Black carbon emissions from traffic contribute substantially to air pollution in Nairobi, Kenya

Air pollution is a leading global environmental health hazard, causing approximately one million premature deaths annually in Africa. Rapid and unplanned urbanization in Sub-Saharan African (SSA) cities, including Nairobi, exacerbates this problem due to inadequate infrastructure and policies. SSA cities are growing at a rate of 4.2% annually, projected to double their populations by 2040. This necessitates effective pollution control measures. Despite the significant health and climate impacts, air pollution in SSA remains understudied. Limited observation sites and short-term measurement campaigns hinder accurate assessments of long-term health and environmental consequences. Available data reveal high and increasing pollution levels, posing a major risk to human respiratory health and regional climate. In Nairobi, high PM2.5 and BC exposure is a significant public health risk. However, the relative contributions of different emission sources are poorly understood, hindering effective mitigation strategies. Black carbon (BC), a primary aerosol from fossil fuel combustion and biomass burning, is a particularly toxic component of PM2.5 and a potent climate warmer. Its sources are poorly constrained globally, especially in SSA due to limited data. Current emission estimates rely on bottom-up approaches with significant uncertainties. An alternative approach is source quantification from source-diagnostic atmospheric measurements. Radiocarbon (14C) analysis is a powerful tool for differentiating fossil and biomass sources of BC, offering a more precise source apportionment.

Existing studies on air pollution in SSA cities, including Nairobi, have highlighted the high levels of PM2.5 and the associated health risks. Previous work in Nairobi has demonstrated high BC concentrations, particularly near roadways, and attempted source apportionment using various chemical tracers. However, these studies often lack the year-round temporal resolution and precise source identification offered by radiocarbon techniques. While bottom-up emission inventories provide estimates of pollution sources, they often suffer from large uncertainties, especially in SSA due to limited activity data and the questionable applicability of emission factors derived from other regions. Therefore, a more direct, top-down approach like 14C analysis is needed to validate and refine these estimates and gain a more complete understanding of the sources of BC in Nairobi's complex urban environment.

This study conducted a year-round (March 2014 to February 2015) campaign to collect PM2.5 samples at an urban background site in Nairobi. A high-volume sampler collected 24-hour filter samples every six days. PM2.5 mass concentrations were determined gravimetrically. Water-soluble inorganic ions (WSII) were analyzed using ion chromatography. BC and organic carbon (OC) concentrations were measured using a thermal-optical transmission (TOT) analyzer. Approximately every second sample was subjected to radiocarbon (14C) analysis to determine the isotopic signature of BC. This involved isolating and cryo-trapping BC, converting it to CO2, and measuring the Δ14C using accelerator mass spectrometry (AMS). The Δ14C signature was used to quantify the relative contributions of biomass burning and fossil fuel combustion to BC using an isotopic mass balance equation. Meteorological data from Jomo Kenyatta International Airport and the Global Data Assimilation System (GDAS) were used to understand meteorological influences. Five-day air mass back trajectories were calculated using the NOAA HYSPLIT model, and remote sensing fire data were obtained from NASA's Fire Information for Resource Management Services (FIRMS).

The year-round average PM2.5 concentration in Nairobi was 27 ± 6 µg m−3, exceeding the 2021 WHO annual mean guideline value by a factor of five and surpassing the 24-hour limit on all sampling days. Limited seasonal variability in PM2.5 was observed. Carbonaceous aerosols (CA) constituted the largest PM2.5 component (64 ± 11%), with organic aerosols contributing 49 ± 7% and BC contributing 15 ± 4%. Water-soluble inorganic ions (WSII) accounted for 13 ± 5%, dominated by sulfates. BC concentrations were consistently high (3.9 ± 1.2 µg m−3) throughout the year, comparable to previous studies but significantly lower than curbside measurements. The BC/PM2.5 ratio in Nairobi was elevated (~15%) compared to other global cities, indicating a unique urban pollution regime. The OC/BC ratio showed limited seasonality, suggesting relatively low biomass burning contributions. Source-diagnostic ratios (e.g., K+/EC, NO3−/EC) showed some influence from long-range transport of air masses from regional fires, particularly during the dry boreal summer. Crucially, the radiocarbon analysis revealed that BC aerosols were highly depleted in 14C (Δ14C = −840 ± 34%), indicating a dominant contribution from fossil fuel combustion (85 ± 3%). This fossil fraction remained consistent throughout the year, suggesting primarily local and constant sources with minimal influence from regional biomass burning.

The consistently high PM2.5 and BC concentrations in Nairobi, coupled with the dominant contribution of fossil fuel combustion to BC, strongly implicate traffic emissions as a major source of air pollution. The high proportion of BC from fossil fuels aligns with the lack of effective transport policies in Nairobi, characterized by traffic congestion, a large fleet of older, less fuel-efficient vehicles, and high fuel consumption in the transport sector. While some influence from biomass burning was detected through chemical tracers, this was not reflected in the 14C data, highlighting the importance of using a direct source apportionment technique like 14C analysis. The findings emphasize the distinct air pollution profile of SSA cities compared to other regions, where the contribution of BC from fossil sources, while significant, is typically lower. The persistent high BC levels pose a substantial health risk to the city’s residents and also contribute to regional climate forcing.

This study demonstrates that traffic emissions are a major contributor to air pollution in Nairobi, particularly through high levels of black carbon. The consistently high PM2.5 levels, far exceeding WHO guidelines, underscore the urgent need for effective air quality management strategies in Nairobi and other SSA cities. Future research should focus on more detailed source apportionment studies in other SSA cities to determine the generality of these findings and on developing and implementing targeted mitigation strategies to address the specific air pollution challenges of the region. This would ideally involve further investigation into the contribution of various transport sectors and the evaluation of different mitigation approaches.

This study focuses on a single urban background site in Nairobi, which may not fully capture the spatial heterogeneity of air pollution within the city. While the sampling site was chosen to represent background conditions, localized variations in emission sources could influence the results. The radiocarbon analysis, while highly precise for BC source apportionment, only accounts for a subset of the overall PM2.5 mass. The relatively low number of 14C measurements compared to the chemical data might also limit the resolution of temporal changes in sources. The chosen methodology may not comprehensively capture the contribution of all relevant sources (e.g., some industrial sources). Further research could address these limitations with more extensive spatial and temporal sampling, increased 14C analysis, and a broader range of source tracers.