the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Apr 2025
Description and evaluation of airborne microplastics in the United Kingdom Earth System Model (UKESM1.1) using GLOMAP-mode
Abstract. Airborne microplastics are a recently identified atmospheric aerosol species with potential air quality and climate impacts, yet they are not currently represented in global climate models. Here, we describe the addition of microplastics to the aerosol scheme of the UK Earth System Model (UKESM1.1): the Global Model of Aerosol Processes (GLOMAP). Microplastics are included as both fragments and fibres across a range of aerosol size modes, enabling interaction with existing aerosol processes such as ageing and wet and dry deposition. Simulated microplastics have higher concentrations over land, but can be transported into remote regions including Antarctica despite no assumed emissions from these regions. Lifetimes range between ~17 days to ~1 hour, with smaller, soluble microplastics having longer lifetimes. Microplastics are well-mixed throughout the troposphere, and the smallest particles are simulated to reach the lower stratosphere in small numbers. Dry deposition is the dominant microplastic removal pathway, but greater wet deposition occurs for smaller soluble microplastic, due to interactions with clouds. Although microplastics currently contribute a minor fraction of the total aerosol burden, their concentration is expected to increase in future if plastic production continues to increase, and as existing plastic waste in the environment degrades to form new microplastic. Incorporating microplastics into UKESM1.1 is a key step toward quantifying their current atmospheric impact and offers a framework for simulating future emission scenarios for an assessment of their long term impacts on air quality and climate.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-1575', Anonymous Referee #1, 02 May 2025
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RC2: 'Comment on egusphere-2025-1575', Anonymous Referee #2, 20 Jun 2025
General comments:
The paper presents a pioneering attempt to incorporate microplastic aerosols into a climate model. This is a valuable and timely work, as such components may become increasingly relevant and will have to be included in atmospheric modeling. However, I have several major concerns regarding the current version of the manuscript. Firstly, a number of key elements are insufficiently described, making it difficult to assess the robustness and validity of the methodology and results, as well as hindering the reproducibility. Additionally, the study relies on many assumptions, many of which are not adequately justified or explored in terms of their uncertainties, which significantly weakens the impact of the findings. A sensitivity analysis or scenario-based approach to explore the possible range of outcomes would strengthen the paper’s scientific contribution.
In particular, it would be important to avoid drawing conclusions on microplastic behaviors at this early stage, such as the longer atmospheric lifetime of small soluble microplastics or the assertion that dry deposition is the dominant removal pathway. These results follow directly from the assumptions built into the model, and we currently lack empirical validation for such assumptions.
Furthermore, the manuscript would greatly benefit from a more transparent discussion of the construction of the emission inventories, including the rationale behind the choices made.
In summary, while the work goes into a promising direction, the manuscript requires additional information to improve clarity, justify assumptions, and better quantify uncertainties before it is suitable for publication.
I detail here below my specific comments on these aspects, plus some minor comments.
Specific comments:
Line 46: “If present in high enough concentrations..” can you be quantitative?
Lines 50-51: Please double-check the structure of this sentence. Do you mean “found in their modelling that atmospheric microplastic, under high emissions scenarios (how high?), can potentially contribute significantly to INP concentrations”. From their paper, it seems that this is true only in specific parts of the globe, where other aerosols are not present. To not be misleading, I would specify it here, for example “…in regions where other aerosol species are absent”.
Line 53-54: This sentence can be misleading in terms of cause and consequences. The contributions and implications of microplastics for total aerosol loading and climate change are not known because we currently do not yet have good information to possibly include them in models. As the authors also discuss, the data are not yet covering enough of the globe and are not coherent. Same with their soluble or insoluble properties, which are still not understood. Hence, assuming their emissions and their behavior in the atmosphere is, indeed, only an assumption, and it has to be treated as such. I think this has to be made clear by the authors here, and throughout the paper.
Lines 118-124: This section needs to include many more details on how the emissions have been created, otherwise preventing the reader from understanding the significance of the results presented. For example, the authors state that the inventories have been updated with respect to previously published emissions (Evangeliou et al. 2022) by considering the ocean gyres, but it is unclear how has this been done. Did you normalize the emissions on the yearly spatial distribution? What was the procedure? Also, from the figure 1a it seems the ocean emissions have a different spatial resolution with respect to land emissions, why is it so?
Similarly, in which way the polar region emissions have been reduced and based on which information? And how, and why have the land emissions been increased? Which of the sectors (agriculture, mineral dust, road dust), all of them as a sum? It will be important to describe these aspects to get a better idea of what is actually computed in the model.
