In this study, the atmospheric PM2.5, increases/decreases of the PM2.5, the PM2.5/PM10 ratio, total PCDD/Fs-TEQ concentrations, PM2.5-bound total PCDD/Fs-TEQ content, and PCDD/F gas-particle partition in Taiwan were investigated for the period 2013 to 2017. In Taiwan, the annual average PM2.5 concentrations were found to be 28.9, 24.1, 21.4, 20.2, and 19.9 µg m–3 in 2013, 2014, 2015, 2016, and 2017, respectively, which indicated that the annual variations in PM2.5 levels were decreasing during the study period. The average increases (+)/decreases (–) of PM2.5 concentrations were –16.7%, –11.1%, –5.75%, and –1.73% from 2013 to 2014, from 2014 to 2015, from 2015 to 2016, and from 2016 to 2017, respectively. Based to the relationship between PM10 values and total PCDD/F concentrations obtained from previous studies, we estimated that in 2017, the annual average total PCDD/Fs-TEQ concentrations ranged between 0.0148 (Lienchiang County) and 0.0573 pg WHO2005-TEQ m–3 (Keelung City), and averaged 0.0296 pg WHO2005-TEQ m–3, while the PM2.5-bound total PCDD/Fs-TEQ content ranged from 0.302 (Kaohsiung City) to 0.911 ng WHO2005-TEQ g–1 (Keelung City), at an average of 0.572 ng WHO2005-TEQ g–1. These values are suggested to be validated in the future study because the sources and formations of PM10 and PM2.5 in different areas were diverse, and may be not closely related to combustion sources. The seasonal variations in the gas fraction of total PCDD/Fs-WHO2005-TEQ concentrations were 68.6%, 86.6%, 82.3%, and 52.3% in the spring, summer, autumn, and winter, respectively. Due to the fact that Taiwan is located mostly in the sub-tropical zone, which had annual average temperatures between 23.0 and 24.4°C, averaging 23.8°C during the study period, the majority of PCDD/Fs-TEQ were dominant in the gas phase.
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