Microstructure-Mediated Electron Transfer Dominates the Photoconversion of NO₂ to HONO on Soot

Area of Science:

• Atmospheric Chemistry
• Environmental Science
• Materials Science

Background:

• Soot's role in nitrogen dioxide (NO2) conversion to nitrous acid (HONO) is crucial but poorly understood.
• Soot microstructure significantly impacts photochemical reactions, affecting air quality and atmospheric processes.

Purpose of the Study:

• To systematically investigate the photochemical pathways of NO2 with soot.
• To identify key structural factors governing soot photoreactivity in NO2 conversion.

Main Methods:

• Characterization of soot microstructure and its influence on NO2 photoreactivity.
• Quantification of NO2 uptake coefficients and HONO yields under varying conditions.
• Analysis of photogenerated electron (e-) dynamics and their correlation with HONO formation.

Main Results:

• Soot samples showed distinct photoreactivity, with NO2 uptake and HONO formation positively correlated with photogenerated electrons (e-).
• Edge and surface defects on soot are critical sites for e- generation, facilitated by oxygen-containing functional groups.
• Directed electron migration from organic carbon (OC) to elemental carbon (EC) enhanced e- utilization and HONO formation.

Conclusions:

• Soot microstructure, particularly defects and carbon components (EC/OC), critically modulates NO2 photoreduction to HONO.
• Understanding these microstructural effects is key to predicting and potentially regulating atmospheric HONO formation from soot emissions.