The new inspiration from the theoretical re-exploration of traditional autoxidation pathways leading to sulfate formation in the haze episode

The high concentration and fast accumulation of sulfate have been identified in most of the haze episodes’ events. It is well known that the autoxidation process is one of the most common and essential routes for sulfate formation. However, the detailed mechanism underlying sulfate formation, especially that from oxygen (O₂) involved autoxidation process, has not been fully established since there is no direct experimental proof available till now. In this work, we thoroughly explored the possible sulfate formation pathways involving O₂ through high-level quantum chemical calculation investigation. It is revealed that in the autoxidation process, the reaction of SO₃⁻ with O₂ will occur on the doublet potential energy surface to form SO₅⁻. We further proposed a pH dependent reaction mechanism in which SO₅⁻ will react with different forms of SO₂ (HOSO₂⁻, SO₃²⁻) through four different pathways to produce sulfate. More importantly, by calculating the branching ratios for different pathways, we found the most dominant path involving O₂ in sulfate formation in almost all the atmospherically relevant pH during pollution period. In this path, SO₅⁻ react with SO₃²⁻ through two steps: a O atom transfer process and a subsequent single electron transfer (SET) process. Detailed calculation results emphasize the importance of autoxidation mechanism in explaining sulfate formation during pollution period, and therefore provide some important and detailed information for understanding the possible reason of haze episode.