Polycyclic Aromatic Hydrocarbons in Urban Soil in the Semi-arid City of Xi'an, Northwest China: Composition, Distribution, Sources, and Relationships with Soil Properties

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the environment. This study collected a total of 62 urban soil samples from the typical semi-arid city of Xi'an in Northwest. They were analyzed for the composition, distribution, and sources of PAHs as well as the relationships with soil properties. The sum of 16 individual PAHs (a16PAHs) ranged from 390.6 to 10,652.8 mu g/kg with a mean of 2052.6 mu g/kg. The average a16PAHs decreased in the order of the third ring road (2321.1 mu g/kg) > the first ring road (1893.7 mu g/kg) > the second ring road (1610.0 mu g/kg), and in the order of industrial areas (3125.6 mu g/kg) > traffic areas (2551.6 mu g/kg) > educational areas (2414.4 mu g/kg) > parks (1649.5 mu g/kg) > mixed commercial and traffic areas (1332.8 mu g/kg) > residential areas (1031.0 mu g/kg). The most abundant PAHs in the urban soil were 3- to 5-ring PAHs. Elevated levels of PAHs were found in industrial and traffic areas from the east and west suburbs and the northwest corner of Xi'an as well as the northeast corner in the urban district of Xi'an. PAHs in the urban soil were mainly related to the combustion of fossil fuel (i.e., coal, gasoline, diesel, and natural gas) and biomass (i.e., grass and wood) (variance contribution 57.2%) as well as the emissions of petroleum and its products (variance contribution 29.9%). Soil texture and magnetic susceptibility were the main factors affecting the concentration of PAHs in urban soil. Meanwhile, this study suggested that the single, rapid, and nondeductive magnetic measurements can be an indicator of soil pollution by PAHs.