Assessment and Mapping of Black Carbon and PM2.5 Variability: A Case Study in Nairobi city

Abstract

Airborne particulate matter is of great concern in the global environment due to associated negative effects on human health and environment, and its contribution to climate change. Reported here is carried out evaluation and mapping of PM2.5 mass concentration and Black Carbon (BC) in Nairobi city. Air sampling was conducted at nine selected sites within Nairobi city between 17/05/2016 to 22/06/2016. Sampling was done for 8 hours and samples analyzed at the Institute of Nuclear Science laboratory for Particulate Matter (PM2.5) and trace elements (Pb, Mn, Zn, Zr, Fe and Cu). BC was analyzed at Gothenburg University, Sweden. The mean 8-h PM2.5 concentrations in the nine sampling sites selected for the study ranged between 147 ± 89 μg m-3 and 52 ± 4 μg m-3. Mean BC concentration for the nine sampling sites ranged from 8.6 ± 3.3 μg m-3 to 2.1 ± 1.2 μg m-3. OLS dispersive models were further used to map out the pollution levels across the city. Elemental content of the sampled fine particulates included Cu, Mn, Zn, Fe, Pb, and Zr which were found to be above 50, 1210, 470, 200, 30 and 480 ng m-3 which was their respective detection limits. The detected trace elements accounted for 15 % of the species in PM2.5, while BC accounted for 8 %. The highest mean concentration was recorded for Fe, followed by Mn and Cu at 2800 ± 3800 ng m-3, 2600 ± 42 ng m-3, and 2200 ± 38 ng m-3, respectively. The WHO health guideline for 24-h mean PM2.5 concentrations is 25 μg m-3. This study represents an eight hour mean hence it was not possible to directly compare the concentrations with the WHO guidelines. However, it is evident that the values are very high if we consider the low activity hours of the night. This means that the population like those driving in jam, traffic police, pedestrians, or individuals living in apartments near the roadways, are all constantly being exposed to high pollution levels. Long-term exposure to these pollutants can have negative effects on public health such as respiratory and cardiovascular ailments, hence there is need for intervention measures.