Scholarly article on topic 'A gas chromatograph system for semi-continuous greenhouse gas measurements at Puy de Dôme station, Central France'

A gas chromatograph system for semi-continuous greenhouse gas measurements at Puy de Dôme station, Central France Academic research paper on "Earth and related environmental sciences"

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Academic research paper on topic "A gas chromatograph system for semi-continuous greenhouse gas measurements at Puy de Dôme station, Central France"

Atmos. Meas. Tech. Discuss., 8, 3121-3170, 2015 www.atmos-meas-tech-discuss.net/8/3121/2015/ doi:10.5194/amtd-8-3121-2015 © Author(s) 2015. CC Attribution 3.0 License.

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This discussion paper is/has been under review for the journal Atmospheric Measurement Techniques (AMT). Please refer to the corresponding final paper in AMT if available.

A gas chromatograph system for semi-continuous greenhouse gas measurements at Puy de Dôme station,

Central France

1* 1** 1 1 2 1 M. Lopez1' , M. Schmidt1' , M. Ramonet1, J.-L. Bonne1, A. Colomb2, V. Kazan',

P. Laj3, and J.-M. Pichon2'4

1 Laboratoire des Sciences du Climat et de l'Environnement (LSCE - UMR8212)' Unité mixte CEA-CNRS-UVSQ' 91191 Gif-sur-Yvette, France

2Laboratoire de Météorologie Physique, Université Blaise Pascal, Clermont-Université, CNRS, UMR6016' 63171, Aubière, France

3Université Grenoble Alpes, CNRS, Laboratoire de Glaciologie et Géophysique de l'Environnement (LGGE - UMR 5183), 38402 Saint-Martin d'Hères, France

4Observatoire de Physique du Globe de Clermont-Ferrand, Université Blaise Pascal, Clermont Université, CNRS, UMS 833, 63171, Aubière, France

now at: Environment Canada, Climate Research Division, Toronto, Ontario, Canada **now at: Institut für Umweltphysik, University of Heidelberg, Heidelberg, Germany

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Received: 28 November 2014 - Accepted: 10 March 2015 - Published: 20 March 2015

Correspondence to: M. Ramonet (michel.ramonet@lsce.ipsl.fr)

Published by Copernicus Publications on behalf of the European Geosciences Union.

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Abstract

Three years of greenhouse gases measurements, obtained using a gas chromatograph (GC) system located at the Puy de Dôme station at 1465 ma.s.l. in Central France are presented. The GC system was installed in 2010 at Puy de Dôme and was designed for automatic and accurate semi-continuous measurements of atmospheric carbon dioxide, methane, nitrous oxide and sulfur hexafluoride mole fractions. We present in detail the instrumental set up and the calibration strategy, which together allow the GC to reach repeatabilities of 0.1 ^olmol-1, 1.2, 0.3nmolmol-1 and 0.06 pmol mol-1 for CO2, CH4, N2O and SF6, respectively. Comparisons of the atmospheric time series with those obtained using other instruments shown that the GC system meets the World Meteorological Organization recommendations. The analysis of the three-year atmospheric time series revealed how the planetary boundary layer height drives the mole fractions observed at a mountain site such as Puy de Dôme where air masses alternate between the planetary boundary layer and the free troposphere.

Accurate long-lived greenhouse gases measurements collocated with Rn measurements as an atmospheric tracer, allowed us to determine the CO2, CH4 and N2O emissions in the catchment area of the station. The derived CO2 surface flux revealed a clear seasonal cycle with net uptake by plant assimilation in the spring and net emission caused by the biosphere and burning of fossil fuel during the remainder of

the year. We calculated a mean annual CO2 flux of 1150t(CO2)km . The derived

CH4 and N2O emissions in the station catchment area were 5.6t(CH4)km yr and

1.5t(N2O)km yr , respectively. Our derived annual CH4 flux is in agreement with the _ national French inventory, whereas our derived N2O flux is five times larger than the g same inventory. U

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1 Introduction

The release of anthropogenic greenhouse gases (GHGs) in the atmosphere leads to a modification of their natural cycles and a strong increase in atmospheric radiative forcing (Myhre et al., 2013). The Intergovernmental Panel on Climate Change (IPCC) reported that the global average temperature increased by 0.89°C between 1901 and 2012 (Hartmann et al., 2013) and will continue to increase during the 21st century (Collins et al., 2013). To limit the global temperature rise, most industrialized countries signed the "United Nations Framework Convention on Climate Change" (UNFCCC) treaty in 1993 to stabilize their GHG emissions between 1990 and 2000. 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 aim to reduce their national emissions of the six main long-lived GHGs by 5.2% between 2008 and 2012, compared to emission levels of 1990: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), sulfur hexafluoride (SF6), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs). The European Union (EU) committed to reduce 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 at 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 sources emissions and to provide an independent verification of the emission inventories reported to the UNFCCC.

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Different methods based on atmospheric measurements can be used to estimate GHG emissions at local to regional scales. These approaches couple atmospheric GHG measurements with measurements of associated atmospheric tracers of air masses, including radon-222 (Biraud et al., 2000; Schmidt et al., 2001), sulfur hex-afluoride (Maiss et al., 1996) or isotopes like radiocarbon in CO2 (Levin and Karsten, 2007; Lopez et al., 2013). 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 (Gloor et al., 2001).

