Insight Into the Formation Paths of Methyl Bromide From Syringic Acid in Aqueous Bromide Solutions Under Simulated Sunlight Irradiation.

Methyl bromide (CHBr) is one of the largest natural sources of bromine in the stratosphere, where it leads to ozone depletion. This paper reported the photochemical production of CHBr from syringic acid (SA) that has been used as an environmentally relevant model compound for terrestrially-derived dissolved organic matter. The formation of CHBr increased with the increase of bromide ion concentration ranging from 0.8 to 80 mmol L. Ferric ions (Fe(III)) enhanced CHBr production, while chloride inhibited it, with or without Fe(III). Meanwhile, methyl chloride (CHCl) was generated in the presence of chloride and was inhibited by Fe(III). The different effects of Fe(III) on the formation of CHCl and CHBr indicate their diverse formation paths. Based on the intermediates identified by liquid chromatography-mass spectrometry and the confirmation of the formation of Fe(III)-SA complexes, it was proposed that there were two formation paths of CHBr from SA in the bromide-enriched water under simulated sunlight irradiation. One path was via nucleophilic attack of Br on the excited state protonation of SA; the other was via the combination of methyl radical and bromine radical when Fe(III) was present. This work suggests that the photochemical formation of CHBr may act as a potential natural source of CHBr in the bromide-enriched environmental matrix, and helps in better understanding the formation mechanism of CHBr.