Secondary Organic Aerosol Formation by Cloud Processing: Accretion Reactions Involving Glyoxal and Methylglyoxal in Evaporating Cloud Droplets

Glyoxal and methyl glyoxal are dicarbonyl compounds found in atmospheric cloud and fog water, typically at low micromolar concentrations. These two compounds are known to form copolymers under certain industrial conditions by the nucleophilic addition of S, N and O-containing molecules. We report ambient FTIR-ATR and particle chamber data on a range of reactions between glyoxal and S, N and O-containing molecules found in cloudwater, some of which are triggered by droplet evaporation. Liquid-phase formation of adducts between glyoxal and S(IV) is seen to halt sulfur oxidation during droplet drying on the ATR crystal. Formation of glyoxal / S(VI) adducts, however, are not observed by ATR. At neutral or acidic pH, droplet evaporation triggers a reaction between glyoxal and amino acids in the residue left behind, forming imines. Glyoxal reacts under similar conditions with glycol compounds, forming cyclic acetals, but not with sugars, perhaps due to a lack of conformational freedom. Glyoxal is not observed to react with carboxylic acids, either in particle chambers or while drying on an ATR crystal.