The first atmospheric CO2 continuous measurements started in the 1950s at Mauna Loa observatory using a Non Dispersive Infra Red (NDIR) analyzer with a repeatability better than 0.1 ^olmol-1 (Keeling et al., 1976). 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 (Rasmussen and Khalil, 1981). The repeatabilities were approximately 10nmolmol-1 forCH4 measurements and 0.7 ^ol mol-1 forCO2 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 1.0nmolmol-1 and 0.1 pmolmol-1, respectively (Weiss, 1981; Prinn et al., 1990; Maiss et al., 1996). Subsequently, the use of two detectors (FID and ECD) has permitted the use of GC to analyze CO2, CH4, N2O and SF6 atmospheric mole fractions simultaneously and on a semi-continuous basis. Since the 1980s, atmospheric monitoring stations that are part of the Global Atmospheric 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 ^olmol-1 for CO2, 2.0 nmol mol-1 for CH4, 0.3 nmol mol-1 for N2O and 0.1 pmol mol-1 for SF6, as shown by van der Laan et al. (2009b), Thompson et al. (2009) or Popa et al. (2010).

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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 infra red spectrometry (FTIR) or off-axis integrated cavity output spectroscopy (OA-ICOS) can be cited. These recent technologies are promising for atmospheric monitoring as they offer real continuous measurements (acquisition frequency on the order of Hertz), 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 Hammer et al. (2013b) have demonstrated their feasibility as "traveling" instruments. They could thus be used for comparisons and quality control purposes to ensure data compatibility through a monitoring network.

Regardless of the benefits listed above, these optical technologies cannot be used to measure yet atmospheric SF6 mole fractions, which is the fourth anthropic GHG in terms of radiative forcing. 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 (van der Laan et al., 2009b). 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 or NDIR analyzers for CO2 and CH4 monitoring in many stations, but GC is still the reference instrument for N2O and SF6 measurements (see WMO-GAW, 2013). Consequently, we installed a GC system in 2010 at the mountain station of Puy de Dome (France) to monitor with high precision the long-term atmospheric trend of the main four long-lived greenhouse gases.

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This paper focuses on three years of ambient air measurements of CO2, CH4, N2O and SF6 obtained using a GC system at the Puy de Dôme station (2010 to 2013). 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, which demonstrated that our measurement system reaches the WMO recommendations (WMO-GAW, 2013). In the last part, we present and analyze our three-year time series of ambient air measurements. Finally, we demonstrate that these time series can be used to estimate the monthly regional flux densities of CO2, CH4 and N2O in the catchment area of Puy de Dôme station, using the radon-222 as an atmospheric tracer.

2 The Puy de Dôme station 2.1 Site description

Puy de Dôme station (45°46'19" N, 2°57'57" E) is located at the top of the Puy de Dôme volcano, 1465ma.s.l., in the Auvergne region 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 statistic and economic studies (INSEE - http://www.insee.fr), Puy de Dôme is surrounded by meadows (36.4%), forests (33.4%) and arable land (17.6%). The major anthropic GHG sources are on 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 396ma.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 Amo-sphérique) reports the French national GHG emissions to the UNFCCC but has also provided a regional inventory of Auvergne for the year 2007 (CITEPA, 2010). According to the CITEPA, the anthropogenic CO2 emissions in the Auvergne region are mainly

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attribuable to road transport, residential and industrial sectors, which represent respectively 45, 25 and 21 % of the total anthropic CO2 emissions of the region. The agricultural sector is responsible for 90% of total anthropic CH4 emissions and 97% of total anthropic 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 with a height of 20 m and a telecommunication antenna with a height of 89 m. 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, aerosols, radon-222, clouds microphysics and radionuclide. The station has international labeling EMEP (European Monitoring and Evaluation Programme), GAW, and is part of the European ACTRIS (Aerosols, Clouds, and Trace gases Research Infrastructure) and ICOS measurement networks.

2.2 Atmospheric conditions at the Puy de Dome 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 (ECMWF, 2012; Seidel et al., 2012) at a three-hour time resolution for the years 2010 to 2012. The grid cell used for the extraction had an area of 15 km x 15 km and was centered at 45.75° latitude and 3.00° longitude at an altitude of 575ma.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. The average difference in wind speed between the meteorological station and the ECMWF

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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 (1465ma.s.l. compared to 575ma.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 one-hour time resolution.

Regarding meteorological parameters, the Puy de Dôme station is primarily influenced by winds from a south-west direction (48.2% of the time) with a mean wind speed of 8.4 ms-1. The wind blows from 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 % during winter time.

Backtrajectories were calculated using the Lagrangian dispersion model Flexpart version 8.2.3, based on ECMWF ERA-Interim data at a horizontal resolution of 1° x 1°, with 60 vertical levels and 3h temporal resolution. Air masses were released 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 1500ma.s.l.) with a lifetime of 3 days. Eight particles were released every 15 min (96 particles every 3 h). 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°x 1° horizontal grid, following the method described by Lin et al. (2003), 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 on Fig. 1 respectively 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. 1a), and between 22:00 and 06:00 UTC when the PBL is below 1400 m and the station is within the free troposphere (Fig. 1b). The grid cells influence is represented as a relative influence compared to the maximum value (in percent). On both maps, the station is located within the black

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grid cell. Boulon et al. (2011) showed that 87% of air masses reaching the station are from the west by analysing backtrajectories from HYSPLIT over 437 days at Puy de Dôme (i.e., from Atlantic and continental Western Europe areas). A statistical analysis of backtrajectories over four years conducted by Venzac et al. (2009) demonstrated that winter air masses reaching Puy de Dôme travel over longer distances from the west than summer air masses.

3 Instrumental setup

The GHG observations at Puy de Dôme started in 2000 with continuous CO2 measurements using a non-dispersive infra-red (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 ô 13C and ô 18O in CO2 at the LSCE in Gif-sur-Yvette, France. In 2010, a GC system for semi-continuous 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

( Rn) analyzer based on the active deposit method. These instruments are housed in a regulated temperature room, and the inlet lines are located on the roof of the station, 10ma.g.l. This section focuses on the setup of the GC installed at the Puy de Dôme station in July 2010.

