Enhanced Wet Deposition of Water-Soluble Organic Nitrogen During the Harvest Season: Influence of Biomass Burning and In-Cloud Scavenging

Globally water-soluble organic nitrogen (WSON) accounts for about one third of water-soluble total nitrogen in atmospheric deposition, yet the key factors influencing WSON deposition remain uncertain. In the present study, precipitation and aerosol samples were collected during 2016–2018 at a forest site in the Pearl River Delta, south China. Rainwater WSON concentrations were found to elevate significantly during fall, and wet deposition of WSON during fall could account for about 50% of its annual total wet deposition. The wet deposition and rainwater concentrations of K⁺ and levoglucosan, which are tracers for biomass burning emissions, also increased substantially during fall. The average WSON/water-soluble total nitrogen ratio in precipitations (0.48) was over two times higher than that in aerosols (0.20) during fall, while during other seasons, the ratios in precipitations (0.17) and in aerosols (0.19) were comparable. Moreover, concentrations of WSON in rainwater, unlike that of inorganic N, did not decrease significantly with the increase of precipitation amount during fall. These results suggested that enhanced biomass burning during fall (harvest season) might be an important source of in-cloud WSON, and in-cloud scavenging, instead of below-cloud washout, likely contributed remarkably to the wet deposition of WSON.