Emmanuel Mahieu

The gradual phase-out of stratospheric ozone-depleting substances (ODS), including chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), has led to the widespread adoption of hydrofluorocarbons (HFCs). HFC-23 (CHF₃) ranks third among HFCs in radiative forcing contribution. As a potent greenhouse gas, it falls under the Kyoto Protocol and Kigali Amendment, motivating careful monitoring. HFC-23 is not intentionally manufactured; its atmospheric release stems primarily from HCFC-22 production. This study presents the multi-decadal trend of HFC-23 derived from a newly developed retrieval methodology applied to high-resolution solar absorption spectra recorded by ground-based Fourier Transform InfraRed (FTIR) instruments. Measurements obtained at the Jungfraujoch high-altitude research station (Switzerland) within the Network for the Detection of Atmospheric Composition Change (NDACC) were combined across three successive FTIR instruments to produce a continuous and consistent time series from 1986 to 2024, including the earliest detected occurrence of HFC-23 at the site. The FTIR time series was compared with two independent observation datasets, surface in situ measurements from the Advanced Global Atmospheric Gases Experiment (AGAGE) network at Jungfraujoch and satellite retrievals from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS, L2 v5.3), as well as simulations from the TOMCAT global three-dimensional chemical transport model. The resulting annual relative trends (2008–2023) are 3.52 ± 0.37 %.yr⁻¹ (FTIR), 3.52 ± 0.15 %.yr⁻¹ (AGAGE), 3.66 ± 0.22 %.yr⁻¹ (ACE-FTS) and 3.53 ± 0.11 %.yr⁻¹ (model). The consistency among datasets supports our retrieval strategy. This methodology can be applied at additional NDACC sites, enabling broader spatial coverage for monitoring this compound using ground-based FTIR instrumentation.

Carbonyl sulfide (COS) is an atmospheric tracer for carbon cycle processes. Global observations reveal a rapid decline in COS concentrations beginning in 2016, but the cause of this decline is unknown. Here we show that this decrease is unexpected. Previous COS trends followed rayon production which emits CS ₂ , a COS precursor. However, the recent COS decline coincides with increasing rayon output. Multiple independent observing systems, including NOAA's global flask network and NDACC FTIR retrievals, show a consistent tropospheric COS trend of −1.2 ± 0.1% yr ⁻¹ from 2016 to 2022, even as rayon manufacturing increased by 4.1 ± 1.3% yr ⁻¹ . Using atmospheric models, we demonstrate that the COS decline is consistent with a 2.5% yr ⁻¹ reduction in emissions, driven by decreasing CS ₂ emission factors. Variations in other sources and sinks are unable to account for this trend. These results highlight the need for improved anthropogenic inventories and point toward robust COS applications to the carbon‐cycle through regional analyses.

The gradual phase-out of stratospheric ozone-depleting substances (ODS), including chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), has led to the widespread adoption of hydrofluorocarbons (HFCs). HFC-23 (CHF₃) ranks third among HFCs in radiative forcing contribution. As a potent greenhouse gas, it falls under the Kyoto Protocol and Kigali Amendment, motivating careful monitoring. HFC-23 is not intentionally manufactured; its atmospheric release stems primarily from HCFC-22 production. This study presents the multi-decadal trend of HFC-23 derived from a newly developed retrieval methodology applied to high-resolution solar absorption spectra recorded by ground-based Fourier Transform InfraRed (FTIR) instruments. Measurements obtained at the Jungfraujoch high-altitude research station (Switzerland) within the Network for the Detection of Atmospheric Composition Change (NDACC) were combined across three successive FTIR instruments to produce a continuous and consistent time series from 1986 to 2024, including the earliest detected occurrence of HFC-23 at the site. The FTIR time series was compared with two independent observation datasets, surface in situ measurements from the Advanced Global Atmospheric Gases Experiment (AGAGE) network at Jungfraujoch and satellite retrievals from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS, L2 v5.3), as well as simulations from the TOMCAT global three-dimensional chemical transport model. The resulting annual relative trends (2008–2023) are 3.52 ± 0.37 %.yr⁻¹ (FTIR), 3.52 ± 0.15 %.yr⁻¹ (AGAGE), 3.66 ± 0.22 %.yr⁻¹ (ACE-FTS) and 3.53 ± 0.11 %.yr⁻¹ (model). The consistency among datasets supports our retrieval strategy. This methodology can be applied at additional NDACC sites, enabling broader spatial coverage for monitoring this compound using ground-based FTIR instrumentation.

