the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Jun 2026
Land-sea breeze contribution to pollutant dispersion from the Guinea Coastal cities of West Africa
Abstract. Urbanization in Africa is often associated with increased air pollution, which affects not only the main cities but also remote regions depending on the prevailing meteorological conditions. Here, we investigate the role of the land-sea breeze (LSB) circulation for the dispersion of pollutants originating from five major coastal cities along the Guinea Coast in West Africa (Abidjan, Accra, Lomé, Cotonou, and Lagos). The study is based on a passive tracer emission experiment using the ICOsahedral Non-hydrostatic model coupled to the Aerosol and Reactive Trace gases module (ICON-ART) for a representative dry-season situation between 08 and 10 January 2021. Pollutants are emitted between 2 and 50 m from the ground from 0600 to 1800 UTC (close to local time) on the first day of simulation, from where they spread horizontally and vertically. The simulation reveals that the LSB starts to intensify near the coast around noon and propagates inland, reaching its maximum latitude at 7° N (approximately 200 km from the coasts) around 2100 UTC. Pollutants are first swiftly transported inland by the southwesterly wind of the LSB. As the planetary boundary layer deepens, particularly above the convergence zone near the LSB front, pollutants can reach the 875–800 hPa layer before being carried towards the coast and the Atlantic Ocean by the prevailing northeasterly return flow. Interestingly, these returning pollutants do not mix strongly down to the surface. Therefore, a well-developed LSB along the coast appears to contribute to attenuating urban coastal pollution by supporting a rapid dispersion of pollutants.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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: final response (author comments only)
- RC1: 'Comment on egusphere-2026-3128', Anonymous Referee #1, 24 Jun 2026
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RC2: 'Comment on egusphere-2026-3128', Anonymous Referee #2, 08 Oct 2026
The manuscript investigates the transport of passive tracers released from five coastal cities along the Guinea Coast using ICON-ART for 8-10 January 2021. Its focus on the relatively understudied dry season is relevant, and the simulated inland transport followed by offshore transport aloft provides a potentially useful account of regional pollutant pathways. However, the evidence does not yet sufficiently isolate the contribution of the land–sea breeze from background circulation and boundary-layer mixing or establish the broader claim that this circulation reduces urban coastal air pollution. The representativeness of the selected episode, validation of the circulation, and numerical robustness also require further attention. I therefore recommend major revision.
Major comments：
- Objective identification of the land–sea breeze and separation from other wind systems (Lines 158–170 and 261–302).
The case selection relies primarily on visual inspection of MODIS cloud imagery and the exclusion of rainy days. Figure 3 provides supporting surface-wind evidence, but neither this qualitative assessment nor a cloud band approximately parallel to the coastline is sufficient to identify the thermally driven circulation objectively. Similarly, temperature and wind-speed gradients may reflect the background monsoon–Harmattan transition or other mesoscale features. The identification should be supported by coast-normal wind evolution, thermal contrasts, convergence, and the timing and inland progression of the front. A complete surface wind reversal need not be imposed where the background onshore flow masks it, but the thermally driven component must be distinguished from that background flow. The broad spatially averaged wind profiles in Figures A7–A8 should be supplemented by coastal cross-sections showing the front, vertical motion, and circulation depth. Explain whether the offshore flow aloft a sea-breeze return branch is, the prevailing Harmattan, or a combination of the two.
- The inference of air-pollution reduction requires stronger attribution and quantitative evidence (Sections 3.3–3.4; Lines 390–440).
The simulations demonstrate tracer transport during an episode with sea-breeze activity, but this does not by itself establish the incremental ventilation benefit attributable to that circulation. Daytime turbulence and the background winds can also disperse tracers; indeed, Lines 328–330 describe transport to 800 hPa before the LSB is well developed. Furthermore, emissions stop at 1800 UTC on the first day, so declining concentrations near the sources cannot be attributed solely to the LSB. The daytime versus nighttime emission experiment is useful but is not a control for LSB occurrence because emission timing and atmospheric stability change simultaneously. Without stronger attribution, the abstract and conclusions should describe potential transport and ventilation pathways under the selected conditions rather than a demonstrated reduction in urban air pollution.
- Representativeness and geographical/seasonal applicability (Lines 41–42, 158–166, and 417–440).
The study analyzes one consecutive three-day episode in January 2021, rather than a representative sample of 2021 or several independent dry-season events. The description of this episode as representative therefore needs evidence.
- Match the air-quality interpretation to the passive-tracer experiment (Lines 154–156, 385–389, 410–415, and 441–444).
