Online Speciation of Glyoxal Multiphase Reactions on Deliquesced Ammonium Sulfate Particles

The reaction between glyoxal and ammonia in bulk Ammonium Sulfate (AS) aqueous solutions is known to be slow under acidic conditions, limiting its atmospheric relevance in wet acidic aerosol conditions. However, previous chamber experiments observed the formation of chromophores during the reactive uptake of gaseous glyoxal on ammonium sulfate particles within minutes. The mechanisms remained unresolved due to the lack of organic speciation and the Relative Humidity (RH) did not allow the seed particles to deliquesce. This study presents chemical speciation of the gas and the particle phases during the uptake of gaseous glyoxal on deliquesced AS seed particles (RH ≥ 80 ± 3%) in the CESAM simulation chamber. The chemical speciation of the particles was performed using AMS measurements complemented by a CHARON-PTR-ToF-MS. The influence of H₂O₂ addition, seed acidity and UV–vis photolysis was explored. Fast reactive uptake of gaseous glyoxal on AS particles was consistently observed in all experiments. The combined measurements from the two mass spectrometers enabled source apportionment analysis through positive matrix factorization of the AMS data and led to the identification and quantification of three dominant processes: (i) glyoxal hydration, (ii) fast dark aging of glyoxal, including the formation of brown carbon and (iii) photochemical aging. The CHARON-PTR-ToF-MS allowed the identification of individual products, such as oxazole and imidazole-2-carboxaldehyde, appearing within minutes during the chamber experiments. A detailed mechanism of glyoxal uptake was proposed, highlighting significant differences in kinetics and dominant reaction pathways compared to the glyoxal reactivity in the bulk aqueous phase.