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.