Also, to comply with prevailing U.S. emissions standards on nitrogenoxides, or NOx, these big diesels require fairly complicated systems of postcombustion treatment with a water-based urea solution, called AdBlue.
In Europe the trucking industry has settled on a technology called selective catalytic reduction, in which urea is periodically injected into the exhaust system to break down the NOx, converting nitrogenoxides into (somewhat) harmless ammonia.