Exploring Photochemical Conversion of NO2 to HONO on N-Heterocycles: Unique Variation Trend of Gases, Kinetics, and Mechanism

The photochemical conversion of NO2 to nitrous acid (HONO) on N-heterocycles and the corresponding benzene compounds was investigated through a flow tube reactor coupled with the methods of spectroscopy, mass spectrum, and theoretical calculation. The average uptake coefficient of NO2 on organics during 1 h (γa) varied in the range of (1.1–8.0) × 10–6. Light led to an increase in γa by a factor of 2.2–11.5 times. HONO was generated by electron transfer from excited organics to NO2, as shown by the linear correlation of NO2 uptake and HONO formation with electron (e–) production from organics. Especially, the temporal variation of NO2 and HONO concentrations from the NO2 uptake on 9-methyl acridine (MA-N) showed a unique trend, where the NO2 uptake and HONO formation increased with time. The photochemical reaction of NO2 with MA-N generated nitrogen-/oxygen-containing species as well as demethylation species. The coexistence of oxygen-containing species, including acridone and hydroxy-methylacridine, had a synergistic effect on the photochemical conversion of NO2 to HONO. According to experimental results, it was speculated that the heterogeneous reaction between NO2 and organics could contribute to a HONO source strength lower than 660 ppt h–1 on the near-ground surface and lower than 92 ppt h–1 in the aloft air.