The spatially resolved cross-correlation spectroscopy (CCS) was used for systematic investigations of the barrier discharge (BD) in N2/O2 mixtures at atmospheric pressure. The spatio-temporal distributions of the microdischarge (MD) radiation intensities were recorded for the spectral bands of the (0–0) transitions of the 2nd positive (λ = 337.1 nm) and 1st negative system of molecular nitrogen (λ = 391.4 nm). The velocities of the cathode-directed ionization waves were evaluated from the CCS data. In the middle of the gap, the MD channel diameter was found to be about 0.3 mm and to expand towards both electrodes. On the dielectrics, outward propagating discharges were observed. A computational model of the BD is proposed, to explain the MD formation in short (1–2 mm) air gaps by a Townsend mechanism. The two-dimensional dynamics of the MD development and the channel radiation for the second positive system of nitrogen are simulated. The proposed model explains satisfactorily the experimental results on the velocity of the cathode-directed ionization wave and the MD radiation of the 2nd positive system.