Theoretical Chemistry Calculations For Secondary Organic Aerosol Formation From Methylglyoxal And Glyoxal

Secondary organic aerosol（SOA）is a major component of atmospheric particulate matter and has broad implications for air quality,human health,and climate change.Dicarbonyl compounds,such as methylglyoxal（MGLY）and glyoxal（GLY）,are considered to be important precursors for SOA formation,especially their oligomerization and free radical reactions are one of the most important sources of SOA in the atmospheric aqueous phase.Currently,several studies focused on the reactions of SOA formation from MGLY and GLY in the aqueous phase,but the understanding of their specific reaction mechanisms is still not comprehensive.Therefore,it is very necessary to investigate the formation of SOA by oligomerization and free radical reactions of MGLY and GLY in the atmospheric aqueous phase.In this study,the typical dicarbonyl compounds MGLY and GLY were selected as targets,and their mechanisms of oligomerization（including the self-oligomerization and the cross-reactions）were investigated by Density Functional Theory（DFT）at the level of M06-2x/6-311++g（d,p）//M06-2x/6-311++g（3df,3pd）.The oligomerization reactions mainly include protonation,nucleophilic addition,hydration,and removal of hydronium ions,and all reactions initialized from protonation are barrierless.The formed oligomers are chain and cyclic（including five-membered and six-membered rings）structures,among which five-membered cyclic oligomers are the main products in this study.The products identified and described by the energetics of their formation pathways are compared in view of available experimental findings from the literature.Many of the products proposed in this study were actually measured and assigned.In addition,diffusion rate constants（kD）of nucleophilic addition were determined to estimate the formation rate of oligomers in the aqueous phase and the calculated kD are in the range of(1.20^-1.31)× 10¹⁰ L mol^-1s^-1.All these findings will help us to gain a better understanding on the oligmerization of the dicarbonyl compounds in the atmospheric aqueous aerosol particles and thus,to provide a scientific support on controlling and reducing the SOA formation.In addition,the rate constants of the H-abstraction reactions of OH/NO₃/SO₄^-radicals with MGLY were investigated by transition state theory（TST）.According to our calculation results,the H-abstractions of OH radicals with MGLY is the most favorable reactions with energy barrier of 8.20 kcal mol^-1,which is lower than the energy barriers of 8.33 and 9.64 kcal mol^-1 for NO₃ and SO₄^-radicals.In terms of kinetics,the rate constants for the reactions between OH/NO₃/SO₄^-and MGLY were 1.34 × 10⁸ M^-1 s^-1,1.28 × 10⁸ M^-1 s^-1 and 1.61 × 10⁷ M^-1 s^-1 at 298 K,respectively,which indicated the high reactivity of OH radicals.Furthermore,T-dependent rate constants in the range of 278-318K were studied by DFT calculations and KiSThelP software.From our results,the rate constants of the OH radicals ranged from（1.59-2.43）× 10⁸ M^-1 s^-1,which was positively correlated with temperature,while the rate constants of the NO₃ and SO₄^-radicals were negatively correlated with temperature and in the range of(1.13^-1.49)× 10⁸ M^-1 s^-1 and（1.34-2.20）× 10⁷ M^-1 s^-1.