The Distribution Characteristics Of Polycyclic Aromatic Hydrocarbons In Soil And Air In China And Numerical Simulation On Large Scale

Polycyclic aromatic hydrocarbons (PAHs) are a group of persistent toxic substances (PTSs) that widespread in the environments. The environmental pollution and fate of PAHs have become a hot research topic in the environmental science domain. In recent years, some researches and studies have been carried out on PAHs in the environmental matrix in China. However, there is still a long way to go with PAH studies in China. The monitoring and modeling research programs about PAHs often focused on small areas only. The information regarding the current status of PAHs at the national scale is deficient, which makes it difficult to understand the large scale transport, transfer and the environmental fate of PAHs. Based on the present situation of PAH studies in China, in this paper, soil samples and air samples at a national scale were collected, and a corresponding multi-medium environmental model was employed to understand the spatial and temporal trend, transport and transfer of PAHs in China. The results of this study would provide a scientific basis for the development of relevant policies and add new contribution to the Stockholm Convention for China.For the residual and spatial distribution of PAHs in soil in China, soil sampling site were selected and the samples were collected in China. The concentration, composition and spatial distribution characteristic of PAHs in soil were investigated, and the influencing factors, toxicity assessment and sources of PAHs in soil were evaluated. Totally 157 soil samples were collected in China, among which there were three types of soil samples; 28 were city samples, 120 were rural samples and 9 were background samples. The collected soil samples were extracted by Soxhlet and cleaned using silica gel column, and PAHs were determined by gas chromatography with mass spectrometer detector (GC-MSD). Sixteen PAHs were detected with different detection levels in soil samples, among which phenanthrene, naphthalene, fluoranthene and benzo[b]fluoranthene were the most predominant compounds. The average concentrations of 617±936 ng/g, 340±676 ng/g,126±74 ng/g for the city, rural and background soils. The PAHs concentrations were compared with a few typical regions in foreign countries. PAHs concentrations on a national scale showed an obvious longitude distribution and urban fractionation effect resulted from the sources. The selected diagnostic ratios and principal component analysis and the multiple linear regression (PCA-MLR) indicate that there are different sources of PAHs for different types of soil samples. The PAHs were mainly from mixed sources of pyrogenic sources (coal combustion and vehicle exhaust) and petrogenic sources in urban soil samples. For rural areas, the PAHs in soil also come from mixed sources of pyrogenic sources (coal and biomass combustion) and petrogenic sources. The nature sources (wildfire) and mixed source (external sources and unknown sources) were the main sources to background soils.For the residual and spatial distribution of PAHs in air in China, air sampling sites were selected and air samples were collected. The concentrations, composition and seasonal variation of PAHs in air were studied, and the influence of temperature, gas-particle partitioning and sources of PAHs in air were investigated. Based on a year round dataset (from August 2008 to July 2009), air samples were collected by high-volume air sampler on a weekly base in ten cities, i.e. Harbin, Beijing, Shihezi, Lanzhou, Xi'an, Chengdu, Lhasa, Nanchang, Kunming and Guanghzou in China. The collected air samples were extracted by Soxhlet and cleaned using silica gel column, and PAHs were determined by GC-MSD. The results indicate that 16 PAHs were detected with different detection levels in air samples in the cities, among which phenanthrene, fluoranthene and fluorene were the predominant compounds. The atmospheric PAHs concentrations exhibited obvious differences among the different cities, within approximately 1 order of magnitude. The average atmospheric concentration of PAHs in the northern 5 cities was higher than that in the southern 5 cities, which were 251±126 ng/m3 and 165±102 ng/m3, respectively. The atmospheric PAHs concentrations in China were comparable with some typical urban cities outside China. Compared with the southern 5 cities, the PAHs composition, fingerprint, gas-particle partitioning, seasonal variation and the influence of temperature in the northern 5 cities also show obvious differences. The selected diagnostic ratios and PCA-MLR indicate that the atmospheric PAHs in Chinese cities mainly came from incomplete combustion of fossil fuel (coal and oil combustion) and petrogenic sources.A multi-medium model on large scale was employed to simulate the residue of PAHs, the spatial and temporal trend in soil and air, to study the relationship between the sources and the receptors of PAHs in China. Firstly, the emission inventory of PAHs with a 1/4°×1/6°longitude/latitude resolution was established from 1990 to 2008. The physical and chemical properties of PAHs and the environmental parameters were used as the input data of the model. The multi-medium model was then improved and optimized for generating the concentrations of PAHs residue in soil and air. The results indicate that during the period from 1990 to 2000, the emission of PAHs kept relatively stable. From 2001, the emission of PAHs kept increasing. There was generally a good agreement between the estimated and the observed concentrations of PAHs in soil and air, and the relative error between input and output was 2%, which shows that the model can be applied to simulate the residue of PAHs in soil and air in China. The simulated PAHs residue in soil and air displayed similar geographical distributions, with high concentrations in eastern China, low ones in middle and western China and lowest in Qinghai-Tibet Plateau. However, compared with PAHs residue in soil, there was small difference for PAHs residue in air. For geographical distribution of PAHs residue in air, relatively two high concentration strips were found: one was called as the middle high concentration strip through the eastern of Gansu, the eastern of Sichuan, and the middle of Guizhou; and the other was called as the east high concentration strip which covered Liaoning, Hebei, Shandong and Jiangsu. Both the two strips were caused by the atmospheric transport when forced by resistance. By separating the emission inventory, the mutual influence between different regions of China and the influence of space heating on PAHs residue in soil were analyzed by numerical simulation. The results indicate that local sources were the remarkable contributor to Pyr residue in soil, and the contribution of local sources could be above 95%. However, as caused by the atmospheric transport, the mutual influence between different regions still exists. And the influence of space heating on Pyr residue in soil in different regions of our country were quite different. The contribution rate shows significant differences between north and south China: from north to south areas, the contribution rate was decreased.