||The deterioration of ambient air quality across the Taiwan strait, including Chinese haze, Asian duststorms and Indochina biomass burning, is highly correlated with industrial emissions, natural soil weathering and swidden agriculture. Under certain meteorological conditions, air pollutants could be blown to the downwind countries/regions and cause poor ambient air quality. Previous literature reported that the northern prevailing winds commonly blow the haze originated from northern China to central and southern China, Taiwan, and even Dongsha Islands. Therefore, the intersectional region of Taiwan Strait and South China Sea is an important air quality monitoring site for long-range transportation.|
This study selected two PM2.5 sampling sites (i.e. Penghu Islands and Dongsha Islands) located at the intersectional region of Taiwan Strait and South China Sea. Twenty-four hour sampling of PM2.5 was simultaneously collected at Penghu Islands and Dongsha Islands for continuous 14 days in four seasons from summer 2015 to spring 2016. PM2.5 samples were simultaneously collected with BGI-PQ200. After sampling, PM2.5 samples were carried back to the laboratory for conditioning, weighing, and chemical analysis. The chemical composition of PM2.5 including water-soluble ionic species, metallic elements, carbonaceous contents, and anhydrosugar. Moreover, the potential sources of PM2.5 and their contribution were further identified by principal component analysis (PCA) and chemical mass balance (CMB) receptor model.
Field sampling results indicated that the spatial distribution of PM2.5 concentration increased from south to north. The lowest seasonal averaged PM2.5 concentrations were observed in summer at both Penghu Islands and Dongsha Islands. PM2.5 concentrations increased gradually since fall, which might be influenced by the northeastern monsoons since air masses could be transported from the north toward Penghu Islands and Dongsha Islands. Air masses blown from South China Sea in summer were much cleaner than those blown from the north in fall, winter, and spring.
Chemical analysis results showed that the most abundant water-soluble ionic species of PM2.5 were secondary inorganic aerosols (SIAs) including SO42-, NO3-, and NH4+ which accounted for 50~70% of water-soluble ions (WSIs). The most abundant metallic elements of PM2.5 were crustal elements (Mg, K, Ca, Fe, and Al), while anthropogenic elements (V, Cr, Mn, Ni, As, Cd, and Pb) concentration increased since fall. Organic carbon (OC) was the main species in all seasons, and OC/EC ratios increased during the northeastern monsoon periods. The levoglucosan concentrations in summer and fall were commonly lower than those in winter and spring, showing that PM2.5 concentrations were highly influenced by biomass burning in winter and spring.
Correlation analysis results obtained from paired t test showed that the p values of PM2.5 concentration and chemical composition were 0.001 and 0.004, respectively, between two subtropic islands, showing that they had high correlation. In addition to spring, both PM2.5 concentration and chemical composition had high correlation in summer, fall, and winter, because the transportation routes toward these two Islands were not similar in spring. The correlation of PM2.5 concentration for different routes showed that the southern routes were generally lower than the northern routes. Oppositely, the correlation of chemical composition for different routes showed that the northern routes were higher than the southern routes and the southern routes was not correlated, showing that the transportation routes were different toward the two Islands for the southern routes.
Results from PCA and CMB receptor modeling showed that major sources of PM2.5 concentrations were at Penghu Islands and Dongsha Islands were sea salts, soil dusts, fuel burning, mobile sources, and secondary aerosols. Since fall, both pollutant sources and their contributions increased, especially for anthropogenic sources including petrochemical plants, steel plants, biomass burning and etc. The contributions of industrial sources (e.g. incinerators, petrochemical, steel, and cement industries) increased almost twice from summer to fall. In winter and spring, biomass burning caused different seasonal trends between Penghu Islands and Dongsha Islands. Levoglucosan concentrations in spring were higher than those in winter at Penghu Islands, while levoglucosan concentrations in winter were higher than those in spring at Dongsha islands, showing that the sources of biomass burning might be different at Penghu Islands and Dongsha Islands. Overall, cross-boundary transport accounted for 28.4~61.0% at Penghu Islands and 36.4~76.8% at Dongsha Islands, respectively, showing that both islands were highly influenced by the cross-boundary transport, especially at Dongsha Islands.