Dear Chathuri,

This seemingly simple question is rather difficult to answer.

First of all it has to do with definitions. Terms like gas, vapour, aerosols, suspended liquid particles are distinct terms but often used somewhat sloppy.

If we talk about tiny particles of water drops then obviously a pH value is possible (how to measure this is another thing and as said by Sebastian Schmitt is depending on individual behaviour of the components in your buffer).

If we talk about a pure gas than I have to agree with Sebastian: pH is non existing since by definition it involves liquid water.

Something that comes close to an answer of your question might be found in atmospheric research.

See for example: https://www.atmos-chem-phys.net/15/5211/2015/acp-15-5211-2015.pdf

The basic message of this paper (in relation to your question) is:

The pH of water in particles/aerosols is predicted based on the equilibrium particle H+ concentration per volume air (the H+air) and with the particle liquid water content (LCW). In other words an indirect measure for ‘our normal’ pH.

So no clear answer, still hope this is of any help to you.

Thank you very much for your answers..really helpful..I appreciate it..