What is the best way to get rid of strong cat pee smell?

[Jin Zhang]

Hi everyone, 

I have a tenant who has been living in one of our units with his two cats for the past two years. Everytime I visit his unit, I could smell a strong cat pee smell in the kitchen where he left the cat litter box. He is leaving soon. What is the best way to get rid of cat pee smell? Do you have any recommendations on product, device, or professional services? 

Thank you! 

Gin