The Altitude-Toxicity Paradox: Biodiesel Blends Reduce Particulate Mass but Elevate Calculated PAH Toxic Equivalency under Simulated Highland Conditions

While biodiesel (fatty acid methyl esters) decarbonizes heavy-duty transport, its impact on unregulated emissions classified as genotoxic by the IARC framework, under high-altitude conditions, remains largely unexplored. This study investigates the theoretical chemical hazard dynamics of biodiesel blends by orthogonally varying intake air pressure (900–1000 hPa) and temperature (15–30 °C) in a Euro V engine equipped with urea-selective catalytic reduction (SCR) over the European Transient Cycle (ETC). We demonstrate that while pure biodiesel (B100) reduced particulate matter (PM) mass by up to 52% relative to a B8 baseline, it substantially modified the integrated tailpipe exhaust chemistry of the combined fuel–engine–SCR system. B100 suppressed light hydrocarbons but significantly increased formaldehyde emissions from 5.45 to 6.80 mg kWh⁻¹. Crucially, despite reducing total polycyclic aromatic hydrocarbon (PAH) mass by 13.7%, fuel oxygenation shifted PAH speciation toward heavier, more potent congeners (e.g., benzo[ a ]pyrene). Consequently, B100 multiplied the calculated Total Equivalent Toxicity (TET) of the particulate phase by 1.6 to 2.6 times, increasing from 6.4–11.2 ng kWh⁻¹ (B8) to 13.9–17.9 ng kWh⁻¹ (B100), with persistent toxicity amplification under simulated high-altitude (900 hPa) scenarios. Mechanistically, established combustion theory suggests that fuel-bound oxygen suppresses primary soot while reducing radiative heat losses, which is expected to increase local flame temperatures and favor the thermal polymerization of heavier PAHs; this in-cylinder hypothesis is consistent with the post-aftertreatment speciation observed at the tailpipe of the integrated fuel-engine-SCR system. These novel findings reveal an “altitude-toxicity paradox”, demonstrating that conventional mass-based regulations fail to capture actual chemical hazards in highland regions. Future emission policies must integrate toxicity-weighted metrics to prevent inadvertently substituting visible soot for highly hazardous compounds. • B100 cuts PM mass by 52% but doubles toxicity equivalence (TET). • Low pressure alters PAH speciation, elevating estimated carcinogenic risks. • Mass-based rules fail to capture biofuel risks in highland cities. • Oxygenated fuels shift emissions to heavier, potent PAH congeners. • Variable intake conditions drive a critical altitude-toxicity paradox.