Dry deposition flux of atmospheric heavy metals and its source apportionment in a typical farmland of Hebei Province

Atmospheric deposition is a major contributor of heavy metals contaminating the farmland, which endangers the quality of soil as well as agricultural products. Because of the paucity of relevant observational data, the dry deposition flux and sources of atmospheric heavy metals in agricultural areas have for long remained unclear. Based on a year of field observation in a typical farmland in Hebei Province, we measured the content of 25 elements in nine size-segregated particles. The dry deposition flux of atmospheric metal elements was estimated by employing resistance modeling. In addition, the sources of atmospheric metal elements were apportioned using the PMF model. The results indicated that there were large differences in the concentrations of 25 metals between the fine particles (D_P ≤ 2.1 μm, aerodynamic diameter, the same below), coarse particles (2.1 < D_P ≤ 9 μm), and giant particles (D_P>9 μm). Heavy metals (such as Zn, Cd, and Pb) were primarily concentrated in fine particles, whereas the crustal elements (such as Al, Fe, and Th) were primarily concentrated in coarse particles. The concentrations of most metals were higher in spring and in winter rather than in summer and autumn. Annual mean concentration of Cr was the highest among the heavy metals both in fine and coarse particles, followed by As, Zn, Pb, V, and Sb. Regarding the dry deposition flux, Cr also had the highest value of 350.7 mg·m^-2·a^-1, followed by As, Sb, and V, which were 153.4, 103.1, and 102.3 mg·m^-2·a^-1, respectively. The primary sources of atmospheric metal elements confined to the study area were road dust, industrial pollution, mineral dust, coal combustion and vehicle emission. The metals in the giant particles were primarily from dust source (62.0%), whereas the metals in the fine particles were primarily from coal, motor vehicles and industrial sources (totally 67.7%). With decrease in particle size, the contribution of anthropogenic pollution increased, thereby increasing the pollution risk (enrichment factor) of heavy metals. In conclusion, the study was critical in considering the input and sources of atmospheric deposition regarding regulation of farmland involving heavy metals.