Introduction
The release of anthropogenic greenhouse gases (GHGs) into the atmosphere
leads to a modification of their natural cycles and to a strong increase in
atmospheric radiative forcing . The Intergovernmental
Panel on Climate Change (IPCC) reported that the global average temperature
increased by 0.89 ∘C between 1901 and 2012
and will continue to increase during the 21st century
. To limit the global temperature rise, most
industrialized countries signed the “United Nations Framework Convention on
Climate Change” (UNFCCC) treaty in 1992 to stabilize their GHG emissions
between 1990 and 2000 and it entered into force in 1994. This convention was
enhanced by the Kyoto Protocol, which was signed in 1997 and was ratified by
182 countries. The countries engaged in the Kyoto Protocol aimed to reduce
their national emissions of the main long-lived GHGs by 5.2 % between
2008 and 2012 compared to the emission levels of 1990. The GHGs in question
are carbon dioxide (CO2), methane (CH4), nitrous oxide
(N2O), sulfur hexafluoride (SF6), hydrofluorocarbons (HFCs)
and perfluorocarbons (PFCs). The European Union (EU) committed itself to a
reduction of its GHG emissions by 8 % for the same period. In addition,
the EU aims to reduce its total GHG emissions by 20 % in 2020, relative
to emissions in 1990. Despite this commitment, it is extremely difficult to
validate the surface GHG fluxes on the country scale using a reliable,
transparent method.
Currently, countries report their respective GHG emissions to the UNFCCC on
an annual basis. These national emission inventories are based on bottom-up
methods and the reliability of these national inventories strongly depends on
the uncertainty attributed to each emission factor. To improve and validate
the bottom-up methods, it is crucial to better characterize the
biogeochemical cycles of the different GHGs, particularly as national
inventories report only anthropogenic emissions to the UNFCCC. Therefore, it
is important to develop new tools to quantify natural-source emissions and to
provide an independent verification of the emission inventories reported to
the UNFCCC.
Different methods based on atmospheric measurements can be used to estimate
GHG emissions on local to regional scales. Some of these approaches couple
atmospheric GHG measurements with measurements of associated atmospheric
tracers of air masses, including radon-222 ,
sulfur hexafluoride or isotopes, such as radiocarbon in
CO2 . A major advantage of this “multigas” approach is that
it avoids the use of complex chemistry-transport models; the tracers
that are used are subject to the same atmospheric transport mechanisms
as the GHGs. Nevertheless, an accurate assessment of the respective
measurement station footprint is required to allocate the estimated
surface fluxes to a specific region .
The first atmospheric CO2 continuous measurements started in the
1950s at Mauna Loa Observatory using a nondispersive infrared (NDIR) analyzer
with a repeatability better than 0.1 µmolmol-1
. Atmospheric CH4 monitoring began in the late
1970s using gas chromatograph (GC) systems equipped with a flame ionization
detector (FID) and a nickel catalyst to enable simultaneous CO2 mole
fraction detection . The repeatabilities were
approximately 10 nmolmol-1 for CH4 measurements and
0.7 µmolmol-1 for CO2 measurements. Coupling an
electron capture detector (ECD) to a GC system enabled the detection of
N2O and SF6 atmospheric mole fractions with repeatabilities
of approximately 1.0 nmolmol-1 and 0.1 pmolmol-1,
respectively . Subsequently, these two detectors (FID and ECD) have permitted the use of GC to analyze CO2,
CH4, N2O and SF6 atmospheric mole fractions
simultaneously and on a semicontinuous basis. Since the 1980s, atmospheric
monitoring stations that are part of the Global Atmosphere Watch (GAW) have
been gradually equipped with GC systems and NDIR analyzers. The GC system
technologies described above have continuously evolved to reach
repeatabilities better than 0.1 µmolmol-1 for CO2,
2.0 nmolmol-1 for CH4, 0.3 nmolmol-1 for
N2O and 0.1 pmolmol-1 for SF6, as shown by
, and .
New types of accurate instruments for CO2, CH4 and
N2O atmospheric measurements have recently become commercially
available. These instruments are based on optical technologies, including
cavity ring-down spectroscopy (CRDS), Fourier transform infrared spectrometry
(FTIR) and off-axis integrated cavity output spectroscopy (OA-ICOS). These
recent technologies are promising for atmospheric monitoring as they offer
high-frequency measurements (on the order
of 1 Hz), require low maintenance and achieve equivalent or
superior repeatability compared to GC systems .
Analyzers based on the CRDS technology are generally used for CO2 and
CH4 atmospheric measurements, OA-ICOS technology is used for
N2O measurements, and FTIR technology is designed to simultaneously
measure CO2, CH4 and N2O. These new types of
instruments are also more easily transportable, and have
demonstrated their feasibility as “traveling” instruments. They could thus
be used for comparisons and quality control purposes to ensure data
compatibility across a monitoring network.
Regardless of the benefits listed above, these optical technologies cannot
yet be used to measure atmospheric SF6 mole fractions; SF6 is
an extremely stable GHG, having a global warming potential of 23 900
. In addition, most of these new technologies need to be
continuously flushed, which makes it difficult to analyze flasks, in contrast
to the GC systems, which employ discrete samples and can analyze four or five
species simultaneously . Studies of optical
technologies are still progressing, and, as noted above, these new
technologies are very promising, particularly for a dense monitoring network,
such as the European infrastructure ICOS (Integrated Carbon Observation
System), which is dedicated to high-precision monitoring of greenhouse gases
over Europe. The CRDS technology is slowly replacing the GC systems and NDIR
analyzers for CO2 and CH4 monitoring in many stations, but GC
is still the reference instrument for N2O and SF6
measurements see. Consequently, we installed a GC system in
2010 at the mountain station of Puy de Dôme (France) to monitor, with a
high precision, the long-term atmospheric trend of the main four long-lived
greenhouse gases.
This paper focuses on 3 years (2010 to 2013) of ambient air measurements of
CO2, CH4, N2O and SF6, obtained using a GC
system at the Puy de Dôme station. After a short description of the
station, the detailed setup of the GC as well as the calibration strategy are
addressed. A paragraph is dedicated to data quality control and atmospheric
measurement comparisons, showing to what extent our measurement system are in
line with the WMO recommendations . In the last part, we
present and analyze our 3-year time series of ambient air measurements.
Finally, we demonstrate that these time series can be used to estimate the
monthly regional fluxes of CO2, CH4 and N2O in the
catchment area of the Puy de Dôme station, using radon-222 as an
atmospheric tracer.
The Puy de Dôme station
Site description
The Puy de Dôme station (45∘46′19′′ N,
2∘57′57′′ E) is located at the top of the Puy de Dôme
volcano (1465 ma.s.l.), in Auvergne in the center of France. This
station is managed by the Laboratoire de Météorologie Physique (LaMP)
and is part of the Observatoire de Physique du Globe de Clermont-Ferrand
(OPGC) located at Clermont-Ferrand, France. According to the French National
Institute of Statistics and Economic Studies (INSEE –
http://www.insee.fr), the ground cover of Auvergne
(26 013 km2) consists mainly of meadows (36.4 %), forests
(33.4 %) and arable land (17.6 %), the Puy de Dôme station being
located in the center of this region. The major anthropogenic GHG sources are
to the east of the station, where the town of Clermont-Ferrand is located:
10 km east of the Puy de Dôme station at an altitude of
396 ma.s.l. Clermont-Ferrand is the largest town in the region,
with approximately 150 000 inhabitants.