The authors explicitly acknowledge that the tracers have infinite lifetimes and do not undergo chemistry, deposition, or aerosol–radiation/cloud interactions. This is a legitimate simplification for investigating transport pathways, but it limits the interpretation in terms of actual air quality. Redistribution of an inert tracer does not establish the response of reactive gases, secondary aerosols, or particles with appreciable deposition. Please identify which classes of pollutants the experiment can reasonably approximate over the simulated transport timescale, and discuss how chemical production, chemical loss, and deposition could modify the inferred coastal and inland impacts. This does not necessarily require a full chemistry simulation, provided the conclusions are appropriately restricted. Statements about wet removal and the degradation of inland air quality should be presented as implications requiring further investigation, rather than outcomes demonstrated by these experiments.
- Validation should prioritize the circulation and mixing that control tracer transport (Lines 123–125 and 232–260; Figures 5–6).
The temperature and pressure comparisons are useful, but correlations in these quantities alone cannot establish that the modeled transport is reliable. The authors acknowledge the 2 m versus 10 m wind-height mismatch and limited observational wind validation. Additional coastal airport METAR records should be considered if available, or their exclusion explained. Report bias and RMSE alongside correlations and clarify the pressure variable and elevation treatment in Figure 5. The weak or negative dew-point correlations also require discussion because moisture transport and vertical mixing are relevant to the proposed mechanism. Explain temporal matching to the six-hourly ECMWF analyses and account for serial correlation when assessing statistical significance. Where vertical observations are unavailable, state the resulting uncertainty explicitly.
- The resolution comparison does not demonstrate robustness of the LSB and tracer transport (Lines 171–184; Figure A3).
The choice to present the 13 km results is justified by differences of less than 10% in daily mean temperature, dew point, and pressure between the 13 km and 6 km domains. These comparisons do not directly test the processes central to this study: frontal convergence, circulation depth, vertical tracer transport, and near-source dilution. Daily averaging can also conceal differences in onset and frontal propagation. Please compare wind components, sea-breeze timing and intensity, front position, PBL height, and tracer transport metrics over the common interior region of the two domains. Since the finer domain does not encompass the complete transport pathway, clarify the role of domain extent and lateral boundaries; resolution and domain size should not be conflated. Define the percentage-difference calculation and report temperature differences in K or °C and pressure differences in Pa or hPa. Percentage differences in temperature are particularly difficult to interpret without specifying the temperature scale. A 13 km experiment is not intrinsically invalid, but its adequacy for the stated mechanism needs more direct evidence.
- Reassess the pressure-gradient diagnosis in Figure 9 (Lines 287–300).
Figure 9 shows pressure differences that remain negative throughout the day, with large offsets of approximately 750–1900 Pa in several curves. Please specify whether these are surface-pressure differences, sea-level-pressure differences, or pressure differences at a common geometric height, and define the reference point and sign convention.
- Provide sufficient information to reproduce and interpret the tracer experiments (Section 2.3; Figures 10–12).
The experimental description does not specify the tracer emission rates or total emitted mass, whether source strengths are identical among cities, or how the 2–50 m injection interval is mapped onto the model layers.
Specific issues:
- Figure 1: The black source diamonds are difficult to distinguish from the coastline and boundary lines. Use contrasting outlines or larger symbols and adjust the arrows. Align the two columns of station labels, identify Abidjan Airport/METAR separately, and provide full station names and codes in the caption or an accompanying table. Expand TAHMO and other unexplained abbreviations.
- Lines 140–142: Explain that “warm rain” refers to precipitation from liquid-phase clouds and why its imperfect detection matters for selecting a non-rainy case. Link this limitation explicitly to the station-rainfall checks. The product/version used should be specified, and the reader should be told how uncertainty in the rain/no-rain classification was handled.
- Figures 2 and A1: These appear to be imagery exported or captured from NASA Worldview. Provide the imagery product, platform, acquisition date/time, and source acknowledgment, and check the reuse conditions of any non-NASA overlays. Prepare clear publication-quality panels, remove unnecessary interface elements, and annotate the cloud feature interpreted as the sea-breeze front. Arrange the panels chronologically. The black swath gap in Figure A1 should be identified as missing coverage. A screenshot appearance alone does not establish that the imagery cannot be used.
- Figure 6: Remove the incomplete outer frame or use consistent panel borders. Revise the caption to identify all three dataset pairs and remove the duplicated “and” before surface pressure. Specify sample sizes and explain whether non-significant correlations are masked; the caption currently describes all displayed values as significant.
- Lines 200–204 and 228–230: Please reconcile the explanation invoking the coarser resolution of ECMWF with the stated nominal grid spacings of 9 km for ECMWF and 13 km for ICON-ART. If the argument concerns effective orographic resolution or different terrain datasets, explain this specifically.
