Atmospheric emissions of carbon tetrachloride (CCl4) are regulated by the Montreal Protocol due to its role as a strong ozone-depleting substance. The molecule has been the subject of recent increased interest as a consequence of the so-called "mystery of CCl4", the discrepancy between atmospheric observations and reported production and consumption. Surface measurements of CCl4 atmospheric concentrations have declined at a rate almost 3 times lower than its lifetime-limited rate, suggesting persistent atmospheric emissions despite the ban. In this paper, we study CCl4 vertical and zonal distributions in the upper troposphere and lower stratosphere (including the photolytic loss region, 70-20 hPa), its trend, and its stratospheric lifetime using measurements from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), which operated onboard the ENVISAT satellite from 2002 to 2012. Specifically, we use the MIPAS data product generated with Version 7 of the Level 2 algorithm operated by the European Space Agency. The CCl4 zonal means show features typical of long-lived species of anthropogenic origin that are destroyed primarily in the stratosphere, with larger quantities in the troposphere and a monotonic decrease with increasing altitude in the stratosphere. MIPAS CCl4 measurements have been compared with independent measurements from other satellite and balloon-borne remote sounders, showing a good agreement between the different datasets. CCl4 trends are calculated as a function of both latitude and altitude. Negative trends of about -10 to -15 pptv decade-1 (-10 to -30 % decade-1) are found at all latitudes in the upper troposphere-lower stratosphere region, apart from a region in the southern midlatitudes between 50 and 10 hPa where the trend is positive with values around 5-10 pptv decade-1 (15-20 % decade-1). At the lowest altitudes sounded by MIPAS, we find trends consistent with those determined on the basis of long-term ground-based measurements (-10 to -13 pptv decade-1). For higher altitudes, the trend shows a pronounced asymmetry between the Northern and Southern hemispheres, and the magnitude of the decline rate increases with altitude. We use a simplified model assuming tracer-tracer linear correlations to determine CCl4 lifetime in the lower stratosphere. The calculation provides a global average lifetime of 47 (39-61) years, considering CFC-11 as the reference tracer. This value is consistent with the most recent literature result of 44 (36-58) years.

The Optimized Retrieval Code (ORM) was originally designed to be the scientific code used as the reference for the ESA operational Near Real Time Analysis of MIPAS measurements on ENVISAT. After the end of ENVISAT mission, occurred on April 2017, this code has been adapted and improved to perform the reanalysis of the full MIPAS mission.

MIPAS on ENVISAT performed almost continuous and global measurements of atmospheric temperature and composition from June 2002 to April 2012. These data are very useful for understanding atmosphere climatology from the upper troposphere to the mesosphere, including trends in composition and variability, as a reference for middle atmosphere ozone and general circulation as well as improvement of tropospheric composition retrievals. The ESA Level 2 processor, based on the Optimized Retrieval Model (ORM), originally designed for the Near Real Time analysis and developed by an European Consortium led by IFAC, is currently improved and used for the reanalysis of the full MIPAS mission. The maintenance and the upgrade of the ESA processor are made in the frame of the MIPAS Quality Working Group, where a fruitful collaboration among Level 1, Level 2 and validation teams is exploited. This collaboration is essential to improve the accuracy of the products and their characterization. This paper is meant to describe the most recent upgrades in the MIPAS processor, in particular the full mission was recently reprocessed with L1 V7 and L2 V7 processors, containing significant improvements with respect to previous version 6, and further improvements are in preparation, that will be collected in version 8 of the ESA processor. The improvements include both L1 and L2 processors, as well as the auxiliary data. In the L1 processor the correction of the instrumental drift caused by the ageing of the detectors has been implemented in order to reduce the non negligible systematic error in the trend estimation. Furthermore, the measured daily gain instead of the weekly gain is now used for the radiometric calibration that allows to better take account for the discontinuities in the gain that occasionally occur in MIPAS band B. Improvements in the L2 processor include a different approach for retrieving atmospheric continuum, the use of an a posteriori regularization with altitude dependent constraint, a better approach for handling interfering species, a reduced bias in CFC-11, the handling of horizontal inhomogeneities and the use of ECMWF altitude/pressure relation to determine more accurate altitudes. Improvements in the auxiliary data consist in the use of microwindows with larger information content, an improved spectroscopic database and diurnally varying climatological dataset. Furthermore, additional species are provided in the new processors, leading to a total of 22 retrieved species by the L2 version 8 processor. Each of these changes has a different impact on the Level 2 products. The individual contributions, as well as the cumulative effects, will be evaluated with a comparison with previous versions of MIPAS products and corresponding results of validation. Improvements in the trend determination will also be evaluated, as well as the quality of the new retrieved species.

