Ethanol

Adding ethanol to gasoline increases the Reid Vapor Pressure (RVP) of the fuel, which, in the summertime, worsens California's ozone problem. Wintertime blending is OK: the extra oxygen lowers carbon monoxide emissions. EPA's judgement to hamper California's flexibility regarding when to blend is really screwed up, and likely reflects the desires of Republican, Midwestern ethanol/farm interests to feather their own nests.