Characterization and Risk Assessment of Atmospheric PM_2.5 and PM₁₀ Particulate-Bound PAHs and NPAHs in Rwanda, Central-East Africa

Abstract

Exposure to airborne particulates is estimated as the largest cause of premature human mortality worldwide and is of particular concern in sub-Saharan Africa where emissions are high and data are lacking. Particulate matter (PM) contains several toxic organic species including polycyclic aromatic hydrocarbons (PAHs) and nitrated PAHs (NPAHs). This study provides the first characterization and source identification for PM10- and PM2.5-bound PAHs and NPAHs in sub-Saharan Africa during a three-month period that spanned dry and wet seasons at three locations in Rwanda. The 24-h mean PM2.5 and PM10 concentrations were significantly higher in the dry than the wet season. PAH and NPAH concentrations at the urban roadside site were significantly higher than the urban background and rural site. Source identification using diagnostic ratio analysis and principal component analysis (PCA) revealed diesel and gasoline-powered vehicles at the urban location and wood burning at the rural location as the major sources of PAHs and NPAHs. Our analysis demonstrates that PM concentrations and lifetime cancer risks resulting from inhalation exposure to PM-bound PAHs and NPAHs exceed World Health Organization safe limits. This study provides clear evidence that an immediate development of emission control measures is required.

Chemical and Biological Components of Urban Aerosols in Africa: Current Status and Knowledge Gaps

“…The majority of studies reviewed in this study found that the mean concentrations of atmospheric PM 2.5 and PM 10 in Africa greatly exceeded the 2006 WHO guideline value of annual and 24-h mean, and those carcinogenic and mutagenic organic pollutants are a major component of PM (Figure 2). Despite available publications of atmospheric NPAHs in Rwanda [34], Egypt [116], and Algeria [117] (Figure 3), the atmospheric concentrations of total PAHs and NPAHs show a large variation among these countries. The PAH concentrations, in descending order, were Senegal, Kenya, South Africa, Mali, Uganda, Rwanda, Sierra Leone, Algeria, and Egypt.…”

“…African automobile emissions (Diesel and gasoline) and biomass burning were considered major contributors of PAHs and NPAHs in urban and rural sites, respectively (Table 2). The [1-NP]/[Pyr] ratios in Kigali, Rwanda were 0.05 (dry season) and 0.04 (wet season) [34], while the values in the Great Cairo Area, Egypt were 0.06 (winter) and 0.03 (summer) [116]. The values in these two African countries were similar to those reported in East Asian cities influenced by large volumes of vehicle emissions [82].…”

“…However, most of the sampling sites in Africa were near the intersection of high traffic volumes, suggesting that these NPAHs were emitted from automobiles. Additionally, several PAH pairs, such as [fluoranthene]/([pyrene] + [fluoranthene]) or [Flu]/([Pyr] + [Flu]), [benz(a)anthracene]/([chrysene] + [benz(a)anthracene]) or [BaA]/([chrysene(Chr)] + [BaA]) and [indeno (1,2,3-cd)pyrene]/([benz(g,h,i)perylene + [indeno (1,2,3-cd)pyrene]) or [IDP]/[BPe + IDP], have been also used as markers of the source of the PAHs in African countries, including Rwanda [34] and Kenya [118]. Biomass burning and automobile emissions were the main sources of atmospheric PAHs in Kenya and Rwanda (Table 2).…”

“…Ambient PM 2.5 (particles less than 2.5 μm in diameter) (left), and ambient PM 10 (particles less than 10 μm in diameter) (right), mean concentration as reported in studies from traffic (back color), urban background (blue), and rural site (green) in African countries such as Algeria [45], Benin [46], Burkina Faso [47], Ethiopia [48], Ghana [49,50], Kenya [51], Mali [52], Morocco [53], Niger [54], Nigeria [49], Rwanda [34], Senegal [54,55], South Africa [56,57], Tanzania [58], and Uganda [59]. …”

“…However, there are still limited studies in Africa on the characterization of chemical and biological components of PM. The few studies that have assessed the chemical components of ambient PM in Africa have demonstrated that PM concentrations and lifetime cancer risks resulting from inhalation exposure to PM chemical composition exceeded WHO safe limits and provide clear evidence that an immediate development of emission control measures is required [34]. The available data from biological composition associated with atmospheric PM comes mostly from Asia [35,36,37], Europe [11], and the United States [38].…”