Size-segregated airborne polycyclic aromatic hydrocarbons down to PM0.1 in urban tropical environment: spatial distribution, health risk assessment, and potential sources

Polycyclic aromatic hydrocarbons (PAHs) are toxic and carcinogenic air pollutants whose health impacts depend strongly on the size of the particles to which they are associated, with smaller particles having greater potential to penetrate deeper into the respiratory tract. This study presents a comprehensive size-segregated investigation of particulate-bound PAHs (PM 10 , PM 2.5 , PM 1.0 , PM 0.5 , and PM 0.1 ) collected at five representative sites across Thailand, encompassing urban, industrial, semi-urban, and coastal environments. Thirteen PAH congeners were quantified, and their sources were identified using diagnostic ratios, hierarchical cluster analysis, principal component analysis, and positive matrix factorization, complemented by back-trajectory analysis. The results revealed pronounced enrichment of total PAHs and high-molecular-weight carcinogenic congeners in ultrafine particles (PM 0.1 ), which accounted for the largest proportion of toxic PAHs despite their relatively low mass contribution. Traffic-related emissions, including distinct diesel combustion modes, were identified as the dominant sources across all size fractions, while biomass burning contributed substantially, particularly during regional transport episodes. Lifetime lung cancer risk (LLCR) assessments consistently increased with decreasing particle size, reaching the order of 10 −4 in PM 0.1 at combustion-impacted sites, indicating that health risks may be substantially underestimated when assessments rely solely on PM 2.5 or PM 10 . These findings highlight PM 0.1 as a critical vector of PAH exposure and underscore the importance of incorporating ultrafine particles into air-quality monitoring and health-risk management frameworks.