Mechanistic insight into the formation of aromatic organosulfates and sulfonates through sulfoxy-radical-initiated reactions in the atmospheric aqueous phase

Abstract

Despite monocyclic aromatic organosulfates (OSs) and sulfonates have been ubiquitously detected in aerosols, their formation processes need to be further analyzed. Monocyclic aromatic hydrocarbon reactions with sulfoxy radical are considered as the significant formation pathway of these monocyclic organosulfur compounds in recent studies, but the reaction mechanism remains unknown. In this work, we selected benzoic acid as a representative monocyclic aromatic hydrocarbon to study its reaction with SO₃^·- and SO₄^·- using quantum chemical calculation. The result shows that SO₄^·- is the main initiator for benzoic acid and its anion (benzoate). And dragging an electron from benzoate (or an H atom from benzoic acid) to produce the radical C₆H₅COO· is the most favorable route in SO₄^·--initiated benzoate (or benzoic acid) reactions due to the lowest activation free energies (5.14 or 5.47 kcal mol⁻¹). In addition, the addition of SO₄^·- to benzoate (or benzoic acid) can also occur because their activation free energies are close to the value of the most favorable route. The analysis on rate constants indicates higher pH can facilitate the formation of these OSs and sulfonates. The produced OSs and sulfonates have different characteristics: only phenyl sulfonate is formed for monocyclic aromatic sulfonates, while both phenyl sulfate and carboxyphenyl sulfate are generated for monocyclic aromatic OSs. This mechanism study can extend our understanding about the formation of monocyclic aromatic OSs and sulfonates, and provide the insight into further studies on aromatic OSs and sulfonates.