Measurement of Atmospheric Black Carbon Concentration in Rural and Urban Environments: Cases of Lamto and Abidjan

Abstract

Black carbon is one of the primary aerosols directly emitted from biomass known to have strong absorbing properties. The INDAAF and PASMU observational field campaigns which took place (2018) in Abidjan (urban area) and Lamto (rural area) allow the analysis of Black carbon concentration at different time scales through real-time measurements using an analyzer named Aethalometer AE-33. Results presented here show at Lamto: 1) for the diurnal scale an average of 1.71 ± 0.3 μg·m−3 (0.34 ± 0.09 μg·m−3) in the dry (wet) season; 2) for the monthly scale an average of 1.14 ± 0.84 μg·m−3; 3) on the seasonal scale, an average of 2.2 ± 0.02 μg·m−3 (0.6 ± 0.19 μg·m−3) in the dry (wet) season. The black carbon variation at Lamto is seasonal with an amplification factor of 85.6. Regarding the urban area of Abidjan, due to sampling issues, our analyses were limited to daily, diurnal and weekly time scales. We observed: a) at a daily scale an average of 5.31.± 2.5 μg·m−3, b) diurnal scale, an average ranging from 6.87 to 13.92 μg·m−3. The analysis indicated that emissions from urban areas are more related to social and economic activities, with weekday concentrations (7.24 μg·m−3) higher than concentrations over the weekend (e.g. Saturday 6.59 μg·m−3 and Sunday 6.00 μg·m−3). Moreover, BC concentration in Abidjan is quite noticeable compared to that of rural areas (Lamto). The ratio between the maximum values of the two areas is of the order of 5.86. In addition, concentrations in some urban areas are slightly above the daily threshold set by the WHO (10 μg·m−3). Therefore, the levels of urban BC concentrations are alarming whilst rural BC concentrations remain below daily WHO thresholds and are of the same magnitude as those of West African megacities. This study underlies that BC concentrations at Lamto are mainly related to biomass combustion sources while those from urban areas are related to traffic sources. The latter is permanently active, unlike those in rural Lamto, which is seasonal.