Pollutants information can be retrieved from visible (VIS) and ultraviolet (UV) diffuse solar spectra exploiting Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) instruments. In May 2021, the Italian research institute CNR-ISAC acquired and deployed a MAX-DOAS system SkySpec-2D. It is located in the “Giorgio Fea” observatory in San Pietro Capofiume (SPC), in the middle of the Po Valley, where it has constantly acquired zenith and off-axis diffuse solar spectra since the 1st October 2021. This work presents the retrieved tropospheric NO₂ and aerosol extinction profiles (and their columns) derived from the MAX-DOAS measurements using the newly developed DEAP retrieval code. The code has been validated both using synthetic differential Slant Column Densities (dSCDs) from the Fiducial Reference Measurements for Ground-Based DOAS Air-Quality Observations (FRM4DOAS) project and real measured data. For this purpose, DEAP results are compared with the ones obtained with three state-of-the-art retrieval codes. In addition, an inter-comparison with satellite products from Sentinel-5P TROPOMI, for the tropospheric NO₂ Vertical Column Densities (VCDs), and MODIS-MAIAC for the tropospheric Aerosol Optical Depth (AOD), is performed. We find a bias of −0.6 × 10¹⁵ molec/cm² with a standard deviation of 1.8 × 10¹⁵ molec/cm² with respect to Sentinel-5P TROPOMI for NO₂ tropospheric VCDs and of 0.04 ± 0.08 for AOD with respect to MODIS-MAIAC data. The retrieved data show that the SPC measurement site is representative of the background pollution conditions of the Po Valley. For this reason, it is a good candidate for satellite validation and scientific studies over the Po Valley.

The Italian peninsula is, as shown by satellite and ground-based measurements, a pollution hotspot. In recent years, ground-based MAX-DOAS commercial systems have been installed in the Po Valley and the area surrounding Rome to monitor NO₂ tropospheric column densities and validate coincident satellite (e.g., TROPOMI) products. Three of the instruments are located in the Po Valley at San Pietro Capofiume (Bologna), Bologna city, and Mount Cimone (Modena), and one is located in Tor Vergata (Rome). The chosen system is the SkySpec-2D from Airyx. All the recorded spectra are saved in the FRM4DOAS format and processed with QDOAS software to obtain slant column densities (SCDs) of NO₂, O₄, and other trace gases. The MAX-DOAS SCD sequences are then analysed with the DEAP code to retrieve tropospheric profiles of NO₂ and aerosol extinction, while zenith-sky SCDs are used to retrieve NO₂ total columns. A dedicated campaign, involving the network instruments, has been conducted in the Po Valley to compare the performance of the individual instruments in the network with respect to the one that participated in the CINDI-3 campaign (Cabauw, The Netherlands). The results of the intercomparison campaign indicated that all instruments showed comparable performance. As an example of obtainable products, one year (from September 2024 to August 2025) of NO₂ tropospheric columns, as well as their comparison with TROPOMI measurements, is presented, highlighting the potential of this network for both air quality studies and satellite validation. Due to Italy’s location in the highly complex Mediterranean hotspot region, these data represent an important contribution to satellite validation efforts, particularly in view of upcoming missions such as Copernicus Sentinel-4, Sentinel-5, and the Copernicus Anthropogenic Carbon Dioxide Monitoring (CO2M) constellation. We found a negative TROPOMI bias relative to SkySpec-2D for NO₂ tropospheric columns ranging from −13% in San Pietro Capofiume, to −25% in Bologna and −44% in Rome Tor Vergata. The comparison between NO₂ total columns from TROPOMI and SkySpec-2D at Mount Cimone shows generally good agreement, with TROPOMI being 15% higher.