The CITEPA (Centre Interprofessionnel Technique d'Etudes de la Pollution
Atmosphérique) reports the French national GHG emissions to the UNFCCC
but has also provided a regional inventory of Auvergne for the year 2007
. According to the CITEPA, the anthropogenic
CO2 emissions in Auvergne are mainly attributable to road transport
and residential and industrial sectors, which represent respectively 45, 25
and 21 % of the total anthropogenic CO2 emissions of the region.
The agricultural sector is responsible for 90 % of total anthropogenic
CH4 emissions and 97 % of total anthropogenic N2O
emissions in the region. Ninety percent of the SF6 emissions are
related to the energy transformation sector.
A military base and a telecommunication center are located
20 m northwest of the station, also on the top of the
volcano. These facilities consist of a main building
(20 m height) and a telecommunication antenna
(89 m height). Since 2010, the only access road to the station has been
closed to the public and has been replaced by a cog train.
The atmospheric research station hosts different analyzers for long-term
atmospheric measurements of GHG, CO, O3, aerosol particles,
radon-222, clouds microphysics and radionuclide. The station is part of the
European ICOS, ACTRIS (Aerosol particles, Clouds, and Trace gases Research InfraStructure) and EMEP (European Monitoring
and Evaluation Programme) measurement networks and of the global GAW network.
Atmospheric conditions at the Puy de Dôme station
Meteorological parameters are monitored at the station, including wind speed,
wind direction, temperature, relative humidity and atmospheric pressure.
A wind shadow area between 300 and 360∘ is clearly observed in the
wind direction due to the building and the telecommunication antenna of the
military base, which both induce local turbulences. The planetary boundary
layer (PBL) height, wind speed and wind direction were extracted from the
European Center for Medium-range Weather Forecasts at a 3-hour time resolution for the years 2010 to 2012. The grid
cell used for the extraction has an area of
15km×15 km and is centered at 45∘45′ N,
3∘00′ E at an altitude of 575 ma.s.l. In this study, the
wind direction from ECMWF was used as the reference because the wind
direction provided by the meteorological sensor is influenced by the local
turbulences caused by the telecommunication antenna located at the military
base. The average difference in wind speed between the meteorological station
and the ECMWF data was 3.4±4.3 ms-1, the wind speed measured
by the sensor was higher because the sensor is located at a higher elevation
than the grid cell used for the ECMWF extraction (1465 ma.s.l.
compared to 575 ma.s.l.). Therefore, the wind speed from the
meteorological sensor was used to correct the ECMWF data. The PBL height and
the wind direction from ECMWF were interpolated using a linear regression fit
to obtain a 1-hour time resolution.
The Puy de Dôme station is primarily influenced by winds from a southwest
direction (48.2 % of the time) with a mean wind speed of
8.4 ms-1. The wind blows from the Clermont-Ferrand sector
(45–135∘) only 7.7 % of the time, with a mean speed of
4.2 ms-1. The PBL height analysis revealed that the Puy de
Dôme station is in the free troposphere during more than 70 % of the
time and up to 81 % of the time during winter.
Back trajectories were calculated using the Lagrangian dispersion model
Flexpart version 8.2.3, based on ECMWF ERA-Interim data at a horizontal
resolution of 1∘×1∘, with 60 vertical levels and a
3-hour temporal resolution. Eight particles were released every 15 min (96
particles every 3 h) in a 3-D box centered around the Puy de Dôme
station (from lower left corner 45.76∘ N, 2.95∘ E to upper
right corner 45.78∘ N, 2.97∘ E; between 1400 and
1500 ma.s.l.) with a lifetime of 3 days. This simulation was
performed for particles arriving at the station between 1 January 2010 and 31
December 2012. The footprints were computed on a 1∘×1∘
horizontal grid, following the method described by and taking
into account the planetary boundary layer height at each particle location.
We considered that a particle is influenced by surface emissions from one
grid cell when its elevation is under the PBL height and that its influence
is inversely proportional to the PBL height. The maps presented in
Fig. show the footprints for air masses arriving at the
station between 14:00 and 16:00 UTC, when the PBL is usually well developed
(Fig. a), and between 22:00 and 06:00 UTC, when the PBL
is below 1400 m and the station is within the free troposphere
(Fig. b). The grid cell influence is represented as
a relative influence compared to the maximum value (in percent). On both
maps, the station is located within the black grid cell. By analyzing back
trajectories from HYSPLIT over 437 days at Puy de Dôme (i.e., from
Atlantic and continental western Europe areas), showed
that 87 % of air masses reaching the station are from the west.
A statistical analysis of back trajectories over 4 years conducted by
demonstrated that winter air masses reaching Puy de
Dôme travel over longer distances from the west than summer air masses.
Instrumental setup
The GHG observations at Puy de Dôme started in 2000 with continuous
CO2 measurements using a nondispersive infrared (NDIR) spectrometer.
Since 2001, a pair of flasks has been sampled once a week by the LaMP team
and analyzed by GC for CO2, CH4, N2O and SF6
mole fractions and by a mass spectrometer for δ13C and
δ18O in CO2 at the LSCE in Gif-sur-Yvette, France. In
2010, the GC system for semicontinuous measurements of CO2,
CH4, N2O and SF6 was installed at the station. In
2011, the NDIR spectrometer was replaced by a CRDS for continuous CO2
and CH4 measurements. Since 2002, the station has also been equipped
with a radon-222 (222Rn) analyzer based on the active deposit
method. These instruments are housed in a regulated-temperature room, the
inlet lines being located on the roof of the station, 10 ma.g.l.
This section focuses on the setup of the GC system running at the Puy de
Dôme station since July 2010.
Description of the GC system
The GC system installed at the Puy de Dôme station is a commercial
HP-6890N from Agilent that was modified and optimized at the LSCE for
automatic and semicontinuous atmospheric measurements of CO2,
CH4, N2O and SF6 mole fractions in dry ambient air
. Similar instrument configurations are installed at the
Gif-sur-Yvette and Trainou stations in northern France
.
Footprint of the Puy de Dôme station from the
Lagrangian dispersion model Flexpart (a) during daytime
(14:00 to 16:00 UTC), when the PBL is usually well developed,
and (b) during nighttime (22:00 to 06:00 UTC), when the
station is usually in the free troposphere.
The ambient air is pumped from the roof of the station (pump KNF: PMF
1433-811) through a 10 m long Dekabon tube with an outside diameter
of 1/2 in. (∼1.27 cm). Three filters (140, 40 and
7 µm TF series from Swagelok) are placed in series to protect the
pump and the analysis system from dust and aerosol particles. After passing
the pump, the ambient air is pressurized and dried in two steps. First, the
air passes through a commercial decanting bowl (40 mL volume) placed
in a refrigerator set at 5 ∘C for preliminary drying. The
water accumulated in the decanting bowl is flushed out every 6 hours for
10 s by opening a solenoid valve. In a second step, the ambient air passes
through a glass trap in an ethanol bath that is maintained at
-55 ∘C by a cryocooler (Thermo Neslab CC-65). The dew point
of the air going in the instrument is approximately -50 ∘C.
The glass trap is changed during the weekly maintenance of the station. An
electronic box is used to regulate the refrigerator temperature and to open
and close the solenoid valve of the decanting bowl. This box also records the
temperatures of the fridge, the ethanol bath and the room. In case of power
failure, the entire GC system is connected to an uninterruptible power supply
(UPS) that allows the system to run for a few hours.
Schematic of the GC system setup (gas flow) at the Puy de
Dôme station.
