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 1.6-2.6 times, increasing from 6.4 to 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.