MIPAS, the Michelson Interferometer for Pas- sive Atmospheric Sounding, is a mid-infrared emission spectrometer which is part of the core payload of ENVISAT. It is a limb sounder, i.e. it scans across the horizon detecting atmospheric spectral radiances which are inverted to verti- cal temperature, trace species and cloud distributions. These data can be used for scientific investigations in various re- search fields including dynamics and chemistry in the alti- tude region between upper troposphere and lower thermo- sphere. The instrument is a well calibrated and characterized Fourier transform spectrometer which is able to detect many trace constituents simultaneously. The different concepts of retrieval methods are described including multi-target and two-dimensional retrievals. Operationally generated data sets consist of temperature, H2O, O3, CH4, N2O, HNO3, and NO2 profiles. Measurement errors are investigated in de- tail and random and systematic errors are specified. The re- sults are validated by independent instrumentation which has been operated at ground stations or aboard balloon gondolas and aircraft. Intercomparisons of MIPAS measurements with other satellite data have been carried out, too. As a result, it

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MIPAS: an instrument for atmospheric and climate research

It is now well established that heterogeneous reactions provide an important mechanism for Antarctic ozone depletion. Recent laboratory studies suggest that the same reactions that occur on HNO3/H2O ice clouds in the cold Antarctic stratosphere can also take place on sulfuric acid particles (e.g., volcanic and background aerosols) typical of lower latitudes, albeit at slower rates. The reduction in stratospheric ozone observed at northern mid-latitudes in late 1982 through 1983 following the volcanic eruption of El Chichon is investigated in terms of ozone loss through heterogeneous chemistry on the aerosol which formed in the stratosphere. The rates of the relevant heterogeneous reactions are believed to be critically dependent on (1) the aerosol surface area density and (2) the percent by weight sulfuric acid in the liquid particles. Direct measurements of both of these important quantities for El Chichon aerosol are described and used as a basis for model calculations of their possible effects on ozone and other trace species. The observed volcanic particle surface area reached a maximum at mid-latitudes of about 50 μm2 cm−3 (above a typical background value of about 0.75) at an altitude of 18–20 km in early 1983. This enhancement of surface area is about the same as that encountered in stratospheric clouds in the Antarctic, suggesting a possible basis for ozone depletion through heterogeneous chemistry. Observations of NO2 and HNO3 also suggest that heterogeneous reactions on both background and volcanic aerosol play a significant role in partitioning reactive nitrogen species in middle and high latitudes in winter. It is shown that heterogeneous reactions similar to those occurring in Antarctica may have been responsible for at least a portion of the anomalous ozone reduction observed at mid-latitudes in early 1983.

Ozone destruction through heterogeneous chemistry following the eruption of El Chichón

The effect of particle precipitation events on the neutral and ion chemistry of the middle atmosphere: II. Odd hydrogen

Ozone plays a very important role in our atmosphere because it protects any living organisms at the Earth's surface against the harmful solar UVB and UVC radiation. In the stratosphere, ozone plays a critical role in the energy budget because it absorbs both solar UV and terrestrial IR radiation. Further, ozone in the tropopause acts as a strong greenhouse gas, and increasing ozone trends at these altitudes contribute to climate change. This review contains a short description of the various techniques that provided atmospheric ozone measurements valuable for long-term trend analysis. The anthropogenic emissions of substances that deplete ozone (chlorine- and bromine-containing volatile gases) have increased from the 1950s until the second half of the 1980s. The most severe consequence of the anthropogenic release of ozone-depleting substances is the “Antarctic ozone hole.” Long-term observations indicate that stratospheric ozone depletion in the southern winter-spring season over Antarctica started in the late 1970s, leading to a strong decrease in October total ozone means. Present values are only approximately half of those observed prior to 1970. In the Arctic, large ozone depletion was observed in winter and spring in some recent years. Satellite and ground-based measurements show no significant trends in the tropics but significant long-term decreasing trends in the northern and southern midlatitudes (of the order of 2–4% per decade in the period from 1970 to 1996 and an acceleration in trends in the 1980s). Ozone at northern midlatitudes decreased by −7.4±2% per decade at 40 km above mean sea level, while ozone loss was small at 30 km. Large trends were found in the lower stratosphere, −5.1±1.8% at 20 km and −7.3±4.6% at 15 km, where the bulk of the ozone resides. The possibility of a reduction in the observed trends has been discussed recently, but it is very hard to distinguish this from the natural variability. As a consequence of the Montreal Protocol process, the emissions of ozone-depleting substances have decreased since the late 1980s. Chlorine is no longer increasing in the stratosphere, although the total bromine amount is still increasing. Considering anthropogenic emissions of substances that deplete ozone, the turnaround in stratospheric ozone trends is expected to take place in the coming years. However, anthropogenic climate change could have a large influence on the future evolution of the Earth's ozone shield.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999RG000059

Ozone trends: A review

[1] Retrieval of abundances of atmospheric species from limb infrared emission spectra requires accurate knowledge of the pointing of the instrument in terms of elevation, as well as temperature and pressure profiles. An optimal estimation-based method is presented to infer these quantities from measured spectra. The successful and efficient joint retrieval of these largely correlated quantities depends strongly on the proper selection of the retrieval space, the selection of spectral microwindows, and the choice of reasonable constraints which force the solution to be stable. The proposed strategy was applied to limb emission spectra recorded with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board the Envisat research satellite in order to validate the instrument pointing information based on the satellite's orbit and attitude control system which uses star tracker information as a reference. Both systematic and periodic pointing calibration errors were detected, which meanwhile have been corrected to a major part. Furthermore, occasional pitch jumps were detected, which could be assigned to parameter uploads to the satellite's orbit and attitude control system. It has been shown that in general, it is justified to assume local thermodynamic equilibrium below 60 km for these purposes. The retrieval method presented has been proven to be suitable for independent monitoring of MIPAS line-of-sight pointing.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003JD003602

