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

Abstract

This study aims to investigate the spatiotemporal variation, chemical composition, and source apportionment of marine fine particles (PM_2.5) 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 PM_2.5. Water-soluble ionic species, metallic elements, and carbonaceous contents were then analyzed to characterize the chemical characteristics of marine PM_2.5. In order to identify the potential sources and their contributions to marine PM_2.5, chemical mass balance (CMB) receptor model was employed along with the backward trajectory simulation to resolve the source apportionment of marine PM_2.5 and to explore their transport routes in different seasons. The results showed that high PM_2.5 concentrations were commonly observed during the northeastern monsoon periods. Additionally, marine PM_2.5 concentration decreased from the west to the east with the highest PM_2.5 at the Nankang Island and the lowest PM_2.5 at the Donyin Island in all seasons, indicating an obvious concentration gradient of PM_2.5 transported from the continental areas to the offshore islands. In terms of chemical characteristics of PM_2.5, the most abundant water-soluble ions of PM_2.5 were secondary inorganic aerosols (SO₄²⁻, NO₃⁻, and NH₄⁺) which accounted for 55–81% of water-soluble ions and 29–52% of marine PM_2.5. The neutralization ratios of PM_2.5 were always less than unity, indicating that NH₄⁺ cannot solely neutralize nss-SO₄²⁺ and NO₃⁻ in marine PM_2.5 at the Matsu Islands. Although crustal elements (Al, Ca, Fe, K, and Mg) dominated the metallic content of marine PM_2.5, 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 PM_2.5 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 PM_2.5 at the Matsu Islands, while, in winter, biomass burning contributed up to 15% of marine PM_2.5. This study revealed that cross-boundary transport accounted for 66~84% of PM_2.5 at the Matsu Islands, suggesting that marine PM_2.5 at the Matsu Islands has been highly influenced by anthropogenic emissions from neighboring Fuzhou City as well as long-range transport from Northeast Asia.

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