Photochemical removal of NO₂ in air at atmospheric pressure using side-on type 172-nm Xe₂ excimer lamp

The photochemical removal of NO₂ in air (1–20% O₂) was studied using a side-on type 172-nm Xe₂ excimer lamp. The removal rate of NO₂ using the side-on lamp (SL), 37.5 min⁻¹, was faster than that using a head-on lamp (HL), 5.4 min⁻¹, with the same input power of 20 W by a factor of 6.9. The energy efficiency for removal of NO₂ using the SL was 3.2 g/kWh, which was 12 times higher than that using the HL. Significant enhancement of the removal rate and the energy efficiency using the SL was attributed to a significant increase in the irradiation volume because of a wider irradiation window. Under the SL irradiation, HNO₃ was a major final product that differed from N₂O₅ obtained under the HL irradiation. To obtain the information related to the conversion mechanism of NO₂ to HNO₃ under 172-nm photolysis in air, computer simulation of the reaction processes was conducted. Results show that the OH + NO₂ reaction is a major pathway for the formation of HNO₃ in the SL, where OH radicals are formed by 172-nm photolysis of H₂O.