Influences of O2 and O3 on the heterogeneous photochemical reaction of NO2 with humic acids

- The γ decreased with increasing the O2 content and the O3 concentration.
- The HONO yield was found to be independent of the O2 content.
- The HONO yield inversely depended on the O3 concentration.
- Aging of HA by O2 and O3 led to the lower reactivity of HA toward NO2.
- Mechanism of effects of O2 and O3 on the reaction of NO2 with HA was discussed.

Oxidizing components in the atmosphere may play competitive roles in the heterogeneous photochemical reaction of NO2 with humic acids (HA). Effects of O2 and O3 on the conversion of NO2 to HONO on HA under simulated sunlight were investigated using a flow tube reactor. The uptake coefficient (γ) of NO2 and the HONO formation rate decreased with the increase of the O2 content (0%-20%) and the O3 concentration (0-100 ppb). The HONO yield was observed to be independent of the O2 content, whereas it inversely depended on the O3 concentration. In addition, the aging process of HA by O2 and O3 under irradiation resulted in the decrease in the reactivity of HA toward NO2, as shown by lower γ and HONO formation rate, while it has little influence on the HONO yield. Finally, the mechanism of role of O2 and O3 in the photochemical reaction of NO2 with HA was discussed in detail.