Theoretical Investigation on the Oligomerization of Methylglyoxal and Glyoxal in Aqueous Atmospheric Aerosol Particles

: Secondary organic aerosol (SOA) is the main component of atmospheric particulate matter with wide implications for air quality, human health, and climate change. However, the formation mechanism of SOA has not been fully understood. Methylglyoxal (MGLY) and glyoxal (GLY) are ubiquitous dicarbonyls in the atmosphere and have been considered as important precursors for SOA formation, especially through the oligomerization in the atmospheric aqueous phase. In this paper, the oligomer formation mechanism through the self-oligomerization of MGLY and GLY, and especially their cross-reactions in the atmospheric aqueous phase were investigated by density functional theory (DFT) calculations. Carbenium ions play a key role in the oligomerization, and all of the reactions take place barrierless under acidic conditions. First-generation oligomers (dimers) lead to the formation of chain and cyclic (including five- and six-membered) oligomers, among which five-membered cyclic oligomers are the dominant products. The products identified and described by the energetics of their formation pathways are discussed in view of available experimental findings from the literature. Many of the products were actually measured and assigned. In addition, the diffusion rate constants (kD) of the nucleophilic addition were determined to estimate the formation rate of oligomers in the aqueous phase, and the calculated kD's are in the range of 1.20-1.31 x 1010 L mol-1 s -1 . All of these findings will help us to gain a better understanding of the oligomerization of the dicarbonyl compounds in the atmospheric aqueous aerosol particles, and thus to provide a scientific support toward controlling and reducing SOA formation.