This study proceeded with regular collections of PM2.5 (aerodynamic diameters equal to or smaller than 2.5 μm) at the Banqiao, Zhongming, Douliu, Chiayi, Xiaogang, and Hualien air quality monitoring sites of Environmental Protection Administration in Taiwan every six days from January to November 2019. In addition, PM2.5 collected at the Fuxing and Sanchong sites for traffic source investigation and the Douliu site for organic compound resolution. PM2.5 mass, water-soluble inorganic ions, carbonaceous contents, and metal elements were resolved plus a semi-quantitative resolution of organic compounds for further analysis. This study investigated the characteristics of temporal and spatial distributions of PM2.5 mass and chemical components (with additional data of December 2018), high concentration events, and potential source contributions by utilizing the analyzed PM2.5 chemical components, air quality monitoring data, environmental factors, and the corresponding model simulation products.
The results showed that seasonal PM2.5 mass levels increased in the order from east, north, to the south of Taiwan during the sampling period except summer. The highest seasonal PM2.5 mass levels at the Hualien and Banqiao sites were in spring, while that of the Zhongming, Douliu, Chiayi, and Xiaogang sites were in winter. The trends of seasonal PM2.5 mass levels in the recent three years were mostly downward but with a sign of leveling off. In general, SO42- and organic carbon were the two most abundant components at all sites for all seasons except NO3- at the Douliu, Chiayi, and Xiaogang sites in winter. The spatial distribution of metal elements varied with PM2.5 mass levels accordingly. The total metal elemental concentrations, individual metal elemental concentrations, or metal elemental fractions of PM2.5 were frequently the highest at the Xiaogang site. However, the concentrations or fractions of metal elements Tl at the Hualien site, Zr at the Zhongming site, and K, Ba, Pb, As, and Ga at the Chiayi site were the highest among the six sites. The volatilizations of NO3- and Cl- were higher in summer and autumn, while NH4+ was relatively stable across seasons. Meanwhile, positive interferences of volatilized organic carbons adsorbed by quartz-fiber filters varied less than negative interferences from the volatilization of the collected carbonaceous particles. For the traffic source collection, PM2.5 mass levels and chemical components were similar at the Fuxing and the nearby Xiaogang site. In contrast, the relative percentage differences of OC1, EC1-OP, and EC2 of carbonaceous contents, and Ba, Cu, Sb, and Zr of metal elements were significant between the Sanchong and the nearby Banqiao sites. The organic compounds resolved from 24 samples collected at the Douliu site revealed that plasticizers and fire retardants were major components and potentially contributed from emissions of weathered plastic materials and thermal pyrolysis volatilization during combustion.
During the sampling period, the environmental characteristic of high PM2.5 concentration events (≧ 35 μg m-3) was with the highest probability for low wind speed in the previous night. In contrast, pollution leftover in the previous night was with the highest probability for the occurrence of high PM2.5 concentration events on the day. Moreover, high PM2.5 concentration events were with more NOx and CO enhancements than SO2 at the Banqiao, Zhongming, and Xiaogang sites, while SO2 enhancement was more evident at the Douliu and Chiayi than the other sites. The percentage increase of NO3- in PM2.5 was 11%, comparing high PM2.5 concentration events with non-event PM2.5 levels (< 35 μg m-3). The analysis of Nitrogen Oxidation Ratio (NOR) showed that the NOR value of high PM2.5 event days in the Douliu and Chiayi sites was the highest among the six sites. The frequencies of excess NO3- in the event days increased from central to south, indicating the abundance of precursor NOx southward of central Taiwan.
For source apportionment using positive matrix factorization (PMF), “sulfate” and “nitrate” were the two most significant factors in the nine resolved source factors at all sites. The factor of “sulfate” was dominant at the Banqiao, Zhongming, Chiayi, and Hualien sites implying regional pollution transport mattered, while the dominance of “nitrate” at the Douliu and Xiaogang sites indicating derivatives from local pollution in control. “Vehicle emissions” less influenced by areas and season, contributed secondly or thirdly at all sites, and increased steadily in the recent three years. For estimating the atmospheric light extinction coefficient (bext), this study inserted PM2.5 chemical components, gas pollutants, and relative humidity into the revised Interagency Monitoring of Protected Visual Environments (IMPROVE) equation. Sulfate contributed bext significantly among all chemical components at all sites in all seasons except nitrate at the Douliu, Chiayi, and Xiaogang sites in winter. The estimated and measured bext values were with moderately high correlations at the Banqiao, Zhongming, and Xiaogang sites. Similarly, SO42- was the significant factor in influencing atmospheric visibility from the statistical regression analysis. Finally, this study reviewed 31 works of literature in the most recent techniques of collection and measurement of PM2.5 chemical components.