Formaldehyde (HCHO) total column densities over the Mexico City metropolitan area (MCMA) were retrieved using two independent measurement techniques: multi-axis differential optical absorption spectroscopy (MAX-DOAS) and Fourier transform infrared (FTIR) spectroscopy. For the MAX-DOAS measurements, the software QDOAS was used to calculate differential slant column densities (dSCDs) from the measured spectra and subsequently the Mexican MAX-DOAS fit (MMF) retrieval code to convert from dSCDs to vertical column densities (VCDs). The direct solar-absorption spectra measured with FTIR were analyzed using the PROFFIT (PROFile FIT) retrieval code. Typically the MAX-DOAS instrument reports higher VCDs than those measured with FTIR, in part due to differences found in the ground-level sensitivities as revealed from the retrieval diagnostics from both instruments, as the FTIR and the MAX-DOAS information do not refer exactly to the same altitudes of the atmosphere. Three MAX-DOAS datasets using measurements conducted towards the east, west or both sides of the measurement plane were evaluated with respect to the FTIR results. The retrieved MAX-DOAS HCHO VCDs where 6  % , 8  % and 28  % larger than the FTIR measurements which, supported with satellite data, indicates a large horizontal inhomogeneity in the HCHO abundances. The temporal change in the vertical distribution of this pollutant, guided by the evolution of the mixing-layer height, affects the comparison of the two retrievals with different sensitivities (total column averaging kernels). In addition to the reported seasonal and diurnal variability of HCHO columns within the urban site, background data from measurements at a high-altitude station, located only 60 km away, are presented.

Since 2009, the Greenhouse gases Observing SATellite (GOSAT) has performed radiance measurements in the near-infrared (NIR) and shortwave infrared (SWIR) spectral region. From February 2019 onward, data from GOSAT-2 have also been available. We present the first results from the application of the Fast atmOspheric traCe gAs retrievaL (FOCAL) algorithm to derive column-averaged dry-air mole fractions of carbon dioxide ( XCO 2 ) from GOSAT and GOSAT-2 radiances and their validation. FOCAL was initially developed for OCO-2 XCO 2 retrievals and allows simultaneous retrievals of several gases over both land and ocean. Because FOCAL is accurate and numerically very fast, it is currently being considered as a candidate algorithm for the forthcoming European anthropogenic CO 2 Monitoring (CO2M) mission to be launched in 2025. We present the adaptation of FOCAL to GOSAT and discuss the changes made and GOSAT specific additions. This particularly includes modifications in pre-processing (e.g. cloud detection) and post-processing (bias correction and filtering). A feature of the new application of FOCAL to GOSAT and GOSAT-2 is the independent use of both S- and P-polarisation spectra in the retrieval. This is not possible for OCO-2, which measures only one polarisation direction. Additionally, we make use of GOSAT's wider spectral coverage compared to OCO-2 and derive not only XCO 2 , water vapour ( H 2 O ), and solar-induced fluorescence (SIF) but also methane ( XCH 4 ) , with the potential for further atmospheric constituents and parameters like semi-heavy water vapour ( HDO ). In the case of GOSAT-2, the retrieval of nitrous oxide ( XN 2 O ) and carbon monoxide ( CO ) may also be possible. Here, we concentrate on the new FOCAL XCO 2 data products. We describe the generation of the products as well as applied filtering and bias correction procedures. GOSAT-FOCAL XCO 2 data have been produced for the time interval 2009 to 2019. Comparisons with other independent GOSAT data sets reveal agreement of long-term temporal variations within about 1  ppm over 1 decade; differences in seasonal variations of about 0.5  ppm are observed. Furthermore, we obtain a station-to-station bias of the new GOSAT-FOCAL product to the ground-based Total Carbon Column Observing Network (TCCON) of 0.56  ppm with a mean scatter of 1.89  ppm . The GOSAT-2-FOCAL XCO 2 product is generated in a similar way as the GOSAT-FOCAL product, but with adapted settings. All GOSAT-2 data until the end of 2019 have been processed. Because of this limited time interval, the GOSAT-2 results are considered to be preliminary only, but first comparisons show that these data compare well with the GOSAT-FOCAL results and also TCCON.

