Significant urban hotspots of atmospheric trace element deposition and potential effects on urban soil pollution in China

Rapid urbanization has profoundly altered the spatial patterns of multiple element cycles. Whether and how urbanization shapes the spatial patterns of atmospheric trace element deposition remains, however, poorly understood. Using a newly compiled database on bulk deposition of eight trace elements (i.e., Cu, Ni, Zn, As, Cd, Cr, Hg and Pb) in China, we assessed the urban imprints on the spatial patterns of trace element deposition. Bulk deposition of the eight trace elements all showed a significant increase with closer distance to the nearest large cities, while the urban effect was also mediated by point emission sources and precipitation. We further compiled a database of urban topsoil (0–10 cm) concentrations of the eight trace elements and found that urban soil quality standards were exceeded in 80% of the studied cities for Cr, 49% for As, and less than 25% for other trace elements, respectively. The urban topsoil concentrations of six trace elements (except As and Hg) showed no significant correlations with their background values for natural soils, while we found a significant correlation between bulk deposition and urban topsoil concentrations of trace elements corrected by background values. We also demonstrated that current levels of trace element deposition would substantially increase urban soil pollution over the coming decades. Our findings confirm the occurrence of urban hotspots of trace element deposition and their impact on soil pollution and highlight a need of emission control of trace elements for safety urban soil quality.