The Picarro G2207-i O2 analyser measures the mole fractions of the two most abundant atmospheric O2 isotopologues, 16O16O and 16O18O, through absorption spectra at 7882.18670 and 7882.050155 cm-1, respectively (Berhanu et al., 2019). The design principles of this analyser have been described in detail by Berhanu et al. (2019). In our study we evaluate only what is called the “O2 concentration” mode, measuring only the 16O16O isotopologue. In the other mode, called the “δ18O plus O2 concentration” mode, O2 mole fraction values are considerably less precise, as the analyser is not optimised for 16O16O measurements (primarily via a different set point for the pressure in the cavity). The analyser reports both “wet” and “dry” O2 mole fraction values. The wet values (O2,NC; NC stands for “not corrected”) do not have any correction applied to them, whereas the dry values (O2,WC; WC stands for “water corrected”) are corrected for the dilution effect of water vapour on the O2 mole fraction, as well as spectroscopic interference, using the analyser's parallel water vapour mole fraction measurements. The G2207-i data sheet states a measurement precision of 5 ppm +0.1 % of the reading (1-σ, 5 s) for the water vapour mole fraction.

The performance of the G2207-i was evaluated in the UEA CRAM Laboratory by measuring a suite of 12 gas cylinders all containing dry natural air with varying O2 mole fractions. The cylinders were stored horizontally in a thermally insulated “Blue Box” enclosure in order to prevent gravitational and thermal fractionation of O2 relative to N2 (Keeling et al., 2007). The O2 composition of each of these cylinders was precisely defined on the Scripps Institution of Oceanography (SIO) O2 scale (Keeling et al., 2007) using a VUV O2 analyser, which is also in the CRAM Laboratory. The CO2 mole fraction was defined on the “WMO CO2 X2007” scale (Zhao and Tans, 2006) using a Siemens Corp. Ultramat model 6F non-dispersive infrared (NDIR) CO2 analyser. Five of these cylinders were working secondary standards (WSSes), which were used to calibrate the G2207-i, one was a reference tank (RT; explained below in Sect. 2.3.2), while the other six were treated as cylinders with unknown mole fractions (Table 1). The six “unknown” cylinders were used to evaluate the performance of the analyser with a CO2 mole fraction range of 375 to 443 ppm and an O2/N2 ratio range of -915 to +435 per meg, a much larger range than would typically be observed in ambient air.

The cylinders were run consecutively, starting with the six “unknowns” and ending with the five WSSes, with the RT run at the beginning and end; this sequence was repeated twice. Each of the gas cylinders was flushed for 20 min prior to running on the G2207-i to allow for removal of stagnant air and equilibration of the pressure regulators; air from each cylinder was then passed through the analyser for 20 min, with the first 8 min of data discarded to allow flushing of the previous cylinder's air from the cavity and to maintain consistency with the flushing time employed in subsequent WAO tests (Sect. 2.3.2). The remaining 12 min for each cylinder was then averaged to give the “raw” O2,NC value for each cylinder as measured on the G2207-i.

The G2207-i has a linear response to O2 mole fraction (Eq. 3) 3y=Bx+C where B and C are the coefficients derived from the slope and intercept of the linear regression calculated from the measurement of the WSSes. Therefore, a minimum of two WSS cylinders are required to determine the B and C coefficients, but by using five we are able to calculate the coefficient of determination (R2), as well as providing more robustness in the fit. The calibration equation was used to convert the “raw” O2,NC values taken from the G2207-i (x in Eq. 3) into what we call “ppm equivalent” (ppmEquiv) O2 units (y in Eq. 3), as described in Kozlova and Manning (2009). A linear interpolation between the RT at the beginning and end of each run was used as a baseline for the run and subtracted from all other cylinder measurements to correct for short-term analyser variations. The calibration curve (Eq. 3) for the G2207-i was also determined relative to the interpolated RT values (WSS – RT); thus, all the unknown cylinder measurements could be converted into ppmEquiv. The ppmEquiv O2 units were then converted to per meg units, providing a δ(O2/N2) value for each unknown cylinder, using Eq. (4). 4δO2N2=δO2+CO2-363.29×SO2SO2×(1-SO2) where, δO2 is the calibrated G2207-i O2,NC value in ppmEquiv units, CO2 is the declared cylinder CO2 mole fraction from the Siemens analyser in ppm, SO2 is 0.2094, which is the standard mole fraction of O2 molecules in dry air (Tohjima et al., 2005), and 363.29 is an arbitrary CO2 reference value in ppm, inherent to the SIO O2 scale (Stephens et al., 2007).