- Lines 285–287 and 427–429: The interval from 1200 to 2100 UTC is nine hours, not six. Also reconcile the stated maximum front position at 7° N around 2100 UTC with the feature described at 9° N around 0000 UTC. Distinguish the sea-breeze front from the ITD and tracer plume, and express penetration as distance from the local coastline rather than a common latitude alone.
- Figure 3 and Figure A2 captions: Use consistent time reference. Figure 3 mixes LT and UTC, although they differ by one hour between the countries listed. Figure A2 is described in the text as showing 9–10 January, but its caption states 8–9 January. Verify the dates and indicate them in each panel. In Figure 12, give the nighttime release explicitly because the interval crosses midnight.
- Figure cross-references: Lines 307–308 refer to Annex A5 for early tracer plumes, whereas Figure A5 shows temperature gradients. Also, Figure 10a–i includes times on both 8 and 9 January, rather than only 0600–1800 UTC on 8 January. The vertical evolution described around Lines 364–365 should be checked against Figure 12e, rather than Figure 12d. Audit all appendix and panel references.
- Vertical transport descriptions: Lines 408–409 place the offshore transport above about 800 hPa, whereas Lines 434–435 describe transport between 875 and 800 hPa. Make the altitude/pressure range consistent and avoid treating pressure levels as fixed heights above ground. In the Figure 11 caption, replace “raw” with “row” and assign 875 hPa to the third column and 800 hPa to the fourth column unambiguously.
- Variables and terminology: Figure 9 displays differences in 2 m air temperature, so Lines 291–296 should distinguish these from land/sea surface temperatures. Define the gradient calculation used in Figures 7–8 and its direction or magnitude. Figures A7–A8 show signed values, which cannot be wind-speed magnitudes; identify the plotted wind component(s) and explain the arrows. Use “LSB” for the circulation as a whole and “sea breeze” for its daytime onshore branch where appropriate.
- Presentation and order: Section 3.1 devotes substantial space to a map-by-map description of Figure 4. Condense this material and foreground validation measures relevant to LSB dynamics. In Section 3.3, organize the Figure 12 discussion consistently as meridional, zonal, and vertical evolution, and reduce repeated descriptions. A concluding paragraph separating diagnosed transport from inferred air-quality implications would improve the transition to Section 3.4.
- Language and references: Correct “clouds parallels to the coast” (Line 159), “northeasterly winds prevails” (Line 197), “the pollutant plume spots growth” (Line 308), “compare to the ocean” (Line 293), “This sensitive study” (Line 381), and “station netowrk” in the acknowledgments. Standardize unit spacing, e.g., “2 m,” “10 m,” and “200 km.” The reference list already contains several recent studies, so its adequacy should be judged by relevance rather than an arbitrary age threshold. Update the discussion where recent work improves the account of sea-land-breeze ventilation.
Citation: https://doi.org/10.5194/egusphere-2026-3128-RC2
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- 1
General assessment
This manuscript investigates the role of land–sea breeze circulation in the dispersion of pollutants emitted from major coastal cities along the Guinea Coast of West Africa, using passive-tracer simulations with ICON-ART for a dry-season case from 8 to 10 January 2021.
The topic is scientifically relevant and well suited to Atmospheric Chemistry and Physics. The dry season over southern West Africa remains less documented than the West African monsoon season, and the interaction between coastal circulations, Harmattan flow, vertical mixing, and pollutant transport is an important question for regional air quality. The manuscript is generally well written and presents a coherent physical interpretation of the simulated tracer transport.
However, I recommend major revisions. The study has clear potential, but several methodological choices need stronger justification, and some conclusions should be made more cautious. The authors should better justify the representativeness of the selected case, strengthen the discussion of the tracer-emission strategy, clarify the link with previous tracer studies, address the possible role of radiatively active aerosols, better support the vertical return-flow mechanism, and make the conclusions on coastal pollution attenuation more cautious.
Major comments
1. Representativeness of the selected case
The study is based on a single three-day dry-season case. The authors describe this period as representative, but this representativeness is not sufficiently demonstrated. The selection is based mainly on the identification of a non-rainy land–sea breeze event using MODIS, IMERG and TAHMO data.
I recommend adding a simple climatological context, for example using ERA5 or ECMWF analyses. The authors could compare the selected period with January or dry-season climatology in terms of ITD position, Harmattan intensity, winds at 925–850–800 hPa, land–sea thermal contrast, boundary-layer height, and land–sea breeze frequency or intensity. This would clarify whether the selected case is typical or exceptional.
