Two-stage aerosol formation in low-temperature combustion

•Aerosols from toluene and n-heptane low-temperature combustion were characterized.•Emission factors were both highly fuel- and temperature-dependent.•n-Heptane exhibited two-stage aerosol formation, reflecting its two-stage ignition.•Aerosols were comprised of organic molecules and are categorized as incipient soot.•Aerosols were light-absorbing and are categorized as brown carbon.