SO₂ Photoaging Enhances the Surface Conversion of NO₂-to-HONO on Elemental Carbon

Abstract

Chemical interactions between soot and NO 2 are believed to play a significant role in the formation of HONO in the atmosphere. Despite extensive studies, the present understanding of how soot chemistry influences HONO formation remains contentious due to the rapid deactivation of surface reactive sites. In this study, we reveal the novel mechanism that the photoaging of SO 2 can notably accelerate the reduction of functionalized elemental carbon (EC) in soot by rapidly removing surface hydroxyl functional groups. The reduced EC can further drive continuous HONO formation due to the rejuvenation of the surface reduction reactivity. We verify that the increase in surface vacancy defects created by the removal of OH groups is the key contributing factor and the reactive centers driving NO 2 adsorption and reduction. This finding challenges the existing notion that fresh soot is rapidly deactivated due to the decline in reductive capacity. Our work suggests that aged graphene-like EC on soot may have a significant effect on the chemical conversion of NO 2 -to-HONO in polluted air, contributing to a better understanding of air pollution chemistry.