The GC system consists of an injection part, a separation part and
a detection part. These three different parts are indicated by different
colors in Fig. . For analysis, an air sample is first filled
into the two sample loops. The sample is then pushed by different carrier
gases to the chromatographic columns, where the species are separated.
Finally, CO2 (via a nickel catalyst) and CH4 are detected by
a FID and a microelectron capture detector (μECD) is used to detect
N2O and SF6. One injection and analysis requires 5.4 min.
Sample analysis
The injection part (outlined by green line in Fig. ) consists
of an eight-port microelectronic valve no. 1 (model DC8WE from Valco vici,
Switzerland) that enables the selection of the samples to be analyzed
(ambient air or gas cylinders). The selected sample is injected into the
system via an electronic pressure control (EPC-Aux5) through two sample loops
located in the room. The sample loops are placed in series on two six-port
two-way Valco valves (no. 2a and no. 2b). The sample loop for CO2 and
CH4 analysis has a volume of 15 mL (sample loop on valve no.
2a), and the one for N2O and SF6 analysis has a volume of
10 mL (sample loop on valve no. 2b). They are both flushed with the
sample gas for 0.75 min at a flow rate of 180 mLmin-1
(corresponding to a pressure of 2.5 psi on Aux5). Before the
injection, the two sample loops are equilibrated at temperature and
atmospheric pressure for 0.5 min by setting Aux5 to 0 psi.
After equilibration, the samples are injected into the columns with the
carrier gases by opening valves no. 2a and no. 2b. The carrier gas used for
the FID is N2 (purity > 99.9999 %), whereas
a mixture of argon and methane (95/5%, ECD quality) is used for
the μECD. A purifying cartridge (Aeronex) is placed after each carrier
gas cylinder.
The columns used to separate the different molecules are placed in an oven
maintained at 80 ∘C (see the section outlined by the yellow
line in Fig. ). A Hayesep-Q (12′×3/16′′SS, mesh
80/100) analytical column is used for CO2 and CH4
separation. For N2O and SF6 separation, a pre-column
Hayesep-Q (4′×3/16′′SS, mesh 80/100) and an analytical column
Hayesep-Q (6′×3/16′′SS, mesh 80/100) are used. The pre-column is
back-flushed between 0 and 0.75 min and between 3.7 and
5.4 min with a 100 mLmin-1 flow rate of the carrier gas
to eliminate heavy electrophilic molecules from the system to avoid an
eventual pollution of the analytical column, which might induce an increase
in the μECD baseline. Between 0.75 and 3.7 min, the N2O and
SF6molecules are injected first into the pre-column and then into
the analytical column, where separation occurs. The analytical column is
directly connected to the μECD. The N2O and SF6 retention
times are 4.3 and 4.8 min, respectively.
The CH4 and CO2molecules are detected by an FID and a
Ni-catalyst used to reduce CO2 to CH4. Methane molecules
elute after 2.7 min and are injected directly into the detector for
analysis. Once the CH4 molecules are released from the analytical
column, the Valco valve no. 4 is opened to connect the nickel catalyst,
allowing CO2molecules to be reduced to CH4 to enable
CO2 detection by the FID. The retention time of CO2 is
3.5 min. The FID temperature is controlled at 300 ∘C,
and the flame is fed with hydrogen at a flow rate of
a 65 mLmin-1 (provided by an NM-H2 hydrogen generator from
F-DBS) and zero air at a flow rate of a 400 mLmin-1 (provided by
a combination of a compressor from June-Air and a 75–82 air zero generator
from Parker-Balston). Hydrogen is also used for CO2 reduction over
the Ni catalyst. The typical efficiency of the catalyst is 97 % in the
CO2 atmospheric mole fraction range.
Figure shows the typical chromatograms obtain by the
FID and μECD detectors. The top panel presents
the FID's response in picoampere. The spike observed between the CH4 peak and the
CO2 peak in the close-up panel is caused by the opening of valve no.
4. The bottom panel of Fig. presents the μECD's
response in Hz. The first large peak observed at approximately
2.7 min is the O2 peak, which is followed by the N2O
peak and finally by the SF6 peak.
Typical chromatograms obtained by the FID (top panel) and by the
μECD (bottom panel).
Analysis management
Data acquisition, valve opening and closing and temperature regulation of the
GC system are entirely processed by Chemstation software (version A.10.02,
Agilent). This software allows controlling the GC system parameters through
the so-called “methods”. A typical method is configured to do the
following:
control the temperature of the detectors, the catalyst and the oven;
regulate the flows of the sample (via Aux5), the carrier gases
(via Aux3 and Aux4), H2 and zero air, all via five distinct
EPCs;
schedule the opening and closing of the four six-port two-way Valco valves,
controlled via the internal events output GC connector;
choose the position of the eight-port microelectronic Valco valve,
controlled via the external events output GC connector; and
integrate the results of the analysis (via the chromatograms).
A typical method is presented in Table and corresponds to
one analysis of a chosen sample. Table summarizes the GC
system setup used between 2010 and 2013. A sequence lasting 3 days is
designed by the sequential arrangement of methods which enables the automatic
selection of ambient air and calibration gas measurement. The created
sequence runs in a loop mode.
The FID and μECD signals (see Fig. ) are expressed
in picoampere and hertz, respectively. The peak
integrations (area and height) of the different chromatograms are
automatically computed by the Chemstation software at the end of each method
and the integration results are stored in “.txt” files. The repeatability
of our GC system (see Sect. ) is improved when the peak
areas for CO2, CH4 and N2O and the peak heights for
SF6 are used. Once a day, the integration results are transferred to
and stored in the LSCE database via ftp, and the mole fractions of the
analyzed samples are automatically calculated. Three to five times each week,
a trained operator evaluates the performances of the GC through a dedicated
graphical application, enabling graphics of the instrument parameters to be
drawn (see Sect. ). Based on these graphics, flags are
manually assigned to the data.
Calibration strategy
The GC system is calibrated using a two-point calibration strategy. Two
working standards containing a known amount of CO2, CH4,
N2O and SF6 in synthetic air (matrix of N2,
O2 and Ar) are used. The mole fractions of the trace gases in
the two working standards are selected to bracket the typical ambient air
mole fractions observed at the Puy de Dôme station and are referred to as
working standard high (WH) and working standard low (WL). These gas mixtures
are used to fill 40 L aluminum cylinders (Luxfer) to 200 bar
by Deuste Steininger (Mühlhausen, Germany). All working standards are
calibrated at LSCE against the laboratory standard scale of the World
Meteorological Organization (WMO scale) provided by the Central Calibration
Laboratories (CCL) of the National Oceanic and Atmospheric Administration
(NOAA). The calibration scales currently used are WMO-X2007, NOAA-04,
NOAA-2006A and NOAA-2006 for CO2, CH4, N2O and
SF6, respectively .
The response function of μECD for N2O analysis is nonlinear,
especially in the range below and above the tropospheric values
see. The nonlinearity of
our μECD was tested by analyzing five cylinders calibrated by the CCL on
the NOAA-2006A scale and with N2O mole fractions between 302.00 and
338.04 nmolmol-1. In this small mole fraction range, which is
important for our measurements, a two-point calibration describes the
response function of our μECD sufficiently. It compares very well with an
exponential fit through five cylinders, with an average difference of
0.01±0.13 nmolmol-1. This result confirms that a two-point
calibration strategy is well adapted to correct for the μECD nonlinearity
in atmospheric mole fraction ranges. Similar tests demonstrated that the FID
response is linear in the atmospheric mole fraction range for CO2 and
CH4 measurements.
