Chemical characteristics, source apportionment, and regional transport of marine fine particles toward offshore islands near the coastline of northwestern Taiwan Strait

This study aims to investigate the spatiotemporal variation, chemical composition, and source apportionment of marine fine particles (PM₂.₅) as well as their regional transport toward the Matsu Islands located near the coastline of northwestern Taiwan Strait. Four offshore island sites located at the Matsu Islands were selected to conduct both regular and intensive sampling of marine PM₂.₅. Water-soluble ionic species, metallic elements, and carbonaceous contents were then analyzed to characterize the chemical characteristics of marine PM₂.₅. In order to identify the potential sources and their contributions to marine PM₂.₅, chemical mass balance (CMB) receptor model was employed along with the backward trajectory simulation to resolve the source apportionment of marine PM₂.₅ and to explore their transport routes in different seasons. The results showed that high PM₂.₅ concentrations were commonly observed during the northeastern monsoon periods. Additionally, marine PM₂.₅ concentration decreased from the west to the east with the highest PM₂.₅ at the Nankang Island and the lowest PM₂.₅ at the Donyin Island in all seasons, indicating an obvious concentration gradient of PM₂.₅ transported from the continental areas to the offshore islands. In terms of chemical characteristics of PM₂.₅, the most abundant water-soluble ions of PM₂.₅ were secondary inorganic aerosols (SO₄²⁻, NO₃⁻, and NH₄⁺) which accounted for 55–81% of water-soluble ions and 29–52% of marine PM₂.₅. The neutralization ratios of PM₂.₅ were always less than unity, indicating that NH₄⁺ cannot solely neutralize nss-SO₄²⁺ and NO₃⁻ in marine PM₂.₅ at the Matsu Islands. Although crustal elements (Al, Ca, Fe, K, and Mg) dominated the metallic content of marine PM₂.₅, trace anthropogenic metals (Cd, As, Ni, and Cr) increased significantly during the northeastern monsoon periods, particularly in winter. Organic carbons (OCs) were always higher than elemental carbons (ECs), and the mass ratios of OC and EC were generally higher than 2.2 in all seasons, implying that PM₂.₅ was likely to be aged particles. During the poor air quality periods, major air mass transport routes were the northern transport and the anti-cyclonic circulation routes. Source apportionment results indicated that fugitive soil dusts and secondary aerosols were the major sources of marine PM₂.₅ at the Matsu Islands, while, in winter, biomass burning contributed up to 15% of marine PM₂.₅. This study revealed that cross-boundary transport accounted for 66~84% of PM₂.₅ at the Matsu Islands, suggesting that marine PM₂.₅ at the Matsu Islands has been highly influenced by anthropogenic emissions from neighboring Fuzhou City as well as long-range transport from Northeast Asia.