3.1 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 semi-continuous atmospheric measurements of CO2, CH4, N2O and SF6 mole fractions in dry ambient air (Lopez, 2012). Similar instrument configurations are installed at the Gif-

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sur-Yvette and Trainou stations in Northern part of France (Lopez et al., 2012; Schmidt et al., 2014).

The ambient air is pumped from the roof of the station (pump KNF: PMF 1433-811) through 10 m long decabon tube with a diameter of 1/2 in. Three filters (140, 40 and 7 pm TF series from Swagelok) are placed in series to protect the pump and the analysis system from dust and aerosols. 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 six hours for 10s by opening a solenoid valve. In a second step, the ambient air passes through a glass trap that is maintained in an ethanol bath at -55°C by a cryocooler (Thermo Neslab CC-65) to remove the remaining water vapor. The glass trap is changed during the weekly maintenance of the station. An electronic box is used to regulate the refrigerator temperature and to switch 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.

The GC system consists of an injection part, a separation part and a detection part. These different parts are indicated by different colors in Fig. 2. 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. A micro electron capture detector (uECD) is used to detect N2O and SF6. One injection and analysis requires 5.4 min.

3.1.1 Sample analysis

The injection part (framed by green in Fig. 2) consists of an 8-port microelectronic valve $ 7 (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

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the system via an electronic pressure control (EPC-Aux5) through two sample loops. The sample loops are placed in series on two 6-port 2-way Valco valves (tf 1a and tf 1b). The sample loop for CO2 and CH4 analysis has a volume of 15 mL (sample loop on the tf 1a valve) and the one for N2O and SF6 analysis has a volume of 10 mL (sample loop on the tf 1b valve). The sample loops are flushed with sample gas for 0.75 min at a flow rate of 180 mL min-1 (corresponding to 2.5 psi pressure on Aux5). Before 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 respective carrier gases by switching valve tf 1a and tf 1b. The carrier gas used for the FID is N2 (6.0 quality), whereas a mixture of argon/methane (95/5%, ECD quality) is used as the carrier gas for the ^ECD. A purifying cartridge (Aeronex) is placed after each carrier gas cylinder.

The columns used to separate the different molecules are placed in an oven regulated at 80 °C (see the portion framed by yellow in Fig. 2). A Hayesep-Q (12' x 3/16"SS, mesh 80/100) analytical column is used forCO2 and CH4 separation. For N2O and SF6 separation, a pre-column Hayesep-Q (4' x 3/16''SS, mesh 80/100) and an analytical column Hayesep-Q (6' x 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 mL min-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 ^ECD baseline. Between 0.75 and 3.7 min, the N2O and SF6 molecules are transported first through the pre-column and then in the analytical column, where separation occurs. The analytical column is directly connected to the ^ECD. The N2O and SF6 retention times are 4.3 and 4.8 min, respectively.

The CH4 and CO2 molecules are detected by an FID. Methane elutes after 2.7 min and is injected directly into the detector for analysis. Once CH4 is released from the analytical column, the Valco valve tf 4 is switched to connect the nickel catalyst to allow CO2 molecules 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

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fed with a 65mLmin-1 flow rate of hydrogen (provided by a NM-H2 hydrogen generator from F-DBS) and a 400mLmin-1 flow rate of zero air (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 catalysis by the nickel catalyst. The typical efficiency of the catalyst is 97% in the CO2 atmospheric mole fractions range.

We used the numbering system used by Agilent for naming each of the valves and the EPCs.

3.1.2 Analysis management

Data acquisition, valve shunting and temperature regulation of the GC system are entirely processed by Chemstation software (version A.10.02, Agilent). This software allows the control of the GC system parameters through the so-called "methods". A typical method is configured to perform the following:

- control the temperature of the detectors, the catalyst and the oven;

- regulate the flow of the sample (via Aux5), the carrier gases (via Aux3 and Aux4), H2 and zero air, all via five distinct ECPs;

- schedule the switching of the four 6-port 2-way Valco valves, controlled via the internal events output GC connector;

- choose the position of the 8-port microelectronic Valco valve, controlled via the external events output GC connector;

- integrate the results of the analysis (via the chromatograms).

A typical method is presented in Table 1 and corresponds to one analysis of a chosen sample. Table 2 summarizes the GC system setup used between 2010 and 2013. A sequence is based on several successive methods. The arrangement of the methods in a sequence enables the automatic selection of the order of samples measurements (ambient air, calibration or quality control cylinders) over several days.

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The FID and ^ECD signals (chromatograms) are expressed in pA and Hz, 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. 3.2) 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, the GC system performance is controlled via a graphical application by a trained operator (see Sect. 3.2), and different flags are manually assigned to the data.

3.1.3 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 Dome 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) by Deuste Steininger (Muhlhausen, 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: Zhao and Tans (2006); Hall et al. (2007); Dlugokencky et al. (2005).

The response function of ^ECD for N2O analysis is non-linear (see Schmidt et al., 2001; van der Laan et al., 2009b; Lopez et al., 2012). The non-linearity of our ^ECD 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. We have deter-

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mined that a linear fit is sufficient to account for the non-linearity of our ^ECD. The two-point calibration strategy compares very well with an exponential fit through five cylinders, with an average difference of 0.01 ±0.13 nmol mol-1. This result confirms that a two-point calibration strategy is well adapted to correct for the ^ECD non-linearity in atmospheric mole fraction ranges. Similar tests demonstrated that the FID response is linear in the atmospheric mole fractions range for CO2 and CH4 measurements.