Measurement method used for the GC system at the Puy de Dôme station.
The “On” position on the valves presented here corresponds to the dashed lines in
Fig. and the “Off” position correspond to the solid lines.
Time (min)
Parameter
Value
Comments
0.00
Aux 3
45.0 psi
Carrier gas pressure for N2
0.00
Aux 4
20.0 psi
Carrier gas pressure for Ar/CH4
0.00
Aux 5
2.5 psi
Sample pressure
0.00
Valve no. 2
On
Flush of the sample loops
0.00
Valve no. 3
On
Backflush of the pre-column (N2O/SF6)
0.75
Aux 5
0.0 psi
Sample pressure
0.75
Valve no. 3
Off
Stop of pre-column backflushing
1.25
Valve no. 2
Off
Sample injection
3.10
Valve no. 4
On
Injection of CO2 into the catalyst
3.60
Aux 3
0.0 psi
Carrier gas pressure for N2
3.70
Valve no. 3
On
Backflush of the pre-column
5.30
Valve no. 4
Off
Opening of the catalyst valve
5.40
Aux 3
45.0 psi
Carrier gas pressure for Ar/CH4
GC system equipment and temperature and flow rate settings.
Detector
FID (CO2/CH4)
μECD (N2O/SF6)
Carrier gas
N2 cylinder (purity > 99.9999 %)
Ar/CH4 cylinder (ECD quality)
+ purifier
+ purifier
Flow rate
100 mLmin-1
45/65 mLmin-1
Loop sample volume
15 mL
10 mL
Oven temperature
80 ∘C
80 ∘C
Pre-column
Hayesep-Q
4′×3/16′′SS, 80/100
Analytical column
Hayesep-Q
Hayesep-Q
12′×3/16′′SS, 80/100
6′×3/16′′SS, 80/100
Detector temperature
300 ∘C
395 ∘C
Catalyst temperature
390 ∘C
Gas supply
H2 generator: 60 mLmin-1
Zero air generator: 400 mLmin-1
The two working standards (WH and WL) are analyzed every 30 min to
correct for atmospheric (temperature and pressure) changes as well as
instrumental drifts, enabling the analysis of five samples between each
calibration. The lifetime of our standards is approximately 3 years using
this calibration strategy. To limit the risk of drift, each working standard
must be replaced before reaching 30 bar pressure. At the end of their
use at the station, all working standards are recalibrated at LSCE to verify
their stability over their lifetimes. At the Puy de Dôme station, the
first set of working standards was replaced on 25 April 2013. Reanalysis of
the standards at LSCE revealed mean differences (2010–2013) of
-0.11 µmolmol-1, -0.03 nmolmol-1,
-0.1 nmolmol-1 and 0.0 pmolmol-1 for CO2,
CH4, N2O and SF6, respectively. The observed
differences are not statistically significant except in the case of
CO2. The calibration cylinders drifted by 0.08 and
0.15 µmolmol-1 over their lifetime for CO2. The
CO2 data presented in this paper are not corrected for the observed
drift on the order of 0.03 µmolmol-1yr-1. The first
measurement period (July 2010 to 24 April 2013) is called “period A” in
this paper, and the second measurement period (after the change of the
working standards) is called “period B” (from 25 April 2013 to 30
June 2013). The mole fractions of the working standards used at Puy de
Dôme are presented in Table .
Trace gas mole fractions of GC working standards used
during period A (July 2010 to April 2013) and period B (May and
June 2013).
Species
Period A – 33 months
Period B – 2 months
WH
WL
WH
WL
CO2 (µmolmol-1)
425.10
372.45
449.60
363.31
CH4 (nmolmol-1)
2179.90
1732.99
2083.38
1663.52
N2O (nmolmol-1)
340.90
322.93
348.03
326.51
SF6 (pmolmol-1)
10.05
5.38
9.86
5.86
Other instrumentation
Flask sampling unit
A flask sampling unit was installed at the Puy de Dôme station in 2002
for weekly sampling. It consists of a pump that pressurizes two 1 L
glass flasks placed in series to 1 bar relative pressure. They are
flushed for 15 min with dry ambient air prior the pressurization.
Ambient air is dried in a distinct cooling trap maintained in the same
ethanol bath as the GC trap (see Sect. ). The flasks are
then shipped and analyzed at LSCE by a GC system for CO2,
CH4, N2O and SF6 and by a Finnigan MAT-252 isotope
mass spectrometer for CO2 isotopic composition (13C and
18O). The calibrations for trace gas analysis are performed in the
same manner as presented in Sect. , and the results are stored
in the same database.
Radon-222 measurement system
Radon-222 (222Rn) is a radioactive noble gas (T1/2=3.8 days)
and is part of the radioactive decay chain of uranium-238. Uranium-238 in the
earth's crust results in the emission of 222Rn by the earth's
surface. Atmospheric radon-222 activity has been monitored at the Puy de
Dôme station since 2002. The analyzer is based on the active deposit
method, which consists of alpha decay counting of 222Rn's solid
short-lived daughters: 218Po, 214Pb and 214Bi.
The measurement technique has been described in detail by
and . To avoid the loss of the solid 222Rn
daughters, the inlet line is a 6 m long straight metal tube with a
31 mm outside diameter. During the first years of measurements, the
inlet line was frequently contaminated by room air, and only measurements
after October 2006 can be used. estimated
a radioactive disequilibrium see at the Schauinsland
station (Germany; 47∘54′ N, 7∘54′ E;
1205 ma.s.l.) of 1.15±0.14. This value was independently
confirmed by . The Puy de Dôme station and the
Schauinsland station are two medium-elevation mountain stations, having the
same geographical environment. They are both frequently above the continental
boundary layer, especially in winter. Based on the similarities between the
Puy de Dôme station and the Schauinsland station, the measured
(222Rn) activity at Puy de Dôme has been corrected for the
radioactive disequilibrium by using the same value (1.15±0.14).
CO2 continuous measurements by in situ NDIR
Continuous CO2 measurements with a NDIR gas analyzer at Puy de
Dôme began in 2000. The instrument is an integrated system constructed
around a LI-COR NDIR (Li-6252 NDIR, LI-COR Inc., Nebraska, USA) analyzer
optical bench. The CO2 measurements are based on the difference in
absorption of infrared radiation passing through two cells: the reference
cell and the sample cell. Infrared radiation is transmitted through both cell
paths, and the analyzer signal is proportional to the difference in
absorption between both cells. The measurement frequency is 1 Hz, and
the cell flow is typically 20 mLmin-1 for the sampling cell and
15 mLmin-1 for the reference cell, which is continuously flushed
with a reference gas. The calibration strategy is based on four cylinders
calibrated on the WMO-X2007 scale. The calibration is performed twice a year
by analyzing each calibration cylinder 30 times for 10 min. Data are then
corrected using a quadratic fit.
Ambient air is pumped from the roof to the instrument through
a 3/8 in. (∼0.95 cm) outside diameter Dekabon line.
The air is dried by passing through a glass trap maintained in a cold ethanol
bath (see Sect. ). Ambient air is analyzed for
50 min following the analysis of the reference cylinder for
10 min, the latter passing through both cells at the same time. The NDIR spectrometer was
replaced by a CRDS analyzer in April 2011.
