One speaker noted that proposed regulations should be carefully assessed to foresee and prevent such unintended effects. Given those difficulties, symposium participants agreed that meeting the regulations for 2007 will require the use of aftertreatment devices that remove pollutants after they leave the engine and before they arrive at the exhaust pipe. Filters are available that can capture PM, but preventing small particles from condensing in the exhaust after release remains a challenge. Capturing NOx is even more difficult. A key problem with existing and potential control technologies is that they are poisoned by sulfur present in diesel fuel.
Sulfur is also a culprit in forming small particles in diesel exhaust streams.
Discussions of the new regulations limiting fuel sulfur occurred throughout the symposium, and the viewpoints expressed diverged considerably. Vehicle engine manufacturers argued that EPA's sulfur cap of 15 ppm is not low enough. While sulfur traps might be developed for individual vehicles, removing the sulfur at the refinery to whatever low levels are set seems preferable. Unless better fuel becomes available, the EMA contends that new diesel engines will effectively be choked out of the market. Fuel industry representatives protested that achieving that level of desulfurization would require enormous investments in refinery modification, and companies would get no payback.