2. Passive-tracer emission strategy
The passive-tracer approach is appropriate for isolating transport mechanisms, but the emission strategy needs better justification. In the manuscript, tracers are emitted only from 06:00 to 18:00 UTC on the first day, with a separate nighttime sensitivity experiment. This design helps isolate daytime and nighttime transport, but it does not represent continuous urban emissions.
This is important because the main conclusions concern inland transport, vertical mixing, and return flow toward the Atlantic Ocean. The plume structure strongly depends on the timing of the release. I recommend adding, or at least discussing, a complementary experiment with continuous emissions over the full simulation period, ideally with a repeated diurnal cycle. This would help distinguish meteorological effects from artefacts linked to the imposed emission time window.
3. Link with previous tracer studies over southern West Africa
The manuscript cites previous DACCIWA-related studies, but the methodological link with earlier tracer experiments should be made more explicit. Previous work already used city-specific tracers emitted from coastal cities such as Abidjan, Accra, Lomé, Cotonou and Lagos to investigate inland transport.
The novelty of the present study is real: it focuses on the dry season, uses ICON-ART, and highlights the role of land–sea breeze circulation and northeasterly return flow aloft. However, the authors should more clearly explain what is inherited from previous approaches, what is new here, and why the tracer-emission protocol differs from earlier studies:
https://acp.copernicus.org/articles/22/3251/2022/
https://acp.copernicus.org/articles/19/473/2019/
https://journals.ametsoc.org/view/journals/bams/99/1/bams-d-16-0256.1.xml
4. Absence of aerosol–radiation and aerosol–cloud interactions
Although ICON-ART can represent aerosols, trace gases, chemistry and interactions with radiation and clouds, the present study uses passive tracers only. This is acceptable for isolating transport, but it is also a significant limitation, especially during the dry season, when Harmattan dust and biomass-burning aerosols are strongly present in the region.
The authors should discuss whether neglecting radiatively active aerosols could affect the simulated meteorology itself, including surface temperature, boundary-layer development, atmospheric stability, low-level winds, clouds, and the strength or inland penetration of the land–sea breeze. A sensitivity test with radiatively active aerosols would be valuable. If this is not feasible, the limitation should be stated more clearly and supported by comparison with aerosol reanalyses or satellite AOD products.
5. Validation of the vertical return-flow mechanism
One of the most interesting results is the simulated transport of part of the plume back toward the Atlantic Ocean between approximately 875 and 800 hPa. This mechanism is physically plausible, but it is mostly supported by the model simulation itself.
Because this result is central to the paper, the authors should provide stronger support for the vertical wind structure. A comparison with ERA5 or ECMWF winds at 925, 850 and 800 hPa would already be helpful. Radiosonde data, if available, or aerosol products such as CAMS, MODIS, MERRA-2 or CALIOP could also help contextualize the presence of elevated aerosol layers during the selected period.
6. Model resolution and land–sea breeze representation
The authors performed simulations at 13 km and 6 km resolution, but the main results are shown at 13 km because the 6 km domain does not capture the full transport pattern. This choice should be better justified. A 13 km grid spacing may be suitable for regional transport, but it is relatively coarse for representing coastal gradients, urban emission sources, and the detailed structure of the land–sea breeze front.
The authors should clarify which aspects of the land–sea breeze are robust at this resolution and which may be smoothed or unresolved. Showing selected 6 km results for the coastal structure of the breeze could strengthen the analysis.
7. Meteorological validation of key variables
The model evaluation shows reasonable agreement for temperature and pressure, but the validation is weaker for humidity and winds. This is important because the conclusions depend directly on low-level winds, vertical mixing and boundary-layer evolution.
The evaluation should be more focused on the variables controlling the proposed mechanism: 10 m wind direction and speed, vertical wind structure at 925–850–800 hPa, timing and inland propagation of the land–sea breeze front, and boundary-layer height.
8. Conclusions on pollution reduction should be more cautious
The manuscript concludes that land–sea breeze activity may attenuate coastal urban pollution. This is plausible in terms of tracer ventilation, but the statement is too strong given the current setup. The study uses passive tracers with infinite lifetime and no chemistry, deposition, aerosol formation, or observational validation of pollutant concentrations.
I recommend using more cautious wording. The study demonstrates redistribution and ventilation of passive tracers in one dry-season case, not necessarily a reduction in real pollutant exposure or chemically active pollution.
9. Figures and color scales
Several tracer figures are difficult to interpret quantitatively. Since the conclusions rely on plume structure, dilution, vertical redistribution and return transport, the color scales and contour levels are critical. The authors should clarify the tracer units, use common color scales where comparisons are intended, consider logarithmic scales for diluted plumes, and distinguish more clearly between column-integrated tracer mass, concentration at individual pressure levels, and near-surface concentrations.