The LSCE laboratory at Gif-sur-Yvette is equipped with two coupled gas chromatographs (HP-6890, Agilent and PP1, Peak Laboratories) which run fully automated, alternating between calibration gas and ambient air, in order to analyse CO, H2, CO2, CH4, N2O and SF6 concentration in atmospheric measurements, flask samples or high-pressure cylinders. Detailed descriptions of the GC system for CO analysis is given by . CO is analysed with the PP1 chromatograph equipped with a reduction gas detector after reduction of mercuric oxide and detection of mercury vapour by UV absorption. Each analysis takes less than 6 min, allowing between two and six injections of ambient air alternating with calibration gases and flask samples. The air is dried before the injection in two steps. First, it passes through a glass trap which is hosted in a commercial refrigerator kept at 5 ∘C in order to remove a large fraction of water vapour, and in the second step air is further dried by passing through a second glass trap cooled in an ethanol bath at -55 ∘C using a cryogenic cooler (designed as the “cooling trap” in the following). An operator is only required to change the cooling trap 2–3 times per week and to restart the acquisition.

The GC is calibrated for CO with cylinders certified by the NOAA Global Monitoring Division (GMD) on the WMO CO X2004 scale . CO concentrations are calculated using regular measurements of one calibration cylinder with a typical atmospheric concentration value (here 168.0 ± 0.8 ppb) and a non-linear correction function of the detector response as described in and . The correction function is determined on an annual frequency using a set of five cylinders with CO concentration ranges from 57± 1.0 ppb to 523 ppb ± 10.9 ppb and applied as a post-run correction . This non-linear correction was validated using flask measurement comparisons between LSCE and NOAA, with a mean difference of 4.5 ± 2.2 ppb for the period of July 2006 to July 2009. For the 1 week comparison campaign with the OF-CEAS instrument in November 2006, the correction function applied to CO in situ measurements by the GC (COmeas) to obtain the calibrated CO concentrations reported in the following is given by ΔCOcorr=11.4+0.077×COmeas-7.110-4×(COmeas)2+1.0310-8×(COmeas)3. It applies a correction for the non-linear behaviour of the analyser in the range of -15 to +15 ppb for measured CO concentration up to 500 ppb.

The calibration cylinder is analysed every 30–40 min along with a quality control gas, a so called target gas, with a CO concentration of 68 ppb, that is treated as unknown. Over the entire comparison period, the repeatability defined as 1σ standard deviation of the target gas is 0.4 ppb.

The flask samples filled during the airborne campaign are measured in a similar way to the ambient air concentration with two injections per flask.

The laser spectroscopy technique going under the name of OF-CEAS was introduced by and has been further detailed in different publications . In particular the OF-CEAS instrument used in this study has been described in . It provides in situ CO measurements with a detection limit of 0.2 ppb in 20 s with no calibration and running unattended. Here we will just recall the basic principle of OF-CEAS.