The TROPOspheric Monitoring Instrument (TROPOMI) on board the Sentinel 5 Precursor (S5-P) satellite provides methane (CH 4 ) measurements with high accuracy and exceptional temporal and spatial resolution and sampling. TROPOMI CH 4 measurements are highly valuable to constrain emissions inventories and for trend analysis, with strict requirements on the data quality. This study describes the improvements that we have implemented to retrieve CH 4 from TROPOMI using the RemoTeC full-physics algorithm. The updated retrieval algorithm features a constant regularization scheme of the inversion that stabilizes the retrieval and yields less scatter in the data and includes a higher resolution surface altitude database. We have tested the impact of three state-of-the-art molecular spectroscopic databases (HITRAN 2008, HITRAN 2016 and Scientific Exploitation of Operational Missions – Improved Atmospheric Spectroscopy Databases SEOM-IAS) and found that SEOM-IAS provides the best fitting results. The most relevant update in the TROPOMI XCH 4 data product is the implementation of an a posteriori correction fully independent of any reference data that is more accurate and corrects for the underestimation at low surface albedo scenes and the overestimation at high surface albedo scenes. After applying the correction, the albedo dependence is removed to a large extent in the TROPOMI versus satellite (Greenhouse gases Observing SATellite – GOSAT) and TROPOMI versus ground-based observations (Total Carbon Column Observing Network – TCCON) comparison, which is an independent verification of the correction scheme. We validate 2 years of TROPOMI CH 4 data that show the good agreement of the updated TROPOMI CH 4 with TCCON ( − 3.4  ±  5.6 ppb) and GOSAT ( − 10.3  ±  16.8 ppb) (mean bias and standard deviation). Low- and high-albedo scenes as well as snow-covered scenes are the most challenging for the CH 4 retrieval algorithm, and although the a posteriori correction accounts for most of the bias, there is a need to further investigate the underlying cause.

Global climate change is one of the most important scientific, societal and economic contemporary challenges. Fundamental understanding of the major processes driving climate change is the key problem which is to be solved not only on a global but also on a regional scale. The accuracy of regional climate modelling depends on a number of factors. One of these factors is the adequate and comprehensive information on the anthropogenic impact which is highest in industrial regions and areas with dense population – modern megacities. Megacities are not only “heat islands”, but also significant sources of emissions of various substances into the atmosphere, including greenhouse and reactive gases. In 2019, the mobile experiment EMME (Emission Monitoring Mobile Experiment) was conducted within the St. Petersburg agglomeration (Russia) aiming to estimate the emission intensity of greenhouse ( CO 2 , CH 4 ) and reactive (CO, NO x ) gases for St. Petersburg, which is the largest northern megacity. St. Petersburg State University (Russia), Karlsruhe Institute of Technology (Germany) and the University of Bremen (Germany) jointly ran this experiment. The core instruments of the campaign were two portable Bruker EM27/SUN Fourier transform infrared (FTIR) spectrometers which were used for ground-based remote sensing measurements of the total column amount of CO 2 , CH 4 and CO at upwind and downwind locations on opposite sides of the city. The NO 2 tropospheric column amount was observed along a circular highway around the city by continuous mobile measurements of scattered solar visible radiation with an OceanOptics HR4000 spectrometer using the differential optical absorption spectroscopy (DOAS) technique. Simultaneously, air samples were collected in air bags for subsequent laboratory analysis. The air samples were taken at the locations of FTIR observations at the ground level and also at altitudes of about 100 m when air bags were lifted by a kite (in case of suitable landscape and favourable wind conditions). The entire campaign consisted of 11 mostly cloudless days of measurements in March–April 2019. Planning of measurements for each day included the determination of optimal location for FTIR spectrometers based on weather forecasts, combined with the numerical modelling of the pollution transport in the megacity area. The real-time corrections of the FTIR operation sites were performed depending on the actual evolution of the megacity NO x plume as detected by the mobile DOAS observations. The estimates of the St. Petersburg emission intensities for the considered greenhouse and reactive gases were obtained by coupling a box model and the results of the EMME observational campaign using the mass balance approach. The CO 2 emission flux for St. Petersburg as an area source was estimated to be 89  ±  28  kt km - 2 yr - 1 , which is 2 times higher than the corresponding value in the EDGAR database. The experiment revealed the CH 4 emission flux of 135  ±  68  t km - 2 yr - 1 , which is about 1 order of magnitude greater than the value reported by the official inventories of St. Petersburg emissions ( ∼  25  t km - 2 yr - 1 for 2017). At the same time, for the urban territory of St. Petersburg, both the EMME experiment and the official inventories for 2017 give similar results for the CO anthropogenic flux (251  ±  104  t km - 2 yr - 1 vs. 410  t km - 2 yr - 1 ) and for the NO x anthropogenic flux (66  ±  28  t km - 2 yr - 1 vs. 69  t km - 2 yr - 1 ).

