The radial profiles of metastable and ground state atomic number densities at numerous sections of a supersonic nitrogen plasma jet have been derived from UV absorption and emission spectroscopy. An original method that provides the ground state number density from the ratio of the intensities of the 119.955, 120.022 and 120.071 nm resonance lines has been developed. Along the jet axis direction, and similarly to the N and N-2(+) excited states, the N ground state density decreases more steeply at the centre of the jet than at its border. At the exit of the plasma source, our results throw doubt upon the nitrogen dissociation rate usually considered in the literature.