Secondary organic aerosol (SOA) from 17 aromatic hydrocarbons is explored in depth for simulated atmospheric photooxidation conditions. The overall objective of this study is to improve our understanding of the chemical and physical processes leading to SOA formation for aromatic systems and identify the key critical parameters necessary to accurately predict SOA formation.

SOA formation potential under low-NO_X atmospheric conditions is found to decrease as the carbon number of parent aromatic hydrocarbon increases while no aerosol formation trends are observed with carbon number for no-NO_X systems. This trend is attributed to the formation of greater fractions of low vapor pressure ring-retaining products from the lighter aromatic hydrocarbons. SOA chemical composition, measured using the Aerodyne High-resolution Time-of-Flight Mass Spectrometer (HR-ToF-AMS), is evaluated for both low-NO_X and no-NO_X conditions. Generally, aerosol formed in this study is observed to be less oxidized than ambient SOA. It is further observed that aerosol formed from aromatics with less substituents attached to the ring has a lower m/z 43 to m/z 44 ratio and a higher O/C ratio, which indicates that lighter aromatic precursors produced a more oxidized SOA than the heavier aromatic precursors. SOA Volatility further supports the observations that lower vapor pressure and more oxidized products of SOA are produced from lighter compounds. A recently proposed empirical relationship between SOA density and elemental ratio (O/C and H/C) is evaluated against real-time density measurements in an effort to extend the applicability of the empirical relationship to aromatic SOA.

Finally, a gas-phase reaction model (SAPRC11) is used to predict the OH and HO₂ levels present for the different aromatic oxidation systems. Generally, it is observed that aerosol formation decreased with increasing [OH]/[HO₂] ratios for both low NO_X and no-NO_X experiments. This trend is possibly attributed to fragmentation of intermediate compounds to form organics with higher volatility in OH radical abundant environments. It is expected that [OH]/[HO₂] ratio can be utilized to help improve the extrapolation of environmental chamber results to prediction of ambient SOA from aromatic precursors.