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 three 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 Puy de Dôme station, the first set of working standards was replaced on 25 April 2013. Re-analysis of the standards at LSCE revealed mean differences (2010-2013) of -0.11 pmolmol-1, -0.03, -0.1 nmol mol-1 and 0.0pmolmol-1 for CO2, CH4, N2O and SF6, respectively. 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 3.

3.2 Quality control and performance of the GC system

A target gas (TGT) is injected into the GC system once per hour for quality control. The target gas cylinder is a 40 L cylinder filled with dry ambient air at Gif-sur-Yvette. After stabilization for at least one 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 5).

Figure 3 shows the time series of the target gas analysis from July 2010 to June 2013. The target gas cylinder was not changed over the three years. The dif-

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ferent data holes 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 holes in N2O and SF6 between April and August 2012 was due to a problem with the power supply of the Valco valve $ 1b. The reproducibility (computed here as the SD at 1-sigma of the target analysis over one year of measurement) and the typical short-term repeatability (SD at 1-sigma of the target analysis over 24 h) are presented in Table 4.

The vertical dark blue lines in Fig. 3 indicate when the working standards were changed and separate the two measurement periods: period A and period B (see Sect. 3.1.3). The average mole fractions of the TGT measurement during period A and period B are presented in Table 5 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 was 0.15pmolmol-1. The same order of magnitude was observed in the difference between the CO2 assigned and measured values in period B, confirming the consistency between the two scales used in periods A and B and emphasizing a bias of the attributed TGT CO2 mole fraction. During period B, the average CH4 mole fraction of the TGT gas was lower by approximately 10nmolmol-1 compared to period A. This decrease was due to a micro-leak in the WH line that was only detected in the CH4 mole fractions because of the use of Ar/CH4 as the carrier gas for the ^ECD. The use of this gas induces a larger CH4 mole fraction in the laboratory than the ambient air level. Target data as well as ambient air measurements for this period were corrected by applying a one point calibration to the four trace gases. This micro-leak was fixed in September 2013, when the target cylinder was replaced by a new one (TG2). The analysis of TG2 since September 2013 has demonstrated that the CH4 scale consistency between Puy de Dome and Gif-sur-Yvette stations agrees with the WMO recommendations (WMO-GAW, 2013).

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3.3 Other instrumentation

3.3.1 Flask sampling unit

Since 2002, a flask sampling unit has been installed at the Puy de Dôme station for weekly sampling. It consists of a pump that pressurizes two 1 L glass flasks in series to 1 bar relative pressure. They are flushed for 15min with dry ambient air prior to pressurization. Ambient air is dried in a distinct cooling trap maintained in the same ethanol bath as the GC (see Sect 3.1). The flasks are then shipped and analyzed at LSCE for CO2, CH4, N2O and SF6 by a GC system and for CO2 isotopic composition

( C and O) by a Finnigan MAT-252 isotope mass spectrometer. The calibrations for trace gas analysis are performed in the same manner as presented in Sect. 3.1.3, and the results are stored in the same database.

3.3.2 Radon-222 measurement system

Radon-222 ( Rn) 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 Rn by the earth's surface. Atmospheric radon-222 activity has been

monitored at Puy de Dôme station since 2002. The analyzer is based on the active

deposit method, which consists of alpha decay counting of Rn s solid short-lived

218 214 214

daughters: Po, Pb and Bi. The measurement technique has been described

in detail by Polian et al. (1986) or Biraud et al. (2000). To avoid the loss of the solid

Rn daughters, the inlet line is a 6m straight metal tube 35mm in 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. Schmidt (1999) estimated a radioactive disequilibrium (see Turner, 1964) at the Schauinsland station (Germany - 47°54' N, 7°54' E - 1205ma.s.l.) of 1.15 ± 0.14. This value was independently confirmed by Xia et al. (2010). Based on the similarities between the Puy de Dôme station

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and the Schauinsland station, the measured ( Rn) activity at Puy de Dôme has been corrected for the radioactive disequilibrium by using the same value (1.15 ± 0.14).

3.3.3 CO2 continuous measurements by in situ NDIR

Continuous CO2 measurements at Puy de Dôme began in 2000 with a NDIR gas analyzer. The instrument is an integrated system constructed around a LICOR 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 20mLmin-1 for the sampling cell, and 15mLmin-1 for the reference cell. 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 decabon line. The air is dried by passing through a glass trap maintained in a cold ethanol bath (see Sect. 3.1). Ambient air is analyzed for 50 min following the analysis of a reference cylinder for 10 min. The NDIR spectrometer was replaced by a CRDS analyser in April 2011.

3.3.4 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 pmol mol-1 for CO2 and 1722 to 2107 nmol mol-1 for CH4. The cylinders are calibrated at the LSCE laboratory on the WMO-X2007 and NOAA-04 scales, respectively. The instrument calibration is performed automatically every 15 days by injection of the following scheme: 4 times

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each calibration cylinder for 30min, 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 15min of the subsequent injections are automatically rejected for equilibration time consideration. The instrument calibration takes 8 h and a linear fit is applied to compute the analyzer response. A target gas is automatically injected in 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 one year were 0.02 pmolmol-1 forCO2 and 0.14 nmolmol-1 forCH4.

Ambient air is injected into the CRDS from the roof (through a 3/8 in decabon line, filtered using 3 filters of 7, 40 and 140 pm) using a pump located after the optical cavity. To avoid the risk of bias caused by interferences between water vapor and trace gases in the CRDS (Chen et al., 2010), the ambient air is dried prior to its injection into the CRDS by the drying system presented in Sect 3.1. 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.