Carbon tetrachloride (CCl?) is a strong ozone-depleting atmospheric gas regulated by the Montreal protocol. Recently it received increasing interest because it was found that at the surface its atmospheric concentration declines with a rate almost three times smaller than its lifetime-limited rate. Indeed there is a discrepancy between atmospheric observations and the estimated distribution based on the reported production and consumption (the so called "mystery of CCl?"). We use for the first time the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) measurements to estimate CCl? distribution, its trend, and atmospheric lifetime in the upper troposphere / lower stratosphere (UTLS) region. In particular, here we use MIPAS product generated with Version 7 of the Level 2 algorithm operated by the European Space Agency. The CCl? zonal means show features typical of long-lived species of anthropogenic origin that are destroyed primarily in the stratosphere: larger amounts are found in the troposphere, monotonically decreasing with altitude. We calculate CCl? trends as a function of both latitude and altitude: negative trends are found at all latitudes in the UTLS, apart from a region in the Southern mid-latitudes between 50 and 10 hPa where the trend is slightly positive. At the lowest altitudes sounded by the MIPAS scan we find trend values consistent with those determined on the basis of long-term ground-based measurements. CCl? global average lifetime of 46(38 - 60) years has been estimated using the tracer-tracer linear correlations approach and the CFC-11 as the reference tracer. This estimated value is consistent with the most recent literature result of 44(36 - 58) years.

Atmospheric emissions of Carbon tetrachloride CCl4 are regulated by the Montreal Protocol due to its role as a strong ozone-depleting substance. The molecule has been the subject of recent increased interest as a consequence of the so called ``mystery of CCl4,'' the discrepancy between atmospheric observations and reported production and consumption. Surface measurements of CCl4 atmospheric concentrations have declined at a rate almost three times smaller than its lifetime-limited rate, suggesting persistent atmospheric emissions despite the ban. In this paper, we study CCl4 vertical and zonal distributions in the upper troposphere and lower stratosphere (including the photolytic loss region, 70-20 hPa), its trend, and its stratospheric lifetime using measurements from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), which operated onboard the ENVISAT satellite from 2002 to 2012. Specifically, we use the MIPAS data product generated with Version 7 of the Level 2 algorithm operated by the European Space Agency.The CCl4 zonal means show features typical of long-lived species of anthropogenic origin that are destroyed primarily in the stratosphere, with larger quantities in the troposphere and a monotonic decrease with increasing altitude in the stratosphere. In the troposphere, the largest concentrations are observed at the latitudes of major industrial countries (20/50°N). The good agreement we find between MIPAS CCl4 and independent measurements from other satellite and balloon-borne remote sounders proves the reliability of the MIPAS dataset.CCl4 trends are calculated as a function of both latitude and altitude. Negative trends are found at all latitudes in the upper-troposphere / lower-stratosphere region, apart from a region in the Southern mid-latitudes between 50 and 10 hPa where the trend is positive. At the lowest altitudes sounded by MIPAS, we find trends consistent with those determined on the basis of long-term ground-based measurements. For higher altitudes, the trend shows a pronounced asymmetry between Northern and Southern Hemispheres, and the magnitude of the decline rate increases with altitude. At 50 hPa the decline is about 30-35 %/decade, close to the lifetime-limited trend.We use a simplified model assuming tracer-tracer linear correlations to determine CCl4 lifetime in the lower stratosphere. The calculation provides a global average lifetime of 46(38-60) years considering CFC-11 as the reference tracer. This value is consistent with the most recent literature result of 44(36-58) years.

