We investigated secondary organic aerosol (SOA) formation in toluene-combustion emissions for combustion temperatures ranging between 1000 K and 1300 K. The experiments involved combusting a premixed toluene and air mixture in an atmospheric-pressure temperature-controlled reactor and then sending the emissions through an oxidation flow reactor (OFR) to simulate atmospheric aging with OH radicals. SOA production dropped sharply with increasing temperature, with a reduction by a factor of ~57 in SOA between 1000 K and 1300 K, consistent with previous studies that showed enhanced SOA formation during cold-start (low-temperature) operation compared to hot-stabilized operation of gasoline vehicles. Chemical speciation analysis of the gaseous emissions (SOA precursors) and SOA particles demonstrated that SOA produced at 1000 K consisted of molecules with higher O/C but less unsaturated and aromatic structures than SOA produced at 1200 K.

These results indicate that SOA precursors at 1000 K were largely comprised of simple aromatic species (mostly unburned toluene) that underwent OH-initiated abstraction followed by oxygen addition ring-opening reactions in the OFR, leading to the high O/C and loss of aromaticity. However, in addition to single-ring aromatics, the SOA precursors at 1200 K likely featured large-molecular-size multi-ring aromatics that retained more of their aromatic structures after oxidation. Overall, the findings reported in this paper highlight the importance of combustion temperature in determining the molecular distribution of the emitted SOA precursors and the consequent SOA formation potential, and provide fundamental reasoning for the high variability in SOA production for different operation conditions of gasoline vehicles.