The chemisorption of NO2 on carbon nanotubes is studied by modeling the interaction between NO2 and N2O4 with an infinitely long (8,0) single-walled carbon nanotube, using planewave/pseudopotential-based density functional theory (DFT). In sharp contrast to the case of graphite, for which NO2 adsorption is physical, a NO2 radical could readily adsorb on the exterior of an (8,0) tube by addition, similar to the reaction between NO2 and alkenes. The process is slightly endothermic and reversible with a low energy barrier, with the NO2 Group in either the nitro or nitrite configuration. Adsorption of a second NO, is considerably exothermic, and desorption of NO2 front such a configuration is much more difficult. The chemisorption of NO2 also increases the conductivity of the (8,0) tube. On the other hand, N2O4 only plays a minor role in the equilibrium between desorption and adsorption processes. These results indicate that the (8,0) tube is more reactive toward NO2 than Graphite, due to the curvature on the rolled graphene sheet.