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Source apportionment and seasonal variation of PM2.5 in a Sub-Saharan African city: Nairobi, Kenya

Journal article
Authors Samuel Mwaniki Gaita
Johan Boman
Michael J Gatari
Jan B. C. Pettersson
Sara Janhäll
Published in Atmospheric Chemistry And Physics
Volume 14
Pages 9977-9991
ISSN 1680-7316
Publication year 2014
Published at Department of Chemistry and Molecular Biology
Pages 9977-9991
Language en
Links dx.doi.org/10.5194/acp-14-9977-2014
Keywords PM2.5, source profiles, urban air quality, Sub-Sahara Africa
Subject categories Meteorology and Atmospheric Sciences

Abstract

Sources of airborne particulate matter and their seasonal variation in urban areas in Sub-Saharan Africa are poorly understood due to lack of long-term measurement data. In view of this, filter samples of airborne particulate 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 13 trace elements. The average PM2.5 concentration at the urban background site was 21±9.5 μg m−3, whereas the concentration at the suburban site was 13±7.3 μg m−3. The daily PM2.5 concentrations exceeded 25 μg m−3 (the World Health Organization 24 h guideline value) on 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, namely traffic, mineral dust, industry, combustion and a mixed factor (composed of biomass burning, secondary aerosol and aged sea salt). Mineral dust and traffic factors were related to approximately 74% of PM2.5. The identified source factors exhibited seasonal variation, apart from the traffic factor, which was prominently consistent throughout the sampling period. Weekly variations were observed in all factors, with weekdays having higher concentrations than weekends. The results provide information that can be exploited for policy formulation and mitigation strategies to control air pollution in Sub-Saharan African cities.

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