Contrasting atmospheric behavior of PCDD/Fs and PBDD/Fs in a subtropical megacity: Source characteristics, regional transport, and gas-particle partitioning.

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are well-characterized toxic atmospheric pollutants, whereas much less is known about their brominated analogues, polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs). Here, a year-round active sampling campaign was conducted in Guangzhou, a subtropical megacity in southern China, to simultaneously investigate atmospheric PCDD/Fs and PBDD/Fs in paired gas- and particle-phase samples. The annual mean total concentration of PBDD/Fs (1,435±1,338 fg m-3; range: 279-10,403 fg m-3) was comparable to that of PCDD/Fs (1,892±1,762 fg m-3; range: 313-11,456 fg m-3), and both compound classes were higher during winter-spring and lower in summer, although the seasonal contrast was weaker for PBDD/Fs. Backward trajectory and concentration-weighted trajectory analyses revealed stronger continental transport influence on PCDD/Fs, whereas PBDD/Fs showed weaker transport dependence and a more regionally confined pattern centered over the Pearl River Delta and coastal South China. Source apportionment associated PCDD/Fs mainly with industrial thermal processes, waste/biomass combustion, and aged regional background transport, while PBDD/Fs were more closely linked to brominated precursor- and material-related processes, possible traffic-related inputs and episodic thermal activities. PBDD/Fs were more strongly particle-bound than PCDD/Fs. However, PCDD/Fs exhibited systematic temperature-dependent partitioning and reasonable agreement with equilibrium-based models, whereas highly brominated PBDFs showed weak temperature dependence and poor model agreement, suggesting additional influences from source-related inputs, particle removal, and other non-equilibrium processes. Mean carcinogenic risks ranged from 6.75×10-7 to 3.29×10-6 across age groups, with the highest value observed for adults, while hazard quotients remained far below unity. These findings demonstrate distinct source, transport, and partitioning controls for PCDD/Fs and PBDD/Fs and support their independent atmospheric assessment.