MIPAS on ENVISAT performed almost continuous measurements of atmospheric composition for approximately 10 years, from June 2002 to April 2012. ESA processor, based on the algorithm ORM (Optimized Retrieval Model), originally designed for the Near Real Time analysis and developed by an European Consortium led by IFAC, is currently used for the reanalysis of the full MIPAS mission. The maintenance and the upgrade of the ESA processor are made in the frame of the Quality Working Group, where a fruitful collaboration among Level 1, Level 2 and validation teams can be exploited. This collaboration is essential to pursue improvements in the accuracy of the products and their characterization. This paper is meant to describe the most recent upgrades in the ESA processor performed to improve the quality of ESA products. In particular, the full mission was recently reprocessed with L1 V7 and L2 V7 processors, containing significant improvements with respect to previous version V6, and further improvements are in preparation, that will be collected in version 8 of the ESA processor. Improvements involve both L1 and L2 processors, as well as the auxiliary data. Improvements in the L1 processor consist in a correction of the instrumental drift, improved spike detection algorithm and new Instrument Line Shape, as well as the use of measured daily gain instead of weekly gain. Improvements in the L2 processor include a different approach for retrieving atmospheric continuum, the use of an a posteriori regularization with altitude dependent constraint, a better approach for handling interfering species, a reduced bias in CFC-11, the handling of horizontal inhomogeneities and the use of ECMWF altitude/pressure relation for determining more accurate altitudes. Improvements in the auxiliary data consist in the use of microwindows with larger information content, new spectroscopic database and diurnally varying climatological dataset. Furthermore, with each new version additional species are provided, leading to 20 the number of retrieved species by the L2 V8 processor. Improvements in the V7 products will be revised on the light of the results of the validation with correlative measurements, and, by comparing the first new L2 V8 products with the L2 V7 ones, a preliminary assessment of the performance of the new V8 processor will be performed.

We present the results of the algorithm modeling the horizontal gradients in the ESA retrieval of MIPAS data ORM v8.

Neglecting the horizontal variability of the atmosphere in the forward model for the simulation of limb emission radiances causes a systematic error in MIPAS retrieved profiles. The horizontal gradient model will be introduced into the Optimized Retrieval Model (ORM) v8, which will be used for the final ESA reprocessing of the whole mission. Several optimizations exploiting the spherical symmetry of the atmosphere can no longer be used. Therefore, both the ray tracing and the radiative transfer integration algorithms have been completely rewritten. We illustrate the choices adopted for the implementation of the horizontal gradient model. We show its performances versus the previous algorithm that assumes the horizontal homogeneity of the atmosphere. Finally we compare our results to those of other retrieval models that take into account the horizontal variability of the atmosphere.

MIPAS on ENVISAT performed almost continuously measurements of the atmospheric composition for almost 10 years, from June 2002 to April 2012. These ten years cover a period when the first effect of the dismiss of the emission of the CFCs after the Montreal protocol ratification in 1987can be measured. Even if ten years constitute a short period to derive trends, it has been proven that useful information on time variation of atmospheric constituents can be derived from the analysis of these measurements. However, previous versions of MIPAS on ENVISAT dataset were characterized by an instrumental drift due to the fact that some detectors used by MIPAS were affected by non-linearities, which change with time due to the ageing of the detectors and this was cause of a non negligible systematic error in the trend estimation. The new full mission reprocessed dataset V7 that will be released very soon uses L1 files where the impact of the ageing of the detectors on non-linearities has been corrected. Furthermore, also the L2 processor has been upgraded with new functionalities improving the performances of the processor. We present the results of study of trends derived from the analysis of the new MIPAS V7 products on several MIPAS target species including ozone depleting species, like CFC-11, CFC-12, CCl4 and HCFC-22.

MIPAS measurements on ENVISAT represent a unique database for the study of atmospheric composition and of the time variation of atmospheric constituents. For trend studies it is important that instrumental drifts are reduced. Some of the MIPAS spectral bands are affected by time-dependent non-linearity which have been recently corrected. In addition to this non-linearity correction, the forthcoming new version of MIPAS products (Version 7) contains several other improvements: a new approach for handling continuum capable of making the retrieval more stable, a new selection of spectral intervals selected for the analysis of the full resolution measurements aimed to reduce the bias between full resolution and optimized resolution measurements, the regularization of the H2O profiles, the products of five new species (HCFC-22, CFC-14, HCN, COF2, CCl4), which makes equal to 15 the total number of retrieved species. The latest improvements implemented in the ESA processor and some of the results on trend will be presented and discussed.

