Competitive secondary formation and evolving sources drive the stage-dependent evolution of PM_2.5-bound PAHs and...

Polycyclic aromatic hydrocarbons (PAHs) and their oxygenated (OPAHs) and nitrated (NPAHs) derivatives are critical carcinogenic constituents of PM_2.5, yet high-resolution observations of their dynamic evolution remain scarce. This study characterized the 4-h spatiotemporal evolution and source-apportioned risks of these compounds across four Guanzhong Plain cities including Xi'an (XA), Baoji (BJ), Tongchuan (TC), and Weinan (WN). The analysis spanned the development (S1), persistence (S2), and abatement (S3) stages of winter haze episode. Average ΣPAHs and ΣOPAHs concentration was 105.18 ± 83.35 ng·m^-3 and 23.76 ± 51.50 ng·m^-3, respectively, and peaked in TC and WN, driven by primary combustion. ΣNPAHs increased to 36.3% in XA during stage S2, highlighting enhanced secondary formation. Stage-resolved source apportionment identified coal and biomass combustion as dominant drivers during stage S1 and S3, while secondary NPAHs increased during S2. Machine learning revealed an episode-specific non-linear framework in which NO₂ and relative humidity serve as the primary regulators governing the competitive balance between inorganic and organic nitration. The humidity-driven promotion of secondary NPAHs emerges only when NO₂-driven competition is weakened, explaining the localized pollution surges observed during the stage S2. Monte Carlo assessments indicated that episode-specific carcinogenic risks were primarily driven by coal combustion and biomass burning during S1 and S3, whereas secondary formation emerged as a prominent risk contributor in XA during S2. These findings underscore stage-specific mitigation strategies that prioritize primary combustion emissions during S1 periods and shift toward precursor reduction during S2 periods, thereby minimizing the episode-specific health risks associated with PAHs and their derivatives.