Spectroscopic measurements of trace gas concentrations require a long light absorption path. Like other spectroscopy techniques, OF-CEAS is based upon the use of a sample cell made with an optical cavity in order to enhance light interaction with the gas sample. Specifically in the spectrometer used here for CO monitoring, the resonant optical cavity composed of high-reflectivity mirrors (mirror reflectivity: R≃ 99.995 %) allows a ∼ 20 km effective absorption length with a compact set-up: the cavity is only 1 m long, folded to a 0.5 m external size. The difficulty of using a resonant cavity with high-reflectivity mirrors (thus very small transmissivity) resides in the coupling of a sufficient amount of light in the cavity by injecting laser light through one of the cavity mirrors. The originality of OF-CEAS is that the optical cavity is made of three mirrors placed in a “V-shaped” configuration. In this way, a fraction of the light trapped inside the optical cavity, and therefore frequency-selected by a resonant mode of the cavity, can be returned to the laser. The non-linear response of the laser then forces it to lase on the exact frequency of the excited cavity mode. This optical-feedback effect is also responsible for a narrowing of the laser emission line width and an increase of the cavity transmission to a level that is orders of magnitude larger than in competing techniques . OF-CEAS absorption spectra are acquired on a small spectral region, as shown in Fig.  for the present case, by scanning the laser frequency at a relatively high repetition rate (6 Hz here). However, the measurement response time for 1/e change in a concentration value is not limited by this rate but by the gas exchange rate inside the sample volume. Therefore, the cell is designed to allow minimal dead spaces and a small internal (sample) volume, which does not exceed 18 cm3. The gas is continuously flowing and the cell pressure is stabilized with a downstream pressure regulator to 200 mbar in this study. The flow is adjusted manually with a needle valve at the inlet of the sampling cell to 250 sccm (standard cm cubes per minute) for ground measurements and around 50 sccm for airborne measurements. The corresponding gas exchange times are then 0.9 and 4.3 s respectively. If needed, a shorter response time can be obtained by using a lower sample pressure or a higher flow rate. The design of the spectrometer is robust and compact: the optical assembly and all the electronics, for real-time control and data acquisition, fit inside a 19 in chassis where the V-shaped cavity is placed in the diagonal as shown in Fig. 1 of . The device is temperature stabilized around 22 ∘C using heating adhesive ribbons.

Importantly, OF-CEAS provides quantitative absorption measurements in real time without the need for a periodic calibration with certified gas mixtures. A normalization procedure of the absorbance scale is realized continuously based on cavity optical loss measurements performed by CRDS . Pressure and temperature stabilization of the sample inside the cavity allows for a real-time numerical fit of the measured absorption spectra with a reduced number of parameters . This enables the selective determination of the concentrations of all compounds that possess absorption lines in the selected spectral window (CO and CH4 here). This is a key point in trace gas monitoring, with atmospheric air being a highly complex gas mixture. To optimize the CO detection limit, an NIR distributed-feedback diode laser is chosen emitting in the [2.3–2.4] µm range (Fig. ), an interesting region that includes an atmospheric window where water vapour absorption lines are sparse and weak, while several light species such as CO display relatively strong absorption bands. The smallest detectable absorption coefficient is typically in the range of several 10-10 cm-1. As a result, an OF-CEAS instrument optimized for CO monitoring has an LOD of 0.2 ppb of CO for an acquisition time of 20 s, a value derived from an Allan variance study in .

Short response time and low LOD allow OF-CEAS instruments to perform fast trace gas monitoring. This has already been exploited in airborne atmospheric measurements of methane in , water isotopes in and , and in other fields using laboratory prototypes and commercial instruments (ProCEAS) as mentioned in the introduction section.

The ring-down calibration included in the OF-CEAS technique allows for direct absolute molecular absorption measurements (in cm-1 unit, Fig. ). Then line intensity is directly converted to CO concentration with a conversion factor specific to the fitted absorption line for the temperature and pressure operation conditions (298.5 ± 1 K and 200 ± 1.8 mbar in this work). This allows to account for temperature and pressure effects on the line intensity parameters. It is important to stress that this factor is a constant that does not depend on the cavity finesse (continuously measured by ring-down) nor on the gas sample composition (the multi-line fit allows independent fit of each species) as far as the foreign pressure broadening effect on CO absorption lines from water can be neglected. This is justified for atmospheric measurements where water concentration remains small (it varies from 0.8 to 1.6 % for ground-based measurements reported in this work). As a consequence, the conversion factor of each molecule needs to be determined for the OF-CEAS spectrometer working conditions only once, and then during operation the instrument delivers absolute concentrations in real time without the need of any calibration with certified mixtures.

The conversion factor can be derived from a spectral database or by direct calibration using certified mixtures. Even if the line intensities for CO in this spectral region are well defined to better than 1 % in the high-resolution transmission molecular absorption (HITRAN) database , in practice calibration with gas standards is found to be more accurate because it cancels sensors pressure and temperature absolute accuracy and allows the minimizing of line profile effects by considering a specific model in the fit procedure – a Rautian model is used as in .

