Volatility and Atomic Ratio of Aromatic Secondary Organic Aerosol: Effects of Aging and Alkyl Substituents

Vaporization enthalpy (ΔH_v) is an essential thermodynamic parameter that governs the phase transitions of organic compounds, linking their volatility to their temperature-dependent gas–particle partitioning. Secondary organic aerosol (SOA) was produced by the photooxidation of aromatic volatile organic compounds (VOCs) using a newly developed Teflon flow reactor. SOA volatility was assessed using a thermodenuder and parametrized using a kinetic mass transfer model. This study examined the effects of aging on the SOA volatility, chemical composition, and mass yield volatility. Variations in the ΔH_v values of SOA driven from different aromatic VOCs were linked to the chemical structure of their aromatic precursors. Elevated hydroxyl exposures increased the oxygen to carbon ratio of SOA, while ΔH_v remained relatively consistent, likely due to fragmentation offsetting the effects of increased oxidation. SOA yields were influenced by the degree of alkyl substitution and the chain length of alkyl substituents in the aromatic VOCs.