ALBERT

Description: Urban air pollution in West Africa has yet to be well characterized. In the frame of DACCIWA (Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa) program, intensive measurement campaigns were performed in Abidjan (Côte d'Ivoire) and Cotonou (Benin), in dry (January 2016 and 2017) and wet (July 2015 and 2016) seasons, at different sites chosen to be representative of African urban combustion sources, i.e., domestic fires (ADF), traffic (AT) and waste burning (AWB) sources in Abidjan and traffic source in Cotonou (CT). Both the size distribution of particulate matter (PM) and their chemical composition including elemental carbon (EC), organic carbon (OC), water-soluble organic carbon (WSOC), water-soluble inorganic ions (WSI) and trace metals were examined. Results show very high PM concentrations at all sites and a well-marked seasonality as well as a strong spatial variation. The average PM2.5 mass concentrations during the wet season are 517.3, 104.1, 90.3, and 69.1 µg m−3 at the ADF, CT, AT, and AWB sites, respectively. In the dry season, PM2.5 concentrations decrease to 375.7 µg m−3 at the ADF site, while they increase to 269.7, 141.3, and 175.3 µg m−3 at the CT, AT, and AWB sites, respectively. The annual PM2.5 levels at almost all sites are significantly higher than the WHO guideline level of 10 µg m−3. As for PM mass, (EC) and (OC) concentrations are also maximal at the ADF site, accounting for up to 69 % of the total PM mass. Such a high content is mainly linked to wood burning for domestic cooking and commercial food smoking activities. Dust contributions are dominant at CT (57 %–80 %), AT (20 %–70 %), and AWB (30 %–69 %) sites and especially in the coarse and fine-particle modes at the CT site and in the coarse fraction at the AT site, which may be explained by the impact of long-range desert-dust transport and resuspended particles from the roads, in addition to anthropogenic sources. The contributions of WSI to the total PM mass, mainly driven by chloride, nitrate, and calcium in the fine and/or large particles, are highly variable according to the sites but remain less than 30 %. Values are generally 1–3 times higher in the wet season than in the dry season. This is due not only to anthropogenic emissions but also to nitrate formation by reaction processes and natural emissions. The concentrations of trace elements reflect well the trends in dust at the traffic and AWB sites, with a predominance of Al, Na, Ca, Fe, and K, keys markers of crustal dust. This study constitutes an original database that characterizes specific African combustion sources.

Description: 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.