Not a chemist but I believe since a diesel ignites from the heat of compression, they form more oxides of nitrogen, due to the high combustion temperatures, and until recently diesel fuel had sulfur in it, so in the oil they formed nitric and sulfuric acids. They lowered the compression ratios on gas engines, in the 70's to lower cylinder temperatures, which stopped oxides of nitrogen. Then they invented the dual bed catalytic converters, which treated the oxides of nitrogen in the 90's and they started raising compression ratios up again, to make them run decent again. Also diesel fuel has more carbon atoms than gasoline which also changes the combustion chemistry as well. Diesel and gas have almost the same stoichiometric ratio around 14 to 1, but diesel has a much wider flammability rate, so they can run very lean like 40 to 1 The reason diesel makes more power but smokes so much when you dump extra fuel in them, is that when you get to stoichiometric, they make tons of soot, due to the extra carbon atoms in the fuel.