3.4 Comparisons of different analyzers

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 recommends a scientific level of compatibility for such a comparison in the Northern Hemisphere (WMO-GAW, 2013). These levels are ±0.1 pmolmol-1 for CO2, ±2.0nmolmol-1 for CH4, ±0.1 nmolmol-1 for N2O and ±0.02 pmol mol-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 6. A 2-sigma filter was applied to the differences to prevent outliers. Comparisons between GC and NDIR were based on 9months of overlapping mea-

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surements (July 2010 to April 2011) and revealed a mean CO2 difference (GC minus NDIR) of -0.14 ± 1.78 pmol mol-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 pmol mol-1 for CO2 and -0.64±5.46 nmol mol-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 pmolmol-1 and -1.27 ± 3.49 nmol mol-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 nmol mol-1 whereas the CO2 difference increased to 0.28 ± 0.75 pmolmol-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 three weeks: a common inlet line for ambient air measurements has been used for the GC and CRDS (the GC one, described in Sect. 3.1). During these three weeks of testing, the difference between GC and CRDS remained constant and equal to 0.28 pmol mol-1, confirming that the inlet lines did not cause bias. Even after the change of the GC working standard in late April 2013, the CO2 difference was still observed.

Comparisons between the GC system and the flasks 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 6 for more details). The Puy de Dôme station also participates in the "Cucumbers comparison programme" (http://cucumbers.uea.ac.uk/) in the frameworks of the European Union CarboEurope project (2000-2005), EU IMECC (2007-2011) and InGOS (20112015) 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)

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and Hegyhatsal (Hungary) stations. Table 6 presents the mean differences between the average analysis of the 3 comparison cylinders at Puy de Dôme and Gif-sur-Yvette (LSCE) between 2011 and 2013. With the exception of N2O, for which the requested specification was not reached, the recommended comparison levels given by 5 the WMO-GAW for CO2, CH4 and SF6 were achieved.

4 Results and discussions

4.1 Three years of ambient air measurements

Figure 4 shows the hourly time series of CO2, CH4, N2O and SF6 ambient air mole

fractions together with the Rn activities at Puy de Dôme from July 2010 to the end io of June 2013. The different gaps observed in the atmospheric GHG time series were explained in Sect. 3.2. These time series are presented with the respective monthly background values (black lines). These monthly background values were calculated

from the monthly average nighttime mole fractions (between 22:00 and 06:00 UTC),

when the station is usually above the PBL (see Fig. 5). The hourly Rn activities are

i5 presented in the last panel of Fig. 4 and varied between 0 and 9 Bq m over the three

years of measurements. From February 2012 to the end of April 2012, the computer for

Rn data acquisition had hardware and software problems, resulting in the observed gap.

Figure 5 presents the mean diurnal cycles per season of CO2, CH4, N2O, SF6, 20 222Rn and for the PBL height (from ECMWF) from June 2010 to June 1013. The diurnal cycles were computed from the detrended hourly time series to the reference of 1 January 2013. 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 because Clermont-Ferrand is located at the lowest altitude of the ECMWF 25 extracted grid cell. The horizontal solid black line on the PBL height panel in Fig. 5 gives the altitude of the station above Clermont-Ferrand and enables a quick obser-

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vation 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 16:00 UTC. After 16:00 UTC, the PBL height began to decrease, reaching a minimum after 20:00 UTC. On a mean annual scale, the GC sampled the trace gases within the PBL between 10:00 and 17:00 UTC with an enlarged time step in summer and a narrowed 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 Rn observed in this study are the typical of mountain sites, as previously described by Schmidt et al. (1996) for the Schauinsland station (1205 ma.s.l.) or Necki et al. (2003) for the Kasprowy Wierch station (Poland, 1987ma.s.l.). The PBL height is a key atmospheric factor, particularly for mountain sites: atmospheric mole fractions of trace gases are generally lower in the

free troposphere than in the PBL because the sources are located at ground level. Due

to its short radioactive lifetime, Rn cannot accumulate in the free troposphere, in

contrast to other long-lived trace gases. The mean diurnal cycles of Rn (Fig. 5) exhibited larger variations in summer than in winter, when the PBL height is maximal and

the inlet line of the station alternates between the PBL during daytime and above dur-

ing nighttime. In winter, Rn activities are lower than in summer because the station is usually in the free troposphere. The mean diurnal cycles for CO2 and CH4 exhibited different shapes. As for the Rn activities, the CH4 mole fractions at the Puy de Dôme station were higher in the afternoon, when the PBL is well developed, compared with the night-time mole fractions. For the yearly average, CH4 afternoon mole fractions were 3.3nmolmol-1 higher than the night-time 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 clearly seen during summertime, when the photosynthetic activity is maximal: the amplitude of the CO2 diurnal cycle was 7.2pmolmol-1 with

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a minimum mole fraction 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. However, 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.

4.2 The marine boundary layer reference

In this section, the background mole fractions of recorded trace gases at the station (see Sect. 4.1) are compared with the respective marine boundary layer references (MBLRs). Here, the MBLRs are the monthly zonal average trace gas mole fractions fora 45.5° N latitude computed from NOAA measurements: Dlugokencky et al. (2013a, b). They were retrieved from the Global Monitoring Division of the NOAA Earth System Research Laboratory. Figure 6 shows the differences between the monthly mean background mole fractions at Puy de Dôme (night time 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 but always positive for N2O, indicating the strong influence of agricultural sources (see Sect. 2.1) in the Puy de Dôme footprint. Finally, the SF6 offset varied between -0.10 and +0.12pmolmol-1, which represents

the same order of magnitude as the GC measurement repeatability. In addition, Fig. 6

shows the monthly Rn continental offset (for night-time selection data between 22:00

and 06:00 UTC) at Puy de Dôme station, relative to marine air. The marine air Rn reference was computed from the mean activity of 15 years of measurements during

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maritime background condition at the European background site of Mace Head (see

Bousquet et al., 1996) and is equal to 168 mBq m . 4.3 The radon tracer method 4.3.1 Method

Once emitted by soils, Rn 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 at the local to regional scales. Detailed descriptions of this method are given in the following studies: Schmidt et al. (2001); Hammer and Levin (2009); Yver et al. (2009); van der Laan et al. (2009a).

