starting in june 2006, petroleum refiners were required by the epa to begin producing ultra-low sulfur diesel (ulsd) fuel, which has 97% less sulfur than the previous low sulfur diesel fuel. when fuel containing sulfur is burned, sulfur dioxide is produced, a main component of acid rain. ulsd, together with new air pollution control technologies required in trucks (starting with model year 2007), will reduce harmful emissions by 90%.