Contrary to the statements put forward in “Evaluation of measurement data – Guide to the expression of uncertainty in measurement”, edition 2008(GUM-2008), issued by the Joint Committee for Guides in Metrology, the error concept and the uncertainty concept are the same. Arguments in favor of the contrary have been analyzed and found not to be compelling. Neither was any evidence presented in GUM-2008 that “errors” and “uncertainties” define a different relation between the measured and true values of the variable of interest, nor does this document refer to a Bayesian account of uncertainty beyond the mere endorsement of a degree-of-belief-type conception of probability.

We show new results from an updated version of the Fast atmOspherictraCe gAs retrievaL (FOCAL) retrieval method applied to measurementsof the Greenhouse gases Observing SATellite (GOSAT) and its successorGOSAT-2.FOCAL was originally developed for estimating the total column carbondioxide mixing ratio ( XCO 2 ) from spectral measurements made by theOrbiting Carbon Observatory-2 (OCO-2).However, depending on the available spectral windows, FOCAL alsosuccessfully retrieves total column amounts for other atmosphericspecies and their uncertainties within one single retrieval.The main focus of the current paper is on methane ( XCH 4 ; full-physics and proxy product), water vapour ( XH 2 O ) and therelative ratio of semi-heavy water ( HDO ) to water vapour( δ D ).Due to the extended spectral range of GOSAT-2, it is also possible toderive information on carbon monoxide ( XCO ) and nitrous oxide( XN 2 O ) for which we also show first results.We also present an update on XCO 2 from both instruments. For XCO 2 , the new FOCAL retrieval (v3.0) significantly increasesthe number of valid data compared with the previous FOCAL retrievalversion (v1) by 50 % for GOSAT and about a factor of 2 for GOSAT-2due to relaxed pre-screening and improved post-processing.All v3.0 FOCAL data products show reasonable spatial distribution andtemporal variations.Comparisons with the Total Carbon Column Observing Network (TCCON) resultin station-to-station biases which are generally in line with thereported TCCON uncertainties. With this updated version of the GOSAT-2 FOCAL data, we provide afirst total column average XN 2 O product.Global XN 2 O maps show a gradient from the tropicsto higher latitudes on the order of 15  ppb , which can beexplained by variations in tropopause height.The new GOSAT-2 XN 2 O product compares well with TCCON.Its station-to-station variability is lower than 2  ppb ,which is about the magnitude of the typical N 2 O variations close to the surface.However, both GOSAT-2 and TCCON measurements show that the seasonalvariations in the total column average XN 2 O are on the orderof 8  ppb peak-to-peak, which can be easily resolved by the GOSAT-2 FOCALdata.Noting that only few XN 2 O measurements from satellites exist sofar, the GOSAT-2 FOCAL product will be a valuable contribution inthis context.

In this paper, we compare Orbiting Carbon Observatory 2 (OCO-2) measurements of column-averaged dry-air mole fractions (DMF) of CO 2 ( X CO 2 ) and its urban–rural differences against ground-based remote sensing data measured by the Munich Urban Carbon Column network(MUCCnet). Since April 2020, OCO-2 has regularly conducted target observations in Munich, Germany. Its target-mode data provide high-resolution X CO 2 within a 15  km   ×  20  km target field of view that is greatly suited for carbon emission studies from spacein cities and agglomerated areas. OCO-2 detects urban X CO 2 with a root mean square different (RMSD) of less than 1  ppm when compared to the MUCCnet referencesite. OCO-2 target X CO 2 is biased high against the ground-based measurements. The close proximity of MUCCnet's five fully automatedremote sensing sites enables us to compare spaceborne and ground-based X CO 2 in three urban areas of Munich separately (center, north,and west) by dividing the target field into three smaller comparison domains. Due to this more constrained collocation, we observe improvedagreement between spaceborne and ground-based X CO 2 in all three comparison domains. For the first time, X CO 2 gradients within one OCO-2 target field of view are evaluated against ground-based measurements. We compare X CO 2 gradients in the OCO-2 target observations to gradients captured by collocated MUCCnet sites. Generally, OCO-2 detects elevated X CO 2 in the same regions as the ground-based monitoring network. More than 90 % of the observed spaceborne gradients have the sameorientation as the X CO 2 gradients measured by MUCCnet. During our study, urban–rural enhancements are found to be in the range of 0.1 to1  ppm . The low urban–rural gradients of typically well below 1  ppm in Munich during our study allow us to test OCO-2's lowerdetection limits for intra-urban X CO 2 gradients. Urban X CO 2 gradients recorded by the OCO-2 instruments and MUCCnet arestrongly correlated ( R 2 =0.68 ) with each other and have an RMSD of 0.32  ppm . A case study, which includes a comparison of one OCO-2target overpass to WRF-GHG modeled X CO 2 , reveals a similar distribution of enhanced CO 2 column abundances in Munich. In thisstudy, we address OCO-2's capability to detect small-scale spatial X CO 2 differences within one target observation. Our results suggestOCO-2's potential to assess anthropogenic emissions from space.

