| 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.
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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.