|dc.description.abstract||Atmospheric suspended fine particles (particulate matter with an aerodynamic diameter less than or equal to 2.5 μm, PM2.5) play an important role in air quality assessment, setting up control strategies for polluting sources, health risk assessment, and environmental change. This study aims to investigate source contributions and causes of polluting events by observing PM2.5 mass and chemical component concentrations at the stations of Taiwan Environmental Protection Administration (TEPA) in the northern (Banqiao), middle (Zhongming), and southern (Xiaogang) parts of Taiwan from July 2015 to May 2016. During the studying period, the deployment of three-filter in series was adopted for PM2.5 collections to correct for the interference of volatilization of semi-volatile species and adsorption of gases from the environment. For the resolved PM2.5 mass and chemical component concentrations, this study investigated their seasonal variation trends and causes of polluting events by combining manual collections with air-quality monitoring data conducted at TEPA stations. In addition, weight percentages of primary and secondary organic carbons in PM2.5 were estimated by EC-tracer and mass reconstruction methods. Contributing factors (i.e., polluting sources) were apportioned by Positive Matrix Factorization (PMF) method and validated by Conditional Probability Function (CPF) with wind direction for high contributing local sources.
The results show that PM2.5 mass levels were lowest at all sampling sites in summer, and highest at stations in the northern, middle, and southern parts of Taiwan in spring, autumn, and winter, respectively. These seasonal variations can be attributed to causes of polluting events, for examples, transboundary transports in the northern Taiwan in spring, upwind regional transport and bad environmental ventilation in the middle Taiwan in autumn, and bad environmental ventilation in the southern Taiwan in winter. Major PM2.5 chemical components are sulfates and organic carbons in most samples. However, the weight percentages of nitrate ion in PM2.5 were apparently enhanced when PM2.5 concentrations were greater than 35 μg m-3 implying precursor sources of nitrate ion played a significant role. The precursor sources of nitrate ion need to be controlled to reduce PM2.5 polluting events.
The results of EC-tracer method in estimating weight percentages of primary and secondary organic carbons in PM2.5 show that the weight percentages of secondary organic carbons are highest in winter with 49%, 48%, and 60%, respectively, across all three sites. It indicates low ambient temperature in winter reduces losses of semi-volatile species and lower winter mixing layer in enhancing precursor pollutant concentrations both favor the process of secondary production. The mass closure results show that the multiplying factors of organic matter (OM) to OC in the northern, middle, and southern metropolis are 1.56, 1.6 and 1.66, respectively. In addition, the factor was apparently increased during high polluting period which implied that the content of OM was enhanced.
The computed values of Enrichment Factor (EF) from the analyzed metal elements across all stations are increased with the rises of pollutant concentrations which reveal that polluting sources mainly contributed from anthropogenic activities. The concentrations of coal burning tracer Se were increased with simultaneous increases of EF values across all three stations when the prevailing wind changed to northeast monsoon during the change of season. The cause was considered to be affected by transboundary or regional transports of nearby metropolis. During low polluting period, the weight percentages of oil combustion element V were enhanced in the middle and southern stations in summer but lowered according to the change of seasons. It indicated oil combustion emissions from local industry were relatively important during low polluting period.
The source apportionment from PMF coupling with CPF reveals that the emissions from local sources are mainly responsible during low polluting period. In contrast, the weight percentages of secondary pollutants were comparatively higher due to bad environmental ventilation or transboundary transport during high polluting period.