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.

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 (PM_2.5) 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 PM_2.5 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.

Carbon monoxide (CO) concentrations in the troposphere are decreasing globally, with Africa as an exception. Yet, the region is understudied, with a deficit of ground-based observations and highly uncertain CO emission inventories. This paper reports multiyear observational CO data from the Mt. Kenya Global Atmosphere Watch (GAW) station, as well as summertime CO isotope observations from both Mt. Kenya and Nairobi, Kenya. The CO variability at Mt. Kenya is characterized by slightly increased concentrations during dry periods and a strong influence of short-Term pollution events. While some data gaps and differences in instrumentation complicate decadal-scale trend analysis, a small long-Term increase is resolved. High-pollution events are consistent with isotopic signal from downwind savanna fires. The isotope fingerprint of CO in Nairobi indicates an overwhelming dominance (near 100%) of primary emissions from fossil fuel combustion with implications for air pollution policy. In contrast, the isotope signature of CO intercepted at the large-footprint Mt. Kenya region suggests that at least 70% is primary sourced, with a predominance likely from savanna fires in Africa. Taken together, this study provides quantitative constraints of primary vs. secondary CO in the eastern Africa region and in urban Nairobi, with implications for satellite-based emission inventories as well as for chemical transport and climate modeling.

Vast black carbon (BC) emissions from sub-Saharan Africa are perceived to warm the regional climate, impact rainfall patterns, and impair human respiratory health. However, the magnitudes of these perturbations are ill-constrained, largely due to limited ground-based observations and uncertainties in emissions from different sources. This paper reports multiyear concentrations of BC and other key PM_2.5 aerosol constituents from the Rwanda Climate Observatory, serving as a regional receptor site. We find a strong seasonal cycle for all investigated chemical species, where the maxima coincide with large-scale upwind savanna fires. BC concentrations show notable interannual variability, with no clear long-term trend. The Δ¹⁴C and δ¹³C signatures of BC unambiguously show highly elevated biomass burning contributions, up to 93 ± 3%, with a clear and strong savanna burning imprint. We further observe a near-equal contribution from C3 and C4 plants, irrespective of air mass source region or season. In addition, the study provides improved relative emission factors of key aerosol components, organic carbon (OC), K⁺, and NO₃^–, in savanna-fires-influenced background atmosphere. Altogether, we report quantitative source constraints on Eastern Africa BC emissions, with implications for parameterization of satellite fire and bottom-up emission inventories as well as regional climate and chemical transport modeling. 

Rapid urbanization and population growth drives increased air pollution across Sub-Saharan Africa with serious implications for human health, yet pollutant sources are poorly constrained. Here, we analyse fine particulate aerosol concentrations and radiocarbon composition of black carbon over a full annual cycle in Nairobi, Kenya. We find that particle concentrations exceed the World Health Organisation's recommended safe limit throughout the year, with little seasonal variability in particle concentration or composition. Organics (49 +/- 7%) and water-soluble inorganic ions, dominated by sulfates (13 +/- 5%), constitute the largest contributors to the particle loadings. Unlike large cities on other continents, the fraction of black carbon in particles is high (15 +/- 4%) suggesting black carbon is a prominent air pollutant in Nairobi. Radiocarbon-based source quantification indicates that fossil fuel combustion emissions are a dominant source of black carbon throughout the year (85 +/- 3%). Taken together, this indicates that black carbon emissions from traffic are a key stressor for air quality in Nairobi.

Gamma spectroscopic analyses of sand and rock samples from Tyaa River report an average activity concentration of 33 +/- 1, 55 +/- 2.8 and 812 +/- 40.46 Bqkg(-1) for Ra-226, Th-232 and K-40, respectively in the sand. A similar analysis of rock samples reveals a mean of 21 +/- 2.47, 49 +/- 2.47 and 782 +/- 39.13 Bqkg(-1) for Ra-226, Th-232 and K-40, respectively. The absorbed dose rate in rocks averaged 75 +/- 3.78 nGyh(-1), while in the sand was 86.2 +/- 4.31 nGyh(-1). The ranges and mean radiological hazard indices (radium equivalent, internal and external) were within the permissible limits and the use of sand and rocks from this mine as construction material pose no health risks. Correlation analysis between the radioactivity in the sand and rocks reveals weak non-linear positive relationships suggesting that they might be from different bedrocks.