What is the difference between ULSD and #2 diesel?

ULSD (Ultra Low Sulfur Diesel) and #2 diesel differ primarily in sulfur content and environmental impact, with relevance to local fuel standards. #2 diesel is a traditional grade of diesel fuel with a higher sulfur content (historically up to 500 ppm or more in older formulations), while ULSD refers to diesel with extremely low sulfur levels—typically below 15 ppm, and in local supply, often as low as 10 ppm to meet Euro 5 emission standards. This key difference translates to lower emissions: ULSD reduces harmful pollutants like sulfur dioxide (SO2), particulate matter (PM), and nitrogen oxides (NOx), aligning with stricter air quality regulations. ULSD is also compatible with modern diesel engines equipped with advanced emission control systems such as diesel particulate filters (DPFs) and selective catalytic reduction (SCR), which can be damaged by high-sulfur fuel. For performance, both fuels have similar energy content, so fuel efficiency remains comparable, though ULSD may include added lubricity agents to compensate for reduced natural lubricity from lower sulfur, ensuring engine components like injectors are protected. In local markets, ULSD is now the standard diesel available, replacing older #2 diesel variants to support cleaner transportation.