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

Soot microstructure significantly influences the photochemical conversion of NO₂ to HONO, and potential regulatory pathways remain insufficiently understood. Here, we systematically investigated the photochemical pathways of NO₂ with soot and identified key structural factors governing the soot photoreactivity. Soot samples exhibited obviously distinct photoreactivity, as evidenced by varying NO₂ uptake coefficients and HONO yields. NO₂ uptake and HONO formation were positively correlated with photogenerated electrons (e^-), suggesting that NO₂ was reduced to HONO by accepting free e^- on soot. Edge and surface defects in soot acted as the critical reactive sites for e^- generation, where oxygen-containing functional groups can facilitate the e^- transfer. Directed migration of photogenerated e^- from organic carbon (OC) to elemental carbon (EC) improved e^- utilization, enabling synergistic effects of EC and OC on HONO formation. The importance of the soot microstructure in modulating e^- generation and transport was well explored for driving NO₂ photoreduction to HONO.