the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Jul 2026
Version 8 of IMK/IAA MIPAS reactive nitrogen reservoir gases
Abstract. Version 8 infrared limb emission spectra provided by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat were used to infer global distributions of HNO3, ClONO2, HNO4, and N2O5 in the altitude range from about 6 to 80 km. Here we describe in detail the analysis of the spectral data by means of constrained non-linear least-squares fitting, and provide information about the averaging kernels, the vertical and horizontal resolutions, and the error budgets of the derived trace gas profiles. For HNO3 and N2O5, the error budgets are dominated by systematic errors, mainly spectroscopic uncertainties, in the relevant altitude range, while for HNO4 and ClONO2 they are dominated by spectral noise. This implies that systematic biases of up to 20% and <10% cannot be excluded for HNO3 and N2O5, respectively, while the information about spatial and temporal variability is quite certain. Random uncertainties of HNO4 and ClONO2 can be favourably reduced by averaging, while systematic uncertainties in the order of 3% (ClONO2) and 20% (HNO4) will remain. The vertical resolution is in the order of 2 to 6 km in the lower and middle stratosphere, depending on the species and the atmospheric situation. Besides the four nitrogen reservoir species, we also present NOyas a derived data product. It is constructed from [HNO3] + [ClONO2] + [HNO4] + 2 x [N2O5] + [NO] + [NO2] and characterised in terms of its random and systematic error budget. Along with the regular data product of the four nitrogen reservoir species, an additional representation of the data on a coarser vertical grid is offered. These data can be used without consideration of the averaging kernels. The new trace gas distributions are compared to the previous data version, and they are discussed along the most relevant signatures of processes to be observed. We find that the new data products provide improved consistency between the full- and reduced-resolution mission periods of MIPAS, as well as between the observation modes covering different altitude ranges, and they exhibit all the analysed features caused by known atmospheric processes.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Measurement Techniques.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 06 Oct 2026)
- RC1: 'Comment on egusphere-2026-4116', Anonymous Referee #1, 12 Sep 2026 reply
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RC2: 'Comment on egusphere-2026-4116', Anonymous Referee #2, 17 Sep 2026
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General Comments
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The manuscript offers a very comprehensive overview and error analysis of the MIPAS nitrogen species. For each species previous validation work, the retrieval method, error analysis, and a light validation is presented. It is well written, thorough, and should be published. I have some comments and suggestions for the authors to consider.
The manuscript would benefit greatly from a little more “compartmentalizing” of the information presented. There seems to be a dual intent of providing an authoritative reference for the data version, but also serving as a guide for users. These two types of readers are often separate, information for both is provided in each section. As much as we would like every data user to read and understand a manuscript like this most of them just want to see a quick summary table with precision/accuracy/resolution and any usage recommendations. I would recommend the authors think about what the most important points they want a data user to take away, and place it together in one summary section.
Specific Comments
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l.71 “[MIPAS] on Envisat has been the only satellite so far that could measure all relevant NOy...” while true, for the reader it may be helpful to mention which nitrogen species have (and continue) to be measured by other instruments, especially since comparisons are performed later on
l.119: “radiance profiles” here is ambiguous since it has not yet been mentioned in this section that MIPAS vertically scans
l. 133 “Major changes refer to the nonlinearity correction”, perhaps it is better to say something like “A major change in the most recent version was an improvement to the nonlinearity correction”
l. 133. A better description of the data version numbering could be useful. The final appended numbers at the end are a mystery to me
Section 2.2 is quite thorough, and a good summary of the performance of previous MIPAS data versions, but I’m left wondering what I am supposed to take away from this section. Most of the data versions mentioned are V3-5, which might be a few revisions behind the currently presented V8. This is further complicated that the data versions may be quite old for the instruments being compared against. If the point is to motivate some of the improvements presented in this manuscript then I would encourage the authors to provide commentary on if the differences presented in this section are thought to be issues with the previous MIPAS processing to motivate the new developments. Related to this, unless I missed it, at this point the reader has not been told that V5 was the previous processing version.
l.243 “this spectroscopic dataset is identical to HITRAN2016” with HITRAN2024 now being out, this statement feels out of date
l.255 Related to the continuum emission… At these upper altitudes is it possible to distinguish a baseline offset versus something like aerosol? I thought the information came from the extra optical depth, but if the atmosphere is thin don’t both terms look the same?
l. 337 “This is a feature of all retrievals performed on retrieval grids finer than the tangent altitude spacing” I know what you are saying, but it’s a little more complicated than just that because of the instantaneous field of view of the instrument.
Sec. 4.2 Perhaps it would be illustrative to show information displacement relative to the actual limb scan varying tangent point instead of a single reference to separate out the actual 2D effect and the geometry effect. I’m not sure what is included in the MIPAS L2 data, but some data providers will include the tangent point at multiple reference altitudes so the user can pick which one is most useful for their application.
Fig. 10. From the figure it looks like the spectroscopy error is greater than the total systematic error? Is that right?
Section 5. The systematic error on NOy is a little harder to interpret than the other species. A single species spectroscopic error is going to be pretty close to perfectly correlated, but different spectroscopic error in each species going into NOy will cause different errors depending on the partitioning and be pseudo-random.
Section 8 I’m a little confused about the pressure grid. The regular MIPAS retrieval is done on an altitude grid correct? So in the coarse grid retrieval you would have a different altitude grid for each scan? Or is the retrieval done on a fixed altitude grid then converted to pressure layer means?
l.735 Maybe this is on my end but the link doesn’t work for me
Citation: https://doi.org/10.5194/egusphere-2026-4116-RC2
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General comments
The manuscript entitled "Version 8 of IMK/IAA MIPAS reactive nitrogen reservoir gases” by G. Stiller et al. presents a comprehensive description of Version 8 of the IMK/IAA MIPAS full-mission retrieval products for four reactive nitrogen reservoir gases, namely HNO₃, ClONO₂, HNO₄, and N₂O₅, together with the combined NOy product.
For each species, the manuscript provides a detailed description of the issues identified in previous versions, the changes introduced in the retrieval algorithm and auxiliary data, the quantities characterizing the products (including vertical and horizontal resolution and the error budget), the differences with respect to the previous version and discussion on the consequent possible impact on the quality of the products. Finally a discussion on the relative signature of processes in global distribution and time series is provided. The paper is a carefully documented reference to the released data product, in the tradition of previous IMK/IAA MIPAS species papers (e.g. Höpfner et al. 2021 for BrONO₂; Funke et al. 2023, 2026 for NO/NO₂).
Overall, I find the manuscript useful and potentially valuable for the atmospheric user community. Information of this type is essential for users who need to assess the quality and suitability of the products for their specific applications. For other satellite instruments, similar information is often provided in dedicated data-quality documents or readme files; for example, the MLS Version 5 Data Quality Document provides a detailed description of the corresponding products, which may have the advantage of containing all relevant information of the products in a consistent way in a unique document. Bringing the relevant information together in a peer-reviewed publication is nevertheless useful, as it provides a citable and comprehensive reference for the MIPAS IMK/IAA V8 products.
Given the length and detailed nature of the manuscript, however, some of the information that is particularly relevant for data users is currently difficult to locate. Furthermore, some general concepts, which are likely described in more detail in other publications, are introduced here only at a rather general level. I therefore suggest a few minor additions and clarifications that, in my opinion, would further improve the manuscript's usefulness as a reference document for current and future users of the dataset.
Specific comments
Technical corrections