The RTM is based on Eq. (1), where Jx and JRn are the respective flux densities of

a trace gas x and Rn. The ACx and ACRn terms are the temporal variations of the

trace gas x mole fraction and of the Rn activity over a period At. Finally, ARn is the

Rn decay constant (Schmidt et al., 2001).

Jx — J

^RnCRn

As shown in Fig. 5, the diurnal variations of trace gases at Puy de Dome are very weak, which makes it difficult to correctly assess the ACx and ACRn terms on a daily basis. In this study, we apply the RTM approach presented by Schmidt et al. (2003), in which

the CO2 flux densities at the Schauinsland station were calculated using the monthly

CO2 and Rn continental offsets (relative to the MBLR). As presented in Sect 4.2, the continental offsets of trace gases reflect the source/sink influence at a continental site relative to a maritime background. In this study, the terms ACx and ACRn (see Eq. 1) were calculated as the monthly offsets of trace gases and radon-222, respectively.

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For a continental mountain site like Schauinsland, Schmidt et al. (2003) determined

that the mean residence time for air masses over the European continent before reach-

ing the station is 3 ± 1 days. This leads to a Rn decay correction (term in brackets in Eq. 1 ) of 0.77. According to Fig. 1, most of the air masses arriving at the Puy de Dôme station and having a lifetime of three days are also from the continent. Based on the

similarities between the Schauinsland and Puy de Dôme stations in terms of altitude,

geographical position and continental air masses influence, the constant Rn decay correction of 0.77 used for Schauinsland is also used for the Puy de Dôme station in

this study.

The Rn emission rate from continental surfaces strongly depends on the type and on the nature of the soils. An ongoing study of Karstens et al. (2015), provided a monthly 222Rn emission map at 0.1° x 0.1° over Europe (U. Karstens and I. Levin, personal communication, 2014). The assessment of this map takes into account the

soil types, the U soil content and the soil moisture evolution over time. According to the mean nighttime footprint at the Puy de Dôme station (see Fig. 1 b), we extracted the

monthly 222Rn average emission from this map for a 300 km x 300 km region centered

on the Puy de Dôme station. Over the years 2010 to 2012, the Rn fluxes range between 75 and 172 Bq m-2 h-1 with minimums in winter, when the soil is wet or frozen.

4.3.2 Uncertainties

The uncertainties of the radon tracer method presented above result from errors in the

Rn exhalation rate, errors in the ACx and ACRn terms and error in the decay correction term (see Eq. 1). 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 Rn exhalation rates is quite difficult. The mean ratio of the spatial variability within the extracted area (300 km x 300km region centered

on the Puy de Dôme station) to the mean flux is 30% (Karstens et al., 2015). This

number is used as a first approximation of the Rn exhalation rates uncertainties. This estimate does not include systematic errors and therefor is likely an underesti-

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mate. Uncertainties in the ACx term have been assessed from the MBLRs and from the background mole fractions uncertainties. The monthly MBLR uncertainties forCO2 and CH4 were provided by NOAA; mean uncertainties over the measurement period have been taken into account and are equal to 0.6pmolmol-1 and 5.1 nmolmol-1, respectively. We used a mean N2O MBLR uncertainty of 0.3nmolmol-1 which was estimated and provided by E. J. Dlugokencky (personal communication, 2014). The uncertainties on the background mole fractions at Puy de Dôme were derived directly from the respective GC repeatabilities (see Table 4). These last two error sources were combined to give the mean absolute continental offset (ACx) over the entire measurement period.

Thus, the mean relative uncertainties in the continental offsets are estimated to be 31,

39 and 42 % for ACCo2, ACCH4 and ACN2O, respectively. The Rn instrument has an absolute error of ±20% for continental measurements (Biraud et al., 2000). Based on

the same approach as for the ACx term, a constant uncertainty of 28% has been at-

tributed to the Rn continental offset term. Finally, Schmidt et al. (2003) reported an error of 7% in the decay correction (term in brackets in Eq. 1).

These uncertainties were combined using the square root over the quadratic sum. The mean relative fluxes 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 Biraud et al. (2000), Schmidt et al. (2001), van der Laan et al. (2009a) or Lopez et al. (2012), 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.

4.3.3 Estimation of GHG surface fluxes in the Puy de Dôme catchment area

Continental CO2, CH4 and N2O surface fluxes were calculated using the radon tracer method at the Puy de Dôme station. As shown by Gloor et al. (2001 ), the knowledge of

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the station footprint is an important parameter in interpreting the large time variability of a trace gas mole fraction observed at a measurement station. Figure 1b shows the integrated nighttime footprint (22:00 to 06:00 UTC) of the Puy de Dôme station between 2010 and 2013 (see Sect. 2.2 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. 7 together with the hourly Rn

exhalation rate (U. Karstens and I. Levin, personal communication, 2014) at Puy de

—2 — 1

Dôme. The units used to express the trace gas density fluxes are: tkm month

2 1 222 for CO2 and CH4 and kg km month for N2O. Because no Rn 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 relative uncertainties calculated

in Sect. 4.3.2.

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 -382 ± 198t(CO2)km- 2 month - 1 in the station catchment area. Schmidt et al. (2003) 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 147t(CO2)km , with a maximum uptake of 550t(CO2)km in spring 1989. These values are 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 bio-spheric (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 21

96 ± 50t(CO2)km month . The CITEPA (French emission inventory) provides only

anthropogenic emissions for the Auvergne region, which is 21 t(CO2)km month . Our approach can not separate biospheric sources and fossil fuel sources; therefore,

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a direct comparison between the atmospheric approach and the emission inventory is not possible.

