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

Methyl bromide (CH₃Br) 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 CH₃Br from syringic acid (SA) that has been used as an environmentally relevant model compound for terrestrially-derived dissolved organic matter. The formation of CH₃Br increased with the increase of bromide ion concentration ranging from 0.8 to 80 mmol L^-1. Ferric ions (Fe(III)) enhanced CH₃Br production, while chloride inhibited it, with or without Fe(III). Meanwhile, methyl chloride (CH₃Cl) was generated in the presence of chloride and was inhibited by Fe(III). The different effects of Fe(III) on the formation of CH₃Cl and CH₃Br 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 CH₃Br 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 CH₃Br may act as a potential natural source of CH₃Br in the bromide-enriched environmental matrix, and helps in better understanding the formation mechanism of CH₃Br.