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.
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.