CO2 and CH4 continuous measurements by
in situ CRDS
The CRDS analyzer (Picarro G1301) was installed in April 2011. It
continuously and simultaneously measures CO2, CH4 and
H2O atmospheric mole fractions. We use four calibration cylinders
spanning the atmospheric range of 366 to 453 µmolmol-1 for
CO2 and 1722 to 2107 nmolmol-1 for CH4. The
cylinders are calibrated at the LSCE laboratory on the WMO-X2007 and NOAA-04
scales. The instrument calibration is performed automatically every 15 days
by injection, according to the following scheme: 4 times for each calibration
cylinder for 30 min, beginning with the one with the lowest mole
fractions and ending with the one with the highest mole fractions. For each
calibration cylinder, the entire first injection and the first 15 min of the
subsequent injections are automatically rejected because of
equilibration time. The instrument calibration takes
8 h and a linear fit is applied to compute the analyzer response. A target
gas is automatically injected into the CRDS every 10 h for 30 min and,
again, the first 15 min are automatically rejected. The SDs at 1 sigma of
the target gas analysis over 1 year are 0.02 µmolmol-1 for
CO2 and 0.14 nmolmol-1 for CH4.
Ambient air is injected into the CRDS from the roof (through a 3/8 in.
Dekabon line equipped with three filters of 140, 40 and 7 µm)
using a pump located after the optical cavity. To avoid the risk of bias
caused by the interference of water vapor and trace gases in the CRDS
, the ambient air is dried prior to its injection into the
CRDS by the drying system presented in Sect. . A problem
occurred in the cavity in August 2011, and the CRDS measurements were stopped
until April 2012. The instrument has been returned to the manufacturer for
repair.
Quality control of the GC system and comparisons with different analyzers
A target gas (TGT) is injected into the GC system once an hour for quality
control. The target gas cylinder is a 40 L cylinder filled with dry
ambient air at Gif-sur-Yvette. After a stabilization time of at least 1
month, the cylinder is analyzed at LSCE using the laboratory primary
standards, and CO2, CH4, N2O and SF6 mole
fraction values are assigned to the cylinder (see Table ).
Figure shows the time series of the target gas analysis from
July 2010 to June 2013. The target gas cylinder was not changed over the 3
years. The different data gaps observed in CO2 and CH4
between March and October 2011 were caused by problems with the hydrogen
generator (leaks in the electrolysis cell). The large data gaps in
N2O and SF6 between April and August 2012 were due to
a problem with the power supply of Valco valve no. 2b. The reproducibility
(computed here as the SD at 1 sigma of the target analysis over 1 year of
measurement) and the typical short-term repeatability (SD of the target
analysis over 24 h) are presented in Table .
Assigned target gas values with the respective mole fractions measured using
the GC system at the Puy de Dôme station over period A and period B. The assigned
values were measured by the GC system at the LSCE central lab against WMO calibration gases.
Species
Assigned values
Period A
Period B
CO2 (µmolmol-1)
402.57±0.07
402.42±0.15
402.38±0.46
CH4 (nmolmol-1)
1973.87±0.73
1973.81±2.12
1963.72±6.64
N2O (nmolmol-1)
325.71±0.23
325.90±0.35
325.76±0.47
SF6 (pmolmol-1)
7.23±0.04
7.23±0.06
7.20±0.07
The vertical dark blue lines in Fig. indicate when the working
standards were changed and separate the two measurement periods: period A and
period B (see Sect. ). The average mole fractions of the TGT
measurement during period A and period B are presented in
Table together with the respective assigned mole fractions.
The measured CH4, N2O and SF6 mole fractions agreed
well with the assigned mole fractions during period A (considering the
uncertainties). The difference between the CO2 assigned and measured
values in period A and period B was 0.15 and 0.19 µmolmol-1,
respectively, confirming the consistency between the two scales used in
periods A and B. This agreement when using two different calibration scales
shows that the problem is probably due to an error in the value attributed to
this target gas. During period B, the average CH4 mole fraction of
the TGT gas was lower by approximately 10 nmolmol-1 compared to
period A. This decrease was most probably due to a micro-leak in the WH line
that affects only the CH4 mole fractions. Target data as well as
ambient air measurements for this period should be recalibrated by applying
a one-point calibration to the four trace gases. This micro-leak was detected
and fixed in September 2013, when the target cylinder was replaced by a new
one.
Time series of the target gas measured with the GC system
at Puy de Dôme. The vertical blue lines in each panel
correspond to the date of the working standards change.
Reproducibility and typical short-term (24 h) repeatability of the GC
system at Puy de Dôme, both within 1 sigma.
Species
Reproducibility
Short-term
repeatability
CO2 (µmolmol-1)
0.14
0.1
CH4 (nmolmol-1)
2.12
1.2
N2O (nmolmol-1)
0.34
0.3
SF6 (pmolmol-1)
0.06
0.06
In addition to internal quality control performed via the target gas
analysis, comparisons of in situ ambient air analysis, flask analysis
and cylinder analysis performed by different analyzers are
relevant. These comparisons enable the validation of the scale
consistency between different instruments and the detection of
possible leaks or biases introduced by the inlet lines. The WMO-GAW
gives recommendations on the scientific level of compatibility for such a comparison
in the Northern Hemisphere . These levels are
±0.1 µmolmol-1 for CO2,
±2.0 nmolmol-1 for CH4,
±0.1 nmolmol-1 for N2O and
±0.02 pmolmol-1 for SF6.
The mean differences between the in situ GC system measurements and weekly
flask sampling, in situ NDIR measurements and in situ CRDS measurements are
summarized in Table . These differences are calculated
from the hourly mean measurements. A 2-sigma filter was applied to the
differences to flag the eventual outliers. Comparisons between GC and NDIR
were based on 9 months of overlapping measurements (July 2010 to April 2011)
and revealed a mean CO2 difference (GC minus NDIR) of
-0.14±1.78 µmolmol-1. Comparisons between GC and CRDS
were based on 20 months of overlapping measurements, from April 2011 to
July 2013 with a break between August 2011 and April 2012. The average
differences (GC minus CRDS) were 0.21±0.78 µmolmol-1 for
CO2 and -0.64±5.46 nmolmol-1 for CH4 over
the total overlapping measurements. Because the CRDS instrument was stopped
for several months and shipped to the manufacturer in 2011, we compared the
results before and after its repair. In the first overlapping measurement
period (April to August 2011), the differences were
-0.13±0.61 µmolmol-1 and
-1.27±3.49 nmolmol-1 for CO2 and CH4,
respectively. In the second overlapping measurement period (April 2012 to
July 2013) the CH4 difference decreased to
-0.26±5.02 nmolmol-1, whereas the CO2 difference
increased to 0.28±0.75 µmolmol-1. Over the second
comparison period, the observed CO2 difference remained constant with
time and did not depend on atmospheric mole fractions. The inlet lines,
including the pumps and the dryer systems, were tested for 3 weeks: a common
inlet line for ambient air measurements has been used for the GC and CRDS
(the GC one, described in Sect. ). During these 3 weeks
of testing, the difference between GC and CRDS remained constant and equal to
0.28 µmolmol-1, confirming that the inlet lines did not
cause the observed bias. Even after changing the GC working standards in late
April 2013, the CO2 difference was still observed. This observed
difference is stable over time and is not concentration dependent. A second
experiment was to analyze the second set of working standards on the CRDS.
The results showed a difference between the assigned value (at LSCE) and the
CRDS of 0.03 µmolmol-1 on the WL and of
0.34 µmolmol-1 on the WH.
