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 nitrogen oxides into (somewhat) harmless ammonia.

FORBES: Clearing the Air

These include the exhaust gases such as nitrogen oxides (NOx), non-methane volatile organic compounds (NMVOCs), and carbon monoxide (CO).

BBC: London's clue to stubborn ozone levels

These gases are then used to clean up smog-producing nitrogen oxides, or NOx.

FORBES: Clearing the Air

Also, to comply with prevailing U.S. emissions standards on nitrogen oxides, or NOx, these big diesels require fairly complicated systems of postcombustion treatment with a water-based urea solution, called AdBlue.

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