One of my off highway diesel suppliers has a 500 ppm sticker on the pump but I talked to the trucker delivering the fuel and he said it is a poorly managed station that hasn't updated their pump. The other supplier in town has a ulsd sticker on the pump and the trucker said he fills both. Unless the station is supplying marine diesel it is illegal to sell anything other than ulsd. Since combustion produces, among other things, carbon dioxide and water, there are oxygen and hydrogen atoms that can combine with the sulfur to produce H2SO4 which is sulfuric acid ergo acid rain. The sulfur also, however, is a nutrient needed by many plants so as acid rain is reduced there is a need to hit plants with sulfur.