Photooxidation browning mechanism of small α-dicarbonyl compounds on natural mineral particle in the presence of methylamine/ammonia

The browning mechanism of glyoxal (GL)/methylglyoxal (MGL) with methylamine (MA) and ammonia (AM) on kaolinite (KAO, proxy of natural mineral particle) surface was investigated using density functional theory. Our results reveal that GL and MGL form mixture with KAO through hydrogen bonding. Notably, the browning reaction of GL and MGL is necessarily initiated by photooxidation. Browning products (imine and diimine) are subsequently formed via reaction of GL/MGL radical with MA/AM on KAO surface. Our results highlight the significant role of photooxidation in browning reaction of small α-dicarbonyl compounds on mineral surface, providing a new insight into Brown Carbon formation during dust storm.