Many volatile organosulfur compounds (VOSCs) are released into the atmosphere from various natural and anthropogenic sources. They have attracted attention because of the important roles they play in global warming, acid rain, and their contribution to the sulfur budget. Number of studies have reported that Allium genus cash crops such as garlic and onions that occupy large acreage on farmlands throughout the world emit high levels of VOSCs such as thiosulfinates into the atmosphere. This raises the question of whether the farming of these crops and their emission of VOSCs into the atmosphere results in organosulfur “hotspots” that may contribute to the sulfur budget and to global warming. In this work, the atmospheric oxidation of two major plant-derived thiosulfinates, dimethyl thiosulfinate (DMTS) and dipropyl thiosulfinate (DPTS) by OH and Cl radicals was investigated using high level electronic structure calculations. The reactions of DMTS+OH/Cl and DPTS+OH/Cl proceeds through: (i) abstraction; and (ii) substitution paths. The barrier heights for the possible H-abstraction and substitution paths suggest that addition of OH/Cl to the sulfur atom of the sulfinyl group (S(=O)) group, followed by simultaneous cleavage of the S(=O)−S single bond, leading to the formation of propanethiyl radical (PTR)+propanesulfinic acid (PSIA), and PTR+propanesulfinyl chloride (PSICl), are the major pathways. The rate coefficients were calculated for all pathways associated with DMTS+OH/Cl and DPTS+OH/Cl using Master equation solver for multi energy well reactions (Mesmer) code in the atmospheric temperature range 200-300 K and P=1 atm. The total rate coefficient for the DMTS+OH, DMTS+Cl, DPTS+OH, and DPTS+Cl reactions were found to be 1.4×10-11, 3.7×10-11, 1.7×10-10, and 8.1×10-10 cm3 molecule-1 s-1 at T=298 K and P=1 atm. In addition, the thermochemistry, branching ratios, and global warming potentials were determined, and the atmospheric implications for all the reactions are discussed.

Computational investigation on the atmospheric oxidation of plant-derived alkyl thiosulfinates initiated by OH and Cl radicals: Reaction mechanisms, energetics, kinetics, and atmospheric implications

ACS Spring 2022

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