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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
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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