Microstructure-Mediated Electron Transfer Dominates the Photoconversion of NO ₂ to HONO on Soot

Soot microstructure significantly influences the photochemical conversion of NO 2 to HONO, and potential regulatory pathways remain insufficiently understood. Here, we systematically investigated the photochemical pathways of NO 2 with soot and identified key structural factors governing the soot photoreactivity. Soot samples exhibited obviously distinct photoreactivity, as evidenced by varying NO 2 uptake coefficients and HONO yields. NO 2 uptake and HONO formation were positively correlated with photogenerated electrons (e – ), suggesting that NO 2 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 NO 2 photoreduction to HONO.