Dark NO₂ Reduction on a Graphene Surface with Implications for Soot Aging and HONO Formation

Area of Science:

• Atmospheric Chemistry
• Environmental Science
• Materials Science

Background:

• Soot, a major component of PM2.5, significantly impacts air quality and atmospheric processes.
• The heterogeneous reaction between nitrogen dioxide (NO2) and soot is crucial for soot aging and nitrous acid (HONO) formation.
• Current understanding emphasizes organic carbon's role in NO2 reduction to HONO on soot surfaces.

Purpose of the Study:

• To investigate the role of elemental carbon (EC) in the heterogeneous reaction between NO2 and soot.
• To elucidate the mechanism of NO2 conversion to HONO on soot surfaces, particularly focusing on EC.
• To challenge and expand the prevailing theory of soot aging and HONO formation.

Main Methods:

• In situ experimental investigations of soot-NO2 interactions.
• Density Functional Theory (DFT) calculations to model reaction pathways.
• Analysis of surface chemistry and defect sites on elemental carbon.

Main Results:

• Elemental carbon (EC), specifically its surface vacancy defects, acts as a reactive site for NO2 reduction.
• A novel mechanism proposes NO2 reduction to HONO via water molecule splitting on EC vacancy defects.
• Simultaneous oxidation of adjacent carbon atoms to form hydroxyl-functionalized EC occurs during the reaction.

Conclusions:

• The redox reaction between EC and NO2, mediated by surface defects, is a significant pathway for HONO formation.
• This finding provides a new perspective on the heterogeneous chemistry of soot, particularly its aging process.
• Expands the understanding of atmospheric chemical mechanisms involving particulate matter and nitrogen oxides.