Abstract. This study evaluates the significance of glyoxal acting as an intermediate species leading to secondary organic aerosol (SOA) formation from aromatic hydrocarbon photooxidation under humid conditions. Rapid SOA formation from glyoxal uptake onto aqueous (NH₄)₂SO₄ seed particles is observed in agreement with previous studies; however, glyoxal did not partition significantly to SOA (with or without aqueous seed) during aromatic hydrocarbon photooxidation within an environmental chamber (RH less than 80%). Rather, glyoxal influences SOA formation by raising hydroxyl (OH) radical concentrations. Four experimental approaches supporting this conclusion are presented in this paper: (1) increased SOA formation and decreased SOA volatility in the toluene + NO_x photooxidation system with additional glyoxal was reproduced by matching OH radical concentrations through H₂O₂ 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 deliquesced (NH₄)₂SO₄ seed resulted in similar SOA growth, consistent with a minor contribution from glyoxal uptake onto deliquesced seed and organic coatings; and (4) the fraction of a C₄H₉⁺ fragment (observed by Aerodyne High Resolution 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 enhanced SOA formation. This study suggests that glyoxal uptake onto aerosol during the oxidation of aromatic hydrocarbons is more limited than previously thought.