Methylglyoxal Forming Triplets via Photolysis: From an aqSOA Precursor to a Source of Photosensitizers

Methylglyoxal (MGly) is a prevalent dicarbonyl and is commonly recognized as a precursor of secondary organic aerosol (aqSOA) in the atmosphere. Through combined laboratory experiments and DFT calculations, this work reveals that aqueous MGly photolysis generates reactive triplets capable of efficiently oxidizing organics. Upon irradiation, MGly transitions to the singlet (¹MGly*) excited state and then the triplet (³MGly*) excited state, and both can undergo photodissociation into formyl (CHO^•) and acetyl radicals (CH₃CO^•). Self-reaction of CH₃CO^• produces biacetyl, a potent photosensitizer that efficiently becomes the triplet excited state (³biacetyl*). These triplets, in turn, might also react with MGly. When glyoxal (Gly) was mixed with MGly or different photosensitizers, aqueous MGly photolysis uniquely induced the photooxidation of Gly, outperforming the well-established photosensitizers of imidazole-2-carboxaldehyde (2-IC, triplet ³2-IC*) and 3,4-dimethoxybenzaldehyde (DMB, triplet ³DMB*). When MGly was mixed with 2-IC or DMB, the decay rates of 2-IC or DMB upon MGly photolysis were ∼5 and ∼80 times higher than their corresponding photolysis rates, indicating MGly photolysis-derived triplets (³MGly* and ³biacetyl*, collectively ³C*) have a higher oxidative capacity than ³2-IC* and ³DMB*. Under atmospheric conditions, the estimated triplet concentration ([³C*]) from aqueous MGly photolysis can reach 10^-14 to 10^-13 M, matching field-observed triplet concentration levels. An empirical relationship between [³C*] and MGly photolysis rate, P_MGly (=j_MGly × [MGly]), was developed as log₁₀ ([³C*]) = 0.4 × log₁₀ (P_MGly) - 10.1. This study highlights a potentially overlooked source of triplets (or photosensitizers) resulting from aqueous MGly photolysis, extending the role of MGly beyond an aqSOA precursor, especially in cloud and fogwater.