For the comparison campaign in 2006, the OF-CEAS spectrometer was calibrated with two high-pressure cylinders containing air whose CO concentration had been certified in 1995 by the Commonwealth Scientific and Industrial Research Organisation (CSIRO). However, the GC was calibrated on the WMO X2004 scale provided by the NOAA. Differences between CSIRO and NOAA CO scales on the order of 6 ppb have been reported by . Therefore, we re-evaluated CO concentrations in the CSIRO cylinders against the WMO CO X2004 scale. This was done in 2014 using another OF-CEAS instrument designed for ice core analysis . This instrument was calibrated with three standards certified in 2011 by the NOAA GMD Carbon Cycle Group on the WMO CO X2004 scale (33.2 ± 0.5, 51.8 ± 0.1 and 102.1 ± 0.1 ppb of CO). The two working standards used to calibrate the instrument for the 2006 campaign were recalibrated to 35.0 ± 1.5 and 104.0 ± 1.5 ppb, while the CSIRO values certified in 1995 were 32.6 ± 0.7 and 98.7 ± 1 ppb, respectively.

These WMO-scaled CO standard gas values were then used to calibrate the entire 2006 dataset using a linear relationship (i.e., without offset adjustment), which is consistent with the fact that the zero of the spectral measurements is intrinsically accurate. Furthermore, the high linearity of OF-CEAS was previously reported for concentrations ranging over more than three decades (data published for water measurements in ). The accuracy of this calibration is estimated to be of 2 %, limited by the accuracy of the NOAA standards. However, the obtained conversion factor corresponds to a 10 % overestimation of the line intensity specified in HITRAN with a 1 % accuracy. CSIRO specifications of the two standards being offset by 5 and 7 % as compared to NOAA standards are not compatible with the HITRAN database. Nonetheless, the good agreement of OF-CEAS and GC measurements reported in the following shows that this calibration on the same reference scale is a crucial point for the inter-comparison.

The LOD of the OF-CEAS spectrometer is much smaller than the accuracy of the NOAA standards, at the level of 0.2 ppb for an acquisition time of 20 s. At longer acquisition times, small drifts prevent a better averaging. It is partly attributed to drifts in the sensors that are used to control sample pressure and temperature, and thus the selection of more stable sensors can decrease the drift. Other causes of drift are changes in parasitic optical etalon effects . However, the drifts associated with these optical effects can be made quite small and cannot increase arbitrarily and remain bounded at all times as shown by the Allan variance of CO measurements in .

Direct comparison of atmospheric CO concentration measurements by GC and OF-CEAS over 1 week (8–14 November 2006) was performed at LSCE in Gif-sur-Yvette, 20 km southwest of Paris (48∘43′∘ N, 02∘09′∘ E; 120 m above sea level). The GC set-up routinely monitors atmospheric concentration with a sampling inlet located on the roof of the LSCE building, 7 m above ground level. The OF-CEAS instrument from LIPhy was set to run in the same building but with an independent sampling line. Sampling lines measured about 20 m and were made of 3/8 in diameter Dekabon tubes. The estimated sample propagation delay along the tube from the roof to the OF-CEAS instrument is about 6 min (with a gas flow of 250 sccm). A larger delay is observed on the GC data due to the use of the cold trap. The volume of this trap corresponds to the sample volume collected over about 15 min by the GC, inducing a smoothing of the signal of the semi-continuous injections. To eliminate the time delay between both instruments, the time shift was fixed to 14 min (Fig. ).

Reported GC CO concentrations are dry air mole fractions. For the comparison, the OF-CEAS CO mole fraction (xCO,air) is converted into dry air mole fraction (xCO,dry) according to xCO,dry=xCO,air1-xH2O=xCO,air1-[RH×e(T)]/P, where the water mole fraction (xH2O) is computed from the relative humidity rate (RH), the atmospheric pressure (P) and the water saturation vapour pressure (e(T)) given by a polynomial function of the atmospheric temperature (T; ). The meteorological data used (RH, P and T) are routinely monitored at the Saclay radar located about 1.5 km north–northwest from the sampling point (data provided by the SPR group from Saclay CEA).

