the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Dec 2024
Decrease of the European NOx anthropogenic emissions between 2005 and 2019 as seen from the OMI and TROPOMI NO2 satellite observations
Abstract. There are great expectations about the detection and the quantification of NOx emissions using NO2 tropospheric columns from satellite observations and inverse systems. This study assesses the potential of the OMI-QA4ECV and TROPOMI satellite observations to improve the knowledge on European NOx emissions at the regional scale and to inform about the spatio-temporal variability of NOx anthropogenic emissions from 2005 to 2019, at the resolution of 0.5° over Europe. Starting from European emission estimates from the TNO-GHGco-v3 inventory for the year 2005, regional inversions using the Community Inversion Framework coupled to the CHIMERE chemistry-transport model and assimilating satellite NO2 tropospheric columns from OMI and TROPOMI have been performed to estimate the European annual and seasonal budgets for the year 2019. Both the OMI and TROPOMI inversions show decreases in European NOx anthropogenic emission budgets between 2005 and 2019 but the magnitude of the reductions differs with OMI and TROPOMI data (-16 % and -45 %, respectively). A TROPOMI in-version giving more weight to the satellite data becomes consistent with the independent TNO-GHGco-v3 inventory for the year 2019, with annual budgets for EU-27+UK showing absolute relative difference of only 4 %. These TROPOMI inversions are therefore in agreement with the magnitude of the decline in NOx emissions declared by countries, when aggregated at the European scale. However, our results —with OMI and TROPOMI data leading to different magnitudes of corrections on NOx anthropogenic emissions—suggest that more observational constraints would be required to sharpen the European emission estimates.
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RC1: 'Comment on egusphere-2024-3679', Anonymous Referee #1, 11 Dec 2024
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Overall, it is clear that a large amount of work was done by the authors. However, in its current state, I cannot recommend publication. I do encourage the authors to carefully consider my comments and resubmit the revised manuscript.
A major concern of mine is that this article doesn’t present new science, so it may be better suited to another journal, such as Earth System Science Data (ESSD). The title and introduction actually sound somewhat appropriate for ACP, but the manuscript content is really not appropriate for ACP.
Another issue is that the manuscript is poorly organized and, therefore, difficult to read. ACP gives handy recommendations on the structure of a manuscript: https://www.atmospheric-chemistry-and-physics.net/policies/guidelines_for_authors.html. I highly suggest that the authors review these recommendations. Here are two examples of what I mean:
- The title doesn’t reflect what the paper is about. The paper title indicates that the paper is focused on discussing the causes of trends in European NOx emissions. At least, that’s what I thought when reading the title. This isn’t the case. Please revise the title to reflect the actual manuscript content.
- The paper is wordy (by 30-50%); the introduction is particularly wordy and needs paragraph indents for clarity. Note that a goal of technical writing is to clearly and concisely convey a particular message. As an example, the purpose of your paper should be given in one sentence, such as in the abstract and as a topic sentence of a paragraph in the introduction. I had to piece it together over a very long paragraph, but I still don’t know the overall goal of the paper given the vague title, vague abstract, and vague conclusions. An overview diagram of the steps in your work would be helpful.
Another major concern is the lack of validation of the data products and emission estimates. Why don’t you compare results with independent in situ observations, such as from the Pandora network? That would certainly strengthen your conclusions about how your method impacts the emissions estimates. In fact, you say in the last sentence of the abstract “…our results —with OMI and TROPOMI data leading to different magnitudes of corrections on NOx anthropogenic emissions—suggest that more observational constraints would be required to sharpen the European emission estimates.” You haven’t even used the existing observational constraints!
Section 2.1: What are the strengths and limitations of this inversion system for your work? Has it been applied and validated with independent observations (e.g., Pandora)? Why are you using it relative to other inversion systems?
Citation: https://doi.org/10.5194/egusphere-2024-3679-RC1 -
RC2: 'Comment on egusphere-2024-3679', Anonymous Referee #2, 16 Dec 2024
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This manuscript presents NOx emission inversion experiments driven by two different satellite NO2 column observational data sets. By performing inversions for 2019 using prior emissions for 2005, this study addresses the question to what extent bottom-up reported NOx emission reductions can be reconciled using satellite NO2 observations in an emission inversion. This study is an important contribution to the literature and is well within the scope of ACP, with the community increasingly focusing on reconciling top-down and bottom-up emission estimates. The methodology is mostly scientifically sound, but the presentation of the methodology and results in the manuscript is unclear. This paper can be considered for publication in ACP after a revision focused primarily on restructuring the manuscript and improving the clarity of writing. Please find below the specific issues to be addressed in a revised manuscript.
Methodological issues:
- While I agree with the authors that soil emissions are a relatively minor term on annual time scales, this is not the case for the spring and summer months when biogenic and agricultural emissions peak and anthropogenic emissions are lowest. Previous studies (e.g., Silvern et al. 2019) found that accounting for soil emissions is important to reconcile simulated and observed NO2 column trends. Please discuss the effect of omitting agricultural soil emissions on the estimated NOx emission reduction trends.
- In my view, this study is missing a section on the performance of the CIF-CHIMERE system using the two assimilated datasets. Please include this, e.g. by presenting error reductions or a comparison with independent (surface) observations.
Writing issues:
- The title does not reflect the contents of the paper: TROPOMI observations were not available in 2005, and the paper does not focus on detecting NOx emission decreases but rather on reconciling top-down- and bottom-up-derived emission trends. Please revise.
- The introduction is too lengthy. Please formulate in a more concise manner (in my view, this section can be shortened by roughly 50%) and rethink the presentation of the research questions towards the end of the section.
- Methodology: there is little coherence between these subsections. Please consider giving more descriptive names to your inversion experiments and including a conceptual diagram summarizing your methods.
Minor issues:
Introduction: please introduce non-anthropogenic NOx emissions before the first use of biogenic emissions in line 55.
Section 2.1: please add more detail on the CIF-CHIMERE setup. Since NOx chemistry is non-linear, how do NOx emission changes impact NO2 columns via the chemistry scheme?
L217-218: what is the goal of reprocessing the NO2 column observations using meteorological data?
L350-351: wouldn’t OMI also underestimate columns in polluted areas, due to the coarse-resolution a priori NO2 profiles? Why is this problem larger for TROPOMI?
L355-360: please explain better what is meant with ‘an indirect comparison of OMI and TROPOMI using the CHIMERE CTM as an intermediate’. I am also missing this paragraph’s key message and contribution to the subsection.
L394-395: this seems specific to the experiment setup with a missing agricultural soil emission term.
References
Silvern, R. F., Jacob, D. J., Mickley, L. J., Sulprizio, M. P., Travis, K. R., Marais, E. A., Cohen, R. C., Laughner, J. L., Choi, S., Joiner, J., and Lamsal, L. N.: Using satellite observations of tropospheric NO2 columns to infer long-term trends in US NOxemissions: the importance of accounting for the free tropospheric NO2 background, Atmos. Chem. Phys., 19, 8863–8878, https://doi.org/10.5194/acp-19-8863-2019, 2019.
Citation: https://doi.org/10.5194/egusphere-2024-3679-RC2
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