Weybourne Atmospheric Observatory (WAO) is located on the north Norfolk coast, UK (52∘57′02′′ N, 1∘07′19′′ E), approximately 35 km north-northwest of Norwich, 170 km northeast of London, and 200 km east of Birmingham. It is part of the European Union's Integrated Carbon Observation System (ICOS) and the World Meteorological Organization's (WMO) Global Atmosphere Watch (GAW) programme. High-precision, high-accuracy, continuous measurements of a wide array of atmospheric gas species (including greenhouse gases, isotopes, reactive gases) are carried out at a fine temporal scale, funded in part through the UK's National Centre for Atmospheric Science (NCAS) long-term measurement programme.

Atmospheric O2 and CO2 have been measured continuously at WAO since 2008 (Wilson, 2013). O2 is measured with an Oxzilla II O2 analyser (Sable Systems International Inc.) (hereafter referred to as the “Oxzilla”), and CO2 is measured with an Ultramat 6E NDIR analyser (Siemens Corp.). These analysers are arranged in series, with the air sample first passing through the Ultramat 6E and then the Oxzilla, with rigorous gas handling and calibration protocols followed (as in Stephens et al., 2007).

The G2207-i was installed at WAO from 23 October 2019–2 November 2019, sampling from a solar shield aspirated air inlet (AAI) at a height of 10 m above ground level (a.g.l.; 20 m above sea level, a.s.l.). The AAI protects the inlet from solar radiation and generates a continuous air flow over the inlet, thus preventing the differential fractionation of O2 molecules relative to N2 molecules due to ambient temperature variations (Blaine et al., 2006) and relatively slow inlet flow rates (Manning, 2001). A diagram of the gas handling set-up for the G2207-i at WAO is displayed in Fig. 1.

In order to further assess the G2207-i's performance in real-world applications, the O2,NC observations from the full-drying regime period at WAO were used to isolate the fossil fuel component of the concurrent CO2 observations and then compared to the ffCO2 values calculated from atmospheric potential oxygen (APO) defined from the Oxzilla O2 observations following the methodology outlined in Pickers et al. (2022).

The tracer APO, derived by Stephens et al. (1998), was first calculated using Eq. (5) (using both G2207-i O2,NC and Oxzilla O2 values); these APO values were then used to calculate ffCO2 using Eq. (6). 5APO=O2+-1.10.2094×350-CO2, where O2 and CO2 are in per meg and parts per million, respectively, -1.1 is the global average O2:CO2 terrestrial biosphere–atmosphere exchange ratio (Severinghaus, 1995), 0.2094 is the standard mole fraction of O2 in dry air (Tohjima et al., 2005), and 350 is an arbitrary reference value for CO2 (in ppm). Multiplying CO2 by -1.1 and dividing by 0.2094 converts the CO2 data from parts per million to per meg units. 6ffCO2=APO-APObgRAPO:CO2, where APO is derived from Eq. (5) in per meg units, APObg is the APO background, or baseline, value determined using a statistical baseline fitting procedure, and RAPO:CO2 is the APO : CO2 combustion ratio for fossil fuel emissions. The APObg values were determined using the rfbaseline function from the IDPmisc package in R, which implements robust fitting of local regression models, with a smoothing window of 1 week (Ruckstuhl et al., 2012). The APO : CO2 emission ratio (RAPO:CO2) used is -0.3 mol mol-1, an approximate mean value for WAO as determined from the COFFEE inventory (a typical value for fossil fuel emissions, given that the APO:CO2 ratio =O2:CO2+1.1; Pickers, 2016; Steinbach et al., 2011). The uncertainty on the ffCO2 mole fractions was calculated using Eq. (6) with the upper and lower uncertainty limit for each variable (where the measurement uncertainty for APO was calculated by summing in quadrature the CO2 and O2 measurement uncertainty for each analyser) and then taking the standard deviation (SD) of the resultant ffCO2 value of each combination for each hourly time stamp.

