# First satellite-based estimate of long-lived halogenated gases radiative forcing

Simon Whitburn, Lieven Clarisse, Hélène de Longueville, Keith Shine, Pierre

F Coheur, Cathy Clerbaux, Andy W Delcloo

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Simon Whitburn, Lieven Clarisse, Hélène de Longueville, Keith Shine, Pierre F Coheur, et al. First satellite-based estimate of long-lived halogenated gases radiative forcing. 2025. hal-04964237

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# First satellite-based estimate of long-lived halogenated gases radiative forcing

**Simon Whitburn**

simon.whitburn@ulb.be

Université libre de Bruxelles

**Lieven Clarisse**

Université libre de Bruxelles

**Hélène De Longueville**

University of Bristol

**Keith Shine**

Department of Meteorology https://orcid.org/0000-0003-2672-9978

**Pierre Coheur**

Université libre de Bruxelles

**Cathy Clerbaux**

LATMOS/IPSL, Sorbonne Université, UVSQ, CNRS https://orcid.org/0000-0003-0394-7200

**Andy Delcloo**

Royal Meteorological Institute of Belgium

## Article

### Keywords:

**Posted Date:** January 29th, 2025

**DOI:** https://doi.org/10.21203/rs.3.rs-5827589/v1

**License:** cc BY This work is licensed under a Creative Commons Attribution 4.0 International License.

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**Additional Declarations:** There is **NO** Competing Interest.
# First satellite-based estimate of long-lived halogenated gases radiative forcing

S. Whitburn<sup>1,2,*</sup>, L. Clarisse<sup>1</sup>, H. De Longueville<sup>3</sup>, K.P. Shine<sup>4</sup>, P.F. Coheur<sup>1</sup>, C. Clerbaux<sup>1,5</sup>, and A.W. Delcloo<sup>2,6</sup>

<sup>1</sup>Université libre de Bruxelles (ULB), Brussels Laboratory of the Universe (BLU-ULB), Spectroscopy, Quantum chemistry and Atmospheric Remote Sensing (SQUARES), Brussels, Belgium

<sup>2</sup>Royal Meteorological Institute of Belgium (KMI-IRM), Atmospheric Composition, Measurements and Modelling division (ACM<sup>2</sup>), Brussels, Belgium

<sup>3</sup>University of Bristol, School of Chemistry, Bristol, United Kingdom

<sup>4</sup>Department of Meteorology, University of Reading, Reading, United Kingdom

<sup>5</sup>LATMOS/IPSL, Sorbonne Université, UVSQ, CNRS, Paris, France

<sup>6</sup>Ghent University, Department of Physics and Astronomy, Ghent, Belgium

* Corresponding author: Simon Whitburn, simon.whitburn@ulb.be

## Abstract

While the atmospheric concentrations of ozone-depleting chlorofluorocarbons (CFCs) are gradually declining following regulatory measures, the levels of other halogenated compounds, such as hydrochlorofluorocarbons (HCFCs) and sulfur hexafluoride (SF₆), continue to rise or are only just starting to stabilize. These halogenated substances are potent greenhouse gases. Their radiative efficiency, which quantifies along with their lifetime their impact on the climate, has until now only been estimated indirectly by means of models. Here, we report the clear-sky instantaneous radiative efficiencies (IRE) of CFC-11, CFC-12, SF₆, HCFC-22 and HFC-134a estimated for the first time directly from an observational dataset. This is achieved by combining trends observed in 15 years (2008–2022) of spectrally resolved infrared radiance fluxes from the Infrared Atmospheric Sounding Interferometer (IASI) on Metop satellites, with concentrations measured from ground and space. Comparisons with literature-reported values generally confirm previous model estimates, with some notable differences. The most significant discrepancies are for CFC-11 and SF₆, with our estimates being 27% lower but, nevertheless, within the bounds of the uncertainty estimates.

From the 1930s to the 1970s, chlorofluorocarbons (CFCs) have been mass-produced as non-toxic refrigerants and propellants. Due to their long atmospheric lifetimes, these compounds accumulate in the atmosphere. However, it was not until the 1970s that their harmful effects on stratospheric ozone became apparent [1]. Following the successful implementation of the 1987 Montreal Protocol and its successive amendments, their concentrations started declining after the mid-1990s (e.g., for CFC-11, CFC-113) and early 2000s (e.g., for CFC-12) [2, 3]. For domestic and industrial applications, CFCs were initially replaced by hydrochlorofluorocarbons (HCFCs), whose atmospheric abundances subsequently saw steep increases after 1995 [3]. In turn, HCFCs are currently being phased out in favor of hydrofluorocarbons (HFCs) and other gases following the production ban in developed countries since 2020 [4].