MIPAS measurements on ENVISAT represent a unique database for the study of atmospheric composition and of the time variation of atmospheric constituents.For trend studies it is important that instrumental drifts are reduced. Some of the MIPAS spectral bands are affected by time-dependent non-linearity that have been recently corrected.In addition to this non-linearity correction, the forthcoming new version of MIPAS products (Version 7) contains several other improvements: a new approach for handling continuum leading to a more stable retrieval, a new selection of spectral intervals for the analysis of the full resolution measurements aiming to reduce the bias between full resolution and optimized resolution measurements, the regularization of the H2O profiles. Furthermore, the implementation of the retrieval of five new species (HCFC-22, CFC-14, HCN, COF2, CCl4) leads to a total of 15 species in ESA products.The latest improvements implemented in the ESA processor, the results of the validation of the products and some preliminary results on the trend of some ozone depleting substances will be presented and discussed.

The ESA-ESRIN contract no. 21719/08/I-OL covered the activities performed between April 2008 and July 2014 by the L2 sub-group of the MIPAS Quality Working Group. The main objective of this project was to monitor the quality of MIPAS L2 products, to improve the performances of the L2 ESA processor, used for both the near real time analysis during ENVISAT mission and for the off-line analysis, and to enlarge the list of retrieved species. In particular, three different versions of the MIPAS operational processor were finalized: IPF V5, ML2PP V6 and ML2PP V7, leading to significant improvements in the quality of the products and in their diagnostics, to an increase of the number of processed scans, and to an enlargement of the list of provided products. With respect to IPF V4.1&2, used to process only the first two years of MIPAS measurements (2002-2004, Full Resolution (FR) measurements) and validated with respect to correlative measurements (Ridolfi et al., 2006, Cortesi et al., 2007, Wang et al., 2007, Payan et al., 2009, Wetzel et al., 2007, Wetzel et al., 2013, all contained in the MIPAS special issue), IPF V5: o performs the analysis of the full mission (both FR and Optimized Resolution (OR) measurements, characterized by a reduced spectral resolution but finer spatial resolution) after the introduction of an a posteriori regularization with self-adapting threshold aimed to reduce instabilities of the retrieval with measurements of the OR phase (due to sampling intervals finer than the instantaneous field of view of the instrument and the choice of using a retrieval grid aligned with the vertical sampling of the measurements); o provides as output Averaging Kernel (AK) for each scan. Products of ML2PP V6 are very similar to the ones of V5 in terms of bias, but are characterized by: o an improved diagnostics, since the Covariance Matrix and the AK matrix are computed taking into account all iterations performed during the retrieval (this is needed if the final Marquardt parameter is different from zero). Furthermore, horizontal AKs are provided for each latitude band and each season. o additional four new species, namely CFC-11, CFC-12, N2O5 and ClONO2. MIPAS ESA processor products V7 are characterized by: o reduced time dependent calibration error due to non-linearities (coming from using new L1 data, V7); o availability of retrieval altitude grid characterized by a very small bias after correction with ECMWF pressure/altitude relationship; o new microwindows for FR measurements with an increased information content and leading to a reduction of the detected bias in the FR measurements for CH4, N2O and CFC-12; o larger stability, given by the new approach for retrieving continuum (the continuum transmission is fitted instead of the continuum cross-section), and new regularization with altitude dependent regularization strength; o lower bias of CFC-11, due to proper handling of COCl2 interfering species in the retrieval o better representation of the measurements, proven by the fact that a smaller chi-square is obtained for most of the species; o better handling of diurnally varying interfering species, coming from the use of diurnally varying Initial Guess profiles; o products of five new species (HCFC-22, CFC-14, COF2, CCl4 and HCN) available. Concerning V7 products, preliminary verifications have been performed only on the diagnostic dataset V7, including more than 4000 orbits: the Non-Linearity correction performed in the L1 files introduces a drift in V7 profiles with respect to previous versions, that corrects for the instrumental drift present in previous datasets. Some problems have also been identified: V7 FR Temperature profiles in polar and midlatitude regions presents a negative bias of about 0.5-1K with respect to both V6 products and correlative measurements coming from the Non-Linearity correction. In the tropics in general V7 Temperature has a positive bias with respect to previous versions, for both FR and OR measurements, and this seems to come from the new approach for handling continuum. A new version of the spectroscopic database has been finalized, with improvements in H2O, HNO3, COCl2 and C3H8. The new version of the spectroscopic database will be used for the final reprocessing of the whole MIPAS mission in the frame of the subsequent contract. A large effort was spent for improving the quality of the MIPAS L2 products, but further improvements have been identified as needed to reduce some systematic errors of the products and to fully exploit the information contained in MIPAS measurements. One of the main problems identified so far is the error due to the horizontal inhomogeneities that are not currently taken into account by the algorithm. The quality of the retrieval is affected by the horizontal gradients but from the analysis of single scans no enough information is contained in the measurements to retrieve the gradients. Considering that the code will be used only for a reanalysis, the choice has been to take advantage of the information on horizontal gradients of both temperature and target species from previous reprocessing or ECMWF reanalysis and to modify the forward model in order to handle the externally provided gradients. Another important objective is the full exploitation of MIPAS measurements that implies the retrieval of weaker and weaker species, whose interference from other species cannot be disentangled. To this purpose the possibility of performing a multi-target retrieval is crucial. Furthermore, we have to consider that the vertical profile retrieval of weak species requires the use of some constraint for the attainment of a fast and useful product and it can be useful to have the possibility of tuning or removing it as needed during the average procedure. To this purpose Optimal Estimation seems to be more apt than the current approach where the constraint is determined with a self-consistent method, difficult to rebuild. For very weak species a posteriori averages of retrieved profiles after removing a priori constrain, as studied in the frame of this project, can be also considered. Validation of ML2PP V7 and further investigations aimed to solve these open issues will be completed in the frame of the subsequent contract.

