Isotopic and Molecular Characterization of Incomplete Combustion Products in sub-Saharan Africa

Abstract: Sub-Saharan Africa (SSA) is a global hotspot for incomplete combustion emissions from large-scale savanna fires, agricultural burning and growing city emissions. However, air pollution is understudied in the SSA region, resulting in a gap in the scientific understanding of regional emissions, source contributions, and impacts of air pollutants on regional climate and health. This thesis aimed to characterize fine particulate matter (PM2.5) and particularly to investigate the sources of black carbon (BC), polycyclic hydrocarbons (PAHs) and carbon monoxide (CO) in both the urban and the remote SSA atmosphere. Field sampling was conducted in Nairobi and at two regional background sites, Rwanda Climate Observatory and Mt. Kenya Global Atmospheric Watch (GAW) station.Air pollution in Nairobi city exceeded the World Health Organization (WHO) health guidelines (Paper I and IV). Of particular concern is the constantly elevated BC and PAHs (particularly toxic PM2.5 components) levels in Nairobi, also seen in some other SSA cities. Radiocarbon-based source quantification shows fossil fuel combustion emissions, specifically from traffic, dominate BC emissions in Nairobi (Paper I). Similarly, molecular source tracers suggested PAHs likely originated from combustion sources (Paper IV). The aerosol regime of the regional background SSA atmosphere was, in sharp contrast, dominated by savanna fire emissions (Paper II). In the remote background regimes integrating large SSA emission footprints, we find strong seasonality for all investigated aerosol species, with highly elevated concentrations coinciding with upwind savanna fires. The dual-carbon isotopes (Δ14C and ẟ13C) show a clear and strong savanna-burning imprint, accounting for up to 95% of BC burden. Similarly for CO, isotopic constraints reveal seasonality with savanna fires contributing over 70% of CO in the background atmosphere. This is significantly higher than the global average primary contribution (55 ± 5%). An almost exclusive primary CO component in Nairobi was observed, largely reflecting the fossil combustion emissions in the city (Paper III). Taken together, these largely isotope-based quantitative source constraints of incomplete combustion emissions in SSA provide evidence for informed policy decisions, with implications also for parameterization of satellite fires and bottom-up emission inventories as well as for regional climate and chemical-transport modelling.