To assess the short-term precision and optimal averaging time of the G2207-i, the Allan deviation technique (Werle et al., 1993) was used whilst sampling a compressed-air cylinder in the laboratory (50 L, 200 bar). The cylinder was run for 24 h with a sample flow rate of 94 mL min-1 and cavity pressure and temperature of 340 mbar and 45 ∘C, respectively. The results of this Allan deviation analysis are in agreement with those obtained by Berhanu et al. (2019), where a precision of 1 ppm (∼4.8 per meg) was achieved after an averaging time of 300 s. Precision then continues to improve until around a 30 min averaging time where a precision of ∼0.5 ppm (∼2.4 per meg) is reached, and it remains around that value for averaging times up to around 1 h (Fig. 2). It should be noted that unlike the hourly average and standard deviation obtained from measurement of cylinder air, the hourly averages of atmospheric data also contain natural variability in addition to analyser-related noise and drift.

To evaluate the analyser drift (i.e. the changing sensitivity of the analysers response with time), O2,NC values from the G2207-i were averaged to 1 h (Fig. 3b; reported in ppm where 1 ppm corresponds to a change of 4.8 per meg in the O2/N2 ratio). The G2207-i data sheet states a maximum drift at STP (standard temperature and pressure) (over 24 h, peak-to-peak, 1 h internal average at 21 % O2) of <6 ppm. We found that over 24 h, the maximum peak-to-peak drift of the hourly averages is ∼1.2 ppm (approximately 5.8 per meg); this is better than stated by Picarro Inc. but does not meet the WMO compatibility goal of ±2 per meg, as the internal drift of the analyser is greater than this goal. The standard deviation of each of these hourly averages is ∼14.5 ppm (∼69.6 per meg) (Fig. 3a), this is caused by the large amount of analyser noise in the raw 1 s data points, spanning ∼100 ppm (∼480 per meg) (Fig. 3c). The overall drift over the 24 h of raw data however is very small, shown by a linear regression slope of -4.26×10-6 ppm s-1 (Fig. 3c).

The G2207-i analyser performance was evaluated by measuring six gas cylinders with precisely defined O2 and CO2 values as measured on a VUV O2 analyser and Siemens Ultramat 6F NDIR CO2 analyser (Table 1). The difference between the O2,NC values (per meg) as measured by the G2207-i and the declared values from the VUV are shown in Table 2 for runs both with and without the RT interpolation applied. This procedure was carried out twice, referred to as “Run 1” and “Run 2” in Table 2

For both sets of runs without the application of the RT interpolation the difference between the VUV declared value and that measured by the G2207-i is very large and far outside of an acceptable range (Table 2), with an average difference from the declared values for all cylinders of 22.0±10.3 per meg. For all cylinders, except for cylinder 5 and 6, a large improvement in the difference is seen after the application of the RT correction. Due to the large differences between the declared and measured values without the RT correction applied, only the results with the RT correction will be discussed hereafter.

