Ground-based monitoring of nitrogen dioxide in Kumasi, Ghana, and its comparison with satellite observations

Résumé

Air pollution poses an increasing public health risk in many African cities, where systematic monitoring is limited by the high cost of reference-grade instrumentation. Satellite-based sensors such as TROPOMI on Sentinel-5P provide global coverage of atmospheric pollutants, including nitrogen dioxide (NO 2 ), but their limited vertical resolution complicates the estimation of surface-level concentrations. Palmes diffusion tubes offer a low-cost, low-tech alternative for measuring NO 2 at the ground level. Combining satellite observations with data from ground-based diffusion tubes presents a promising approach for generating regional air quality maps, particularly in resource-limited settings. This study reports on the implementation of a pilot network of Palmes tubes in Kumasi, Ghana. To ensure sustainability and scalability, a local air quality laboratory was established to prepare and analyse the tubes using locally sourced materials and equipment. Validation against tubes prepared and analysed by an accredited external laboratory demonstrated satisfactory agreement. Measurements from March 2025 revealed a wide range of NO 2 concentrations, from an average of 7 µg m −3 in residential areas away from traffic and industry to peak values of 88 µg m −3 at heavily trafficked intersections – substantially exceeding the WHO 2021 guideline limits of 25 µg m −3 for 24 h exposure and 10 µg m −3 for annual mean concentrations. Averaged satellite data showed the accumulation of NO 2 plumes downwind of the city, though the relationship with surface-level measurements remains complex and needs further investigation. Column-to-surface ratios in Kumasi are significantly lower than those typically observed in European cities. Using a parametrization method based on ground-based measurements and TROPOMI retrievals, we find that TROPOMI underestimates tropospheric NO 2 column densities by roughly a factor of 2.5 in March 2025. This underestimation is attributed to higher aerosol loading over Ghana, which reduces retrieval sensitivity in the troposphere and increases reliance on unrepresentative a priori profiles.

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