These long-lived halogenated species are also potent greenhouse gases (GHG) with strong absorption features in the infrared spectral domain. The effects of changes in the concentration of atmospheric compounds on the Earth's radiation budget are quantified using the concept of radiative forcing (RF, in W m⁻²). When only the initial radiative flux imbalance at the top-of-the-atmosphere (TOA) is considered in response to a change, this
is called the instantaneous RF (IRF, W m⁻²). The initial imbalance leads to a series of rapid adjustments (or short-term feedbacks) such as changes in atmospheric temperature, humidity, or clouds. The sum of the IRF and the radiative response of all rapid adjustments is referred to as the effective radiative forcing (ERF). The latter has been shown to be closely related to the long-term surface temperature response [5].

Until now, the radiative forcing of halogenated compounds was evaluated mostly from radiative transfer model calculations [e.g., 5–12] often for a limited number of idealized atmospheric profiles. While these models are generally considered well-characterized, uncertainties may still arise due to the potential lack of representativeness of the input atmospheres [7, 12, 13].

The outgoing longwave radiation is currently measured directly from space by a few high spectral resolution sounders. Several studies have already shown that their measurements contain the spectral signature of a whole series of halogenated compounds, which can be used to derive their concentrations and their trends [e.g., 14–22]. However, to date, no attempts have been made to estimate their RF from these measurements, or that of other species for that matter. In this study, we derive for the first time the clear-sky IRF (IRF, W m⁻²) and IRE (instantaneous radiative efficiency, W m⁻² ppbv⁻¹)—the radiative forcing per unit change in mole fraction of the target species—of five halogenated gases directly from 15 years of measurements from the Infrared Atmospheric Sounding Interferometer (IASI, [23]). In addition, we also identify the effects of the concentration changes in ozone (O₃) and nitric acid (HNO₃), as well as three additional halogenated compounds (CCl₄, CF₄, HFC-152a) for which the signature was not strong enough to derive the IRF.

## Spectral Analysis

The starting point of our analysis is the daily 15-year long IASI dataset of clear-sky spectrally resolved outgoing longwave radiation (SR-OLR) from [24] in units W m⁻² (cm⁻¹)⁻¹. For land and ocean, and for each spectral channel separately (sampled at 0.25 cm⁻¹), daily time series of averaged SR-OLR were established, on which linear trends (LTs) were fitted (in W m⁻² yr⁻¹ (cm⁻¹)⁻¹) (see Methods and example in Extended Data, Fig. 2a and b). The land-sea average trends as a function of channel number are shown in Fig. 1 (gray line). For most of the considered spectral range (atmospheric window from 750 to 1400 cm⁻¹), the trends average around 8.5×10⁻⁵ W m⁻² yr⁻¹, which is equivalent to about 20 mK yr⁻¹, and is consistent with current observed trends in the global increase in surface temperature [25, 26]. Because of the relatively limited time period, this change in temperature results here from a combination of short-term natural phenomena (e.g., El Niño) and the longer-term anthropogenic warming [24].

On top of the overall increase in OLR, finer spectral features are apparent, which closely match the spectral signature (Jacobians) of a number of greenhouse gases as indicated on the figure (see Methods). The impact of the increasing CO₂ concentration is seen in the ranges 790–825 cm⁻¹, 920–995 cm⁻¹ and 1025–1100 cm⁻¹. In addition to CO₂, five halogenated compounds are identified unambiguously in the LTs, namely CFC-11 (CFCl₃), CFC-12 (CF₂Cl₂), HCFC-22 (CHF₂Cl), HFC-134a (CH₂FCF₃) and sulfur hexafluoride (SF₆). Note that in the atmospheric window spectral range shown here, increasing concentrations lead to an increase of the amount of Earth's radiation absorbed, and vice versa. For CFC-11 and CFC-12, as their concentrations are decreasing since the late 1990s, early 2000s (Fig. 2), relative positive LTs are observed. Conversely, for SF₆, HFC-134a and HCFC-22, relative negative LTs are observed, consistent with their current atmospheric increase.

Somewhat less obvious, signals consistent with carbon tetrachloride (CCl₄), carbon tetrafluoride (CF₄) and HFC-152a (CH₃CHF₂) are also detected. While the spectral signature of most of these species is apparent in the LTs, eliminating the contribution of CO₂, CH₄ and N₂O (blue line, see Methods) makes their detection unambiguous, in particular for SF₆ and CCl₄. For CCl₄ and CF₄, because their main spectral region of absorption are located on the edges and outside of the window regions, and for HFC-152a because of the relatively limited change in concentration over the considered period (Fig. 2), the trend signals are less certain but are nevertheless in agreement with the sign of their concentration changes in the atmosphere over the 2008–2022 period (decrease for CCl₄ and increase for CF₄ and HFC-152a). For these three species, no attempts were made to derive an IRE.