In this study, an extension on the previously reported status of theCOllaborative Carbon Column Observing Network's (COCCON) calibrationprocedures incorporating refined methods is presented. COCCON is a globalnetwork of portable Bruker EM27/SUN FTIR spectrometers for derivingcolumn-averaged atmospheric abundances of greenhouse gases. The originallaboratory open-path lamp measurements for deriving the instrumental lineshape (ILS) of the spectrometer from water vapour lines have been refinedand extended to the secondary detector channel incorporated in the EM27/SUNspectrometer for detection of carbon monoxide (CO). The refinementsencompass improved spectroscopic line lists for the relevant water lines anda revision of the laboratory pressure measurements used for the analysis ofthe spectra. The new results are found to be in good agreement with thosereported by Frey et al. (2019) and discussed in detail. In addition, a newcalibration cell for ILS measurements was designed, constructed and put intoservice. Spectrometers calibrated since January 2020 were tested using bothmethods for ILS characterization, open-path (OP) and cell measurements. Wedemonstrate that both methods can detect the small variations in ILScharacteristics between different spectrometers, but the results of the cellmethod indicate a systematic bias of the OP method. Finally, a revision andextension of the COCCON network instrument-to-instrument calibration factorsfor XCO 2 , XCO and XCH 4 is presented, incorporating 47 newspectrometers (of 83 in total by now). This calibration is based on thereference EM27/SUN spectrometer operated by the Karlsruhe Institute ofTechnology (KIT) and spectra collected by the collocated TCCON stationKarlsruhe. Variations in the instrumental characteristics of the referenceEM27/SUN from 2014 to 2017 were detected, probably arising from realignmentand the dual-channel upgrade performed in early 2018. These variations areconsidered in the evaluation of the instrument-specific calibration factorsin order to keep all tabulated calibration results consistent.

This work employs ground- and space-based observations, togetherwith model data, to study columnar abundances of atmospheric trace gases( XH 2 O , XCO 2 , XCH 4 and XCO ) in two high-latitude Russian cities, St. Petersburg and Yekaterinburg. Two portable COllaborative ColumnCarbon Observing Network (COCCON) spectrometers were used for continuousmeasurements at these locations during 2019 and 2020. Additionally, a subset of data of special interest (a strong gradient in XCH 4 and XCO was detected) collected in the framework of a mobile city campaign performed in 2019 using both instruments is investigated. All studied satellite products (TROPOMI, OCO-2, GOSAT, MUSICA IASI) show generally good agreement with COCCON observations. Satellite and ground-based observations at high latitudes are much sparser than at low or mid latitudes, which makes directcoincident comparisons between remote-sensing observations more difficult.Therefore, a method of scaling continuous Copernicus AtmosphereMonitoring Service (CAMS) model data to theground-based observations is developed and used for creating virtual COCCONobservations. These adjusted CAMS data are then used for satellitevalidation, showing good agreement in both Peterhof and Yekaterinburg. The gradients between the two study sites ( Δ Xgas) aresimilar between CAMS and CAMS-COCCON datasets, indicating that the modelgradients are in agreement with the gradients observed by COCCON. This isfurther supported by a few simultaneous COCCON and satellite Δ Xgasmeasurements, which also agree with the model gradient. With respect to thecity campaign observations recorded in St Petersburg, the downwind COCCONstation measured obvious enhancements for both XCH 4 (10.6 ppb) and XCO (9.5 ppb), which is nicely reflected by TROPOMI observations, which detect city-scale gradients of the order 9.4 ppb for XCH 4 and 12.5 ppb for XCO .

Accurate observations of atmospheric ozone (O 3 ) are essential to monitor in detail its key role in atmospheric chemistry. The present paper examines the performance of different O 3 retrieval strategies from FTIR (Fourier transform infrared) spectrometry by using the 20-year time series of the high-resolution solar spectra acquired from 1999 to 2018 at the subtropical Izaña Observatory (IZO, Spain) within NDACC (Network for the Detection of Atmospheric Composition Change). In particular, the effects of two of the most influential factors have been investigated: the inclusion of a simultaneous atmospheric temperature profile fit and the spectral O 3 absorption lines used for the retrievals (the broad spectral region of 1000–1005 cm −1 and single micro-windows between 991 and 1014 cm −1 ). Additionally, the water vapour (H 2 O) interference in O 3 retrievals has been evaluated, with the aim of providing an improved O 3 strategy that minimises its impact and, therefore, could be applied at any NDACC FTIR station under different humidity conditions. The theoretical and experimental quality assessments of the different FTIR O 3 products (total column (TC) amounts and volume mixing ratio (VMR) profiles) provide consistent results. Combining a simultaneous temperature retrieval with the optimal selection of single O 3 micro-windows results in superior FTIR O 3 products, with a precision of better than 0.6 %–0.7 % for O 3 TCs as compared to coincident NDACC Brewer observations taken as a reference. However, this improvement can only be achieved provided the FTIR spectrometer is properly characterised and stable over time. For unstable instruments, the temperature fit is found to exhibit a strong negative influence on O 3 retrievals due to the increase in the cross-interference between the temperature retrieval and instrumental performance (given by the instrumental line shape function and measurement noise), which leads to a worsening of the precision of FTIR O 3 TCs of up to 2 %. This cross-interference becomes especially noticeable beyond the upper troposphere/lower stratosphere, as documented theoretically as well as experimentally by comparing FTIR O 3 profiles to those measured using electrochemical concentration cell (ECC) sondes within NDACC. Consequently, it should be taken into account for the reliable monitoring of the O 3 vertical distribution, especially over long-term timescales.