Results of the comparisons between in situ measurements obtained using the GC system,
NDIR, CRDS and flask measurements. The mean differences in the cylinder analysis at the Puy de Dôme
and Gif-sur-Yvette stations are presented in the last column. Flasks and cylinders were analyzed
at the LSCE central lab at Gif-sur-Yvette.
Date
July 2010–July 2013
July 2010–April 2011
April 2011–July 2013
July 2010–July 2013
Comparisons
GC in situ – flasks
In situ:
In situ:
Cylinders:
Average
No. of flasks
GC–NDIR
GC–CRDS
PUY–GIF
CO2 (µmolmol-1)
0.11±1.19
55
-0.14±1.78
0.21±0.78
-0.02±0.11
CH4 (nmolmol-1)
0.04±4.30
57
n/a
-0.64±5.46
0.64±0.26
N2O (nmolmol-1)
0.12±0.55
47
n/a
n/a
0.21±0.47
SF6 (pmolmol-1)
-0.01±0.07
53
n/a
n/a
0.03±0.03
Comparisons between the GC system and the flask analysis or comparison
cylinders provide information on the scale consistency between different
laboratories. For the four analyzed long-lived GHGs, the comparisons between
GC in situ measurements and flask analyses reached the desirable comparison
levels (see Table for more details). The two GC
measurements bracketing each sampled flask measurements are linearly
interpolated in order to match the time of the flask sampling. The Puy de
Dôme station also participates in the “Cucumbers comparison programme”
(http://cucumbers.uea.ac.uk/) in the framework of the European Union
CarboEurope project (2000–2005), EU IMECC (2007–2011) and InGOS
(2011–2015) infrastructure projects. Three cylinders are alternately
analyzed on the GC at Puy de Dôme and at the Gif-sur-Yvette, Trainou
(France), Kasprowy Wierch (Poland) and Hegyhatsal (Hungary) stations.
Table presents the mean differences between the
average analysis of the three comparison cylinders at Puy de Dôme and
Gif-sur-Yvette (LSCE) between 2011 and 2013.
The different comparison methods presented in this section showed that the GC
system installed at the Puy de Dôme station matches the WMO-GAW
recommendations for CH4 and SF6 measurements. The CO2
comparison shows different results depending on the method used. The
recommendations are met by considering the comparison with cylinders or
flasks, whereas they are not if we consider only the comparison between in
situ instruments. The WMO-GAW recommendations concerning the N2O
measurements are ambitious considering the repeatability obtained with our
GC. The N2O measurements are not in line with the WMO-GAW
recommendation but we note that the different comparison methods used show
differences lower than our instrumental repeatability.
Results and discussions
Three years of ambient air measurements
Figure shows the hourly time series of CO2,
CH4, N2O and SF6 ambient air mole fractions together
with the 222Rn activities at Puy de Dôme from July 2010 to the
end of June 2013. The different gaps observed in the atmospheric GHG time
series are explained in Sect. . These time series are
presented with the respective monthly background values (black lines). The
monthly background values were calculated from the monthly average nighttime
mole fractions (between 22:00 and 06:00 UTC), when the station is above the
PBL (see Fig. ). The hourly 222Rn activities are
presented in the last panel of Fig. and varied between 0
and 9 Bqm-3 over the 3 years of measurements. From February 2012
to the end of April 2012, the computer for 222Rn data acquisition
had hardware and software problems, resulting in the observed gap.
Hourly mole fractions of CO2, CH4,
N2O and SF6 atmospheric ambient air and hourly
activities of 222Rn at Puy de Dôme from July 2010 to
June 2013. The black lines are the respective monthly GHG
background mole fractions of the Puy de Dôme measurements
(between 22:00 and 06:00 UTC) and the 222Rn background
activity at Mace Head (Ireland).
Mean diurnal cycles of CO2, CH4,
N2O, SF6, and 222Rn together with the
planetary boundary layer height (relative to Clermont-Ferrand
altitude – 396 ma.s.l.) at the Puy de Dôme station
for each season. Trace gas mole fractions were detrended based on
1 January 2013.
Figure presents the mean diurnal cycles per season of
CO2, CH4, N2O, SF6, 222Rn and for
the PBL height (from ECMWF) from June 2010 to June 2013. The GHG diurnal
cycles were computed from the detrended hourly time series to the reference
of 1 January 2013. The mean yearly increase rate was subtracted from the time
series, before computing the seasonal mean diurnal cycles. To represent the
thickness of the PBL relative to the ground level, we used the altitude of
Clermont-Ferrand (396 ma.s.l.) as the reference altitude to plot
the PBL height, Clermont-Ferrand being located at the lowest altitude of the
ECMWF extracted grid cell. The horizontal solid black line on the PBL height
panel in Fig. gives the altitude of the station above
Clermont-Ferrand and enables a quick observation of whether the station is
within or above the PBL. The mean diurnal cycles of the PBL height exhibited
the same pattern for each season, with an increase in height from 06:00 to
12:00 UTC followed by a stable height until 15:00 UTC. After 15:00 UTC,
the PBL height began to decrease, reaching a minimum after 21:00 UTC. On
a mean annual scale, the GC sampled the trace gases within the PBL between
09:00 and 18:00 UTC with an enlarged time step in summer and a narrower time
step in winter. In winter, the Puy de Dôme station is often above the PBL
during several consecutive days.
The mean diurnal cycles of the long-lived GHGs and 222Rn
observed in this study are typical of mountain sites, as
previously described by for the Schauinsland
station (1205 ma.s.l.) or for the Kasprowy
Wierch station (Poland, 1987 ma.s.l.). The PBL height is
a key atmospheric factor, particularly for mountain sites where measurements
alternate between the free troposphere and the PBL.
The atmospheric mole fraction variabilities of trace gases are generally
larger in the PBL because of the combination between its diurnal variability
and the emissions from surface sources.
Due to its short radioactive lifetime, 222Rn cannot
accumulate in the free troposphere, in contrast to other long-lived
trace gases. The mean diurnal cycles of 222Rn
(Fig. ) exhibited larger variations in summer,
when the PBL height is maximal and the inlet line of the
station alternates between the PBL during daytime and above during
nighttime. In winter, 222Rn activities are lower than in
summer because the station is usually in the free troposphere.
The mean diurnal cycles for CO2 and CH4 exhibit different
shapes. As for the 222Rn activities, the CH4 mole fractions
at the Puy de Dôme station are higher in the afternoon, when the PBL is
well developed, compared with the nighttime mole fractions. On a yearly
average, CH4 afternoon mole fractions are 3.3 nmolmol-1
higher than the nighttime mole fractions. We observed an opposite diurnal
cycle trend for CO2: the biosphere is a sink for CO2 during
the daytime and counterbalances the atmospheric effects. This is seen clearly
during summertime, when the photosynthetic activity is maximal: the amplitude
of the CO2 diurnal cycle is 7.2 µmolmol-1, with
a minimum mole fraction of around 16:00 UTC. The CO2 mole fractions
are maximal in winter when the biosphere acts as a CO2 source, mainly
driven by soil respiration.
The amplitudes of the N2O and SF6 diurnal cycles are
very small and nearly undetectable, except for the N2O
in summer which exhibits an amplitude of 0.25 nmolmol-1.
The CH4, N2O and SF6 mole fractions are largest in spring and
lowest during summertime because their respective mole fractions are
mainly driven by the PBL height and the associated vertical mixing.
