Abstract. 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.