Discharge current pulse profiles have been recorded with a subnanosecond time resolution for a corona discharge from a negative-polarity needle functioning in the Trichel pulse regime. The optical characteristics of plasma glow have been measured with a high spatial resolution. This corona discharge has been numerically simulated within a two-dimensional theoretical model taking into account plasmochemical kinetics. It has been shown that Trichel pulses are initiated in atmospheric-pressure air when positive ions are concentrated near the needle to the critical concentration for an avalanche-to-streamer transition, in particular, at a dc voltage across a gap (in several or several tens of seconds). It has been established that the nanosecond pulse front is determined by the time of avalanche multiplication of electrons in the region of a locally enhanced field. It has been shown that a short duration of a Trichel pulse is due to the fast displacement of the electric field from the dense-plasma region and to the escape of free electrons from this region with the subsequent disappearance through recombination and attachment. It has been established that the density of the ion–ion air plasma in the local region in front of the needle in an interval between Trichel pulses remains at a level of 10 11 cm –3 , which ensures the subsequent generation of new Trichel pulses.