The marine boundary layer reference
In this section, the background mole fractions of recorded trace gases at the
station (see Sect. ) are compared with the respective
marine boundary layer references (MBLRs). Here, the MBLRs are the monthly
zonal average trace gas mole fractions for 45.5∘ N computed from
NOAA measurements . They were retrieved from the
Global Monitoring Division of the NOAA Earth System Research Laboratory.
Figure shows the differences between the monthly mean
background mole fractions at Puy de Dôme (nighttime values between 22:00
and 06:00 UTC) and the respective monthly MBLRs for CO2,
CH4, N2O and SF6. These comparisons enable the direct
quantification of the influence of sources and sinks on trace gases at the
station relative to oceanic air masses. These differences are called
continental offsets. The CO2 continental offset has negative values
in spring, indicating the influence of the continental biosphere, which acts
as a sink. During summer, autumn and winter, the offsets are positive,
revealing the importance of continental fossil fuel and biospheric sources in
the Puy de Dôme catchment area. The continental offsets are usually
positive for CH4 and always positive for N2O, indicating the
strong influence of agricultural sources (see Sect. )
in the Puy de Dôme footprint. Finally, the SF6 offset varied
between -0.10 and +0.12 pmolmol-1, which represents the same
order of magnitude as the GC measurement repeatability. In addition,
Fig. shows the monthly 222Rn continental offset
(for nighttime selection data between 22:00 and 06:00 UTC) at the Puy de
Dôme station relative to marine air. The marine air 222Rn
reference was computed from the mean activity of 15 years of
measurements during maritime background conditions at the European background
site of Mace Head see and is equal to
168 mBqm-3.
Differences between the monthly background at Puy de
Dôme and the respective monthly MBLR at 45.5∘ N latitude for CO2,
CH4, N2O and SF6. The last panel is the
222Rn offset relative to marine air.
The radon tracer method
Method
Once emitted by soils, 222Rn is an excellent tracer of continental
air masses due to its physical and chemical properties. Thus the radon tracer
method (RTM) has been used in numerous atmospheric studies to estimate trace
gas surface emissions on local to regional scales. Detailed descriptions of
this method are given in the following studies:
.
The RTM is based on Eq. (), where Jx and JRn are
the respective fluxes of a trace gas x and 222Rn. The ΔCx and ΔCRn terms are the temporal variations in the trace
gas x mole fraction and in the 222Rn activity over a period
Δt. Finally, λRn is the 222Rn decay
constant.
Jx=JRnΔCxΔCRn1-λRnCRnΔCRnΔt
As shown in Fig. , the diurnal variations in trace gases at
Puy de Dôme are very weak, which makes difficult to correctly assess the
ΔCx and ΔCRn terms on a daily basis. In this study,
we apply the RTM approach presented by , in which the
CO2 fluxes at the Schauinsland station were calculated using the
monthly CO2 and 222Rn continental offsets (relative to the
MBLR). As presented in Sect. , the continental offsets of trace
gases reflect the source and/or sink influence at a continental site relative
to a maritime background. In this study, the terms ΔCx and ΔCRn (see Eq. ) were calculated as the monthly offsets
of trace gases and radon-222, respectively.
The term in brackets in Eq. () corresponds to the radioactive
decay correction factor which depends on the mean residence time of air
masses over the European continent before reaching the station. In
Sect., it was shown that most of the air masses arriving at the
station are from the western part (oceanic air masses) and have an average
wind speed of 8.4 m s-1. Based on this, it takes at least
10 h for the oceanic air masses to reach the station from the closest
oceanic coast. Considering also that some of the air masses are from other
directions, we estimate an average transit time for the air masses arriving
at the station of 1 day, leading to a decay correction of 0.91.
The 222Rn emission rate from continental surfaces strongly depends
on the type and on the nature of the soils. A study of
provides a monthly 222Rn emission map at a resolution
0.083∘×0.083∘ over Europe. The assessment of this map
takes into account the soil types and properties, the 238U soil
content and the soil moisture evolution over time. According to the mean
nighttime footprint at the Puy de Dôme station (see
Fig. b), we extracted the monthly 222Rn average
emission from this map for a 300km×300km region
centered on the Puy de Dôme station (U. Karstens and I. Levin, personal
communication, 2014). Over the years 2010 to 2012, the 222Rn fluxes
range between 75 and 172 Bqm-2h-1, with minimums in winter,
when the soil is wet or frozen.
Uncertainties
The uncertainties of the radon tracer method presented above result
from errors in the 222Rn exhalation rate, errors in the
ΔCx and ΔCRn terms and error in the decay
correction term (see Eq. ). This section describes how
these errors have been assessed to derive a mean relative uncertainty
of the flux estimation of each trace gas.
A systematic assessment of the 222Rn exhalation rate is quite
difficult. The mean ratio of the spatial variability within the extracted
area (300km×300km region centered on the Puy de
Dôme station) to the mean flux is 30 % and U. Karstens and
I. Levin, personal communication, 2014. This number is used as
a first approximation of the 222Rn exhalation rates uncertainties.
This estimate does not include systematic errors and therefore is likely an
underestimate. Uncertainties in the ΔCx term have been assessed
from the MBLRs and from the background mole fraction uncertainties. The
monthly MBLR uncertainties for CO2 and CH4 were provided by
NOAA; mean uncertainties over the measurement period have been taken into
account and are equal to 0.6 µmolmol-1 and
5.1 nmolmol-1, respectively. We used a mean N2O MBLR
uncertainty of 0.3 nmolmol-1, which was estimated and provided
by E. J. Dlugokencky (personal communication, 2014). The uncertainties
regarding the background mole fractions at Puy de Dôme were derived
directly from the respective GC repeatabilities (see
Table ). These last two error sources were combined to
give the mean absolute continental offset (ΔCx) over the entire
measurement period. Thus, the mean relative uncertainties in the continental
offsets are estimated to be 31, 39 and 42 % for ΔCCO2, ΔCCH4 and ΔCN2O, respectively. The 222Rn instrument has
an absolute error of ±20 % for continental measurements
. Based on the same approach as for the ΔCx
term, a constant uncertainty of 28 % has been attributed to the
222Rn continental offset term. Finally,
reported an error of 7 % in the decay correction (term in brackets in
Eq. ) estimated at Schauinsland.
These uncertainties were combined using the square root over the quadratic
sum. The mean relative flux uncertainties derived for our RTM approach were
52, 57 and 59 % for CO2, CH4 and N2O,
respectively. The uncertainties estimated here using this continental RTM
approach are larger than those found by ,
, and , which
are all close to 35 % for the CO2, CH4 and N2O
flux estimates. The uncertainties presented here are mainly driven by the
continental offset uncertainties.
The uncertainties in the SF6 emissions are up to 300 %.
Therefore, we do not present any SF6 emissions in this study.
Estimation of GHG surface fluxes in the Puy de Dôme catchment area
Continental CO2, CH4 and N2O surface fluxes at the
Puy de Dôme station were calculated using the radon tracer method. As
shown by , knowledge of the station footprint is an
important parameter in interpreting the large time variability in a trace gas
mole fraction observed at a measurement station. Figure b
shows the integrated nighttime footprint (22:00 to 06:00 UTC) of the Puy de
Dôme station between 2010 and 2013 (see Sect. for more
details). The station is mainly influenced by regional air masses, which are
well distributed all around the station during nighttime, when the
measurements are usually performed in the free troposphere.
