Optimization of the removal efficiency of nitrogen oxides in the air using a low-pressure Hg lamp

The photo-oxidation of gaseous nitrogen oxide (NOx) by a low-pressure Hg lamp (UV254+185 nm) irradiation was studied. Nitric oxide gas (NO) was rapidly converted to nitrogen dioxide (NO2) to form nitric acid (HNO3) by continuous reactions with ozone and hydroxyl radicals which are produced by UV254+185 nm irradiation. Because HNO3 is water-soluble, it can easily be removed by water spray. It was found that high removal efficiency of NOx was recorded at the reaction conditions of humidity above 40% and the reaction temperature below 100 °C. NOx removal efficiency increases with lower inlet concentration and longer residence time. The conversion of NO2 to HNO3 by UV254+185 nm irradiation was easier than that of NO, which indicates that the pre-oxidation of NO to NO2 might be effective for enhancing the amount of NOx removal.