Abstract
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.
Highlights
The impact of anthropogenic pollution on environment and health has been demonstrated by numerous studies in Europe and North America, which have contributed to the implementation of emission reduction policies
Let us note that C particle contribution in bulk is relatively higher in the traffic and waste burning sites than at the Abidjan domestic fire (ADF) site (40 %), whereas F and UF particle contributions are on the order of 60 %
This paper presents the mass and the size-speciated chemical composition of particulate matter (PM) obtained during the dry and wet seasons in 2015, 2016, and 2017
Summary
The impact of anthropogenic pollution on environment and health has been demonstrated by numerous studies in Europe and North America, which have contributed to the implementation of emission reduction policies. Air pollution in Africa is far from being well characterized, it is suspected to be responsible for negative health outcomes (WHO, 2014) This is a major problem since Africa is an intense emitter of pollution from anthropogenic sources that include domestic fires, vehicular traffic, waste burning, and growing oil and mining industries. It has one of the fastest growing urban populations in the world, especially in West and East Africa (United Nations, 2019). It is important to recall the impact of biomass burning and dust sources in the African atmospheric composition, especially occurring during the dry season All of this results in a major degradation of urban air quality and an impact on the health of exposed populations. A few studies on this subject have been conducted in West Africa (Val et al, 2013; Dieme et al, 2012; Kouassi et al, 2009) despite the high atmospheric pollutant concentrations already measured to be on the same order as in Asian megacities and well above WHO international standards (WHO, 2014)
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