Perhaps you should try a calming air freshener such as Feliway.

Feliway uses synthetic feline facial pheromones to end urine marking and scratching and to comfort cats in stressful, unfamiliar situations. The Plug-In unit disperses a consistent flow of Feliway in areas up to 650 square feet. Place the unit in the room(s) your cat frequents for best results. Feliway is odorless to humans.

I've never tried it but I plan on getting some when we introduce a couple of more kitties to our family. It's supposed to work wonders. Other than that, I have no suggestions.