Spatial profiles of N 2+ concentration are measured in an atmospheric pressure nitrogen glow discharge by cavity ring-down spectroscopy. At discharge currents of~ 100 mA, we measure centreline ion concentrations on the order of 10 12 cm-3, and radial profiles with radial-half-maximum of~ 1 mm. The measurements have sub-ppm concentration sensitivity, and sub-millimetre spatial resolution. Using a collisional-radiative model we relate the ion concentrations to electron number densities, and we find good agreement with spatially integrated electrical measurements. The measured concentrations indicate significant nonequilibrium in the discharges.