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Chamber studies of SOA formation from aromatic hydrocarbons: observation of limited glyoxal uptake

by S. Nakao, Y. Liu, P. Tang, C.-L. Chen, J. Zhang, D. R. Cocker III
Atmospheric Chemistry and Physics ()

Abstract

This study evaluates the significance of glyoxal acting as an intermediate species leading to secondary or- ganic aerosol (SOA) formation from aromatic hydrocarbon photooxidation under humid conditions. Rapid SOA forma- tion from glyoxal uptake onto aqueous (NH 4 ) 2 SO 4 seed par- ticles is observed in agreement with previous studies; how- ever, glyoxal did not partition significantly to SOA (with or without aqueous seed) during aromatic hydrocarbon pho- tooxidation within an environmental chamber (RH less than 80 %). Rather, glyoxal influences SOA formation by raising hydroxyl (OH) radical concentrations. Four experimental ap- proaches supporting this conclusion are presented in this pa- per: (1) increased SOA formation and decreased SOA volatil- ity in the toluene + NO x photooxidation system with addi- tional glyoxal was reproduced by matching OH radical con- centrations through H 2 O 2 addition; (2) glyoxal addition to SOA seed formed from toluene + NO x photooxidation did not increase SOA volume under dark; (3) SOA formation from toluene + NO x photooxidation with and without del- iquesced (NH 4 ) 2 SO 4 seed resulted in similar SOA growth, consistent with a minor contribution from glyoxal uptake onto deliquesced seed and organic coatings; and (4) the frac- tion of a C 4 H + 9 fragment (observed by Aerodyne High Reso- lution Time-of-Flight Aerosol Mass Spectrometer, HR-ToF- AMS) in SOA from 2-tert-butylphenol (BP) oxidation was unchanged in the presence of additional glyoxal despite en- hanced SOA formation. This study suggests that glyoxal up- take onto aerosol during the oxidation of aromatic hydrocar- bons is more limited than previously thought.

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