Southeast Asian biomass-burning influence on PM2.5-bound PAHs and toxicity-equivalent burden across receptors in Southwest China.

Biomass burning can transport hazardous organic aerosols over long distances, yet its influence on the toxicity-equivalent burden of receptor-site PM2.5-bound PAHs remains poorly constrained. We measured 16 priority PAHs at two border sites, one mountain-background site, and one urban site in Southwest China across four seasons, with particular emphasis on springtime influence from Southeast Asian biomass burning (SEA-BB). Fire activity, backward trajectories, biomass-burning tracers, and source apportionment collectively supported substantial SEA-BB influence during spring. Compared with the other seasons, ΣPAHs and BaPeq at the border and mountain-background sites were approximately 2.6-3.4 times the corresponding other-season means, whereas no significant enhancement was observed at the urban site. PAH composition and toxicity responses also varied among receptor environments. The border sites showed elevated combustion-related PAHs and higher toxicity-equivalent burdens, whereas the mountain-background site accumulated PAH mass but exhibited lower toxicity per unit mass. Source apportionment indicated that the relative contribution of biomass burning to BaPeq was generally slightly higher than its contribution to total PAH mass. Under hypothetical sustained-exposure scenarios, modeled incremental lifetime cancer risk increased at the border and mountain-background sites during the SEA-BB-influenced period. Overall, SEA-BB influence produced receptor-dependent changes in PAH loading, composition, toxicity-equivalent burden, and modeled risk.