Interestingly, apart from halogenated species, evidence of increasing O₃ and HNO₃ forcing is apparent.
<figure><img src="image_2.png" /><figcaption>Fig. 1: Global average mean spectrally resolved outgoing longwave radiation (SR-OLR) linear trend from 15 years (2008–2022) of clear-sky SR-OLR (W m⁻² yr⁻¹) with (gray) and without (blue) the contribution of CO₂, N₂O and CH₄. Hatched areas highlight the changes in the SR-OLR due to the change of the concentration of five long-lived halogenated species. The forcing rate of change (FRC) values for the fitted Jacobians integrated over the entire 750–1400 cm⁻¹ spectral region are reported in the legend.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Fig. 2: Global mean surface concentrations (ppt) of the detected halogenated species derived from the globally averaged surface mean data provided by the AGAGE network over the period 2008–2022 [3].</figcaption></figure>

For HNO₃, weak absorption features are observed in the LTs around 870–900 cm⁻¹ in agreement with what had already been observed in the IASI spectrum in [27]. The IASI channels corresponding to the ν₃ O₃ band (centered around 1042 cm⁻¹) are mostly sensitive to the stratosphere [19] and the negative LTs likely result from the combination of the changes in both the global upper tropospheric and stratospheric temperature and O₃ concentrations [19, 28].

## Forcing rate of change

For each of the five halogenated compounds of interest, a mean clear-sky forcing rate of change (FRC, in W m⁻² yr⁻¹) was derived after fitting and integrating a spectral Jacobian to the linear trends. The fits are carried out over a reduced spectral range to avoid contamination from interfering species, while the integration was performed over the entire 750–1400 cm⁻¹ spectral domain (see Methods). Importantly, while the Jacobians are calculated with a forward model, only their normalized spectral shape is used here, not their absolute quantitative values. As we show in the Methods, the shape of the Jacobian is mostly independent of the chosen atmosphere and the specific absorption cross sections that are used, so that the resulting dependence on the forward model is small (mostly below 10%). This is where the current method deviates from previous methods, which rely on the forward model for the calculation of the forcing itself.

The combined clear-sky FRC for the five halogenated compounds is negative (-15 mW m⁻² over the 15 years). Decreasing CFC-11 and CFC-12 concentrations (positive FRC) therefore do not compensate the increase of SF₆, HFC-134a and HCFC-22 (negative FRC). HCFC-22 and HFC-134a show here a comparable rate of change of -1.17 mW m⁻² yr⁻¹ and -1.20 mW m⁻² yr⁻¹, respectively. Note that these are averaged values for the 2008–2022 period. In recent years, HCFC-22 concentrations have levelled off (see Fig. 2) and its FRC is therefore likely to become positive in the coming years.

## Instantaneous Radiative Efficiencies

As the atmosphere is optically thin for the species under consideration here, their FRC is linearly correlated with concentration changes ([10, 29], see also Extended Data, Fig. 2c in the Methods where this is illustrated directly from the IASI data). The FRC estimates can therefore be converted to an Instantaneous Radiative
<figure><img src="image_4.png" /><figcaption>Fig. 3: Clear-sky TOA IRE and associated uncertainties derived from the SR-OLR (diamonds) and converted from the 2022 WMO O₃ assessment report (circles) for CFC-11, CFC-12, SF₆, HCFC-22 and HFC-134a.</figcaption></figure>

Efficiency (in W m⁻² ppbv⁻¹) by dividing by the concentration change over the considered 2008–2022 time period. The concentration data were sourced from the highly accurate ground-based measurements of the AGAGE network (see Fig. 2 and Methods). The radiative efficiency calculated in this way is an instantaneous rather than an effective one, as we consider only the direct spectral radiative effects on the outgoing longwave radiation and not secondary effects such as the effect of the change of these compounds on the atmospheric temperature that in turn affects the radiative effect of the principal greenhouse gases. Finally, again relying on linearity, a total Instantaneous Radiative Forcing (IRF, in W m⁻²) at the top-of-the-atmosphere (TOA) can be obtained by multiplication of the IRE with their mean present day concentrations. For each quantity an associated uncertainty was estimated from a comprehensive uncertainty budget (see Methods). The results are given in Fig. 1 (for the FRC) and in Table 1 (for the IRE and the IRF).

Despite their decreasing concentrations, about 70% of the present day IRF for the five halogenated compounds is due to CFC-11 and CFC-12 (0.242 W m⁻² against 0.355 W m⁻² in total) (Table 1). The third and fourth largest contributors are HCFC-22 with an IRF of 0.077 W m⁻² and HFC-134a with an IRF of 0.030 W m⁻². SF₆ has the largest IRE (0.57 W m⁻² ppbv⁻¹) but currently the lowest IRF (0.006 W m⁻²) because of its much lower atmospheric concentration compared to the other halogenated species (Fig. 2). In the coming decades, these IRF will change in response to changes in concentrations, both in absolute terms and relatively to each other. Based on the minimum and maximum concentrations provided by [30] for nine future concentration scenarios, the total RF of the five halogenated species considered here is expected to decrease by 50% between 2050 and 2150. In terms of their relative importance, the RF of SF₆ is expected to surpass that of CFC-11 by 2075-2100 and HCFC-22 by 2050-2075. For HFC-134a, concentrations vary significantly across projections. In the worst-case scenario (a sharp increase in concentration until 2100), the RF of HFC-134a could exceed that of CFC-11 and HCFC-22 by 2050 and become the highest among the five by 2100, before being overtaken by SF₆ around 2200.

