Diurnal and seasonal variation, chemical characteristics, and source identification of marine fine particles at two remote islands in South China Sea: A superimposition effect of local emissions and long-range transport

Abstract

This study explored the spatiotemporal variation, chemical characteristics, and long-range transport of marine fine particles (PM2.5) at two remote islands in the South China Sea (SCS). Daytime and nighttime PM2.5 was simultaneously sampled at the Dongsha and Nansha Islands for determining its mass concentrations and chemical fingerprints. Field measurement results showed that high PM2.5 level occurred in winter and spring, particularly during the northeastern monsoon periods (NMPs), which blew anthropogenic PM2.5 to the SCS via continental outflow. In terms of diurnal variation, daytime PM2.5 concentrations were higher than those at nighttime in all seasons. Chemical analysis indicated that PM2.5 was dominated by water-soluble ions (WSIs) which were dominated by secondary inorganic aerosols (SIAs) (NO3−, SO42−, and NH4+). Crustal elements (Ca, K, Mg, Fe, and Al) were abundant of elements in PM2.5, but enrichment factor (EF) showed that trace elements (Ti, Cr, Mn, Ni, Cu, and Zn) were mainly originated from anthropogenic sources. The concentrations of organic carbon (OC) in PM2.5 were always higher than those of elemental carbon (EC). High concentrations of levoglucosan and organic acids were observed in winter and spring. The major source of PM2.5 was sea salts (16.8–24.8%), and followed by fugitive dust, secondary aerosols, oil-fired boilers, automobile sources, and biomass burning. Clustered trajectories indicated that polluted air masses were transported from mainland China, Korean Peninsula, and Japan Islands, particularly passing through industrialized and urbanized regions in East Asia during the NMPs. A superimposition phenomenon of local emissions and long-range transport (LRT) was observed in the SCS. The LRT of anthropogenic aerosols accounted for 54–79% of marine PM2.5. Overall, PM2.5 at the Dongsha Islands was highly influenced by LRT from Asian continental outflow, while PM2.5 concentrations observed at the Nansha Islands did not vary seasonally.