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Airborne black carbon (BC) particles have serious implications for human health and climate change and thus represent a prime target for mitigation policies. The sources of BC include vehicles burning diesel fuel, which are common in urban areas in low-income countries. The objective of this study was to examine the possible relationship between traffic and airborne BC concentrations near roadways in Nairobi, Kenya. We measured personal exposure levels of BC in Nairobi in a field campaign in summer 2009, and subsequently confirmed our ability to estimate BC quantitatively by conducting a co-location experiment in Nairobi in the fall of 2011. The central business district of Nairobi recorded 11-h average daytime BC concentrations in the range 20-42 mu g m(-3) while the main highways feeding into Nairobi recorded BC levels of 17-79 mu g m(-3). As far as we are aware, the data reported here include the highest multi-hour BC concentrations ever reported in Africa. Samples from gradient sampling showed rapid reduction of BC concentrations with distance away from traffic. The measured BC near the curbside of roadways was estimated to be in the range of 34-56% of PM2.5, implying traffic was a dominant source of PM2.5 emissions in Nairobi. The high concentrations of BC that we observed raises concern regarding potential health threats to workers, residents, and visitors, and highlights the need for policies to address traffic-related air pollution in Nairobi.