Photochemical removal of NO2 in N-2 or air (5-20% O-2) mixtures was studied by using 172-mm Xe-2 excimer lamps to develop a new simple photochemical aftertreatment technique of NO2 in air at atmospheric pressure without using any catalysts. When a high power lamp (300 mW/cm(2)) was used, the conversion of NO2 (200-1000 ppm) to N-2 and O-2 in N-2 was >93% after 1 min irradiation, whereas that to N2O5, HNO3, N-2, and O-2 in air (10% O-2) was 100% after 5 s irradiation in a batch system. In a flow system, about 92% of NO2 (200 ppm) in N-2 was converted to N-2 and O-2, whereas NO2 (200-400 ppm) in air (20% O-2) could be completely converted to N2O5, HNO3, N-2, and O-2 at a flow rate of 1 l/min. It was found that NO could also be decomposed to N-2 and O-2 under 172-nm irradiation, though the removal rate is slower than that of NO2 by a factor of 3.8. A simple model analysis assuming a consecutive reaction NO2 NO -> N + O indicated that 86% of NO2 is decomposed directly into N + O-2 and the rest is dissociated into NO + O under 172-nm irradiation. These results led us to conclude that the present technique is a new promising catalyst-free photochemical aftertreatment method of NO2 in N-2 and air in a flow system. (C) 2008 Elsevier B.V. All rights reserved.