Cylinders 5 and 6 contain O2 values far higher than that found in ambient air (411.7 and 434.6 per meg, respectively) and outside of the range spanned by the WSSes used for calibration. For these two cylinders, the difference between the declared value and that measured by the G2207-i is far larger than the other cylinders and also more variable between the two runs, with a standard deviation of the absolute values between the two runs of ±14.9 and ±19.5 per meg, respectively (Table 2). Berhanu et al. (2019) found that the accuracy of the G2207-i was reduced when the CO2 mole fraction was much higher than that of ambient air but did not observe the same reduction in accuracy with high O2 mole fractions. Ignoring the two cylinders with positive O2, the average absolute difference for the remaining four unknown cylinders and the declared values over the two runs is 3.4±2.5 per meg, this is slightly greater than the WMO compatibility goal of ±2 per meg but does fall within the extended goal of ±10 per meg and is similar to what can be achieved with an Oxzilla II (Pickers et al., 2017). There is also no correlation between the accuracy and the declared O2 value when excluding the two cylinders with positive O2 (R2=0.07 for run 1, R2=0.53 for run 2).

Although the accuracy of the O2 values measured by the G2207-i for these cylinders is variable, particularly for the cylinders with high O2, the standard deviation of the 2 min data points used to calculate the final cylinder O2 value as defined by the G2207-i within each run is more consistent. However, the repeatability, used as a proxy for precision, and defined here as the ±1σ standard deviation of the average of the two measurements of each cylinder, is variable. For the two cylinders with high O2 (cylinders 5 and 6) the repeatability is more than 5 times greater than the WMO extended repeatability goal of ±5 per meg. For the remaining four cylinders the repeatability is far lower, with cylinder 1 and cylinder 3 both falling within the extended repeatability goal.

In order to further assess the G2207-i's performance in real-world applications the fully dried, RT corrected, O2,NC observations from WAO were used to isolate the fossil fuel component of the concurrent CO2 observations and then compared to the ffCO2 values calculated from the Oxzilla O2 observations following the APO methodology outlined in Pickers et al. (2022). The resultant ffCO2 values calculated from each analyser are displayed in Fig. 9.

The measurement uncertainty was calculated as the average hourly SD on 30 October 2019, this date was chosen as it was a particularly stable period with little variation in the TT results for both analysers (Fig. 8); the resultant uncertainty for the G2207-i is ±11.2 per meg compared to ±4.9 per meg for the Oxzilla. The uncertainty in the baseline determination (±28 %) and the emission ratio uncertainty (±22 %) are significantly larger than these measurement uncertainties (Pickers et al., 2022), but as these are the same for both analysers the additional measurement uncertainty for the G2207-i caused by analyser noise increases the uncertainty of the calculated ffCO2 values. The average final calculated uncertainty on the ffCO2 values calculated from the Oxzilla measurements is ±5.8 ppm, compared to ±13.0 ppm on the G2207-i.

The average ffCO2 value over the entire full-drying period for the Oxzilla is 5.1 ppm, compared to 7.9 ppm on the G2207-i (Table 4); the calculated ffCO2 from the G2207-i is higher than that of the Oxzilla 73 % of the time. This difference is predominantly due to the higher O2 values reported by the G2207-i as discussed in Sect. 3.3.2; some of this difference also comes from the jumps in the G2207-i O2 values, which means that the calculated baselines used for each analyser follow different trends. For example, on the 27 and 30 October 2019 the largest difference between the calculated ffCO2 values is observed (Fig. 9); on both of these dates there is a large jump in O2 values from the previous day measured by the G2207-i following a WSS calibration (Fig. 7). Although the O2 difference between the two analysers on these days is low, there was a large difference the preceding day, the days with the larger difference (due to a higher O2 value reported by the G2207-i) in observed values pull the baseline to become more positive, thus making the difference between the ffCO2 calculated from the two analysers larger on days where the observed O2 difference is smaller.

Although the G2207-i calculated ffCO2 values that are often higher than those from the Oxzilla, they still follow the same trend (with some jumps in the G2207-i values); however, the maximum and minimum values occur at different times. The differences in ffCO2 calculated from the G2207-i and the Oxzilla would become problematic if using the G2207-i analyser for top-down ffCO2 quantification on an hourly basis.