## Comparison with previous estimates

IASI-derived clear-sky IREs are compared to those obtained from the RE values reported in the 2022 WMO O₃ assessment report [31]–the current reference for halogenated species–after appropriate conversion to clear-sky and top-of-atmosphere conditions (see Methods). A general good correspondence is found for the five species (Fig. 3 and Table 1), with differences consistently falling within the bounds of their uncertainty estimates. The best agreement in the central estimate is found for HFC-134a and HCFC-22, with differences of 0% and 3%, respectively. Interestingly, larger differences are observed for CFC-11, CFC-12 and SF₆ while these three species show the most evident signature in the trend spectrum and suffer from least interferences with other species (Fig. 1). For these, the central estimate of the IASI-derived IREs are, respectively, 27%, 18% and 27% (about 0.10, and 0.08 and 0.22 W m⁻² ppbv⁻¹) lower. This could indicate an overestimation of the modeled values or
<table><caption>Table 1: Clear-sky Instantaneous Radiative Efficiency (W m⁻² ppbv⁻¹) derived from the IASI measurements and converted from the values reported in the literature (± absolute uncertainty). Original values (STA-RE) and multiplicative conversion factors are provided as well (see Methods). The bottom line gives the present day IRF derived from the IRE.</caption><thead><tr><th></th><th>CFC-11</th><th>CFC-12</th><th>SF<sub>6</sub></th><th>HCFC-22</th><th>HFC-134a</th></tr></thead><tbody><tr><td>WMO O<sub>3</sub> rep. (2022) [31] STA-RE</td><td>0.280</td><td>0.330</td><td>0.574</td><td>0.223</td><td>0.173</td></tr><tr><td>Conversion factor</td><td>1.31</td><td>1.36</td><td>1.38</td><td>1.34</td><td>1.31</td></tr><tr><td>WMO-converted IRE</td><td>0.37 ± 0.07</td><td>0.45 ± 0.09</td><td>0.79 ± 0.15</td><td>0.30 ± 0.06</td><td>0.23 ± 0.04</td></tr><tr><td>This work - IRE (W m<sup>-2</sup> ppbv<sup>-1</sup>)</td><td><strong>0.27 ± 0.05</strong></td><td><strong>0.37 ± 0.05</strong></td><td><strong>0.57 ± 0.22</strong></td><td><strong>0.31 ± 0.07</strong></td><td><strong>0.23 ± 0.05</strong></td></tr><tr><td>This work - present day IRF (W m<sup>-2</sup>)</td><td>0.059 ± 0.011</td><td>0.183 ± 0.026</td><td>0.006 ± 0.002</td><td>0.077 ± 0.018</td><td>0.030 ± 0.007</td></tr></tbody></table>

an underestimation of the IREs reported here.

## Discussion

While hyperspectral infrared satellite measurements have been exploited for climate applications before [14, 15, 19, 32, 33], in this study we have shown that their measurements can be used directly for the determination of gaseous radiative forcing components. Making use of 15 years (2008–2022) of IASI-derived spectrally resolved OLR, and the ground-based measurements of AGAGE, we have presented for the first time a measurement-based approach for deriving the clear-sky instantaneous radiative efficiency (IRE, W m⁻² ppbv⁻¹) and forcing (IRF, W m⁻²) of a series of long-lived halogenated compounds. The main strength of the method is its minimal reliance on representative average atmospheres and radiative transfer model calculations, which are used here only to characterize the normalized shape of the infrared absorption features for specific species. For this reason, the results offer a way for the validation and the identification of possible biases in model-based RE. Despite its advantages, the method has two main limitations. First, in the way it was applied here, its applicability is largely restricted to species that absorb in the atmospheric window region (750–1250 cm⁻¹). Outside these spectral regions, interference with other species (particularly H₂O, CO₂, N₂O and CH₄) complicate identification and fitting of the spectral Jacobians. However, this challenge could potentially be mitigated through the simultaneous fitting of the different interfering species. The second limitation is more fundamental. As the method inherently depends on observing changes in the OLR, it can only be used to estimate the IRE for species whose atmospheric concentration are undergoing (long-term) changes. However, we hope that this study inspires future work on an independent experimental verification of the radiative forcing of the main greenhouses gases (mainly H₂O, CO₂, N₂O, CH₄).

