Polycyclic aromatic hydrocarbons in urban soils of Beijing: Status, sources, distribution and potential risk
Research highlights
► The primary sources of PAHs in Beijing soil were vehicle exhaust and coal combustion and the secondary source was the atmospheric deposition of long-range transported PAHs. ► Three areas of high PAH concentration soils are identified by Kriging interpolation included the industrial zone, the historical Hutong district and the university district. ► The estimated incremental lifetime cancer risks associated with exposing to soil PAHs in Beijing for both adults and children are acceptable.
Introduction
Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants that once released may remain in the environment for a long time and undergo long-range transportation (Sun et al., 2009). PAHs are generated during the combustion processes and, in urban environment, emitted primarily by anthropogenic sources, such as vehicle emissions, coal and fossil fuel powered generation, petroleum refining, straw and firewood burning, industrial processing, chemical manufacturing, oil spills and coal tars (Masih and Taneja, 2006, Nam et al., 2003). PAHs tend to accumulate in soils as they are sparingly soluble, readily adsorbable by soil particles, and difficult to be degraded (Ping et al., 2007, Tang et al., 2005, Tang et al., 2006). PAHs have become a major type of pollutant of urban areas (Agarwal, 2009, Jiang et al., 2009) and present potential carcinogenic risks to urban residents (Szabová et al., 2008).
PAHs in the urban soils came from fallouts of PAH containing particulates suspended in the atmosphere and adsorption of gaseous form PAHs. PAHs that were removed from the soils through volatilization, leaching, fugitive dust, and plant absorption may reenter the soils at a later time and/or another location (Ma et al., 2005). The concentration and distribution of PAHs in soils would reflect the extent of urban environmental pollution (Motelay-Massei et al., 2004, Wang et al., 2007). Their distributions in urban soil provide positive information for assessing pollution, identifying emission sources, and evaluating environmental health risks associated with PAHs.
Exposures to PAHs in the atmosphere not only adversely affect the well being of frontline occupation such as traffic policemen (Hu et al., 2007), factory employees (Tsai et al., 2001) and Buddhist temple attendants (Liao and Chiang, 2006), but also are potentially harmful to ordinary urban residents (Kameda et al., 2005, Yu et al., 2008). The health risk of being exposed to PAH in soils is less understood, especially urban soils that are in close proximity with high-density population.
Beijing, the capital of China, is the political, cultural and commercial hub of China. The city center at the ancient Forbidden City and expands outward in six concentric circles, each separated by a ring road for express automobile traffic. The population of Beijing has exceeded 18 million and most of them reside and work in urban districts inside the fifth ring road that covers roughly 650 km2 in surface area. In preparation for the 2008 Olympic Games, Beijing initiated and implemented a series of measures to improve urban environmental quality. Nevertheless, Beijing remains as a rapidly urbanizing and densely populated city and consumes over 84 000 tons of coal and 20 300 tons of petroleum fuels per day (Beijing Municipal Statistics Bureau, 2007).
It is inevitable that a city like Beijing has to deal with numerous PAH emission sources. Li et al. (2006) reported the PAHs contents of 30 soils obtained inside of the fourth ring road. Tang et al. (2005) measured PAHs in 31 soil samples obtained outside of the fifth ring road. Two additional studies evaluated the soil PAHs concentrations in the outskirts of Beijing metropolitan area and soil around the Guanting Reservoir, the source of Beijing’s water supply (Jiao et al., 2009, Ma et al., 2005). How did PAHs accumulate and distribute in Beijing’s soils, especially the spatial variation and relationships with land use patterns? To assess the human health risks of exposures, the distribution characteristics of PAHs in the city has to be delineated.
We hypothesized that the emission sources and land use patterns determine the spatial distribution of PAHs in soils of Beijing. In this study, we undertook a systematic investigation to characterize PAHs in urban soils of Beijing. The specific objectives were to: (1) measure concentrations of PAHs in surface soils throughout metropolitan Beijing; (2) identify possible sources of PAHs in urban soils of Beijing using principal component analysis; (3) analyze the spatial distribution of PAHs in Beijing using the Kriging interpolation; and (4) evaluate incremental lifetime cancer risks due to exposure to PAHs in urban soils of Beijing.
Section snippets
Soil sampling
Beijing was first divided into 2600 (500 m × 500 m) grids and one tenth of the grids were then randomly selected for sample collection. In this manner, we were able to establish citywide sampling coverage without being overextended in terms of time and labor. At 233 out of the 260 selected locations, surface soil samples (0–10 cm) were obtained in September 2008 at permeable surfaces most representative of land use pattern of the grids (Fig. 1). We were unable to obtain samples at the remaining
PAH concentrations in Beijing urban soil
The total PAHs concentrations of Beijing’s soils showed a wide range (Table 1). The maximum concentration was over 130 times greater than the minimum concentration, varying from 93 to 13 141 μg kg−1. The mean concentration of lower molecular weight 2 to 3 ring PAHs (LMW PAHs) was 206 μg kg−1, accounting for 17% of the total PAH mass, and the mean concentration of higher molecular weight 4–6 ring PAHs (HMW PAHs) was 1022 μg kg−1, accounting for 83% of the total PAH mass. The total PAH
Conclusions
Polycyclic aromatic hydrocarbons are pervasive in the soils of metropolitan Beijing. Their concentrations based on a citywide comprehensive sampling could vary by two orders of magnitude, from 93 to over 13 000 μg kg−1. About 2/3–3/4 of the mass in the soils are high molecular weight PAHs (4–6 ring). Coal combustion and vehicular emission are the primary sources of PAH accumulated in the soils of metropolitan Beijing.
Based on the PAH distribution patterns, three areas of high PAH concentration
Acknowledgment
We gratefully acknowledged financial supports provided by the National Natural Science Foundation of China (Grant No. 41030744), the Knowledge Innovation Program of the Chinese Academy of Science (Grant No. KZCX2-YW-422), the Technical Supporting Programs of China (Grant No. 2007BAC28B01) and the Special Foundation of State Key Lab of Urban and Regional Ecology.
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