Intercity Comparison of Aerosol Optical Depth and Modelled Surface Dust Concentrations across Seven West African Cities (2019–2025)

Abstract

Satellite aerosol optical depth (AOD) is widely used as a proxy for surface particulate pollution where ground monitoring is scarce, including in West Africa. This study tests whether this substitution holds for mineral dust by comparing MODIS MAIAC AOD (550 nm) with MERRA-2 modelled surface dust concentrations in seven cities (Dakar, Bamako, Niamey, Korhogo, Bouaké, Cotonou and Abidjan) over 2019–2025, on days matched by city and date, in November–March and April–October, using stationary-bootstrap 95 % confidence intervals. City rankings differ between the two quantities: Niamey has the highest surface dust in both windows, whereas the highest AOD occurs in Cotonou in November–March and in Dakar in April–October. Surface dust is higher in November–March in all seven cities, but AOD follows this seasonality only in Cotonou and Abidjan; it is opposite in Dakar and Bamako, and no clear contrast is detected elsewhere. The daily AOD–dust correlation is positive everywhere in November–March but becomes indistinguishable from zero in Abidjan in April–October. These dissociations persist even though MERRA-2 assimilates MODIS AOD. In April–October, AOD is less often available in the southern and central cities, where observed days are dustier than average. In practice, AOD captures part of the day-to-day dust variability within each city in November–March, but it should not be used to rank cities by surface dust concentration, to infer the seasonal cycle of surface dust, or as a proxy for surface dust on the Gulf of Guinea coast during the monsoon.