In the past, the possibility to retrieve the total column of water vapour (TCWV) from the thermal infrared (TIR) day and night measurements above water surfaces of the dual-view Along Track Scanning Radiometers (ATSR) has been demonstrated, and an algorithm, named Advanced InfrarRed Water Vapour Estimator (AIRWAVE), was developed and successfully applied to the measurements of the (A)ATSR instrument series. A similar instrument, the Sea and Land Surface Temperature Radiometer (SLSTR), is currently operating on board the Sentinel 3 satellite series. In this paper, we demonstrate that the AIRWAVE algorithm can be successfully applied to the SLSTR instrument to obtain reliable TCWV measurements. The steps performed for upgrading the algorithm are thoroughly described. The new AIRWAVE algorithm makes use of parameters computed offline with a state-of-the-art radiative transfer model using the most recent spectroscopic data and continuum model. For the parameters calculation, a new climatology capable of representing the average atmospheric and sea surface status during SLSTR measurements has been developed. The new algorithm, named AIRWAVE-SLSTR, has been implemented in both IDL and Python languages. In the frame of an EUMETSAT contract, AIRWAVE-SLSTR has been applied to a full year of SLSTR measurements (2021) and the retrieved TCWV have been validated with the help of both satellite- and ground-based measurements. The correlation of the retrieved TCWV with satellite MW measurements is 0.94 and the average bias is of the order of 0.66 kg/m${}^2$. When compared to ground-based measurements, the average correlation is 0.93 and the bias −0.48 kg/m${}^2$. The obtained accuracy is well within the requirements set for both numerical weather predictions (1–5 kg/m${}^2$) and for coastal altimetry applications (1.8–3 kg/m${}^2$). Therefore, the AIRWAVE-SLSTR algorithm can be safely applied to obtain a long time series of reliable TCWV above water surfaces.

We report the installation and first two years of operation of an EM27/SUN Fourier transform spectrometer at the CNR-ISAC facility in Bologna, Italy, the first instrument of its kind operating in the Po Valley, one of the European region mostly affected by anthropogenic pollution. The instrument is integrated into the COCCON network and it retrieves total columns and column-averaged dry air mole fractions of CO₂, CH₄, CO, and H₂O using the PROFFAST processing chain as per standardized COCCON protocols. The data is filtered with the fitted solar gas shift in the O₂ absorption band for quality controls, and here we study the seasonality of the retrieved products. To assess the relationship between column-integrated and near-surface measurements, we present a case-study comparison against a co-located Cavity Ring-Down Spectroscopy analyzer for CO₂ and CH₄ over a single day, using ceilometer-derived Mixed Aerosol Layer height to interpret the differing diurnal variability between the two techniques, with CRDS showing substantially larger amplitude swings driven by boundary-layer dynamics and surface emission/sink processes. We further validate the EM27/SUN against TROPOMI satellite retrievals of XCH₄ and total-column CO over the full observing period, finding low systematic biases (-0.13% and 0.60% respectively) consistent with satellite mission requirements and comparable to TCCON stations at similar latitudes. As a case-study, we show that both instruments jointly detected an anomalous CO enhancement in July-August 2024 attributable to long-range transport of Canadian wildfires smoke, illustrating the capability of ground-based FTIR observations to identify high-altitude pollution intrusions undetectable by in-situ instrumentation alone. These results establish the EM27/SUN in Bologna as a robust reference for satellite validation and a valuable tool for characterizing atmospheric transport events in the Po Valley.