The gradual phase-out of stratospheric ozone-depleting substances (ODS), including chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), has led to the widespread adoption of hydrofluorocarbons (HFCs). HFC-23 (CHF3) ranks third among HFCs in radiative forcing contribution. As a potent greenhouse gas, it falls under the Kyoto Protocol and Kigali Amendment, motivating careful monitoring. HFC-23 is not intentionally manufactured; its atmospheric release stems primarily from HCFC-22 production. This study presents the multi-decadal trend of HFC-23 derived from a newly developed retrieval methodology applied to high-resolution solar absorption spectra recorded by ground-based Fourier Transform InfraRed (FTIR) instruments. Measurements obtained at the Jungfraujoch high-altitude research station (Switzerland) within the Network for the Detection of Atmospheric Composition Change (NDACC) were combined across three successive FTIR instruments to produce a continuous and consistent time series from 1986 to 2024, including the earliest detected occurrence of HFC-23 at the site. The FTIR time series was compared with two independent observation datasets, surface in situ measurements from the Advanced Global Atmospheric Gases Experiment (AGAGE) network at Jungfraujoch and satellite retrievals from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS, L2 v5.3), as well as simulations from the TOMCAT global three-dimensional chemical transport model. The resulting annual relative trends (2008–2023) are 3.52 ± 0.37 %.yr−1 (FTIR), 3.52 ± 0.15 %.yr−1 (AGAGE), 3.66 ± 0.22 %.yr−1 (ACE-FTS) and 3.53 ± 0.11 %.yr−1 (model). The consistency among datasets supports our retrieval strategy. This methodology can be applied at additional NDACC sites, enabling broader spatial coverage for monitoring this compound using ground-based FTIR instrumentation.

Large uncertainties and variability in individual ground-based instrument records limit the detection of statistically significant ozone trends, particularly in the lower stratosphere. Available merging studies are typically performed by latitude bands on satellite-based data records. This study derives correlation-based regional composites of ground-based timeseries towards reducing trend uncertainties. We address fundamental heterogeneities resulting from grouping individually homogenized ground-based datasets to enable robust merging. Uneven temporal and vertical resolutions of five ozone measurement techniques (Ozonesondes, FTIR, Dobson Umkehr, Lidar and Microwave radiometers) are handled by integrating monthly mean ozone profiles in two sets of four independent partial columns. Spatial heterogeneity is resolved by defining coherent regions using the Copernicus Atmosphere Monitoring Service (CAMS) reanalysis. Regional timeseries are merged by the BAyeSian Integrated and Consolidated (BASIC) algorithm, adapted to consider propagated measurement uncertainties and the agreement between individual timeseries by Principal Component Analysis (PCA). Trends for the 2000–2024 period are then estimated by Multiple Linear Regression using the LOTUS model. We compare BASIC with a conventional weighted mean. While the weighted mean fails to capture variability during periods of low instrument consensus, BASIC produces more representative timeseries by robustly handling outliers. Accordingly, for the selected regions, BASIC reduces average uncertainties of the trend estimates by 9.4 % relative to the weighted-mean approach. Our results support positive trends in the upper stratosphere, predominantly negative trends in the middle stratosphere and non-significant trends in the lower stratosphere. This study establishes a consolidated ground-based reference to be used for comparison with global satellite-based ozone trends.

Since 1991, continuous, consistently calibrated and openly archived ground-based measurements from the Network for the Detection of Atmospheric Composition Change (NDACC) have been collected to investigate processes responsible for decadal-scale changes, anomalies in atmospheric composition, and to validate satellite observations and model simulations. These measurements, from nearly 120 stations, support fundamental research in the area of stratospheric and tropospheric processes impacting ozone chemistry, greenhouse gases, atmospheric radiative forcing, air quality, and interactions with solar radiation and the entire Earth system. NDACC data are supplemented by observations from eleven global Cooperating Networks. The operational principles of Cooperating Networks are well aligned with NDACC objectives and protocols, focusing on data that (a) are high-quality, uniformly processed and traceable to reference standards; and (b) capture short-term (daily to interannual) anomalies and long-term trends. This paper summarizes the NDACC organizational structure. We also review the major accomplishments of NDACC since De Mazière et al. (2018), collaborative research with Cooperating Networks, and interactions with the satellite and modeling communities. Ground-based atmospheric composition monitoring is at a crossroads. Challenges include sustainability of human and financial resources required for complex and intensive data collection, technical issues including aging instrumentation, requirements for FAIR (findable, accessible, interoperable, reusable) data, and lack of data over large parts of Asia, Africa and South America. NDACC is well-positioned to adopt a three-pronged strategy going forward: protecting and modernizing existing stations; promoting the growing use of NDACC data; expanding the number of measured species and network coverage in under-sampled or under-reporting regions.