Lines 143-145: Do you choose this bin because it is the largest number of microplastics from your emission inventories or from observational data?
Lines 156-160: How is the solubility process represented from MP in the model? Is it the same as any specific aerosol included in the model? It has to be emphasized that for MP this is also a process of which, in reality, we have very little knowledge, so the resulting wet removal or CCN outcomes are also speculative.
Lines 168-169: This is a bit unclear: what size and size distribution do you then assume for fibers? You mention an original emission inventory with fibers of lengths between 10-3000 um and diameter 1-10 um. How do you convert that here?
Line 176: “homogeneous in composition” means that you assume they are all assimilated in a single polymer? If that's the case, which one do you use and why?
Lines 183-185: Just to clarify: The Mulchay et al paper refers to other aerosols simulations, do you mean that the model have already been well tested for that same period for other aerosols, so that you expect it to behave properly also for microplastic transport?
Paragraph 2.6: It would be necessary to get more information on this section to get a proper interpretation of your model validation. For example, can you provide the actual list of the papers you got from your Scopus search and which ones you selected? And how many data points did you get for validation at the end for land and for ocean?
Line 205: The microplastic fibers distribution of concentrations is, for example, not easy to interpret without more information on how you considered these particles. Aside the settling velocity correction to be implemented for the shape, it would be important to know also which size distribution you associated with them, since big spherical particles would of course settle down very close to the emission points.
Line 214: What is meant by “surface concentration” (up to which altitude)?
Line 227: I am also not sure to have understood how the solubility or insolubility of the particles is chosen. For the microfibers for example, are they only in the insoluble mode because they belong to the biggest particle size, which in the model is currently only associated to insoluble dust?
Line 244: Why do you say that number concentrations of less than 1/m3 are unrealistic? There are several studies that measured concentrations below 1 (e.g. Abbasi et al. 2022, Chandrakanthan et al 2023, Jia et al. 2025)
Abbasi, S., Rezaei, M., Ahmadi, F., & Turner, A. (2022) 'Atmospheric transport of microplastics during a dust storm', Chemosphere, 292, pp. 133456-133456. 10.1016/j.chemosphere.2021.133456
Chandrakanthan, K., Fraser, M.P., Herckes, P. (2023) Airborne microplastics in a suburban location in the desert southwest: occurrence and identification challenges, Atmos Environ, 298, 10.1016/j.atmosenv.2023.119617
Jia, Q., Duan, Y., Song, Z., Han, X., Wang, X., Wang, L., Song, D., Chen, J., & Guangli Xiu. (2025b). Seasonal Patterns and Environmental Drivers of Atmospheric Microplastics in a Coastal Megacity. Environ Research, 121763–121763, https://doi.org/10.1016/j.envres.2025.121763Line 289: This is a bit counterintuitive. Can you give an explanation on why the soluble particles are actually the ones that have the longer lifetime?Also, I was wondering if coagulation in any way acting on the “removal” of these particles? You mention that the MP class is also able to coagulate in your model (being assimilated to the behavior of which particles? And how realistic is that?), but you didn’t discuss the effect of this process on them.
Lines 324-325: This is one of the reasons why you should indicate which studies you included in your comparison. From what you say, it seems that you take the super-coarse class from the model and compare the number concentrations measured in the various studies, but some will have upper limits at certain diameters, and others at different ones. This can lead to quite some big differences in the concentrations. How do you handle that? That would also be quite complicated if you also include the data on fibers.
Lines 334-335: Looking at the scatter plot, though, there are high deviations also for the observations over land, which suggests that the model gives higher values with respect to the data overall.
Lines 339-340: One of the main reasons of discrepancy is also the modelling itself. It is important to recognize that our knowledge at the moment is limited, hence the assumptions that are needed to feed the model may contain important biases (e.g. on the transport behavior, such as the solubility or coagulation properties, the distribution of the sources, their intensity, the size distribution). While the points raised by the authors are valid, those modelling uncertainties are most likely the biggest reason for the discrepancies with data. This is also why I would invite the author to add sensitivity studies or some assessment of the uncertainties related to the various assumptions introduced with this new aerosol species.
Technical comments:
Line 37: I’d suggest removing the parenthesis and say “..by the Earth with a warming effect.”
Line 95: space missing after 3.3um.
Citation: https://doi.org/10.5194/egusphere-2025-1575-RC2 -
AC1: 'Comment on egusphere-2025-1575', Cameron McErlich, 31 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1575/egusphere-2025-1575-AC1-supplement.pdf
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