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. Southern West Africa (SWA) is influenced by large numbers of aerosol particlesof both anthropogenic and natural origins. Anthropogenic aerosol emissionsare expected to increase in the future due to the economical growth of Africanmegacities. In this paper, we investigate the aerosol optical depth (AOD) inthe coastal area of the Gulf of Guinea using sun photometer and MODISsatellite observations. A network of lightweight handheld sun photometershave been deployed in SWA from December 2014 to April 2017 at five differentlocations in Côte d'Ivoire and Benin. The handheld sun photometermeasures the solar irradiance at 465, 540 and 619 nm and is operatedmanually once per day. Handheld-sun-photometer observations are complementedby available AERONET sun photometer observations and MODIS level 3 time seriesbetween 2003 and 2019. MODIS daily level 3 AOD agrees well with sunphotometer observations in Abidjan and Cotonou (correlation coefficient R=0.89and RMSE = 0.19). A classification based on the sun photometer AOD andÅngström exponent (AE) is used to separate the influence of coarsemineral dust and urban-like aerosols. The AOD seasonal pattern is similar forall the sites and is clearly influenced by the mineral dust advection fromDecember to May. Sun photometer AODs are analyzed in coincidence with surfacePM2.5 concentrations to infer trends in the particulate pollutionlevels over conurbations of Abidjan (Côte d'Ivoire) and Cotonou(Benin). PM2.5-to-AOD conversion factors are evaluated as afunction of the season and the aerosol type identified in the AEclassification. The highest PM2.5 concentrations (up to 300 µg m−3) are associated with theadvection of mineral dust in the heart of the dry season (December–February).Annual means are around 30 µg m−3, and 80 % of days inthe winter dry season have a value above 35 µg m−3, whileconcentrations remain below 16 µg m−3 from May to September.No obvious trend is observed in the 2003–2019 MODIS-derived PM2.5time series. However the short dry period (August–September), when urban-likeaerosols dominate, is associated with a monotonic trend between 0.04 and0.43 µgm-3yr-1 in the PM2.5 concentrations overthe period 2003–2017. The monotonic trend remains uncertain but is coherentwith the expected increase in combustion aerosol emissions in SWA.

atmospheric chemistry and physics

PM&amp;lt;sub&amp;gt;2.5&amp;lt;/sub&amp;gt; surface concentrations in southern West African urban areas based on sun photometer and satellite observations