Ammonia (NH₃) is an important constituent in the global nitrogen cycle, present in both urban and remote environments. It is a source of reactive nitrogen and a precursor for particulate matter, thereby affecting atmospheric chemistry and radiative forcing. This work presents the seasonal and diurnal variability, along with long-term trends, of atmospheric NH₃ total columns retrieved from Fourier transform infrared (FTIR) spectroscopic solar absorption measurements at 22 ground-based sites, globally distributed from 45° S to 80° N. Comparisons are made with simulations from the GEOS-Chem High Performance (GCHP) chemical transport model and the Tropospheric Chemistry Reanalysis (TCR-2) NH₃ product. The mean NH₃ total columns from the FTIR time series ranged from 0.12×10¹⁵ to 19.20×10¹⁵ molecules cm⁻², with the smallest columns found at the Arctic and high-altitude sites, and the largest in urban areas. Significant enhancements were attributed to biomass burning, and NH₃ emissions from volcanic eruptions were detected at the Izana site. The seasonal patterns are similar across most sites, with maxima mainly related to the volatilization of NH₃ due to higher temperatures. The diurnal variability differs significantly and depends on the characteristics of each site and local sources. Most sites have positive trends in the total column, with a mean value (and 95 % confidence interval) for all sites of 3.82 (3.29–4.35) % for the FTIR measurements, 3.66 (3.35–3.97) % for GCHP, and 6.49 (2.00–10.98) % for TCR-2. GCHP exhibited a better general agreement with the FTIR observations than TCR-2; potential reasons for this are explored, including a sensitivity test on the emissions used.

We present updated evaluation of stratospheric ozone profile trends in the 60° S–60° N latitude range using long-term ground-based and satellite climate data records, as well as simulations by chemistry-climate models. The trends are evaluated using the LOTUS (Long-term Ozone Trends and Uncertainties in the Stratosphere) regression model. Analyses of satellite data confirm the statistically significant positive ozone trends in the period 2000–2024 in the upper stratosphere of ∼ 1–3 % per decade, with larger trends at mid-latitudes compared to the tropics. The trends are slightly positive or close to zero in the middle stratosphere, and mostly negative, −1 to −2 % per decade, in the lower stratosphere, but they are not statistically significant. The morphology and magnitude of ozone trends are similar to previous analyses (2000–2020 trends). Ozone trends in 2000–2024 predicted by chemistry-climate model simulations are in good agreement with combined satellite trends. In the upper stratosphere, models predict a slightly stronger ozone recovery than observations. In the lower stratosphere, both models and satellite observations report negative trends in the tropics, while modelled ozone trends are slightly positive at mid-latitudes. Ozone profile trends over several stations estimated from ground-based records capture the same overall vertical pattern of ozone trends as merged gridded satellite datasets. Analyses of regional ozone profile trends in 2003–2024 using merged satellite datasets confirmed the previous observations of a longitudinal structure in ozone trends in the NH mid-latitude stratosphere, with positive trends over Scandinavia and negative trends over Siberia. However, the magnitude of this dipole-like structure is reduced compared to previous analyses.

We employ an updated retrieval of space-based methanol (CH₃OH) column measurements from the Infrared Atmospheric Sounding Interferometer (IASI) and an emission optimisation framework built on the MAGRITTE chemical transport model to assess terrestrial emissions of methanol to the atmosphere between 2008 and 2019. We first carry out a IASI CH₃OH validation study based on concentration measurements from three airborne campaigns, using the model and the IASI averaging kernels to compute aircraft-based columns directly comparable to IASI data. IASI is found to underestimate high columns in the considered region. A linear regression gives Ω_IASI = 0.46 Ω_airc + 10.6·10¹⁵ molec.cm^-2, with Ω_IASI and Ω_airc the IASI and aircraft-derived columns, respectively. Inverse modelling of terrestrial methanol emissions using MAGRITTE and bias-corrected IASI columns leads to much-improved overall agreement against in situ measurement campaigns and column data at eight FTIR stations. The optimised global biogenic methanol emissions (~160 Tg yr^-1) are 22–60 % higher than previous top-down estimates, due to (1) column enhancements caused by the IASI bias-correction and (2) higher dry deposition velocities over land, compared to previous model studies, based on a parametrisation constrained by extensive campaign data. The inversion results are less reliable over boreal forests due to shortcomings of both the bias-correction and the dry deposition scheme over these regions. The optimisation suggests large changes in the seasonality of emissions. Over tropical ecosystems, radiation and temperature appear to exert a stronger control on biogenic emissions than is currently accounted for in the MEGAN model.