Retrieval of temperature and tangent altitude pointing from limb emission spectra recorded from space by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS)

An optimized code to perform the near-real-time retrieval of profiles of pressure, temperature, and volume mixing ratio (VMR) of five key species (O3, H2O, HNO3, CH4, and N2O) from infrared limb spectra recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) experiment on board the European Space Agency (ESA) Environmental Satellite ENVISAT-1 was developed as part of a ESA-supported study. The implementation uses the global fit approach on selected narrow spectral intervals (microwindows) to retrieve each profile in sequence. The trade-off between run time and accuracy of the retrieval was optimized from both the physical and the mathematical points of view, with optimizations in the program structure, in the radiative transfer model, and in the computation of the retrieval Jacobian. The attained performances of the retrieval code are noise error on temperature <2 K at all the altitudes covered by the typical MIPAS scan (8–53 km with 3-km resolution), noise error on tangent pressure <3%, and noise error on VMR of the target species <5% at most of the altitudes covered by the standard MIPAS scan, with a total run time of less than 1 min on a modern workstation.

Optimized forward model and retrieval scheme for MIPAS near-real-time data processing.

Measurements of stratospheric NO and NO2 from the last four flights of the Oxford radiometer are compared with predictions by a diurnal photochemical model. The rapid sunrise and sunset changes predicted by the model and detected on previous flights are confirmed. The observations suggest agreement with model predictions of the night time decay of NO2 associated with N2O5 production but do not fully verify them. For three of the flights this is due to insufficient measurement precision and short flight duration, and in the other flight it is due to disruption of the diurnal cycle by viewing parts of the stratosphere of different composition. The 1982 flight showed both NO and NO2 to be greatly reduced between 30 and 35 km. We attribute this to the eruption of the El Chichon volcano. Possible mechanisms for this reduction are discussed.

Simultaneous measurements of stratospheric NO and NO2 and their comparison with model predictions

OXIDES of nitrogen are generally accepted to be important for regulating the atmospheric ozone content through catalytic reactions first described by Crutzen1 and Johnston2. In the stratosphere, nitric oxide is produced naturally by the reaction of nitrous oxide, emanating from the biosphere, with excited oxygen atoms generated by ozone photolysis at wavelengths shorter than about 310 nm. As Crutzen et al.3 have pointed out, large amounts of nitric oxide may also be produced by high-energy solar protons, mainly at geomagnetic latitudes greater than 60°, and altitudes above about 35 km. The ozone measurements reported here made during an intense solar proton event in August 1972 suggest that NO production rates due to ionising particles are higher than those calculated by Crutzen et al.

The August 1972 solar proton event and the atmospheric ozone layer

Satellite observations of atmospheric infrared spectra can be modeled accurately with line-by-line calculations, but these are too slow to be incorporated into operational retrieval schemes. However, a monochromatic calculation is still feasible if the line-by-line summation is replaced by pre-tabulated absorption coefficients, requiring a three-way optimization of storage space, accuracy and access time. Such a scheme is used for the operational processing of data from MIPAS, a limb-viewing interferometer. The tabulated data are compressed to a manageable size using singular value decomposition, although the reconstruction adds a small overhead. The number of monochromatic radiative transfer calculations is reduced by determining suitable quadrature points in the spectral domain, which reduces both processing time and data storage requirements. An important aspect of such optimizations is the control of the associated errors in the forward model calculation. The result is an acceleration of monochromatic forward model calculations by one or two orders of magnitude compared to a line-by-line calculation without any significant loss of accuracy, while obtaining data compression factors of 100 or more compared to a direct tabulation of absorption coefficients.

Fast monochromatic radiative transfer calculations for limb sounding

The IR emission limb sounder MIPAS will be operated as an ESA core instrument on the ENVISAT-1 satellite. Near real time retrieval of pressure, temperature and volume mixing ratio of five key species from calibrated spectra will be performed in the Level 2 processor of the ENVISAT Payload Data Segment. In order to develop an optimized retrieval algorithm suitable for the implementation in MIPAS Level 2 processor, an ESA supported study is being carried out. In the framework of this study, an optimized forward/retrieval code based on the global fit approach was implemented. In this approach all the spectra of a limb-scanning sequence are simultaneously fitted, so that error propagation in the altitude domain is avoided. The attained accuracy performances of the retrieval code are the following: (i) temperature accuracy < 2 K at all the altitudes covered by the standard MIPAS scan; (ii) tangent pressure error: < 3 percent; (iii) error on the retrieved VMR of the key species: < 5 percent at most of the latitudes of scientific interest covered by the standard MIPAS scan. The run-time required to perform p,T and VMR retrieval of the five MIPAS target species from a limb-scanning sequence of 16 limb-views is less than 6 minutes on a SUN SPARCstation 20. The most effective code optimization were implemented in the radiative transfer model and in the computation of the jacobian of the retrieval.

Robert J. Wells

Papers

Optimized forward model and retrieval scheme for MIPAS near-real-time data processing.

Simultaneous measurements of stratospheric NO and NO2 and their comparison with model predictions

The August 1972 solar proton event and the atmospheric ozone layer

Fast monochromatic radiative transfer calculations for limb sounding

Optimized forward and retrieval model for MIPAS near-real-time data processing