Reactivity of NO2 in air with urea supported on pitch based activated carbon fiber (ACF) was examined by its concentration from 50 to 1000 ppm at room temperature to establish a basic scheme for its reduction in an open atmosphere. NO2 was found to be reduced into N-2 with urea supported on the ACF and its reduction continued until the complete consumption of urea. A small amount of NO was liberated during at the same time. The conversion of NO2 appears to depend on the amounts of both urea and the ACF, although too much concentration of urea on ACF of limited amount decreased the NO2 conversion. O-2 concentration in air was not influential on the reduction of NO2, whereas humidity accelerated the formation of HNO3 over the ACF, resulting in the extension of NO2 removal period. The constant conversion of NO2 was obtained regardless of NO2 concentration, effective reduction of very low concentration of NO2 being expected. Following reactions are suggested to take place over the ACF: 2NO(2) --> NO(3)ad + NO(disproportionation); NO2 + NO + (NH2)(2)CO --> 2N(2) + CO2 + 2H(2)O; NO(3)ad + (NH2)(2)CO --> 1.5N(2) + CO2 + 2H(2)O; being summed up as the following overall reaction: 6NO(2) + 4(NH2)(2)CO --> 7N(2) + 4CO(2) + 8H(2)O. Approximately 1.8 molecules of NO2 were reduced by one molecule of urea except for the following reaction: NO2 + NO(3)ad + H2O --> 2HNO(3) down arrow Through above reaction, NO2 was removed as nitric acid. (C) 2004 Elsevier B.V. All rights reserved.