Stuff stands on end in a vacuum too, though. I don't know about the effect of the presence of air exactly, but the basic phenomenon doesn't depend on it. In electrical engineering where you mostly care about voltage it's convenient to pick a relative ground, but in physics Coulomb's law is pretty unambiguous:

|F| = ke*q1*q2/r^2^

Where q are the charges in question, measured in Coulombs, r is distance and ke is a fundamental constant. For contrast voltage is energy per distance per Coulomb. If we were to add a constant charge to both sides:

|F|=ke(q1+1)(q2+1)/r^2^

|F|r^2^/ke=(q1+1)(q2+1)

|F|r^2^/ke=q1q2+q1+q2+1

You'll notice that even if we assume no charge was present in the first place, the +1 means that now the two objects will repel. Doing the same thing subtracting from one of them, assuming they're both the same, produces a difference of squares and will decrease repulsion or add attraction, again without requiring any charge in the first place.

The Earth probably does gain a very slight electric charge as it interacts with the solar wind, but it's tiny and I'm not sure if it has ever been measured.