The calculated monthly fluxes are presented in Fig. together
with the hourly 222Rn exhalation rate (U. Karstens and I. Levin,
personal communication, 2014) at Puy de Dôme. The units used to express
the trace gas fluxes are tkm-2month-1 for CO2 and
CH4 and kgkm-2month-1 for N2O. Because no
222Rn activities were recorded between January and the end of
April 2012, no fluxes could be derived from the RTM. The vertical grey lines
on each curve are the absolute uncertainties calculated in
Sect. .
Monthly CO2, CH4 and N2O fluxes at
the Puy de Dôme station derived from the radon tracer
method. The last panel presents the hourly 222Rn
exhalation rate (U. Karstens and I. Levin, personal
communication, 2014). The vertical grey
lines are the respective flux uncertainties.
The CO2 fluxes integrate the signals from all CO2 sources and
sinks in the nighttime footprint of the station. These are the contributions
of the biosphere (emissions and uptakes) and of the anthropogenic emissions
(fossil fuel and biofuel). The derived CO2 fluxes present negative
values in spring, emphasizing the net uptake by the plant assimilation with
a monthly average value between April and June of
-435±226 tCO2km-2month-1 in the station catchment
area. calculated CO2 fluxes at the Schauinsland
station from 1980 to 2000 and observed a long-term monthly mean CO2
uptake between May and June of 147 tCO2km-2, with a maximum
uptake of 550 tCO2km-2 in the spring of 1989. These values
are of the same order of magnitude as the estimations of this study. In
summer, fall and winter, the fluxes are positive, indicating that the
CO2 signal is dominated by the biospheric (predominantly soil
respiration) and fossil fuel emissions. The monthly average CO2 flux
over the total measurement period in the Puy de Dôme station footprint is
109±57 tCO2km-2month-1. The CITEPA (French emission
inventory) provides only anthropogenic emissions for Auvergne; these were
21 tCO2km-2month-1. Our approach cannot separate
biospheric sources and fossil fuel sources; therefore, a direct comparison
between the atmospheric approach and the emission inventory is not possible.
The CH4 fluxes exhibit large variabilities, with monthly values
between -1.04±0.59 and
1.65±0.93 tCH4km-2month-1. Negative values occurred
in April, September and November 2011 due to biases in the calculated
background induced by the many data gaps during the months considered.
Therefore, these negative fluxes are not taken into account in the average
flux calculation. The average CH4 emission was
7.0±4 tCH4km-2yr-1 over the total measurement
period. The N2O estimate emissions varies between 84±50 and
360±213 kgN2Okm-2month-1, with a mean annual
emission of 1760±1040 kgN2Okm-2yr-1.
Several studies have used the radon tracer method to estimate CH4
and/or N2O emissions over western Europe . The results of these estimations are summarized in
Table together with the CH4 and N2O
emissions estimated by this study and the estimate provided by the CITEPA for
Auvergne. The estimates of CH4 emissions in the cited literature
agree well over western Europe, with the exception of the estimation of
, who calculated much higher CH4 emissions
for the Netherlands. Following Fig. , the grid cells
contributing the most to the signal measured at the station at night cover an
area of approximately 300km×300km. Auvergne covers
an area of approximately 150km×250km also centered
on the station, but the neighboring regions are also rural areas presenting
roughly the same land cover and similar GHG fluxes, which allows a direct
comparison between the fluxes estimated by our atmospheric approach and those
estimated by the CITEPA for Auvergne.
The CITEPA estimates a yearly CH4 emission of
6.0 tCH4km-2yr-1 for Auvergne, indicating good agreement
between the inventory and the atmospheric approach. However, our study
overestimates the N2O emissions by a factor of 5 compared with the
CITEPA estimations. The N2O fluxes are mainly driven by agricultural
sources in Auvergne , and such fluxes strongly depend
on the soil characteristics, soil temperature, and amount and type of
fertilizer used. Thus, soil N2O emissions are extremely
heterogeneous, which explains the distribution of results obtained in the
different studies cited in Table . The high N2O
emissions observed in this study may be attributable to the influence of
a local agricultural source. These differences are also linked to significant
uncertainties, which are strongly driven by the small continental offsets
between 0.6 and 1.5 nmolmol-1. Despite this difference, the
atmospheric approach presented provides an independent estimation of GHG
emission over the station footprint as well as new information on flux
seasonality.
Summary of CH4 and N2O flux estimations and their respective
uncertainties over western Europe using the RTM (this study; ; and regional
emission inventory of CITEPA).
Study
Station
Catchment area
Years
CH4
N2O
tCH4km-2yr-1
kgN2Okm-2yr-1
This study
Puy de Dôme (night)
Auvergne
2010–2012
7.0 ± 4.0
1760 ± 1040
Emission inventory
Auvergne
2007
6.0 ± 3.0
320 ± 640
Mace Head
western Europe
1996–1997
4.8–3.5 ± 1.5
475–330 ± 120
Schauinsland
western Europe
1996–1998
1180 ± 345
Lutjewad
the Netherlands
2006–2009
15.2 ± 5.3
900 ± 300
Trainou (180 ma.g.l.)
central region (France)
2009–2012
520±156
Conclusions
Semicontinuous measurements of four long-lived GHGs at Puy de Dôme
started in 2010 with the installation of a GC system. This GC is designed to
automatically measure CO2, CH4, N2O and SF6
atmospheric mole fractions. We described in details three methods which have
been used for comparison purposes. They are based on a direct comparison
between two in situ analyzers, flask measurements and cylinder measurements.
For CH4 and SF6, all comparisons show that GC measurements at
Puy de Dôme are in agreement with the WMO-GAW compatibility goals. For
N2O, our measurements do not match the ambitious WMO-GAW
compatibility goal. For CO2, the comparison based on ambient air
flasks and reference cylinders analysis between the GCs operated at Puy de
Dôme and at LSCE reaches the desirable comparison level, showing there is no
bias in the scale transfer between the two sites. Nevertheless, it does not
do so for the in situ comparison with other analyzers (NDIR and CRDS). The
comparisons between the GC and the CRDS in situ measurements indicate a
constant offset of 0.21 µmolmol-1 CO2 over 20 months
of overlapping measurements. Several tests have been performed and are
described in the study, but the reason for the observed constant bias is not
yet clear. We are continuing to work on this issue and are therefore aware of
the order of magnitude of bias that is possible.
At stations that typically run only one analyzer, a bias of
0.25 µmolmol-1 might not be detected when the target gas and
the comparison cylinders yield good results. For consistency, we thus
recommend using different methods based on flask or cylinder comparisons but
also based on in situ comparisons to check whether the considered
measurements match with the WMO-GAW recommendations.
The diurnal cycles of CO2 and CH4 observed at Puy de Dôme
are mainly driven by the PLB height, and they present the typical
shape of a mountain station, such as the Schauinsland or Kasprowy
Wierch stations, while the N2O and SF6 mean diurnal
cycles present flat behaviors that are difficult to interpret.
Radon-222 was used in this study as an air mass tracer to estimate the
monthly continental fluxes of CO2, CH4 and N2O
relative to the maritime background layer references. We derived a yearly net
emission of 1310 tCO2km-2, 7.0 tCH4km-2 and
1.7 tN2Okm-2. The derived CO2 and CH4 fluxes
compare well with other European studies or with the national inventory
(CITEPA). However, it remains difficult to compare the N2O fluxes
with other studies due to large errors. Compared to the GC system presented
in this study, the new analysis technique based on CRDS, FTIR or OA-ICOS
achieve better precision and require less maintenance. Consequently, the use
of these new technologies enables the development of a dense measurement
network, such as ICOS, which will further improve uncertainties in the flux
estimates.