Formation of secondary organic aerosol from the reaction of styrene with ozone in the presence and absence of ammonia and water

We report on a comprehensive investigation of the influence of ammonia and water on secondary organic aerosol (SOA) formation from the styrene–ozone system. The presence of ammonia and water each affected the gas-phase chemistry leading to SOA formation, thereby impacting the total aerosol yield for the system. Two lumped products using the classic semi-empirical gas–particle equilibrium model (α1, K1, α2, and K2 were estimated as 0.0490, 0.3410, 0.1439, and 0.0040, respectively) were sufficient to predict SOA formation in the dry styrene–ozone system. We propose 3,5-diphenyl-1,2,4-trioxolane and a hydroxyl-substituted ester as the major aerosol-forming products in the dry, ammonia-free styrene ozonolysis system. Addition of excess ammonia after SOA formation rapidly and significantly reduced the aerosol volume suspended in the chamber. We believe this is due to rapid decomposition of the 3,5-diphenyl-1,2,4-trioxolane and the hydroxyl-substituted ester by nucleophilic attack from the ammonia molecule. Additional experiments with α-methylstyrene/ozone produced SOA that was unaffected by ammonia, suggesting that the addition of a methyl group led to SOA that stearically hindered nucleophilic attack by the ammonia molecule. The presence of water vapor prior to styrene oxidation was found to reduce SOA formation, likely due to inhibition of the formation of 3,5-diphenyl-1,2,4-trioxolane.