The CH4 fluxes exhibit large variabilities, with monthly values between -0.91 ±

— 2 — 1

0.52 and 1.45 ± 0.82 t(CH4) km month . Negative values occurred in April, Septem-

— 2 — 1

ber and November 2011. The average CH4 emission was 5.6 ± 3.2t(CH4)km yr

over the total measurement period. The N2O estimate emissions varies between

74 ± 44 and 316 ± 187kg(N2O)km month , with a mean annual emission of 1546 ± 912 kg(N2O) km-2yr-1.

Several studies have used the radon tracer method to estimate CH4 and/or N2O emissions over Western Europe: Biraud et al. (2000); Schmidt et al. (2001); Lopez et al. (2012). The results of these estimations are summarized in Table 7 together with the CH4 and N2O emissions estimated by this study and the estimate provided by the CITEPA for the Auvergne region. The estimates of CH4 emissions in the cited literature agree well over western Europe, with the exception of the estimation of van der

Laan et al. (2009a), who calculated much higher CH4 emissions for the Netherlands.

The CITEPA estimates a yearly CH4 emission of 6.0 t(CH4) km yr for the Auvergne region, indicating a good agreement between the inventory and the atmospheric approach. However, our study overestimates the N2O emissions by a factor of five compared with the CITEPA estimations. The N2O flux density is mainly driven by agricultural sources in the Auvergne region (CITEPA, 2010), 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 cited studies in Table 7. 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 nmol mol-1. Despite this difference, the presented atmospheric approach provides an independent estimation of GHG emission over the station footprint as well as new information on flux seasonality with lower fluxes as expected.

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5 Conclusions

Semi-continuous measurements of four long-lived GHGs at Puy de Dôme started in 2010 with the installation of a GC system. This GC was designed to automatically measure CO2, CH4, N2O and SF6 atmospheric mole fractions. Many comparisons between the in situ GC system and the GC system located at LSCE (Gif-sur-Yvette) via flask or cylinder analysis showed comparison levels in agreement with those recommended by the WMO-GAW. The comparisons between the GC and CRDS in situ measurements indicated a high degree of compatibility for CH4 whereas a constant offset of 0.28 pmolmol-1 was observed for CO2 ambient air comparison. Several tests have been performed and explained in the study, but the reason for the observed constant bias is not yet clear. We continue 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 pmol mol-1 might not be detected when the target gas and the comparison cylinders yield good results.

The diurnal cycles of CO2 and CH4 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 flux densities of CO2, CH4 and N2O relative to the maritime background layer

references. We derived a yearly net emission of 1150 t(CO2) km , 5.6 t(CH4) km and 1.5t(N2O)km . 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. In the future, new analysis techniques based on CRDS, FTIR or OA-ICOS will help achieve better precision and fewer data gaps at remote stations with only weekly maintenance visits, which will further improve uncertainties in flux estimates.

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Acknowledgements. The authors thank the LaMP team for weekly maintenance of the station and weekly flask sampling. We also acknowledge the support provided by the RAMCES group at LSCE for regular maintenance of the analyzer and for data management. We thank the ECWMF and I. Pison for providing and extracting the meteorological conditions at the Puy de Dôme pixel. We also want to thank I. Levin and U. Karstens for providing and sharing the European 222Rn map emissions. We used the marine boundary layer reference kindly provided by E. Dlugokencky and K. Masarie from NOAA. The authors acknowledge the Regional Council of Auvergne for the financial support. This work was partly funded by the InGOS EU project (284274).

References

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Schmidt, M., Lopez, M., Yver Kwok, C., Messager, C., Ramonet, M., Wastine, B., Vuillemin, C., Truong, F., Gal, B., Parmentier, E., Cloué, O., and Ciais, P.: High-precision quasi-continuous atmospheric greenhouse gas measurements at Trainou tower (Orléans forest, France), Atmos. Meas. Tech., 7, 2283-2296, doi:10.5194/amt-7-2283-2014, 2014. 3131 Seidel, D. J., Zhang, Y., Beljaars, A., Golaz, J.-C., Jacobson, A. R., and Medeiros, B.: Climatology of the planetary boundary layer over the continental United States and Europe, J. Geophys. Res.-Atmos., 117, D17106, doi:10.1029/2012JD018143, 2012. 3128 Thompson, R. L., Manning, A. C., Gloor, E., Schultz, U., Seifert, T., Hansel, F., Jordan, A., and Heimann, M.: In-situ measurements of oxygen, carbon monoxide and greenhouse gases

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report series No. 143, WMO-TD 1073, Global Atmosphere Watch, Geneva, 2001. WMO-GAW (Ed.): 17th WMO/IAEA Meeting of Experts on Carbon Dioxide, Other Greenhouse Gases, and Related Tracer Measurement Techniques, vol. 213, Global Atmosphere Watch, Beijing, China, 10-14 June 2013. 3126, 3127, 3136, 3139 Xia, Y., Sartorius, H., Schlosser, C., Stohlker, U., Conen, F., and Zahorowski, W.: Comparison of one- and two-filter detectors for atmospheric 222Rn measurements under various meteorological conditions, Atmos. Meas. Tech., 3, 723-731, doi:10.5194/amt-3-723-2010, 2010. 3137

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gen, carbon monoxide, and radon-222 semicontinuous measurements, J. Geophys. Res., 114, D18304, doi:10.1029/2009JD012122, 2009. 3144 Zhao, C. L. and Tans, P. P.: Estimating uncertainty of the WMO mole fraction scale for carbon dioxide in air, J. Geophys. Res.-Atmos., 111, D08S09, doi:10.1029/2005JD006003, 2006. 3134

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Table 1. Measurement method used for the GC system at the Puy de Dome station.