OF-CEAS and GC raw data were post-treated as detailed in Sect. . In Fig.  the typical variations of atmospheric CO dry concentration measured by both the GC and the OF-CEAS analysers during a weekday (Fig. a), a Sunday (Fig. b) and a Wednesday night (Fig. c) are shown. During night-time and most of the day on the weekends, CO concentration slowly varies within typically 100–300 ppb. But during weekdays emissions from nearby traffic usually induce two rush hour peaks in the morning at around 08:00 local time (LT) and in the evening after 17:00 LT. The persistence and higher intensity of the second CO peak could be related to air mass change or to the Friday evening traffic jams all around the Paris suburbs (Fig. a). During daytime and in the evening, the fast response time of the OF-CEAS instrument (1 s averaged here to 2 s) allows the recording of many short but very strong peaks (sometime rising up to more than 1 ppm for only 1 or 2 min). These are due to local pollution of vehicles passing by the laboratory. The 14 November 2006 night (Fig. c) is characterized by the fact that CO concentration remains quite stable around 100 ppb (concentration fluctuations smaller than 20 ppb are measured over 7 h), thus allowing a comparison over several hours with nearly no effect from the slower GC response time.

Overall, OF-CEAS and GC measurements show an excellent agreement. When CO concentration varies slowly, such as during night-time and Sunday measurements shown in Fig. , the agreement is within about 2 ppb rms over several hours for concentration values ranging from 100 to 300 ppb for the whole comparison period of 1 week. This difference is fully compatible with the calibration accuracy of the two instruments reported before. When CO concentration is subject to fast changes such as in Fig. a, the strong difference in the GC and OF-CEAS measurements is explained by the slower response time of the GC instrument due to the buffering effect of the cooling trap. The air sample is continuously flushed in the cooling trap, and the effect on CO concentration measurements by the GC is not equivalent to a simple time average. A more complex weighted moving average could be performed on the faster OF-CEAS measurement to try to mimic the GC measurement, but a study concerning this averaging issue appears to be beyond the scope of this paper.

The OF-CEAS instrument measures CO and CH4 simultaneously (Fig. ). Contrary to CO, CH4 concentration is not sensitive to traffic pollution. Its daily variability is usually less than 10 %, with a background value of about 1900 ppb. The rms noise of the OF-CEAS measurements for CH4 is 4 ppb for an averaging time of 20 s. A good agreement between OF-CEAS and GC is also found with maximum deviations of ± 20 ppb, corresponding to about 1 % in relative units. Such performance has been previously reported in with a similar OF-CEAS instrument compared to the same GC.

In the framework of the French RAMCES observation network for greenhouse gas monitoring, regular weekly flights have been carried out by LSCE since 1996 above the Orléans forest, located about 100 km south of Gif-sur-Yvette. This flight program aims to improve our understanding of transport processes into the atmospheric boundary layer and to better assess the relative role of local, regional and continental anthropogenic and biospheric fluxes on the observed trace gas concentrations. In particular, vertical profiles of trace gases are very useful for assessing atmospheric transport model performances. During the flights, air samples are collected in flasks, as described in , and later analysed at LSCE by GC to measure the concentration of CO2, CH4, CO, N2O and SF6 . Glass flasks are filled at 10 different altitudes (between 100 and 3000 m above ground level). Those used for the present comparison were analysed 1 week after collection. On 15 November 2006, the OF-CEAS instrument was installed inside the aircraft to perform in situ measurements of atmospheric CO concentration during the entire flight. The OF-CEAS instrument was mounted in a 19 in rack fixed in place of a seat.

Tropospheric air was sampled upwind of the aircraft engine exhaust: a 2 m Dekabon inlet line carried outside air to the set-up entrance, passing through a customized window of the aircraft. The same inlet was used for the OF-CEAS instrument. Regarding the flasks set up, the sampling unit consisted of a diaphragm pump which drew air through a chemical drying cartridge filled with Mg(ClO4)2. Air was collected in 1 L glass flasks sealed with PTFE O-rings. Flasks were collected in pairs and pressurized to 2 bar absolute pressure. The filling step took between 30 s and 1 min, during which the plane covered a typical horizontal distance of 5 km.