High-resolution ground-based Fourier Transform InfraRed (FTIR) spectroscopy is a key technique for monitoring atmospheric composition, in particular, greenhouse gases, providing vertically integrated information essential for climate studies, emission estimates, and satellite data validation. Within the framework of the PNRR EMM and ITINERIS projects, two new ground-based FTIR spectrometers have been installed at the CNR research area in Bologna, Italy, significantly enhancing the national observational capability in these fields.The first instrument, a Bruker IFS 125HR, operates in the infrared spectral range from approximately 850 to 9000 cm⁻¹, with a spectral resolution of about 0.0036 cm⁻¹, enabling the retrieval of a wide set of atmospheric trace gases and minor atmospheric constituents, including CO₂, CH₄, N₂O, CO, O₃, HNO₃, HCl, HF, C₂H₆, HCN, and HDO. These measurements are crucial for investigating changes in atmospheric composition and their impact on the Earth’s radiative balance, as well as for deriving greenhouse gas emissions and trends.The second instrument is a Bruker EM27/SUN, which operates in the NIR range from 4000 to 12000 cm⁻¹, thus allowing the retrieval of CO2, CH4, CO, and H2O. Being part of the COCCON network, its data are analyzed on a daily basis, and the resulting GEOMS files are regularly delivered to the network for public distribution.The FTIR spectrometers are located in the Po Valley, one of the most polluted regions in Europe, offering a unique opportunity to observe greenhouse gases and air pollutants under conditions of strong anthropogenic influence. The IFS 125HR is fully compliant with the NDACC and TCCON network requirements, ensuring standardized data acquisition, calibration, and processing procedures and thereby allowing direct comparison with other ground-based FTIR sites worldwide and with satellite observations. This installation represents the first FTIR facility of its kind in Italy and the central Mediterranean region, providing significant scientific value at both national and international levels.

Water vapor (WV) plays a fundamental role in tropospheric processes, as most atmospheric moisture is confined to this layer. However, homogeneous and globally distributed observations of the lower troposphere—up to about 5–6 km altitude—remain limited. Filling this observational gap would significantly improve short-term climate analyses and the performance of numerical weather prediction (NWP) models.Within theoretical activities supported by ESA, a novel retrieval concept called Normalized Differential Spectral Attenuation (NDSA) was developed to estimate integrated water vapor (IWV) from microwave attenuation measurements in the 17–21 GHz frequency range along tropospheric propagation paths. The method is based on the estimation of a spectral sensitivity coefficient (S), defined as the differential attenuation between two closely spaced carrier frequencies with a relative separation smaller than 2%. We demonstrated a linear relationship between S and IWV, enabling a simple and robust retrieval scheme. These investigations also highlighted the suitability of NDSA for spaceborne applications, including co- and counter-rotating Low Earth Orbit (LEO) satellite geometries. The Italian Space Agency funded the SATCROSS project to assess the technological feasibility of a dedicated satellite mission and to develop a ground-based prototype capable of performing NDSA measurements on terrestrial microwave links at 19 GHz.A critical step toward an operational space-based system is the quantitative assessment of the accuracy and reliability of IWV estimates derived from the prototype through validation against independent observing techniques. A first validation campaign was in 2024, comparing IWV retrieved by the NDSA prototype with measurements from a MAX-DOAS instrument observing the same atmospheric volume along a 91 km link between the meteorological station “Giorgio Fea” (San Pietro Capofiume, 10 m a.s.l.) and the climate observatory at Mount Cimone (2165 m a.s.l.). Additional reference data were provided by radiosondes, hygrometers, and GNSS. While the results were encouraging, significant signal scintillation affected the NDSA measurements due to a large fraction of the link remaining within the terrain boundary layer.The present work focuses on a second campaign carried out in 2025 along a 160 km high-altitude microwave link connecting Mount Cimone to Mount Amiata (1738 m a.s.l.). For the first time, the NDSA prototype was tested on a link with nearly constant height and limited ground influence, closely approximating the geometry of a LEO-to-LEO satellite link with a tangent height of about 2000 m. This setup enabled verification of the theoretical relationship between the spectral sensitivity parameter S and IWV, with particular attention to the linear model coefficients reported by the authors in previous papers. ERA5 reanalysis data (25-km linear res.), integrated along the full link, were also compared with in situ hygrometer measurements and GNSS-derived IWV. Overall, IWV estimates from the different techniques show good agreement in capturing daily and seasonal variability, while ERA5 systematically underestimates IWV due to its coarser resolution. At shorter timescales, discrepancies increase during periods of enhanced tropospheric turbulence, induced by air mass movements. Criteria for real-time identification of high-scintillation conditions were defined, demonstrating the capability of NDSA to detect precipitation while preserving WV information.