Unravelling the chemical speciation and sources of PM2.5-bound polycyclic aromatic hydrocarbons (PAHs) in Nairobi, Kenya

Air pollution is a major environmental human health risk in African cities, largely due to the rapidly growing urban population, unregulated traffic and industrial emissions, and inadequate regulations and pollution control policies. Currently, about a million premature deaths are linked to air pollution in Africa, and the related health burden is projected to increase. However, data on PM_2.5 chemical characterization and source contribution, needed to address the air pollution challenges and inform policies, is currently limited and/ or inadequate for most African cities. In this view, year-round PM_2.5 quartz filter samples were collected in Nairobi city and analyzed for mass concentration and PAHs (known for their carcinogenic and mutagenic properties). The average PM_2.5 concentration was determined at 27 ± 6 µgm^-3, exceeding the World Health Organization 24-h health guideline. The PAHs concentration ranged between 5 - 20 ng m^-3 and were dominated by the heavy molecular weight PAHs (>4 rings). Molecular diagnostic ratios further revealed that the PAHs predominantly originate from combustion sources, such as traffic emissions. Overall, this study signal to a severe health concern, and provide information that can be exploited for policy formulation and air pollution mitigation strategies in Nairobi, as well as other African cities.