The Bruker EM27/SUN is a portable, low-resolution Fourier transform spectrometer (FTS) with a built-in solar tracker using a camera trained on the detector aperture to provide active feedback. The instruments measure DC-coupled interferograms with a spectral resolution of 0.5 cm-1 using two indium gallium arsenide (InGaAs) detectors at room temperature, one with a spectral range of 5500–11 000 cm-1 and a second, wavelength-extended detector measuring in the 4000–5500 cm-1 range ( and ).

The raw interferograms are processed and Xgas retrievals made using the PROFFAST software of the COCCON-PROCEEDS framework which was developed on behalf of the ESA; it is open-source and freely available. It has previously been described by and will be summarised herein. The DC-coupled, double-sided interferograms are processed into spectra by the PROFFAST–PREPROCESS code which includes a DC correction, phase correction and quality checks of the resulting spectra. Separately, cross sections of the molecular absorption are calculated for each day by the PCXS module based on the meteorological and trace gas priors generated using the TCCON method and for a representative surface pressure. The trace gas retrievals are then computed by the INVERS module which uses a least squares fitting algorithm to scale the prior profiles, with the option of adjusting the surface pressure to that which was actually measured at the time of the observation.

A further correction is applied to the retrieved XCO2 values. This is to account for a compounded correction factor applied by PROFFAST, which had been derived for an earlier version of the code. The earlier version included a bug that caused a pointer offset of magnitude of 1 in the spectrum handling. This bug was fixed in the PROFFAST version 2020-08-10, and while it was found that the correction factors applied to all other retrieved species remained valid, it was necessary to apply a further, solar zenith angle-dependent correction to XCO2 of the following form: 1XCO2corr=XCO2×(1.0018-0.001×(SZA90)2), where XCO2corr and XCO2 are the corrected and uncorrected values and SZA is the solar zenith angle in degrees. This correction has been applied to the R02 version of the dataset described in this work. A previous dataset version (R01) was published before the need to apply the correction was known. Further details of this correction and the background to it are available in “Technical note on XCO2 bias in current PROFFAST distribution” (https://www.imk-asf.kit.edu/english/3225.php, last access: 27 January 2022).

The Lauder TCCON station (45.04∘ S, 169.68∘ E, 370 m a.m.s.l.) is the southernmost in the network and has one of the longest continuous records. The station has been previously described in . In this work, we have used the standard output from the GGG2014 data-processing version which is publicly available from the TCCON data archive .

The ESA’s Sentinel-5 precursor (S5P) satellite, which carries the TROPOMI instrument as its sole payload, is orbiting in a Sun-synchronous, low-Earth polar orbit and has an observation swath of 2600 km wide across track, resulting in daily global coverage and a pixel size of 5.5 × 7 km for CH4 and CO.

The S5P operational XCH4 data are retrieved using the RemoteTeC–S5P algorithm , which produces retrievals of XCH4 only under cloud-free conditions. In this work, we compared both the standard product and a bias-corrected version of the dataset. The details of the bias correction can be found in the algorithm theoretical baseline document (ATBD) for S5P methane retrievals . The S5P operational total column density of carbon monoxide (CO) is retrieved using a short-wave infrared carbon monoxide retrieval (SICOR) algorithm . The retrievals of CO are performed simultaneously with interfering trace gases and effective cloud parameters, such as cloud height and optical thickness. The offline (OFFL) operational data version 01.03.02 was used in this paper.

In order to monitor the stability of the alignment of an FTS, the ILS can be retrieved .

For the Lauder TCCON instrument, regular (approximately monthly) measurements are made of an internal cell, containing a calibrated quantity of HCl, illuminated by a lamp source. From these spectra, the ILS is retrieved using version 14.5 of the LINEFIT software described in .

The EM27/SUN does not have the capability to measure HCl cells, but an ILS retrieval can be achieved by taking long (4 m) path measurements of a lamp source and making use of water vapour adsorption as described in and updated by . For the EM27/SUN instrument used in this study (serial number 53), ILS measurements were conducted at the Karlsruhe Institute of Technology before and after the instrument was shipped to New Zealand.