The high resolution radiance record of IASI now almost spans two decades, and is going to be extended well beyond with the measurements of its successor IASI-NG [34], guaranteeing increasingly accurate estimates of the IRE of the main long-lived halogenated species. In particular, CFC-11 and CFC-12 concentrations are expected to continue to decline and SF₆ and HFC-134a to rise in the coming decade [30] and will therefore exhibit a stronger signature in the spectral linear trend. In addition, new signatures related to other halogenated compounds should appear more clearly (e.g., CF₄) enabling the retrieval of their IRE.

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# Methods

## Experimental Data

The clear-sky SR-OLR are derived from Infrared Atmospheric Sounding Interferometer (IASI, [23] [35]) radiance measurements. IASI is a hyperspectral sounder flying on board a suite of three Metop satellites (namely, Metop-A, -B and -C). Its measurements cover a wide and continuous part of the thermal infrared region, between 645 and 2760 cm⁻¹, with an apodized spectral resolution of 0.5 cm⁻¹ (and spectral sampling of 0.25 cm⁻¹). Since the launch of its first version in 2006, IASI has shown an exceptional stability over time and between the three instruments making its dataset an excellent fundamental climate data record [36, 37]. The algorithm for the conversion of the IASI radiance to the clear-sky SR-OLR is detailed in [24] and relies, for the selection of the cloud-free scenes, on a cloud mask developed by [38]. The SR-OLR are derived in the range 645–2300 cm⁻¹ at the 0.25 cm⁻¹ native spectral sampling of the instrument and with a precision generally within ±0.005 W m⁻² (cm⁻¹)⁻¹ depending on the wavenumber [24]. Here, we focus our analysis on the region between 750 and 1400 cm⁻¹ where halogenated compounds mostly absorb. This corresponds roughly to the spectral window region and the low wavenumber section of the H₂O ν₂ band (1250–1400 cm⁻¹). The raw dataset consists of 15 years (2008–2022) of daily global clear-sky SR-OLR averaged on a 2° × 2° grid, separately for day and night observations. Prior to 2017, the SR-OLR are derived from a complete reprocessed dataset of IASI/Metop-A radiance measurements (with the version 8.0 of the L1C). From 2017 onwards, the SR-OLR are derived from IASI/Metop-B to avoid biases due to the drift in the orbit of Metop-A starting from 2018, before its decommissioning on 30 November 2021.

For the construction of the Jacobians, representative mean atmospheric vertical profiles are required. We use a standard atmosphere (US 1976, [39]), with the vertical profiles of CO₂, CH₄, N₂O scaled to match their 2015 surface concentrations. 2015 was chosen as the reference year as it is in the middle of the 2008–2022 period considered in this study. We use the globally averaged marine surface data provided by the National Oceanic and Atmospheric Administration Earth System Research Laboratory (NOAA/ESRL; https://gml.noaa.gov/ccgg/) for its CO₂ measurements and the Advanced Global Atmospheric Gases Experiment (AGAGE) network [3] for N₂O and CH₄. For the vertical profiles of the halogenated species, we follow the approach from [22]. First, representative vertical profiles are obtained by averaging 2008–2022 global ACE-FTS v5.0 Level2 satellite data [17] [40]. Next, these were shifted to match their 2015 concentrations from AGAGE (Fig. 2).

## Trends estimation

The main steps for deriving respectively the LTs, the FRC and the IRE from the IASI observations are illustrated in Extended Data, Fig. 1. From the native clear-sky SR-OLR (W m⁻² (cm⁻¹)⁻¹) on 2° × 2° daily grids, daily area-weighted averages were calculated for land and ocean separately. Next, for each IASI channel between 750 and 1440 cm⁻¹ (2601 channels in total), global linear trends (LTs, in W m⁻² (cm⁻¹)⁻¹ yr⁻¹) in the SR-OLR were computed using the approach described in [41]. The method consists of fitting to the dataset a low order Fourier series (here n = 3) capturing the intra-annual variability in the SR-OLR plus a linear term characterizing the long term trends. The confidence limits associated with the trend estimates are determined using a bootstrap resampling. The procedure results in two trend spectra, for land and for ocean, which are subsequently averaged into a single spectrum by weighting them by their respective surface area. It was found that this way of obtaining a global average was better than the alternative of fitting a LT on a single global average OLR. For pure linear behaviour, both methods are equivalent, however, because of the stronger seasonal cycles and intra-annual variability over land than over ocean, the fits are better when land and ocean are treated separately. An example of area-weighted average OLR time series before and after removing the intra-annual variability is given in Extended Data, Fig. 2a and 2b for two channels in the spectral region of absorption of CFC-12. The associated fitted LTs are shown as well.
## FRC, IRE and IRF

As explained in the main text, the spectral signature of a series of halogenated species can be seen in the LTs. The identification is achieved by comparison with spectral Jacobians, calculated as the derivative of reference radiance spectra with respect to the species under consideration. For its calculation, the Atmosphit line-by-line radiative transfer code was used [42]. The atmosphere for the reference scene is described above and is representative for the year 2015 (see Experimental Data). Only the normalized Jacobians are used, which are largely insensitive to specific reference scenes (see Uncertainties and assumptions).

