Contact-passivated sensor devices allow one to measure the response of individual ultraclean single-walled carbon nanotubes to 1 ppm NO2, and show that the activation energies for desorption from nanotubes of diameters in the 1.5-3.5 nm range are of the order of 1 eV. DFT calculations based on several exchange-correlation functionals are presented and critically examined. The nature of the molecular binding is thus clarified for NO2, N2O4, and NO3, and also the dependence on the size of the nanotube. The binding strength of physisorbed NO3 is consistent with the experimental data on desorption. (C) 2016 AIP Publishing LLC.