Polycyclic aromatic compounds (PACs) in tree barks and tree cores of a national large-scale coal-fired power base of China: Sources, atmospheric toxicities, and pollution histories.

Polycyclic aromatic compounds (PACs) are important hazardous air pollutants in China due to the country's coal-dominant energy structure. In order to reveal the pollution characteristics, sources, toxicity, and pollution historical trends of PACs in the atmosphere of the middle reach of the Huaihe River (MRHR), a large-scale coal-fired power base of China, tree barks and tree cores were collected and employed as passive air samplers and historical trend recorders, and 76 PACs were identified for the first time. ΣPACs in tree barks ranged from 170 to 3800 ng g-1 (mean = 700 ± 720 ng g-1), with the high concentrations observed mainly in the coal-mining and coal-bearing area. 16 priority PAHs (PriPAHs) were the predominant substances and accounted for 59 ± 8.3 % of ΣPACs. The combustion of coal and fuel oil was the most significant source of PACs, accounting for 43 % of ΣPACs, followed by the combustion of biomass (30 %) and non-combustion sources (27 %). Based on a bark-air partitioning model, volumetric air concentrations for ΣPACs were calculated to be 450-11,000 ng m-3 (mean = 1600 ± 2000 ng m-3). The BaP-toxic equivalent concentrations (TEQBaP) of ΣPACs (mean = 9.7 ± 15 ng m-3) were significantly higher than the Chinese guideline (1 ng m-3) and were mostly caused by coal & fuel oil combustion (55 ± 13 %). High molecular weight PACs were detected in lower percentages in tree cores than in tree barks, indicating that PACs in the particle phase were difficult to enter the tree core. Major PACs decreased in tree core samples between 2000 and 2020 as pollution control efforts improved, however, some PACs showed different trends when influenced by point sources.