Source apportionment and seasonal variation of PM<sub>2.5</sub> in a Sub-Sahara African city: Nairobi, Kenya

Abstract

Abstract. Sources of airborne particulate matter and their seasonal variation in urban areas in Sub-Sahara Africa are poorly understood due to lack of long-term measurement data. In view of this, airborne fine particles matter (particle diameter ≤ 2.5 μm, PM2.5) were collected between May 2008 and April 2010 at two sites (urban background site and suburban site) within the Nairobi metropolitan area. A total of 780 samples were collected and analyzed for particulate mass, black carbon (BC) and thirteen trace elements. The average PM2.5 concentration at the urban background site was 20 ± 8 μg m−3 whereas the concentration at the suburban site was 13 ± 8 μg m−3. The daily PM2.5 concentrations exceeded 25 μg m−3 (the World Health Organization 24 h guideline value) 29% of the days at the urban background site and 7% of the days at the suburban site. At both sites, BC, Fe, S and Cl accounted for approximately 80% of all detected elements. Positive Matrix Factorization analysis identified five source factors that contribute to PM2.5 in Nairobi; traffic, mineral dust, secondary aerosol, industrial and combustion. Mineral dust and traffic factors were related to approximately 74% of PM2.5. Identified source factors exhibited seasonal variation though traffic factor was prominently consistent throughout the sampling period. The results provide information that can be exploited for policy formulation and mitigation strategies to control air pollution in Sub-Sahara African cities.

Characterization of Size-Fractionated Particulate Matter and Deposition Fractions in Human Respiratory System in a Typical African City: Nairobi, Kenya

“…3(D)). This similarity was an indication of contribution from multiple sources, namely metal smelting industries and traffic (Chueinta et al, 2000;Chow et al, 2004;Were et al, 2012;Gaita et al, 2014). One noteworthy observation in this study was the pattern of Cl concentrations in the different size ranges.…”

“…S, Zn and Br had approximately 50% of their concentrations in the fine size range compared to Pb and Zn, which had approximately 75% of their mass concentrations in the same size range. K had approximately equal proportions in both coarse and fine size mode pointing to its sources as mineral dust and biomass burning as reported by Gaita et al (2014). S, K, Cu and Br had between 10 and 31% of their mass concentration in the ultrafine size range.…”

“…Given the increase of severe droughts as a consequence of climate change in Africa, the mineral dust component will continue to play a key role in the air quality issues as highlighted by the dominance of the crustal elements; Si, Fe and Ca (WMO, 2013;Gaita et al, 2014). From the size distribution data, the majority of the elements are in PM less than 2.5 µm and thus have higher probability of penetrating deeper into the respiratory system (Anderson et al, 2012).…”

“…The impact of PM on human health in Nairobi city, as a representative of a typical African urban centre, can be discussed in the context of estimating deposition fractions of both coarse and fine PM. A study in Nairobi city by Gaita et al (2014) identified sources of airborne PM to include traffic, mineral dust, industrial activities and combustion. The said study reported 24 h average PM 2.5 concentration of 21 µg m -3 from an urban background site.…”

“…The urban centres in Africa are growing at exceedingly high rates whereas the provision of prerequisite social services and amenities are not commensurate with this growth (UN-HABITAT, 2006;UNDESA, 2010;Karanja and Makau, 2012). The urban population is reported to be exposed to wide range of air pollutants (Gulis et al, 2004;Ngo et al, 2015) whose sources have been identified as vehicular emissions, biomass burning and mineral dust (Formenti et al, 2003;Gatari and Boman, 2003;Kinney et al, 2011;Petkova et al, 2013;Gaita et al, 2014). Therefore, the objectives of this study were to obtain a size distribution of PM and elemental concentrations in a typical African city and to determine the deposition fractions of measured PM and trace elements in the respiratory system.…”