Impact of Acidic Aerosol on Deceleration of SOA Formation from the Photooxidation of Phenol or Benzene

Jiwon Choi, MYOSEON JANG, University of Florida

     Abstract Number: 580
     Working Group: Aerosol Chemistry

Abstract
Understanding phenol’s oxidation mechanisms is essential to accurately predict a secondary organic aerosols (SOA) formation from the atmospheric process of benzene. The atmospheric oxidation of phenol produces a considerable amount of a Persistent Phenoxy Radical (PPR), which can catalytically consume ozone via a NO_x cycle. This PPR can negatively influence SOA formation due to the reduction of OH radicals and hydrocarbon consumption. Unexpectedly, we discovered that phenol SOA growth was considerably retarded by acidic seed. It hypothesizes that the formation of PPR from phenol oxidation is accelerated via a heterogeneous acid-catalyzed reaction of phenols in acidic seed, and also retards SOA formation. In this study, the explicit gas mechanisms of phenol were developed as a function of aerosol acidity to simulate the suppressed phenol oxidation and improve benzene oxidation mechanisms. Then, resulting gas products were incorporated into the UNIfied Partitioning Aerosol Reaction (UNIPAR) model to predict SOA formation via multiphase reactions of hydrocarbons. Gas products were lumped based on volatility-reactivity characteristics and involved in their multiphase partitioning and aerosol phase oligomerization. The suitability of gas mechanisms and SOA model was examined by simulating data obtained in the UF-APHOR chamber under different conditions (NO_x levels, humidity, and aerosol acidity) with ambient sunlight. Both chamber data and model simulation show that with sulfuric acid seed, ozone formation, the consumption of phenol and benzene, and SOA growth were significantly suppressed, compared to those without inorganic seed. The impact of aerosol acidity on phenol SOA mass is invisible due to the net effect, the acceleration of acid-catalyzed oligomerization of reactive products and the deceleration of product aging by PPR. In benzene oxidation, about 53% oxidation pathway are connected to phenol formation in the current gas mechanism. Hence, benzene is more impacted by acid-catalyzed oligomerization than phenol showing the increased SOA mass with acidic seed.