Physico-chemical characterization of urban aerosols from specific combustion sources in West Africaat Abidjan in Côte d’Ivoire and Cotonou in Benin in the frame of DACCIWA program

Abstract

Abstract. Air pollution in West Africa is far to be well characterized. It was the rationale of the Air Pollution and Health work package in the DACCIWA (Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa) program. Intensive measurement campaigns were running in two West African capitals (Abidjan in Côte d’Ivoire and Cotonou in Benin), to examine size distribution of the particulate matter (PM) concentrations and their chemical composition (Elemental Carbon (EC), Organic Carbon (OC), Water-soluble organic carbon (WSOC), Water-soluble inorganic ions (WSI) and trace metals). This work aims to characterize PM from different sites in Abidjan, the economic capital of Cote d’Ivoire, typical of Domestic Fire (ADF), Traffic (AT) and Waste Burning (AWB) and Cotonou, the capital of Benin, representative of Traffic (CT). These selected sites, impacted by a large amount of pollution levels, are representative of the main combustion sources prevailing in South West Africa during dry and wet seasons. To address this concern, intensive campaigns in Abidjan and Cotonou have been conducted in July (2015 and 2016) and January (2016 and 2017). Results show a well-marked seasonality, inter-annual and spatial variabilities and the PM levels at the studied areas are generally higher than the WHO guidelines. The average mass concentrations in the wet season were 90.3, 104.1, 69.1 and 517.3 μg m−3 at AT, CT, AWB and ADF sites, respectively. The largest value at ADF site is due to the contribution of smoking and roasting activities. By contrast in the dry season, the concentrations increase to 141.3, 269.7 and 175.3 μg m−3 at AT, CT and AWB site, respectively whereas at ADF site concentration decreases to 375.7 μg m−3. The chemical aerosol mass closure shows that dust contributed for 25–65 % at the both traffic and AWB sites, and 10–30 % at ADF with a clear seasonal cycle. A large variability of POM is observed with contribution range of 37–68 % at ADF, 20–42 % at AT, 10–34 % at AWB and 15–22 % at CT. The contribution of WSI to bulk PM (lower than 20 %) is 2–3 times larger in wet season than dry season, except at ADF site where no season variation is observed. The most dominant species in WSI fraction at ADF are chloride (18–36 % of the total ions), potassium (8–22 %) and calcium (13–25 %), while at the rest of the sites, nitrates (21–36 %), chlorides (6–30 %) and sulfates (9–20 %) are higher. At all sites, the proportion of EC is twice higher in dry season than in wet season. Carbonaceous aerosol (sum of EC and POM) and dust particles are the two major contributors to the different particle fractions with carbonaceous aerosol predominant at Abidjan and dust at Cotonou. The highest carbonaceous aerosol contribution is obtained at ADF (up to 75 % of total PM), while at the other sites its contribution ranges between 18 and 35 %. WSOC levels are higher at the traffic sites during the dry season, while during the wet season they are maximum at ADF and AWB sites. Element trace characterization is also determined, showing predominance of Al, Na and Ca followed by Fe, K and Mg. Our study highlights the contribution of different traffic emissions in two major West African cities in atmospheric aerosol composition but also the one of domestic fire and waste combustion sources. It constitutes an original database to characterize urban air pollution for specific African combustion sources.