Time (min) Parameter Value Comments c CO i

0.00 Aux 3 45.0 psi Carrier gas pressure for N2 o

0.00 Aux 4 20.0 psi Carrier gas pressure for Ar/CH4 P 03

0.00 Aux 5 2.5 psi Sample pressure T3 CD

0.00 Valve ( 1 Off Flush of the sample loops —\

0.00 Valve j 3 On Backflush of the pre-column (N2O/SF6) —

0.75 Aux 5 0.0 psi Sample pressure

0.75 Valve 3 Off Stop of pre-column backflushing

1.25 Valve 1 On Sample injection o Œ. CO

3.10 Valve 4 On Injection of CO2 in the catalyst i o

3.60 Aux 3 0.0 psi Carrier gas pressure for N2

3.70 Valve 3 On Backflush of the pre-column P 03

5.30 Valve 4 Off Switch of the catalyst valve T3 r

5.40 Aux 3 45.0 psi Carrier gas pressure for Ar/CH4

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Table 2. GC system equipment and temperature and flow rate settings.

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Detector

FID (CO2/CH4)

,uECD (N2O/SF6)

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Carrier gas Flow rate

Loop sample volume Oven temperature Pre-column

Analytical column

Detector temperature Catalyst temperature Gas supply

N2 cylinder (6.0 quality) + purifier

100mLmin-1 15 mL 800 C

Hayesep-Q

12'x 3/16''SS, 80/100 300 0C 390 0C

H2 generator: 60mLmin-1 Air zero generator: 400mLmin-1

Ar/CH4 cylinder (ECD quality) + purifier

45/65 mLmin-1 10mL 80 0C

Hayesep-Q

4' x 3/16''SS, 80/100

Hayesep-Q

6' x 3/16''SS, 80/100

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Table 3. 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 (^molmol- -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 (pmol mol- 1) 10.05 5.38 9.86 5.86

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Species Reproducibility Short-term repeat

CO2 (^molmol- -1) 0.14 0.1

CH4 (nmolmol- 2.12 1.2

N2O (nmolmol- 0.34 0.3

SF6 (pmolmol- 0.06 0.06

Table 4. Reproducibility and typical short-term (24 h) repeatability of the GC system at Puy de Dome. a

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Table 5. 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 (nmolmor1) CH4 (nmolmol-1) N2O (nmolmol-1) SF6 (pmolmol-1)

402.57 ± 0.07 1973.87 ± 0.73 325.71 ± 0.23 7.23 0.04

402.42 ± 0.15 1973.81 ± 2.12 325.90 ± 0.35 7.23 0.06

402.38 ± 0.46 1963.72 ± 6.64 325.76 ± 0.47 7.20 0.07

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Table 6. Results of the comparisons between in situ measurements obtained using the GC system, NDIR, CRDS and flasks 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.

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Date Jul 2010- Jul 2013 Jul 2010-Apr 2011 Apr 2011-Jul 2013 Jul 2010-Jul 2013

Comparisons GC in situ - flasks In situ: In situ: Cylinders:

Average Nb of flasks GC-NDIR GC-CRDS PUY-GIF

CO2 (limolmor1) 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 NA -0.64 ± 5.46 0.64 ± 0.26

N2O (nmolmol-1) 0.12 ± 0.55 47 NA NA 0.21 ± 0.47

SF6 (pmolmol-1) -0.01 ± 0.07 53 NA NA 0.03 ± 0.03

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Table 7. Summary of CH4 and N2O fluxes estimations over Western Europe using the RTM (this study, Biraud et al., 2000; Schmidt et al., 2001; van der Laan et al., 2009a; Lopez et al., 2012, and regional emission inventory of CITEPA).

Study Station Catchment area Years CH4 t(CH4)km-2yr-1 N2O kg(N2O)km-2yr-1

This study Puy de Dome Auvergne region 2010-2012 5.6 1546

CITEPA (2010) Emission inventory Auvergne region 2007 6.0 320

Biraud et al. (2000) Mace Head Western Europe 1996-1997 4.8-3.5 475-330

Schmidt et al. (2001) Schauinsland Western Europe 1996-1998 1180

van der Laan et al. (2009a) Lutjewad the Netherlands 2006-2009 15.2 900

Lopez etal. (2012) Trainou (180ma.g.l.) Central region (France) 2009-2012 520

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Figure 1. 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.

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INJECTION

SEPARATION

DETECTION

Figure 2. Schematic of the GC system setup (gas flow) at the Puy de Dome station.

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Figure 3. Time series of the target gas measured with the GC system at Puy de Dôme. The vertical blue lines on each panel correspond to the date of the working standards change.

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Figure 4. Hourly mole fractions of CO2, CH4, N2O and SF6 atmospheric ambient air and hourly

activities of Rn 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).

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* Autumn

Spring

CO O Œ. CO

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Figure 5. Mean diurnal cycles of CO2, CH4, N2O, SF6, and Rn together with the planetary g 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.

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Figure 6. Differences between the monthly background at Puy de Dôme and the respective monthly MBLR (Dlugokencky et al., 2013b, a) at 45.5° N latitude for CO2, CH4, N2O and SF6.

The last panel is the Rn offset relative to marine air.

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Figure 7. Monthly CO2, CH4 and N2O fluxes at the Puy de Dôme station derived from the

radon tracer method. The last panel presents the hourly Rn exhalation rate (U. Karstens and I. Levin, personal communication, 2014). The vertical grey lines are the respective flux uncertainties.

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