Atmospheric pressure air plasma discharges generate potential antimicrobial agents, such as nitrogen oxides and ozone. Generation of nitrogen oxides was studied in a DC-driven self-pulsing (1-10 kHz) transient spark (TS) discharge. The precursors of NOx production and the TS characteristics were studied by nanosecond time-resolved optical diagnostics: a photomultiplier module and a spectrometer coupled with fast intensified camera. Thanks to the short (similar to 10-100 ns) high current (>1 A) spark current pulses, highly reactive non-equilibrium plasma is generated. Ozone was not detectable in the TS, probably due to higher gas temperature after the short spark current pulses, but the NOx production rate of similar to 7 x 10(16) molecules/J was achieved. The NO2/NO ratio decreased with increasing TS repetition frequency, which is related to the complex frequency-dependent discharge properties and thus changing NO2/NO generating mechanisms. Further optimization of NO2 and NO production to improve the biomedical and antimicrobial effects is possible by modifying the electric circuit generating the TS discharge.