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Atmospheric trends of long-lived halogenated gases derived from 15 years of IASI measurements

par De Longueville, Hélène ;Clarisse, Lieven ;Whitburn, Simon ;Clerbaux, Cathy ;Lecomte, Gilles ;Coheur, Pierre
Référence Journal of quantitative spectroscopy & radiative transfer, 311, page (108755)
Publication Publié, 2023-12-01

Article révisé par les pairs

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Résumé :

Atmospheric emissions of chlorofluorocarbons (CFCs), their hydrogenated derivatives (HCFCs, HFCs) and other long-lived halogen-containing substances perturb the chemical and radiative equilibrium of our atmosphere. A global network of ground-based stations (AGAGE) monitors the concentrations of most of these species. Supplementing these, upper tropospheric and stratospheric concentrations are available from satellite measurements of the ACE-FTS solar occultation sounder. Measurements derived from nadir viewing infrared sounders can potentially complement both observational datasets, offering extensive spatial coverage and temporal sampling. With a preferential sensitivity to the middle troposphere, they also cover the vertical altitude range that is not covered by other means. However, fluctuations in surface temperature and the concentration of interfering atmospheric constituents render the retrieval of halogenated species particularly challenging. Relying on previous work on spectral whitening, we present an unconstrained generalized least squares estimation retrieval methodology, which largely allows to overcome the problem of interference. We demonstrate that it can be used to retrieve monthly anomalies of all halogenated species previously observed in spectra from the Infrared Atmospheric Sounding Interferometer (IASI/Metop). Focussing on northern mid-latitudes, we derive the monthly concentrations of CFC-11, CFC-12, HCFC-22, HCFC-142b, HFC-134a, CF4, SF6 and CCl4 between 2008 and 2022. Trends are compared to the observations from AGAGE and ACE-FTS. A good match is obtained with both, with especially remarkable agreement in the linear trends for CF4, SF6 and HFC-134a, and in the non-linear trends of CFC-11 and HCFC-22. Large discrepancies and unexplained variations are seen in the time series of HCFC-142b, CFC-12 and CCl4, necessitating further optimization of the retrieval technique. The results demonstrate the potential of IASI and follow-up missions for establishing a unique long-term time series of the most important long-lived halogenated species.