...here's what EPA says:
• EPA regulates the
volatility/Reid Vapor Pressure (RVP) of conventional gasoline sold at retail stations during the
summer smog season (
June 1-September 15) to reduce evaporative emissions that contribute to smog.
•
Winter Oxygenated Fuel programs increase fuel oxygen and are mandated in certain areas for carbon monoxide control. The
winter oxygenated fuel season is generally
October through February or
March.
Source:
http://www.epa.gov/otaq/fuels/gasolinefuels/index.htm