Impulse is usually defined as a change of momentum. In your question it is more like the result of lightening strike on a high voltage power system, peak voltage and time duration of peak compared to survival of the system. .

Here is a reference to air gap breakdown that might be helpful or give some useful references.

@ Jerry Decker Sir Thanks a lot for your kind response, I couldn't find any reference in your reply.

Yes Sir I am interested to find the relationship between pulse width and the peak voltage for air breakdown at a certain pressure.

Maybe I intended to reference

http://ethesis.nitrkl.ac.in/2875/1/Full_Thesis_Print_04.07.2011.pdf

Or

http://file.yizimg.com/175706/2012061612003140.pdf

Or

http://www.journal.esrgroups.org/jes/papers/8_2_8.pdf

Thanks a lot Sir @Jerry Decker. I couldn't find any material related to my question.