Abstract
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.
Highlights
The air quality in major cities within Sub-Saharan Africa (SSA) is deteriorating due to several factors, which include an increasing urban population, unregulated traffic activities, poorly maintained vehicles, inadequate regulations and air pollution control policies (Zachariadis et al, 2001; van Vliet and Kinney, 2007; UN, 2010)
This study has shown that traffic-related emissions and mineral dust are both significant contributors to PM2.5 in Nairobi
It can be concluded that policies such as banning of Pb in gasoline have shown positive effects on reducing Pb concentration in PM2.5 and reducing the risk of Pb toxicity to the urban inhabitants
Summary
The air quality in major cities within Sub-Saharan Africa (SSA) is deteriorating due to several factors, which include an increasing urban population, unregulated traffic activities, poorly maintained vehicles, inadequate regulations and air pollution control policies (Zachariadis et al, 2001; van Vliet and Kinney, 2007; UN, 2010). In order to draft environmental regulations to control air pollution, high-quality and longterm measurement data are required. Such data are not readily available in the SSA region due to a lack of longterm air pollution monitoring (Petkova et al, 2013; UNEP, 2013). A study conducted in Nairobi by Kinney et al (2011) reported that concentration levels of PM2.5 at street level, next to a city road, frequently exceeded the World Health Organization (WHO, 2000) h guideline of μg m−3.
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