Both for the identification of halogenated species and the estimation of their clear-sky forcing rate of change (FRC, W m⁻² yr⁻¹), the contribution of changes due to CO₂, N₂O and CH₄ was first removed. This was achieved by carrying out a least squares fit of the Jacobians onto the LTs and subsequent removal of the scaled Jacobians. In the next step, similar type of fits were carried out for the halogenated species. The fits were systematically performed over a limited spectral region (see Extended Data Table 1) to avoid as much as possible spectral overlap with interfering species, and so that fits can be performed independently of each other. The fitted and scaled Jacobians were then integrated over the entire spectral range 750–1400 cm⁻¹ to obtain FRC values.

The signatures of halogenated species in the LTs are much smaller outside the window region because of the overlap with the main absorbers (H₂O, CO₂, CH₄ and N₂O). Nevertheless, the good match between the LTs and the adjusted Jacobians outside the fitting spectral range (Extended Data Table 1), in particular for CFC-12 around 1160 cm⁻¹ and HCFC-22 around 810 cm⁻¹ (see Fig. 1), adds confidence to the derived FRCs.

For the conversion to the IRE and IRF at the TOA (W m⁻²), the clear-sky FRC of each halogenated species is first multiplied by the period length (15 years) to get the total radiative change between 2008 and 2022 (W m⁻² 15yrs⁻¹) and divided next by the change in concentration between 2008 and 2022 (Fig. 2) which gives the Instantaneous Radiative Efficiency (IRE) (in W m⁻² ppbv⁻¹). The present day IRF is then obtained by multiplying the IRE by the 2022 concentration assuming a zero non-anthropogenic background. This assumes that the IRE varies linearly with concentration, which is valid for the present concentrations and perturbations of the halogenated compounds [10, 29]. The linearity is explicitly demonstrated on Extended Data, Fig. 2c where the time series of the OLR difference between two neighbouring channels in the CFC-12 absorption range (at 1161 and 1161.5 cm⁻¹) closely follows the decrease of CFC-12 derived from the AGAGE network.

## Uncertainties and sensitivity to assumptions

A complete uncertainty budget is calculated along with the IRE of each halogenated compound. In total, six sources of uncertainties related to the assumptions made in the calculation of the IREs were characterized. These are detailed below and summarized in Extended Data Table 2.

A first source comes from the fitting of the Jacobian to the SR-OLR trend. The derived IREs are to some extent sensitive to the choice of spectral fitting range. To evaluate this sensitivity, four different fits were performed by extending and reducing the fitting range by 10 cm⁻¹ on either side. For SF₆ and HFC-134a, as the original fit was performed on a very narrow range (less than 20 cm⁻¹), the fitting range was only reduced by 5 cm⁻¹. An uncertainty was then estimated from the mean of the absolute values of the relative differences between the resulting IREs and the original IREs. While for CFC-11 considering different spectral ranges only affects the derived IRE by 5% on average, the sensitivity to the fitting range is higher for the four other species with mean differences of about 10–15% for CFC-12, HCFC-22 and HFC-134a and 21% for SF₆.

The next sources of uncertainties are related to the uncertainties in the normalized Jacobians. These depend in first instance on the used absorption cross sections. For the forward model, we used the latest available cross sections [43–47], which for the five species under consideration all have an estimated systematic uncertainty of 3 %. Systematic shifts in the overall absorption strengths, however, do not impact the results since only normalized Jacobians are used. However, variations within the absorption cross section, where some parts are overestimated or underestimated, could affect the accuracy of our results. To estimate the possible order of magnitude of such uncertainties, normalized Jacobians were obtained for the five species from an independent source of absorption cross sections [48, 49]. The differences with the original Jacobians are shown in the Extended Data, Fig. 3. The largest impact on the IRE was for SF₆ for which the use of an older version of the cross
sections [48] shows a derived FRC 24% higher. While the differences between the two versions of the cross-sections are limited to a few percent, these affect the scaling factor of the Jacobians obtained when fitting the spectral signatures in the trends, which in turn results in larger differences in the FRC. In contrast, the results for CFC-12 and HCFC-22 differed by only a few percents. For HFC-134a, while the two Jacobians show large discrepancies around 825–860 cm⁻¹ and 950–990 cm⁻¹, the derived IRE is not greatly affected as these two regions only account for a small fraction in the integrated Jacobians. These uncertainty estimates were retained for the calculation of the total uncertainty budget.

