Acidity-Driven Singlet Oxygen Production in Atmospheric Aerosols through Photosensitized Oxidation of Dicarboxylic Acids and Phenols

Singlet oxygen (1O2) plays a critical role in the oxidative aging of atmospheric organic aerosols, yet the pH-dependent production mechanisms and molecular drivers remain poorly constrained. This study investigates 1O2 generation from atmospherically relevant dicarboxylic acids (pyruvic acid, 2-ketobutyric acid) and phenolic compounds (guaiacol, catechol, o-cresol) photosensitized by 3,4-dimethoxybenzaldehyde (3DMB*) across a pH range (2.5–6.5) mimicking atmospheric waters. Using furfuryl alcohol as a selective probe, we quantify steady-state 1O2 concentrations ([1O2]ss) and demonstrate that acidic conditions (pH 2.5) enhance 1O2 yields by up to 40% for dicarboxylic acids and an order of magnitude for methoxy-substituted phenols (guaiacol: 4.32 × 10–11 M vs catechol/o-cresol). Structural analysis reveals that protonation stabilizes triplet-state intermediates, while electron-donating groups (e.g., −OCH3) promote energy transfer to O2. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) further elucidates pH-dependent product formation: acidic conditions favor oligomers and polycarboxylic acids (Xc > 2.7), whereas neutral pH shifts pathways toward fragmentation. These findings highlight aerosol acidity as a key control on 1O2-driven oxidation, with implications for secondary organic aerosol (SOA) formation and organic pollutant degradation in atmospheric waters.