Can citric acid be used instead of cream of tartar?

Yes.

It is roughly four times as strong, so you have to reduce the amount used by four. If your recipe calls for a teaspoon of cream of tartar, use 1/4 teaspoon citric acid.

However if it only calls for a pinch -- for instance when you're making meringue -- then you can use a pinch of citric acid in place of a pinch of cream of tartar.