suggestions in the development of the CO2 sonde and the observations.
Financial support
This research has been supported by the Grant-in-Aid for Scientific Research (grant nos. KAKENHI 20310008 and KAKENHI 24310012), the Green Network of Excellence, Environmental Information (GRENE-ei) program from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), the Development of Systems and Technology for Advanced Measurement and Analysis Program from the Japan Science and Technology Agency (JST), and the joint research program of the Solar-Terrestrial Environment Laboratory (now new organization: the Institute for Space-Earth Environmental Research), Nagoya University.
Review statement
This paper was edited by Christof Janssen and reviewed by four anonymous referees.
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