Severe ozone pollution in metro Denver and the northern Front Range will lead to more expensive gas prices this summer as stations switch to a specialized fuel blend that will burn cleaner and release fewer toxic emissions into Colorado’s air. But exactly how much prices will increase remains a source of debate, with estimates ranging from less than a nickel to 60 cents per gallon — which would push gas above $4 a gallon for the first time in two years. Gas stations in a nine-county region must sell reformulated gasoline from June 1 to Sept.