As air is made of many kinds of molecules (nitrogen, oxygen, carbon dioxide and a host of others), I suspect that it does not have a single resonance frequency. I also suspect that the physical properties of gaseous molecules are such that it is probably nearly impossible to create such a tuning fork anyway.
However, before you create the next sci-fi superweapon, do a little research. I may be dead wrong in my suspicions; it does happen, though with fairly low, um, frequency.