The Jacobians further depend on the choice of reference atmosphere, which is here the US 1976 standard atmosphere (US 1976, [39]) with the concentration of the main greenhouse gases and halogenated compounds scaled to their 2015 concentrations. Deviating temperature and water vapour profiles could have an impact on the shape of the normalized Jacobians. Jacobians calculated for three different standard atmospheres (tropical, mid-latitudes summer and sub-arctic summer) yielded results within 1 to 5 %, indicating a high degree of independence of the normalized Jacobians on the atmosphere. Likewise, the dependence on the choice of reference year was tested by considering two other reference years (2008 and 2022). Only for SF₆ a small difference of 1% was observed (see Extended Data Table 2).

The effects of the long-term changes in CO₂, CH₄ and N₂O concentrations are taken into account by removing their contribution to the SR-OLR trends before deriving the IREs. However, possible long term changes in H₂O and atmospheric temperature are not explicitly incorporated. These can affect both the observed change in OLR in the fitting range, and the calculated Jacobian over the entire range. However, the radiative impacts of the latter occur mainly beyond 1250 cm⁻¹, where the absorption of the detected halogenated species is limited. The experiments that were carried out using different reference atmospheres indicate that these effects are small. Also consistent with this, [7] showed a reduction of 10% in H₂O mixing ratio results in only about 1% increase in the radiative forcing of the halogenated compounds suggesting that uncertainty on the IRE due to H₂O adjustment is negligible. Conservatively, we added an additional 5% uncertainty to account for this source for all species.

The variation of the IRE due to the uncertainty on the slope of the LTs is comprised between 2% and 5% depending on the species. However, to assess more comprehensively the sensitivity of the results to the use of LTs, we calculated the IRE and FRC from fitting the Jacobians to the difference between the mean SR-OLR of two distinct years. This method has the advantage of providing an independent verification of the results, even though the sensitivity to interannual variations of interfering species is larger, and we do not take advantage of the noise reducing properties of the LTs that are calculated from the daily values. For the test, we used the difference between 2008 and 2021 or 2022. The largest difference between the IRE (in percent) was retained as the uncertainty value. The corresponding figures are provided in the supporting material. For CFC-11 and CFC-12, the estimated uncertainty associated to the methodology is 11% and 6% respectively, while for SF₆, HCFC-22 and HFC-134a, they are close to 20% (Extended Data Table 2).

With this, the main sources of uncertainty have been quantified. Other sources of uncertainty, such as those on the average concentrations of the halogenated species from the AGAGE network are assumed to be negligible. Finally, a total uncertainty was calculated as the root-sum-square of the individual absolute uncertainties. Uncertainties range between 14% for CFC-12 to 39% for SF₆ (Extended Data Table 2).

## Conversion of RE from the literature

The IRF and IRE can be evaluated at different atmospheric altitude levels. By convention, it is generally defined at the tropopause (TRP) [5] while, in our case, they are derived at TOA. The magnitude and the sign of the differences on IRF/IRE depend on two effects in response to an increase in stratospheric concentration of a greenhouse gas: (1) an increased absorption by the stratosphere of the upwelling tropospheric irradiance and (2) an increase of the stratospheric emittance. In spectral regions relatively transparent to the Earth's radiation, such as the window region considered in this study, the increase in the stratospheric absorption of the upwelling radiation exceeds the increase in the stratospheric emittance, resulting in a RF at the TOA that is larger than at TRP [5].

For comparison with the literature, we consider the values provided in the 2022 WMO Ozone Assessment
Report [31], deemed the most up-to-date values for the RE of halogenated species. Reported RE values do not only account for the initial direct radiative impacts due to a change in the species concentration (i.e. the IRE) but also for secondary stratospheric temperature adjustments (STA-RE). To enable comparison, the all-sky STA-RE values are first converted to clear-sky IREs at TOA (Extended Data, Fig. 1). For this conversion, a multiplicative factor was derived for each of the five species by simulating both their STA-RE and clear-sky IRE using an updated version of the 300 narrow-band (10 cm⁻¹) radiative transfer model of [5] (using the line parameters from HITRAN2020 [50] and the continuum model MT-CKD version 4.3 [51]). The conversion factors, which range from 1.31 to 1.38, are provided in Table 1.

As for the SR-OLR IASI-derived IRE, the modeled RE values are also subject to uncertainties due to the potential errors in the absorption cross sections, the radiative forcing calculations and the assumptions made on the atmospheric state. [10, 11] evaluated the total radiative forcing uncertainty for halogenated compounds to 13-15% for species with lifetimes ≥ 5 years. A value of 14% was retained here for all reported RE values for the comparison with the IASI-derived IRE. Moreover, the assumptions made for the conversion of the RE values to the IRE at TOA also add to the total uncertainties. To account for this, an additional 5% is considered which seems a reasonable approximation given the variability in the different conversion ratios calculated from [11, 29] [52].

