Chemistry Transport Model

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Eryn

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Aug 3, 2024, 5:48:31 PM8/3/24

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Abstract. Satellites monitoring air pollutants (e.g., nitrogen oxides, NOx = NO + NO2) or greenhouse gases (GHGs) are widely utilized to understand the spatiotemporal variability and evolution of emission characteristics, chemical transformations, and atmospheric transport over anthropogenic "hotspots'". Recently, the joint use of space-based long-lived GHGs (e.g., carbon dioxide, CO2) and short-lived pollutants has made it possible to improve our understanding of emission characteristics. Some previous studies, however, lack consideration of the non-linear NOx chemistry or complex atmospheric transport. Considering the increase in satellite data volume and the demand for emission monitoring at higher spatiotemporal scales, it is crucial to construct a local-scale emission optimization system that can handle both long-lived GHGs and short-lived pollutants in a coupled and effective manner. This need motivates us to develop a Lagrangian chemical transport model that accounts for NOx chemistry and fine-scale atmospheric transport (STILT-NOx); and investigate how physical and chemical processes, anthropogenic emissions, and background may affect the interpretation of tropospheric NO2 columns (tNO2).

Satellite retrievals of NO2 columns are used to determine NOx emissions from power plants and cities. They are increasingly used alongside CO2 retrievals to calculate emissions from these sources. However, the effect of NOx chemistry and transport on NO2 columns is often overlooked. To address this issue, Wu et al. have developed a model that incorporates a simplified representation of NOx chemical loss within the STILT Lagrangian particle dispersion model. It includes additional features such as a column weighting module to account for retrieval averaging kernel profiles and an error analysis module. The model is evaluated against TROPOMI NO2 observations from three power plants and two cities. The manuscript covers the model's advantages, limitations, and applications such as using NO2-to-CO2 enhancement ratios to estimate CO2 emissions and identifying wind biases in meteorological data.

We thank reviewer #1 for the constructive comments and have addressed all comments with additional analyses and clarifications to the manuscript. Please refer to the supplementary document for our point-by-point response.

This paper introduces STILT-NOx, a Lagrangian chemical transport model, evaluates it against satellite based column observations of NOx, and presents various sensitivity studies. The paper is well written, and I recommend publishing after the following minor comments are addressed.

Rotation of wind: As the wind changes significantly within the atmospheric boundary layer with height (the Ekman spiral), differences between modelled wind direction and the direction apparent from the observed plume can also be related to inaccuracies in the plume release height distribution, potentially associated with plume rise of the buoyant exhaust. I would recommend this to be discussed.

We thank reviewer #2 for the constructive comments and have addressed all comments with additional analyses and clarifications to the manuscript. Please refer to the supplementary document for our point-by-point response.

You state in your manuscript that you have not published the code used in your work and that you will do it later. This is highly irregular, and I must make clear that your manuscript should have never been accepted for Discussions given this issue. In this way, you must publish in a prompt manner the code of the model (and any other code that you use to produce your work) in one of the repositories listed in our policy. I should note that you mention GitHub in your submission; however, GitHub is not a suitable repository for scientific publication, which is clearly mentioned in our policy too. GitHub itself instructs authors to use other alternatives for long-term archival and publishing.

Therefore, please, publish your code in one of the appropriate repositories, and reply to this comment with the relevant information (link and DOI), and a new title for your manuscript, as soon as possible, as it must be available for the Discussions stage. Also, please, include all the relevant primary input and output data.

We apologize for not providing the model script timely and have now released a fixed version of the STILT-NOx model code on Zenodo ("STILT-NOx v1 for GMD submission") with a doi of The key input/output data including the preprocessed EDGARv6 emissions and the essential NOx curves are included in the /data subdirectory. The NOx chemistry module contains functions under r/src/chem_lifetime.

- First, I recommend that you clarify in the reviewed version of your Code and Data Availability section that the numbers that you provide for TROPOMI and OCO data are DOIs, something that, with the current wording, is unclear.

- In the Zenodo repository for the code, in the Readme file, it states "in this GitHub repository". I understand that this is inherited from moving the code from GitHub to Zenodo; however, it would be good if you could fix it and, in the information in the Zenodo repository, refer to it.

- Also, in the Zenodo repository, there is no license listed for the code. If you do not include a license, despite what you state saying that the license is "Other", the code continues to be your property, and nobody can use it, precluding the possibility of reproducing your work. Therefore, you should include a license for your code. You could want to choose a free software/open-source (FLOSS) license. We recommend the GPLv3. You only need to include the file ' -3.0.txt' as LICENSE.txt with your code. Also, you can choose other options that Zenodo provides: GPLv2, Apache License, MIT License, etc.