Mass concentration, optical depth and carbon composition of particulate matter in the major southern West African cities of Cotonou (Benin) and Abidjan (Côte d'Ivoire)

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Abstract

Air quality degradation is a major issue in the large conurbations on the shore of the gulf of Guinea. We present for the first time PM<sub>2.5</sub> time series collected in Cotonou, Benin and Abidjan, Cote d'Ivoire from February 2015 to March 2017. Measurements were performed in the vicinity of major combustion aerosol sources: Cotonou/Traffic (CT), Abidjan/Traffic (AT), Abidjan/Waste Burning (AWB) and Abidjan/Domestic Fires (ADF). We report the weekly PM<sub>2.5</sub> mass and carbonaceous content as Elemental (EC) and organic (OC) carbon concentrations. We also proceed to the measurements of the Aerosol Optical Depth (AOD) and the Angström exponent in both cities. The average PM<sub>2.5</sub> mass concentrations were 32&amp;thinsp;&amp;plusmn;&amp;thinsp;32&amp;thinsp;&amp;mu;g/m<sup>3</sup>, 32&amp;thinsp;&amp;plusmn;&amp;thinsp;24&amp;thinsp;&amp;mu;g/m<sup>3</sup>, 28&amp;thinsp;&amp;plusmn;&amp;thinsp;19&amp;thinsp;&amp;mu;g/m<sup>3</sup> at traffic sites CT and AT and waste burning site AWD, respectively. The domestic fire site shows concentration of 145&amp;thinsp;&amp;plusmn;&amp;thinsp;69&amp;thinsp;&amp;mu;g/m<sup>3</sup> due the contribution of smoking and grilling activities. The highest OC and EC concentrations were also measured at ADF at 71&amp;thinsp;&amp;plusmn;&amp;thinsp;29&amp;thinsp;&amp;mu;g/m<sup>3</sup> and 15&amp;thinsp;&amp;plusmn;&amp;thinsp;9&amp;thinsp;&amp;mu;g/m<sup>3</sup>, respectively. While the other sites present OC concentration between 8 and 12&amp;thinsp;&amp;mu;g/m<sup>3</sup> and EC concentrations between 2 and 7&amp;thinsp;&amp;mu;g/m<sup>3</sup>. The OC&amp;thinsp;/&amp;thinsp;EC ratio is 4.3 at CT and 2.0 at AT. This difference highlighs the influence of 2-wheels vehicles using 2-stoke mix in Cotonou compared to Abidjan. AOD were rather similar in both cities with a mean value of 0.58 in Cotonou and 0.68 in Abidjan. The seasonal cycle is dominated by the large increase in surface mass concentration and AOD during the main dry season (Dec.&amp;ndash;Feb.) as expected due to mineral dust advection and biomass burning activities. The lowest concentrations are observed during the minor dry season (Aug.&amp;ndash;Sept.) due to an increase in surface wind speed leading to a better ventilation. On the other hand, the high PM<sub>2.5</sub>&amp;thinsp;/&amp;thinsp;AOD ratio in the minor wet season (Oct.&amp;ndash;Nov.) indicates the stagnation of local pollution.

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Djossou, J., Léon, J. F., Barthélemy Akpo, A., Liousse, C., Yoboué, V., Bedou, M., … Awanou, C. N. (2018). Mass concentration, optical depth and carbon composition of particulate matter in the major southern West African cities of Cotonou (Benin) and Abidjan (Côte d’Ivoire). Atmospheric Chemistry and Physics, 18(9), 6275–6291. https://doi.org/10.5194/acp-18-6275-2018

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