A time series of the results of these ILS retrievals, parameterised in terms of the modulation efficiency (ME) at maximum optical path difference (OPD) and the maximum value of the phase error, are shown in Fig.  for both instruments. A noticeable feature of Fig.  is a step change in the ME at max OPD for the TCCON instrument in May 2019. This is related to a change of the instrument's metrology laser on 28 May. Before this change, the mean ME at max OPD was 0.9991 (with a standard deviation of 0.0006), after the change, it was 0.9902 (0.0010). This 1 % change in ME at max OPD will not have a significant impact on Xgas retrievals since estimated that a 4 % change would result in only a 0.035 % error in XCO2. The TCCON max phase error remained virtually the same across this change at -0.0019 (0.0004) before and -0.0022 (0.0007) after. The EM27/SUN had an ME at max OPD and max phase error of 0.9891 and 0.0006, respectively before leaving Karlsruhe and 0.9875 and 0.0010 when it returned. We therefore conclude that both instruments maintained their alignment throughout the presented dataset.

At Arrival Heights, the EM27/SUN was positioned on a bench inside the laboratory. A small amount of the beam from the solar tracker that is used for a Bruker IFS125HR as part of the Network for the Detection of Atmospheric Composition Change (NDACC, ) was diverted into the EM27/SUN's solar tracker which was fixed in position.

Measurements were taken on clear-sky days when a technician was present at the laboratory. Over the course of the 2019–2020 summer season, between 6 November 2019 and 9 March 2020, measurements were made on 41 d.

Under normal operations, 10 interferogram scans over a period of about 70 s are co-averaged into a single EM27/SUN measurement. To halve the amount of data needing to be transferred, it was decided to co-average 20 interferograms at Arrival Heights. Since the rate-of-change in solar zenith angle is relatively small at high latitudes, this change is not expected to have any impact on retrievals.

The pressure data used in the retrievals at Arrival Heights is taken from the weather station at the nearby Scott Base (NIWA electronic weather station, EWS, Scott Base, ID:12740, 77.85∘ S, 166.76∘ E, 20 m a.m.s.l.) and corrected for the altitude difference of 185 m between the two locations.

Figure  shows the time series of each of the Xgases retrieved from the EM27/SUN and the Lauder TCCON station before, during and after the measurement campaign at Arrival Heights.

The XCO2 measured at Arrival Heights is systematically lower than at Lauder. This is an expected result as high-latitude southern hemispheric CO2 concentrations are less than at mid-latitudes . Also, the seasonality that can be seen at the lower latitude is not obvious in these measurements. However, over the period of deployment, the growth rate of XCO2 is also not seen at Arrival Heights. This is potentially because the seasonal drawdown is of a similar magnitude and obscures the growth over this short period.

The XCH4 shows considerable variability over the period of deployment and a negative trend which contradicts the positive trend seen at lower latitudes. To investigate this structure further, we examined the isentropic modified potential vorticity (MPV) over Arrival Heights derived from the Modern Era Retrospective-Analysis for Research and Applications reanalysis product (MERRA2) . Although not an absolute diagnostic of the position of the polar vortex relative to Arrival Heights, the MPV value will be negatively correlated with the influence of the vortex . The top panel of Fig.  shows the MPV at the 460 K isentropic level (corresponding to the lower stratosphere) over the course of the instrument campaign, while the lower panel shows the daily averaged XCH4 retrievals for the same period. There is clear correlation between the two (r=0.82, 95 % CI: 0.68–0.90). The correlation of XCH4 with MPV confirms a weak barrier effect of the polar vortex .

The lower panel of Fig.  shows XCO. The data collected at Arrival Heights are clearly measuring baseline XCO concentrations whilst at lower latitudes, the Lauder TCCON data show spikes caused by stratospheric transport of air masses affected by biomass burning in the tropics.

Before and after the EM27/SUN was deployed to Arrival Heights, it was operated for a period at Lauder, alongside the TCCON station there. Between 8 March and 22 July 2019, the instrument was in a workshop where it could be moved outside on fine days to make observations using the built-in solar tracker. An automatic scheduling application was used to run the measurements and this was able to interrogate the output from the EM27/SUN's Camtracker software and pause measurements when clouds obscured the Sun. On 22 July, the EM27/SUN was relocated to the same laboratory as the TCCON instrument; here, the built-in solar tracker was parked and illuminated by a small amount (approximately 5 %) of the parallel solar beam from a solar tracker coupled to another Bruker 125HR. The difference in altitude between these locations is 10 m and corrections have been applied to account for this in the surface pressure used for trace gas retrievals. This pressure measurement is from the Vaisala PTB100A sensor that is part of the Lauder climate station.

