Dominant Contribution of Pyrogenic Sources to PM2.5-Bound Polycyclic Aromatic Hydrocarbons in Nairobi, Kenya

Abstract

Air pollution is the leading environmental cause of premature death and an impediment to sustainable development in Africa, where exposure levels are high, yet data are scarce. This study provides year-round speciation and source identification for PM_2.5-bound polycyclic aromatic hydrocarbons (PAH) in the East African city of Nairobi. The ∑₁₉PAH concentrations ranged between 5–20 ng m^–3 with an average of 11 ± 4 ng m^–3 in the urban background and were dominated by heavy molecular weight compounds (four or more fused benzene rings). The PAH loadings in Nairobi stayed rather invariant through the year with limited seasonal variability. The observed PAH concentrations in Nairobi were higher than those reported in cities with more stringent emission controls. Furthermore, the calculated benzo[a]pyrene equivalent in Nairobi’s background atmosphere exceeded 1 ng m^–3, signaling a severe health concern. Source identification using molecular diagnostic ratio analysis suggests the PAH in Nairobi are predominantly of pyrogenic (combustion of fossil fuels and biomass burning) origins. Overall, this study provides a baseline reference data set for future local and regional studies and contributes to scientific underpinnings to motivate the urgent need to develop emission control initiatives in Nairobi, among other rapidly growing African cities.

Scanty research exists on air pollution status and sources in rapidly growing African cities. This study reports high PAH emissions from combustion sources with implications for human health and pollution mitigation policies.