Atmospheric wet deposition of trace elements to forest ecosystem of the Qilian Mountains, northwest China

Atmospheric deposition is the main pathway by which trace elements (TEs) enter the terrestrial ecosystems, but few studies have investigated atmospheric TE deposition in remote forests. To understand the concentration, fluxes, influencing factors, and sources of TEs in atmospheric wet deposition, 104 samples from 13 rainfall events in two forest stands of the Qilian Mountains between July and September 2018 were collected and analyzed for eight TEs. The order of volume-weighted mean (VWM) concentration of TEs was Cu > Pb > Ni > Cr > Cd > As > Hg. Cu was the most abundant TE, with VWM concentrations of 6.33-10.90 mu g L-1 in Qinghai spruce (Picea crassifolia Kom.) forest and 3.21-7.48 mu g L-1 in Qilian juniper (Junipenis przewalskii Kom.) forest, which were 2 or 3 orders of magnitude higher than those of Hg, respectively. The wet deposition fluxes of TEs in Qinghai spruce forest were larger than those in Qilian juniper forest. The rainfall amount, rainfall intensity, rainfall duration, relative humidity, and wind speed were found to be important factors influencing atmospheric TE deposition. Based on correlation analysis, Principal Component Analysis (PCA) and Positive Matrix Factorization (PMF) modeling, three major sources of particulate matter (PM) were identified, namely industrial activities and waste incineration, traffic emissions and coal combustion, and biomass burning. Industrial activities and waste incineration were the dominant contributors, accounting for 79% of PM mass. Our results confirm that long-range transport of TEs has a significant impact on remote forest ecosystems in the Qilian Mountains.