In this paper, we compare column-averaged dry-air mole fractions of water vapor ( XH 2 O ) retrievals from the COllaborative Carbon Column Observing Network (COCCON) with retrievals from two co-located high-resolution Fourier transform infrared (FTIR) spectrometers as references at two boreal sites, Kiruna, Sweden, and Sodankylä, Finland, from 6 March 2017 to 20 September 2019. In the framework of the Network for the Detection of Atmospheric Composition Change (NDACC), an FTIR spectrometer is operated at Kiruna. The H 2 O product derived from these observations has been generated with the MUlti-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water (MUSICA) processor. In Sodankylä, a Total Carbon Column Observing Network (TCCON) spectrometer is operated, and the official XH 2 O data as provided by TCCON are used for this study. The datasets are in good overall agreement, with COCCON data showing a wet bias of ( 49.20±58.61 ) ppm (( 3.33±3.37 ) %, R 2 =0.9992 ) compared with MUSICA NDACC and ( 56.32±45.63 ) ppm (( 3.44±1.77 ) %, R 2 =0.9997 ) compared with TCCON. Furthermore, the a priori H 2 O volume mixing ratio (VMR) profiles (MAP) used as a priori information in the TCCON retrievals (also adopted for COCCON retrievals) are evaluated with respect to radiosonde (Vaisala RS41) profiles at Sodankylä. The MAP and radiosonde profiles show similar shapes and a good linear correlation of integrated XH 2 O , indicating that MAP is a reasonable approximation of the true atmospheric state and an appropriate choice for the scaling retrieval methods as applied by COCCON and TCCON. COCCON shows a reduced dry bias ( −14.96  %) in comparison with TCCON ( −19.08  %) with respect to radiosonde XH 2 O . Finally, we investigate the quality of satellite data at high latitudes. For this purpose, the COCCON XH 2 O is compared with retrievals from the Infrared Atmospheric Sounding Interferometer (IASI) generated with the MUSICA processor (MUSICA IASI) and with retrievals from the TROPOspheric Monitoring Instrument (TROPOMI). Both paired datasets generally show good agreement and similar correlations at the two sites. COCCON measures 4.64 % less XH 2 O at Kiruna and 3.36 % less at Sodankylä with respect to MUSICA IASI, whereas COCCON measures 9.71 % more XH 2 O at Kiruna and 7.75 % more at Sodankylä compared with TROPOMI. Our study supports the assumption that COCCON also delivers a well-characterized XH 2 O data product. This emphasizes that this approach might complement the TCCON network with respect to satellite validation efforts. This is the first published study where COCCON XH 2 O has been compared with MUSICA NDACC and TCCON retrievals and has been used for MUSICA IASI and TROPOMI validation.

Although optical components in Fourier transform infrared (FTIR) spectrometers are preferably wedged, in practice, infrared spectra typically suffer from the effects of optical resonances (“channeling”) affecting the retrieval of weakly absorbing gases. This study investigates the level of channeling of each FTIR spectrometer within the Network for the Detection of Atmospheric Composition Change (NDACC). Dedicated spectra were recorded by more than 20 NDACC FTIR spectrometers using a laboratory mid-infrared source and two detectors. In the indium antimonide (InSb) detector domain (1900–5000  cm −1 ), we found that the amplitude of the most pronounced channeling frequency amounts to 0.1  ‰ to 2.0  ‰ of the spectral background level, with a mean of ( 0.68±0.48 )  ‰ and a median of 0.60  ‰ . In the mercury cadmium telluride (HgCdTe) detector domain (700–1300  cm −1 ), we find even stronger effects, with the largest amplitude ranging from 0.3  ‰ to 21  ‰ with a mean of ( 2.45±4.50 )  ‰ and a median of 1.2  ‰ . For both detectors, the leading channeling frequencies are 0.9 and 0.11 or 0.23  cm −1 in most spectrometers. The observed spectral frequencies of 0.11 and 0.23  cm −1 correspond to the optical thickness of the beam splitter substrate. The 0.9  cm −1 channeling is caused by the air gap in between the beam splitter and compensator plate. Since the air gap is a significant source of channeling and the corresponding amplitude differs strongly between spectrometers, we propose new beam splitters with the wedge of the air gap increased to at least 0.8 ∘ . We tested the insertion of spacers in a beam splitter's air gap to demonstrate that increasing the wedge of the air gap decreases the 0.9  cm −1 channeling amplitude significantly. A wedge of the air gap of 0.8 ∘ reduces the channeling amplitude by about 50  % , while a wedge of about 2 ∘ removes the 0.9  cm −1 channeling completely. This study shows the potential for reducing channeling in the FTIR spectrometers operated by the NDACC, thereby increasing the quality of recorded spectra across the network.