The MIPAS instrument on the ENVISAT satellite has provided vertical profiles of the atmospheric composition on a global scale for almost ten years. The MIPAS mission is divided in two phases, the full resolution phase, from 2002 to 2004, and the optimized resolution phase, from 2005 to 2012, which is characterized by a finer vertical and horizontal sampling attained through a reduction of the spectral resolution. While the description and characterization of the products of the ESA processor for the full resolution phase has been already described in previous papers, in this paper we focus on the performances of the latest version of the ESA processor, named ML2PP V6, which has been used for reprocessing the entire mission. The ESA processor had to perform the operational near real time analysis of the observations and its products needed to be available for data assimilation. Therefore, it has been designed for fast, continuous and automated analysis of observations made in quite different atmospheric conditions and for a minimum use of external constraints in order to avoid biases in the products. The dense vertical sampling of the measurements adopted in the second phase of the MIPAS mission resulted in sampling intervals finer than the instantaneous field of view of the instrument. Together with the choice of a retrieval grid aligned with the vertical sampling of the measurements, this made ill-conditioned the retrieval formalism of the MIPAS operational processor. This problem has been handled with minimal changes to the original retrieval approach but with significant improvements nonetheless. The Levenberg-Marquardt method, already present in the retrieval scheme for its capability to provide fast convergence for non-linear problems, is now also exploited for the reduction of the ill-conditioning of the inversion. An expression specifically designed for the regularizing Levenberg-Marquardt method has been implemented for the computation of the covariance matrices and averaging kernels of the retrieved products. The regularization of the Levenberg-Marquardt method is controlled by the convergence criteria and is deliberately kept weak. The resulting oscillations of the retrieved profile are a-posteriori damped by an innovative self-adapting Tikhonov regularization. The convergence criteria and the weakness of the self-adapting regularization ensure that minimum constraints are used and the best vertical resolution obtainable from the measurements is achieved in all atmospheric conditions. Random and systematic errors, as well as vertical and horizontal resolution are compared in the two phases of the mission for all products, namely: temperature, H2O, O3, HNO3, CH4, N2O, NO2, CFC-11, CFC-12, N2O5 and ClONO2. The use in the two phases of the mission of different optimized sets of spectral intervals ensures that, despite the different spectral resolutions, comparable performances are obtained in the whole MIPAS mission in terms of random and systematic errors, while the vertical resolution and the horizontal resolution are significantly better in the case of the optimized resolution measurements.

