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)

Abstract

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 2.5 time series collected in Cotonou, Benin, and Abidjan, Côte 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/landfill (AL) and Abidjan/domestic fires (ADF). We report the weekly PM 2.5 mass and carbonaceous content as elemental (EC) and organic (OC) carbon concentrations. We also measure the aerosol optical depth (AOD) and the Ångström exponent in both cities. The average PM 2.5 mass concentrations were 32 ± 32, 32 ± 24 and 28 ± 19 µg m −3 at traffic sites CT and AT and landfill site AL, respectively. The domestic fire site shows a concentration of 145 ± 69 µg m −3 due to the contribution of smoking and roasting activities. The highest OC and EC concentrations were also measured at ADF at 71 ± 29 and 15 ± 9 µg m −3 , respectively, while the other sites present OC concentration between 8 and 12 µg m −3 and EC concentrations between 2 and 7 µg m −3 . The OC / EC ratio is 4.3 at CT and 2.0 at AT. This difference highlights the influence of two-wheel vehicles using gasoline in Cotonou compared to that of four-wheel vehicles using diesel fuel in Abidjan. AOD was rather similar in both cities, with a mean value of 0.58 in Cotonou and of 0.68 in Abidjan. The seasonal cycle is dominated by the large increase in surface mass concentration and AOD during the long dry season (December-February) as expected due to mineral dust advection and biomass burning activities. The lowest concentrations are observed during the short dry season (August-September) due to an increase in surface wind speed leading to a better ventilation. On the other hand, the high PM 2.5 / AOD ratio in the short wet season (October-November) indicates the stagnation of local pollution.

Personal exposure to PM<sub>2.5</sub> emitted from typical anthropogenic sources in Southern West Africa (SWA): Chemical characteristics and associated health risks

“…Total carbon (TC) was the most important chemical species in PE PM2.5, accounting for 24.4%±4.5%, 16.6%±2.0% and 17.8%±4.9% of PE PM2.5 mass of women, students and drivers, respectively. High level of OC proves the strong contribution of combustion sources to PE PM2.5 in SWA in this study(Djossou et al, 2018;Ouafo-Leumbe et al, 2017). The OC and EC concentrations varied significantly, ranging from 28.3 to 460.0, 8.0 to 189.9 and 14.7 to 65.1 μg m -3 for OC and 1.5 to 31.1, 0.8 to 35.1 and 1.9 to 18.2 μg m -3 for EC for women, students and drivers, separately.…”

“…In a previous study of Djossou et al (2018) about OC/EC at ambient (A) environment, OC/EC in personal exposure samples were about 1.2 and 2.5 times of the ambient values in dry and wet seasons for women at DF, 1.7 and 2.8 times for students at WB, and 1.1 and 2.0 times for drivers at MT. Therefore, the higher OC/EC values in personal exposure samples resulted from some specific individual's activities and potentially contaminated microenvironments (Crist et al, 2008;Meng et al, 2009).…”

“…( Figure S1d), leading to a very high personal PM2.5 exposure level indoors during the cooking time in this area (especially for student B who was in charge of cooking at home sometimes recorded in the activity logging and questionnaire). Meanwhile, the ambient PM2.5 sampling equipment at WB was neither fixed very close to (blue marker in Figure 1C) nor in the downwind direction of the landfill (Djossou et al, 2018), which direct suggests the huge differences between the ambient and personal exposure PM2.5 concentrations.…”

“…PE PM2.5 concentrations reducing could be attributed to the occurrence of increased levels of rainfall in wet season in SWA (Table 1), which causes the large reduction of road dust exposed to drivers and limits the garbage spontaneous combustion significantly around students. Moreover, large scale transport of mineral dust and combustion aerosols emitted by savannah wild fires contribute significantly to the aerosol load during the dry season (Djossou et al, 2018), which is more important at WB and MT than at DF (crowded community environment).…”

“…The average PE PM2.5 levels are compared to the weekly ambient PM2.5 concentrations (Djossou et al, 2018) obtained in the same area and similar sampling period. The average PE PM2.5 were 3.0 and 2.0 times the ambient values found at DF, and 6.1 and 8.8 times at MT in dry and wet seasons, respectively.…”