Effects of octane boosters on the formation of oxygenated primary organic aerosol in low-temperature combustion

We have previously demonstrated that n-alkanes that exhibit two-stage ignition also exhibit two-stage primary organic aerosol (POA) formation. The first-stage POA, or oxygenated POA (OxyPOA), forms via alkylperoxy radical chemistry that involves sequential oxidation reactions of cyclic ethers generated during first-stage ignition. Here, we perform controlled-combustion experiments and chemical kinetics simulations to investigate the effect of octane boosters (toluene, ethanol, and 2,5-dimethylfuran (DMF)) on OxyPOA emissions from n-heptane as the primary fuel. Both ethanol and DMF exhibited synergistic blending for OxyPOA suppression (i.e., suppressed OxyPOA beyond what is calculated based on molar displacement of n-heptane), in concordance with their reported synergistic blending for research octane number (RON), while toluene exhibited antagonistic blending for OxyPOA suppression. DMF was the most effective at suppressing OxyPOA formation, also in concordance with its reported efficiency at scavenging OH radicals produced during first-stage ignition. This finding indicates that DMF, an advanced drop-in biofuel with superior octane boosting characteristics, also has the added benefit of reducing aerosol pollution. Chemical analysis using electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry showed that the reactions of the octane boosters with OH radicals formed OxyPOA molecules via pathways different from those produced from n-heptane. These reactions are similar to the OH-oxidation reactions that produce secondary organic aerosol (SOA) in the atmosphere. This suggests that, contrary to current understanding of atmospheric aerosols, some POA emitted from low-temperature combustion (i.e., OxyPOA) can be chemically similar to SOA. Copyright © 2024 American Association for Aerosol Research