PM<sub>2.5</sub> surface concentrations in southern West African urban areas based on sun photometer and satellite observations

Abstract. Southern West Africa (sWA) is influenced by large amounts of aerosol particles of both anthropogenic and natural origins. Anthropogenic aerosol emissions are expected to increase in the future due to the economical growth of African megacities. In this paper, we investigate the aerosol optical depth (AOD) in the coastal area of the Gulf of Guinea using sun photometer and MODIS satellite observations. We use a lightweight handheld sun photometer measuring the solar irradiance at 465, 540 and 619 nm operated manually every day from December 2014 to April 2017 at 5 different locations in Côte d'Ivoire and Bénin. Handheld sun photometer observations are complemented by available AERONET sun photometer observations and MODIS level 3 time series between 2003 and 2018. MODIS daily level 3 AOD agrees well with sun photometer observations in Abdidjan and Cotonou (correlation coefficient R = 0.89 and RMSE = 0.19). A classification based on the Angstrom Exponent is used to separate the influence of coarse mineral dust and urban-like aerosols. The AOD seasonal pattern is similar for all the sites and is clearly influenced by the mineral dust advection from December to May. AODs are analyzed in coincidence with surface PM2.5 concentrations to infer trends in the particulate pollution levels over conurbation of Abidjan (Côte d'Ivoire) and Cotonou (Bénin). PM2.5 to AOD conversion factors are evaluated as a function of the season and the aerosol type identified in the AE classification. Highest PM2.5 concentrations (up to 300 μg/m3) are associated to the advection of mineral dust in the heart of the dry season (December–February). From December to March the median concentration above Abidjan and Cotonou is around 40 μg/m3, while it is around 20 μg/m3 during the rest of the year. Considering only the days during which the AOD belongs to the urban-like aerosol category, we observe a significant trend S = 0.32 μg/m3/year in the PM2.5 concentrations over the period 2003–2017. This trend leads to an increase of 5 ± 3 μg/m3 over 15 years and is coherent with the expected increase in combustion aerosol emissions in sWA.