MIPAS measurements on ENVISAT represents a unique database for the study of atmospheric composition and of the time variation of atmospheric constituents and trends, in combination with other data. With the end of the ENVISAT mission, the importance of this decadal set of measurements justifies any efforts for their full exploitation. The maintenance and the upgrade of the ESA processor are made in the frame of the Quality Working Group, where a fruitful collaboration between L1, L2 and validation teams can be exploited. This collaboration is essential to pursue improvements in the accuracy of the products and their characterization. Recently a new version of the Level 2 processor (ML2PP V7) has been finalized, containing a new approach for handling continuum capable of making the retrieval more stable, a new selection of spectral intervals selected for the analysis of the full resolution measurements aimed to reduce the bias between full resolution and optimized resolution measurements, the regularization of the H2O profiles, the products of five new species (CFC-22, CFC-14, HCN, COF2, CCl4). The addition of these 5 species leads to 15 the number of species processed by the operational processor, but the list of retrieved species can be further enlarged. The latest improvements implemented in the ESA processor and the results of the feasibility studies for additional upgrades will be presented and discussed.

The MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) instrument on the Envisat (Environmental satellite) satellite has provided vertical profiles of the atmospheric composition on a global scale for almost ten years. The MIPAS mission is divided in two phases: the full resolution phase, from 2002 to 2004, and the optimized resolution phase, from 2005 to 2012, which is characterized by a finer vertical and horizontal sampling attained through a reduction of the spectral resolution. While the description and characterization of the products of the ESA processor for the full resolution phase has been already described in previous papers, in this paper we focus on the performances of the latest version of the ESA (European Space Agency) processor, named ML2PP V6 (MIPAS Level 2 Prototype Processor), which has been used for reprocessing the entire mission. The ESA processor had to perform the operational near real time analysis of the observations and its products needed to be available for data assimilation. Therefore, it has been designed for fast, continuous and automated analysis of observations made in quite different atmospheric conditions and for a minimum use of external constraints in order to avoid biases in the products. The dense vertical sampling of the measurements adopted in the second phase of the MIPAS mission resulted in sampling intervals finer than the instantaneous field of view of the instrument. Together with the choice of a retrieval grid aligned with the vertical sampling of the measurements, this made ill-conditioned the retrieval problem of the MIPAS operational processor. This problem has been handled with minimal changes to the original retrieval approach but with significant improvements nonetheless. The Levenberg-Marquardt method, already present in the retrieval scheme for its capability to provide fast convergence for nonlinear problems, is now also exploited for the reduction of the ill-conditioning of the inversion. An expression specifically designed for the regularizing Levenberg-Marquardt method has been implemented for the computation of the covariance matrices and averaging kernels of the retrieved products. The regularization of the Levenberg-Marquardt method is controlled by the convergence criteria and is deliberately kept weak. The resulting oscillations of the retrieved profile are a posteriori damped by an innovative self-adapting Tikhonov regularization. The convergence criteria and the weakness of the self-adapting regularization ensure that minimum constraints are used and the best vertical resolution obtainable from the measurements is achieved in all atmospheric conditions. Random and systematic errors, as well as vertical and horizontal resolution are compared in the two phases of the mission for all products, namely: temperature, H2O, O-3, HNO3, CH4, N2O, NO2, CFC-11, CFC-12, N2O5 and ClONO2. The use in the two phases of the mission of different optimized sets of spectral intervals ensures that, despite the different spectral resolutions, comparable performances are obtained in the whole MIPAS mission in terms of random and systematic errors, while the vertical resolution and the horizontal resolution are significantly better in the case of the optimized resolution measurements.

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) is a mid-infrared emission spectrometer which is part of the core payload of the Envisat satellite, launched by ESA in March 2002. It provides unique observations of the atmospheric spectral radiances in the 4.15 -14.6 ?m spectral interval with innovative limb scanning capabilities for the three dimensional observation of the atmospheric composition and processes. The species, the processes and events that have been studied with this instrument in its 10 years of operation are briefly reviewed.