What is electric field required at the tip of sharp needle to ionize air?
See here for details:
http://en.wikipedia.org/wiki/Paschen's_law
You can use that the mean free path for electrons in air is about 5*10^(-7) meters. You always have ions and electrons in the air, so what needs to happen is that an electron has to be accelerated due to the applied electric field so that it has enough energy to ionize an atom. This means that the potential difference over 5*10^(-7) needs to be a few volts (ionazation energy is typicxally of the order of a few eV), so the electric field needs to be of the order of 10^7 Volt/meter.