Ultra-Low Sulfur Diesel (ULSD) 15 ppm (15 parts per million) is the new EPA standard for refiners and importers for sulfur content in diesel fuel for highway use that went into effect on June 1. The new regulation requires that 2007 and later model year trucks reduce nitrogen oxide emissions by 50% and particulate matter emissions (soot) by 90%.

Currently, CITGO Petroleum Corporation manufactures and distributes ULSD that meet the new requirements. Stations are not required to carry ULSD but those who choose to carry it must stock fuel that meets the new ULSD standards and label their pumps accordingly.