After returning to New Zealand from Antarctica, the instrument was returned to Lauder where it was installed under another, permanently installed solar tracker and further measurements were collected from 5 June until 3 September 2020 when it was returned to the Karlsruhe Institute of Technology (KIT).

In total, measurements were collected on 72 d alongside the Lauder TCCON station. To make a meaningful comparison between the EM27/SUN and TCCON, we first average the retrievals from both instruments into bins of 10 min. The window of 10 min. was chosen to include sufficient measurements to reduce the effects of random uncertainties whilst not aliasing in slowly varying signals due to e.g. air mass dependence.

Figure  shows a time series of the differences between the 10 min averages before and after the deployment for the three species, with box and whisker plots of these in the inset panel. A summary of the before-and-after statistics is also given in Table . There is a small, but statistically significant, difference for all three species between the before-and-after comparisons. As described previously, retrievals of the instrument line shape (ILS) for both instruments spanning the period in question show no large drifts in the alignment of either. Therefore, it is reasonable to expect that this is due to a seasonally dependent bias between the two instruments. While it is difficult to draw conclusions from this limited time span plotted in Fig. , it is not unreasonable to suggest that this is the case. Such an effect was seen previously by when comparing low- and high-resolution instruments. This effect is likely caused by the averaging kernels of the low- and high-resolution instruments aliasing different errors in the common priors into the respective retrievals. In any case, the change in offset with respect to TCCON is 0.07 % for XCO2 (0.16 % for XCH4 and 1.72 % for XCO), which is below the 0.1 % precision value generally accepted as the requirement for carbon cycle studies and satellite validation .

The spatial and temporal coincidence criteria used to select S5P measurements corresponding to EM27/SUN observations are the same as , i.e. within a radius of 100 km (50 km) around the site for methane (carbon monoxide) validation and with a maximal time difference of 1 h for EM27/SUN observations around the S5P overpass time. Because of the high latitude of the Arrival Heights laboratory and the inclination of the S5P orbit, this often results in coincidences on more than one S5P orbit for a particular day's EM27/SUN observing period.

The average S5P pixel values are compared to EM27/SUN retrievals which have had the a priori alignment applied to compensate for or correct its contribution to the smoothing equation . The co-located pairs are selected only if a minimum of five S5P pixels were found in applying the co-incidence criteria.

Figure  shows the comparison between the ground-based EM27/SUN and the S5P retrievals for bias-corrected XCH4 and standard XCO (left and right plots, respectively) with all values retrieved by each instrument (pale symbols) as well as co-located overpass means (bold) (upper panels) and the relative differences (lower panels).

The bias-corrected S5P XCH4 product shows a bias of 2.14 %, which exceeds the S5P bias requirement (1.5 %). The bias of the standard S5P XCH4 product without the albedo-dependent correction is 1.05 %. This shows that the S5P XCH4 product at this location is strongly dependent on the surface albedo and the respective correction applied to the product. The standard deviation of the relative bias, which is a measure of the random error, is below 0.3 % for both standard and bias-corrected S5P XCH4 products, which is well below the requirement of 1 %. This high bias at Arrival Heights was also seen by when they performed a comparison with the NDACC station there. This was attributed to the highly variable topography in the region. Similar effects have also been noted at high northern latitudes p. 129, again attributed to variability on surface albedo and topography. These results further highlight the value of having reliable, high-quality, ground-based measurements which are independent of surface effects to validate satellite retrievals, particularly in challenging regions such as these.

For S5P XCO, the mean bias is 3.77 %, which is well within the S5P bias requirement (15 %). The standard deviation of the relative bias is 4.73 %, which is well below the requirement of 10 %. Applying a cone co-location criterion to the S5P data following the ground-based EM27/SUN line of sight, as recommended in Sha et al. (2021) for high-latitude sites, we find a mean bias of 5.89 % compared to the EM27/SUN. The bias with respect to the EM27/SUN is less than the bias of 11.99 % that Sha et al. (2021) found when comparing to the NDACC data using a common a priori for a period of about 3 years. The discrepancy is mostly because of the different time periods between the NDACC and EM27/SUN comparisons, a priori difference or spectroscopy limitations. The S5P bias shows a seasonality with respect to NDACC data with high values in September and October, a period when the EM27/SUN did not measure. Furthermore, a bias change of about +3.7 % has been reported for the S5P validation with NDACC when comparing directly versus using the S5P a priori as the common prior, whereas this change is about -0.26 % for EM27/SUN comparisons.