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Data availability

Monthly means of IASI-derived spectrally resolved OLR [53] are freely available through the IASI-FT website: https://iasi-ft.eu/data-access/OLR/. Daily IASI-derived spectrally resolved OLR can be made available upon request to the corresponding author. Global mean IASI clear-sky SR-OLR trends are available in supplementary material.

Code availability

The analysis codes can be made available upon request to the corresponding author.

Acknowledgments

IASI has been developed and built under the responsibility of the Centre National d'Études spatiales (CNES, France). It is flown on board the Metop satellites as part of the EUMETSAT Polar System. The IASI L1C data are received through the EUMETCast near-real-time data distribution service. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement 742909). It was also supported by the Prodex arrangement IASI.FLOW (Belspo-ESA). Simon Whitburn is grateful to the Belgian Science Policy (BELSPO) for funding his research work through the FED-tWIN programme (Prf-2021-014). L. Clarisse is Research Associate (Chercheur Qualifié) with the Belgian F.R.S.-FNRS. C. Clerbaux is grateful to CNES for scientific collaboration and financial support. K. Shine acknowledges funding from UK Natural Environment Research Council (Grant Reference NE/X004198/1). We also thank the AERIS, AGAGE and NOAA/ESRL teams for the production and the distribution of the data used in this paper.

Author contributions

S.W. and L.C. conceptualized the study. S.W. performed the analysis, interpreted the data, drafted the paper and produced the figures. K.P.S. performed the RT simulations for deriving the multiplicative conversion factors. All authors contributed to the discussion and writing of the manuscript.

Competing interests

The authors declare no competing interests.
<figure><img src="image_5.png" /></figure>

Extended Data Fig. 1: Conceptual flowchart of the clear-sky TOA IRE calculation from the IASI observations (blue) and converted from RE values reported in the literature (red). all stands for all-sky, clr for clear-sky, STA for stratospheric temperature adjustment.
<figure><img src="image_6.png" /></figure>

Extended Data Fig. 2: (a) OLR time series of two neighbouring channels (at 1161 and 1161.5 cm⁻¹) in the CFC-12 absorption band. (b) Deseasonalized OLR time series. (c) Difference between the OLR of the two channels and CFC-12 mole fraction (ppt) from the ground-based measurements of AGAGE.
<figure><img src="image_7.png" /></figure>

Extended Data Fig. 3: Normalized Jacobians derived using two different sources of absorption cross sections.
<table><caption>Extended Data Table 1: Fitting spectral range of the Jacobians and interfering halogenated species.</caption><thead><tr><th>Target</th><th>Spectral range (cm<sup>-1</sup>)</th><th>Interfering halogenated species</th></tr></thead><tbody><tr><td>CO<sub>2</sub></td><td>961–987</td><td></td></tr><tr><td>CH<sub>4</sub></td><td>1320–1400</td><td></td></tr><tr><td>N<sub>2</sub>O</td><td>1223–1330</td><td></td></tr><tr><td>CFC-11</td><td>830–860</td><td>HCFC-22, CCl<sub>4</sub></td></tr><tr><td>CFC-12</td><td>900–939</td><td>CFC-11, HCFC-142b, SF<sub>6</sub></td></tr><tr><td>SF<sub>6</sub></td><td>939–948.25</td><td>CFC-12</td></tr><tr><td>HCFC-22</td><td>1084–1134.25</td><td>CFC-11, CCl<sub>4</sub></td></tr><tr><td>HFC-134a</td><td>1165.5–1186</td><td>CFC-12, HCFC-142b</td></tr></tbody></table>

<table><caption>Extended Data Table 2: Uncertainties on the IASI-derived IRE.</caption><thead><tr><th></th><th>CFC-11</th><th>CFC-12</th><th>SF<sub>6</sub></th><th>HCFC-22</th><th>HFC-134a</th></tr></thead><tbody><tr><td>Fitting range</td><td>5%</td><td>11%</td><td>21%</td><td>14%</td><td>12%</td></tr><tr><td>Jacobian (cross section)</td><td>14%</td><td>3%</td><td>24%</td><td>4%</td><td>10%</td></tr><tr><td>Jacobian (std atm.)</td><td>4%</td><td>5%</td><td>4%</td><td>1%</td><td>5%</td></tr><tr><td>Jacobian (ref. year)</td><td>0%</td><td>0%</td><td>1%</td><td>0%</td><td>0%</td></tr><tr><td>Long-term T and H<sub>2</sub>O changes</td><td>0%</td><td>0%</td><td>0%</td><td>5%</td><td>5%</td></tr><tr><td>Methodology</td><td>11%</td><td>6%</td><td>21%</td><td>18%</td><td>16%</td></tr><tr><td>Total (RSS)</td><td>19%</td><td>14%</td><td>39%</td><td>24%</td><td>23%</td></tr></tbody></table>
**Supplementary Files**

This is a list of supplementary files associated with this preprint. Click to download.

* HaloXRFvfinalsuppl.pdf