Role of glyoxal in SOA formation from aromatic hydrocarbons: gas-phase reaction trumps reactive uptake

Abstract

This study evaluates the significance of glyoxal acting as an intermediatespecies leading to SOA formation from aromatic hydrocarbon photooxidationunder humid conditions. Rapid SOA formation from glyoxal uptake ontoaqueous (NH4)2SO4 seed particles is observed; however, glyoxal didnot partition to SOA or SOA coated aqueous seed during all aromatichydrocarbon experiments (RH up to 80%). Glyoxal is found to onlyinfluence SOA formation by raising hydroxyl (OH) radical concentrations.Four experimental approaches supporting this conclusion are presentedin this paper: (1) increased SOA formation and decreased SOA volatilityin the toluene + NOx photooxidation system with additional glyoxalwas reproduced by matching OH radical concentrations through H2O2addition; (2) glyoxal addition to SOA seed formed from toluene +NOx photooxidation did not increase observed SOA volume; (3) SOAformation from toluene + NOx photooxidation with and without deliquesced(NH4)2SO4 seed resulted in similar SOA growth, consistent with acoating of SOA preventing glyoxal uptake onto deliquesced (NH4)2SO4seed; and (4) the fraction of a C4H9+ fragment (observed by AerodyneHigh Resolution Time-of-Flight Aerosol Mass Spectrometer, HR-ToF-AMS)from SOA formed by 2-tert-butylphenol (BP) oxidation was unchangedin the presence of additional glyoxal despite enhanced SOA formation.This study suggests that glyoxal uptake onto aerosol is minor whenthe surface (and near-surface) of aerosols are primarily composedof secondary organic compounds.