The entire set of measurements is shown in Fig.  starting from the airport of Toussus-le-Noble (48∘45′∘ N, 2∘08′∘ E; 164 m a.s.l.), during the flight to the Orléans forest area (47∘50′∘ N, 2∘30′∘ E; 135 m a.s.l.) where the plane starts a routine flight that consists of legs at the 10 pre-defined altitudes for the flask samples collection, and during the flight back to the airport. During the flight above the Orléans forest, CO concentrations remained at around 90 ppb (± 10 ppb) above 1000 m, while an increase at lower altitudes was clearly measured up to 150 ppb at 100 m due to surface CO sources like traffic and heating.

It should be noted that the OF-CEAS instrument is robust enough to operate in the harsh environment of a small aircraft including during takeoff and landing phases. On the tarmac, CO rises up to 16 ppm due to airplane exhaust gases. This illustrates the wide sensitivity range of the measurements, of about 4 orders of magnitude. During the whole flight, the instrument ran unattended. The only not automatized action by the operator required during the flight consisted of adjusting the flow in the sampling cell with a needle valve. Given that this valve was placed at instrument inlet, the flow changed linearly with pressure, thus decreased with altitude. However, for GC comparison, measurements were taken during constant altitude sections, allowing averaging on time scales largely exceeding the sample exchange time for flow around 50 sccm. Later, flow regulation was automatized using a numerically controlled flow regulator. During the flight, the flow was slowly varying between 40 and 70 sccm. OF-CEAS data were averaged for 5.5 s to be consistent with the largest value of the response time. It corresponds to a space resolution of 300 m according to the aircraft velocity. Due to the harsh environment in the plane, the standard deviation of the measurements was increased to typically 2 ppb (zoom in Fig. ), while for ground-based measurements it was 0.6 ppb for 2 s averaging time (Fig. c).

As explained in the previous sections, for the comparison of GC and OF-CEAS measurements, post-data processing was performed to correct for the RGD non-linearity and to bring both instruments on the same calibration scale. Additionally, OF-CEAS concentrations have to be expressed in dry air. The water mole fraction was not monitored during the flight but was derived from a model allowing the computing of the specific humidity q from meteorological data (analysis from the European Centre for Medium-Range Weather Forecasts, ECMWF). Water mole fraction is then given by xH2O=q×MdryMH2O+q×(Mdry-MH2O), where MH2O and Mdry are respectively the molar mass of water and dry air. During the flight, the values obtained for the water mole fraction varied typically from 0.2 to 1 % at respectively high and low altitude, inducing a correction on CO values between +0.2 and +1.6 ppb. The model was compared to the meteorological data (RH, P and T) provided by the closest station that is the radio sounding of Trappes (48∘46′∘ N, 2∘1′∘ E; 168 m a.s.l.). These data were recorded by Meteo-France and are available on the SIRTA website at http://sirta.ipsl.polytechnique.fr/sirta.old/data_policy.html. Humidity rates derived from ECWMF model and Trappes data are in agreement within 20 %, which means that corrections obtained from one or the other model will be closer than the measurement error.

In the bottom of Fig.  the difference between OF-CEAS and GC CO concentration measurements is plotted. To be consistent with the typical filling duration of the flasks, OF-CEAS values were averaged for 1 min around the flask filling times. A good agreement is obtained for the set of 10 measurements recorded at different altitudes, the difference has a mean value of -2.2 ppb with a standard deviation of 1.7 ppb. This small systematic difference could not be explained even when different artefacts like a residual effect of the non-linearity of the RGD or the humidity correction of the OF-CEAS measurements were examined. The agreement between the OF-CEAS spectrometer and the GC measurements is very close to the 2015 World Meteorological Organization compatibility goal of 1 σ for CO that is ±2 ppb (see WMO–GMA report edited by ).